Weasel
Red Skull Member
Oh and the pic of the White TJ/JK is a great one of why we don't worry much about ackerman except for maximing the outside tire. We don't want to comprimise the spring rates for compliance so we live with lifting tires.
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Ifs 101
- Thread starter patooyee
- Start date
TRINDU
PBB Orphan
Speaking of which, is there such a thing as pre fab knuckles for IFS? Or a preferred production one?
Isdtbower
In for Tech
I put this here for those that want a different preacher. It's going to clutter up the thread but so-be-it. This was written several years ago
********************************************
Sometimes it is the Cold Holidays where we have more time to relax and catchup on thoughts and background reading. (Print this for the library???) Damien has been on my IFS "team" of designers since
Shannon introduced it to KOH racing. He is a treasure of knowledge and unfortunately he sits with a 4x4 desert truck unfinished in Paso Robles, CA. It is actually pretty special and contains some of the "new" technology being thrown at high dollar TT, Pro and KOH cars. Plus he added adjustability depending on the "compromises."
Minibuggy.net has always been a site where I go back to every several months. It is easier for them to experiment and report out as comp is not a part of their domain. Damien has a stickie in the suspension section that is excellent GENERAL reading and sums up tech that can easily go un-noticed and un-engineered. This is not an easy read, but if you are into IFS or contemplating it, it is something that you and your designer need to relate to.
Most of us get the simple geometry, but Damien underscores the need to add body roll and tire compression during jumping and turning into the engineering. Suspension geometry to the ground...not within the car.
Minibuggies are also not generally 4X4 so you need to add in the thinking of the front end pulling you around if that is how you are rolling. Searching ideas from Rally and Front Wheel Drive drifters, and applying to our huge dimensional changes are interesting.
He was the first to teach me to design from the tire and wheel inward. That pushed me over my pay grade (no CAD simulation) and I have been really lucky to be connected with the few knowledgeable designers, builders asking questions and trying things, parts suppliers adding their expertise, and drivers that can feel and express the differences.
Damien:......I would not necessarily recommend the milliken and gilespie books, although I own and have read most of both. Unless you are an engineer you will probably find them very dry and not a lot of info that is directly applicable to minibuggies. There is not a lot written about designing offroad stuff. The concepts do transfer but it does take a lot of thought to compare the two, and I think a lot of the research that has been done on road has not been replicated off road, and if it has there are so many terrain types the info is a best guess at best. I would recommend easier reading like chassis engineering (I think thats what its called) by herb
adams or tune to win or any of the ...to win books by carol smith. Those are pretty straight forward. (Edit 2021: RC cars have some good info but don't come close to the same power to weight ratios. A few other sites are out there also. BB)
as far as your parameters-- I would definitely think about camber through the suspension travel as well as in roll. As far as max roll-- I found a front view picture of Robby Gordon in his TT sliding sideways on asphalt, and I took a protractor to my screen and measured it. I know its ghetto but it put me in the ball park. Getting your rollcenter height where you want it is probably not as important as hitting your travel target. Also your roll center height/ cg height/ roll couple only deals with roll between your chassis and wheels, not the ground, a good amount of roll will be due to your outside tire compressing and inside decompressing. I've heard that keeping your rollcenter relatively fixed in roll "feels" better in a corner. roll centers can move all over the place in roll even outside the track due to suspension design. I don't know that you could feel this offroad though. Also as someone mentioned before, often in a corner the rear is sliding and the front wheels are counter steered. the front outer wheel is losing camber due to caster. if they are not counter steered like in a berm situation then the front outer in gaining camber due to caster. One thing you forgot to mention is bump steer, caster, and scrub radius. getting those three parameters right will definitely be very important for the drivability of your car.
**********
As you probably can figure, designing a suspension is full of trade offs. you make one aspect better and that screws something else up. as far as prioritizing everything. obviously if you get one of those things way off it may be undrivable. I think you may have the wrong definition of scrub radius. it is the distance between the center of the contact patch of the tire on the ground and a point on the ground made by a line going through the upper and lower pivot points on the spindle. typically you shoot for zero to 1 inch inboard of the contact patch center.
So my procedure for design is (everyone's is different, I probably left something out) : Figure out how much travel you want and what the car is going to be used for. A desert car may give up some camber control in turning for more wheel travel. A forest trail car may do the opposite. Decide on your rolling radius of your tire, then fix the scrub radius and caster (or trail total). your scrub radius will be a combination of king pin inclination (kpi), wheel offset, hub width, and tire rolling radius. Those two things don't have a lot of trade off or compromise, they are what they are. This forms some constraints on your upright. Then fix your outer lower A-arm pivot point along your KPI line where you want it. (Typically where the snout intersects the kpi line is a good place. Then fix your inner lower A-arm pivots. Depending on frame considerations there are probably a lot of restrictions of where it will go. You generally want the A arms as long as possible without compromising the strength of the design, this reduces scrub which is good.
