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Author Topic:   Cross weight %
racer13
Dirt Full Roller

Total posts: 27
posted April 05, 2002 06:23 PM
I have heard both sides! So which is right?
If I rise cross weight above 50% does that tighten or loosen the car. So if I ws at 53% would that be looser or tighter than if I was at 48%. Thanks!

BK19
Dirt Freak

Total posts: 285
posted April 05, 2002 07:41 PM UIN: 40696038
i would think that you would be tighter at 53 %

sdhnc29
Dirt Freak

Total posts: 467
posted April 05, 2002 10:08 PM
The higher you run your cross weight , the looser the car will be on entry into the corner due to the LR still being loaded , and the tighter it will be coming off . The less cross weight you run , you will see the opposite reaction .

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BK19
Dirt Freak

Total posts: 285
posted April 06, 2002 03:55 AM UIN: 40696038
you are correct that is the exact thing that was going on with my car last year it was going in nice but about hitting the wall with the front of the car going out

SLEEPY GOMEZ
Dirt Maniac

Total posts: 199
posted April 06, 2002 11:46 PM
First the front end should be properly set up. I like 4-5 degrees caster stagger with 7-9 degrees in the right front. Maybe no more than 2 1/2 degrees camber in the right front.
Your springs must be right for your car and track and I'm not sure what you have.

Now set the cross weight to make the corner entry correct. Set whatever it takes, but use only as much cross weight as necessary.

Only when corner entry is correct can you begin to work on corner exit. Use rear tire stagger and wheel offset then to work on corner exit. More stagger will loosen the car up and keep it off the wall on exit while having little effect on entry. Extending the right rear out with wheel offset will have a similar effect but stagger is more effective. Yes, stagger wil have a small effect on cross weight. Remember, everything is a compromise on a race car. Hope this helps, SLEEPY.


racer13
Dirt Full Roller

Total posts: 27
posted April 07, 2002 05:46 PM
Thanks for the help you guys. It is nice to fine out the right answer. Thanks

c21
Dirt Freak

Total posts: 346
posted April 09, 2002 03:31 PM
quote:
Originally posted by sdhnc29:
The higher you run your cross weight , the looser the car will be on entry into the corner due to the LR still being loaded , and the tighter it will be coming off . The less cross weight you run , you will see the opposite reaction .


Steve, isn't that relationship more from roll couple distribution rather than straight crossweight changes (front vs. rear roll resistance)? If a guy is running a stiff spring in the RF (as many hobbystocks and streeters do)I would think more cross will tighen him in and out.

SLEEPY GOMEZ
Dirt Maniac

Total posts: 199
posted April 09, 2002 11:18 PM
Hi c21, cross weight is not affected by the strength of the springs until after the suspension begins to move. This means roll couple distribution is not affected by STATIC cross weight settings. The heaviest corner will have the most traction. Remember cross weight is a STATIC setting. As the suspension begins to move, a stiffer spring will not allow as much weight to transfer to the right front as a softer one. Thus,the cross weight could be correct allowing the car to begin its turn. But with too strong of a rt front spring, not allowing enough weight to continue transferring, there might be a push in the middle of the turn. The necessary amount of cross weight can be different for each car. Several factors of car design affect it. Left weight seems to have the biggest effect. Hope I've not confused you too much, SLEEPY

c21
Dirt Freak

Total posts: 346
posted April 10, 2002 10:33 AM
Hi Sleepy,
Nothing is affected by spring stiffness until the suspension begins to move. Don't know about your car, but mine is moving when I enter a turn. Cross weight is a static setting mainly because it is impractical to measure it dynamically.
The heaviest corner is the heaviest corner. Once you exceed the maximum traction ...
F = L x Cf (where F is the traction force, L is the load or weight on the tire and Cf is the coefficient of friction for the rubber)you will push (or be loose if we are talking about your rear tire).
Too stiff of a spring on the right front will make the front end's roll resistence too high (relative to that of the rear) placing TOO MUCH weight on the right front (transferring too little to the right rear when the car is in roll), causing a push. That is roll couple distribution,"Hope I've not confused you too much".
c21

sdhnc29
Dirt Freak

Total posts: 467
posted April 10, 2002 12:41 PM
I think you hit it c21 . The stiffer the spring , the faster it loads and unloads . If your RF is too stiff , regardless of cross weight % , the immediate weight transfer during the initial corner entry will be taken , and deflected . Also I think a problem with the "hobby stock push condition" ,that is often overlooked and difficult to deal with , is the below ground front roll center involved with having to use stock mount's and components .

SLEEPY GOMEZ
Dirt Maniac

Total posts: 199
posted April 10, 2002 10:56 PM
Hi again c21, The first paragraph you have right. Let's leave the second paragraph for a book sometime. However, in the last paragraph about the right front spring there is a problem. A heavier right front spring will NOT allow as much weight to transfer to that corner as a softer one, period. Please don't let me confuse you. SLEEPY

c21
Dirt Freak

Total posts: 346
posted April 11, 2002 10:13 AM
Sleepy,
while a stiffer RF spring will reduce the total amount of weight transfered right (or forward in pitch).... the portion of weight being transferred is borne by the RF purportionatly to the amount of roll resistance it's corner creates (thru spring rate,sway bar, shock valving, geometry, etc.) relative to the RR (or LF in pitch)..... Bottom line: Stiffer spring = increased percentage of weight being transferred goes to that corner. PERIOD "Please don't let ME confuse YOU".

Maybe your chassis guy can explain this to you in terms you are more familiar with.

