HeavyChevy,

I also like you have heard in many places all the benefits related to rotational mass etc, but it has always been only speculation and sort of a black box. What I do know is that I run heavy Cargraphic racing wheels, and with my other lighter wheels (Magnesium) , I had no impact on lap times that I could attribute to the wheels, the same with 60-130mph runs.

I am not sure I understand the relationship of this with drivetrain losses. Drivetrain losses are due to component resistance and friction (heat) mainly, the rotational mass in a torque tube is minimal , being so light and close to the center of rotation. The 2WD to 4WD conversion has been calculated to increase drivetrain losses by about 6%, based on datalogged runs, excluding weight savings.

Maybe as a point of reference, Cargraphic has been winning for three years in a row the German Tuner Grand Prix with GT3 cars fitted with their very heavy wheels, with the amount of money invested on suspension, aerodynamic and engine improvements, I would have thought that they would have started with the wheels!

Tradin1, Thanks. I posted above some names.

Sechsgang, I know what you mean, there is all sorts of speculation going on at the tracks, mostly related to butt dyno feelings or hearsay, but seldom proven with data. I doubt the impact is really meaningful other than the benefit of mass reduction of the wheels itself and some more as a result of unsprung weight. Steering input, I am definitely there, so is a reduction of rebound/better tire contact on bumpy tracks.

 

wross, very nice comment thank you. nice to hear from someone on the forum I do respect!

Personally I have no problem with anyone on this forum regardless of how they may feel about me, not really something I would loose sleep over!

thanks again

tom

 

GD, who gives a damn already. I asked a simple question. I am pretty sure the majority of topics have been discussed, screw the search function

I wanted to get some more opinions

Christian

 

Well Christian that's fair, wross demonstrated the search function for you and the link he put in this thread is where to go, virtually all of us have replied, you should find what you need there.
good luck.
tom

 

In bike racing, ignoring the drug issues, the lightest wheels are not always best because of:

durability

stability at speed

ability to hold roundness (stiffness in climbing/cornering).


In cycling, data exhist for why light stuff is better in terms of work needed to go up hill, in particular, and in sprinting (acceleration). At steady state, it is not as big a deal.

Perhaps, and only perhaps, these issues impact real world use of car wheels.

When I got my new 996TT, I asked if the hollow wheels were ok. Mechanic who had a lot of scca time said to me: light and strong as hell. In some ways, probably the best wheel you'll ever find for the money. So I stuck with them. I am sure there are better wheels, etc. But these are quite strong and pretty light.....to me, I want a reliable wheel. And, once in a while one can get them for 300 or so each in sets of four. :-)


JB

 

you arent in Hershey PA are you?

 

I simply labeled it as drivetrain because the wheels are next in line to add weight and friction (tire). Your drivetrain has to turn the tires and heavier tires are going to produce more resistance to turning so there is a bit of loss where the drivetrain is concerned (though maybe not directly, but supports my point) You cant say that if you put wheels that weigh 100 lbs total more weight on a car that it wont slow down, and I bet it would slow down more than 100 lbs in the passenger seat. Though I cant say for sure if it's weight or size of the wheels that makes the biggest impact because most people that notice the loss have added not only heavier but bigger wheels as well.

 

HeavyChevy

Different diameter wheels will certainly have an impact on acceleration rate, however it is also speed dependent, while it has a benefit in off the line acceleration (provided you can keep them gripping), it does not give any improvement at higher speeds where acceleration levels have milded down.

It is a simple formula in fact:

Torque put to the ground= Torque at the axle / (rolling diameter (In.)/2/12 )
So If the torque coming to the wheel is 1000 lbs, then the thrust put to the ground for example is 961lbs with a 295/30 x18 and 988lbs with a 265/35 x 17 , not much difference.

With a taller tire you will have a bit less acceleration and more weight but much bigger contact patch and sidewall, which is why they prioritize that over smaller diameter wheels and tires that are lighter.

996-997 RSR run the tallest possible tires they don't need the additional torque coming from a smaller diameter tire.

Sorry to go OT Christian. I will stop here with the wheel/tire thing.

 

How does weight translate in that formula.

 

It doesn't, I am talking about acceleration impact due to rolling circumference difference only per the previous question

Weight will come in the next step, when you want to measure acceleration impact in ft/s.sq. , you take weight and circumference into the equation.

We can take it offline if interested.
Jean