Re: Car physics: was MK1 Drag results...

["Gary A. Huff" <scirocco86@home.com>]

Scott Williams wrote:
> 
> "Gary A. Huff" wrote:
> 
> > Not true, High displacement RWD torque monster cars are jacked up for a
> > reason; that is weight transfer.
> 
> Gary is right on point with this. The higher the ride height the greater the amount of weight that is
> transfered. His observation about the RWD cars exemplifies this succinctly.

Yep, it's all about traction on RWD cars. With all the low RPM torque of
oh lets say a big block camaro, or something equally as evil, the thing
would just sit and spin. The real fix is massive slicks, but you can get
away with raising the rear end on a street car to avoid running huge
tire, and possibly keeping that sleeper look....


> Chris DeLong wrote:
> 
> > The center of gravity is more centered to the front wheels increasing
> > traction. Provided the rear struts are stiffer than the front for more ideal
> > weight distribution.

FWD car's have a limit of traction, I think the fastest FWD import cars
are *BARELY* breaking the single digits in drag racing, because of said
limit. There are RWD full body GM's that run well into the single
digits, infact some crazy guy in a twin turbo 3rd gen mustang killed
himself a few years ago in ocean city I think, it was supposedly a
street legal low 9 second car, but again *supposedly* I don't believe it
until I see it


> Incorrect, the center of gravity changes from front to rear only with ballast movements. Raising the
> rear of the car does not move the cg up towards the front wheels at all. Alright, let me hit y'all off
> with a little bit of physics. What actually does change is the *moment* or the degree by which torque
> can be applied. The torque caused by a part is equal to the weight of the part multiplied by the
> distance from the reference axis -in this case the cg.

> Raising the car increases the levering action with the cg being the fulcrum upon which the parts act.
> Thus raising the rear of the car *increases* weight transfer off the front -not the reverse. Check out
> the most powerful fwd drag racing cars. Tell me, do you see rear ends that are much higher than the
> fronts? Higher than stock, that is.


> Ask a road racer about weight jacking. You know those height-adjustable coil over kits that people
> think look really cool? Well, their legitimate purpose is to transfer weight around to achieve a closer
> to ideal load distribuition. The old axim is "lower is lighter; higher is heavier". Since weight  moved
> farther (vertically) from the cg increases the moment, weight transfer is thusly *increased* towards
> the direction that the chassis is moving around the fulcrum -rearwards.
> 
> This raising the back method would make the car rule in backwards dragracing, though. Hey, the reverse
> gear is a pretty damned low ratio, already. Maybe we're on to something?..

I did that in my friends rabbit :) It flew out of reverse at about 25mph
and made bad noises :)

> Jasin wrote:
> 
> > The fastest NA rabbit is using solid rear suspension and I am considering making something
> > similar to replace the rear struts once I get to the track .

Yep, and I bet he's running massive slicks up front too :) 

> >
> Now you're cooking with gasoline. The solid rear suspension eliminates nearly all weight transfer to
> the rear. Only the compression and extention of the tire sidewalls will contribute. To minimize
> compression and to decrease the rolling resistance, you jack up the tire pressure back there to the
> maximum "safe" level. I put that in quotes, though, because this formula may be a recipe for
> oversteering disaster.

> As for the autocross performance, the raised rear end may help a bit in making the car rotate. The same
> physics apply here, too. In a turn, centrifugal force acts on the car's weight. The raised rear of
> Jasin's car is "heavier" because moment is larger. Instead of understeering, the back-end "kicks" out.
> That's a really simple explanation but you get the picture... When the car is properly set up, though,
> this change will be a liability. It would be better to simply increase the rear roll stiffness with
> higher rate springs or rear anti-roll bar.

In road racing/autox the raised rear in a RWD torque monster helps too.
Have you ever seen a car with 450ft lbs of torque at 2300rpm try and
corner without massive wheelspin? It's funny, but it can be done.......
In fact my next project after my current 16v car, and this GTi I may be
getting is going to be a third generation Z28. Contrary to popular
belief, with a panhard rod, and some frame connectors the car's handle
well.

> Amen.

Yeah, I agree amen, and I've been doing too much reading on physics and
cars :(

> Scott F. Williams (impersonating Iain Mannix)
> NJ Scirocco nut
> SCCA ProRally driver
> Hotrod Rabbit GTi
> 
> Check out our rally team's website!
> http://www.usrallyteam.com
> 
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-- 
===================================================================================================
Gary A. Huff scirocco86@home.com
86.5 16v Scirocco "Red Rocket" (120k miles)
80 Gas Rabbit Pickup; grazing the driveway. (Needs strut tower welding
and
title, 50k original miles)
===================================================================================================

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