At this point (on paper, ) you have a lower A-arm connected to an upright with wheel attached, no steering, upper a-arm or upper a arm mount on the upright. Ignore the steering for right now and pretend that there is no bump steer. Now you have to place your upper A- arm. We know the outer pivot has to be on the kpi line, it has to be above the lower pivot by enough to make room for steering etc. also the further apart you set them the less the slop in the joints matter and the less load (in some circumstances) your arms and pivots see. A vertical separation between the lower and upper outer pivots of about 20%-40% of the tire diameter seems pretty normal. Fix the outer upper pivot. Then we have to fix the upper inner pivots. This is typically where personal preference comes into play. The inner upper pivots are almost always at least as far out board as the lower inner pivots. The vertical separation between the upper and lower inner pivots is usually the same or less than the outer pivots. This is where a kinematic suspension design program is great. I use wingeo 3, people use solid works, some people use 2d scale front view paper models with push pins for pivots. Move the inner upper pivot around and go through ride and roll iterations and combinations of both till you feel like you have the best control of your wheel possible. Placing your front view swing arm length at about 1.8-2.2 track widths seems to yield a good compromise. Keeping your outer tire upright in roll will be easy but then you end up with massive positive camber in droop, and vice versa, its all a compromise. If you end up with positive camber in droop, you may be able to cure it with a couple degrees of negative camber at static ride height. You may end up with negative camber in droop neutral at ride height and negative in bump. This is because your front view IC is moving from inboard to out board.
I would look at your roll center height compared to your cg height guess. Your RC will probably be between your cg and the ground. That is fine. Now you are all done except for steering. You have to think of Ackerman now, or consciously ignore it. There is the top view method, its pretty easy to point the steering arms at the center of the rear axle and be done with it. If you are steering behind the spindle its easy, if you are steering in front you may hit the tire with your outer steering pivot. There are all sorts of theories on Ackerman in conjunction with slip angles... (Data on tire slip angles vs. load on offroad tires are non existent so it would be a guess.) probably low on the priority list, I ignore slip angles. You can also get Ackerman by moving your rack fore and aft. If you are going for 100% Ackerman or 100% plus slip angles, I'd say its most important to get it right at full lock, and let it be off everywhere else.
********************************************
Sometimes it is the Cold Holidays where we have more time to relax and catchup on thoughts and background reading. (Print this for the library???) Damien has been on my IFS "team" of designers since
Shannon introduced it to KOH racing. He is a treasure of knowledge and unfortunately he sits with a 4x4 desert truck unfinished in Paso Robles, CA. It is actually pretty special and contains some of the "new" technology being thrown at high dollar TT, Pro and KOH cars. Plus he added adjustability depending on the "compromises."
Minibuggy.net has always been a site where I go back to every several months. It is easier for them to experiment and report out as comp is not a part of their domain. Damien has a stickie in the suspension section that is excellent GENERAL reading and sums up tech that can easily go un-noticed and un-engineered. This is not an easy read, but if you are into IFS or contemplating it, it is something that you and your designer need to relate to.
Most of us get the simple geometry, but Damien underscores the need to add body roll and tire compression during jumping and turning into the engineering. Suspension geometry to the ground...not within the car.
Minibuggies are also not generally 4X4 so you need to add in the thinking of the front end pulling you around if that is how you are rolling. Searching ideas from Rally and Front Wheel Drive drifters, and applying to our huge dimensional changes are interesting.
He was the first to teach me to design from the tire and wheel inward. That pushed me over my pay grade (no CAD simulation) and I have been really lucky to be connected with the few knowledgeable designers, builders asking questions and trying things, parts suppliers adding their expertise, and drivers that can feel and express the differences.
Damien:......I would not necessarily recommend the milliken and gilespie books, although I own and have read most of both. Unless you are an engineer you will probably find them very dry and not a lot of info that is directly applicable to minibuggies. There is not a lot written about designing offroad stuff. The concepts do transfer but it does take a lot of thought to compare the two, and I think a lot of the research that has been done on road has not been replicated off road, and if it has there are so many terrain types the info is a best guess at best. I would recommend easier reading like chassis engineering (I think thats what its called) by herb
adams or tune to win or any of the ...to win books by carol smith. Those are pretty straight forward. (Edit 2021: RC cars have some good info but don't come close to the same power to weight ratios. A few other sites are out there also. BB)
as far as your parameters-- I would definitely think about camber through the suspension travel as well as in roll. As far as max roll-- I found a front view picture of Robby Gordon in his TT sliding sideways on asphalt, and I took a protractor to my screen and measured it. I know its ghetto but it put me in the ball park. Getting your rollcenter height where you want it is probably not as important as hitting your travel target. Also your roll center height/ cg height/ roll couple only deals with roll between your chassis and wheels, not the ground, a good amount of roll will be due to your outside tire compressing and inside decompressing. I've heard that keeping your rollcenter relatively fixed in roll "feels" better in a corner. roll centers can move all over the place in roll even outside the track due to suspension design. I don't know that you could feel this offroad though. Also as someone mentioned before, often in a corner the rear is sliding and the front wheels are counter steered. the front outer wheel is losing camber due to caster. if they are not counter steered like in a berm situation then the front outer in gaining camber due to caster. One thing you forgot to mention is bump steer, caster, and scrub radius. getting those three parameters right will definitely be very important for the drivability of your car.
