Chassis guys please help with my mopar race car (scale ?)

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Tom Cannon
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Chassis guys please help with my mopar race car (scale ?)

Post by Tom Cannon » Sat Nov 01, 2008 9:58 pm

I race in a class called F.A.S.T(factory appearing stock tire) I have a T/A Challenger with a 340 stroker and a 4 speed.The car runs around 119-120 mph but only ets in the high 11s.I feel like I need to get the et to reflect the speed.When the class was young it was about making power with stock intake and exhaust manifolds.Now it seems like we have the power thing figured out and a few guys have the car really working.

So my car has a slippery clutch and I think it is geared pretty good, it weighs 3250 with out any weight in it.I recently bought a set of scales and I'm trying to figure out what to do with the info.

The first thing I did was to weigh it empty
LF 903 RF 920
LR 739 RR693

Then I put 100 lb behind the RR wheel and sat in it
LF934 RF951
LR851 RR800
this brought the cars weight to 3350 without me and 3536 with me.
53% on the nose 47%on the tail

So,What do I do now? We race on reproduction bias plys.The fastest 60 ft in the class is 1.56 out of a Vette that went 134 mph.That makes me think that the rubber or the tires isn't my problem. I would say my average 60 ft would be 1.85 1.95 with my best 60 of 1.73.

Any help would be great

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Post by Tom Cannon » Sat Nov 01, 2008 10:03 pm

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Post by RCJ » Mon Nov 03, 2008 10:11 pm

If your car has torsion bars you can raise the lf and lower the rf.That will put equal wieght on the rear tires or preload if you want.Have you tried any tire softners?I always try to remove every ounce. of wieght from the front tires forward

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Post by Tom Cannon » Mon Nov 03, 2008 11:04 pm

Thanks for the reply.

Is that the way to adjust that? I've heard the RR should show 30lbs more than the left.Any thought on that?

I have never used any tire softeners.And I try to keep the nose as light as possible

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Re: Chassis guys please help with my mopar race car (scale ?

Post by David Redszus » Mon Nov 03, 2008 11:18 pm

Then I put 100 lb behind the RR wheel and sat in it
LF934 RF951
LR851 RR800
this brought the cars weight to 3350 without me and 3536 with me.
53% on the nose 47%on the tail

So,What do I do now?
The next step is to determine the center of gravity height above the ground. This can be done by raising the car on an incline and weighing the rear, then plugging the weights and height into an equation.

Or you can simply sit in the car, stick your thumb into your belly button and measure the height of your belly button from the ground. The rule of thumb and careful measurement come out to be about the same.

Now we can calculate the rearward weight transfer during acceleration by using the following formula:
WT = weight * CGH (ft) * Gforce / wheelbase (ft)

assuming:
WT = 3536 * 1.5 (ft) * 1.2 g / 9 (ft) = 707 lbs. Which means that 707 lbs has shifted to the rear during hard acceleration. Your new axle weights become 1178 front (33%), 2358 rear (67%). You have 2358 lbs of downforce at the rear tires to produce tire grip. If you send more than 2358 lbs of torque to the wheels, you will experience wheel slip. If you obtain more than about 15-20% wheel slip, you will lose acceleration force.

How much traction force can your tires handle?

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Post by Tom Cannon » Mon Nov 03, 2008 11:40 pm

I'm sorry David,I'm not sure what traction force is or how to calculate it.

I'm really lost here :oops:

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Post by RCJ » Tue Nov 04, 2008 12:47 pm

Most torrsion bar have a bolt at the back of them you can jack wieght with.Is the car hooking good or are you slipping the clutch, killing power etc..to get it to leave.What are the rules on traction bars?

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Post by Tom Cannon » Tue Nov 04, 2008 1:49 pm

I'm kinda all ready nose up.The clutch does slip with about 650lbs of base pressure.The car doesn't hook, which is why I am after it :lol:

No traction bars allowed and no cal tracs either :cry: No rule against ballast though

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Post by Tuner » Tue Nov 04, 2008 3:37 pm

Remove the slip pads from the leaf tips and solidly clamp the leaves together at each tip from the axle forward. Use at least 2 or 3 clamps or what clearance allows on each side. Perhaps one more on the right than the left will help bias the torque reaction.

viewtopic.php?p=108165#108165

Can you use a pinion snubber? You would benefit from a copy of the Mopar Chassis Book. I got one at the dealer about 10 years ago under part # P4452791. Mopar’s leaf spring program works so well most people I know are in denial over it (except happy Mopar users). You might benefit from slightly relocating the vertical positions of the front spring eyes. The Mopar front spring mounting bracket is easily fabricated or modified and makes fine tuning the anti-squat sweet spot easier than brands C and F. The chassis book gets into some detail on the matter.

Billy Shope is the guy to answer your questions. Check his website for information. There’s a link at the bottom of this post of his. viewtopic.php?p=107122#107122
Last edited by Tuner on Wed Nov 12, 2008 6:03 am, edited 1 time in total.

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Post by Tom Cannon » Tue Nov 04, 2008 5:37 pm

Wow Tuner, there is some juicy stuff in there

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Post by af2 » Wed Nov 05, 2008 12:05 am

Thanks Tuner, that is some good reading!
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Post by jeff swisher » Wed Nov 05, 2008 8:40 am

I was waiting for someone to meantion Pinion snubber. Or MOPAR super stock springs....
I cant say enough about the springs set up that way and clamped in the front :D

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Post by David Redszus » Fri Nov 07, 2008 9:12 pm

I'm sorry David,I'm not sure what traction force is or how to calculate it.
Not too hard.
Take the:
engine torque (ftlbs) x trans ratio x diff ratio / tire radius (ft)= traction force in lbs.

If the tration force exceeds the vertical load on the tire, the tire will slip. More vertical load is needed or a tire with more grip is needed.

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Post by jim_ss409 » Tue Nov 11, 2008 11:12 pm

I wonder if softer front shocks and torsion bars would help.
I once replaced the oil in a set of shocks with ordinary diesel fuel. The shocks still worked but they were quite a bit softer. You could probably think of better fluids to use than diesel fuel but that worked for me.
Removing or modifying the bump stops to increase the amount of front end travel might also help.

street monster

mopar traction

Post by street monster » Sat Nov 29, 2008 4:27 pm

glad somebody mentioned ss springs rs 1 leaf thicker and different rate also six cyl torsion bars are good bet if you read that book you got it. also mopwer moved bat to ps trunk with factory kit if thats allowed

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