I wanted to post my thoughts on this topic that I have given a lot of thought recently and get everyone's input.
Assumptions:
- Camber plates, big brake kits, pads and fluids are out of scope of this discussion.
- I am focused on attaining an ideal track day setup, not a racing one as I wanted to keep compliance with club racing rules out of scope.
- Wheel budget is limited to $2,000 for a set of 4 wheels
- Tire budget is limited to to $1,200 for a set of 4 tires
I have couple of concerns/questions regarding the proper function of the electronic and mechanical systems. Then there are subsequent decisions driven mainly by cost, performance, practicality, weight and aesthetics in descending order:
1a. How much can tier diameter deviate from OEM setup before ability of electronic systems to function as designed is impacted - abs, traction control, stability control? Does this matter with DCS and ESC turned off?
This is a non-issue. Dropping down 1" in diameter in wheel which will likely result in shorter tires will not be an issue.
1b. How much can tier diameter deviate from OEM setup before ability of mechanical systems to function as designed and their longevity is impacted - rear differential is the only one that comes to mind.
Saame response as above. Smaller diameter wheels/shorter tires don't adversely impact the differential
Cost:
2a - assuming comparison of same make/model or across manufacturers looking at similar material and build process, 18" wheels are cheaper. Same can be said for tires.
Yes to both. Wearables are cheaper which would be the #1 reason to do this provided you can access tires in the sizes you want
2b - 18" square setup will give you the ability to rotate tires front to back, saving you money. Arguably, this is true for a 19" setup.
Square setups such as the 18x10 to the extent they fit into your wheel well are ideal for rotation and more even wear. The car will also be easier to make it more neutral provided you're fine with that setup
2c - a square setup with the right tires will also allow you to flip those tires (run them backwards - counter to their intended rotation), giving you additional days of track time on the same tires, assuming there is usable rubber left.
If you get uneven wear, say shoulder wear from insufficient front camber or inside wear in the rear due to toe in, my suggestion is to not flip them and run them backwards but flip them on axel so the driver's front is now the passenger front and vis versa
2d - you can buy a set of takeoffs for ~$500-$600; you will usually have 2/3 of the heat cycles left in them. Similar tire setup will run at least $1,000-$1,200 new, but your selection will be limited to the offering of the reseller.
Take offs have their trade offs. You need to factor in the cost of mount/balances versus what you are saving. Some people, especially toward the end of the summer, sell low run take offs either because they've won tires on contingency, and don't want to store them over the winter or want them cleared out.
Performance
3. I think this one is a wash. Many competitive teams, including the Pirelli World Challenge M235iR cars run 18" square setup. Some may argue that 19" tires have a superior wall stiffness, resulting in less tire roll. There may be other arguments and I am interested in hearing those.
Most of the really good tires have pretty damn stiff sidewalls. IMO -- going to the lower aspect of 19" tires really compromises suspension compliance which arises from the sidewall. Here -- I'd say 18" wheels/tires is better.
Practicality
4. If you invested in a big brake kit or are considering one, tour wheel choice may be impacted based on which BBK manufacturer you chose. However, most manufacturers kits are by design compatible with 18" wheels.
No opinion as I have not run BBK which could not fit with 18" wheels
5. Weight - 18" vs 19" wheel/tire combo will generally save 2.5-5 pounds of rotational weight per wheel, 10-25 pounds total, which translates to roughly 20-50 pounds weight savings of sprung weight (assuming 1:2 ratio of sprung to unsprung weight for argument's sake - meaning a weight saving of one pound on a sprung component is an equivalent of saving two pounds on an unsprung component). Of course, one can buy expensive 19" wheels made of exotic materials that can be significantly lighter than 18" wheels, but I am assuming a budget of roughly $500/wheel, and not $2000/wheel or more. I created a spreadsheet to track a variety of tire 18" and 19" Bridgestone RE71R tire sizes and dimensions, including contact patch and weight, as well as sample wheel weight (I used APEX wheels as an example) -
https://docs.google.com/spreadsheets...it?usp=sharing
Weight can be felt and = time
6. Aesthetics - least, but somewhat important, as you will live with tour buying decision for months and years to come. 18" wheels are arguably still more popular for racing purposes, and there is a great, if not better, selection of competition wheels in this size that are pleasing to the eye.
Thanks for reading and I look forward to your input!