TOTM: Rear Suspension

[rstrucks]

We've covered front suspensions pretty well, time to move onto the rear suspesion. If you are running a bolt on system, what works well, what doesn't. Custom or one-off suspension - what are the numbers (anti-squat, roll center height, link length)?

Post pics of your set-up if you have them.

[Ken L]

Uppers are 35" long, lowers are 44" long. I have rod ends (RuffStuff Specialties) on both ends of all the links. Uppers are triangulated, lowers slightly angle out from the unibody to the axle but I couldn't really call them "triangluated" (33.5" apart at the unibody, 39" at the axle). Lowers are 2.5" forward of the axle centerline, uppers are 1" forward of the centerline. 9" vertical separation at the axle, 4 1/4" at the unibody.

Anti-squat is 98%, roll center is at 21".

I really like how this suspension works. I have the same length arms for the front radius arms, I'm running RE 4.5" springs. The suspension really works well, I am impressed by how it worked a couple weekends ago on Chinaman Gulch. At this point there is nothing on the suspension that I would change.







I have more pics of the unibody sleeves with the brackets on, too.

[SirFuego]

For those that have run an upper tri 4 link (like Clayton's) and a double tri 4 link -- did you notice any major improvements? Curious primarily about stability and/or if the lack of "flex steer" was noticeable.

[Horus]

For those that have run an upper tri 4 link (like Clayton's) and a double tri 4 link -- did you notice any major improvements? Curious primarily about stability and/or if the lack of "flex steer" was noticeable.

I ran a ZJ with Clayton's front and SAs in the rear on 5 inches of lift for about a year on the street and trail. With no other changes made I swapped in the Clayton rear LA upgrade after the first year with the rig, giving me a good direct comparison between the two rear setups.

The flex steer you mention was not really a noticeable difference between SA and Clayton's single tri'ed rear LAs. The torque roll through the chassis was insane with the rear SAs in place. The whole rig would roll four or five inches if I got into the throttle. I was used to it but passengers found it very scary. This was completely remedied with the Clayton LA rear kit.

We duplicated the Clayton rear kit on my buggy. The Clayton upper arms are being reused its so close. If I was worried about flex steer I'd have located the lower arms closer together. This will eliminate the majority of the flex steer issue. I see many newer local rigs with single triangulated lower rear setups now for this reason. Triangulate the lower links so they are close at the frame end and wide at the axle. Then locate the upper parallel links about two or three feet apart.

With this design you all but eliminate flex steer and create more than enough room for rear steer without link interference to the tires. It makes sense on most rigs.

[grin]

I'm runnning a standard Clayton triangulated 4-link rear in my WJ, utilizing Clayton's over the pumpkin truss rather than the stock D44a mounting location since I swapped in a JK D44 rear. I have noticed absolutely no torque steer, nor any significant flex steer (though my off-road seat time in this rig has been limited). I used the same location as the JK lower control arm mounts on the axle side, but elevated about 3" from stock JK location, which I believe is close to replicating the stock WJ LCA location, but spaced perhaps an inch wider at the axle end... Coming from the XJ leaf-sprung world, it has been weird to get used to having ZERO body roll offroad, since the Clayton/coil suspension in the rear just sucks up whatever obstacle it encounters. Me likes.

[Jeeptech01]

I have Durango bastard packs and .75" shackles. Its predictable and locates the axle quite well. No links reqd

[rstrucks]

I didn't have my Grand long enough after the rear triangulated four link to give too much feedback about how it performed but it did do well the one time I wheeled it. I wish I had gotten more seat time in it with it done. Anyway, I used to run a suspension from Rusty's and it did susprisingly well but I thought it was time to upgrade. I had to notch the rear floorboards for upper link clearance to get the links sitting close enough to where I wanted them.



I have more pics of the 4 link on my home computer (I'll try to add those later) but here are my "numbers" -

anti-squat - 99%
roll center height - 26"
roll axis (oversteer) - 3 degrees

[SirFuego]

I have more pics of the 4 link on my home computer (I'll try to add those later) but here are my "numbers" -

anti-squat - 99%
roll center height - 26"
roll axis (oversteer) - 3 degrees

How did you figure out the CG height and unsprung mass? I was planning to put together writeup for a homebrew method of estimating unsprung weight without using any scales (just need to know your spring rate).

The reason I ask (you probably know this, but I'm mentioning this for tech's sake), is that anti-squat is a function of the height of the center of gravity. So if your COG estimate is wrong, your anti-squat is going to be wrong, too. That's why it's always suggested to try to make your setup adjustable (usually by having multiple mounting holes for the uppers). The purpose is twofold -- (a) it allows you to play around with different anti-squat values and see which works best and (b) it helps compensate for any errors in estimating COG and weight.

I have a Vehicle Dynamics book (it's basically a text book) by Milliken and Milliken that has some great diagrams and simple explanations for concepts like instant center and roll axis -- and all are discussed in regards to practically any possible suspension geometry, including independent suspension and leaf sprung suspensions. It also includes a couple chapters of tuning steering/suspension to achieve desired effects.

