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New Auto-xer

JT_TT

By JT_TT
in General Motorsports Talk

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90RioGS Newbie

90RioGS

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Re: New Auto-xer

and they stated that the wider rear track took caused the rear tires to have more loading due to fact that they are takeing the largest moment force (furthest away from the center of the car)

Wider rear track should cause less loading on the rear tires, since the moment will remain the same while the moment arm is larger.
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Corey Newbie

Corey

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Re: New Auto-xerKarts are different beasts though: they have solid rear axles, gobs of scrub radius in the front geometry, and are designed to unweight or even lift the inside rear tire. None of those three factors exist in cars. (aside from some FWD cars picking up an inside wheel)No matter what the answer is, I doubt a 1/4" spacer is going to make any effect you can feel. Please get 1/4" longer lug bolts if you do run spacers! ;)

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CDN-SiR-02 Newbie

CDN-SiR-02

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Re: New Auto-xerHere's what I got from a few articles.1. A wider rear track width will make lifting the inside rear wheel off the track more difficult. So, the grip of two tires is better than one, which should decrease oversteer.and...2. Increasing rear track width changes the motion ratio of the suspension, which effectively reduces spring and anti-roll bar rates, which decreases oversteer.

A good guideline is to increase the track width and lower the car more on the end that slides first in a corner. An understeering, nose-heavy, front-wheel-drive car can use more track width and a lower ride height in the front. A powerful rear-engine car can be lower and have more track width in the rear. This play on physics will help reduce weight transfer in both cases.

So, it is dependant on which end your icreasing/decreasing the track width but ultimately if you increase the track width you get more grip on that end.http://www.sportcompactcarweb.com/tech/0506scc_suspension/index4.htmlhttp://racingarticles.com/article_racing-24.html
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conebasher Apprentice

conebasher

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Re: New Auto-xer

Karts are different beasts though: they have solid rear axles' date=' gobs of scrub radius in the front geometry, and are designed to unweight or even lift the inside rear tire.  None of those three factors exist in cars. (aside from some FWD cars picking up an inside wheel)No matter what the answer is, I doubt a 1/4" spacer is going to make any effect you can feel.  Please get 1/4" longer lug bolts if you do run spacers!  ;)[/quote']Corey is right, karts corner better when the inside rear tire lifts off the ground, I think it's because the axle is solid and the rear tires fight with each other when both are on the ground.
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mzf Newbie

mzf

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Re: New Auto-xer

No matter what the answer is' date=' I doubt a 1/4" spacer is going to make any effect you can feel.  Please get 1/4" longer lug bolts if you do run spacers!  ;)[/quote']wouldn't want a wheel to fall off......... :osorry JT, i had to.
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ForesterCarlin Newbie

ForesterCarlin

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Re: New Auto-xerKevin and Slip Angle I agree in principle with what you are saying about decreasing of load because of increasing the moment arm and effectively reducing your center of gravity but I also agree that widening the rear track steals load from the front of the car increasing it again. We all agree the widest point/or stiffest point of the car is what is going to take the largest cornering load. If it is the rear tires that have to do that and accelerate you will be using up all the grip of your tires and they will start to slide. Although we are dealing with cars from different hemispheres so the could react completely different.....like toilets flushing opposite in Austrailia, who knows?JT I actually compared the tracks of the 180hp Quattro to the 225hp Quattro and the tracks were different. I find it strange that Audi wouldn't use the same parts on both Quattros unless they meant to do that.

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yofa Newbie

yofa

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Re: New Auto-xernarrower rear tracks adds understeer, since the front tires have to take up more of the roll distribution. narrowing out the rear of the car decreases its roll resistance, but that need for roll resistance doesn't disappear. it's transfered to the front, just like the swaybar theory.fwd's usually have wider rear tracks than rear or all-wheel driven cars, because the fwd's have all the understeer they can manage already.race cars typically have narrower rear tracks than front, but the illusion of a wider rear end is created by the wider tires in the back.

