Dezert suspension 101

[Nick]

With permission from PaulW, i'm posting some of his thoughts on suspension set up. Thank you Paul!

Originally posted by PaulW

Desert suspension 101

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As you guys know I did not design my suspension. And I did not build it. Thus I find the discussion on how it works very interesting. Below are a couple of threads to follow for tech info. Lots you you guys already know all this, but maybe it is of value for the new guys.

First is the discussion about dual springs at
http://www.race-dezert.com/forum/showthread.php?t=60528
And it explains to someone who never thought about $2 springs$ why is is a good way to go. The extra cost is worthwhile and is common for the Bronco TTB.

Second is all about leaf springs and why it is hard to get tuning correct.
http://www.race-dezert.com/forum/showthread.php?t=55843
This thread has good info and agrees with the fab guys I had help me. The leafs are hard to get correct due to the rate change. The whole time I had my leaf setup I constantly was tightening up my rebound. Even with my C/O setup the rebound had to be tightened to get rid of rear kick in the whoops. The answer for leafs is $bypass$ shocks. More important for the short WB Bronco compared to a long WB pickup.

My observations
-- I was at RM12 at the SCORE unannounced check at SF and right beside where I logged the cars was a deep hole. I was surprise how many 1 & other buggies had inadequate rebound. The result for the guys that did not know about the hole was the vehicles kicked up in the rear almost vertical on their front wheels. Some of the racers had the hole marked and just drove around it. (both Broncos and the FSs went around - lots of room). The fast cars were correctly dialed in and just floated thru the hole. No TTs had any issue and the front runnig 1s had no issue. The fast guys hit the hole and went airborne, but the cars stayed horizontal and they just flopped back on the track with all 4 wheels hitting the ground at the same time. Perfectly dialed in.
-- Its real sad to see a Bronco pre-runner all wadded up due to inadequate rebound. What happens is if one goes to fast in the whoops with low rebound in the big whoops and endo is possible and happens. Two of them in 2008. I did not see any this year.
-- It works in the front as well. Another example is my good friend driving a Jeep TJ. Single coils in front and hardly any rebound with his adjustable Ranchos so as to give the thing a smooth ride. When going up a gentle smooth rock on the trail the front started to bounce. He backed off and tried again with more pedal. I sat there and watched. The first bounce raised the front wheels about 1-2' the second bounce resulted in a back flip and he landed on his roof. The reason for the action was there was so little rebound in the shocks that the springs just threw the thing in the air and over. Extreme example of a bad setup with to little rebound in front. All was fine, nobody hurt, the stock rollover cage was adequate & undamaged and the diy fixup was less than $2k.
Paul

[PaulW]

I refer you all to --
http://www.race-dezert.com/forum/showthread.php?t=65153
Ther are several Dezert links started by "Atomicjoe23". He is a vet and an engineering student and is designing his own off road vehicle. The advice he is being given is from whos who in desert truck design.
I try to absorb all teh info. The threads are great reads.
PW

[PaulW]

Air Bump discussion
http://www.race-dezert.com/forum/showthread.php?t=60793
This one wont answer your specific questions but does describe the operation.
PW

[SteveG]

This thread has good info and agrees with the fab guys I had help me. The leafs are hard to get correct due to the rate change. The whole time I had my leaf setup I constantly was tightening up my rebound. Even with my C/O setup the rebound had to be tightened to get rid of rear kick in the whoops. The answer for leafs is $bypass$ shocks. More important for the short WB Bronco compared to a long WB pickup....

-- Its real sad to see a Bronco pre-runner all wadded up due to inadequate rebound. What happens is if one goes to fast in the whoops with low rebound in the big whoops and endo is possible and happens. Two of them in 2008. I did not see any this year.

As I said over there, I disagree with some of the rebound comments. In my experience, leaf springs have so much friction that they require little rebound valving and too much will cause suspension packing. On my first "prerunner" build I was having a hell of a time getting the rear of my truck to stop bucking. I added more and more rebound valving... ordered shims several times and it got worse and worse. I talked to a shock-guru friend of mine that contradicted everything I had been thinking... but it made sense. He advised very little rebound valving in the rear and the front. I made the changes and the truck REALLY woke up!

There was a specific dirt road with a relatively large and blunt bump that would ALWAYS send the rear of my truck airborne. The bump was large enough to just about bottom the front at speed but the nose would still stay pretty level. The rear would do the same on compression but instead of staying level it would buck like mad due to lack of wheel travel. The front was staying too low and the rear was packing... by the time I hit that bump, the rear had lost wheel travel because it had been packing in all the previous bumps. The answer was to speed up rebound in the rear and front.

After the valve change, the front would push off the bump and come up. The rear would still go up after the bump, but now I was utilizing all the available rear travel and because the front had come up as well, it would basically match the front then it would come down relatively flat.

Since that lesson I have valved all my trucks in similar fashion. My Bronco handles very much like that first truck. At speed through rough terrain my Bronco carries the front end. For me, it's confidence inspiring and just feels really, really good.

Now, of course LOTS of things come into play here. Driving style, set-up, weight, etc... what works for one truck won't necessarily work for another.

[baja-chris]

I understand your comments regarding spring stiction and agree. The real answer is it depends (on many things). You really have to deal with each setup based on all the circumstances. A great tool is to video tape the vehicle and study the tape. This is the only way I've been able to tell the difference between kicking due to packing and kicking due to lack of rebound. In the case of my bronco (and my dad's when he had leafs), the real problem is the angle the shocks are at. It has a falling rate so at full compression the shocks just don't have enough leverage to control the spring. Some day I'll do a bell crank shock to get a flat or rising rate out of it. BTW, on our leaf setups we custom machine both our bushings and sleeves (using 4130 tubing) and make the sleeves longer than the bushings so we can torque the spring bolts super tight and still have free movement of the spring to minimize stiction. My rear suspension droops out nicely but nothing like a 4-link coilover does.

[randomthoughtsracing]

Thanks for compiling those links in one place. Over the past year I have been doing a number of setups with leaf sprung race vehicles (class 3, 7s/sx trucks, Jeepspeed, 1450). I have found that these classes which are supposed to be the cheapest actually require the most thought out (and custom) suspension packages. Most of the performance difficulties seem to come from applying theories (and setups) from non leaf spring vehicles. I have come to the following conclusions and am busy making changes to address them.

-rear end performance is dependant on front end performance.
-bumpstop travel length and tuning is important.
-Standard bypass layouts (while they might work well laid over under a Bronco) are not ideal for mounting near vertical with a leaf spring.
-Shock tuning makes a poor bandaid for bad leafspring and shackle setup.

[randomthoughtsracing]

Here is a video a customer took last weekend testing a pile of non-standard Bilstein parts. Interesting to see what is going on with the rear leaf spring through its range of motion. Enjoy!

Behind footage.
http://www.youtube.com/watch?v=6nfJGig_22U

Undercarraige footage.
http://www.youtube.com/watch?v=4b7uV_SjU2o

[SteveG]

Thanks for joining and commenting, Joel. Your input is much appreciated.

The leaf springs on that truck look really unhappy at times.

Here are a couple videos showing my Bronco at work. The leaf spring video degrades toward the end... the camera case did not take well to being sand blasted.

http://www.youtube.com/watch?v=xyZj5kPObDE

http://www.youtube.com/watch?v=OImRB8vZZMY

[monkei]

^ Those have to be some of the coolest videos, EVER. Big inspiration to me as someone who will probably be staying leafsprung for a good while.