**********
As you probably can figure, designing a suspension is full of trade offs. you make one aspect better and that screws something else up. as far as prioritizing everything. obviously if you get one of those things way off it may be undrivable. I think you may have the wrong definition of scrub radius. it is the distance between the center of the contact patch of the tire on the ground and a point on the ground made by a line going through the upper and lower pivot points on the spindle. typically you shoot for zero to 1 inch inboard of the contact patch center.
So my procedure for design is (everyone's is different, I probably left something out) : Figure out how much travel you want and what the car is going to be used for. A desert car may give up some camber control in turning for more wheel travel. A forest trail car may do the opposite. Decide on your rolling radius of your tire, then fix the scrub radius and caster (or trail total). your scrub radius will be a combination of king pin inclination (kpi), wheel offset, hub width, and tire rolling radius. Those two things don't have a lot of trade off or compromise, they are what they are. This forms some constraints on your upright. Then fix your outer lower A-arm pivot point along your KPI line where you want it. (Typically where the snout intersects the kpi line is a good place. Then fix your inner lower A-arm pivots. Depending on frame considerations there are probably a lot of restrictions of where it will go. You generally want the A arms as long as possible without compromising the strength of the design, this reduces scrub which is good.
At this point (on paper, ) you have a lower A-arm connected to an upright with wheel attached, no steering, upper a-arm or upper a arm mount on the upright. Ignore the steering for right now and pretend that there is no bump steer. Now you have to place your upper A- arm. We know the outer pivot has to be on the kpi line, it has to be above the lower pivot by enough to make room for steering etc. also the further apart you set them the less the slop in the joints matter and the less load (in some circumstances) your arms and pivots see. A vertical separation between the lower and upper outer pivots of about 20%-40% of the tire diameter seems pretty normal. Fix the outer upper pivot. Then we have to fix the upper inner pivots. This is typically where personal preference comes into play. The inner upper pivots are almost always at least as far out board as the lower inner pivots. The vertical separation between the upper and lower inner pivots is usually the same or less than the outer pivots. This is where a kinematic suspension design program is great. I use wingeo 3, people use solid works, some people use 2d scale front view paper models with push pins for pivots. Move the inner upper pivot around and go through ride and roll iterations and combinations of both till you feel like you have the best control of your wheel possible. Placing your front view swing arm length at about 1.8-2.2 track widths seems to yield a good compromise. Keeping your outer tire upright in roll will be easy but then you end up with massive positive camber in droop, and vice versa, its all a compromise. If you end up with positive camber in droop, you may be able to cure it with a couple degrees of negative camber at static ride height. You may end up with negative camber in droop neutral at ride height and negative in bump. This is because your front view IC is moving from inboard to out board.
I would look at your roll center height compared to your cg height guess. Your RC will probably be between your cg and the ground. That is fine. Now you are all done except for steering. You have to think of Ackerman now, or consciously ignore it. There is the top view method, its pretty easy to point the steering arms at the center of the rear axle and be done with it. If you are steering behind the spindle its easy, if you are steering in front you may hit the tire with your outer steering pivot. There are all sorts of theories on Ackerman in conjunction with slip angles... (Data on tire slip angles vs. load on offroad tires are non existent so it would be a guess.) probably low on the priority list, I ignore slip angles. You can also get Ackerman by moving your rack fore and aft. If you are going for 100% Ackerman or 100% plus slip angles, I'd say its most important to get it right at full lock, and let it be off everywhere else.