[AgitatedPancake]

I've heard that the height of the crankshaft is a decent rough estimate for COG height, don't know how true it actually holds but sounds good to me haha

[rstrucks]

How did you figure out the CG height and unsprung mass?

Geusstimation method for the CoG. The unsprung mass is a little more exact. I had corner weighted my ZJ with scales and then estimated the weight of the axles tires/wheels.

I've heard that the height of the crankshaft is a decent rough estimate for COG height, don't know how true it actually holds but sounds good to me haha

I have heard that the cam height is the better estimate point. I actully added a couple of inches to that for my top heavy Grand.

[AgitatedPancake]

LOL my instant reaction to that was WTF as I was thinking about my overhead cam engine...yeah haha. You're probably right though, having it higher would account for the extra weight up high

[5.9 ANDY]

i have heard top bolt of the tanny bell housing.

[OverlandXJ]

Is the torque steer with SA rear something everybody is experiencing? I'm not liking the sound of that..

I was planning this with a LA front for the next year or so till i can scrape together Claytons rear setup. The front suspension is coming from my XJ, and i have everything but a tracbar for the rear currently.

[IndyZJ]

Is the torque steer with SA rear something everybody is experiencing? I'm not liking the sound of that.

Torque steer is usually a term referring to FWD cars. I'm guessing you have a bit of lift and the panhard bars ar at exaggerated angles. When the rear end squats under weight transfer, the rear axle is pushed to the driver side by the panhard bar. Since the axle can't move sideways very easily, it pushes the ass end of the body the other way. Since the front panhard bar is opposite of the rear, the effect can be even further exaggerated. The axles are tracking parallel to eachother, but you effectively end up crab walking down the road. The only real solution is flatter panhard bars or eliminating them all together.

[OverlandXJ]

Indy, thanks for the explanation.. it helped. Mine is not done yet, soon but i saw the mention by Horus that peeked my interest.

What i get from this is that it's to be expected, cure being triangulated rear uppers and the TB delete..correct?

[UTChavok]

Is the torque steer with SA rear something everybody is experiencing? I'm not liking the sound of that..

I was planning this with a LA front for the next year or so till i can scrape together Claytons rear setup. The front suspension is coming from my XJ, and i have everything but a tracbar for the rear currently.

Torque steer is usually a term referring to FWD cars. I'm guessing you have a bit of lift and the panhard bars ar at exaggerated angles. When the rear end squats under weight transfer, the rear axle is pushed to the driver side by the panhard bar. Since the axle can't move sideways very easily, it pushes the ass end of the body the other way. Since the front panhard bar is opposite of the rear, the effect can be even further exaggerated. The axles are tracking parallel to eachother, but you effectively end up crab walking down the road. The only real solution is flatter panhard bars or eliminating them all together.

Great explanation. Im running radius arms up front, but short arms in the rear also. Adjustable RE lowers and teraflex uppers. The body roll is outrageous with no swaybars, a 4 link is the next project for my zj to cure this. Im used to it but when passengers ride and I take a decent turn at the normal speed like you would in your moms minivan it feels like im stuffing my 35s on one side and I get some interesting looks. To be honest with anyone debating trying to get away with short arms on 5" of lift, dont do it. Im running 5.5" coils and it seems no matter how much I tighten every bolt and recently replaced the bushings there always seems to be something loose and honestly it drives like crap.

[SirFuego]

Another thing to consider is that when you are running short arms on 5+" of lift, your control arm angles are at a severe upwards angle. This has multiple effects:

1) Rougher ride because the angle is transmitting some of the force directly through the control arm onto the chassis.
2) More suspension wear because the shorter arms creates more binding (which is why many short arm people tend to go through a lot of bushings once they start to wheel their rig).
3) Increased anti-squat. This essentially means that the body should go up when you hit the gas. I'd imagine this can also contribute to the body roll since you are essentially increasing your COG a bit from the body lifting up. The effects of anti-squat are debatable as there is a wide range of AS values on successful competition rigs in the past (although I wonder how accurate the COG heights were estimated on the rigs that theoretically have high AS).

Here is a diagram I modifed to get a better understanding of what Instant Center is and how it relates to Anti-squat. You can see how to determine instant center for a linked suspension, but you can just think of it as an imaginary "pivot point" for the entire suspension. The white line from the bottom of the rear tire to the COG is simply the 100% antisquat line for reference.

[Horus]

Indy, thanks for the explanation.. it helped. Mine is not done yet, soon but i saw the mention by Horus that peeked my interest.

What i get from this is that it's to be expected, cure being triangulated rear uppers and the TB delete..correct?

What I was describing was torque induced chassis roll. It is effected by squat/antisquat characteristics but is IMO a separate issue from torque steer or flex steer. If you have ever owned a solid 11 second street pounder you know what this is. Your engine's power is rotating across the width of your chassis all the way down until it is spun 90 degrees by the ring and pinion. From R@P to the tires that power (multiplied substantially of course) now works down the length of the chassis, this is controlled by control arms set at particular angles to the chassis (squat numbers.)

