Toyo RA-1 Tire Review
This tire review is for Toyo RA-1 in the "post-R888 era". If there are grip and wear differences between the original RA-1 and the current RA-1 as some claim, I have not personally experienced them and thus will not speak to them.
The tires were tested on NA/NB and NC Miatas.
Applications & Tread Depth
RA-1, depending on its tread depth, can serve in one of three applications: full wet, moist and dry. This makes it unique among R compound tires, but has some downsides too as we'll see in a moment.
RA-1 starts out with 8/32" of tread, and the outer 4/32" are composed of independent shouder blocks. From 8/32" to 6/32" of tread RA-1 is a solid full wet competition tire.
From 6/32" to 4/32" the tire is suitable for intermediate wet or moist conditions. At 4/32" and below RA-1 is a dry tire, with performance slightly increasing as the tread depth is decreasing assuming the same number of heat cycles.
Dry grip of RA-1s depends primarily on how much tread the tire has.
At 4/32" and below the tire is a semi-slick with multiple circumferential grooves. These grooves are larger than those in Toyo RR or Hoosier R7/A7/SM7 and there are more of them on the RA-1, giving RA-1 somewhat lower grip than Toyo RR and consequently significantly lower grip than Hoosier offerings.
Even below 4/32" RA-1 has its tread organized into relatively narrow strips, making the tire more susceptible to tread squirm than other R compound tires. Shaving RA-1 down to 3/32", 2/32" or even 1/32" reduces the height of the tread bands thus reducing the squirm, and makes the tire faster.
At full tread depth of 8/32", RA-1 has less rubber in contact with the pavement due to the top layer consisting of independent blocks, making the tire slower and squirmier.
At 4/32" and below, RA-1 seems quite similar to RR conceptually; the RA-1 drives like a narrower RR. This applies to both lateral and forward grip.
At 8/32", RA-1 are somewhere in between a street tire and a (semi-)slick R compound. RA-1 have less grip and more rollover, but remain progressive and controllable.
RA-1 deals fairly well with braking, accounting for it having less rubber in contact with the pavement than (semi-) slick R compounds. There is less braking grip compared to other R compounds but RA-1 is well-behaved at the limit in non-ABS cars.
At 4/32" and below RA-1 has reasonable lateral grip. It's not as good as that of RR/Hoosiers and is generally on par with NT-01/RC-1 of the same size. RA-1 is fairly sensitive to camber, especially in the front; having insufficient camber makes the tire roll over more and RA-1 is already susceptible to rollover due to squirmier tread.
RA-1 is not as responsive as Toyo RR. Some of it undoubtedly has to do with tread squirm especially at higher tread depths. I do not have enough experience with RA-1 yet to positively say that its sidewall is softer or as stiff as RR's sidewall.
The practical consequence of this is that coming from RRs to RA-1s the turn in points need to be moved slightly earlier to compensate for delayed steering response.
At 8/32" and low heat cycle count, RA-1 does very well in rain. It is not as sticky as new Hoosier rains or new non-DOT rain tires, but provides much more grip compared to street tires or slicks.
Full tread RA-1s have excellent water evacuation properties - the tires maintain contact with the pavement while driving over small puddles, maintaining most of the steering and braking control over them. When driving over large areas of water the tires still maintain some contact with the pavement, allowing the car to turn, brake and accelerate over water.
At 3/4 tread depth, RA-1 begin to lose contact with the pavement when driven over standing or running water. In most cases the loss of contact is momentary until the car exits the water: when hitting the water with the front tires and some steering angle in the car the car will plow straight for a second then resume the turn, and when htiting the water with the rear tires the car would yaw but would be easy to bring back. The tires exhibit a progressive breakaway on both front and rear ends of the car, making the car relatively easy to drive near the limit. The tires also recover quickly once the car exits the water and returns to (wet) pavement.
As the tires approach half tread depth and the independent tread blocks are disappearing, even small differences in tread depth can make a large change in how the car handles. At Barber, after driving on the RA-1s for about 200 miles in one weekend, I found that having 1/32" less tread depth on the front tires compared to the rear tires made the car terminally understeer in corner entries and almost impossible to get into the corners. Rotating the tires front to rear made the car a bit more tail happy than I would prefer but it became capable of turning in again.
At 1/2 tread depth the independent tread blocks are gone, replaced by the 5 ribs. At this point RA-1 would still do decently well in wet conditions with minimal standing water but would struggle in extended water areas like large lakes. The loss of independent tread blocks, besides reducing the amount of void area in the tread, also reduces the ability of the tread to flex and bite the pavement. Unlike a full race rain tire like Hoosier H2O that has the tread blocks running the entire depth of tread, and is thus usable as a rain tire for more of its life, RA-1 are usable as full rain tires from full tread to 3/4 tread and as an intermediate rain tire to 1/2 tread. At 1/2 tread and below RA-1 are best suited to dry conditions.
At 4/32" and below, the tire has decent wear but not as good as that of Toyo RR. One reason for this is the shoulders of RA-1 continue to have tread blocks even past the 4/32" tread depth, making the shoulders in particular run relatively hotter and thus wear faster.
The biggest factor in dry wear is how hard the tire is driven, and consequently how hot the tread gets. In competitive scenarios with hard driving the wear of RA-1 is worse than that of Toyo RR and possibly on par with Hoosiers; in HPDE scenarios where the car is driven at 7-8/10ths most of the time with occasional bouts into 8.5/10ths range the tires will last much longer. Heat cycled out RA-1s last a very long time, although at that point they do not perform very well.
At more than 4/32" of tread the same relationship holds between tire temperature and longevity, except higher temperatures are attainable due to increased tread squirm. Aggressive driving on RA-1 with 6-8/32" of tread results in rapid tire wear; at the same time it is very possible to street drive and run HPDE events with full tread RA-1s in the dry as long as they are not driven as hard.
RA-1 wear very slowly in wet conditions, being a relatively hard compound tire. A full tread RA-1 used in wet weather only will last a long time.
The primary concerns with full tread RA-1s used only in the wet is them aging and heat cycling out. If the tires do get used fairly often, they still accumulate heat cycles without wearing the tread down; eventually they will start to lose grip. Tires that are not used often will nonetheless slowly lose grip over time in storage, especially in winter if not stored at room temperature.
Price & Value
When used in dry conditions competitively, RA-1 must be shaved to 4/32" or below which makes it a rather expensive tire to run. It is generally more expensive than all other Toyo and Nitto offerings as well as Hoosier SM6/SM7. Hoosier R6/R7/A7/A6 and non-DOT slicks do cost more than RA-1.
When used in dry conditions non-competitively, RA-1 can be used as a track tire as well as a street tire for short trips to and from the track. In this application RA-1 is similar to NT-01, with RA-1 having smaller and more flexible shoulder blocks at full tread making it more suitable for street driving in wet conditions but also making the tire easier to overheat on a dry track.
When used in wet conditions competitively, I expect RA-1 to be useful over a longer time compared to Hoosier H2O and Hankook non-DOT rains as RA-1 retain their grip better both in face of heat cycles and in face of age. RA-1 also can be used as a practice tire in the dry when they get worn down to 4/32" or get heat cycled out at higher tread depth, as again the longevity of the tire is a function of tread temperature and a heat cycled out tire will run at a lower tread temperature.