Tire Testing

If I was going to pick one tire to use for everything, I’d use the Hankook RS4. It’s not the fastest, but it’s fast enough, decent in the rain, has great feedback, lasts forever, and is available in three 15” Miata sizes. The 195 is really a 205 width, and a good choice for Miatas on stock power and skinny wheels. The 225 is an ideal choice for most upgraded Miatas on 8-9″ wheels. The 245 is good for tight courses and for more powerful cars on 9-10″ wheels. This is my favorite tire and I see no reason to do HPDEs or endurance race on anything else.

And yet… I keep buying different tires! Some of this is wanderlust, to just see what else is out there. Some of this is bargain hunting: I got Yokohama S.Drives on closeout and was out the door for $200 mounted and balanced; I bought 225 Maxxis RC1s on closeout for $108 each, with free shipping; I’ve bought Douglas all-season tires for $36 for sliding around and training. And then I’ll stumble across cheap take-off slicks that are priced so well that I don’t have a choice.

As a result, I have 36 tires in my garage right now, and 28 of them are mounted. After upgrading my NA6 to NB Sport brakes, I gave up on 14″ wheels and so all seven sets are 15″ wheels. One set of wheels is 7″ wide and has street tires on it, all the others are 8-9″ wide, with track rubber. Shockingly, only two sets of wheels have RS4s, the rest are rotated regularly for tire testing.

Tire testing is a hobby, but also something of a responsibility. I wrote the rules for the Pineview Challenge Cup, and I rank every tire individually. Some of that ranking comes from online tire reviews, but a lot comes from my personal testing. The primary way I evaluate tires is to look at the lateral Gs in Turn 2 at Pineview Run, a long right hander.

Turn 2 for tire testing

I export the data to a CSV file, and then average the lateral Gs through 200 feet of that corner. This removes the peaks and valleys from the 10hz GPS data, and also idiosyncrasies of driving style and line, and gives me a solid number I can use to compare to other tires.

Tires that I have personally tested and have the lateral G data for are the following: Achilles ATR Sport 2, BFG Rival 1.5 S, Bridgestone RE71R, Champiro SX2, Continental ECS, Douglas all-season, Dunlop DZ102, Falken 615K+ and RT660, Hankook RS4 and Z214, Hoosier A7, R7, and SM7, Maxxis RC1 and VR1, Nexen N’Fera Sur4G, Nitto NT01, Pirelli PZero PZ4, Toyo RR, and Yokohama S.Drive. I’m sure I’m forgetting some, but you get the idea. I often put one of my Aim Solos in other people’s cars, and the list of tires that I have lateral G data for, but didn’t personally drive, is about twice that many.

I don’t know of anyone else who has such an extensive database of tire grip, but then I wouldn’t expect to! It takes a lot of time, money, and effort to gather and collate this data, so this is the kind of thing one keeps to themselves. Or at least it’s not something that’s shared with other people without some means of compensation.

Tire Test: Accelera 651 Sport

I recently tested the Accelera 651 Sport, and I’m not shy about sharing this data, because it’s not a competition tire. Most of the people using this tire are drifting, and even though the treadwear carries a 200 UTQG rating, this is a mid-300 TW tire, grip wise.

I’ve driven on the 651 Sport previously, but in a Honda minivan, in a 24 Hours of Lemons race. That car is brutal on tires, and I didn’t feel it was a fair test of this tire. You can read that report over here, but the gist of it is that the 651s were fast in a straight line, but lacked cornering grip compared to the N Fera Sur4G.

The reason I hadn’t previously tested the 651s on a Miata is because Accelera only made them in a 195 width. Given that these tires run narrow to begin with, I wasn’t interested. But starting this summer Accelera started offering the two most popular Miata sizes: 205/50-15 and 225/45-15. I ordered a set of 225s as soon as I’d heard about them.

I had the tires mounted at Shade Tree Auto, my favorite car mechanics in Ithaca. Jack, the owner, races in Champcar, and his team regularly kicks my team’s ass, and so I know that he knows what he’s doing. Jack did my alignment at the same time, and when he was finished called me to report something very strange going on. The car pulled to the right on acceleration. He knew it wasn’t the alignment, and suspect diff bushings, a worn out Torsen, or something else in the drivetrain.

I picked up the car, drove it home, and noticed the same thing. On the gas, and on the brakes, the car would veer to the right. At 80 mph and on the gas, the car felt unsafe.

I really didn’t want to take my rear end apart, but I had to see what was going on. When I pulled the tires off, I set them up against the wall instead of stacking them on top of each other, as usual. I don’t know why I even looked, but I noticed they were different heights. Two of the tires were about 1/4” taller than two others, weird. I installed two of the tires with the tread facing backwards, so that I could get the same size diameters on the front and rear, and viola, the on-throttle steering returned to neutral!

