Undersized vs. Proper sized rubber... significanly affects speed?

[shiftbmw]

Ah, so you're saying it will be SLOWER after I change from 215/40/16 to 225/45/16. I hope you are wrong as I beg to differ. My explaination below...



Interesting. Here is how I see it...

215/40/16's - 22.77 = diameter = 71.278 circumference = 888.9 rotations/mi
225/45/16's - 23.97 = diameter = 75.2658 circum = 841.8 rot/mi

Took an example at 120mph. I chose 120 because an indicated 120mph on the speedo only registered 111mph on the gps.

I caculate that 120mph = 2112 ft/s.

gives us,

215/40/16 = 29.63 rotations/sec @ 120mph
225/45/16 = 28.06055 rotations/sec @ 120mph

and I see this two ways.

1st way is 1.6 rotations/sec (difference @120mph) x 3.9878 inch circumference difference = 6.38048 inches/sec at 120mph.

so I should be gaining 6 inches per second with the larger tires at 120mph. Half of that at 60mph. Figure a pull lasts 10 seconds, avg 5 inches/sec, give you 50 inches or apprx 4 ft, about a 1/3 of a car. Don't know if that logic is quite right though...

2nd way is the smaller tires have to rotate 29.63 times per second to move the car at 120mph. The larger tires only have to be going 28 rotations per second. The difference in weight of the rubber is only 2 lbs (not including weight of more volume of air), so I assume that is neglible. I can't imagine the 2lbs affecting anything enough to make the rate at which the wheel gets to that speed any slower, so they wheels should get to 29.63 rotations/sec in the same amount of time.

The 215/40's at 29.63 rotations is at 120mph.
The 225/45's at 29.63 rotations is at 126.7mph.

I don't see how it could make the car slower.

Per rpm, with a larger circumference tire you will be traveling at a higher speed. In terms of acceleration, your logic is flawed.

You essentially apply a certain torque to your rear axles(so lets say about the axis of the wheel).

Your torque remains the same about the axis of the wheel no matter what tires you have, because that constant is driven by your motor and driveline(gearing, frictional loss). You are concerned with force applied at the ground and the distance from the wheel axis to the ground is affected by tire size(this distance is tire radius)

Torque = Force times distance. The higher the distance(larger the radius), the lower the force you apply at the ground.


It's more complicated than that in real life, more variables but that's the high school physics explination.


The pic below diagrams what I was talking about. It's not a great diagram, but it shows you the moment, radius, and force at least:

[Mossman]

I think you've figured it out but yeah you're thinking was flawed. The engine only has so much power, it is not going to accelerate at the same rate no matter what diameter tire is on it, if that was the case we wouldn't need transmissions we'd just gear it to go 200 and hammer down. Gearing is very important, and probably with all that whp already aside from tuning gearing and weight reduction are about all you can do to go faster, aside from forced indicution.

[MIKe30]

Not only will you effectively make your gearing longer with a much larger overall diameter but you'll also increase drag and (unsprung) weight from going with a bigger tire. They also tend to be more expensive and may rub the body. You have to weigh the pros and cons. Are the gearing and weight penalties worth more traction? From your post it sounds like traction isn't the issue.

Just slightly rolled the fenders, no rubbing with my 225/45/16's... but my 235/40/17's just barely rub the rear ugh. Need a flat roll to fit the 17x9's :) This topic is only in regard to the 225/45/16's though.

And as far as your question goes...

He's got 260whp in a 27-2800lb E30 on 215s, my guess is traction is a bit of an issue.

This.

Yes, 1st gear is useless. 2nd gear would be useless if I didn't feather the throttle after ripping through 1st.

Per rpm, with a larger circumference tire you will be traveling at a higher speed. In terms of acceleration, your logic is flawed.

You essentially apply a certain torque to your rear axles(so lets say about the axis of the wheel).

Your torque remains the same about the axis of the wheel no matter what tires you have, because that constant is driven by your motor and driveline(gearing, frictional loss). You are concerned with force applied at the ground and the distance from the wheel axis to the ground is affected by tire size(this distance is tire radius)

Torque = Force times distance. The higher the distance(larger the radius), the lower the force you apply at the ground.


It's more complicated than that in real life, more variables but that's the high school physics explination.


The pic below diagrams what I was talking about. It's not a great diagram, but it shows you the moment, radius, and force at least:

Thanks for the clairification. It makes a little more sense now. Can't possibly make the same amount of force to the road with the same amount of torque being applied. Should probably make my dyno numbers go down a point or two (with the understanding that dyno peak numbers are useless)...

I did take AP physics in hs, just been a while :p

I think you've figured it out but yeah you're thinking was flawed. The engine only has so much power, it is not going to accelerate at the same rate no matter what diameter tire is on it, if that was the case we wouldn't need transmissions we'd just gear it to go 200 and hammer down. Gearing is very important, and probably with all that whp already aside from tuning gearing and weight reduction are about all you can do to go faster, aside from forced indicution.

Time to clean up this little vac leak that's driving me nuts and fine tune the bitch. Anyway, this is what it looks like now with the 225's on it. All this talk about them we might as well see them right?? Dirty and phone pic but you get the idea...

[pbr87]

The 215/40's at 29.63 rotations is at 120mph.
The 225/45's at 29.63 rotations is at 126.7mph.

I don't see how it could make the car slower.

Because has a similar effect as raising your gear ratio. You can go faster in every gear, but your acceleration is a little slower.

[pbr87]

per rpm, with a larger circumference tire you will be traveling at a higher speed. In terms of acceleration, your logic is flawed.

qft

[Philo]

Then you need wider tire, not taller ones. lol.

Got grip? :)

[SpyHunter]

Then you need wider tire, not taller ones. lol.

Got grip? :)

Actually, that is incorrect. While a wider tire would help with acceleration, it will only help marginally. Wider is primarily better for lateral forces. A taller tire, if widths are the same, will have a greater impact on acceleration grip.