curlybandit

Currently I have a set of 16" rims with 205/45R16 General Exclaim UHP's that I've been using for the last couple of Time Attack and Autocross events since I started attending this summer.

Eventually I'm going to have to replace them and have considered going down to 15" rims since there's a greater choice of tires and from what I've read can help improve speed because there is less rotating mass.

But the tires I've considered getting are the Bridgestone Potenza RE-11's and they are available for both 15" and 16" rims.

I want to keep one set of rims dedicated for track and have a different set for street use. I may keep my current 16" for track and get a different set for street, or get a set of 15" rims for track and keep my 16" for street.

Any suggestions?

ol Dusty

iTrader: (2)

First, are your 16's nice? if so I'd keep them for street and get some track worthy 15's, but regardless a good set of lighter 15's for the track would be better then the 16's anyway. Less rotating mass true, but not only are the tires available cheaper and more variety but also to low of a profile isn't great for handling anyway. F1 racing is probably one of the most advanced racing leagues ever and do they run elastic band sized tires? get some 205/50R15's and something lite but wider like the new Enkie classics are a 15X8" wheel, they'll get you a ton of traction and still be lighter then the 16's. Then you can keep your 16's to look baller in a EK for the meets.

D.T.P

+1 with Dustin.

16"s for street, light 15"s for track

Sundown

The advantage of running a smaller wheel doesn't only come from reduced weight, but where that weight is.

If you had a 15" wheel and 16" wheel of the same weight, the 15" wheel would be easier to accelerate and decelerate as the weight is nearer the center of rotation as a function of the reduced diameter. It has less rotational inertia.

I'm going to have to strongly disagree with ol Dusty about the sidewall height.

Formula One is a poor example to draw upon, and I'll explain why in a minute. One should look more closely at an analagous saloon car (GT) series like the WTCC, BTCC, DTM, V8 Supercar, FIA GT, Super GT, World Challenge etc. Top level tin-tops use the smallest profile they can get away with, this keeps the sidewall as stiff as possible, allows for protection against punctures (there is typically more contact in closed-wheel series) and less volume of air (nitrogen in top series).

Less volume of air is desirable as it will have a smaller effect on tyre profile as it expands and contracts due to changes in temperature. You also just have to carry less of it around with you to achieve a given tyre pressure vs a taller sidewall profile.

Tall sidewalls act like springs, but the problem is that there is no (affordable) way to dampen that oscillation. Tall sidewalls lead to the wandering of the contact patch, poor road holding due to whumps and bumps, and a difficulty in maintaining consistent pressure and temperature characteristics.

As for why F1 is a bad example of high aspect ratio (tall sidewall) tyres being effective... First and foremost must be the fact that they HAVE to run tall sidewalls. The wheel size is limited to 13" by the sporting regulations for F1 by the FIA, along with the overall dimensions of the tyre. Secondly, the tyre is built by a single source supplier and is the same for all teams. This negates and design flexibility in the realm of sidewall selection.

I mentioned before that tall sidewalls act like springs, springs need dampers to control their oscillation. F1 cars have essentially fixed suspension arms to provide the most stable platform possible for the aerodynamics. The primary loading on these points is from the aero and accounts for 20% or less of the entire suspension travel of the vehicle. The other 80% comes from that tall sidewall, like the spring in your car. To dampen the oscillation of the sidewall, F1 cars use a neat device called an 'Inerter'. The inerter is the mechanical analogy of a capacitor and soaks up additional oscillatory motion from the sidewall to make the tall sidewall act like a low-profile tyre.

In the end, your real question should be, do I have the power, brakes, suspension and budget to work with a bigger wheel? If not, run a smaller wheel, save some cheddar on tyres and enjoy peppier dynamics all around, but be ready for a "barely" (driver dependent) noticeable increase in mid-corner disturbance from surface fluctuations (bumps).

MPR

^Note all those GT series type cars are much heavier and require much larger brakes, also thus, larger rims to fit over the brakes...

The type of tire makes a huge difference as well. If the tire is designed for the track, it's going to have a much stiffer sidewall and perform like a race/track tire, and the sidewall will make very little difference. We've autocrossed and lapped with 185/60R14 track tires for years and the stability was stellar. No issues with road holding over bumps or anything. That type of issue comes about from a tire not designed for that type of use.