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frs_jeremy Newbie

frs_jeremy

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Re: New Auto-xerJT if I were you I would ignore everything written in this entire thread and just try it and see.I made a change in spring rates to my car late last season which I was certain would cause understeer, but would prevent my front suspension from bottoming.You could ask on ANY automotive forum on the internet: "What will happen if I increase my front spring rates?" and the consensus would be "Understeer."I dont know if you noticed or not but in the last few events of the season I typically had 2 or 3 spins per event. Thats not what I would call understeer. So if I had to make a prediction on what increasing your rear track width would do, my prediction would be:"It depends."

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yofa Newbie

yofa

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Re: New Auto-xerjer brings up a good point. we can make all the generalizations we want about steady-state, but autocross is mostly transients. imbalanced cars, with a front-biased roll distribution, can have better turn-in and ultimately quicker lap times. you're better off tuning dampers to get time out of your car than modifying springs, sway bars and track widths.but... if we can learn anything from nascar, since the drivers spend so much time through "corners," i think they know it best. they often complain that "the car's so tight that it's loose." this is a condition when there is so much understeer that the front loses grip, scrubs away the speed, and when the front tires recover, catch and abruptly turn the car, the driver goes for a spin. did that make sense?that could be the case with jer's car. the other theory is that the car may have such wicked turn-in that the rear can't keep up. either way, adding rear roll stiffness may help in this extreme case.

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KevinT Newbie

KevinT

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Re: New Auto-xerI think we all know that there is nothing to be learned from nascar in an Auto-x forum. A giant, super fast, steady, left corner is in no way applicable to our super fast transitions. They also race on suspension designs from the 1950's which are no longer used on street cars.(Am I showing too much bias?)

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KevinT Newbie

KevinT

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Re: New Auto-xerAccording to the Good old Carroll Smith, his knowledge says: "A relatively narrow front track width will cause understeer", although he doesn't go on to say the opposite, but I would assume that it is also true.Here is a rudimentary formula I found on a technical site regarding weight transfer.Weight transfer = ( Lateral acceleration x Weight x Height of CG ) / Track widthMore weight transfer at the rear, would generally yield oversteer, which is the same thing a bigger rear sway bar would accomplish.

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Corey Newbie

Corey

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Re: New Auto-xer

that could be the case with jer's car

Another option is that the stiffer front springs allowed the car to corner without engaging the massively stiff bumpstops/supplemental springs. I know the Focus 'bumpstops' were measured at around 1200 lbs/in, I'd guess most modern cars are similar.
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frs_jeremy Newbie

frs_jeremy

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Re: New Auto-xer

Another option is that the stiffer front springs allowed the car to corner without engaging the massively stiff bumpstops/supplemental springs. I know the Focus 'bumpstops' were measured at around 1200 lbs/in' date=' I'd guess most modern cars are similar.[/quote']I cant see this being the case. When I calculated the wheel rates, I found that the front was in the neighborhood of double the rear. That should be an understeering car. I think its more like what yofa, suggested, that the front end of the car turned in so violently that there was no time for the outside rear tire to have load tranfered to it and generate a slip angle. This is why I normally only spun when I had to turn the wheel quickly (as in slaloms). I could fly through sweepers no problem. I could even brake and turn at the same time! (as long as I did it smoothly). I was thinking later that I should have tried some rear toe-in instead of leaving it at zero. Oh well, at least now I know what springs to use if we ever have a gymkhana event. :grin:
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yofa Newbie

yofa

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Re: New Auto-xer

According to the Good old Carroll Smith' date=' his knowledge says: "A relatively narrow front track width will cause understeer", although he doesn't go on to say the opposite, but I would assume that it is also true.Here is a rudimentary formula I found on a technical site regarding weight transfer.Weight transfer = ( Lateral acceleration  x  Weight  x  Height of CG ) / Track widthMore weight transfer at the rear, would generally yield oversteer, which is the same thing a bigger rear sway bar would accomplish.[/quote']dammit! i'm so completely sure that the real carroll smith wrote and published a line that goes roughly; "i prefer a narrower rear track to induce a light understeer, since it'll give the rear end an available grip reserve for the upcoming corner exit," but i can't find it! i'm missing my "drive to win" book, so i think it might be hidden in there.anyway, that formula would only be good for a trike. for 4-wheeled cars, any roll resistance removed from one axle is transferred to the opposite axle. for sway bars, that means stiffer rear bar, and hence more rear anti-roll, means more rear roll distribution and oversteer.for rear track width, less width, and hence less anti-roll, means less roll distribution and, with the roll distribution moved up front, you get understeer. ultimately, you get less rear weight transfer because the fronts are forced to do more of the work.p.s. carroll smith, the god, should not be confused with carroll shelby, the guy who takes too much credit for others' work, including the very carroll smith that i credit for the gt40's successes.
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KevinT Newbie

