I pulled this one out of the ethernet...searched MoI and wheels. Seems to indicate that the writer feels there is a definite benefit in acceleration for lighter wheels, given the same MoI. I'm not sure I'm buying it, though. He doesn't seem to really support his arguement with any real tech or data.


The effect of rotating mass can be calculated using Moment of Inertia (MOI). MoI is related to not only the mass of the rotating object, but the distribution of that mass around the rotational center. The further from the center, the higher the MoI. The higher the MoI, the more torque required to accelerate the object. The higher the acceleration, the higher the torque required.

Because of this, the weight of rotating mass such as wheels and tires on a car have a bigger effect on acceleration than static weight such as on the chassis on a car. When purchasing new wheels and tires for a performance car, it can be useful to compare the effects of different wheel and tire combinations. This is especially true when considering upgrading to larger wheels or tires on a car.

The use of light-weight alloys in wheels reduces rotational mass. This means that less energy will be required to accelerate the wheel. Given that each pound of rotational mass lost provides an equivalent performance gain as a 10 pound reduction in vehicle weight, the benefits of light alloy wheels on vehicle performance cannot be overlooked.
For example:
***A reduction in the weight of the rim/tire assembly of 5lbs x 4 (all around the car) is equivalent to a 200lb weight reduction in vehicle weight (thats worth 0.200 in the 1/4 mile)***

 

If you're not trying to compete at the highest level of competition don't bother with the lightweight wheels. Even then it's probably not going to mean the difference between winning and losing especially when you're talking about a car which comes from the factory with wheels that are pretty light.

 

MOI is a representation of weight relative to radius, weight alone doesn't cut it

BTW, those 285s add a lot of MOI over 245's since a tire's weight is concentrated at a radius larger than that of a wheel

 

I found this extremely intriguing. So if mwood's figures are correct, that 90 lbs equals half of one passenger or .24-.3 second according to Jason's figures. That also seems to jive with the Goeke example mentioned earlier.

But I actually don't buy the 90 lbs because that would mean 5 times static weight from 18 lbs of wheel weight savings. Even with SSR and 28 lbs difference from stock, 90 lbs may be on the high side at 3 times static weight. But clearly it is a substantial advantage....and far different than true stock.

For instance, it appears that without those lightweight pricey wheels, using Jason's own figures (60 lbs= two tenth)s, he would have finished 2nd in B Stock this year, with only a tenth a second advantage over Carter Thompson. So please quit feeding me this line of bull that such expensive stock mods make no difference.

 

Mike, your earlier formula yielded a 90+ lb weight from 18 lbs of wheel weight savings. By this formula, it should have been 180 lbs of weight savings. I agree completely that it is some multiple of static weight but it sure isn't 10 times. I got fried one time for daring to claim it was as much as 4 times the static weight savings which I had also found claimed in a road test. Nobody wants to admit their dollar bought goodies are an unfair advantage....or if they are, it is because they are true competitors.

 

Cole, please, I'm begging you, DO NOT set up your soap box here...for the love of all that is good and decent in this world...

 

Just ignore him. He doesn't even understand that "stock" is a prep level.

I'll say it again, for most people the lightweight wheels are probably not even a worthwhile investment. But if you're prepping a car to the limit of the rules, trying to give yourself every legal advantage you can, it's absolutely worth it. The magnitude of the advantage may vary from course but it is an advantage.

 

There's also the advantage of less effort required for wheel changes and moving the wheels around while they're off the car. When you have to run around the outside of your house to get them to and from the basement (particularly after an entire humid, 90 degree plus day of standing around a parking lot), it's an even bigger advantage.

On course...it's just one more place to make the car a little better. Whether the driver can make full use of the advantage (however large or small it may be), is a different story.

 

If anyone is looking for a set of wheels to fit over brakes similar to Stoptech's 12.9" rotor setup (factory rotor size, but 4 piston, I will be selling in about a month a set of Billit Alum 18 x 9 with the proper offset. These were custom made by Complete Custom Wheel in Daytona. Cost $650 each. Used for 6 track events. Selling for $450 each. Light and strong.

Will also be selling at that time or soon thereafter suspension parts (tein flex with electronic controler and other parts.

If interested email me at jayg@printwithbudget.com or call 561-252-0443

Jay Goldfarb

 

Lots of stuff to read here but I dont think I saw anybody mention the gyroscopic effect yet. Like if you've ever spun a bicycle wheel, held it by the hub, and moved it right/left on a vertical axis. Considering the large amount of turning in autox, especially on a slalom, it couldnt hurt to have a lighter wheel in that respect. Its the aspect I think I felt the most when I switched to lighter wheels on a former STS car. But who knows, its about as scientific as the butt dyno.

 

^^^ Angular momentum.

Though, I doubt that the phenomenon would have that great of an effect with modem power steering systems.

 

somebody better snap these up:

https://www.rx8club.com/showthread.php?t=105424

 

Keep in mind that T3 does not have the offset rules that we do. I have not seen that wheel listed in the US in a BS legal offset. However it could have been a special order.

 

If they are 42mm offset, I wonder how much the machining would cost to bring them into compliance?

 

It is not a matter of how much.... It is if the wheel can stand to have 1.65mm taken off. We need to see a pic of the back of one.

 

I was looking at Tirerack a few minutes ago, thinking I'd buy myself a Christmas present...anyhow, OZ has a new wheel for our car, Chrono HT. Apparently, it is going to weigh about 8.7kg vs. 8.4kg for the Ultraleggera. Nice looking wheel, though. Also, the Ultraleggera is now available in a black finish...not sure how that would look, I've been leaning towards a classic silver finish, but the black may be kind of cool and not show brake dust

Finally, am I correct in assuming that there is absolutely no need to run TPMS sensors on autocross rims?

 

no need for TPMS

 

Thanks.

I finally did it and just bought a set of OZ's with nice, new V710s.

Total cost was $2336, mounted, balanced and delivered to my door. Not too bad, considering. The OZ's are pretty darn good bang for the buck...maybe not optimal, but a good value.

 

Possible new BS wheel from Buddy Club

P1 Racing II
18x8.0 45mm 5x114.3



GunMetallic, HyperGold, and SBC (extra $$$) colors




Need to verify hubcenter diameter and weight, would expect 16 lbs based on their other wheels, seen them around $500/each

http://www.buddyclub.us/buddyclub/p1r2/p1r2.html




.

 

^Wonderful. My OZ's arrived on Friday

 

here's another one, new Enkei offering, kind of heavy and expensive, just logging it in the thread

GTC01
18x8.0 48mm 5x114.3

http://www.good-win-racing.com/Mazda.../60-13752.html

 

another one for the thread


http://www.edgeracing.com/2005/Mazda/RX8/wheel/1906/

 

.and sometimes...taking them back off helps your car run...

 

no affiliation or knowledge about them:

http://cgi.ebay.com/ebaymotors/18-x-...QQcmdZViewItem

 

From seller: