Why 18" vs 19" wheels/tire for track and Auto-X

phunk · 08-27-2014, 02:15 PM

Why do you reference gearing and speeds in regards to wheel diameter?

If you plan on putting tires on your wheels before going to the track, I would worry more about tire diameter in regards to gearing.

wstar · 08-27-2014, 04:48 PM

Basically, a smaller wheel diameter is beneficial because it has less mass (unsprung weight) and less rotational inertia (which is more than just the rest mass difference; there's a multiplier in there for wheel radius for the mass at the outer rim), all other things being equal.

In the net you basically always want the smallest wheel/tire diameter possible. The reason we don't run e.g. 15" wheels on this car (well, aside from it looking stupid) is because the other balancing factor is being able to fit the brakes in the wheel. Even if you custom-built a brake system for your 370 that could fit in a 15" wheel, it probably would not have enough mass or surface area in the rotors to take the heat/abuse. A 3000lb+ car needs fairly beefy brakes.

Given that most 18" wheels fit over the stock/sport brake systems, the step from 19 -> 18 is a pretty obvious, easy win.

Rusty · 08-27-2014, 09:17 PM

Quote:

Originally Posted by wstar

Basically, a smaller wheel diameter is beneficial because it has less mass (unsprung weight) and less rotational inertia (which is more than just the rest mass difference; there's a multiplier in there for wheel radius for the mass at the outer rim), all other things being equal.

In the net you basically always want the smallest wheel/tire diameter possible. The reason we don't run e.g. 15" wheels on this car (well, aside from it looking stupid) is because the other balancing factor is being able to fit the brakes in the wheel. Even if you custom-built a brake system for your 370 that could fit in a 15" wheel, it probably would not have enough mass or surface area in the rotors to take the heat/abuse. A 3000lb+ car needs fairly beefy brakes.

Given that most 18" wheels fit over the stock/sport brake systems, the step from 19 -> 18 is a pretty obvious, easy win.

Custom brake set-up. Inboard rotors. $$$$$$$

Fit anything you want.

j-rho · 08-27-2014, 05:06 PM

If there were an infinite world of tires the wheel decision would be more pure. In reality, a given model of tire is only available in a finite set of sizes. In racing there are usually rules governing what you can do with wheel sizes and what tires you can run.

Within those constraints the best possible choice may be a sub-optimal wheel diameter, to allow for a superior tire and thus overall better wheel/tire combination.

wstar · 08-28-2014, 01:46 PM

An old thread that covers some interesting angles on why not to use an inboard brake system: Are inboard brakes outlawed ? - Forum - F1technical.net

It would probably be easier, given sufficient budget to do either, to do a more-advanced brake system that's still in the wheel but works well at a smaller diameter. Exotic disc/pad materials, much better airflow/cooling, etc? I bet even engineering a multi-disc system (as in 2x discs in parallel inside each wheel with 4 pad surfaces) would be easier than engineering inboard brakes.

Rusty · 08-28-2014, 10:10 PM

Quote:

Originally Posted by wstar

An old thread that covers some interesting angles on why not to use an inboard brake system: Are inboard brakes outlawed ? - Forum - F1technical.net

It would probably be easier, given sufficient budget to do either, to do a more-advanced brake system that's still in the wheel but works well at a smaller diameter. Exotic disc/pad materials, much better airflow/cooling, etc? I bet even engineering a multi-disc system (as in 2x discs in parallel inside each wheel with 4 pad surfaces) would be easier than engineering inboard brakes.

Well, it WAS an idea. LOL

08-27-2014, 02:15 PM	#1 (permalink)
phunk A True Z Fanatic Join Date: Mar 2009 Location: Chicago Posts: 3,669 Drives: 370 Rep Power: 974725	Why do you reference gearing and speeds in regards to wheel diameter? If you plan on putting tires on your wheels before going to the track, I would worry more about tire diameter in regards to gearing. __________________ Charles @ CJ Motorsports : Website \| Facebook \| Instagram \| YouTube Home of the 9 second, stock longblock, stock drivetrain 6MT 370z. 9.91 @ 142mph Last edited by phunk; 08-27-2014 at 02:27 PM.

08-27-2014, 04:48 PM	#2 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3596	Basically, a smaller wheel diameter is beneficial because it has less mass (unsprung weight) and less rotational inertia (which is more than just the rest mass difference; there's a multiplier in there for wheel radius for the mass at the outer rim), all other things being equal. In the net you basically always want the smallest wheel/tire diameter possible. The reason we don't run e.g. 15" wheels on this car (well, aside from it looking stupid) is because the other balancing factor is being able to fit the brakes in the wheel. Even if you custom-built a brake system for your 370 that could fit in a 15" wheel, it probably would not have enough mass or surface area in the rotors to take the heat/abuse. A 3000lb+ car needs fairly beefy brakes. Given that most 18" wheels fit over the stock/sport brake systems, the step from 19 -> 18 is a pretty obvious, easy win. __________________ 7AT Track Car! Journal thread / Car setup details

08-28-2014, 01:46 PM	#5 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3596	An old thread that covers some interesting angles on why not to use an inboard brake system: Are inboard brakes outlawed ? - Forum - F1technical.net It would probably be easier, given sufficient budget to do either, to do a more-advanced brake system that's still in the wheel but works well at a smaller diameter. Exotic disc/pad materials, much better airflow/cooling, etc? I bet even engineering a multi-disc system (as in 2x discs in parallel inside each wheel with 4 pad surfaces) would be easier than engineering inboard brakes. __________________ 7AT Track Car! Journal thread / Car setup details

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
18"x10" and 18"x10.5" Track Wheels Options?	scruffydog	Wheels & Tires	100	03-30-2017 09:00 AM
[WTB] 18" Tire and Wheels (S.F. to L.A.)	DivZero	Parts for sale (Private Classifieds)	1	01-03-2014 02:56 AM
20" XXR 526 staggered wheels - 11" rear wheels with 5.25" lip - starting @ $ 1216.00	speed depot	Tires/Wheels	48	12-05-2012 12:05 PM
18" Lightweight Track Tire/Wheel setup - 1500	Mike	Parts for sale (Private Classifieds)	14	10-06-2010 11:56 AM
oem 18" wheels for track use	Luigi	Wheels & Tires	12	08-31-2009 08:11 AM

08-27-2014, 05:06 PM	#4 (permalink)
j-rho Base Member Join Date: Jul 2014 Location: San Diego Posts: 154 Drives: 1967 Camaro Z28 Rep Power: 13	If there were an infinite world of tires the wheel decision would be more pure. In reality, a given model of tire is only available in a finite set of sizes. In racing there are usually rules governing what you can do with wheel sizes and what tires you can run. Within those constraints the best possible choice may be a sub-optimal wheel diameter, to allow for a superior tire and thus overall better wheel/tire combination.