Sizing Tires in ITS/ITA

[Andy Bettencourt]

As many of you know, we are taking an SM and converting to ITA. I was wondering what others have experienced in terms of lower powered cars and tires size. I will be running 9lb 15x7 SSR's with a 4.88 rear. I figure I have 2 choices in tire size - 205/50 and 225/45.

I see so many lower powered ITB and ITC cars with as much tire stuffed on those 6" rims...and most ITA cars run 225's on their 7's.

Anybody actually run the smaller tire with data to back it up? I really think the Miata is going to work better with the 205's on it as it really won't have a crazy amount of power compared to the Nissan 2L's and the Honda 1.8's. Many have said that on the bigger tracks the rolling resistance is really a factor of a bigger tire.

We have both here, NHIS and LRP are small and twisty with low top speeds while WGI and Pocono run big numbers. Maybe I will have to run 2 set-ups.

Thoughts and experiences?

AB

[ITANorm]

I run different sizes, depending on the track. As light and as well balanced as our cars are, it's hard to get a 225 up to temp on the higher speed courses.

[Greg Amy]

Andy, you may recall I ran my NX's 225/50-14 Hoosiers on my 1.8L Spec Miata in ITS at LRP once last year. I did it just for fun, and (unfortunately) I was solo so Matt wasn't there to take tire temps.

Granted I have minimal time in Spec Miatas, but I was never able to break into the 1:06s. I was really disappointed in the grip, and I suspect it was from not getting the tires up to temp (they didn't look or feel that hot after a session). If that's correct (an unqualified "if") then there's three options:

- One, get more track time so I can be more aggressive and not be such a little girl,
- Two, use a tire with smaller section width in order to reduce contact width and increase temps,
- Three, go with a softer compound (e.g., Hoosier A3S05)

If I were playing with the 1.8L today in ITA, I suspect I'd try the "known" setup first, the 205s in R3S05 SM compound (buy 'em while you can...) Based on feedback and temps achieved I'd consider the 225s in A3S05 compound, and if those get too hot I'd go for the 225s in R3S05.

Testing, testing, testing...

[dickita15]

We all know that testing is the answer but with the Miata having good corner speed and a little limited on the long straights. The smaller tire might race better. Giving up a little corner speed in a car that will still corner faster that the competition in exchange for some more straight line speed could be worth it.

Greg I thing the spec suspension does a poor job of using the grip from the Hoosiers. I am guessing you need more spring and shock to use that grip without just binding the car

[JLawton]

Andy,
No real hard data, just seat of the pants: I ran a 205 and then switched to a 225 on my GTi. I was getting the 225 up to heat (at least in the front) but I went from hitting 5th on the straight at LRP with the 205 to keeping it in 4th with the 225.......But with faster lap times....... Not sure how well this applies to your situation......If I had the money and I was going to a WGI or Road Atlanta, I would have gone back to the 205s....

[zracre]

at the ARRC i dont think I saw a CRX with 205's...less power and less weight but they still seemed to run the big tires. I would think a front driver would need the larger tires as they use more fronts...but judging by the times SM's were running on 205's...I would probably get a set of 205's fresh then go to 225 and see how different they are. I dont think you will have a problem getting them up to temp.

[Ggerg1186]

Getting tires up to temp has less to do with horsepower of your motor and more to do with the horsepower of your brake system, and steering system.

Remember, Force multiplied by Velocity equals Power. So if your engine cannot heat up your tires, then your brakes (brake later) and steering wheel (turn harder) can. I seriously doubt you will find a tire wide enough to actually provide more traction power than your brakes.

[philstireservice]

- Three, go with a softer compound (e.g., Hoosier A3S05)

If I were playing with the 1.8L today in ITA, I suspect I'd try the "known" setup first, the 205s in R3S05 SM compound (buy 'em while you can...) Based on feedback and temps achieved I'd consider the 225s in A3S05 compound, and if those get too hot I'd go for the 225s in R3S05.

Testing, testing, testing...
[/b]


Greg,

The A3S05 ideal tire temp is about 140, which is why its designed for autocross. But it might make a perfect tire for qualifying (hint hint). As far as for a full race, unless it is very cool out, which in the North, spring and fall can be freakin freezing as we all know, it might not make the entire race.

my .02....

[MMiskoe]

Good question Andy. When you figure it out let me know.

We've been running a 1.8 in ITS for some time now, but only in enduros. We have always run 205-55-14 w/ a couple of different rear ends when it came to races. We have messed around w/ 225-50's, but have not been able to do a side by side comparison. The 225's appeared to offer a bit more grip, but it was not quatifiable since the gearing was changed, and the 205's we had that day were suspect for heat cycles. (and I didn't drive the car, so you're getting this 2nd hand).

