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Old 03-04-2011, 10:29 AM   #1
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Choosing a tire size

A friend emailed me yesterday, asking about tire choice, and I figured I might as well share the info with everyone since I'm bothering to type it out.

Feel free to ask my opinion on any of this, ask for pics of any of the tires I have run, or tell me why I'm an idiot, or that this is a re-post, or whatever you'd like to do! I won't be offended.

Friend's question: Why do you race on 16's with a larger sidewall?

My answer:

I'm not sure what you mean by "a larger sidewall" ? Larger than a 215/45-17?

When choosing an autocross racing tire, the most important things are width and overall diameter. Wheel width and tire design have more to do with the contact patch width than many people realize. For example, a 205 on a 7" wheel is probably going to have a wider contact patch than a 225 on a 6" wheel. For autocross, sidewall size and wheel size are not really important. They only affect overall weight and amount of tire flex - which, actually, you want SOME tire flex. Tire flex allows for a bit more "wiggle room" with driver inputs, and allows the tire to better conform to rough surfaces for maximum grip. I keep going with smaller overall diameters because I want shorter gearing for the track. My car doesn't have much torque, and I hate waiting for power when I'm coming out of slow corners in second gear. As I keep trying smaller and smaller diameters, I have decided to go with 15" wheels, because they are lighter than 16" wheels, and because I want to keep some sidewall flex, and because tires are cheaper in 15".

For daily driving tires, I like the tallest one that will fit under the fender, because taller gearing gives me better mileage and a lower/quieter cruising RPM. Also, for daily driving, I like a taller (50-60) sidewall for comfort. Taller sidewalls give you more flex over potholes, and they make it a lot harder to hurt your wheels on curbs and things.

Here are some tire size specs:

215/45-17 has a 24.62" diameter and 3.81" sidewall
225/50-16 has a 24.86" diameter and 4.43" sidewall
205/50-16 has a 24.07" diameter and 4.04" sidewall
205/55-16 has a 24.88" diameter and 4.44" sidewall - Favorite DD size
205/60-16 has a 25.69" diameter and 4.84" sidewall
225/50-15 has a 23.86" diameter and 4.43" sidewall
205/50-15 has a 23.07" diameter and 4.04" sidewall
225/45-15 has a 22.97" diameter and 3.99" sidewall

The tires that I have chosen to use for autocross racing, in order, are

On a 16x7" wheel: 225/50-16 (24.86", 4.43") BFGoodrich g-Force Sport
On a 16x7" wheel: 205/50-16 (24.07", 4.04") Dunlop Direzza Sport Z1 Star Spec
On a 16x7" wheel: 225/50-16 (24.88", 4.34") Hoosier A6
On a 15x7" wheel: 225/50-15 (23.86", 4.34") Kumho Ecsta V710
On a 15x7" wheel: 205/50-15 (23.07", 4.04") Bridgestone Potenza RE-11
On a 15x7" wheel: 225/45-15 (22.97", 3.99") Hankook Ventus R-S3 (I have not used this tire yet)

The tires that I have chosen to use for daily driving, in order, are

On a 17x7" wheel: 215/45-17 (24.62", 3.81") Kumho Ecsta MX
On a 17x7" wheel: 215/45-17 (24.62", 3.81") Goodyear Eagle F1 All Season
On a 16x6" wheel: 205/55-16 (24.86", 4.44") Hankook Icebear W300 (Winter)
On a 16x7" wheel: 225/50-16 (24.86", 4.43") BFGoodrich g-Force Sport
On a 16x7" wheel: 205/50-16 (24.07", 4.04") Dunlop Direzza Sport Z1 Star Spec
On a 16x6" wheel: 205/60-16 (25.69", 4.84") General Altimax Arctic (Winter)


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Old 03-04-2011, 11:18 AM   #2
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I figured you picked a 16" for gearing reasons. By larger side wall I meant you have a 55 instead of a 45. Even though overall diameter isnt different you side wall is about .5" larger. Less steering response, like you said more flex. I hate that "float" feeling.

15" wheels will not clear your caliper so not sure how you plan to do that?


Thanks for sharing
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Old 03-04-2011, 11:26 AM   #3
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There are a few and very rare 15" wheels that DO fit the SVT. But again RARE.

Like the 15" Rota Slipstreams and 15" Motegi TrakLites.

Yeah, alot of guys get the 16" for gearing since we have a long 1st gear. I went with 16s but have a 50 side. I was told it's even better to go with 45side on a 16" wheel. But I was going for grip and the grippy tires I found for the 16s only had a 50 side wall tire.

-Ray-
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Old 03-04-2011, 11:40 AM   #4
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I picked up some 15x7 Team Dynamics Pro Race 1's. They fit great and weigh 14.00 pounds each. Here are a bunch of pictures of them





















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Old 03-04-2011, 11:43 AM   #5
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Quote:
Originally Posted by tukaniSVT View Post
There are a few and very rare 15" wheels that DO fit the SVT. But again RARE.

Like the 15" Rota Slipstreams and 15" Motegi TrakLites.

Yeah, alot of guys get the 16" for gearing since we have a long 1st gear. I went with 16s but have a 50 side. I was told it's even better to go with 45side on a 16" wheel. But I was going for grip and the grippy tires I found for the 16s only had a 50 side wall tire.

-Ray-
The reason for going with smaller diameter wheels/tires is not because 1st gear is long, but because we don't have enough torque to provide strong acceleration coming out of corners in 2nd gear. It would be much slower to go to 1st for most of these corners, because entry and exit speeds are normally about 35-50mph

Oh, another factor in all of this is that I have a Torsen. FWD cars with open diffs will not want shorter gearing, because they will spin the inside tire way before a similar car with a Torsen would lose traction. Good examples of this are stock Focus, Mazda3, etc.

Also interesting to note is that Pete's wagon (zetec with shaved head, cams, torsen) has more torque and does spin 205/50-16 (I drove it using the StarSpecs) coming out of the same types of slow corners in 2nd gear. Does anybody know offhand the overall gearing difference in 2nd gear from an MTX75 to the Getrag 6-speed?
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Old 03-04-2011, 05:35 PM   #6
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^interesting, I just learned a lot.....Never knew bout the 15's and that you had them already.

I have so work to do before track season or I'll be spanked lol!
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Old 03-04-2011, 05:54 PM   #7
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Hehe, your car is already in SMF, so you should just put some R-comps on one of your sets of 17" wheels - you have plenty of power to run big tires!

225/45-17 Hankook Z214 C71 Soft are $180.00 each

Or for less chance of rubbing you could get 16x7" wheels and run the 225/50-16
Or for even more grip you could get 16x8 or 16x9" wheels and run 245/45-16 !!
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Old 03-04-2011, 06:01 PM   #8
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I have noticed the crappy exit speeds in corners ... I am going to try some play during my autoX run this weekend with these new tires I got last month.

The Last time I ran them was wet ... it should be dry this weekend.

-Ray-
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Old 03-05-2011, 03:22 AM   #9
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Charles run 245/40-17 right now. It's a CHEAP rcomp size. Problem is 17x8 or 17x9 4x108 are big $$$





PS I'm the guy who spins 225/50-16 with Torsen on corner exits with his non-SVT zetec massive TQ's (lol). (not with that^ car, with this one:





)

(and the BMW I race furrealz is there too )
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Old 03-05-2011, 07:47 PM   #10
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thanks pete!
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