KevinT

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Re: New Auto-xerI agree, the roll distribution is moved around when you change the track width. The question is, is that relevant compared to how much increased weight transfer you are receiving from the shorter track width? So long as you have sufficiently stiff anti-roll bars and springs, decreasing the rear track width will cause oversteer.As to fwd cars having a smaller front track width? Can you tell me any performance fwd cars that have this? I know my RSX actually has 2mm more front track width than the rear. If you look into it, you will find that only the CSX in the acura car lineup has more rear than front.That formula is effective for a 4 wheeled vehcle. It doesn't take everything into account I agree, but its basic physics.The line I quoted from Carroll Smith is from his engineer in your pocket guide. I know you own this guide so maybe you should check out the narrow front track width causing understeer section! BOOYAH!

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yofa Newbie

yofa

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Re: New Auto-xer

I agree' date=' the roll distribution is moved around when you change the track width.  The question is, is that relevant compared to how much increased weight transfer you are receiving from the shorter track width?  So long as you have sufficiently stiff anti-roll bars and springs, decreasing the rear track width will cause oversteer.As to fwd cars having a smaller front track width?  Can you tell me any performance fwd cars that have this?  I know my RSX actually has 2mm more front track width than the rear.  If you look into it, you will find that only the CSX in the acura car lineup has more rear than front.That formula is effective for a 4 wheeled vehcle.  It doesn't take everything into account I agree, but its basic physics.The line I quoted from Carroll Smith is from his engineer in your pocket guide.  I know you own this guide so maybe you should check out the narrow front track width causing understeer section!  BOOYAH![/quote']i never said that a fwd's rear track width is larger than its front. i said that compared to rear or awd cars, fwd's usually have the wider rear track. simple geometry will show that the rear wheels track a smaller radius than the fronts, so making the tracks equal-width, or wider at the rear will just mean your rear wheels will be doing lots of curb-hopping.yes, i do have carroll smith's "engineer in your pocket," and the bigger version's "tune to win's" full explanation says:I believe that the front track should be considerably wider than the rear track. My reasons have to do with turning the car into corners and jumping on the power coming out. The wider the front track, the more resistance there is going to be to diagonal load transfer and the lesser will be the tendency for the car to "trip over itself" on corner entry and/or to push into the wall from the effect of the drive on the inside rear wheel when the power is applied. I believe that most of our present road racing cars, with roughly equal front and rear tracks, would benefit from an increase in front track width. The slower the corners to be negotiated, the more important this relative track width becomes.in other words, the abrupt change in the outside front tire's vertical load due to the combined lateral and diagonal weight tranfer causes an initial understeer. he's not talking about steady-state at this point. the assumption that "the wider the better" is wrong, and i clearly remember reading the line about narrow rear track for light understeer from carroll smith himself. it must be in "drive to win" which i borrowed to egilson way too long ago. i'll booyah you once i get it back.
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KevinT Newbie

KevinT

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Re: New Auto-xerYou have in essence already proved the point that I was trying to make, a wider front track decreases understeer. Reducing the rear track increases oversteer. I made no stipulations about transient or steady state conditions and neither did you when the argument was started.I also don't understand how you can compare the track widths between different cars, saying a fwd car usually has a wider rear track than a rwd car means nothing. Only in comparison to the front track does it mean anything. A hummer usually has a wider rear track than an echo.We were discussing whether changing the track width would affect understeer/oversteer. Both Wray and I agree reducing the rear track width increases oversteer.

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