R-compound tire recommendation?

[Fair!]

I have a couple comments but no real answer to your question. If you put on tires with a lot of grip, you'll generate a more body roll in the corners, more dive on braking and more squat on acceleration, than you had using tires with less grip. If you get a lot of lean/dive/squat, the contact patch will become less than optimal. You may be able to make camber changes as a workaround for cornering but you can only go so far in the front before you'll start to mess up your braking. And if you have a lot of power, too much negative camber in the rear will impact traction on acceleration. Stiffer sway bars will help control roll; stiffer springs will help control roll, dive and squat. Stiffer springs may also mean you need to alter the damping (shocks) if you want to avoid bouncing around and losing traction. It's can be a vicious/expensive circle.

You also need to consider what you are doing. AutoX needs tires which are consistent from run to run. Track tires require a significant workout in order to get warmed up to the point where they work properly. Consequently track tires tend to change a lot from run to run at autox's so they may not be your best bet. Something like a Toyo RA1 or the very similar Nitto NT01, might be a good compromise. There are some very sticky/soft tires, Hoosier makes some and so does Kuhmo which might be better for autox but it might depend on how close together your runs are.

Big tires are not always the best tires. Bigger tires are heavier than small ones so they add rotational mass and this reduces acceleration (and deceleration). Bigger and/or wider rims tend to do the same thing. If you have enough torque to spin up heavier tires/wheels and enough brake to slow them down, go for it. But remember that the rubber needs to stay properly planted on the the road surface in order to work effectively.

Heavier tire/rims also increase unsprung weight. A big increase in unsprung weight can have an pretty nasty impact on your car's handling if the course is bumpy.

In a perfect world, more/stickier rubber would always mean faster times but it doesn't work that way. It's all about finding the right compromise.

Hmm... I agree with most of what you wrote but not all. I've heard the same arguments that "too much camber can negatively affect braking", and on paper it makes sense... but it just doesn't seem to pan out when we have tested this. We've data logged hundreds of runs over 3 years of competitive events (25+ separate events and 3+ drivers per year) on our E36 M3 and we ran camber from -2.5 to -4.1° up front (and ever wider tires, from 245->255->265->275mm)... and the car just got faster and faster and still peaked at 1.1g braking and held 1.0g constant... with 140 treadwear "ST" class tires.

Camber works. On McStrut cars, camber is crucial. "As much as you can get" is usually the right amount. :D

As we test similar set-ups on softer R compound and wider overall tires (275->285->305->315mm, in various Kumho/Hoosier/Hankook R compound flavors) we have bumped camber up over the -5° mark... with an increase in grip and even better tire wear. Similarly, we run a lot of rear camber as well (more grip -> more lean -> more need for camber) and the car "hooks up well" (315mm wide Hoosiers).

And wider tires do tend to always add more grip, if you can keep them on a wide enough wheel. Too many folks stuff gigantic tires on too narrow wheels for autocross (esp. Stock class, yuck!) and this rarely adds significant or any grip over a properly sized tire on the same wheel. Rotational inertia, quicker heatup, proper carcass control, etc... but if you keep adding tire and wheel width the car gets faster and faster. I don't know if there's a real limit... we run 315mm wide R compounds on 11" wide wheels on our XP car and the lateral grip is "painful". I would love to try them on 12" wheels, too. Sure, there has got to be a point of diminishing returns somewhere.... somewhere...


Left: 18x10s with 305/30/18 Hoosiers. Right: 17x11s with 315/35/17 Hoosiers. At these grip levels its easy to overpower the spring rate, swaybar effectiveness, etc. We've since added more droop travel to the struts and shocks as well as stiffened spring rates significantly.

I've gone to great lengths to minimize track width, because this can negatively affect slalom speeds and hurt you on courses with tight gate widths - every 6" of additional offset added .1 sec per slalom gate, in tests we've done. For our class we can modify inner fender sheet metal and it helped to move the wheel/tire package inboard a bit here, a bit there. Every little bit helps. For road racing the tendency is to widen the track to increase lateral grip, which works, but remember: a wider track also worsens aero drag. The F1 teams all went faster (broke every track record) the year after the FIA restricted the formula from 18" to 15" rear tires... in the hopes to slow them down by removing track width (and grip) they did just the opposite, all due to aero: The biggest gains were on the fastest tracks.

Also, anyone that chooses to run Toyo RA1s or Toyo NT01s over a Hoosier A6/R6 or Kumho V710 is choosing to go slower. A lot slower. Solo, road race, whatever - these harder tires are just downright slow. We've made equal grip data on a stinkin Falken RT615 "street tires" compared to a Toyo RA1 "R compound", same car, same wheels, same surface, same temps, from data logging. The Toyos had significantly worse wear, though. Since we've run on them for many years (way back in the mid 1990s), and have tested them against many other tires, I am not afraid to say this: Toyo RA1s SUCK.

Sure, the (ancient) Toyo RA1 type "mid-level" R compounds tend to last longer than a "top tier" R, and for a D.E. car where "you aren't really racing" they are great, and better/faster than most street tires. Or in spec classes, where everyone is on the same hard/slow tire, good stuff. But for outright speed : softer compound FTW.

Just my $.02. :)

[thull]

I think you are right in that slicks are the way to go for track use, but if you're on the street they aren't an option. For a compromise the RA1 is a great choice considering its wear rate, wet traction, and low cost.

Saying that they suck is a little overboard, IMO, for their intended purpose they are a great tire.

I had a set that lasted me 3 years and over 10k miles including a few DEs. It wasn't until last week that I had to replace the RA1s and I chose the new Toyo R888s. So far I like them but they are barely scrubbed in.

BTW - I have your camber plates in my garage ready to go on the car and I am looking forward to maxing them out :D

[whiltebeitel]

Yeah, I'm running RA-1 scrubs at AX, and they barely got warm with a co-driver a few weeks ago here in Texas. They are better than street tires, but they make poor AX tires.

[Fair!]

Yeah, I'm running RA-1 scrubs at AX, and they barely got warm with a co-driver a few weeks ago here in Texas. They are better than street tires, but they make poor AX tires.

Agreed. Again, we've tested them side by side with things like Falken 615s and they were nearly identical. A good, low cost, fun track tire with decent grip... or a good spec series tire that will last a long long time.

Just not an extremely competitive and fast autocross or track tire compared to 1st tier R compounds (Kumho, Hoosier, or even Hankook). But if you aren NOT out trying to set a lap record, or win your autocross class, Toyo's are all good.