I got two tires flipped on the rim so that my tread pattern is all going the same way now, and now as long as I run the larger ones on the rear (or front), the car tracks perfectly. I contacted the tire importer Tire Streets to tell them about this, and they rectified the situation with a new set of tires. They didn’t have the 225s in stock, so I got 205s instead, but in the 100 TW compound instead of 200. A+ for customer service, even if it did take several pictures and measurements before they believed me. The replacement tires are in a heated basement waiting for next year and another tire test.

I had no idea that 1/4” in diameter would have this kind of effect. I posted about this on the HPDRE group on Facebook and found that other people have had similar problems with different brands of tires, even well known name brands! FWIW, three of the tires all had the same date codes, one was different, but two of them measured undersized.

If I can pass along this one piece of knowledge – measure the diameter of your tires – and keep someone else from disassembling their car looking for answers, then my work is done here.

When mounted on 15×8 wheels, a stack of four 225 Acceleras measured exactly the same height as a stack of 225 Maxxis RC1s. The 651s I drove on the minivan were narrower by comparison, and so perhaps new Accelera sizing is more in line with other tires now?

My initial plan was to use these tires in a Lemons race, but that plan when to shit, quite literally. My first chance to test the tires was at the final Pineview Challenge Cup race. I hadn’t driven in 14 weeks, and still not feeling 100%, but I figured that getting back on track might kickstart my system.

My first few laps with the tires were tentative, and I initially thought the 651s might be similar to all-seasons. But I think it was just cobwebs, since I got better and better as the day went on. The time trial race is just three laps per session, one warm up, and then two hot laps. In my first run I barely broke a 1:20. My second run I did a low 19, and started feeling more myself. In the final run I did a 1:18.6, and that felt like a pretty good lap. I initially set the tires to 28 psi cold, but after checking the pyrometer, added 4 psi all around to bring the center of the tire up, and that helped lap times as well.

The tires felt really good, with great audible feedback and predictable breakaway. They are a little vague on turn in, but are responsive to mid-corner steering and throttle inputs. I was a little disappointed in the lap times, being 1 second off the time I set using Continental ExtremeContact Sport (ECS). But like I said, I’ve been out practice for a bit, and I’m comparing to a different day, with different conditions.

So let’s look at the data. The following image is lateral Gs comparing four laps on Conti ECS (red) vs four laps on Accelera 651 Sport (blue).

Lateral Gs – 651 (blue) vs ECS (red)
  • A – This is Turn 2, where I do most of my lateral G data gathering. If I went by peak lateral Gs, then the 651s would win, and this is why I average the Gs over 200′ of distance
  • B – This is a right/left going down the hill, and it’s not a peak G corner, but it’s a good measure of driver confidence. On average, the ECS are giving me a bit more confidence.
  • C – This is T11, also called the Knuckle, a long left hander. You can see red and blue overlap quite a bit here, and lateral Gs are just about the same.
  • D – This is the Blind Hairpin, a cambered 180 with a downhill braking zone that requires a lot of turn-in confidence. In this section, the Conti ECS has more grip.
  • F – This is the S-trap, a super tight right-left switchback that loads the tires a lot. Again, a very slight advantage to the ECS.

Now this wasn’t a back-to-back comparison; track conditions were different both days. The wheels were different as well. The 205/50-15 Contis were mounted on 15×6.5 wheels, while the 225/40-15 Acceleras were mounted on 15x8s. If the Conti was available in a 225, and it was on a 8″ wheel, I think the Conti would be a clear winner, but as it is, there’s not much to choose between them.

While I haven’t tested the Accelera 651 Sport for longevity, they are so similar to the Conti ECS I bet they are the same in that way as well. Like the ECS and many other 300 TW tires, the 651s have three center grooves, and I would guess they make a good rain tire. All in all, I think the Accelera 651 Sport is a solid alternative to the Continental ECS, Firestone Firehawk Indy 500, Michelin Pilot SuperSport, PS4S, and other 300-340 TW tires. The 651s are a step above the older 300 TW summer tires like Yokohama S.Drives, which Miata people generally liked.

I guess it all comes down to this: for those situations when I don’t care about lap times, and just want a playful and responsive tire with great feedback, I would be happy to drive these tires all the time. I’ll even go on board to say they are my second favorite tire right now.

Accelera tires are imported by Tire Streets, and have a unique 30-day money-back guarantee. They are priced well, and are a bargain in the larger sizes. I plan to use up this set as a dual-duty tire, and then mount the Accelera 651 Sport Xtra tires I got in exchange. These carry a 100 UTQG rating, but based on the 200 TW 651 Sport being more like a 300 TW, I’m betting the Sport Xtra 100 TW is more like an actual 200 TW. But I’ll only know that after I flog them through Turn 2.

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