Tire Size and Its Influence on Grip - Mustang Evolution

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Old 04-12-2015, 08:33 PM   #1
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Tire Size and Its Influence on Grip

How much more grip is provided by a 275 than a 255 or 235? Is it a night and day difference? Also how much of a difference does the sidewall height make? I know a shorter sidewall will have less flex and therefore be more predictable and handle better overall, but how much is to much sidewall? I like the look of tall/meaty tires, but I don't want to make my car handle like s***. I'm asking because I currently have a set of 17x8 wheels from American Muscle and I'm wondering if I should look into getting new wheels. All input is appreciated, thanks guys.

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Old 04-12-2015, 08:51 PM   #2
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With all other things being equal (treadwear, tread design, tire diameter, etc), going from a 235 wide (9.25") to a 255 (10") or 275 (10.8") is mainly in the total area of the contact patch.

If you have 4" of tire circumference on the ground and it's a 235 tire, you have 37 sq in of contact surface. With a 255 you have 40 sq in and with a 275 you have 43.2 sq in. That doesn't seem like much but if you look at as a percentage, you get a 7.5% or 15.5% increase in surface contact respectively.

On my car I went from the stock 235/50/18's to Nitto 555's in 275/35/20 and the handling is night and day. It's much crisper than with the taller sidewalls of the stock tires and doesn't feel like it takes as long to react to steering inputs.
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Old 04-12-2015, 09:30 PM   #3
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From your experience would it be better to buy new wheels and tires? Or put that money towards another suspension part?

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Old 04-13-2015, 03:15 AM   #4
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What are your goals for the car?

What do you NEED the car to do every day?

What do you WANT the car to do ocassionally?

What DON'T you like about the car now?
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Old 04-13-2015, 07:45 AM   #5
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Well right now the car is just my daily driver and I would like to daily drive it for another 3 or 4 years. I'm hoping to get into autocross in a few months. There isn't a lot I don't like about the car currently, It has some oversteer and a lot of wheel hop. I would like to put some kind of FI on the car, but that should be a couple of years away.

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Old 04-13-2015, 08:38 AM   #6
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Your biggest immediate impact is going to be new tires with less sidewall. You can have a bunch of suspension work done but if the tires are subpar all you'll do is spin and slide. Without getting ridiculously technical, the coefficient of friction between the tire and pavement is what ultimately dictates how hard you can launch and corner. The suspension can be fantastic, but if the tires can't cope with the forces applied.....well, you get the picture.

If you're going to daily drive the car, I'd do the tires first, some good shocks and sway bars, and MAYBE lower the car an inch, but that would be the last thing I'd do. Good shocks and sway bars go a long way in controlling wheel hop and obviously how the car reacts in corners and accelerations.
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Old 04-14-2015, 02:22 AM   #7
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Please describe the difference in harshness or stiffness between the different tire sizes.
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Old 04-14-2015, 06:29 AM   #8
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It's not really harsher, it's more of a firm feel. It's not like you're driving down the road and you feel every little pebble that the tires roll over, it just feels like it floats less.


Where the stock tires tended to feel like they would have delay to steering inputs, these tires feel almost instant and it doesn't feel like the car wallows when you get to sawing on the wheel.


Now, when you hit the rumble strips before stop lights and stuff, you definitely notice those more, but that's the only real difference in ride quality actually transmitted into the cabin.
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Old 04-17-2015, 01:03 PM   #9
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Tire construction plays a role. Some tires have stiffer sidewalls, positively impacting steering precision without necessarily having to go with a shorter sidewall.

I'm not sure what tires you have on the car now, but something in the max performance summer category will certainly improve handling, steering precision, etc. Again though, each tire is different, and some have stiffer sidewalls then others. Tire Rack has a good selection of tires that they test themselves. A good place to look at various comparisons.

Just know that going with a shorter tire will impact other variables such as gearing, speedometer accuracy, and even fuel economy. You'll also have a larger fender well gap which some people might not like. My personal suggestion is to stick with the recommended tire size, but get something that is performance oriented.
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Old 04-17-2015, 01:32 PM   #10
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You don't just go down in sidewall height though, you increase the wheel size to keep the overall diameter the same, or at least close.

Applied torque is highly dependent on the length away from the axis to which it's applied (i.e. sidewall height). rFsin(theta) gives you the actual torque force applied at the axis (bead).

If you have a 3" tall sidewall (76.5mm or .0765m) and you apply the cars weight at 1G (1600kg)(9.81 ms^2) at a 90* angle to the wheel, you end up with a torque force on the bead of 1200 Nm (~885 lb/ft).

Now, drop that down to a 2" sidewall (50.8mm or .0508m) and keep everything else the same. You end up with a force on the bead of 797 Nm (~590 lb/ft)

That's a reduction of 403 Nm (~300 lb/ft) on the bead of the tire.....about 1/3.

That's a HUGE difference that you don't make up with sidewall construction alone, unless the tire is significantly heavier in construction.

All of that translates into what happens to the contact surface of the tire.

So, if you want better handling, a stiffer sidewall is one thing, but reducing the sidewall height goes a LOT farther. Deflection is not your friend when it comes to handling.
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