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

[MIKe30]

My significanly undersized tires and some pulls with an e46 m3 got me thinking and brings us to this thread.

Long story short, I am only pulling 2 cars on an e46 m3 - and frankly, I think it should be more. I know my motor has yet to be fine tuned, but dyno'd 261whp/231wtq so it shouldn't be as close as it is. Sometimes it's only 1 car.

My tires are pretty undersized. I was running 215/40/16's in the rear, I just bought and now have mounted 225/45's. The 215's looked like rubber bands compared to the 225's. According to my logic and calculations, there should be a noticeable difference in acceleration (not by the butt, but by comparison when running alongside another car) and even a noticeable difference on the dyno.

I am not going to post my calculations or my logic quite yet... I want to see your thoughts on the matter first and what the general consensus is.

Discuss.

[Flash]

With a shorter overall rolling diameter, it WILL be faster, it is just changing your axle ratio (read: not gear ratio) even further. A lot of the old 2002 guys used to do that. I could feel the difference on the 5 with going from 225/60/15 to 225/50/15, HUGE difference.

[lifeiskaos]

So you're saying he's going to be slower with bigger tires?

[Mossman]

So you're saying he's going to be slower with bigger tires?

Yes...if he had traction before he will have less acceleration with the new tires. If the compound/tread depth/width gives him more traction with these tires than he might be faster now, and if the race had him shifting once more than the e46 with the old tires he might be faster now, but 5% shorter tires are like going from a 3.73 to a 3.91...thus more acceleration and shorter gears. Also smaller tires are lighter but that's harder to quantify, but will give more acceleration as well.

[MIKe30]

With a shorter overall rolling diameter, it WILL be faster, it is just changing your axle ratio (read: not gear ratio) even further. A lot of the old 2002 guys used to do that. I could feel the difference on the 5 with going from 225/60/15 to 225/50/15, HUGE difference.

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...

Yes...if he had traction before he will have less acceleration with the new tires. If the compound/tread depth/width gives him more traction with these tires than he might be faster now, and if the race had him shifting once more than the e46 with the old tires he might be faster now, but 5% shorter tires are like going from a 3.73 to a 3.91...thus more acceleration and shorter gears. Also smaller tires are lighter but that's harder to quantify, but will give more acceleration as well.

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.

[Raxe]

Smaller overall diameter increases acceleration, larger overall diameter increases speed.

[Philo]

The key here is traction. If you were getting traction with the smaller tires, then . . .

[Flash]

Dont get me wrong, if you google it you will see the information out there, but people have been using a shorter axle ratio to increase acceleration. You will end up using more revs to achieve the same speed as a taller rolling diameter thats the only downside is fuel economy but lets be honest if you are (people in general) wanting to do motor swaps, and increase power-fuel economy is the last thing to complain about :D
ive been going to the dragstrip and the size alone makes a HUGE difference, and i will never see the high end of 100+ MPH so it has no downside other than indicating that you are going faster than you actually are. In my case 80 MPH on my speedo means im really going 76 (with GPS).
Unsprung weight is always a key player but if you could keep the same weight (lets say a mag wheel) then you will accelerate quicker still with a overall lower rolling diameter.

[MIKe30]

ahh fuck. So instead of pulling only 2 cars on this damn e46 m I'll be looking at 1.5 cars? bahh. So many factors. guess the only real answer is get this bitch fine tuned and hopefully blow his doors off? lol

[ethree]

What are your times? Drivers have a lot to do with drag racing not just cars.

[Rustyduktape]

the sad thing is, i just learned more reading this thread than ive learned in the past 2 days of school.

[Mde36]

Smaller overall diameter increases acceleration, larger overall diameter increases speed.

thats basically all you need to know

[reelizmpro]

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.

[Wh33lhop]

ahh fuck. So instead of pulling only 2 cars on this damn e46 m I'll be looking at 1.5 cars? bahh. So many factors. guess the only real answer is get this bitch fine tuned and hopefully blow his doors off? lol

It depends, there are many factors (traction, number of shifts, etc) but in-gear acceleration will be better with the smaller tires in your case.

Also keep in mind the E46 is more slippery than the E30, you most likely have a good aerodynamics disadvantage.

[Wh33lhop]

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.

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