Isdtbower
In for Tech
Now you need to place the outer steering pivot vertically. This is packaging, remember about halfway between the outers will place the rack end about half way between the inners. Once you have the outer fixed in a convenient place, which considers your Ackerman goal. Then you have to fix the inner to reduce bump steer. The easiest way to do this with a minimum of technology is with a 2d cad program like AutoCAD. Draw the upper and lower inner and outer A-arm pivots, and outer steering pivot in front view (so that your upper arm inners are a point and your lower inners are a point) in cad. Repeat the outer points at ride height full droop and full bump. Then you will have three points for your outer steering pivot, at different points in your travel. You can then draw a circle that these 3 point lie on the circumference of, in cad. The center of that circle is where your inner steering pivot needs to be for zero bump steer, at these travel points. You probably want to cycle the suspension to a few more points and see how close that the steering points are to the circle. Any spot that is off will translate to bump steer. There are some A-arm configurations where there will inevitably be some bump steer but its usually manageable. If your front view IC moves from one side of the car to the other during travel, you will have some bump steer, no way around it. Often the bumpsteer in the greatest at full droop or full bump. You may want to limit travel if you cant bring bumpsteer under control at the end of the travel.
Then of course as you are going through this you will make changes which will make you start over, again and again. But eventually you come out with a pretty good design. There are other aspects of A-arm design like anti- squat and dive, and wheel recession but that gets more complicated, and a lot of people think it’s a crock. The rear is similar, except you don't have steering but you do have a half shaft. Getting zero plunge on your half shaft is a lot like getting zero bump steer, you may not care about it very much. Bump steer in the rear is easy to get rid of by locating your inner toe link pivot collinear with your inner upper or lower pivots, or on the arm, then locating your outer pivot on a line going through your outer upper or lower pivot that is parallel to the line going through your inners.
I think that about wraps it up.
***************88
Rolling radius-- basically decide on what tire you are going to run then figure out what the actual radius of the tire is when loaded. The distance from the ground to the center of the wheel. the tire will squish a little when loaded and the advertised diameter isn't always what it measures out to on your rim.
Fixing the scrub radius-- usually its just a number like .5" inboard of the center of the contact patch. once you know where that point is then you can draw a line from it which will become your steer axis.
front view swing arm length--- look at a front view of your car. lets start with the right side suspension (when you are looking at the car from in front of it) , extend an infinite line through your right upper A-arm joint on the spindle through the upper A-arm frame joint. Do the same for the bottom. They will likely intersect at some point. This intersection is called the instant center of rotation or IC. this is the point that the suspension is rotating about at that point in the travel. the distance from your wheel to the IC is the front view swing arm length. typically the IC of your right side suspension will be located about a track width distant to the left of the vehicle. The length of the front view swing arm FVSA is then 2 track widths. If the arms are parallel then you have an infinite length FVSA, that will mean that there in no camber change in roll, relative to the chassis, for that point in your suspension. if you IC occurs on the other side of the car it means that the tires are leaning the wrong way in a turn. The shorter your FVSA is the higher the rate of camber change you have, the opposite is true as well.
The guideline I gave was just an opinion and a good starting point. I should clarify that this number is at ride height. often the FVSA length will shorten in bump and lengthen in droop.
Hope that clarifies things.
Damien
*****************
bullnerd--- caution rambling ahead! as far as a setup for short course vs. desert. I would take your approach as short course is all about turning whereas desert not so much. that being said i dont have any short course experience so getting info from someone who does would be valuable. also your FVSA length will likely change a lot in your suspension travel range so my idea on the length was at ride height. I don't think there is an exact number you can design to with suspension. I think getting to basics is what's important. Think about the goal, which is keeping a large tire tread contact patch on the ground. as a tire corners it the tread deforms, dialing in a little negative camber into the outside front tire sets the tire up for this deformation, and keeps the contact patch large. too little and the tire can buckle over and lift the tread on the inside of the tire, too much and the cornering force doesn't deform the tire enough to get a good contact patch. obviously the sweet spot in camber for the outer tire will be opposite for the inner. if you run a lot of negative camber for turning you will be running on the inside of the tire, small contact patch, for straight line acceleration and braking. low profile tires are usually stiffer and don't deform as much. low pressure large side wall tires are opposite. look at the old MTEG trucks. they run lots of static negative camber. That means that there is a combo of low stiffness tires, higher importance put on cornering than braking, lots of lateral weight transfer in a corner putting much more importance on the outside tire than the inside. or maybe a significant amount of body roll which stands the outside tire up in a corner.
I looked at a bunch of action shots of class one cars, I looked at body roll steering angle combos in hard corners. I looked at my suspension in those combos and looked at what the tire orientations were, estimating how that would affect the contact patch.