We did do Hoosiers vrs Toyos, where the difference in width for any given size is pretty noticable (like almost the same as 205 to 225 w/in same makes). The Hoosiers did offer more grip, but were harder to drive and fell off in a much more spectacular fashion making us abandon that idea.

Get me offline if you have other questions.

Matt

[iambhooper]

On my CRX we have used the 185 Toyo's, the 215 Khumo's and a pair of 205 Yokohma's, all at VIR. success has depended o the configuration of th track.

I had been told since the begining that the smaller tire was the faster option... here's what we found. At a full course test day, last August, both my codriver and I played around wtih the 185's and the 205's. We found that the smaller tire was actually quicker on the full course becuase of it's lower profile, and that it allowed you to use 4th and 5th gear better. The 205 was great on the Grand Course, a course with more turns and less high speed, becuase it changed the gearing enough that I could leave the car in lower gears. Grip was marginally improved, however these were slightly flatspotted takeoff's.

We purchased the 215's in October. The same weekend, I turned my personal best lap time at VIR... ironically, on the Toyo's. I was a second to a second and a half slower with the 215's. However, I don't attribute that to the tire, so much as I give credit to the Sciracco that I couldn't get around.

There are people in ITC that run 225 Hoosier's, and have moment's when they are blindingly fast. However, I as only a few second's behind them with the smaller Toyo's (with 2/3rd the investment in tire to).

So, did I answer your question? Hope so. The data is there... the smaller tire is quicker than the big tire on certain tracks. The wider lower profile tire should be quicker than the others becuase of superior grip. My suggestion... buy a few takeoffs and play with it till you find what you are happiest with.

[charrbq]

I've seen a lot of stuff in ITC, as you refer to, for a number of different reasons, but the consensus at the front of the ARRC was more is better. We went from 225/50 to 225/45 due to clearance problems. We'd have to realign the car for each track. The diameter is greater than the 205's and 185's which affects the final drive and the grunt out of the turns, but on a long course like Atlanta, it makes up for it at the end of the straight. Our main reason for running fat tires is for the cornering speed. We have no horsepower to create acceleration, so we just can't slow down for the turns. I've never driven a Miata, but that's my understanding of their position in ITA. No go so don't slow. My tire temps and pressures are usually perfect from the start of a session to the end.

[Tkczecheredflag]

Andy - With my "just sold" CRX "C" car, I've run Hossier 185, 205, and 225's. I used to run the 225x45x13 all the time, but when the new Hoosier's came out a couple of years ago(R3S04) and the tire dimensions changed I started to look at alternatives, being concerned about the tires sticking outside the bodywork. I also kept hearing that the Rivergate Honda "C" cars were running the "skinny" tires at the ARRC. Logic was longer straights, less tire to push down the straight, made them quicker. The things I evaluated in my thinking were tire temps, overall car feel (braking and cornering), and lap times. Keep in mind that my rear tires are just along for the ride, never get any decent heat in them, and have been reported by corner workers waving to the crowds during race events :unsure: . Also after speaking with the front runners at the ARRC I found that some drivers mix and match 225 up front and 185 in the rear.

My limited data told me this: Looking at temps, the 225's ran 142 - 129 degrees inside - outside, the 185's were 144 - 138 (I run 2.8 neg camber 1/8 TO). The car felt great under braking and in the corners with both sets fo tires (probally means that I am not driving fast enough). Lap time were better with the 185's (we hold the track record at NHIS with chicane and got with in 2/10ths of LRP record with 105.1, and had great straight away speed at RA, moving from 7th to 2nd in a couple of laps then I lost my front header panel not to mention control of the car in 10a, and slid back to and finished 7th, first time at RA and I am a slow learner of new tracks). We like the 185's for the gearing (4.9 gear) too.

I'm not sure if you can explore different size fronts and rears but that seemd to have merit for the 2nd place car at RA a CRX. Not sure if that creates an over steer problem with a smaller tire or not but I like the concept of pushing a smaller tire down the straights. I know very little about RWD set up.

Hope that helps, dispite the FWD RWD car differences. For my underpowered car/class "bigger things came in a smaller package", 185 is my tire of choice.

[Andy Bettencourt]

Thanks to all. One thing to understand about my example is that my two tire options have the same overall diameter. So I am looking specifically at the more grip vs. rolling resistance on an 'underpowered' car.

