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Re: effect of inclined roll axis on handling?

To: Smokerbros@aol.com
Subject: Re: effect of inclined roll axis on handling?
From: james creasy <jcreasy@perforce.com>
Date: Fri, 21 Jul 2006 11:18:17 -0700
charlie,

i dont see how lowering the rear roll center on its own can make the car 
rotate better.

that change alone will increase the CG to RC distance and acts like 
softening the rear bar.  for example, when i lowered my front roll 
center, i calculated how much more front bar i needed to balance that 
change- so i changed from a 1.25" OD, .095" wall to a  .188" wall front 
bar which rebalanced the car.

in the scenario you describe, was there also a stiffer rear bar added?  
how much stiffer?  was the geometry changed, e.g. from trailing arms to 
a panhard? 

but now we are describing steady state cornering- which is tuned with 
bars and springs. 

its the possible initial effect of the roll axis before steady state 
that i am interested in.  ive gotten several different, but impassioned, 
arguments from various experts.  unfortunately there is no agreement on 
what the dominant effect/s is/are. 

james
OSP - Octane's Sole Purpose


Smokerbros@aol.com wrote:

>In a message dated 7/20/2006 5:59:57 P.M. Pacific Standard Time,  
>nihal@berkeley.edu writes:
>
>I don't  think that it would work this way. If the rear roll center is
>lower, this  does not change the total weight transfer at that axle. It
>only changes the  roll angle. The roll angle is purley a function of the
>distance between the  RC and cg height (the lever arm you speak of) and the
>lateral acceleration.  With the lower rear roll center the car will become
>"lazy" and be more  stable at that end.
>The overloading of a tire doesn't make senes to me. The  weight that went
>onto that tire had to come off of another wheel (usually  inside). These
>inside wheels are what govern the oversteer and understeer  characteristics
>of a car. I would suspect if the front outside was in the  situation you
>describe (kind of like overdriving a FWD car) the front  inside wheel has
>very little weight, and thus cannot produce the lateral  accelration it
>needs to, and therefore you have  understeer.
>
>
>
>Okay, say it however you want to.  My experience is with strut  front/solid 
>axle rear cars.  I know the following to be true.  
> 
>1)  MacPherson strut suspensions have a very low roll center, usually  
>between 2" above ground and somewhere below ground.  
> 
>2)  Solid axles without a Panhard or Watts have a roll center that is  right 
>through the center of the axle.  So, on a car with 24" tall tires,  this is 
>12".  
> 
>3)  That's a steep roll axis and the car will push.  Lowering the  rear roll 
>center to say 6" above ground causes the rear to rotate more and the  car to 
>push less.  
> 
>this has been proven time and time again in numerous autocross cars.   Cloak 
>it in whatever math or verbage you want to, but the end result  is #3.
> 
>Charlie 




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