Before that happens the engine power is rotating across the chassis as it traverses the space between the t-case output and rear pinion. Your motor is trying to twist its way out of the chassis. The motor is bolted to the chassis, the rear pinion (axle housing) is not. The result is the axle assembly also tries to twist away from the chassis under power. The driver rear tire will push down hard into the ground whereas the passenger rear tire tries to pull up and tuck into the wheel well. When you watch a drag car launch it only appears as though the tucked side of the chassis is hooked up harder.

[Horus]

When you have a V8 powered ZJ you already have a fun and powerfull vehicle. Add power with intake and exhaust or even cam and cylinder head mods and these things are wicked. Now put it up five inches plus the height gained from 35 inch tires and you have a recipe for a very easy to roll over rig. Ask me how I know.

I drive with a pretty heavy foot as it is. Honestly the excessive torque roll thing was fun much of the time. It was never an issue offroad nor was torque or flex steer, short arm or long. Torque roll on the street with rear SAs sucked when I had to take a left hand turn as one would through a traffic intersection. I've picked up the driver front tire a couple times just by laying into the throttle a bit hard turning left.

When I did the Clayton rear upgrade the torque roll all but disappeared. The rig felt much safer to drive on the street. I kinda missed being able to scare the shit out of an unknowing passenger with a blip of the throttle during a turn.

[SirFuego]

One other thing that I should mention is that the parameters regarding a suspension most people talk about (anti-squat, roll center height, and roll axis) are STATIC measurements on flat ground. Meaning that as the suspension cycles, these values can change -- which a suspension designer may take into account. The newest 4-link calculator allows you to bump and droop the suspension to see how the values change.

The best way to visualize this is to look at the diagram I posted above and imagine the rear wheels drooping down a couple inches. Assuming the instant center is not at the control arm mounts (i.e. uppers and lowers mount at the same chassis height), the control arm angles will change the instant center, thus the anti-squat now changes.

I almost wonder (meaning that I don't know if it's true, but it's an unproven hypothesis I have) if this is the reason some rigs "bounce" when trying to throttle over rocks and hill climbs (whereas others just crawl right over it with no problem). My thought is if the AS characteristics change from <100% to >100% (or vice versa) that could cause the suspension to eventually oscillate back to <100% AS, then back to >100% then to <100%, etc. At each change, the suspension/body are reacting differently, so it causes the rig to bounce. Whereas the rigs that have no trouble have a suspension where AS is consistently above or below 100% throughout the travel used in that obstacle...

[IndyZJ]

When you have a V8 powered ZJ you already have a fun and powerfull vehicle. Add power with intake and exhaust or even cam and cylinder head mods and these things are wicked. Now put it up five inches plus the height gained from 35 inch tires and you have a recipe for a very easy to roll over rig. Ask me how I know.

I drive with a pretty heavy foot as it is. Honestly the excessive torque roll thing was fun much of the time. It was never an issue offroad nor was torque or flex steer, short arm or long. Torque roll on the street with rear SAs sucked when I had to take a left hand turn as one would through a traffic intersection. I've picked up the driver front tire a couple times just by laying into the throttle a bit hard turning left.

The problem was not because of the "wicked" power of a slightly modified v8 (~300 hp). A lifted short arm 4 cyl. TJ will do the exact same thing. It is a function of the atrocious suspension geometry that comes with a moderately lifted, but otherwise stock suspension. Look at the diagrams above and pay attention to the concepts of antisquat and instant center. With a lot of lift on short arms, those values can virtually be through the roof.

An 11 second car (or any vehicle) that torque rolls like that would also have a "bad" suspension setup. It might get the job done, but it is far from optimized.

[ZJ TINS]

So based on the drawing the ideal anti squat set up would be never over 100% and as you cycle to minimize how far below 100% it gets. (e.g stay as level as you can but rear squat a little at worst case)

True? or is there a reason to get further from 100% either way and under what condition?

[SirFuego]

Posts 17 and 25 in the thread below may be of interest:
http://www.pirate4x4.com/forum/showthread.php?t=168577

[UTChavok]

Thought Id bump this up. What length are you guys running on your lowers? I found a pair of RE lower arms that are 33.5 bolt hole to bolt hole and can get these a lot cheaper then buying 4 johnnys, tube inserts, jam nuts and DOM and then just make my uppers. Tomorrow I plan to sit outside with the calculator and see what I can come up with but figured id post up today and ask. I already have brackets for the frame side cut up like Claytons kit so basically was going to copy that design.

[blackbeer]

can we get into brands? pros and cons?

i have claytons front and i have seen people remove the pasenger upper arm to make it a 3 link- ill save that for another topic,
point being im debating if i should stick w the same brand for rear LAs.

Im really digging TnT frame stiffeners vs claytons and i also like the LA setup they have as seen on AndyZJs rig being built.

thoughts on TnT vs Claytons? for all fairness you could include IRO in the debate but i absolutely refuse to use a bolt on long arm kit- + not have uni stiffeners. like to hear the experts chime in im really leaning towards tnt