DOT R Compound Tires
DOT R compound tires, usually called just R compound tires or R compounds, are tires made to comply with DOT requirements with a very soft tread compound.
A bit of history will help understand their existence and explain their name: A long time ago there were two types of tires - street tires and race tires. Street tires used hard tread compounds designed to last for tens of thousands of miles, had tread pattern and many voids in the tread to evacuate water, and offered relatively low grip. Race tires were slicks for dry applications and as such had no tread pattern, and came with much softer tread compounds lasting 1000 miles or less. The cost of running race tires could easily have been 10x or more the cost of running street tires. Naturally, race tires had a lot more grip than street tires.
To contain costs some sanctioning bodies (like SCCA) instituted rules requiring that only street tires be used in certain classes of competition. The definition of a "street tire" was one that was DOT approved, as all tires used on public roads must be DOT approved.
Tire manufacturers, of course, also compete with one another and it did not take long for them to start looking at DOT requirements very closely to see how they might produce a street tire with more grip than other street tires had. The result were "R compound" tires - tires that look like street tires but have a much softer tread compound, often with less void area for water evacuation, designed to have much more grip.
Dry Vs Wet
R compound tires come in three varieties:
- Dry and wet
Tires designed for dry use have minimal tread pattern. The historical standard was to have two grooves running along the circumerence of the tire, however BFGoodrich and Hoosier are now making tires with a dashed line for the pattern and Hoosier's does not even run the entire circumference. This lack of tread pattern means the tire has more rubber in contact with the pavement which increases grip. Obviously, lacking a tread pattern the dry tires are unable to evacuate water, and do not perform well on wet surfaces. Examples of dry surface R compounds are Toyo RR, Hoosier A7/R7 and BFGoodrich g-Force R1.
Some R compound tires like Toyo RA-1, Toyo R888 and Nitto NT-01 are made with a real tread pattern and with voids in the tread to evacuate water. At full tread these tires then are suitable for wet weather driving. Unlike street tires, on most R compound tires the tread pattern is not present through the full depth of tread - when the tire gets worn down to 2/32" to 4/32" depth (depending on the tire), only the two circumferential grooves remain. This permits using the mixed dry/wet R compounds in dry competition when the tires are shaved to the circumferential grooves, but has the flip side that the tire is only suitable for wet weather driving when it is close to full tread. Naturally, the ability of a tire to evacuate water diminishes as its tread depth gets reduced due to wear.
Interestingly, the dry/wet R compounds often have harder tread compound than both full dry and full rain tires. This is because having the softer compounds in blocks would produce too much deflection, causing heat build-up and overheating of the tire in dry conditions. This affects both dry and wet grip of these compromise tires - in wet conditions a softer tread with more deflection would be faster, and in dry conditions lack of tread with a softer rubber would also be faster.
Finally, full wet R compound tires have the softest tread of them all combined with many blocks/void channels for evacuating water. Due to the softness of the tread compound and the large void space the full wets have incredible wet grip. The trade-off is that these tires tend to blister and wear out extremely rapidly when they are not running on water to cool them down.
R compound tires are positioned between street tires and full race tires in cost, longevity and grip. Full race tires are often designed to last a single event, race or even a session. At professional motorsport level (i.e. televised races) when announcers are talking about tire degradation over the course of a 45 minute race, the tires are designed to last maybe an hour or 100-150 miles. Full race tires are even softer than R compounds, can have less tread depth when new, and are not built to withstand multiple heat cycles.
A tire wears out in two ways: tread depth and heat cycle count. Tread depth reduction is obvious, while heat cycle wear is something that affects R compound tires and can become the dominating factor in race tire wear.
When driving on track, tires are running at about 150-250°F. Going from the 50-80°F ambient temperature to the track temperature and back constitutes a heat cycle. Generally speaking, every track session counts as a heat cycle.
Each heat cycle hardens the rubber in a tire, thus reducing the tire's grip. This effect is most pronounced in softer tires like Hoosier A7 and non-DOT race tires. These tires often have significant drop-off in performance throughout their first 5 heat cycles, and beyond the 5th heat cycle their performance is poor enough that the tire becomes useless for competition.
