Ride height vs. cross weight

[John_Schwemler]

A text book answer from a couple different chassis books would have cross anywhere in the 50-54% area on a highbank track, depending on car etc.  On a highbank track you shouldn't need as much weight transfer from the RF to the LR as say a flat track. (a high cross percent will transfer more weight, adding "bite" into the LR, which can easily introduce a push to a push/loose symptom into a car depending on other variables)

We run on a flat track (4-5* banking) and are in the 56-60% range.

Back to the main topic,
If you have an ideal ride height number given to you from a setup, an ideal cross weight number, and access to a set of scales, I would recommend as another responder has said and take the time to set the car to both.  I would give the cross weight priority and your attention to detail will probably depend on time and patience.  After it has been done once, its generally easy enough to maintian throughout the year and other chassis changes.

[TomTom]

  THANKS GUYS for all the help. I was thinking of running about 52%, just wanted to confirm my rookie thoughts before I put 52% cross in & then it push like a dump truck, I should know I drive one for a living 

THANKS AGAIN

Tom #3

[John_Schwemler]

Just remember that cross weight is only part of the package to make a car handle well.  Proper weight placement, including control of weight transfer through springs, as well as alignment can help/hinder any race car.

After you have the ride heights and cross weight where you want it, and you feel the desire to make a change at the track, you can keep the ride height pretty close by putting in the same amount on one cross set of corners and the taking out on the opposite cross set.  (put 1 rd in the LR and RF, take 1 rd out of RR and LF)  As you might have found in the raceshop setting the car up, 1 rd in all 4 corners can be a large jump in numbers on the scale (% wise).

Good luck, if you have decent notes and don't like it, you can always come back.

[gramps05]

John, what is your opinion on a car with incorrect stagger ( to much or to little) causing either a push or loose condition, can it be corrected with more or less cross?

[John_Schwemler]

Yes.....     but.... 

I truly believe that the shaved tires we run on our car are just slightly under staggered taken in consideration the narrowed track width compared to say a late model.  I'm not sure about your track, or if more stagger is needed on dirt... 

Before I would fully commit to the use of crossweight to overcome the numbers I would like to see for stagger (and a stagger change at the track will affect your cross %'s) I'd like to consider that a loose or tight condition is not being made by a misalignment, tire pressure issue, failed spring or another mechanical change that has happened.  If the car has always had this particular handling problem w/ that particular cross and stagger, your rearend is straight and your right side tires are inline, tire temps are fairly even across each tire (7* split is pretty good) then I am all for that chassis adjustment.  Take notes, if the driver isn't comfortable w/ the change, undo it.  If your still looking to make a change afterwards try something else like changing a rear spring (and resetting that corner's ride height)

Using the crossweight to overcome a slight handling problem due to insufficient stagger would be one of the first changes I would make, within the "rules" that I have read about in various chassis set up books.  I would shy against cross under 50% and over 60%, and for some reason people w/ more experience than me say that cross % doesn't work at the 56% and 57% range.

Remember to take notes and not to try to change too much at one time.  If you don't get the expected result, you can always come back to your baseline and try something else instead. 

[TomTom]

Say if you want to increase your cross % but dont want to screw up your ride heights, you would stiffen 1/4 turn RF & LR, and soften 1/4 turn LF & RR. Am I correct ?

THANKS John for all the help

Tom #3

[John_Schwemler]

Thats exactly the idea Tom.  It should keep the ride heights either the same or very close to what they were before adding the cross. 

[SMS]

John,Scott, Dan,JJ,
I have to disagree about using cross as a primary adjusting tool. All cross is, is a number, it is used for fine adjusting. Cross is used to distribute  the weight tranfer across the RF and RR. Cross is simply a balancing act, tuned to the needs of the situation. As far as using cross to fix a stagger problem, that's not going to happen. No matter how much cross you put in or take out of a car is not going to help if you don't have the right stagger. Higher  cross does not increase transfer, it actually preloads the RF. We have won with the cross at 47%, and we have won with cross at 52%. I think that you should set your ride heights in the front and leave it, because adjusting the height in the front changes so many other things. If you spend the time in the garage during the week to find out what moves when you make the changes, the further ahead you will be when you are at the track.

