Wheel / Tire Size and Gearing Question

[47AJV8]

I'm not super experienced with running a wheel / tire combo that results in a different size than stock, but when I saw the Z/28 come out on those 19s I knew I wanted something like that, now I recognize I won't be able to run a 305 in the front as the fenders aren't wide enough, but I was looking at running a 35 aspect ratio tire on 19s and was wondering if by changing my gearing this would correct for the fact the overall wheel / tire diameter would be so much smaller? Or am I completely looking in the wrong place? Any feedback would be very much appreciated, thank you!

[SUX2BU]

Smaller tire will give you more gearing. More gearing is better for our fat cars.

[47AJV8]

Originally Posted by SUX2BU

Smaller tire will give you more gearing. More gearing is better for our fat cars.

Thanks, I see you went with 30 aspect ratio on 18s and then 40s on 18s, except with less wide tires, do you think 30 all around with 285 up front and 305 or 315 in the rear will work well? Or should I go 35 / 30 front to rear?

[Witchdoctor92]

I am running 19X10 with 285/40 all the way around. The tire is only about 3/4 of an inch shorter than stock, but the wheel/tire weighs about 12 lbs less per wheel and that makes the car quicker. I love the setup with the taller sidewall too.

[SUX2BU]

Originally Posted by nbrigdan

Thanks, I see you went with 30 aspect ratio on 18s and then 40s on 18s, except with less wide tires, do you think 30 all around with 285 up front and 305 or 315 in the rear will work well? Or should I go 35 / 30 front to rear?

I think you may be making the same mistake that a lot of people make. The aspect ratio is not a size but a ratio. It is a percentage of the tire's width. So a 35 series 315 tire is the same height as a 40 series 275 tire. Honestly I am not a fan of staggered set ups for street use. I also like to run the absolute lightest set up. My 18x10 with 285/40 all the way around are 50 lbs lighter than stock.

[jbrown13]

Here's my take on it, which may be wrong, since I've been wrong before. The diameter of the tire has nothing to do with gearing or torque. The only thing that can change the gearing of a driveline are the ratios of the transmission gears and rear end gear. What is changed by varying the tire diameter is the pounds of thrust where the rubber meets the road. An example would be a car that makes 400 ft-lbs of torque at the rear axle with a 24" diameter tire. Since you have a 1 foot moment arm you end up with 400 lbs of thrust. If you shorten or lengthen that moment arm the thrust will go up or down proportionately. If your tire diameter is too small you run the risk of more thrust than the friction of the surfaces can hold and you get wheel spin. This can be overcome by a larger diameter tire to lower thrust or a wider tire to increase the friction surface. Of course, each track and day is going to vary in friction factor, and ideally you want to look for a combination that dead hooks in all situations. There are a lot of other factors, such as percentage of rubber contacting the track (i.e. a treaded tire vs a slick), rubber compound, sidewall height and stiffness, etc, which come into play. That's way it's a lot of of trial and error to find the right combo for your car. IMHO, from a physics standpoint, the notion that tire diameter changes gearing/torque is not accurate.

[2SS45th]

Diameter changes effective gearing, and physics backs that up.

[jbrown13]

Originally Posted by 2SS45th

Diameter changes effective gearing, and physics backs that up.

The key word in your statement is effective, which is subjective, which physics does not back up. Show me the textbook and formula backing up your statement. You can't change gearing without changing gears, just not possible in the real world. The only thing that tire diameter affects (not effects) is thrust. Does thrust change what your mind senses, yes, but it is not a gear change. If this is true why are so many people changing out their rear gears and why aren't purpose built drag cars running tiny wheels?

Here are a couple of links re rear (drive) tire/wheel diameter changes:

http://www.hipermath.com/cars/rear_tire_diameter

http://books.google.com/books?id=NNW...matics&f=false

http://www.nightrider.com/biketech/c...mulas_page.htm

[47AJV8]

Originally Posted by SUX2BU

I think you may be making the same mistake that a lot of people make. The aspect ratio is not a size but a ratio. It is a percentage of the tire's width. So a 35 series 315 tire is the same height as a 40 series 275 tire. Honestly I am not a fan of staggered set ups for street use. I also like to run the absolute lightest set up. My 18x10 with 285/40 all the way around are 50 lbs lighter than stock.