Harder, more durable tires like Toyo RR or Nitto NT-01 also lose some performance with each heat cycle but the delta per heat cycle is less and it takes more heat cycles - on the order of 10-30 - to lose enough grip to where the tire becomes uncompetitive.
Heat cycling is why racers often put brand new tires on for qualifying - this is a single session when tenths and hundredths of a second can make a difference, much more than they can in a race. Racers with a larger budget will run brand new tires in qualifying and for each race to give themselves an edge at the start of the races. Obviously, using up multiple sets of tires in a weekend is not cheap.
For a typical HPDE driver, the important thing to keep in mind is that a heat cycle equals a session, not an event. In a typical HPDE weekend with 4 sessions per day the tires will get 8 heat cycles put on them. A Hoosier tire on a DE car generally has nowhere near the performance of the same tire on a race car ready to go on track to race.
Full Tread R Compounds In Dry
Recall that the "dry/wet" R compounds are necessarily compromise tires. Some tires, notably Toyo RA-1, are not recommended to use in dry competition. This is because such tires have smaller tread blocks (with a corresponding increase in void area to help evacuate water) which would deflect more and hence generate more heat.
Can full tread RA-1s be used in the dry? This depends primarily on the driver's aggression level and the driving style. Full tread RA-1s have much less grip than shaved RA-1s in the dry, because there is a lot less rubber in contact with the pavement and because the tread blocks deflect, again reducing the amount of rubber touching the pavement. A driver used to the dry grip level of shaved RA-1s will be disappointed with the grip level of full tread RA-1s in the same conditions. Driving full tread RA-1s aggressively in the dry will wear them rather quickly, however by the time they get to 4/32" they will already have several heat cycles on them and hence will not be as quick as brand new RA-1s shaved to 4/32".
That said, a driver with an easy or moderately aggressive driving style who does not slide the car much can get away with running full tread RA-1s in the dry. Naturally such driver will not be as quick but running RA-1s instead of street tires can offer more heat resistance as well as higher grip level, leading to more enjoyment at the track.
Cold Weather Operation
R compound tires are intended to be used in warm weather (let's say above 50°F). They are designed to operate with a tread temperature of somewhere in the 150-250°F range, and with surface temperatures below 50°F the tire generally cannot achieve such tread temperature.
When R compound tires are operating at less than their ideal tread temperature, they have less grip. Exactly how much grip they have depends on the exact temperature, tire compound and the number of heat cycles that the tires have gone through. Softer R compound tires (the "autocross" compounds for instance, as well as soft non-DOT slicks) generally have more grip in colder weather because the tread, being softer, deforms more and hence attains a higher temperature compared to a harder compound tire. Tires which have gone through many heat cycles have hardened rubber which deforms less, hence gains less heat at the same ambient temperature, therefore becoming less grippy than the same tire with fewer heat cycles.
Even though grip of R compound tires is reduced in ambient temperatures below 50°F, the tires will often work at temperatures down to 30°F and possibly even 20°F. In very cold weather like this in my experience heat cycle count makes the most difference followed by tire compound. For example, fresh Toyo RR will work OK at 40°F and will be usable at 30°F; a well worn Hoosier A6 will struggle even at 40°F and will be barely usable at 30°F.
The key to driving R compound tires in cold weather is to allow time for the tires to come up to temperature, put enough energy into them to get them to warm up, and recognize that their grip level is going to be reduced regardless. The second point here is very important - the car needs to be driven aggressively enough to put substantial loads, and therefore heat, into the tire, otherwise the tire will never grip.
The combination of cold weather and wet track surface makes many R compound tires, especially slicks and double so slicks with a high heat cycle count have dangerously low grip levels and extremely sudden breakaway. This is again because moisture on the surface cools the tires and the tires are barely getting enough heat in them as it is.
Cold Weather Cracking
Tire manufacturers warn against storing R compound tires in near-freezing or freezing temperatures, as the rubber may crack. From my experience and confirmed by several other track day participants, it seems that R compound tires are likely to crack when they are stored in near-freezing or freezing temperatures for extended periods of time, especially if they are mounted on a vehicle and are therefore bearing the vehicle's weight. The same tires seem much less likely to crack if they are being operated in near-freezing ambient temperature, or are briefly stored in freezing temperatures when not mounted on a car.