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Ifs 101

mobil1syn said:
care to elaborate ...
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I loved my IRS when I had 4" up and 4" down in a Flatty. (built in '65) So when I started a "new build" (2000ish) , I saw that I could now get 12" up and 12" down. Great. I also worked at minimizing the toe change within that range and got to 1/2* with longer 56" trailing arms. All was cool. Then we started to work on IFS and quickly realized that on a "G-out" the diff hit the ground. We backed off on travel from 24" to 18" and set the suspension up with the half shafts higher at the diff. No biggie when you build another chassis to lower the CG back down. Back to my IRS: In my original 4up-4down my diff rarely hit the ground. But my original drawings of 12up-12down, the diff would be 5" under the ground on full bump. OUCH.

About the time I was working out the IRS, but not noticing the G-out dilemma, the rear Straight Axle guys started to figure out the double triangle links to get rid of the rear steer on articulation and work the anti squat numbers. (Mobil1syn was a big part of that). A straight axle does not have a droopy diff on bump. And bumps HURT with IRS as the diff is attached directly to the frame! The TT world was also showing us that they could get 36" of travel (with minimal articulation) and I quickly got the picture. (Before the 4 link builds, KOH max speeds were a white knuckle, rear steering, ride between 60-80mph. I feel the #1 help for offroad speed was the double triangle link suspensions. Front and rear. The shocks just followed along with good tuners and by-passes)

I have gone too far on my build to ditch the IRS, but would not do it again. As also proven with all KOH and other buggies. (But have been rumored to be more comfortable...until) I am lucky I caught moving the diff up in the frame with the result of 8" less overall travel (16"') with maybe 7-8" before grounding out.

I think I can also make the case that is why a TT is faster than a Class 1 and ultimately a liability for a SXS. ..........Hurts. ( I was told that a IRS or center skid g-out hit travels up your back. A IFS G-out gets somewhat absorbed with your belts. The front skid on an IFS should not be steeper than 45* in my opinion as very close to the ground) (I have never seen Goodby put a rockpile after a jump)

EDIT: And the TT's didn't move to IFS until portals, and another 4" of clearance. (And today we hear that Menzies, with a 15 minute lead at vegas to Reno was sidelined by "loosing his front diff on a rocky roller coaster section....???)
 
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Isdtbower said:
But my original drawings of 12up-12down, the diff would be 5" under the ground on full bump. OUCH.
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this is something that a lot of builders miss on the front end as well, even short course builders. cant fix a harsh bottom out when its the belly slapping the ground. you cant have IRS without portals and id even argue you cant have IFS without them. seriously a game changer.

i wheeled a stock tacoma for a little bit after having a SA minitruck for a number of years. it was a learning curve dropping off stuff and finding a rock wtih the crossmember because the center drops with ifs.
 
mobil1syn said:
it was a learning curve dropping off stuff and finding a rock wtih the crossmember because the center drops with ifs.
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The skid has become the most important frame part of IFS AND IRS. Doesn't "solve" anything. Just lets the other exist.

I think we all got wrapped up in the cool tech, but IFS was, and still is, proven to be quicker above 60mph.

The SA guys have caught up significantly (Ultra4) in the last couple years with hydraulic improvements. But man are those axles HEAVY.
 
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Provience said:
this was done with a toyota setup for a...????? class legal system. whitetrashfab? DWT? DirtyWhiteRacing? can't remember who the heck it was now. but basically tough truck type stuff and kept ripping up idler arms, added a RHD steering box to the frame rail and problem solved, on that front.

they weren't both powered, just one. small tire class
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That was me. ECORS truck. RHD box for an idler. I still have the box and the guts from it...

That truck had upper ball joint spacers which effectively make no difference except keeping ride height with less torsion bar preload.

I cranked the cam bolts on the lower arms to max caster/max negative camber and welded them in place.

Check out that sweet toe in. Had I kept racing it I would have built a new center link and longer tie rods. The truck needed 100 more HP to keep up with the stockish shitbox cherokees I was racing against too. There was one guy racing a Formula Toy with some high zoot leaf springs and bypass shocks and and a built ass big money 22r, and he was barely competitive. When we raced at Durhamtown GA, I was hitting ~65mph in 5th gear redline (low range, the truck wouldn't pull high range in the woods at all) and would get passed by everyone on the straights.
.staticflickr.com%2F8107%2F8586223055_1967beb060_b.jpg
 
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The thoughts on geometry haven't changed much in our minds. What is notable is that many new builds are not understanding or regarding the advancements in geometry thinking over the last several years.

The new development has been in the addition of portals. The geometry thinking of the tire to the ground stays the same but definitely changes the physical designs. There also seems to be some significant feedback from the portals back into the steering systems. I have not been close to the designs but the 4" offset between the hub center and wheel stub center could be adding some stress, either in accel or decel/braking, that is taking out different parts of the steering systems. We have seen it in both Ultra4 and Trophy Truck. Maybe they are just going faster........
 
patooyee said:
I've come back to it several times to re-read and re-digest. I still want to build with IFS but life just continually gets in the way.
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I am half way thru the build in years, and bogging on just the amount of bushings and their design. Design was easily a year to move from CroMo to Billet. Very different thinking on strengths of materials. $$$$ start creeping up on you also.
 
I agree that the geometry actually applied to builds hasn't changed dramatically in the last year aside from one or two notable builds (Loren healy's new IRS configuration makes me drool).

The portal and leverage is definitely an interesting topic. We've of course talked with it across multiple threads in the past few years, but something very interesting I've observed - getting the axle shaft off spindle centerline has allowed people to build the latest 4wd trucks with many 2wd front suspension principals. The biggest being the lower control arm essentially meeting the "knuckle" / upright at the spindle centerline like 2wds have been configured for a long time. Sure they probably should be changing the geometry to accommodate the new torque input (and change in braking torque reaction through the suspension), but the forces seen by the outer joints shouldn't be too much more extreme than 2wds see.

