2013 Scion FR-S: Wheel Selection Conundrum, Part II
July 17, 2012
When I broached the subject of aftermarket wheels for Project FR-S, our 2013 Scion FR-S, it quickly became apparent by a few of the comments that more background was needed.
First, why add grip? Won't this ruin the car's balance?
Oh, absolutely it will. In fact, the argument could be made that the stock FR-S already has too much grip for its grunt. Adding more tire will only exacerbate the need for more power... an area we don't plan on ignoring either. As of yet there isn't much in the way of power adders, but we want to keep the ball rolling anyway.
Project cars are a stepladder no matter which aspect you tackle first.
Why not do something more modest, like 225s on stock wheels?
That's... uhmm.. zzzz... sorry, dozed off there for a sec. I'd turn that question around and ask: why not give it something it can really grow into? This is a project car. It's supposed to be fun, something with which to experiment and share those observations and tradeoffs.
But for sure there's no way we're keeping the stock wheels on this car. Those we'll leave for our upcoming BRZ, which will remain stock.
Wheel widths. Why go wide? Why not just put wide tires on narrow wheels?
When it comes to grip and steering precision, wider wheels make a significant difference. The reason is multifaceted. For a given tire width, a wider wheel "preloads" the sidewall with tension, reducing the amount of sidewall deflection when the driver makes a steering input. This reduces yaw delay.
Wider wheels provide more uniform contact patch loading. The same tires on narrow wheels require higher pressures as a means of "support" to offset distortion-induced camber loss. Higher pressures reduce the tread's ability to conform to the road surface, reducing grip. Wider wheels require less pressure, increasing grip. Less reliance on static camber is important too, since with the stock suspension we're currently limited on our ability to adjust camber.
The wider wheel also provides a larger internal air volume. Especially during track use, this reduces temperature excursions, so the tire pressure remains closer to optimum more of the time. OEMs like air volume because it provides sound damping (reducing that tire 'pinggg' sound you hear with baloney skin-type fitments), but we don't really care about that in this instance.
It is true that, all else equal, a wider wheel is usually a heavier wheel, and unsprung mass is bad, m'kay? It's important to use quality wheels not only for safety reasons but also to minimize the addition of unsprung mass.
Wide tires will kill the steering!
Wider tires will tend to follow cambers more closely, as they put more tread width on the ground. We'll see how significant this really turns out to be. Past experience with more extreme changes (like Project Miata for example) than what we're planning with this car suggests it's something you can notice, but doesn't amount to a big deal.
More important when it comes to steering is to retain a reasonable / stockish scrub radius, which is why you don't want to drastically reduce the wheel offset compared to stock. Sure, small offsets push the wheels out more and look tougher but that's not a tradeoff I'm willing to make.
OMG aren't you going to have super skinny sidewalls and a huge rolling diameter at the same time which will make your gearing tall and your teeth fall out?
Aspect ratio is apparently a very misunderstood topic, as the earlier comments revealed. When I say that the new tires will (probably) be 245/40-17 rather than the stock 215/45-17s, the drop in aspect ratio (i.e., going from 45 to 40) does not automatically mean that there is any reduction in sidewall height. In fact, this change in tire size results in almost no change at all in sidewall height (and rolling diameter) compared to stock.
Remember, this is a ratio -- the stock sidewall height is 215mm times 0.45, or 96.8mm. The replacement tires' sidewalls are 245mm times 0.40, or 98mm. In terms of overall rolling diameter, this amounts to a difference of just a tenth of an inch; a negligible difference.
Jason Kavanagh, Engineering Editor