I was curious why people choose 17s over 18s. Always thought the less sidewall flex would be good for handling? Figured I'd post this here as those of you who autoX/track would know best.

Also how much of a difference is there when dropping 4-5lbs from stock rims when you're going to a wider tire that weighs more? Is there really a noticable difference or is it noticable because of the wider tire?

 

less rotational mass. from a physics standpoint there is also less interia. where i think I=mr^2. not too sure of that.

i remember hearing that a 1lbs loss will feel like a 10hp increase. not measurable though. maybe i have those numbers mixed up.

less sidewall flex is good for handling. but having the least sidewall doesn't necessairily correlate with the least sidewall flex.

the more contact area the tire has with the road, the better.

 

Go to autocross and race or watch. Talk to the drivers. Some racers prefer 17-inch wheels and tire with fat sidewalls. You can pick-up the PSI to 50, and still NOT ride on the sidewalls. There’s nothing wrong with a fat sidewall that will flex before the tire breaks traction.

 

Auto-X: A good part of the situation has a lot to do with the tire sizes that are available in the two most competitive tire brands. They are limited. In the stock class, the 710 Kumho is quite a tire, their 17" offering seems to be a best match with the stock wheel widths (comparing 17s vs. 18s). The 18" offerings, by default land the stock class Z driver on the Hoosier 275/35-18. Which is what I have been running in B-Stock. The tire is very good for a very few events. The tread depth isn't the issue, it's the compound cycling out rapidly. This 18" Hoosier on stock wheels requires the use of sub-optimal air pressures to make them work, while not cording the outer edge prematurely.

Given a choice, I'd run a 17" wheel, but my car came equipped with 18"s, so that's it for me. I'm thinking of sucking up my desire to win and merely enjoy A/X'ing by going to BSP. If I do, I'll go with a set of 17"s on wheels with a 9.5" width.

BC

 

losing 1 lb rotational = 10hp increase?

bit much?
not that i know but it sounds kind of over..

 

1 lb of rotational mass savings is approximately the same as taking 8 lbs of static mass off the car.

Put it this way -- saving 8 lbs per wheel on my SpecV moved me from a good 3 seconds off of ITR times to 1.2 seconds behind -- on a nationals-style course.

 

less weight faster wheel rotation

 

bump

 

Here is what I know.

1 # of UNSPRUNG wieght is worth 8 # of sprung wieght. So if you can shave off wieght on the tire/wheel combo, than it is worth 8x as much. This also counts for your brakes, A-arms, Shocks and dampers, ... This has to do with keeping the mass on the ground when you hit a bump. Your car has the shocks and dampers to help, your wheels and tires are on their own.

So I don't think you can say that 17s or 18s are better, look for the combination to see the overall wieght. Yet, for race tires today, there are more in 17s than 18s.

 

I've taken BSP in my region (Old Dominion Region, SCCA) for the past two years, so going BSP isn't all that bad unless you've got some national level C4 corvettes around you. Now, that being said, take a look at the 2006 PAX for BSP. BSP lost another .004 this year, making it even harder to keep up against the general population.

Back on topic. I run stock 17" (enthus.) wheels with 275/35 Khumo 710s for autocross which took a few events to get used to, but now I'm extremely happy with that combo. I run the stock 18" rims (touring) with Michelin Pilot Sport Cups (265/30) for track events and I'm really happy with that tire there - I'm about to order my second set of Mic PSCs after 9 track weekends and a season of Auto-X. BTW, don't use the Mics for Auto-x unless it's over 100 degrees outside!

 

the intertial formula also takes into account radius, other than mass, as well as location of mass... wheels aren't made of equal density... Smaller radius means less interial weight. light 17's are better than same weight 18's b/c of radius. Also, 17s w/ most of the mass in the center are better than 17s w/ most of the mass outside...

 

this is also true. good point!

jim - when are you doing some fender modifications and throwing some 315s on that bad boy!!

 

So I guess the question is, what do run on the street?

Do you run the PSC 265/30 all the way around? Did you have a problem fitting them to the 18" touring rims?

 

265 on the 8" shouldnt be a problem for any diligent tire mounter.. some give you lip over it though!

he runs a nismo 18 on the street. car looks sick!

 

What they all said... + there are a lot more tire choices in the 17" size, and they are cheaper per tire!

 

This is generally true for street tires, but there are diminishing returns. Using this theory, going to 19 or 20" wheels with very small sidewalls would yield better results than 17 or 18" wheels. This is almost never true for street cars, especially as the weight of the wheel moves further from the center, thus increasing the moment of inertia.

If you look at F1 cars, the sidewalls of the tires are actually quite tall, and the wheels are rather small in diameter. The tire flex of the sidewall is actually a vital part of the suspension system. There are many factors in determining the optimal wheel/tire combo.

If you look at the tires used in Super GT (formerly JGTC) for our Z cars, the tires are quite wide, but more tall than you might expect. They are something like 280/40-18's on 11" wide wheels in the GT300 class, and 330/40-18's on 13" wide wheels in the GT500 class.

Choice of wheel size also depends on brake size. If you have 14" or larger brake rotors, you are probably going to need at least 18" wheels.

Being able to buy 17 inch tires for us poor guys who don't have big factory sponsors can save us big money. I almost paid for my FN01R-C 17" wheels with what I saved in the cost of the Falken RT-615 tires in 275/40-17 vs. 275/35-18 (over $100 a tire!). These tires have a very stiff sidewall, so I don't feel like I was giving up much by having a slightly larger sidewall.

 

F1 wheel sizes are dictated by the rules, not optimal performance. Incorporating sidewall flex into the suspension system is more a function of "it's there, might as well use it" than "I'd rather use sidewalls than springs".

I agree with the general message that plus-sized wheel / tire combinations are not necessarily better, though.

Steve

 

True. I didn't mean to imply that F1 wheel/tire sizes were optimal. I was making the point via example that just because you have a larger/taller sidewall doesn't mean that handling will necessarily be worse than a smaller/shorter sidewall and vice versa.

But I disagree with your statement of sidewall flex being more a function of "it's there, might as well use it" than " I'd rather use sidewalls as springs." If you had NO flex in sidewalls, you would slide off the road the first corner! Tire/sidewall flex is an intergral part of any suspension system on any car.

 

Hi Knightwhite,

I'm researching 17x9" wheel/tire choices for the upcoming season and only found the FNO1R-C in +35 offset, is that what you're using with 275's? Any rubbing issues? Those wheels are definitely on my radar, but I thought a +30 all around would be required for best results. Any feedback would be appreciated.

Thanks,

Tim

 

I just ordered a set of Motegi Touge 17x9 23 offset. Also going 275s, from I could find it should be fine.