From 18's to 19's, should i be accelerating faster or slower?
 

I know your all reading the title and thinking duh, slower. But i went from 18's with 245/45s in the rear that weighed a combined 58 lbs.

And now im running 19's with 275/35s in the rear that weigh 48 lbs combined.

WHat do you guys think?

What kind of difference will it make?

You will probably hardly feel the difference. When I switch between light 18's and heavy 19's, I notice a handling/braking difference but not much of a straight line acceleration difference.

If you weigh less you should be faster, even if it is imperceptible or by a marginal amount.

To be honest, it's a toss up.

rotational inertia

doesn't matter if the static mass of your 19s is lower than the static mass of your 18s

the fact that MORE of the weight has been moved further OUT towards the circumference of the rim means that your car WILL have a harder time turning it

I agree, however i too will say that I doubt you wil notice much of a performance/acceleration difference. I wouldn't worry about it much in all honesty.

That's true but most of the weight is in the tires. And the overall diameter of the tires is about the same.

I honestly think this one's a toss up, not much difference either way.

+1 :)

The wheels with the stickiest tire will make the car accelerate faster... ;)

I know there has been a discussion on this and something about sprung and unsprung weight. I thought that if you were able to reduce overall wheel weight, it's effect is magnified since it is sprung weight. I don't know much more, but I think someone said it was 10:1. I think that is way too much, but I'm no expert. I do think if you can reduce 40lbs off the wheels it is more important than reducing 40lbs of body. However, it all adds up.

Yes, reducing unsprung weight is “better”.

The suspension will react faster, steering also. Acceleration and braking will benefit from it.

But it’s not as simple as adding (or subtracting) the weights and putting in a multiplier. It might give a general idea, but nothing conclusive.

We’d have to dyno the individual wheels/tire combo to get the inertia. And again, the tire choice will affect the handling more than a couple pounds saved in the rim. Side wall compliance, grip, feedback, slip angle characteristics, bla bla…

Edit: The main reason track guys go to 17in wheels is tires availability and price, not weight saving. If I could afford it, I’d go for a 19in magnesium wheels with giant brakes and call it a day! The performance benefits would be quite appreciable.

Before anybody points to F1 running 13in wheels with balloon tires, know that it’s a cost and performance reduction regulation put in by the FIA. Bigger rim is preferable provided you can keep the weight down.

Well i did also lose weight in the tires.

The Potenza RE040's weigh 32lbs in size 245/45/18
The Pilot Sport PS2's weigh 27lbs in size 275/35/19

Im pretty sure the PS2's grip much better then the RE040's.

Well, it's kind of a tossup isn't it? Lets exaggerate for the purpose of illustration.

Lets pretend for a second you have 20 inch rims that weigh 1lb a piece, and also pretend they are super strong. I would guess that the car with the 1lb rims would accelerate significantly faster in my opinion, than say, 16 inch rims that weigh 15lbs a piece.

I've went from 16's to 22's and noticed a dif and even 22's to 24's but thats alot more weight. loosing 10 pounds of rotating mass will help its why people buy lightened driveshafts and flywheels.

But it doesn’t work that way in life.

What’s the geometrical difference between a 16in wheel and a 19in wheel? Let’s imagine where using the same design for both, one just bigger.

The hub is virtually the same.

The spokes will be 1.5in longer, adding material near the circumference of the rim

The rim itself will have more volume (assuming we use the same rim cross-section x circumferance)

So if both wheels are cast the same way, the 19in will be much heavier.

Now the fun part. Even if we used a different material to make both wheels weight the same, the 19in wheel will still have more inertia because of the longer spokes.

Thanks for letting me know it didn't work that way in real life ;) I exaggerated for the purpose of illustrating a point. My highschool physics professor taught me that.

But your explanation doesn't account for rims that are significantly lighter. Lets take 25 lb 17's and 15 pound 18's for example. The benefits of the light weight may be cancelled out by the added rotational intertia, thereby making it equal to the 25lb 17, but there's no way the 17" rim is going to be faster. I would still be willing to bet that a 15lb 18 would help the Z accelerate quicker than a 25lb 17.

my wheel and tire combo was 10 pounds lighter and i was .2 sec faster in 0-60

Again, in real life (:icon39: ) we won't know what the rotationnal inertia will be unless we dyno both wheels.

Obviously, a 40% weight differance will offset most common wheel diameter. So you can win this one. :cool:

But if the 17in/25lb guy used the money saved on his wheels to buy better tires, he might still "win" the race ;)

But, but, but...

My "cheap" 5Zigen 17in wheels are lighter than my OEM forged 18in wheels. So I "win" anyways ! there! hehe :p

That was my whole point. I exaggerated with the 1lb wheel example to show that a weight difference is goingg to offset any benefit of a wheel diameter. I'm sure If i wanted to pay out the ASS I could find a ridiculously light pair of rims, even if I had them made custom.

Side question: But anyways, I forgot from the other hreads, do you run your 17s all the time? Or just at the track?

We'll, in a straight line and assuming no wheel spin, sure, the lighter the better.

But there are so many forces at work dynamically, that a generalization can't take all situation into account. Cornering for exemple. The less sidewall, to more talkative the steering will be. The rim width will play also, as well as the offset and it's effect on the srcub radius...

The problem for us is that we don't custom design our wheels. We have to take a set with fixed compromises. That's why I say that taking the weight only into account is not great.

My 17" are for the track only. They are 255/40R17 and 275/40R17
For the street, I have my OEM wheels with 235/40R18 and 255/40R18

Rotational inertia (Moment) will affect the accleration while weight/mass alone affects suspension and steering(also has gyro effect).

Rotational inertia is a function of Mass and distance of the mass to center of rotation (SI unit Kg.m^2). Thus the location of mass about the center of rotation affects inertia. Since its a function of square of distance (radius of gyration), larger wheel of the same weight/mass will have higher inertia and will increase will increase 4 times if the radial increase was 2. This will in effect be sluggish, thus the ideal (theoretically) is mass lumped in the center.

It will be desirable to do a trade off of which tire/wheel combo is optimal for both mass and inertia (Moment). Lighter mass for good suspension response and smaller diameter for better accleration response.