THSCC Forums

Ok, for starters, I am pretty much locked into my current wheels due to $$$ issues (Kosei 7.5” x 16”)

But I am pretty curious about why it seems everyone gravitates toward 15” wheels. I am assuming that much of this is due to this being a common size for “stock” classes, but even in SP or ST* classes I don’t hear much of people moving to larger diameter wheels. Before I bought my current wheels I had read an article in Grassroots Motorsports (a few years ago) in which they tested 15, 16 and 17 inch wheels on a single car (I think it was a Honda Civic). I also think that wheel width and tire brand stayed the same. Also the tire was sized so that overall diameter of each type was as close as possible. I think that the results showed that the 16” was slightly better (feel and time) than the 15”, but that the 17” didn’t perform as well. So assuming this article is true, why don’t more people use 16” wheels or at the least something other than the magical 15”?

I know that ultimately you want as low as weight possible for your wheel tire combo, but from what I can tell the difference in weight between a 15” and 16” wheel is about 1 lb (using Kosei as example with my 7.5” x 16” at about 15lbs). So far it seems to me that the tire itself is the heavy part and can be very different between brands. My 215/45-16 Falken Azenis are about 23lbs when new which is at least 2 lbs heaver than the Kumho 712 that they replaced. So the Azenis is 8lbs heavier than the wheel itself. If I went with a 15” wheel I would expect the Kosei to be about 14lbs or so, but I can see that the Azenis would have to do nothing but increase in weight due to the slightly larger sidewall. I would expect that that it would eat up at least the pound or so saved by using the smaller wheel?

Someone please educate me on this. Am I missing something?

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Richard Casto
1972 Porsche 914
2013 Honda Fit Sport
2015 Honda Fit EX
http://motorsport.zyyz.com
Money can't buy happiness, but somehow it's more comfortable to cry in a Porsche than a Kia.

Here is the GRM article you were talking about. link

The main thing you will notice is that they are doing the test with street tires R comp tires are another ballgame. The reason I am going to a 15 is so I can reduce my final drive without having to open up the tranny. My selection of R comp tires is non existant in a 16 unless I want taller gearing (that is untill Hankook releases the 215/40). The other downside to the bigger tires is cost. Lets take your Azenis 215/45-16 $110 from discount tire direct, and in the 205/50-15 $77. That $132 I could have saved on tires could have bought me a day at the track.

Cost is certainly a big factor. Another one, if you're assuming wheel width and overall diameter stay basically the same, is the width of tire you can fit on the rim. Base Neons came stock with 185/65-14 tires- on those same 14x6 rims I can squeeze 225/50-14s (overall diam is a bit larger). If I "upgraded" to 17" wheels I could probably only fit a 205 width tire because the sidewalls would already be as short as they could get.

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Carl Fisher

Be Cool to the Pizza Dude:
http://www.npr.org/templates/story/stor ... Id=4651531

People are leaving out the math here.

Take a 15 and a 16 wheel that weigh the same. The weight of the 16 is further away from the center, hence more rotational mass with the lever arm. Also that 1 -3 lbs diff in most wheel/tire combos from 15 to 16 is multiplied. Depending on which source you follow unsprung weight adds as a multiplier of 5 to 10 times the actual weight. So that 1-3 lbs looks like 20-120 lbs of weight to your car.

And as mentioned the availability of tires in the 16 is lacking compared to 15.

I did it for the money as stated before, and for wheel width. I have an awkward bolt pattern (4x108), and my stock wheel was a 16x6.5. I was able to get 15x7 in a wheel and tire package with the 205/50 15 Azenis that after I sold the stock wheels was a wash with getting the 215/45 16 Azenis. Which means that every time I buy tires in the future, I save >$100 (how bout that autox logic kicking in?)

edit: just checked out the times in the trial -- at first glance the 16s seem to be better, but the wheels used in the 15s are 6.5" wide, not 7" and the tire is a 195/55 15 instead of a wider 205/50 15. I speculate that if the wider tire and wheel in the 15 would get back those couple hundredths of seconds from the 16.

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Has no responsibility whatsoever.

I know you know this.. but of course anyone running in a stock class must run wheels that are the same size (diameter, width and offsets) as the stock class wheel.