As far as rear a-arm. I did it on my mini. this is where i started my entire car design. I wanted high travel 20" so I went with non plunging cv's and let the plunge happen between the star and axle. this made it critical to reduce plunge as the primary objective. I got it to about .100". I went with a micro stub and a rpm box because they were the narrowest within my budget. I set my track width to get the travel numbers without over arcing my cv's. with those pieces fixed then I worked on the geometry, every time I moved points I had to re optimize for plunge. wheel camber adds or subtracts to the outer cv angle which has to be figured as well. I completely ignored scrub radius and ended up with a large one. the unintended consequence was a lot of twisting (when viewed from above) stress on my rear upright due to braking and acceleration. This bent my upright, which surprised me. I would try to minimize scrub radius or strengthen my upright if I had to do it again. I ended up with a similar RC height and FVSA length similar to the front, (I'll check it on Monday). I designed some wheel recession in the front and some anti dive. This made for a upper a arm tilt higher than the lower a arm. this made a short side view swing arm length (SVSA. I matched the rear arm tilt with the front lower. since the upper and lower rears are parallel in side view the rear SVSA length is infinite. the side effect is some anti squat which is fine.
Hope this helps.
P.S. I still feel that the greatest advancement in suspension at KOH has been the solid rear double triangle geometry. That all but eliminated the rear roll steer that drivers were fighting in the go-fast uneven desert.
With builders adding more camber to the straight axles, and recent advances in hydraulic steering, the seemingly huge advantages of IFS will erode somewhat. No substitute for preparation either.
BB
Then of course as you are going through this you will make changes which will make you start over, again and again. But eventually you come out with a pretty good design. There are other aspects of A-arm design like anti- squat and dive, and wheel recession but that gets more complicated, and a lot of people think it’s a crock. The rear is similar, except you don't have steering but you do have a half shaft. Getting zero plunge on your half shaft is a lot like getting zero bump steer, you may not care about it very much. Bump steer in the rear is easy to get rid of by locating your inner toe link pivot collinear with your inner upper or lower pivots, or on the arm, then locating your outer pivot on a line going through your outer upper or lower pivot that is parallel to the line going through your inners.
I think that about wraps it up.
***************88
Rolling radius-- basically decide on what tire you are going to run then figure out what the actual radius of the tire is when loaded. The distance from the ground to the center of the wheel. the tire will squish a little when loaded and the advertised diameter isn't always what it measures out to on your rim.
Fixing the scrub radius-- usually its just a number like .5" inboard of the center of the contact patch. once you know where that point is then you can draw a line from it which will become your steer axis.
front view swing arm length--- look at a front view of your car. lets start with the right side suspension (when you are looking at the car from in front of it) , extend an infinite line through your right upper A-arm joint on the spindle through the upper A-arm frame joint. Do the same for the bottom. They will likely intersect at some point. This intersection is called the instant center of rotation or IC. this is the point that the suspension is rotating about at that point in the travel. the distance from your wheel to the IC is the front view swing arm length. typically the IC of your right side suspension will be located about a track width distant to the left of the vehicle. The length of the front view swing arm FVSA is then 2 track widths. If the arms are parallel then you have an infinite length FVSA, that will mean that there in no camber change in roll, relative to the chassis, for that point in your suspension. if you IC occurs on the other side of the car it means that the tires are leaning the wrong way in a turn. The shorter your FVSA is the higher the rate of camber change you have, the opposite is true as well.
The guideline I gave was just an opinion and a good starting point. I should clarify that this number is at ride height. often the FVSA length will shorten in bump and lengthen in droop.
Hope that clarifies things.
Damien
*****************
bullnerd--- caution rambling ahead! as far as a setup for short course vs. desert. I would take your approach as short course is all about turning whereas desert not so much. that being said i dont have any short course experience so getting info from someone who does would be valuable. also your FVSA length will likely change a lot in your suspension travel range so my idea on the length was at ride height. I don't think there is an exact number you can design to with suspension. I think getting to basics is what's important. Think about the goal, which is keeping a large tire tread contact patch on the ground. as a tire corners it the tread deforms, dialing in a little negative camber into the outside front tire sets the tire up for this deformation, and keeps the contact patch large. too little and the tire can buckle over and lift the tread on the inside of the tire, too much and the cornering force doesn't deform the tire enough to get a good contact patch. obviously the sweet spot in camber for the outer tire will be opposite for the inner. if you run a lot of negative camber for turning you will be running on the inside of the tire, small contact patch, for straight line acceleration and braking. low profile tires are usually stiffer and don't deform as much. low pressure large side wall tires are opposite. look at the old MTEG trucks. they run lots of static negative camber. That means that there is a combo of low stiffness tires, higher importance put on cornering than braking, lots of lateral weight transfer in a corner putting much more importance on the outside tire than the inside. or maybe a significant amount of body roll which stands the outside tire up in a corner.
I looked at a bunch of action shots of class one cars, I looked at body roll steering angle combos in hard corners. I looked at my suspension in those combos and looked at what the tire orientations were, estimating how that would affect the contact patch.
