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Old 10-19-2012, 01:49 PM   #1
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Default Discuss the pros and cons of 13" wheels

I am thinking about making a change to 13" wheels on my fp miata. Remember this car is a low horse power car and the greatest speeds it will ever see are just north of 130.

All things being equal wouldn't 13" wheels provide better handling? Im thinking that going from 15's to 13's will lower the cars cg and allow better cornering. Not to mention the benefit of less unsprung weight and reciprocating mass for my measly 112# of torque to turn.

Am also on the fence as to which tires to run, I will be on Hoosiers, just don't know if I will be running cantilevers or radials. I have been told that radials have less rolling resistance, but how much less, and does it make up for the loss of corning offered by the cantilevers? I have this posted on the prod site I belong to, but was looking for input from you guys as well.
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Old 10-19-2012, 01:59 PM   #2
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We can blindly speculate, but at the end of the day, you need to buy both setups and test if you actually want to know which one is faster.

Having said that, you're going to need to push the 13s way out on backspacing to clear the control arms, and that's going to induce aero drag and scrub radius issues. If you can get a wide enough tire in 15", I wouldn't bother.
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Old 10-19-2012, 02:15 PM   #3
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Old 10-19-2012, 02:20 PM   #4
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Old 10-19-2012, 02:27 PM   #5
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Nice looking car Hustler. Rules only allow me 13x7 or 15x7.

Sav the problem testing is my current gears are 4.77's and I will need to run 3.63's with the 13's, makes back to back testing virtually impossible.
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Old 10-19-2012, 02:54 PM   #6
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I think you're looking at a ton of work for minimal gain. I've got a set of 13x7's for my D-Prepared project, and have been contemplating the opposite. Here's my thoughts:

Pros:
Less unsprung mass (in my case around 5#/corner)
Possible lower static ride height without screwing up your geometry (~2" lower)
Cheaper tires(highly dependent on brand/ rules)

Cons:
Less steering input available with stock arms. (ruleset allows nonstandard arms)
Less mass, and possible tire overheating issues.
Less possible brake mass/diameters.

In Between:
Gearing.


What kind of revs are you running with your motor? North of 8K? The hoosier 20x9.5x13 (which is what I think you'll want to run) with your 4.77 gearset, and assuming the stock 5speed box will net you in the 130 range @ 8500rpm. This doesn't even start to take account of individual gearing for specific tracks, which can be another nightmare.

I don't think aero will be an issue, it's been awhile, but I'm pretty sure the 13x7's tuck nicely into the stock fenders.
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Old 10-19-2012, 03:04 PM   #7
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Old 10-19-2012, 04:04 PM   #8
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Needs moar solid forklift tires on that jeep.
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Old 10-19-2012, 04:05 PM   #9
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I have three sets of 13" FS, Panasports 3P 2 sets 13x7.5 with 50% 710's and 1 set 13x8 50% fat Hoosiers.
600.00 ad 800.00 you pickumup!

Vince
Auburn CA
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Old 10-19-2012, 06:09 PM   #10
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Quote:
Originally Posted by tpwalsh View Post
What kind of revs are you running with your motor? North of 8K? The hoosier 20x9.5x13 (which is what I think you'll want to run) with your 4.77 gear set, and assuming the stock 5speed box will net you in the 130 range @ 8500rpm. This doesn't even start to take account of individual gearing for specific tracks, which can be another nightmare.

I don't think aero will be an issue, it's been awhile, but I'm pretty sure the 13x7's tuck nicely into the stock fenders.
I run a alternate gear box; quaife dog box, I'm not sure about 5th but I believe it is overdrive. 130 mph at 8k with 215 580r the diameter is 22.9"
I will need to calculate gears for the diff if I go to 13's


Quote:
Originally Posted by compaddict View Post
I have three sets of 13" FS, Panasports 3P 2 sets 13x7.5 with 50% 710's and 1 set 13x8 50% fat Hoosiers.
600.00 ad 800.00 you pickumup!

Vince
Auburn CA
I am allowed max width of 7"
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Last edited by hingstonwm; 10-19-2012 at 07:59 PM.
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Old 10-19-2012, 07:36 PM   #11
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Quote:
Originally Posted by hingstonwm View Post
Nice looking car Hustler. Rules only allow me 13x7 or 15x7.

