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I must have missed that GL copied NASA again and allows the RR as a 100tw. Why don't more people run 245/40/15 RR?
It is a new-ish thing for NASA (because the year 2020 doesn't exist) and you still get murdered if someone that can drive in a prepped car shows up on Hoosiers, at least with NASA points.
I can get the RR shipped for about the same as the AR1 shipped.
I'm in for it and running my theory crafted ~2251lbs at -1% modifier.
275 is way too much tire at GLTC lbs/hp & 2251 lbs. On high speed tracks, a 205 on 8" wheel will be faster. Everywhere else, 225 on 9".
You need enough power to maintain the optimum slip angle at all four contact patches in 80-110mph turns. If not, you're just pushing around extra rolling and aero drag. We have a lot of experience in this area. When in doubt, run the smaller tire.
So you are saying a 205 AO52 on an 8 is the right tire for COTA? A lot of people run it on a 9 and say it works out better.
Id think a 225 r7 is best suited to a 9.5 wheel.
Ideal contact patch to weight ratio is power and track dependent. I don't have a formula, just experience putting too much tire on a more than once over the years.
But yeah, I could see a 205 being faster overall at COTA on a 2150-2300# Miata without a lot of power.
Again, your go/no-go should be being able to sustain optimum slip angle at all four tires above 80mph. If if there is too much grip for the available power, add power or reduce grip. Most tracks will have avg speeds between 85-100mph for 130-200whp Miatas.
being able to sustain optimum slip angle at all four tires above 80mph. If if there is too much grip for the available power, add power or reduce grip
Im trying to wrap my head around this. There is always going to be a grip limit. The more grip, the faster you go until you hit that limit. Once you reach the limit, the car will exhibit slip angle in some way.
So why would I want to reduce the grip of the car? Wouldn't that just make the point of slip angle much lower?
Im trying to wrap my head around this. There is always going to be a grip limit. The more grip, the faster you go until you hit that limit. Once you reach the limit, the car will exhibit slip angle in some way.
So why would I want to reduce the grip of the car? Wouldn't that just make the point of slip angle much lower?
Not speaking for E, but it's the trade-off. Assuming the same compound, if your min speeds thru the 80-100mph turns is in a small percentage delta between the two sizes, but the percentage delta on the straights is much larger between the two sizes.
Then there are compound differences, rules, track, weight of race, type of race, yada yada. I wouldn't be surprised if your car with 205 AO52s with GLTC rules would be it's fastest setup for a race at COTA.
Also not speaking for him, but if you've got the whole car slipping you need less steering input (which is effectively a brake). If you don't have enough power to keep the rear tires slipping from the initial entrance then you end up pushing the front.
I was under the impression he was talking about car dynamics in cornering specifically, and not the tradeoff in straight line speeds. I would be surprised if an a052 is the right tire given the car will need to be run at around 2500lbs. I would imagine it would get greasy real quick, but its all speculation. I have no real data to say for sure.
Any tire, be it slick, DOT, radial or bias will generate it's greatest lateral acceleration (cornering speed) at a certain slip angle, not rolling without slip.
That slip angle generates drag, which requires power to overcome.
If your car does not have enough power to sustain that slip angle at all four contact patches at the typical min speeds on your track, then you simply have too much grip for your power.
Thought exercise:
Miata that can just barely sustain a four wheel drift at 85mph min speed on a 205 tire, 3° slip angle say, which is ideal for that particular tire.
Bump to a 225 tire which the car no longer has enough power to slide and the min cornering speed could actually be lower. Add to that lower speeds on the straights and a general difficulty in adjusting cornering attitude with weight transfer because you can't make the tires slide.
This isn't conjecture, it's a simple logic problem.
Auto-x cars can survive being over tired because their avg min speeds are like 45mph where torque multiplication allows a low powered car to sustain slip angle. They also rely on transient slip angle meaning, they go into a turn a bit too fast generating initial slip angle (max G), car begins to bog but before it loses too much speed the driver changes direction and begins the process again. Road courses have longer duration turns and aero drag.
Back to the original point, if you can't sustain some slip angle, you have too much tire for your power/weight.
Keep in mind, lap times are made up in the fast turns, not the slow ones. So the most critical turns are the faster and longer duration turns.
I would pick whatever tire has you at limit of adhesion around 16 through 18. If you have so much tire that you're flat with ease, then you brought too much tire for COTA imo (in keeping with what Emilio is talking about). FWIW my car in full TTE trim (2436lbs / 136whp) was easy flat around that bit of track on sticker 205 R7 and it's apparnetly higher grip now since repave.
The closest thing to that section of track is Sonoma t8-10. My car feels really good through there in its current setup with a slight tendency to push. The only time it wants to ride rear slip angle is low speed corners.
I think this is t10, you can see the front tires scrubbing.
we use 245, 200tw/100tw tires and after comparing data, we'll keep using them.
i'm not sure we could get 275's up to temp unless the surface was super hot on a twisty track... and even if we could, i'm still not terribly convinced that they'll match the g-load of 225 r7's, and that they won't totally negate the 3% power advantage
A 245 usually wouldn't be as big of an issue at the slower tracks, but COTA in particular tends to put a wrench in things.
Its still on the table for me to run a 100/200tw 245, but I don't know if the drag penalty is worth the power.
The only way for me to know at this point is just pick a setup and run with it. Right now my brain tells me skip the hoosier, run my current 245 AR1, and a low drag aero setup.
Where are you getting the Toyo RR for that price? They are about $200/set more than AR1's (Tirerack & Phil's). Is there a cheaper source?
TrackdayTire with a sponsor code (anyone can get one) and free shipping is $788 shipped. Maybe I am missing the AR1 deal, but I haven't seen cheaper than $774 a set shipped.
02-15-2021, 02:30 PM
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