As far as rear a-arm. I did it on my mini. this is where i started my entire car design. I wanted high travel 20" so I went with non plunging cv's and let the plunge happen between the star and axle. this made it critical to reduce plunge as the primary objective. I got it to about .100". I went with a micro stub and a rpm box because they were the narrowest within my budget. I set my track width to get the travel numbers without over arcing my cv's. with those pieces fixed then I worked on the geometry, every time I moved points I had to re optimize for plunge. wheel camber adds or subtracts to the outer cv angle which has to be figured as well. I completely ignored scrub radius and ended up with a large one. the unintended consequence was a lot of twisting (when viewed from above) stress on my rear upright due to braking and acceleration. This bent my upright, which surprised me. I would try to minimize scrub radius or strengthen my upright if I had to do it again. I ended up with a similar RC height and FVSA length similar to the front, (I'll check it on Monday). I designed some wheel recession in the front and some anti dive. This made for a upper a arm tilt higher than the lower a arm. this made a short side view swing arm length (SVSA. I matched the rear arm tilt with the front lower. since the upper and lower rears are parallel in side view the rear SVSA length is infinite. the side effect is some anti squat which is fine.
Hope this helps.
P.S. I still feel that the greatest advancement in suspension at KOH has been the solid rear double triangle geometry. That all but eliminated the rear roll steer that drivers were fighting in the go-fast uneven desert.
With builders adding more camber to the straight axles, and recent advances in hydraulic steering, the seemingly huge advantages of IFS will erode somewhat. No substitute for preparation either.
BB
Isdtbower
In for Tech
I believe Jesse Haines cut his teeth on this car and JT Taylor owns it now. He has run Baja and some hill climbs with it. Definitely not afraid of gong fast in it.
vetteboy79
It's bent. #ttb
Just a few pics I remember from working on Tim's car.
Like I said, always got a lot of questions when that thing came through the pits 
Last edited:
arse_sidewards
Contrary to everything
- Joined
- May 19, 2020
- Member Number
- 71
- Messages
- 8,209
I wish we just knew what type of wheel motion was "best". Toe change is well understood (basically you want none of it but there's a couple edge cases where some is an acceptable trade off). Caster is pretty well understood (everyone seems to have settled on numbers in the 9-15 range). Nobody is sharing why they're doing what they're doing when it comes to camber though since there's still a lot of variation between the top performing teams it's kind of a proprietary secret. Once you know what "best" looks like it's easy to build to that.
By using unequal spacing on the inboard and outboard arms (assuming equal length for now to keep things simple) you are effectivly building a swing axle that pivots about some hypothetical point in space beyond the chassis side end of the pivot in the same way that link separation on a 4-link rear makes the vertical movement of the rear end happen about a point in space well beyond the upper link ends. Since your CV center is never at that point the outer CV center is going to move relative to that point and you're always gonna have plunge. You can get it close by using narrower inboard spacing but the closer you get it the shorter your theoretical swing axle is and the more aggressive your camber change is.
I'm not sure if that's correct but it's 7am on a Monday and thinking is hard.
How competitive are the Cambell cars? I don't really follow ultra4 closely
I think that the difference between unequal length arms and equal(ish) length arms with unequal inboard and outboard pivot spacing is going to wind up being a very big deal because while they both do the same thing in bump they do opposite things in droop and I'm really not sure which is better. Of course if you're taking a turn in the desert the inboard dropping tire has no traction so it doesn't really matter but I think in the rocks where you often find yourself forcing a fully dropped tire forward into a rock the difference in behavior would make a big difference in performance. I just have no idea which is better.
Stupid amounts of plunge angle on the center shaft do no result in long life for the center joint.
That said, I can't think of anyone who's built a competition quality TTB who's regretted it. Seems like most of the problems happen in builds that are spending enough for serious travel but not spending so much that RCVs are just a checkbox, those are the guys who just can't get the center shaft to live long enough.
By using unequal spacing on the inboard and outboard arms (assuming equal length for now to keep things simple) you are effectivly building a swing axle that pivots about some hypothetical point in space beyond the chassis side end of the pivot in the same way that link separation on a 4-link rear makes the vertical movement of the rear end happen about a point in space well beyond the upper link ends. Since your CV center is never at that point the outer CV center is going to move relative to that point and you're always gonna have plunge. You can get it close by using narrower inboard spacing but the closer you get it the shorter your theoretical swing axle is and the more aggressive your camber change is.
I'm not sure if that's correct but it's 7am on a Monday and thinking is hard.
How competitive are the Cambell cars? I don't really follow ultra4 closely
I think that the difference between unequal length arms and equal(ish) length arms with unequal inboard and outboard pivot spacing is going to wind up being a very big deal because while they both do the same thing in bump they do opposite things in droop and I'm really not sure which is better. Of course if you're taking a turn in the desert the inboard dropping tire has no traction so it doesn't really matter but I think in the rocks where you often find yourself forcing a fully dropped tire forward into a rock the difference in behavior would make a big difference in performance. I just have no idea which is better.