The application should dictate the size of rims and tire you use. In this application, there really isn't a huge difference between 15" and 16" rims. Ultimately, if the OP is not making much more than stock power, then a brake upgrade is not necessary and he should use a lighter, wider 15" rim with a good track tire like RA-1's or similar.

Sundown

I agree about the brakes MPR, but in terms of weight, most top GT cars weigh in around 1000-1100kg (~2200-2400lbs) on the grid. Seeing as my '92 VX rolled into the dealership at 2050lbs, that's not too much of a difference from the flyweight (CRX excluded) of the family.

At most AutoX events the duration of sustained loading on the tyre is very short, thus the effect very minimal. But a mid-corner disturbance will cause porpoising with a higher aspect ratio tyre in a very high-speed, sustained-loading, situation due to the undamped oscillation of the sidewall. High- speed sweepers will produce a condition where this phenomenon has a high potential of occurring.

If the car is truly being driven at the limit, the upset in balance fore and aft as a result of the undamped sidewall oscillation, and thus weight transfer fore and aft, will make the car 'twitchy', or 'loose'. This is particularly dangerous as you can imagine as the car is already at some considerable speed and most likely in proximity of armco/barriers/walls/other competitors.

With a lower aspect ratio tyre, these disturbances still exist, but the event lacks the tumble-down effect of the tall sidewall induced oscillation. (and in top GT cars is usually smashed out by the aero anyways)

Really in Ontario the only corners fast enough to produce this effect are turns 1, 2 & 4 at Mosport (Grand Prix Track) and the entrance to the carousel at Callaboogie (Turn 22 or something?). You might be able to argue Turn 1 at Shannonville, though the preceeding straight is pretty short.

A final downside of high aspect ratio tyres is the loss of 'crispness' to the turn in response. This is more personal and driver specific, but I personally like to slide the car on entry and turn in response is essential to being able to balance a slide well.

We could go into the differences between bias-ply and radial construction, etc, etc, but admittedly, for the purposes of curlybandit, this is all far too involved and a good 15" wheel should do just fine. That's what I use on my Civic!

MPR

Well as I've mentioned, we've track at several different locations, shannonville probably the most high-speed of the lot, on those 185/60R14 tires, and there was no instability or "porpoising" that you speak of. That come more as a result of tires not suited for the use and or a poorly setup suspension with incorrect spring rates, incorrect setup dampeners and or incorrect alignment. Trust me, we were frequently at or beyond the "limit". We're not scared to push it. That's how you find out where the limits are and how well setup your car is.

We've setup our car properly from the start with optimal spring rates, dampeners that can handle them and that have been adjusted to match and proper alignment. I believe that is the key to proper stability and optimal mechanical grip. If you have the proper tires, your suspension setup is paramount in achieving high-speed mid-corner stability.

You say you like to let your car slide a bit. I must say I believe any sliding means you're beyond the limits of grip and means you have an unbalanced and unoptimized setup. Drivers often tailor their setup to the style and handling characteristics they prefer. Sure a little sliding is ok. And it's fun . But it's slower than it could be. The time it takes for you to correct is time you are losing on each lap. A fast neutral car will not necessarily feel fast, but the lap times will speak for them selves.

Sure there is slightly more flex and oscillation on a taller tire, but if you're using a proper track tire, the issues you speak of should not be that extreme unless the suspension setup on that car is incorrectly setup and not optimized to begin with. Then it exacerbates the situation and becomes an issue. So two points here: 1, setup your suspension for optimal mechanical grip and 2, use the right type of tires for the application.

GT class and GT challenge weigh in at 2480-2600+ lbs. They need big brakes to not only stop the car again and again from high speeds but also dissipate massive amounts of heat and be able to last an entire race.

Myz

Quiet an informative & interesting read.
All i can offer is what i've learned from my track experiences.
I raced mainly Autocross & i'd switch between wheel sizes for the sole purpose of lengthening or shortening gear ratios depending on the way the track was setup for the day.
IMO, if your power is close to stock or just over, I'd run 15's. Infact, even if you had a complete head package I'd still run 15's. But that's just my opinion.

Regamaro

Run 15s but go for r compounds or rs-3s over re-11s. I rthird-11s this summer and while they are good for ~10 heat cycles they still are nowhere near an r-compound.

PurpleStuff

forget re-11's if you're using them only for track, use that same money and buy some proper r-compounds, the traction is incomparable to re-11s.

