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Jumping your truck

Generally you want the dampers as vert as is reasonable. The main reason to angle them is to get more travel than the actual damper's stroke. For some reason most dez racers angle them top forward in the rear. What this means is that the amount of damper piston travel per unit of housing travel lessens the closer the axle gets to full bump. The ramification of this is that the piston velocity slows down as it nears the top of the stroke, not something you usually want in a dez machine. Translation: This is falling rate damping where you really want rising rate damping as it aids the bumpstop and tends to act a bit like a hydro-bump.

My thot is to angle them slightly rearwards. As the spring flattens out in bump the housing will move back. This will put the housing closer to directly under the upper mount. I would go as far as cycling it with the main leaf only (& spacer blocks to simulate the other leaves if spring over). Set the mounts truly vert, as seen from the side, while at full bump.

You also want the mount on the housing to be as close to the wheel as is reasonable. I would settle for a mild inward tilt if it got the lower end closer to the wheel. The reason for this is that the damping acts more in a 1:1 relationship with the wheel. The further inboard the mount is, the firmer the damping has to be to get the same effect at the wheel.

I'm looking at all of this from a dezert machine aspect, I've no good idea of what a crawler will want.
 
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Dont forget that unsprung weight is a bad thing. That is all the weight not sprung by the springs, stuff like this is axles, tires and wheels.

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This one has me wondering still. Why is unsprung weight a bad thing? I'm not sure I quite understand.
 
This is some good info. I like your thoughts on how to set the top mount. I guess it should work for both the front and rear.

As far as the shock placement for a crawler goes as long as the shocks aren't limiting the travel in any way I don't think it would much matter. You might get a softer feel by inboarding them but you would loose the dampening and even in a crawler I don't think I would care for that trade-off much.

So would you mount the shock even closer to the wheel than the spring mounts?






/forums/images/graemlins/thinking.gif /forums/images/graemlins/grin.gif Is there anything a crawler can do that a racer can't lol /forums/images/graemlins/deal.gif
 
Weight is mass (well, it is for this purpose). When you have more mass, you have more inertia, which is basically the tendency to resist change in speed (accelerate) in any direction.

The unsprung mass will accelerate in different directions based on inputs. For instance, consider a pot-hole or speed bump. The less unsprung mass, the less it resists this change in direction of travel. More importantly, the much greater mass above, separated by springs, will remain relatively stable as the springs compress or extend to allow the unsprung mass to move. Then that stored energy in the springs can change the direction of the unsprung mass easier due to it's lower mass resulting in less inertia. More unsprung mass means more of the spring's stored energy that will be transferred back into the sprung mass above as the unsprung mass resists changing directions. That sprung mass also has it's own inertia which wants to keep it moving in exactly the same way. The higher the ratio of unsprung to sprung mass, the more stored energy from the spring will be transfered into the sprung mass.

That's the simplified Newtonian Mechanics version, but should be complete enough to cover the bases while still making sense.
 
Hmm so I guess my planned 42" trail tires would be traded in for some 37" a/t's before a trip to the dunes then. /forums/images/graemlins/tongue.gif
 
put evan more simply the less unsprung weight you have the better your suspinsion will react to changes in the road surface.... right?
not trying to step on your toes i know that you know ALOT more than me about this stuff!!!!!
 
I just wanted to provide some of the "why". /forums/images/graemlins/laugh.gif
 
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My thot is to angle them slightly rearwards. As the spring flattens out in bump the housing will move back. This will put the housing closer to directly under the upper mount. I would go as far as cycling it with the main leaf only (& spacer blocks to simulate the other leaves if spring over). Set the mounts truly vert, as seen from the side, while at full bump.

You also want the mount on the housing to be as close to the wheel as is reasonable. I would settle for a mild inward tilt if it got the lower end closer to the wheel. The reason for this is that the damping acts more in a 1:1 relationship with the wheel. The further inboard the mount is, the firmer the damping has to be to get the same effect at the wheel.

I'm looking at all of this from a dezert machine aspect, I've no good idea of what a crawler will want.

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I agree w/ this completely. I also believe that getting the upper mount as high as is practical helps to stabilize during jumps/landings.
3081Buggy5_081803.jpg

The only drawback for this shock setup in the "crawler" application is that it's hard to find a shock long enuf to be mounted in this way that will allow full articulated travel. My "double shock" setup has 20" of stroke, and in the rocks it gets "bottomed" and "topped".

Marv
 
So as far as shock length I should buy an 18" travel shock and mount it as high as I can without having it top out when drooped?
 
No, when you mount your shocks and design your suspension you want to have the most amount of controlled travel you can have. At least thats what i've always been told. I would stick with a 16" travel shock, and mount it so you can achieve decent travel, but dont over do it. Use limit straps and bump stops to control the suspension, you can have all the travel in the world but if its not controlable its pointless in having.
 
How would using an 18" travel shock turn it into uncontrolled travel? /forums/images/graemlins/confused.gif If the suspension can cycle through that much wouldn't it remain controlled?

I plan on using 63" rear springs. They should be able to handle that much travel if not very close. It might not all be necessary for the go fast end of things but you could limit the travel with straps as you mentioned and then for the rocks unhook the straps and you could use the extra 2" right?

Am I missing anything /forums/images/graemlins/dunno.gif The front would only need a 16" shock but I couldn't see the rear needing the same as the front...
 
