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Author Topic:   How much is to much?
Hollis Ray
Dirt Roller

Total posts: 6
posted June 09, 2004 07:00 PM  
Rear percentage that is?

Dirt Forum Racer

Total posts: 97
posted June 09, 2004 08:26 PM  
On some of your smaller tracks running up to 61% isnt abnormal. I am starting to think that more rear % isnt always the answere.


Dirt Freak

Total posts: 370
posted June 09, 2004 08:54 PM  
It isn't. I'd rather have the hammer swung to drive the nail rather than just lay the hammer head on it!!!!!

unregistered Total posts: 370
posted June 09, 2004 11:50 PM           send a private message to juliaferrell   Edit/Delete Message   Reply w/QuoteBBBBBB
Generaly speaking, if you go over 62% to try to get more bite, there is probably something else wrong with the setup. The more rear % the more cross you should have. (to a point, over 54% cross would be too much also)

Dirt Freak

Total posts: 334
posted June 10, 2004 07:50 AM  
Can you explain why you want more cross if you have high rear percent? Do you do it to help tighten up the car?


Dirt Maniac

Total posts: 115
posted June 10, 2004 09:08 AM  
This is probably a better question than you realize. I bet you get a lot of different opinions. There is nothing better than rear % for getting forward bite on a dry slick track especially on IMCA hockey pucks. At what point is the rear % so high that you a hurting your corner speed. Sprint cars don't seem to have a probem with really high rear % so why should a mod. I have a D&M (for sale in the classifieds by the way) that can be 63% full of fuel at 2500 lbs. I don't know if it might be better with less or not, but it's **** fast like it is.

Dirt Forum Champ
Total posts: 796
posted June 10, 2004 09:12 AM  
Does more rear percenent produce push as well, the higher you go?

Dirt Forum Champ
Total posts: 1875
posted June 10, 2004 10:07 AM  
here is my advice on rear percent.
no lower than 55 but i only go to like 57.5 on my car, but i would just keep adding to it till the car got tight.
if the car is tight all over then take some rear out of it, and if loose all over add some rear.
as far as cross. i dont ever figure cross anymore. i go off lrbite only, the driver feels lrbite.
also cross or lrbite will loosen entry and tighten exit, not tighten overall.
i would say just ruff guessing that 60-61 would be the max i would go, but if 63 works good for you then by all means do it.
i would try to get more bite from other means, spring changes(to change which tire gets what), thrust angles, throttle control, tire sanding.
but like i said if 63 works then do it.
my overall advice add some rear if loose all over take it away if tight all over.
good luck

unregistered Total posts: 1875
posted June 10, 2004 10:21 AM           send a private message to zeroracing   Edit/Delete Message   Reply w/QuoteBBBBBB
Zero's got it. Reducing rear % will lossen up the car, as well as make it eaiser to wheel spin. When you add rear % you are able to use more throtle because the rear tires are planted better. However this also adds to weight transfer in the rear, which will load the RR more than the left. That is why you add LR split (or bite) to the car, to compensate for and even out the weight distrubition across the rear tires.

Dirt Maniac

Total posts: 115
posted June 10, 2004 03:56 PM  
You guys are correct that all that rear weight makes a car tight, and it rolls over hard on the RR. You definitely have to run a heavy enough RR spring and I always run about 2" stagger in the rear. The rest is all done with the bars and I never have
trouble turning the car into the corner unless I miss the track really badly. However, I have never felt that I was getting beat by someone having better forward bite off the corner. If I'm off the pace it's because I missed the setup and my corner speeds are down.

Dirt Freak

Total posts: 370
posted June 10, 2004 06:14 PM  
Yes sprints run alot of rear %. But they only weight 1400 pounds. That computes out to 1/8 the pounds of inertia at 1.2 lateral g's. Yes, a sprint car corners 8 times easier than your IMCA and hooks at least that much better. Yes weight matters. That's why the ultimate is to have all the weight directly above the rear wheels but this is not possible on 4 wheels and wheelbases in the 108+. Remember optimum cornering capability comes from a neutral 50/50 car. And this compromise is what drives us racers nuts.

Hollis Ray
Dirt Roller

Total posts: 6
posted June 10, 2004 08:27 PM  
Thank's for all the information.But i forgot to add that we can run a 8" spoiler that's why i was thinking that we could cut back on the rear percentage.Thinking that the spoiler would have enough downforce.

Dirt Freak

Total posts: 376
posted June 10, 2004 08:28 PM  
Ok, if you were to add a chunck of lead,(say 25-30 lbs.) to the rear at the track, would you need to bring your ride heights back to where they were prior to adding the weight?

Dirt Forum Champ
Total posts: 755
posted June 11, 2004 06:05 AM  
Xhub- if you just add the weight at the track, I probably wouldn't worry about the ride heights. 25-30lbs isn't going to change things much as far as ride height is concerned. Worry about correcting the ride height when you get back to the shop and have a level floor to make accurate measurements.

[This message has been edited by NJantz (edited June 11, 2004).]

unregistered Total posts: 755
posted June 11, 2004 09:19 AM           send a private message to NJantz   Edit/Delete Message   Reply w/QuoteBBBBBB
Look at is this way, 3 gallons of fuel weighs just over 27 lb and adding that much fuel hardly moves the chassis. You should be fine.

Dirt Forum Champ
Total posts: 2007
posted June 11, 2004 09:31 AM  
If I add around 25# i dont usually worry about ride heigths if i add 50+ i might put an extra turn in the rear weight jacks or check the height before adding the weight to see how much it settles the car compared to how it was in the shop then adjust accordingly if needed.

Dirt Maniac

Total posts: 149
posted June 11, 2004 01:04 PM  
do you guys scale your cars full of fuel, or scale it like it would come off the track, ive heard both ways.

unregistered Total posts: 149
posted June 11, 2004 01:52 PM           send a private message to thebomb   Edit/Delete Message   Reply w/QuoteBBBBBB
I get my base numbers with the car setup exactly as I would have it rolling out on to the track. Fuel, air press, lead, all set ready to race. I also do a few scales with light fuel, different air pressures, different lead weights and placements, etc so I know just what I'm doing to the car when I change these things at the track.

Dirt Freak

Total posts: 324
posted June 11, 2004 07:26 PM  

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