Pat I hear ya man. I'd really like to build my interpretation as well, but I can't commit that time to a personal project unless it has a potential business case these days. I hope I can get on the other side of that in the coming couple years, but I have way too many irons in the fire haha.
 
AgitatedPancake said:
getting the axle shaft off spindle centerline has allowed people to build the latest 4wd trucks with many 2wd front suspension principals. The biggest being the lower control arm essentially meeting the "knuckle" / upright at the spindle centerline like 2wds have been configured for a long time.
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I fully agree and I believe Ultra4 will learn from the TT guys as they have more vehicles in process and racing to learn from. I heard from a birdie that a OEM put a lot of stress sensors on an IFS rig to collect data. Maybe why the uprights are mostly surviving but the steering boxes/racks have not been perfected yet.

It could be that the rigs and trucks are just going faster!!
 
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Isdtbower said:
Maybe why the uprights are mostly surviving but the steering boxes/racks have not been perfected yet.
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i suspect that lack of steering stops at the knuckle are causing the issues with steering, would explain all the sheer rack bolts.
 
mobil1syn said:
i suspect that lack of steering stops at the knuckle are causing the issues with steering, would explain all the sheer rack bolts.
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Many moons ago...Maybe even Shannons first IFS.....there was the same problem. Now the tech is to trap the rack into a pocket on the bulkhead. Now the F9 bolts are only stressed in tension.

Not sure how to design steering stops on a IFS upright. Probably your point.

As the bolts would loosen on the rack, the turning could/would go beyond the CV turning capability. The balls would fall out and explode the whole thing. Capture plates were made. I think RCV now adds more outside shoulder to help that problem???? And that learning is maybe why the UTV's have such good travel now.
 
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Isdtbower said:
Many moons ago...Maybe even Shannons first IFS.....there was the same problem. Now the tech is to trap the rack into a pocket on the bulkhead. Now the F9 bolts are only stressed in tension.
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OEMs have been doing that since forever (albeit with squishy bushings in the mix) because it lets them cheap out on the rack casting and mounting points on the crossmember.

Funny how stuff like that got missed initially because people build stuff based on stuff they knew that just by luck didn't have a feature they wound up needing.
 
Saw this picture on the Ultra4 feed and was curious about what's going on with the outside front wheel.

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Doesn't look to be at a drastically different point in the travel compared to the inside tire. Reverse ackerman and agressive caster/KPI? Seems like the opposite of what I'd expect, but I also expect Horschel knows what he's doing...
 
vetteboy79 said:
Saw this picture on the Ultra4 feed and was curious about what's going on with the outside front wheel.

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Doesn't look to be at a drastically different point in the travel compared to the inside tire. Reverse ackerman and agressive caster/KPI? Seems like the opposite of what I'd expect, but I also expect Horschel knows what he's doing...
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from here it looks like a combination of tire deflection, and broken/deformed parts.
 
That picture is from the Visions race at MAO in July during qualifying. Paul rolled the car and ripped both pivot points for the upper a-arm off the chassis. Paul could still steer so he kept going and got a qualifying time. His lead fabricator Mitchell cleaned up the damage and tig welded back together for the race the next day.
 
spidr said:
Somehow I hadn't found that number and its actually super helpful.
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Those are generally good numbers for short term or intermittent use.

26* for a "u-joint" style half shaft is the max that the desert guys ran....past tense.

Now they have moved up to the 934 or 935 (35 spline) rezeppa style CV's (IRS) but probably holding to 30* on a regular basis and possibly going to 45* when on the air bumps. A IRS has no turning...or compounding of the angle. I would venture that once you get above 5-600 hp, that the CV's would move toward the series 30 size which are significantly bigger and costly.

A series 30 CV is usually used in a IFS outer. That is because misalignment bushings can allow 45* actuation for 30ish degrees of vertical and 40ish degrees of upright turning. 935 CV's have been run on the inside/diff-side with success as only in the 30ish degree vertical movement. HP and car weight is a factor in "moving up in size and cost.

Straight axle guys are now putting "Rezeppa joints" at the hub to turn past 40* to 50* number. But depends if the chassis is far enough away for that.

And etc. Just some confirmation and for the new interested. There is definitely more to this. One being boot survival at those numbers.
 
Thanks @lsdtbower
I'm hoping to leave the factory CVs in mine for a little until I upgrade to something better, in my head I was thinking I could go 35* on a tripod joint, so I'm going to have to back down some.
Last go around Rcv was good enough to send me cad models so I had something to reference, I'll probably talk to them again early so that I can make sure everything's ready for them, they where one of rhe best companies I've ever worked with for technical data, which is the only reason I know a lot of what you just explained but I love that it's all right there.
 
spidr said:
Thanks @lsdtbower
I know a lot of what you just explained but I love that it's all right there.
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I re-wrote the above thread.

I got to thinking...BS... so went back to remembering years ago when the development was happening. Heat is an enemy during articulation and moves the numbers down significantly. CV greases are expensive and NEEDED. CV's usually require maintenance every race. Even short ones. Best cleaner is WD40 in a tub.
 
AgitatedPancake said:
Man I just surfed through like 2/3rds of this thread again. So good. Those GM uprights still look as appealing as ever.

Also recently stumbled onto the fact that Dodge has used the same aluminum uprights on their 1500 rams for 20 years. Pretty attainable, and that vertical separation is solid

For Dodge Ram 1500 Pickup 4WD Front LH Driver Side Steering Knuckle Spindle | eBay

s-l1600.jpg
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Same with btoyota but they are steel and has a readily available gusset from TC.
 
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