Sav the problem testing is my current gears are 4.77's and I will need to run 3.63's with the 13's, makes back to back testing virtually impossible.
I doubt that you would go with that much of a change. Try this.

Speed (MPH) = Tire Circum. X RPM / axle ratio X 1056

Why not run a tire that had the same diameter or slightly smaller as what would be on the 15 inch wheel anyway? I would think too small a diameter would sacrifice grip by reducing the contact patch.

13s were the norm when Miatas first started in motorsports.
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Old 10-19-2012, 08:02 PM   #12
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^^^ I ran the numbers using the formula provided at 949 racing's web site and came up with 4.30's which is perfect for a back to back test. All I need to do is find a 99 differential from a 5 speed car.
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Old 10-19-2012, 09:01 PM   #13
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Quote:
Originally Posted by miata2fast View Post
ame diameter or slightly smaller as what would be on the 15 inch wheel anyway? I would think too small a diameter would sacrifice grip by reducing the contact patch.
This is a pretty common misnomer, contact patch size is influenced by weight of the car, and tire pressure, not wheels size. You can alter the shape of the contact patch by varying the wheel size, but not the size. width influencing width, and height influencing length. Generally the longer the patch, the better it will put down power, and the wider, the better the cornering grip.
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Old 10-23-2012, 10:28 PM   #14
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Quote:
Originally Posted by tpwalsh View Post
This is a pretty common misnomer, contact patch size is influenced by weight of the car, and tire pressure, not wheels size. You can alter the shape of the contact patch by varying the wheel size, but not the size. width influencing width, and height influencing length. Generally the longer the patch, the better it will put down power, and the wider, the better the cornering grip.
In this case is "wheel" refering to tyre size changing rolling diamter or rim sizes with the same rolling diameter.
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Old 10-24-2012, 02:06 PM   #15
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Quote:
Originally Posted by tpwalsh View Post
This is a pretty common misnomer, contact patch size is influenced by weight of the car, and tire pressure, not wheels size. You can alter the shape of the contact patch by varying the wheel size, but not the size. width influencing width, and height influencing length. Generally the longer the patch, the better it will put down power, and the wider, the better the cornering grip.
Makes sense. I am assuming you are actually refering to the overall diameter of wheel and tire combo. Right?

I was suggesting to run a 13" wheel with a tire that was tall enough to equal the same height of the 15" wheel and tire combo. I kind of assumed the 13" combo would be a tad lighter than the 15" combo with the same height tire. Particularly if used with bias tires. Am I right on that assumption? Anyone care to comment?
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Old 10-24-2012, 02:09 PM   #16
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Quote:
Originally Posted by miata2fast View Post
I kind of assumed the 13" combo would be a tad lighter than the 15" combo with the same height tire. Particularly if used with bias tires. Am I right on that assumption? Anyone care to comment?
Doubtful. Tires are heavier than wheels - a new 225/45 NT-01 is ~21-22lbs, and even a non-DOT Hoosier radial slick is 16 or 17lbs. A 13" combo that's as tall as a 15" combo will have a lighter wheel, but a heavier tire, and the tire weight matters more (rotational mass further away from the center of rotation).
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Old 10-24-2012, 02:16 PM   #17
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Originally Posted by Savington View Post
Doubtful. Tires are heavier than wheels - a new 225/45 NT-01 is ~21-22lbs, and even a non-DOT Hoosier radial slick is 16 or 17lbs. A 13" combo that's as tall as a 15" combo will have a lighter wheel, but a heavier tire, and the tire weight matters more (rotational mass further away from the center of rotation).
Don't forget that a 23.5" tall tire on a 13" rim is going to move around A LOT compared to a 20"/13" combination. Generally bias ply slicks have pretty soft sidewalls compared to DOT tires. Granted I look for this more in my world than on track, but it's something to think about.
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Old 10-24-2012, 05:47 PM   #18
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My first set of 13" wheels will be here tomorrow. Jongbloed 330's, they weigh in at 8 pounds, only down side is they are black in color, may be making a trip to get powder coated. I will be getting a second set in natural aluminum. Need to figure out if I am going to run bias or radials, I need to make a call to Hoosier and have a chat with them.
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Old 10-25-2012, 10:37 PM   #19
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Got my wheels in today. Have a total of 8, it should make time management easier, having 8 like wheels. Wheels weigh in at approx 8#.
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Old 10-25-2012, 11:13 PM   #20
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purdy
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