R-compound rules in street-legal race tire

There's a new tire out that is about as close to a gummy, sticky racing slick as any amateur race driver will ever get.

Its grip is so extreme that its cornering limits will never be fully explored on a street car with regular suspension settings, even when lapping on track day.

The Toyo Proxes R-888 replaces the RA1, the previous generation street-legal R-compound track tire.

It's available as small as 13-inch, but a more typical 16-inch tire costs about $240 after the standard 35 per cent discount. A 19-inch tire for a Porsche 911 will cost about $478 with the discount.

R-compound rubber used to be seen only rarely at track or autocross days. In the past five years, as auto suspensions grew sophisticated and more performance-oriented, R-class tires have become more common. Common to the point where some street-model Jaguar, Aston Martin, Ferrari and Porsche cars leave the factory gates shod with this almost disposable rubber.

This class of tire was developed to go racing and win in classes where cars were not allowed true racing slicks, and instead had to run on U.S. Department of Transport-approved street tires.

Since the strengths and weaknesses of race tires are basically the opposite of those for street tires (see box), marrying these two breeds posed a challenge, to say the least.

And so we have the R-compound tire. It has a stronger carcass than a race tire so it's less prone to going flat after running over debris. It operates and provides grip over a huge rubber temperature range (typically from 65C to over 120C, depending on brand).

Longevity is also much improved over the race tire. Most R compounds have a tread-wear rating of 60 to 80, comparing well with street ultra-high performance tires, with ratings of 140 to 180. Sedan all-season tires have ratings of 400 to 600. (The bigger the number, the longer the tread life; double the number means twice the tread life.)

So a well-cared-for R-compound tire can last several thousand kilometres if driven by a smooth driver or be used up in a day at the track if muscled around by an oaf.

In making R tires, companies have chosen where on the spectrum from full street to full race they want their product placed. Some opt for grip sooner as the tire warms; some take ride quality into consideration; some concern themselves with noise; some engineer tread patterns for wet conditions; and some just place their tire as close to the racing slick as it can get. This is the R-888, an extreme example of its genus.

This Toyo has so much stick, the car's suspension becomes the weak link in the grip equation. The R-888 allows the car to be the best it can be in terms of lateral grip and stopping power. It can make your arms, shoulders and neck ache from turning the wheel at G-loads never before experienced on a street-legal tire. Steering response is instantaneous. Unlike the RA1, this tire doesn't require a suspension alignment with a lot of negative camber. That promises a longer tread life.

But before track life can begin, the Toyo must be broken in by several hard-heat cycles. All tires, even the lowliest all-seasons, sweat out some manufacturing chemicals when they are new. Usually, a few hundred kilometres on the highway will clean them up.

This tire needs a full track day of several hours' use and some severe heat cycles to get any grip working. So you can write off your first track day as on-the-job training for you and your new tires. They will get a greasy feeling, and lack precision and grip. Don't worry: with every heat cycle they'll get better.

(A heat cycle involves starting with the tires at air temperature, getting them to a maximum tire temperature on-track, then letting them cool back down. The heat-up of the tire does not take long; the cooling takes forever.)

Overall, grip and tire stability seems to increase with every heat cycle – great. But so does noise – not so good. Noise is the only serious flaw with the R-888. I am not talking a bit of roar or buzz, but rather a heavy thrumming drone like on old WWII Lancaster bomber coming in for a landing.

On track, no one expects a tire to be quiet. Drivers want the feedback of noise to know how the tires are doing. However, on the drive home from the track, the growling drone sounds as if the differential is low on oil and about to meet its demise.

Interestingly, once the tires cool, the droning diminishes somewhat. On the subsequent drive to the track, the tires will be louder than the last drive to meet their heat cycle fate, and then on the drive home, they will be louder again than the previous time.

I'd expected perhaps this noise cycle would peak after a certain number of heat cycles, but so far that has not been so. The good news is the heat cycles only make this tire grip better. After six track days, I don't think this tire has reached its performance apex. It certainly hasn't shown any significant wear either. When hot, the tread rubber has the consistency and feel of a kitchen sponge. The rubber stretches but doesn't come off when pulled. That bodes well for longevity.

This Toyo has stiff-as-steel sidewalls – excellent for fast steering response and high cornering loads. Not so great when cruising on uneven roads. This effect is called tram-lining; the tire just wants to follow the road contours because it's so stiff. This effect, too, increases after each track day.

Wet conditions are always a concern with R-compounds. Generally, all have just suggestions of grooves to move water out from under the centre of the contact patch. Standing water and any serious speed are a bad combination.

However, these Toyos will attack a damp track with enthusiasm. Since their operating temperatures are 80C to 120C, they can slice across a damp track with élan, provided they are hot.

But there's the rub. Consider this: rain cools pavement, driving fast around corners in the rain means sliding off the track if the tires are cold, and getting the tires hot requires sliding around corners without sliding off the track. This is not an exercise for the faint of heart.

In summary, awesome grip in dry, better grip than average for R compounds in the wet, louder than a freight train when hot and good value on a per-kilometre basis. This is the ideal tire for those who drive to the track, lap for a day, drive home, then park the car until the next track day. But if you like to use your sports car as a daily driver between track days, this tire is not for you.

thetireguy_1@hotmail.com

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