Car physics: was MK1 Drag results...

[Scott Williams <sfwilliams@home.com>]

"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.

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.
>
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?..

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 .
>
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.

Amen.
--
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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