Stupid amounts of plunge angle on the center shaft do no result in long life for the center joint.
That said, I can't think of anyone who's built a competition quality TTB who's regretted it. Seems like most of the problems happen in builds that are spending enough for serious travel but not spending so much that RCVs are just a checkbox, those are the guys who just can't get the center shaft to live long enough.
He just sold it again right before Christmas
Wherever this came from, thank you. I'd say it's the most clearly explained, logical approach to the suspension design for a noob that I have seen. I'm literally reading it over and over again and taking notes.
Isdtbower
In for Tech
Damien was very active on the forums several years ago. He studied suspension at Cal Poly, I believe. His passion was offroad and during that study he saw that offroad had limited connection to what was written and studied for asphalt. We became digital pals as he was good at what we were seeing, and how he attacked the issues. His playground was Dezert. Ours was Desert and rocks. But he could explain the anti's and about how far we should go before giving up wheel chatter, etc. When I visited him to see his car, he had just bought a CNC to build more parts. He was also just starting a family and I believe the decision to give up the race thing for awhile. I get it. The information is from several threads and personal notes. I too have read that many times. You do need to reference it from time to time to make sure you haven't overlooked something. When you first start out you don't realize what you don't know, so you don't know what is relevant until later in the process. I believe the reason for the information was because he was just a recreational racer and fabricator. He had nothing to hide, and a lot to offer others that were trying to go the right direction "the first time." He helped us, as I am trying to pass on that karma.
I talked about adjustable anti's. Here was his CAD. You will notice a quick change for the front diff (7" flange to flange). I took that idea from him for my front engine, rear diff. "Good for 1800HP" and about the same weight as a billet 9" which I am using for the front as packaged smaller but 12 3/4" flange to flange. You will see where he usedd quite a bit of kick to the front arms. The bottom was at 10* and the top arm was to be adjustable for the anti's. You can see the extra holes. They were for 0 to 50% anti's. Today that kick would be considered excessive, but many 2WD Dez guys use 7*. The dates go back to 2010 to 2012. And yes, much on Race-dezert.com.
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Ghetto Fab.
No idea what I'm doing
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I'm glad someone posted that thread from Damien. I don't know where I ran into it, I thought it was over on Racedezert.com, but it might have just been a link there. It was a very well thought out post about how to go about a practical way of actually designing an IFS suspension. You can come up with all the geometry in the world, but at some point you have to put it into a vehicle and his approach really summed it all up nicely.
Tech Tim
Your AD Here....
iirc Damien changed his view point on the kick later and says that it was excessive. Maybe it was you that told me that?
michael.gonzalez
Banned
Where is this Damien guy? I'd like to pick his brain on a few things.
Tech Tim
Your AD Here....
Arm separation on IFS is similar to arm separation on a live axle: To help get the geometry right and to combat leverage.
Now if you are talking about the 2' tall spindle uprights you see on the big street poser trucks, they are only doing that to bridge the gap they created by keeping the upper A-arm in the OE position and dropping the lower A-arms in a cradle 1' below the OE position.
Tech Tim
Your AD Here....
That's cool but why did we all stop using boxes in the first place?
Look at the amount of real estate in front of the diff. That may work fine for a desert ride, but would be a rock plow in our sport.
Isdtbower
In for Tech
I think it was from watching the SXS's going over backwards in the rocks in the early days. When it was brought up in "our group" we talked about lift from the front geometry. The CG moving back. And the rear anti's wanting to lift the front. I am not sure we got resolution and kept building the iFS "relatively" level, and didn't overdo the rear anti's. I have noticed a couple U4 multiple car builders adding a few degrees of front kick/rake in the bottom arm and a little more in the top. I haven't seen any big advantage in their race results though.....Not that that means anything as the driver, build, and testing has more to do with winning at the top. I am pretty sure the "Kick" is there for constant desert pounding not any wheeling that contains verticals. And, I suppose, if you think abut it, a desert guy would want the front to lift on accel. I am not close enough to TT to see what they are doing in 4x4.
So......... It depends where your spindle rotation is when you caster gain or loose. Is that rotation around the spindle or the bottom omni? It makes a difference in wheel regression, bite, etc., etc.
I was reminded of a sentence from someone who talked about hard braking or landing and the pitch of the vehicle went down in front by 6* and the static caster was 6*...and how crappy the steering would be at that point. The level of tech can be never ending.....and thus the word..."compromise" meaning ?????
Here is Damien's ability to adjust the front anti's. I bet I have 6-10 pages of discussion on anti's from the "group" and snips in pirate and other threads. Damn informative, Basically you want to front anti's so you don't slam into the bumpstops on braking but not too much as they "lock" the front so the shocks loose their tune during braking or accel. This led us into the discussion and importance of left foot, or trail brakng...whee you can "assist: the shocks during chatter and etc. As I was looking for his anti's, I also saw that the 10* number came from early sxs's and thinking they had great engineering.........................