AB

[zracre]

since they are both on 15X7 wheels, I would start with the 205's and use them completely...get the alignment right and get comfortable with the feel...then go to the 225's and see if there is any difference in feel. Lap times on different days really are not valid unless they are drastic changes. I dont think the difference will be too drastic it will be more of feel and judging top speeds and exit speeds. In ITA you can run 14's or 15's I believe, as the car came with 14's... test test test

[JIgou]

This will probably just be more info leading to no conclusion, but:

1. I've read about data (I believe Jason Holland gathered - Jason, you watching this?) that showed 205 Toyos were 2-3 MPH faster down the backstraight in Atlanta than a 205 Hoosier. Whether that's from section width or stickiness of rubber or construction of tire/sidewall is up to you to decide....but I know (as we all do) that the stack of 4 Hoosier 205/50 was significantly taller than the stack of 205/50 Toyos.

1.a. In St. Louis last October I was clocked with about a 2 MPH advantage down the front straight of the roval. I was on Toyos, while pretty much everyone else was on Hoosiers. There was one other car with similar trap speeds back in the pack a bit; not sure what tire he was running.

2. A friend tried the Avon tires in both 205 and 225 width last year on his SM. The Avon tires have roughly the same true section width as the Hoosiers (W-I-D-E). He took the 225s off after a single session - didn't like the feel vs the Hoosier. A couple weekends later he tried the 205 Avon vs the 205 Hoosier, and was happier. Not enough to make the switch permanent, but happier. Unfortunately, no direct comparison of 225 vs 205 Avon.

Sorry there's nothing scientific in this to truly answer the question, but you may find correlating data that begins to paint a picture eventually....

Jarrod

[MMiskoe]

Another thought for you Andy. Although it would take a bunch of time w/ pencil, paper & calculator, you could start breaking down a track into segments such that you can discern the amount of time you spend braking, turning and accelerating. (obviously you have to assume you only do one at a time). Then start looking at distance traveled vrs time it took to travel it. Then you can start playing w/ amount of difference an acceleration rate while under power has vrs a higher speed in a turn and its exit speed. (My guess is that braking rates are not going to change dramatically enough to have a huge impact since you are not on the brakes as long as you are doing the other two). This could point you at weather you gain more from all out straight line speed vrs cornering speed. I am pretty sure you will find it varies w/ track layout, the stop & go track at NHIS will be different than a place like Mosport where you've got long periods of running flat.

You would need to know some actual distances between points on the track, then compare to a video tape to get times to use as a starting point. A little bit of D=RxT and D=1/2AT^2 and you might learn something. I guarantee that you will learn that you remember why you didn't enjoy high school homework.

The more I think about it, the more I think it would be better time spent to just buy two sets of tires and pay for a practice day.

[Andy Bettencourt]

We will be doing the testing, you can bet on that! I just thought some of you may have an educated guess as to the outcome.

I am betting the 225's would be good at LRP...205's at Pocono.

AB

[Ggerg1186]

Another thought for you Andy. Although it would take a bunch of time w/ pencil, paper & calculator, you could start breaking down a track into segments such that you can discern the amount of time you spend braking, turning and accelerating. (obviously you have to assume you only do one at a time). Then start looking at distance traveled vrs time it took to travel it. Then you can start playing w/ amount of difference an acceleration rate while under power has vrs a higher speed in a turn and its exit speed. (My guess is that braking rates are not going to change dramatically enough to have a huge impact since you are not on the brakes as long as you are doing the other two). This could point you at weather you gain more from all out straight line speed vrs cornering speed. I am pretty sure you will find it varies w/ track layout, the stop & go track at NHIS will be different than a place like Mosport where you've got long periods of running flat.

You would need to know some actual distances between points on the track, then compare to a video tape to get times to use as a starting point. A little bit of D=RxT and D=1/2AT^2 and you might learn something. I guarantee that you will learn that you remember why you didn't enjoy high school homework.

The more I think about it, the more I think it would be better time spent to just buy two sets of tires and pay for a practice day.
[/b]

Try google Earth, or http://www.terraserver.microsoft.com/ use the scale to determine curve radiuses and straight lengths. You will have to estimate the heights of hills and valleys though.

[gran racing]

[quote] I just thought some of you may have an educated guess as to the outcome. [quote]

No hard data but based on many discussions with others, here is my bet. LRP - strongly bet on 225s. NHIS - bet on 225s. The Glen - 205s. Pocono - ? (didn't do enough research on that) You didn't ask, but RS - 225s.



[quote] I just thought some of you may have an educated guess as to the outcome. [quote]

No hard data but based on many discussions with others, here is my bet. LRP - strongly bet on 225s. NHIS - bet on 225s. The Glen - 205s. Pocono - ? (didn't do enough research on that) You didn't ask, but RS - 225s.