As long as the wheel diameter (and tire diameter)remain the same, it's more than one but frequently LESS than 2.

I can run the numbers for you using a comparative acceleration spreadsheetif it's really necessary to do so.

A one to ten ratio is more closely related to flywheels/flexplates/balancers rather than wheels and tires. Tranny and axle gearing figure into the calcs and apply to things that rotate (and accelerate) at multiples of wheel rpm. A 1.5 factorx 3.31 axle x 2.00 second gear is 9.93 . . . close enough?


Norm

 

"exx", if I was "that worried about it", I would just get the Vette and wind up with way more power than your blower. Good idea though, thanks for the help.

 

Norm, aside from the weight calculations for rotational mass and sprung vs unsprung weight formulae - do we simply have the weight of the wheel/tire combo for the stock 18's with bullit wheels?

thanks

 

Unfortunately, I don't have that information. The FRPP catalog gives extensive wheel dimensional data, but itdoesn't provide any wheel weights.

If it's a rough "bottom line effect" that you're after, even 5 lbs per corner on a Miata only amounts to 0.1 - 0.2second on a 115-ish second road course, as per testing done for the SCCA's own monthly publication "SportsCar". Figure half that at a typical auto-X, and less still on the quarter mile. Almost no value whatsoever on the street beyond bragging rights.

Strip or auto-X timers would be able to pick the time difference up. So if you're really that consistent as a driver (most of us aren't) it's a clear benefit to go as light as is safe. It'll win for you a very close race that you'd otherwise have just barely lost, but that's about it. Methinks this little piece of physics has been blown all out of proportion to its actual effect, like it's some sort of magic bullet or something. It's not. Just a little tweak that's good for a little help.

A really sensitive driver might notice a difference in throttle control required to avoid wheelspin (Sam claims to be able to, I certainly don't). But I don't think that even Sam feels it as a difference in acceleration sensationthrough SOTP forward pressure.


Norm

 

The stock bullets 18" with tires were 52 lbs.

 

Norm, I don't actually race the car at the track, however, I have heard several, if not many, forum people talk about noticeable loss of power and performance after getting bigger or heavier tire/wheel combinations.

Having to fight (and pay dearly), for every single horsepower addition, I just don't want to give it back by adding weight unnecessarily.

 

Sawyer....thanks.

 

Damn, that's exactly what the fanblades with tires weigh.

 

Unfortunately, the math/physics/engineering does not support this "popular wisdom". "Everybody" tends to develop an opinion based on the increased difficulty in lifting a wheel (it weight) and swinging it over and onto the lugs (crudely, a measure of rotational inertia unless you lift straight up and install perfectly straight on without difficulty).

Anyway, that's the smallest part of car resistance to acceleration when you consider the 3500 lb weight plus its a 150 - 250 lb driver. Just because I had the spreadsheet handy, 5 lbs difference per wheel with the same size tires costs you less than 1% acceleration. Nobody short of maybe a professional test driver would ever notice. Imagine it, yes.

(hopeful edit) The problem with "popular wisdom" is that it isn't fully described, controlled, or measured with instruments. Too many other things have greater effect.

Taller tires hurt you worse with the effect of diameter on overall effective gearing, and again if the speedometer wasn't recalibrated, since a slow-reading speedometer will stretch out the time it takes to reach what you think is the target speed. Then it all gets blamed on wheel weight if that's what you're interested in at the moment.

I hear you on not wanting to give anything back. Sacrifice any of your newly-found torque to parasitic losses, to be more accurate. But when the "cost" is only about one foot and 0.016 seconds in a second gear sprint from 35 to 55 (for the same 20 extra total wheel lbs) it hardly matters unless you're being timed, the competition really is that close, and you've paid attention to all the other little 1% things, too.


Norm
(having huge trouble getting this posted)

 

I'd have to see the science behind that. There is absolutely no doubt in my mind that I dropped time in the 1/4 based on a wheel swap. Just sitching from the 18s to the 17s shaved almost 2 tenths off of my ET. True, you won't notice that kind of difference in the seat of your pants, but bigger wheels definitely hurt my car's performance.