[John_Schwemler]

I'm going to stick to my guns on using cross as an adjustment tool in this case for stagger.  Unfortunately w/ the spec tire, I know that we cannot get the stagger numbers (that the Legends track width combined w/ the sharp radius corners and nearly nonexistant banking that our local track has) to make the car "stagger neutral".  When I look up the effects of cross and the effects of stagger the two have a very linear relationship w/ handling symptoms causing the car to be imbalanced. 

Corner entry oversteer - excessive rear stagger/insufficient cross (plus several others, some relating to mechanical problems, others alignment or improper springs/tire pressures)
Mid-Corner oversteer - excessive rear stagger/insufficient cross (plus geometry problems, spring rate, and mechanical problems as well as alignment and driver faults)
Corner exit oversteer - excessiver rear stagger/insufficient cross (plus pressures, spring rate, geometry and alignment problems)

Understeer is generally the same, with main causes of entry understeer being more mechanical and geometry related, but mid-corner and exit directed more towards cross/stagger issues.

(information paraphrased from Steve Smith's Trackside Tuning Guide)

If your taking care to keep the ride heights the same while making cross/spring changes, your alignment should not be affected (within reason) because the frame rails are still at the same height relative to the rest of the suspension and the contact patches.  Change of alignment at the track could happen just as easily while changing a spring as while changing cross.  Measure ride heights before making a change, adjust, and recheck. 

"The Crossweight Percent"
Having an excessive amount of crossweight percent, or bite as it is referred to in dirt racing, causes too much weight to be supported be the left-rear and right-front tires and can cause a car to be tight in, through the middle, and off the turns.
Running a crossweight number that is too low is a distinct indication of a tight car,  If a car needs 51.2% cross to have a proper weight transfer, and is only running 48.8% the team has needed to take cross out of the car because the setup was tight.
There is an optimum percentage of weight supported by the cross corners (RF and LR) that will make the car neutral.  Remember, a neutral car is not necessarily a winning car.  It must remain neutral throughout the entire race.

"Rear Stagger"
Insufficient rear stagger will cause the car to "point" towards the outside wall on exit.  There is a correct amount of rear stagger for each track based on the overall tire diameters, track width of the rear tires, the radius of the race track, and the track banking angle.
Too little stagger will cause the car to drive to the right as get back to the throttle and the rear end moves in the arc that has a greater radius than the track at that point.  The larger the radius, if drawn onto the racetrack, would probably lead into the grandstands and that is definately not where we want to go.

("The Crossweight Percent" and "Rear Stagger" borrowed from Circle Track & Racing Technology - Chassis & Suspension Handbook (from the editors of Circle Track magazine))

I'll finish with mentioning that following the INEX rules, one cannot overly change the Moment Center on the front of a Legends car like can be done on a late model, nor is there an adjustable (vertically)  panhard bar to raise or lower the rear Moment Center, and springs have a minimum 15lbs difference in spring ratings available for use.

If there was something I could make small adjustments at the track to help balance a car, other than tire pressures, cross and springs, I would be very grateful.  Until then I'm only smart enough to use the resources set out infront of me.

(nothing personal SMS just defending the reasoning behind my thoughts with literature)

SMS does have a good point on keeping the same ride height (really try to keep to your notes on ride heights while making changes) and that time in the garage at home generally equals positions on the track come race night!    On avg our Legends car is receiving 8 hrs (+) of maintenance per race session and we have a 40% feature win ratio thus far in '09.

[qweedqwag]

try this
LF 3 3/4 
RF 3 3/4
LR 4
RR 4

I would tell you who ueses this but it does work.