Designing a 4-link rear for the desert vs. a trail rig???

atomicjoe23

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Just a quick question for those of you in the know. . .

I have a collection of 4-link articles from various sources but most of them are for trail trucks or rock crawlers so I was wondering what would be the major difference in geometry from one these rigs that are mostly concerned with maximizing articulation and a desert truck that has long travel at high speeds. . .

. . .my educated guess would be that the angles that you install the links (particularly the upper links) at (as you are looking down at the truck from above) would be shallower in a high speed truck than it would be for a slow speed, articulating truck, but what angles should one shoot for for a desert truck???
 

Giant Geoff

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Your are always starting threads asking good questions, what are you going to build or have you started?
 

atomicjoe23

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Thanks for the compliment!!!

PM sent. . .response was kinda long and off topic so I decided to PM you instead of posting it here. . .

the short answer is I currently have a '94 Jeep YJ Wrangler and a '79 F-150 4x4. . .but I want to build a 2-seat, rear V8/RWD play buggy for the desert. . .
 

Kritter

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big one being a rockcrawler should be designed to transfer weight forward or rise in the rear on acceleration since you are going up steep obstacles. Desert truck that rises in the rear on acceleration is rare but I know at least one that used to really fast but you had to know how to drive it. A desert truck you want to transfer to the rear.

the above difference have to do with anti squat and how much you want.
 

Motiracer38

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Eehhh, I'm not sure about your theory on desert trucks. I have always been under the assumption that the rear rising would aid in stiffening your rearend and keep you on top of whoops. Isn't that the reason you pin it when you hit a section with too much speed? Lately all the TTs I've seen have been pulling their forward trailing arm mounts up in the chassis, leading to more anti- squat.
 

Kritter

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Eehhh, I'm not sure about your theory on desert trucks. I have always been under the assumption that the rear rising would aid in stiffening your rearend and keep you on top of whoops. Isn't that the reason you pin it when you hit a section with too much speed? Lately all the TTs I've seen have been pulling their forward trailing arm mounts up in the chassis, leading to more anti- squat.
so you want to plow the front end into the whoops when you hammer the gas? Or do you want to keep it nose light?
 

Triaged

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big one being a rockcrawler should be designed to transfer weight forward or rise in the rear on acceleration since you are going up steep obstacles. Desert truck that rises in the rear on acceleration is rare but I know at least one that used to really fast but you had to know how to drive it. A desert truck you want to transfer to the rear.

the above difference have to do with anti squat and how much you want.
If you are accelerating having the rear rise would transfer more weight to the rear than if it squatted.
 

atomicjoe23

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I was really more interested in the angles between the upper control arms (when viewed from the top) on a desert racer vs. those on say a rock crawler or a trail rig. . .and not so much in a discussion of anti-squat (or lack thereof) characteristics. . .

. . .what I have seen in the past is that the desert racers seem to run little to no angle on the lower trailing arms (once again viewed from the top) and that both the upper and lowers mount to the main frame while it seems that rock crawlers and trail rigs seem to mount both sets of links to a crossmember between the main frame rails. . .

. . .what I would like to know is what ~angle are the desert racers running on the upper control arms. . .and what effect on chassis handling does the angle of the upper control arms have (once again speaking of the angle when viewed from the top of the vehicle not from the side).

Thanks!!!
 

Triaged

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Normally rock crawlers have higher belly height and shorter links than desert race trucks. That makes the links more angled when looking from the side. If the links are parallel (top view) than the axle roll axis is parallel to them in side view. In order to get the rear axle roll axis (which directly affects rear roll steer) away from roll oversteer (unstable) rock crawlers normally triangulate the lower links. Some desert race trucks don't seem to care about the axle roll axis at all or maybe even want roll oversteer in the rear (see VW TT)

The angle of the upper links just comes down to the length. You can only put the mounts so wide on the chassis and so close together on the axle so longer upper links lead to less angle. The less angle you have on the upper links the more load will go through them from side loads on the axle.
 

Haycock

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triaged is correct. rock crawlers deal more with articulation, with one tire fully drooped and one stuffed you will get alot of roll steer (the drooped tire will swing forward) with strait lower links. desert trucks usually see more strait up-down motion plus with a longer lower arm they dont need to worry about roll steer as much. both suspensions need atleast the upper or lower arms triangled or to run a panhard bar to keep the axle centered. i believe that strait lower links is a stronger setup for high speed stuff. as far as the angle of the upper ams go, more angle gives you more side load stength.
 

Chris_Wilson

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In addition to the high speed handling issues that can be caused by too much roll steer, the link geometry also determines the amount of drive shaft plunge.

Drive shaft plunge is more important to minimize in desert racing than in rock crawling because the suspension moves faster for long periods of time and this builds up a lot of heat in the splines. Too much heat and the splines don't want to slip and this can damage the drive shaft, transmission or rear end.

This is also why plunging axles are not common in desert racing (they get hot and don't want to slip which can damage the transaxle etc.)
 

atomicjoe23

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Thanks guys. . .that makes a lot of sense. . .I wasn't thinking about the roll steer when I was trying to think about exactly what the differences in handling were. . .

. . .I knew about the articulation bit, but hadn't taken the roll steer into account. . .or the driveshaft plunge.

Greatly appreciated!
 

MNGSX

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http://www.spike.com/full-episode/suspension-101-rock/30579

Pretty good video here.

Right now I am looking at the 80 series landcruiser in the drive and I just might be moving to CA since a potential employer is flying me in just for an interview!

So I am thinking of turning it into a cross trainer. A rock runner or pre-rocker. It won't dominate the desert or the rock garden but it can excell at being a jack of all trades.

I want to replace the front radius arms with a 3 link. The rear already has a 4 link but it is parallel and has a panhard. So I am looking to build a 4 link that can do both desert and the trail well.

Of course there is the handy dandy 4 link calculator!

http://mysite.verizon.net/triaged/files/4BarLinkV3.1d.zip

Here you can play around with your link lengths and angles in 3D and see the effects on roll steer and antisquat/squat!
 

ironbenderii

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Uhhhh, unless I misunderstood the canadian he is confused on squat vs. anti-squat. He has it reversed, saying the point where they converge being above the centerline causes squat and below it causing anti-squat. I'll have to go back and watch it again...
 

Triaged

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Uhhhh, unless I misunderstood the canadian he is confused on squat vs. anti-squat....
He did botch up a thing or 2 in that show. I would have to watch it again to tell you what all of them were but I think you nailed the biggest one.
 
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