Grip Levels / Tire Choice 

The Toyota GT86 / Scion FRS / Subaru BRZ came equipped with Michelin Primacy HP 87W GreenX summer touring tires sized at 215/40/17.

 

Given the shorter final drive and corresponding increase in torque, more grip is required, but too much will make the car harder to modulate and lose its fun to drive character. This whole build diary is concerned with fun, but I have decided to include a track setup if you are interested. 

 

Analysis from Hot Version 116

 

Japan car program Hot Version 116 (source below) mounted the widest tire size they could on the car to experiment with increased grip. Keiichi Tsuchiya seemed to like the grip on the time attack, and the fast lap time reflected this. However, when asked to drift, the grip of the car made it very tricky. As mentioned the car was not developed to clock the most number of G's on the skidpad, instead engineered for fun. 

Rim Width by Tread Width

 

I came across an article on Moto IQ which presented Billy's rule of thumb: 

 

“For ideal handling: when sizing a tire for a given wheel, I usually target a tire’s tread width to be as wide as the wheel width, or 0.5” narrower than the wheel width.”

From my personal experience working on and driving countless track days, time attacks, street cars and race cars, this typically 
optimizes the tire’s carcass for response and outright grip.  It also gives the sidewall a very slight ‘stretch’ or preload, which will improve the tire’s response and break-away characteristics.  

OEM’s have to size tires to meet a 
ton of different targets like kerb to wheel damage protection, cost, snow chain clearance (almost all cars have to take this into consideration) all which affects the decision on the size of the tire and is usually a compromise at the expense of peak performance.

 

Naturally, I was curious to see what OEM manufacturers were up to and did a quick bit of research. Measuring the rim width of each of the cars, I subtracted it from the tread width (taken from tirerack.com) here were the results:

 

Mazda ND MX-5          0.5

Porsche GT4              -0.2

Scion FRS                  -0.5

Lotus Elise                 -0.7

Honda FD2 Type R    -1.2
 

The cars above were selected due to their reputation as 'drivers cars' as opposed to outright performance. We can see manufactuters leaning towards tires with a slightly smaller treadwidth comparted to the rim width. You might be on to something Billy! 

 

Road Wheel / Tire Setup  

 

Wheels - TSW Nurburgring 17x8 ET35 (8kg)

1kg less per wheel for an inch more of width and a taller offset than the stock wheels. Ideally would have liked to go forged for stiffness, but I do like the CSL style design of these spokes. 

Tire - Michelin Pilot Sport 4 (215/45/17) 

The balance of grip and slip is about perfect, and 215 width on an 8 inch tire aligns with above. 

 

Race Wheel /Tire Setup

 

My race tires go the opposite direction, maximising tire width and coupling with rim stiffness. Initially, I experimented with the RE71 tire as a street tire but quickly realised that its lack of progression, compromised wet performance, noise and raw grip make it more suited as a track tire. 

 

Wheel - Yokohama Advan RC2 @ 17x8.5
Reason: Was sold to me at a price I could not refuse. This is a tarmac and gravel rally wheel which is perfect for some over the curb action on track days. Weight is: 8.4 kg. 

 

Ideal Wheel - Rays Volk Racing CE-28N @ 17x9
Reason: Weighing 7,2kg, the CE28 is a forged monoblock wheel for stiffness and weight. 

Tire - Bridgestone Potenza RE-71 @ 245/40/17
Reason: Winner of tirerack track oriented tire shootout  for July 2016. The tread width of the 245/40 RE-71 is 8.6 inch (the 245/45 has a treadwidth of 8.2") which is a perfect match for the 8.5" rim. 

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Resources: 


Road & Track's Tire Analysis - Does incremental upgrades of tires and reports findings. 

Hot Version 116 - Explores basic modifications to improve lap times including tires. 

 

Auto Express TRD Toyota GT86 - Review of the TRD version of the GT86 in the UK. 

 

Size tyres for Performance - MotoIQ's article on proper tire selection 

 

Rim Stiffness - A great article on the overemphasis of lightweight rims. 

 

Track Tire Shootout July 2016 - Tirerack 

 

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