suspension help, dmax 4x4 launch

Krazeeun

Krazeeun

Hardway Performance
Joined
Feb 4, 2008
Messages
2,369
lets say, hypothetically, we have a 700+rwhp lmm duramax that we want to go fast in a straight line.

this duramax has full billet steering, rear lowering shackles, stock spring pack, caltracs and 33" proxe ST's on 22x9.5 XD Hoss's.

when the truck launches (25-30psi) it jumps straight up in the air shakes violently and pulls all over the place until throttle's back out, it hooks then the allison goes into limp and we run down the track cruising in 3rd gear only :bs:

We're going to remove 2 or 3 of the rear leaf's, mess with the rear shackle to get ride height dialed in and try it again... trying to get some weight transfer to happen... am I on the right track?

suggestions? looking to run in the high 11's with the truck, current best at the 1/8th is a 7.68 when we could get it to hook :doh:
 
Are the torsion bars all the way down? Its basic, but i have to ask. Wheel bearings tight in the front? I'd pull the overloads, but watch they you don't get to much front end rise, a buddy had to put limit straps on his LMM because it transferred really well, and was lifting way to high in the front. I'd try pulling the overload, and making sure the bars are all the way down then try it again
 
The basic's is what I need...

it's going to the redneck nationals next weekend and if we can't get it to hook it's just going to be a waste of time
 
Ok drop the torsion bars all the way down, undo the bolts till they are not touching anything. Check the wheel bearings, make sure the idler and pitman arm are tight. It sounds like a lot of your hopping issues are coming from the torsion bars. They launch like **** if the bars are tight.
 
25 or 30 psi launch could be your problem also!!!!thats alot of boost your leaving at!!!
 
Try 10 psi boosted and go up from there till it gets worse then back down till when it was good. Pull the overload and you can pull the 2nd spring from the top , then install the stock shackle to regain most of your ride height. Your trans limp is most likely to do with the high boost launch.
Not sure what full billet steering is but make sure it has either an SD centerlink or Cognito braces and some sort of HD tie rods in addition to dropping the front end completely.
What hole are the caltracs in on the front mount?
 
First of all, 25-30 psi is way more than you can effectively launch at unless you are running some big slicks that you can get lots of heat in before you launch. I've had the best results launching my truck at around 15 psi. Secondly, your wheels and tires are not helping you at all. If you have or can find a set of stock aluminum wheels, mount some 265s on them and bolt those up for the track. Also, like was mentioned previously, make sure your torsion bars are lowered as far as you can get them (as in take the bolts out). When you say you have full billet steering, does that include a straight center link? If not, you will probably want to get a Cognito pitman/idler arm brace kit.
 
rhilton109 said:
25 or 30 psi launch could be your problem also!!!!thats alot of boost your leaving at!!!
Click to expand...

I'm not totally sure why he has to leave that hard... I'll ask & find out. Charger is a S366/74, variable went out the window a while back.

JD4440 said:
Try 10 psi boosted and go up from there till it gets worse then back down till when it was good. Pull the overload and you can pull the 2nd spring from the top , then install the stock shackle to regain most of your ride height. Your trans limp is most likely to do with the high boost launch.
Not sure what full billet steering is but make sure it has either an SD centerlink or Cognito braces and some sort of HD tie rods in addition to dropping the front end completely.
What hole are the caltracs in on the front mount?
Click to expand...

Forgive my ignorance with the steering, I'm a cummins guy :poke:... I know he's replaced everything up front so it's not supposed to break. Sometime this week, we'll be doing as you suggest with the rear-end, front's already dropped as far as it can go. We think we have the trans issue pinpointed, if we can get it to hook, even at 30psi.. it doesnt limp.

JoshH said:
First of all, 25-30 psi is way more than you can effectively launch at unless you are running some big slicks that you can get lots of heat in before you launch. I've had the best results launching my truck at around 15 psi. Secondly, your wheels and tires are not helping you at all. If you have or can find a set of stock aluminum wheels, mount some 265s on them and bolt those up for the track. Also, like was mentioned previously, make sure your torsion bars are lowered as far as you can get them (as in take the bolts out). When you say you have full billet steering, does that include a straight center link? If not, you will probably want to get a Cognito pitman/idler arm brace kit.
Click to expand...

Big slicks arent that far out of the picture. The tires he's running are proxe's with a ton of rubber for the size wheel its on, they're about 33" tall. You think the 285/17 nitto highway terrains from my truck would hook better???
Again on the steering, I'm pretty sure he's got it taken care of there's no hint of bend/break after repeated 30psi launches on the strip

horrible video of the launch... but the near lane is the truck in question
http://www.youtube.com/watch?v=txUipChpN6Q
 
indeed, just didnt want to show my truck running a 10.x in the 1/8th... LOL
 
Lower the T-Bars all the way, clamp the rear spring pack even if you have cal-tracs. Make sure it has a straight center link or cognito braces like Josh H mentioned.

I cut 1.6 -60' times with stock suspension, cognito braces, and spring clamps using street tires, 1.70 60' with 265 mud tires on a 675hp street truck.
 
Thanks fellas, anything else you can think of let me know, will be doing this stuff this week, prior to the races. Thanks for the tips.
 
Mine done the same thing, lower the T-Bars, and leave with a little less Boost. Trust be you will bend Tie-Rods and all kinds of stuff if you don't drop the T-Bars.
 
by T-bars you mean the front "springs" or torsion bars correct? They're already slammed out... looking to do this stuff tomorrow evening. Won't be able to tell you if it worked or not, any time we try and launch on the street or the pavement around the shop it just blows all 4 off
 
I'd like to point something else out guys. I know a lot of you mention leaving with less boost. We've got big intentions with this truck and frankly don't really care what we break. As of now, we'll just replace what explodes and keep tweaking until we can get it to launch half decent and go real fast for 1/4 mile...
 
Yea but it's just not nearly as fun!!

We'll be doing the recommended changes to the rear tomorrow evening... any tips on pre-load for the cal-tracs?

If this no worky we'll be forced to order some slicks...
 
If you actually have the torsion bars lowered all the way, then you need better shocks because in the video I posted the front end looks like it is pretty high. I've had good luck out of the Rancho 9000s, but several other guys have had success using double adjustable QA1 shocks. Bilsteins don't seem to work very well for drag racing as they don't have enough rebound damping to keep the front end down.

The reason I said you should leave with less boost is because of traction, not because of parts breaking concerns. Street tires just can't hold that kind of power. Leaving at 15 psi allowed me to cut a 1.68 60' time on a set of cheap all terrain tires on an 11.13 second 1/4 mile run.
 
I stand corrected, we've only left at 30psi once and it did not turn out well, most launches and all of this is occurring at approx 20psi of boost.

Although I understand the traction concept, the proxe's seem to hook up very well and watching the truck from behind... if you could slow it down slow motion it looks like a toddler crawling the way it hops up front left/right....

I'm hoping the suspension stuff we're going to do helps it out
 
Hey guys, thanks for the input! I do have a billet centerlink and everything is tight and aligned up-front (just took it to alignment shop 2 weeks ago). I plan on taking the bolts out of the t-bars, removing 2 leafs and the overload spring, replacing front shocks, and running a little pre-load on my cal-tracs. Hopefully all this will allow for a better showing of the power my truck is making :)
 
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