I finally got the RA1s (255/40/17) and almost instantly I noticed that they felt a little more numb/squirmy than the A046s. I didn't get them shaved so the level of grip is probably not much more than the A046s either. Ran them on Sunday with around 36psi up front and around 34psi at the rear. Almost felt like I could have been equally fast with the A046s. What pressures do you guys run the RA1s, and for comparison what did you used to run for your A046s?

 

I thought RA1s aren't preferred for autox use?

 

With respect to RA1s -- I had a set of them as "every day" tires from June 2004 until January 2007, except for a few months on snow tires. (They lasted that long because I was putting very light mileage on the car.) So while I'm no expert, I am qualified to talk about them as a fairly happy user.

Unlike most R compounds, they seem to get better the more they wear down -- I suspect some of that is due to the tread squirm going away as the tread goes away. Still, my RA1s felt super grippy after 2 1/2 years right up until I corded them -- not something that you can say about Hoosiers.


The "slip angle curve" of RA1s is vastly different than Hoosiers or A046s. RA1s love to slip, and exhibit very forgiving breakaway behavior. You're not getting everything you can out of your RA1s unless you're "driving it like you stole it". Learning to trust RA1s in that "slip zone" takes a while, and is probably useless as "practice" for tires like Hoosiers -- I've always told people that RA1s aren't really a suitable "beginner" R compound as a first step toward "serious" autocross R compounds (Hoosiers/V710s) for this reason. (They're a good budget R compound tire if you want to make 1 set last for 2 seasons, and they're certainly capable of winning at the local level -- unless you've got a ringer in your class locally.)

As you mentioned, the tires do feel a bit numb -- less road feedback through the tires than some other tire models. That goes away a bit as the tread wears down.


My biggest complaint about the RA1s had nothing at all to do with the dry behavior, which was always grippy and predictable. It was the fact that they were basically useless in the rain even at about half of the original tread depth. The car became a dry-weather only car, which always seemed a bit silly for an AWD car.

 

I ran about 36 psi all around on my RA1s. However, that was on a Street Mod car with a ton of static negative camber, that might not be the sweet spot for an A Stock car.

 

What size of wheel and tire? Because I'm running a 255 on an 8 inch wheel I'm tempted to increase the pressure so that the tire doesn't flex too much. But then with more pressure comes a smaller contact patch. I didn't really think about it until someone pointed it out to me.

Contact Patch = Weight per tire / Tire pressure

So by running a similar or higher pressure on a wider tire, you're getting a wider but shorter contact patch.

Thanks for the info.

 

My RA-1 wheel/tire combo was "stock size" (235/45-17 on 17x8 wheels).


Also, talking about the shape of the contact patch... Realize that if you have insufficient negative camber (like most A Stock Evos), the contact patches of the outside loaded tires are decidedly "non-optimal"... Most of the load is on the outermost inch of the tire, and the sidewall is probably not really holding the shape you'd like. Adding pressure helps with both of those -- moves the contact patch just a slight bit inward toward the crown of the tire, and stiffens the sidewall.

I tested RA1s up into pressures over 50 psi, and at such high pressures, they cornered great mid-corner, but there were also some bad side effects -- grip in a straight line definitely suffers due to the small contact patch, so initial turn-in was compromised and braking in a straight line suffered as well.


One more thing that is overlooked sometimes when talking about Stock class air pressures -- if you don't have adjustable shocks or adjustable swaybars, the tire pressure is your only suspension adjustment. Adding pressure = adds spring rate. Lowering pressure = lowers spring rate. Don't forget this aspect; leave yourself a bit of room to adjust pressures to balance the car between front grip and rear grip.

 

Makes sense.

Indeed, and I seem to always prefer a slightly lower pressure than others. Maybe coz the RS is slightly lighter than a regular Evo

 

Actually, that formula is only accurate for tires with no sidewall strength. For real tires, the contact patch doesn't change as much with car weight or tire pressure. And for racing tires, with their much stiffer sidewalls, the change is even less. Finally, the extreme counter-example is run-flat tires, where the contact patch stops changing at all, well before you get down to 0 tire pressure.

I'm not saying that the increased pressure doesn't reduce the contact patch, it does, but not by as much as in the formula above. It's impossible to say whether the traction-lowering effect of the reduced contact patch is more or less important than the traction-increasing effect of less sidewall flexing. Best way to determine that is probably to run experiments using a skidpad and a G-force meter.

I have the same tire setup and I plan to do just that this weekend.

 

I see you point, but would think that a tire with a very stiff sidewall but running a low pressure would have most of the pressure applied to the ground very close to the walls, and the center of the tire wouldn't be exerting much force on the ground. It'll hold it's shape better, but I'm willing to bet that you'll see extensive tirewear at the edges. But, this is all my speculation.

Excellent. Let's try and keep this thread updated with our setups and results.

 

Okay, sure.

By the way, your intuition about most of the pressure being applied close to the tire walls when your pressure is too low, is right. The rule of thumb in autocross is that you know you have your pressure set for optimum traction when, after a run, the heat on the tire is even across the width of the tread. The heat is related to how hard that part of the tire was working, i.e. how hard it was pressing down on the pavement. So if you have too little pressure, your tire will be hot on the outside edges and cooler in the middle. Too much pressure, and the hottest point will be in the center. This is all because of the sidewall stiffness.

There's also the possibility that your tire will be hotter on the left side than the right side, due to not enough, or too much camber, but that's another story, and not something you can do anything about without camber adjustment.

 

Had my first event with this setup on Saturday . I ran between 35 and 37 psi. Must say that the increased level of grip is very nice Came in 2nd to an experienced driver running Kumhos, I was off by 0.5s on a 30 second course. I made a stupid mistake on my final run and had an off course, but I know I can get more speed out of these. Just have to tell my right foot to stay down and not ease off thinking the tires will lose grip through the corners. Looking forward to finding their limit, but I kinda wish I got them shaved.

 

Glad to hear you were so close to 1st place. I just did my second event yesterday, with my new setup (Stance coilovers + Kumho V700 255/40-17). At the first event, I was understeering, due to having too much rear camber (-1.6). I reset it down to -1.2, and now the car can be made neutral by adjusting tire pressures. (I'm at -2.5 camber up front, by the way.)

On the first run, I tried 33F/38R and the car was way too loose (this may also have been because the tires were cold and didn't have enough traction). On the next two runs I moved to 34F/37R and it was pretty well balanced. Would have loved to get a few more runs, but this being our overcrowded SCCA club at Devens, three runs was all we got. I think I took 2nd place in BSP, but the Evo who beat me was 3 secs ahead of me (on an 80 sec course). He has Kumho V710s, 275/40-17, clearly a better tire size. Also he has the standard stage 1 power mods. I can't decide which I need to do first -- new wheels and tires, or a power upgrade.

 

Was this the NER event on Sunday? My friend ran his Evo in A-stock. Said the course was nice and long. I have to come up and try it sometime.

 

i ran 40 up front, 44 in the rear when i had the same exact sized ra1s, anything under 40 i would always have rubbing on the side-walls, on a road course i had them at 32psi front 34 psi rear and they would heat up pretty nicely tad over 40 when it got warmer

 

What's up dude, did you move yet?

I didn't notice any rubbing, could you hear it on yours or did you have to inspect the inside of the fender?