8 replies to this topic

[ChrisA]

Why is the corner balancing ratio alway between LR and RF? I can see RF being good for Ovals (counter-clockwise) to help focus on front end bite, and I can see LR being focused on for planting the LR out of corners on Road Courses (clockwise w/primarily right turns), However, on low torque & Hp cars like ours, wouldn't switching to RR and LF be advantageous? Since the car lacks much torque for planting out of corners the focus for setup balance would go toward having the car bite into and around righthand corners. I'm in the process of installing the Penskes and doing my car and started pondering "Why?"

[Bench Racer]

For my you get:

 

IMHJ, common/prevalent term, could be either as long as term is common/prevalent. Cross weight/wedge may be adjusted via spring or tire pressure at any corner of car. My guess, the term started within oval racing tech speak. Asphalt racers concern themselves with cross weight (diagonal weight) and dirt racers concern themselves with bite weight (right rear versus left rear) specified as left rear weight in pounds.  

[ChrisA]

I kind-of figured it came out a specific racing genre and/or was it just to avoid confusion in the shop, amongst mechanics, teams, etc? Since we don't have purpose built race cars and rules only allow ballast to be placed on the the passenger floorboard, we will never have cross-weight that is equal LR/RF and LF/RR. If the car were a table it will alway teeter one direction or the other. This got me wondering if the "defacto" LR/RF target is the best for our circumstances or would we be better served having the car stable in the LF/RR direction?

[Ron Alan]

Corner balance/wedge...just terms to describe/compare the same ratio car to car. The first person who put a car on 4 independent scales had to come up with a way of differentiating...coin toss...LR/RF was heads...ok as the percentage goes up it is more weight on these 2 wheels! Every track in theory will have a percentage above, at, or below 50% that drivers/data seem to indicate is fastest. And this of course is after you have considered all the other factors that go into developing a fast car on any given day! Why do we drive on the right side of the road?

[Dave D.]

Best Answer

 "Cross Weight" is referred to the amount of weight PERCENTAGE the LR and RF have to the TOTAL weight of the car. The LR/RF direction of diagonal is used as an industry standard reference point. The percentage can either be "wedged" or "de-wedged" depending on which diagonal(LR/RF or RR/LF) is carrying a higher percentage of the car's total weight. The car is "wedged" if the number is greater than 50%, and de-wedged if the number is less than 50% cross. The number to use is the number that serves you and your car the best at the tracks you race at. There are suggested numbers to start at, but the rest depends upon the stopwatch and your driving style.

[ChrisA]

Dave, that makes sense and clarifies my thinking. I checked my past numbers I had stored and it's essentially 50/50 in both crosses, +/- the small wedge or de-wedge of the LR/RF cross. Just finish balancing and everything is fine. My wacky logic started when I was getting frustrated around 10pm last night chasing numbers on the scale. Plus, prior to beginning the process found I had a scale pad that was about 1/4" too low; unless that was garage floor setting, then all my past scalings were wrong    Now onto alignment.

[Bench Racer]

Plus, prior to beginning the process found I had a scale pad that was about 1/4" too low; unless that was garage floor setting, then all my past scalings were wrong    Now onto alignment.

Ouch!!!

 

Again for my and Drago (water level, it's been way to quite Jim) likes to disagree. It's all about consistency in setup. All four scale top faces in a single plane. Drum roll, a water level which is consistent.

[Jim Drago]

 "Cross Weight" is referred to the amount of weight PERCENTAGE the LR and RF have to the TOTAL weight of the car. The LR/RF direction of diagonal is used as an industry standard reference point. The percentage can either be "wedged" or "de-wedged" depending on which diagonal(LR/RF or RR/LF) is carrying a higher percentage of the car's total weight. The car is "wedged" if the number is greater than 50%, and de-wedged if the number is less than 50% cross. The number to use is the number that serves you and your car the best at the tracks you race at. There are suggested numbers to start at, but the rest depends upon the stopwatch and your driving style.

+1

[SaulSpeedwell]

Its been said on here before, but I think what *almost* every person misses is that *almost* every 4 wheeled ground vehicle operates like a diagonal teeter-totter - or - like a wobbly barstool or kitchen table.  Put 4 quarters under both rear feet of your barstool - what happens?  Almost nothing.  A teeny tiny amount of forward weight transfer, but no "wobble".  Put 4 quarters under both left feet of your barstool?  Same thing - teeny tiny amount of rightward weight transfer, but no "wobble".  But 4 quarters on the diagonal?  4 under the RF and 4 under the LR?  Now you have well over 50% cross weight.  If you lean a little left or right, the barstool "slams" down onto ONE of the front corners (the LF), or ONE of the rear corners (the RR).  That's how % cross works, at least in *almost* every car that doesn't have some odd extenuating circumstance of mass distribution and roll centers.  An SM or *almost* any other race car is little more than a barstool sitting on adjustable coilovers.  

So why does the car hook right and push left if we made the RF heavy with >50% wedge?  Shouldn't a heavy RF make the car hook left?    Shouldn't a heavy LR make a car push right?

No.  Because the car transfers weight across it's diagonals when cornering.  When the car is pushing down into the ground on its RF-LR diagonal, the weight will transfer about the roll centers, just like the annoying wobble barstool ….  with the static "wedge" being in control at the moment you turn the wheel.  As a result, the % cross while cornering *tends to* have an inverse relationship to the static % cross.  Carroll Smith doesn't cover this, as I recall.

 

Understanding the details doesn't really matter for the mechanic or the driver, because *almost* all of us learn that more % cross equates to more rightward oversteer and more leftward understeer.  Knowing the rule of thumb is good enough. 

Once at the track, ignoring the scale numbers and being directionally correct on your wedge adjustment based on driver feedback will put you in the upper 90%-ile (of understanding, if not laptime).  I watched Flying Lizard scaling a car in the grass on wobbly plywood at Mid Ohio in 2004 - it seems they've gotten closer to SM frontrunner prep levels since then.