MPR

RA-1's

Sundown

MPR, I think we agree in the resulting wheel selection, but the level of resolution of the decision making process is where we are differing in opinion. I'm offering some analysis behind the selection, as I can not be sure that the individual asking for guidance will always be running a Civic, and that 15's will always be the correct solution.

Shannonville, while technical, is not high-speed, potentially barring turn 1 and the entrance off the drag strip. I am also going assume that the majority, or entirety of your track experience is in converted saloon cars and that you have minimal or no experience on bias-ply slicks. Hop in a formula car for a few sessions on bias-ply rubber and your hyper-set-up GT car on R-Comps will feel like sitting on a pile of laundry. In tying that back to our discussion about the mid-corner unsettling, it is far more noticeable in something like a formula car with very, very stiff suspension (to cope with the aero) but is also minutely noticeable in a saloon car, and more so if your moving directly from one to the other. To draw on an analogy I've argued isn't valid, (take what you will from that!) check out the Singapore GP this weekend and the resulting wiggles and twitches in the F1 machinery as they negotiate the whumps and bumps with their tall sidewalls!

I also agree wholeheartedly, MPR, that correct suspension selection and adjustment are key, but the engineer in me has to examine even the most minuscule of events. A quality r-comp will do the grassroots racer just fine and in most series the tyre selection is fixed, which renders most of this moot.

To correct a poorly worded note on my personal 'sliding' of the car, I'm not talking about big, lurid, D1 GP, hand-out-the-window slides, but rather, unsettling the car on the brakes and catching it with the throttle and some minor steering input. Simply used as a technique to rotate the vehicle resulting in a slip angle of 2-5 degrees. Though I have been known to drive sideways for fun.

To get back to the root of the situation, run a 15, and you may want to stay away from the RE-11 as PurpleStuff said (I have them on my road car) as they're not a true R-Comp).

Keep it rubber-side down.

- Sundown

MPR

^We race a MKI MR2 (AW11) with full suspension/bushings, nearly everything done to it handling-wise with 205/50R15 and 225/45R15 RA1's. It was a 4agze which we converted to a 4agte (remove restrictive supercharger (which we maxed out) for a properly sized turbo (TD04-13T from a WRX) plus water to air I/C and tuned with megasquirt stand alone). Full boost at 3000rpm and holds all the way to the limiter at 7500rpm. She goes like stink and handles like a go-kart!

The mid-rear layout presents interesting handling characteristics that most people aren't used to. We actually use a setup that does not use sway bars. I am 110% convinced this setup method works better and allows for more mechanical grip than any setup on the same vehicle using sway bars for a pure track setup. But that's a whole other discussion. Just wanted to give you some of my background experience.

I'm very experienced with autocrossing and lapping a variety of cars from fwd, rwd and mid. I have no awd racing experience, and no "touring car" experience.

I understand all you have said and mostly agree. I don't doubt the tall sidewalls on F1 cars contribute significantly to the "twitchiness" of their handling. I think one important thing to remember though is that a certain amount of sidewall flex vs. no flex can be the difference of maintaining grip and pushing past the limit. We're talking about a very fine line here though...lol.

Will be watching the Singapore race for sure. Have always loved F1.

Xscorpio

omg so much info on tires, i never knew...thanks you so much

F8i

iTrader: (10)

My 2 cents, hope you don't feel ripped off
15's are lighter (Bonus), 205 is on 15" is the largest you would need to go. Grass root motorsport had an artic on this years ago. They said you wouldn't be able to heat up the tire enough to make it more benificial than a henderiance.

2nd point to MPR, AWD for the win

NOTE: THEY'RE CALLED WHEELS NOT RIMS! WHEELS CAN HAVE RIMS THOUGH!
-B

MPR

^AWD is for cheaters... lol :P

curlybandit

Haven't checked this post in a while and there's been some serious activity!

Just to update, I was fortunate enough to find someone selling a set of Rota Circuit 10's (bronze) with practically unused Yokahama Advan Neova AD08 tires. I've used them in three autocross events and at Shannonville so far.

I'm very please with the performance of these tires. When compared with the 16" General Exclaim UHP's that I used earlier in the season, I feel these offer better grip and feel than the UHP's did.

Thanks everyone for the solid input. This is a never-ending learning process - as I quickly found out just from going to a few events this year - and I appeciate everyone's contributions.