4 link front questions vs 3 link

[Waternut]

I have a stretched YJ with a semi triangulated 4 link rear running 14" Fox coilovers. I currently am running XJ leafs up front and they work well but I think I've hit the limit on what the leafs can do. Sometimes the leafs work great and sometimes they don't. Basically, if the Jeep is loaded hard on opposite corners, the leafs can get about 14" of travel. However, if I just roll up on a 2 foot tall obstacle with the front axle, the rear will hit bumps before the front even starts to flex. I'd really like to see a more balanced suspension setup so I'm considering linking the front as well.

So my first question is... I know a triangulated 4 link up front with conventional steering will cause a lot of bump steer. I already have full hydro steering so will I notice a difference in drive-ability on the road between a 3 link and a semi-triangulated 4 link front suspension?

I actually think the 4 link will be easier to install on my rig instead of a 3 link so I'd prefer to go that route if there aren't any major negatives to it that I'm missing. So with that said, here are the calculators I'm looking at. Please comment if you see something that I need to address.

 

[Mouse]

With full hydro go triangulated 4-link up front. No trackbar is necessary. The hard part will be getting the links to clear everything through cycling the suspension. This is the setup I have in my TJ. Does not drive on the street much worse than it did with a 3-link and hydro assist...much better on the trail.

 

[Waternut]

Thanks. I know full hydro drives a little funny on the road so "not much worse than 3 link and hydro assist" is probably more to do with full hydro than the suspension geometry itself.

 

['84 Bronco II]

I have a stretched YJ with a semi triangulated 4 link rear running 14" Fox coilovers. I currently am running XJ leafs up front and they work well but I think I've hit the limit on what the leafs can do. Sometimes the leafs work great and sometimes they don't. Basically, if the Jeep is loaded hard on opposite corners, the leafs can get about 14" of travel. However, if I just roll up on a 2 foot tall obstacle with the front axle, the rear will hit bumps before the front even starts to flex. I'd really like to see a more balanced suspension setup so I'm considering linking the front as well.

So my first question is... I know a triangulated 4 link up front with conventional steering will cause a lot of bump steer. I already have full hydro steering so will I notice a difference in drive-ability on the road between a 3 link and a semi-triangulated 4 link front suspension?

I actually think the 4 link will be easier to install on my rig instead of a 3 link so I'd prefer to go that route if there aren't any major negatives to it that I'm missing. So with that said, here are the calculators I'm looking at. Please comment if you see something that I need to address.

The only reason to choose a 3-Link over a 4-Link with full hydraulic steering is if you are trying to get a crazy high roll center like the moon buggies at crawling comps. For most applications, you don't want a roll center that high, so a 4-Link is the better choice since the axle won't shift laterally throughout its travel. Both 3 and 4 link suspensions can drive beautifully on the road depending on your geometry. I personally wouldn't consider a 3-Link for your application with fully hydraulic steering, so I'll only comment on your 4-Link numbers.


I am not an expert, so don't take my word as the gospel, but your anti dive is really high and I think it is likely excessive since over 100% means your front end will try to jack up under braking and will suck down pretty drastically under power (which can be a good thing for crawling) and eat up compression travel. Your pinion angle change seems problematically high for a front application since that means you are also losing over 10 degrees of caster. I would try to minimize your caster/pinion angle change within what your driveshaft can tolerate. Your roll center height seems decent assuming your C.G. height is accurate (biased more towards crawling rather than hauling, but not at one extreme of the spectrum). Your upper links are too long; a general rule of thumb is ~80% of the lower length. You could also use some more vertical separation at the axle end since you only have 7", and the 25% of tire diameter rule of thumb would put you at 9.5". More vertical separation at the axle is better assuming you have clearance and you don't sacrifice other aspects of your suspension geometry.

Busted Knuckle did a really good 3-part video series on link suspension design which can be found here: https://irate4x4.com/general-4x4/92089-roll-center-explained-suspension-design

 

[Waternut]

The only reason to choose a 3-Link over a 4-Link with full hydraulic steering is if you are trying to get a crazy high roll center like the moon buggies at crawling comps. For most applications, you don't want a roll center that high, so a 4-Link is the better choice since the axle won't shift laterally throughout its travel. Both 3 and 4 link suspensions can drive beautifully on the road depending on your geometry. I personally wouldn't consider a 3-Link for your application with fully hydraulic steering, so I'll only comment on your 4-Link numbers.


I am not an expert, so don't take my word as the gospel, but your anti dive is really high and I think it is likely excessive since over 100% means your front end will try to jack up under braking and will suck down pretty drastically under power (which can be a good thing for crawling) and eat up compression travel. Your pinion angle change seems problematically high for a front application since that means you are also losing over 10 degrees of caster. I would try to minimize your caster/pinion angle change within what your driveshaft can tolerate. Your roll center height seems decent assuming your C.G. height is accurate (biased more towards crawling rather than hauling, but not at one extreme of the spectrum). Your upper links are too long; a general rule of thumb is ~80% of the lower length. You could also use some more vertical separation at the axle end since you only have 7", and the 25% of tire diameter rule of thumb would put you at 9.5". More vertical separation at the axle is better assuming you have clearance and you don't sacrifice other aspects of your suspension geometry.

Busted Knuckle did a really good 3-part video series on link suspension design which can be found here: https://irate4x4.com/general-4x4/920...pension-design

Thanks for the insight. You do seem more knowledgeable than most. This is a crawler primarily. I was actually shooting for anti-dive over 100 so that the front will squat slightly with the rear when hitting the gas. The rear already rises pretty heavily under braking so I don't want the front to drop as well. Although it may feel a little weird to have the whole vehicle rise under braking on the street.

