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Author Topic:   Cross % -vs- Fuel Cell Placement (Long ?)
STOCK64
Dirt Roller

Total posts: 23
posted April 21, 2004 08:12 PM  
Ok, here's the bases of the question. I run an IMCA Stockcar. I bought this chassis new and let them set everything up last year. It was ok, but was horrible coming off on a dryslick race track.

On this chassis they place the 16 gal. fuel cell on the rr. They said that fuel will burn off and "TIGHTEN" the car as the race is run. So over a A-feature I will lose aprx. 40# on the rr. The problem I have with this is when weight is physically move from rr to lr, this takes away cross (on the scales). Which i agree with b/c the more weight on the lr, pulls down on the lf/rr taking cross away.. Thus I moved the cell to the lr. Now when that same 40# is lost during the race, it adds cross. This is b/c weight is coming off of lr and rr gains, thus pulling down on the rf/lr, adding cross.
This was all proven on the scales.

So my question is.... Doesn't cross tighten a car???? If so, why was this chassis builder saying that it tightens the car when fuel is burnt off of the rr, when the scales show the opposite??? The only thing i could possible see is with the fuel cell on the rr, left rear bite (split) will increase in the rear. What the **** am I missing here, or am I seeing this correct??? Ive racked my brain pretty hard on this and possible confused myself with it also..lol
Anyway thanks for the help...

c21
Dirt Forum Champ
Total posts: 521
posted April 22, 2004 08:55 AM  
it's a common misconception that weight placed on a given corner of a chassis will be directly reflected as an increase in corner weight (as seen on the scales) on that corner.

therefore, if one assumes that taking weight off the rr removes "counter-cross" (or more to the point .... adds cross) then it stands to reason that it will tighten the chassis.

I think some of the confusion may stem from the fact that in general, removing weight from the rear of the car will tighten it (during pure cornering).

it's been my experience that people who state that removing RR weight from the chassis increases cross ..... have NEVER done this on there scales or .... are racing a 3-wheeler

however, I won't rule out the posibility that this might be the actual case in some unusual circumstance ..... I've just never seen it, and I might add that I have witnessed the phenomenon (cross increasing as LEFT rear chassis weight is removed) on set-ups that had spring combos/roll-couple at both ends of the reasonable spectrum.

to answer your question, yes cross does tighten the car ..... generally ..... however,if you run a high rear brake bias and high cross, more cross can loosen your entrance (car will tend to turn-in on LR)

c21

stockcar5
Dirt Forum Champ
Total posts: 578
posted April 22, 2004 11:14 AM  
on a normal imca stockcar you want more lr or wedge as the track drys out. by placing the cell on the lr you will lose weight as you burn fuel...by adding to the rr you will not have this problem. i think he went to far in saying the car will tighten...i just think it will be more consistent and not fall off at the end of the race.

luke

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juliaferrell
Dirt Freak

Total posts: 370
posted April 22, 2004 06:30 PM  
stocker 5 is (I believe) right on. He wasn't refering directly to the cross weight percentage concept. But remember as weight disappears (cause that's what it does) the ride height changes and with the increase in cross % *not weight change* is the meat behind the right side cell philosophy. Very simply spoken

racerguy500
Dirt Maniac

Total posts: 168
posted April 22, 2004 07:32 PM  
As a chassis builder I deal with this everyday, the problem you have is what you are led to believe in %'s and how it all works doesn't apply to every track condition, every driver, every night and every car. By the cell being on the right it adds left side % as you burn fuel. Now if you move the cell forward, backwards, left or right or even raise it or lower it, you change it all. My point being, if your cell is to the right, to the left or wherever it is, you are not gonna be hooking up and fast on every track condition there is. Talk to who built your chassis and they should be able to point you in the right direction.

c21
Dirt Forum Champ
Total posts: 521
posted April 23, 2004 08:59 AM  
.....since track moisture content, lateral acceleration and fuel load normally drop simultaneously as the mains progress ..... I'm not sure I understand why (anyone would think) increasing left percentage as the race progresses would be considered desirable. More cross as the race progresses makes sense, but mounting the cell on the rt will give you the opposite effect.

My normal reaction to a drying out track is to move the load to the right relative to the drive force (by way of sticking out the LR or sucking in the RR).

If the argument is made that a dry slick track is more like pavement and calls for a pavement type set-up (more left) I would counter by saying that higher lateral acceleration calls for a higher static left and ..... you be the judge as to when your lateral acceleration is highest.

but in the end, I defer to racerguy500's statement about solutions being car/driver/track/condition specific.

.......and will close by saying that if your track is still coming in during the main .... then Rt side would probably be the place to mount the cell (IMHO).

c21

    

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