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No, when you mount your shocks and design your suspension you want to have the most amount of controlled travel you can have. At least thats what i've always been told. I would stick with a 16" travel shock, and mount it so you can achieve decent travel, but dont over do it. Use limit straps and bump stops to control the suspension, you can have all the travel in the world but if its not controlable its pointless in having.

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Jason,

I was gonna ask for some clarification on this too... /forums/images/graemlins/ears.gif
By contolled - do you mean w/ in the range of the shock stroke, or do you mean a point where the axle locating ability of your system starts to change quickly?

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So as far as shock length I should buy an 18" travel shock and mount it as high as I can without having it top out when drooped

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Max,
At the risk of showing my ignorance /forums/images/graemlins/ignore.gif, I would say that this would be a decent 1st swing to get you in the ballpark. If you wished to limit the system from "topping, bottoming, or radical geometry changes" w/ straps and such - I think that would be wise.... But one point I was trying to make is that I think it's better to use an 18" travel shock and limit it to 16" of usable stroke, than to use a 16" stroke shock and use the full length. Why...? Only reason - cuz the top mount is higher (of course this assumses that damping rate is constant across the shock stroke...).

Marv
 
Well from talking with a friend that is running ford 57's in the back that was capable of swinging close to 20" of travel, but had it limited to around 16" or so. Anyways we were talking that there is the possibility that when you are hauling ass and the back end hits max droop with the spring extended all the way out that its possible for the back end to grab traction all of the sudden and bust a spring. While this wouldn't be so bad on military wrap springs it would be horrible on stock springs with only one main leaf. Plus ive read from spring manufactures that the only usuable travel is when the spring is still together. As in you wont gain any usuable travel if you open your spring clamps all the way up so the spring droops way out.

Also when you start to hit around 18" of travel your going to need some sort of trackbar to keep the axle centered under the truck at full droop. And not to mention driveline issues and an 18" travel shock is monsterous. With an 18" travel shock your looking at a shock that is nearly 4 feet tall extended and over 2 feet long at full compression. Yeah the mounts will be high, but I think they may be overkill. So I dont know, thats just my reasoning.
 
Ya know, Kreg Donahoe said something a few years ago that really made a lot of since.

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Dont think MORE... Think perfect... Perfect spring Rate.. Perfect shock Valving... Most of the little 7s trucks racing right now have under 12" of wheel travel yet they can still take a 3ft whoop at a good clip of speed. SHOCKS Are the key... Get a good set of shocks Like Sway-a-ways.. By-pass is better. If you get 12" of travel out of the front and 18" or so out of the rear with a good set of shocks you will have a great runner there...

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I've seen trucks that didn't appear to have a lot of travel, yet when they were running, it didn't matter what they were running through, the fluidity (is that really a word?) of the suspension was just amazing, and this wasn't no Trophy Truck either.
 
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Ya know, Kreg Donahoe said something a few years ago that really made a lot of since.

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Dont think MORE... Think perfect... Perfect spring Rate.. Perfect shock Valving... Most of the little 7s trucks racing right now have under 12" of wheel travel yet they can still take a 3ft whoop at a good clip of speed. SHOCKS Are the key... Get a good set of shocks Like Sway-a-ways.. By-pass is better. If you get 12" of travel out of the front and 18" or so out of the rear with a good set of shocks you will have a great runner there...

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I've seen trucks that didn't appear to have a lot of travel, yet when they were running, it didn't matter what they were running through, the fluidity (is that really a word?) of the suspension was just amazing, and this wasn't no Trophy Truck either.

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Thats what I was trying to explain. That quote explains it better than I ever could. Thanks bubba. /forums/images/graemlins/thumb.gif
 
I understand what you guys are saying....

Proper spring rate and damping goes a long way. Basically, a properly "sprung/damped" truck w/ 12" of wheel travel would work better than a truck w/ 18" of travel and spring rates/dampers all outta whack!

correct me if I'm misunderstanding....

Marv
 
Ok.. I'm not thinking in the terms of more is better, so much as, it's there so use it.

I can see how too much travel can become an issue as far as keeping the axle properly located etc. but in the case of a dual purpose wheeler would it not be possible to have an 18" damper dialed in for high speed use through 16" travel, limit the droop to 16" and have the pack clamped together to maintain proper usable travel while blasting around at desert speeds but then unbolt the clamps, remove the limiting straps and use the extra 2" travel for crawling?

I am going on the assumption that the usable travel as well as the damping and axle location is more of an issue at speeds.


On a side note I'm also curious if the spring rate is going to become a compromise for the dual purpose thing? Or do you simply dial in a spring for the weight of your vehicle and it will excel in both applications?
 
Reading my post again, am I overrating droop while crawling? Is that extra 2" worth fighting for. You would probably only need it maybe 2% of the time you are on the trail.

It just goes back to the if it's there might as well use it. I would hate to be in one of the 2% situations knowing that my suspension is capable of the extra 2" but I didn't make use of it. /forums/images/graemlins/dunno.gif /forums/images/graemlins/dunno.gif What do you guys think?
 
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I understand what you guys are saying....

Proper spring rate and damping goes a long way. Basically, a properly "sprung/damped" truck w/ 12" of wheel travel would work better than a truck w/ 18" of travel and spring rates/dampers all outta whack!

correct me if I'm misunderstanding....

Marv

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Bingo.............................

But remember, this is two different animals we're talking about. Even though there is a lot of crossover technology and some guy's rock rigs can handle the speed thing pretty good, they're not gonna be able to compete against most desert rigs in that facet, and I seriously doubt there's a desert rig (since most of 2wd) that can handle rocks decently.

You're right on target.
 
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