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

Ok, so after doing a obligatory search I cant find any threads discussing performance of 18" vs 19" wheels on the 370z. However, I see a lot of folks talking about running 18" wheels on their track & auto-X cars.

I am assuming that on a track the loss of gearing is more than made up by benefits reaped by...

1.) less rotational mass
2.) bigger better cheaper tire & wheels options

Is that it?

But for auto-x I see guys running 18" too...which is stranger to me since 2nd gear top speed is such a critical factor for most Auto-X cars.

Anyone care to tell me why they choose 18" over 19" for Auto-X?

Cheaper tires to burn...

1. Better tire options for 18" wheels.
2. Weight reduction, although the wider wheels most of us run end up bringing the weight back up.
3. Rotational inertia is better on 18" wheels.
4. Even with 315/30-18's on my car I still redline around 67mph which is plenty for most courses to not need 3rd gear.

Nismo has same gearing as base w/sport? Also, is around 67mph captured with data or just your best SWAG?

Thanks for linking, some good info there. But mainly covers the points I assumed were true between the two options...and a bunch of other typical off topic forum piss matching<sigh>

I never found reasonable 18" wheels that would be legal for CS, but if I had, this would've been the main reason for me.

At current prices, RS3s are $400 a set cheaper in 18" sizes. From the looks of things, I'm going to burn through 2.5 sets in one year of autocross in my car.

Granted, that's doing every regional event including practices (in SoCal, so one a month year round), 2 ProSolos, 2 National Tours, and Nationals next week, often with co-drivers.

I'm getting around 120-150 runs out of my RS3s, flipping them and rotating them.

Noted, that's valuable feedback as I was intending to get RS3's as well.

P.S - Good luck at Nats buddy...now that your picked for a potential CS winner a lot of eyes will be on you. :tup:

GPS reading on data logger for speed capture. Also the gearing (both trans and diff) are the same between the Sport and Nismo.

Sounds like a reason just to come play in ASP.

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

https://c2e17d9c09c94c22a24fa62f65b9...oQ00/photo.PNG

Thanks :tup:

Yeah I was hitting 69 on the big 285 35 19, the 285 35 18 only hit like 64 or 65

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. :) :)

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.