I bet we spent 1000's of hours on tech back then. Neat stuff. Several pages on not just the displacement of the spindle, but the rotation of the spindle and uprights. We have forgot so much. At the time we started to consider Portal geometry and spindle rotation forces. We moved on to improving what we had. I have not seen anyone go to that extent with portals but expect that will come out of TT development now. Nothing better than getting Trucks out there and listening to the driver feedback and making rational adjustments. I wish we could get into the minds of the MAson's, Geiser's, etc. But I have had some friendly discussions where "they" say that "We don't know why, but it works."
michael.gonzalez
Banned
This is my favorite thread here by far. Lots of tech and discussion.
One day I'll build an IFS/IRS buggy...
One day I'll build an IFS/IRS buggy...
Weasel
Red Skull Member
We ran a whole bunch of different ranges of tilt on our arms and determined that somewhere in the 5-7 range did well enough. At the time I liked 10 and I think we ran up to 12 at one point. Would soak up the impacts amazing but touching the brakes would also bottom out the skid which has loads of other issues.
And I forgot about the castor gain issues, ran through those as well. Had one car that went positive and that things was a handful!
I've been of the opinion that bucking in the rear is exaggerated by raking the front arms due to the position of the car as it hits a bump.
And I forgot about the castor gain issues, ran through those as well. Had one car that went positive and that things was a handful!
I've been of the opinion that bucking in the rear is exaggerated by raking the front arms due to the position of the car as it hits a bump.
I'm starting to fall behind with some of the terms being thrown around. By "tilt" you're referring to the angle of the chassis mounts of the arms in relation to level? And by 5-7 you're referring to 5-7* tilted back? So that the tires would move slightly backwards on hits? And this is called wheel regression? And you're mainly referring to lower arms, not uppers, right? Since uppers would be different to set anti-dive? Am I getting all that right?
Tech Tim
Your AD Here....
Weasel
Red Skull Member
Correct, although our upper and lower arms were always parallel to each other.
I can see above that the drawing has the arms not parallel to each other on the mounting points, from what I remember the anti were determined by the angle the arms are rotated of the horizontal plane. Not if they are parallel or not to each other in the side profile?
I can see above that the drawing has the arms not parallel to each other on the mounting points, from what I remember the anti were determined by the angle the arms are rotated of the horizontal plane. Not if they are parallel or not to each other in the side profile?
ScottRS
Not @Rockstomper today
My 9" center housing (still have it, actually) was made from a junkyard rear 9", I found the Dutchman centersection that somebody else had posted, back in the day, and while that was cool (and narrow) it was also mighty expensive for about-20-year-old me, so I built what I could that was narrow-enough. Housing ended up about 13.5" wide, CV flange to flange was about 18.5" IIRC. The Dutchman or others posted would be better options to keep the diff narrower. They've also developed a lot more since then, a 35 spline 9" then was borderline unobtainium and finding something other than a spool for it just about took special blessing from the pope.
The rest of the IFS in that truck, sadly, was more of a packaging "thing" than a design/engineering excercise; it was hit the frame points, hit the knuckle points, OK, move the knuckle points 2" out and 1" forward in space relative to the frame points, now connect the dots. It worked pretty well for what it was, the drive was the weak link. Toy IFS knuckles didn't allow bigger than 26 spline outers, and basically a 930 size (28 spline) joint, and making new/bigger/better knuckles was beyond my capabilities/confidence at the time.
Isdtbower
In for Tech
Both.
arse_sidewards
Contrary to everything
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ScottRS
Not @Rockstomper today
Set 20 tapered roller bearings and seals, it was just a teeny tiny semifloat 9" rear end. I'll dig around in my shelves and decide if I want to take pictures, the idea is a lot prettier than the "learning to weld on borrowed equipment" fabwork.
arse_sidewards
Contrary to everything
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So you just cut down OEM axles super short and then had them splined?
We need more backyard fab around here. I think all the tube work and fancy tig welds make people get out of touch with the kind of fab that's tolerable on a trail rig.
YotaAtieToo
Thick skull
I've thought about doing something like this for more of an oem upgrade, so stubby as possible on one side, with a short tube on the other. Say replacing a stock diff in a 3rd gen 4runner for a "mid travel" type setup. The rcv kit is pretty strong, but the 7.5" center section isn't
I feel like center mount for longer CV's isn't really a huge advantage when your a arms are mounted outside the frame. Having done that, would you agree?It may be better at that point to just buy the aftermarket narrow diff and then make a double flanged shaft with some type of support bearing. But as we've discussed, those are damn pricey.