Miles

I thought offset could vary by as much as 1/4" and still be eligible for stock

That is correct.

Ok, I understood the stock class issue. And it seems that like for the non-stock folks it mostly boils down to cost, availability and fitment issues.

I understand what Adam is saying about moving the mass away from the center of rotation and the affect this has on rotational acceleration. I may even try to create a spreadsheet that models this with variables for wheel and tire cross sectional area, wheel diameter, width, tire size, etc.).

I am still puzzled since (for me) tire weight (22lbs) is still much larger than wheel weight (15lbs). I keep wondering if you do a plus 1 I would expect a slight increase (+1lb) in wheel weight and a slight reduction (-1 or 2 lbs) in tire weight. I keep thinking that they might balance each other out with respects to change in rotational acceleration? I need to run the numbers to see if this works using some generic tire and wheel cross sectional area assumptions.

I guess what I am really curious at this point is that I see a number of people who spend a lot of money on the sport. They may have a number of different wheels and have probably spent a great deal of money on tires over the years. So I just don’t understand why people are not spending more for light weight 16” wheels and tires. Assuming you don’t have availability/fitment issues, why not go for the 16” wheels? Are there diminishing returns on the money spent vs. performance increase? Is there any performance increase? Why not switch to 14” wheels (assuming you can find the tires for them)?

_________________
Richard Casto
1972 Porsche 914
2013 Honda Fit Sport
2015 Honda Fit EX
http://motorsport.zyyz.com
Money can't buy happiness, but somehow it's more comfortable to cry in a Porsche than a Kia.

Integrals make my head hurt.

_________________
Has no responsibility whatsoever.

Underpowered cars make the inertia problem with larger diameter wheels painfully obvious. In my old Scirocco, which usually ran on 14 x 6 wheels, I would occasionally use the 15 x 6 wheels from the GTI 16v. The car handled like a pig with the larger wheels. Note that my contact patch _and_ overall diameter was virtually identical with both. Almost all of the difference was from higher rotational inertia.

On the GTI, switching to the small 14" wheels made a noticable improvement in acceleration and nimbleness.

I never ran a formal test to measure the difference but it was quite obvious, even driving on the street.

--Kevin H.

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2003 WRX (again!)

Honestly I think this assumption may be incorrect. Simply because there is less side wall won't mean a tire of the same width weighs less. I would think that lower profile tires actually have more metal,cords, etc than the higher profile one.

Just a thought.

If I were buying wheels I would start by selecting the tire that I want to run, then buy the best wheel to go with that tire.

When a 205/50-15 any-brand is often half the cost of any other size, it makes the choice really easy...

Scott

Well, here's my input...

The first half of 2003 I used my stock wheels (16X7, 16 lbs) with 215/45/16 Azeni. The second half of 2003 I used Rota Subzeroes (17X7.5, 17 lbs) with 225/45/17 Azeni. I couldn't really tell much of a difference in times compared to other folks' (meaning Aaron's) times - but I never compared them back-to-back. The smaller ones made the car respond to steering & throttle much much much quicker, but the bigger ones kept me out of 3rd gear sometimes, increased grip slightly, and looked a lot cooler.

It has been about 13 yrs, but triple integrals using cylindrical and spherical coordinate systems use to be all I did. Hopefully it will come back to me.



You are probably right. Or at a minimum the difference in weight is very small. I did some digging (not easy to find tires that are only different by the wheel diameter AND have published weights). I found some info on the Kumho 712 that I am semi-familiar with....

Kumho 712

<pre>Size Section Tread Diameter Weight

205/50-15 8.4" 7.3" 23.1" 20lbs
205/45-16 8.1" 7.4" 23.2" 20lbs
205/40-17 8.6" 7.4" 23.4" 21lbs
</pre>


These are about as close as I can find in sizes. With regards to the overall size (diameter/width), the 17 > 16 > 15 so that can explain the slight weight gain as the size increases. But overall the weights are nearly identical and did not decrease as I expected (hoped).

_________________
Richard Casto
1972 Porsche 914
2013 Honda Fit Sport
2015 Honda Fit EX
http://motorsport.zyyz.com
Money can't buy happiness, but somehow it's more comfortable to cry in a Porsche than a Kia.