SLEEPY GOMEZ
Dirt Maniac

Total posts: 199
posted April 11, 2002 10:35 AM
doesn't happen

jammin
Dirt Administrator

Total posts: 5556
posted April 11, 2002 11:10 AM UIN: 16262997
C21, the spring is going to compress so far to reach a certain rate that the chassis is looking for when in the turns. The centrifigal force on the car in the turns is going to require this spring rate dynamically. If your car is reaching a 900 lb spring roll resistance in the turns and your have a 700 lb spring, the car will compress that spring approx 2 or 3 inches to get to the 900 lbs. If you put a 900 lb spring on the rf, its gonna compress 1 inch to acheive that 900 lb resistance. Still getting the same weight on that spring, just different movement and roll in the car. The weight transfers yes, but your spring tension is only going to determine the amount that it rolls. The only way you can increase the weight to that specific corner is to put a taller spring in it or to decrease put a shorter spring it it. The spring rate has absolutely nothing to do with weight distribution providing they are the same height spring.

Hope this helps.

jammin

c21
Dirt Freak

Total posts: 346
posted April 11, 2002 11:38 AM
Jammin, thanks for jump'n in.
Not wanting to seem coy, but don't you suppose that in your example (not certian if I entirely agree with 1 vs. 2 or 3" is a reasonable comparison for a 900 vs 700 lb./inch, but that aside...)the dynamic crossweight would be dramatically different for the two scenarios? In your example (with a 700 lb.spring) where the car settles an inch or so more on the RF, hasn't it dynamically un-wedged the car as compared to the 900 lb. spring example?
Maybe I don't understand your post?

thanks again for jump'n in
c21
P.s. Those Digatron Guages helped me Iidentify a low fuel pressure problem at the end of the strait, gotta love the technology!

jammin
Dirt Administrator

Total posts: 5556
posted April 11, 2002 12:13 PM UIN: 16262997
Yes, it will loosen the car, but you have to remember, at the same time the front spring is compressing, the rear spring is also extending. It is more of a weight transition. The weight might transfer to the front, but the same crossweight applies. Doesn't actually change the wedge. It changes front to rear. That is unless you have something in the car that dynamically changes the crossweight, then you are in another ballgame. This situation I am speaking of is a generic setup with springs on top of the rearend. But, on entry, your still in static mode because your suspension returns to static state because your off throttle. On exit is another story.

Jammin

c21
Dirt Freak

Total posts: 346
posted April 11, 2002 01:00 PM
Jammin,
The fact that your car has different wheel rates (wheel spring rates) means that you will ALWAYS have dynamic wedge change any time that you have ANY suspension movement (any weight transfer) whether it be pitch, roll, squat, etc. ... or any combination. No magic external devices, just contantly changing wheel pair stiffnesses.

If you could calculate where your weight transfers on turn entry for example .... you could move this weight in your car while it is sitting on the scales to get an approximation of the effect on wedge and corner weights (for some real fun try it again with different springs!). If you were to then simulate the various weight transfer scenarios your car normally encounters (entry high/low, exit on throttle braking,etc.) and this doesn't change your crossweight, I'll eat my keyboard
c21

jammin
Dirt Administrator

Total posts: 5556
posted April 11, 2002 01:57 PM UIN: 16262997
The exact same scenario would be to change the spring rate on the right front....put your car on the scales, change the spring on the right front(same height spring) and tell me what you see.

jammin

jammin
Dirt Administrator

Total posts: 5556
posted April 11, 2002 02:11 PM UIN: 16262997
The reason that this doesnt happen is simple. The numbers do change, but the actual relationship of the weights stays the same in the car, mathematically, it will always work out the same because each of the wheels changes. Upon entry, both front wheels take on more load. This is true. But in a crossweight situation, upon entry, the car gains weight on the right front because of the heavier spring, true and the left front also takes on more weight, but usually people are running a little bit lighter spring on the left front. The reason for this is that they want to keep the left rear of the car down. When weight transfers to the right front, it lightens the left rear, but your wedge is the same(more weight on right front, less weight on left rear, they equalize), will not change. Same with the numbers of the left front to right rear. These settings are what makes these cars work. Thats why you add wedge to the car to make it tighter off of the turns generally, because your putting more of the total car weight on the left rear and the right front. Either or but they both make wedge, not just the left rear.

jammin

c21
Dirt Freak

Total posts: 346
posted April 11, 2002 04:08 PM

Jammin,
I will agree that in the example where the left is the stiffer rear and the right is the stiffer front (common set-up) that most of the weight transfers from the left rear to right front during straight line braking (I usually rotate a bit on entry. so it clouds the issue as to which direction the weight is actually transferring on entry in your example,so I substitute straight line braking for the term "entry" in this scenario), but since the wheels have different wheel spring rates I would call it a wild coincidence if the weight transferred proportionately rear to front.

If you complicate the example by rotating the car on entry (or even just adding cornering forces to the example)you also have to factor in roll,warp and heave (and then there is your anti-dive characteristics. Even if your anti-dive is the same left and right you are dealing with non-symetrical forces acting on your front suspension as you go into pitch and roll simultaneously)....
c21


[This message has been edited by c21 (edited April 11, 2002).]

jammin
Dirt Administrator

Total posts: 5556
posted April 12, 2002 01:10 AM UIN: 16262997
Play with the scales more and do things to your car while on the scales it will show you a few things. Such as, just jack the front weight jacks all the way up in the front, see if it makes a difference in your rear percentage. Or let them down, see if it changes, some of these are characteristics of exactly what you are talking about.


jammin

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