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

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.

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.

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.

ConeKilla · 08-28-2014, 04:48 PM

Quote:

Originally Posted by stuy486

My GPS data shows high 68ish MPH top end in 2nd. Captured using RaceTrace ;-)

The two runs displayed are my right side runs at the Crows Landing ProSolo: https://www.youtube.com/watch?v=NFEB...EGGtLCB9guMgJA

exactly what I was looking to satisfy my curiosity. Thanks! P.S. You ever heard or seen Petrel data logging software? It's basically THE data logger for Auto-Xer's I highly recommend you make the switch. The ability to instantly review a run definitely gives a competitive edge.

stuy486 · 08-28-2014, 05:08 PM

Quote:

Originally Posted by ConeKilla

exactly what I was looking to satisfy my curiosity. Thanks! P.S. You ever heard or seen Petrel data logging software? It's basically THE data logger for Auto-Xer's I highly recommend you make the switch. The ability to instantly review a run definitely gives a competitive edge.

I'm a developer for RaceTrace, and I'm pretty happy with the software I wrote myself

This screenshot is from my iPhone, and I regularly review data between runs when I'm autocrossing. Plus, the ability to compare my data to co-drivers' on the fly really helps to learn where we're gaining and losing time to each other.

There's a thread from a few weeks ago with some more info on the topic: Lap recording software.

Also, SoloStorm (The Petrel product) is $200 and Android only, RaceTrace is $15 and (currently) iPhone only.

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

08-28-2014, 01:46 PM	#18 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3594	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

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08-27-2014, 05:06 PM	#16 (permalink)
j-rho Base Member Join Date: Jul 2014 Location: San Diego Posts: 154 Drives: 1967 Camaro Z28 Rep Power: 10	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.