Thanks, I know it's a ratio, but I was under the impression some drivetrains prefer certain overall diameters, ya just did more reading, I had always been interpreting it as the aspect ratio was the tire's sidewall in relation to the diameter/height of the rim, not the section width. Woops. Thanks for the suggestion, I am looking at running a very light setup as well, the only reason I shied away from running 18s is I thought they might look too small, they look good on your car in pictures but in person do they still look aggressive as 19s or 20s?

[SUX2BU]

Originally Posted by jbrown13

Here's my take on it, which may be wrong, since I've been wrong before. The diameter of the tire has nothing to do with gearing or torque. The only thing that can change the gearing of a driveline are the ratios of the transmission gears and rear end gear. What is changed by varying the tire diameter is the pounds of thrust where the rubber meets the road. An example would be a car that makes 400 ft-lbs of torque at the rear axle with a 24" diameter tire. Since you have a 1 foot moment arm you end up with 400 lbs of thrust. If you shorten or lengthen that moment arm the thrust will go up or down proportionately. If your tire diameter is too small you run the risk of more thrust than the friction of the surfaces can hold and you get wheel spin. This can be overcome by a larger diameter tire to lower thrust or a wider tire to increase the friction surface. Of course, each track and day is going to vary in friction factor, and ideally you want to look for a combination that dead hooks in all situations. There are a lot of other factors, such as percentage of rubber contacting the track (i.e. a treaded tire vs a slick), rubber compound, sidewall height and stiffness, etc, which come into play. That's way it's a lot of of trial and error to find the right combo for your car. IMHO, from a physics standpoint, the notion that tire diameter changes gearing/torque is not accurate.

Originally Posted by jbrown13

The key word in your statement is effective, which is subjective, which physics does not back up. Show me the textbook and formula backing up your statement. You can't change gearing without changing gears, just not possible in the real world. The only thing that tire diameter affects (not effects) is thrust. Does thrust change what your mind senses, yes, but it is not a gear change. If this is true why are so many people changing out their rear gears and why aren't purpose built drag cars running tiny wheels?

Here are a couple of links re rear (drive) tire/wheel diameter changes:

http://www.hipermath.com/cars/rear_tire_diameter

http://books.google.com/books?id=NNW...matics&f=false

http://www.nightrider.com/biketech/c...mulas_page.htm

You are right that tire diameter does change thrust, but it also changes gearing just like changing gears does. Using your logic 2 identical M6 Camaros at 6,000 RPMs in 5th gear, with the only difference being one in on a 26" rear tire and one is on a 29" tire, would be traveling at the same speed. This is just not true.

The 29" tire car would be going about 180 mph and the 26" tire car would be going about 162 mph. This is the same effect as swapping from a 3.42 gear to a 3.80 gear

[jbrown13]

Originally Posted by SUX2BU

You are right that tire diameter does change thrust, but it also changes gearing just like changing gears does. Using your logic 2 identical M6 Camaros at 6,000 RPMs in 5th gear, with the only difference being one in on a 26" rear tire and one is on a 29" tire, would be traveling at the same speed. This is just not true.

The 29" tire car would be going about 180 mph and the 26" tire car would be going about 162 mph. This is the same effect as swapping from a 3.42 gear to a 3.80 gear

I guess I'm doing a poor job explaining myself. I agree that they would be traveling at different speeds because of the circumference difference of the two tires. However, the rpm of the axles would be the same, hence the torque would be the same, and the rear gear and transmission gear is what determines the axle speed and torque at the rear axles. My point is the gear ratio has not changed, it is the tire ratio that has changed. To equate it to a gearing change could possibly create confusion in my opinion, which along with a $ (used to be 5 cents) you could buy a cup of coffee. But again, you are correct that the two cars would be traveling at different speeds.