I am a little worried about the pinion angle drop as well but I'm also a little worried about link length. As the upper lengths get shorter, the lower link placement starts getting more and more finicky. I know everything changes a little bit when you start placing it on the vehicle and 1/2" upper or down on any of the lower link mounts starts changing things drastically. If I shorten the upper links 2 inches (roughly 80%), the pinion angle gets better by about 3 degrees but the roll axis angle goes up as well. I do struggle grasping the concept of roll axis angle so maybe going from 1.5 degrees ti 3 degrees isn't a big deal but I've always thought you want that close to zero.

My front driveshaft is 47" long and the u joint angle at the TC will be pushing around 25-28 degrees (including 3 degrees of TC up angle) at 9-10 inches of droop so it's close but in the working range of a 1310 u joint.

Granted, I've only ever built my rear 4 link but I've never understood the justification behind axle link separation rules that people have adopted. I totally understand that more axle link separation will lower the forces on your links and mounts. However, if your forces are under control based on your link material and heim joints, then what's the big deal? I know there was a post from triaged on some forum when I was researching this a couple years ago and his comment was something along the lines of... rules of thumb are irrelevant; just make the numbers work. That's a gross over simplification but IMO I think the rules are in place to help out the guy who's got a 6k lb rig trying to run low link separation and focuses only on the first page. In that scenario, his loads are approaching what even a 1.25" heim joint can handle with a 3/4" grade 8 through bolt.

edit: I don't really have the ability to calculate my vehicle CG. I'm using the basic "top bolt on the transmission" as my number but I suspect that CG number may be slightly lower since I'm always topless and doorless.

 

['84 Bronco II]

Thanks for the insight. You do seem more knowledgeable than most. This is a crawler primarily. I was actually shooting for anti-dive over 100 so that the front will squat slightly with the rear when hitting the gas. The rear already rises pretty heavily under braking so I don't want the front to drop as well. Although it may feel a little weird to have the whole vehicle rise under braking on the street.

I am a little worried about the pinion angle drop as well but I'm also a little worried about link length. As the upper lengths get shorter, the lower link placement starts getting more and more finicky. I know everything changes a little bit when you start placing it on the vehicle and 1/2" upper or down on any of the lower link mounts starts changing things drastically. If I shorten the upper links 2 inches (roughly 80%), the pinion angle gets better by about 3 degrees but the roll axis angle goes up as well. I do struggle grasping the concept of roll axis angle so maybe going from 1.5 degrees ti 3 degrees isn't a big deal but I've always thought you want that close to zero.

My front driveshaft is 47" long and the u joint angle at the TC will be pushing around 25-28 degrees (including 3 degrees of TC up angle) at 9-10 inches of droop so it's close but in the working range of a 1310 u joint.

Granted, I've only ever built my rear 4 link but I've never understood the justification behind axle link separation rules that people have adopted. I totally understand that more axle link separation will lower the forces on your links and mounts. However, if your forces are under control based on your link material and heim joints, then what's the big deal? I know there was a post from triaged on some forum when I was researching this a couple years ago and his comment was something along the lines of... rules of thumb are irrelevant; just make the numbers work. That's a gross over simplification but IMO I think the rules are in place to help out the guy who's got a 6k lb rig trying to run low link separation and focuses only on the first page. In that scenario, his loads are approaching what even a 1.25" heim joint can handle with a 3/4" grade 8 through bolt.

edit: I don't really have the ability to calculate my vehicle CG. I'm using the basic "top bolt on the transmission" as my number but I suspect that CG number may be slightly lower since I'm always topless and doorless.

The roll axis inclination determines whether the car will tend to have a geometric tendency to oversteer or understeer, but over/understeer is affected by other factors such as sway bars, spring rates, and weight distribution as well. I would say as long as it isn't something drastic (3 degrees isn't drastic), you probably won't notice or be able to tune it out. The safer bet is to err on the side of understeer (front roll center lower than the rear roll center) than oversteer (front roll center higher than the rear roll center).

Sure, theoretically speaking, if your mounting brackets, joints, and links are strong enough, you can get away with less link separation, but the forces go up exponentially as your separation decreases. It is difficult to accurately account for the forces your links and mounts will see, so that is why it is good to follow rules of thumb. Personally, I have seen more bracket failures than link or joint failures on the trail. I don't think there is a good reason to run too little separation unless there is no way to physically get the separation required by the 25% rule.

 

[Waternut]

The roll axis inclination determines whether the car will tend to have a geometric tendency to oversteer or understeer, but over/understeer is affected by other factors such as sway bars, spring rates, and weight distribution as well. I would say as long as it isn't something drastic (3 degrees isn't drastic), you probably won't notice or be able to tune it out. The safer bet is to err on the side of understeer (front roll center lower than the rear roll center) than oversteer (front roll center higher than the rear roll center).

Sure, theoretically speaking, if your mounting brackets, joints, and links are strong enough, you can get away with less link separation, but the forces go up exponentially as your separation decreases. It is difficult to accurately account for the forces your links and mounts will see, so that is why it is good to follow rules of thumb. Personally, I have seen more bracket failures than link or joint failures on the trail. I don't think there is a good reason to run too little separation unless there is no way to physically get the separation required by the 25% rule.

I agree that making your link separation low because you can isn't the smartest move but creating a ton of work and fitment problems for yourself just to make sure the rule of thumb works out isn't the best move either.

I had a couple epiphany's tonight as well. I recently ordered new stiffer springs to bring the rear of the Jeep up an inch or two to level it out with the front. However, if I lower the front an inch or so, that actually fixes some issues that would've otherwise been hard or weird to fix. Wheelbase and tire size has changed a bit since I did my rear 4 link and I know I made some trivial changes to the old calculator to correct it slightly, it may need to be more accurately corrected to make sure the rear will play right with the front.