Re: Top speed and vehicle weight

["Graydon D. Stuckey" <graydon@apollo.gmi.edu>]

On Thu, 13 Mar 1997, Bertenyi, Tamas wrote:

> I hate to sound like an engineer here, but several postings discussing 
> the top speed of the 90 or others referred to the weight of the 
> vehicle. While the mass of the car is important when considering 
> acceleration, handling, and deceleration, it does not affect the top 
> speed. Top speed is really only a function of aerodynamic drag (which 
> is low for the 90) and available power or gearing (depending on 
> whether the car is drag limited or rpm limited).
Tamas,

	I must confess, I've ignored the top speed thread, but your 
comment here is interesting.   Top speed does depend on weight, although 
not as much as any acceleration-based performance parameter.   Extra 
weight causes greater wheel bearing resistance, greater tire rolling 
resistance, and greater suspension resistance.   The first two are pretty 
obvious, but the third point may be a little more obscure.   When a car 
going down the road hits a bump, the suspension takes up the bump, and 
hopefully, the car chassis should stay pretty much steady.   
Obviously the greater the bumps, the greater the amount that the car will 
be jolted.   Think of the car travelling across a plowed field.   If the 
bumps are very high, then the engine must pull the car up the bumps, 
which requires more energy.    If the road was perfectly smooth, then I 
would expect weight to have a very small influence on top speed, but 
since all of our roads are not perfect, it takes more power to traverse a 
road with a heavy car than with a light car.   Conversely, a lighter car 
can achieve a higher top speed than a heavy car, all other factors being 
equal.

Later,
Graydon D. Stuckey 
'86 5KCSTQ, getting more toys every day...