Wheel sizing vs. power question
 

I recall that when I went from 18" to 19" wheels with my 350Z, I could definitely feel a loss of power, likely due to the increased rotational enertia of a bigger wheel.
Has anyone who has gone from 19's to 20's on their 370Z felt the same thing?

Aside from the raw effects of wheel weight and wheel diameter in terms of rotational mass / inertia, the effective gearing is also changed. Making the wheels larger has the same gearing effect as changing your LSD's ratio in the bad direction (closer to 1:1), whereas making them smaller moves the ratio back in the good direction (closer to larger_number:1).

I went from the stock 19's to a lighter (but wider) set of 18's and the difference was very noticeable.

circumference stays the same. gross weight is wat matters...even that you're using the right size wheels and tires.

The circumference has to be larger by definition when you upsize the wheels, right? (well, unless you offset larger wheels with thinner tires, or vice-versa)

The diameter would, of course, stay the same.....tires with 1" lower profile would be necessary. However, I've found that a larger wheel, even though the diameter of the tire stays the same, will sap some power. Am I wrong about this?

the tire's most outer circumference stays the same. you offset the larger diameter wheel by using shorter sidewall tires.

remember that the purpose of using larger diameter wheels it to allow larger brakes. quicker turn-in response is just a nice sideeffect.

i switched up from 19in te's to 20in ssr's hoping it wud slow me down...it didn't.:ugh2:

and yes, as everyone above has stated, circumference stayed the same

kenchan is right. ive heard this reffered to as "rolling diameter". usually they will offer you the thinner tire to compensate the growth in wheel size, unless you specify otherwise. and vegas, right again about the power sap on the bigger rim, regardless of the "rolling diameter" remaining the same. why? aside from obvious weight gain? i dont know. but i have experienced this on two vehicles. must, maybe the rims you chose were close enough to the weight of your old rims, so you didnt get power loss? just guessing

I am thinking of replacing my rears with 285/35,i wonder what effect wiil have.??

i never changed width and i have no idea how that would be. maybe affect handling? my biggest concern (for myself) about going up on rim size is not any marginal power loss, but ride comfort. the less rubber under you, the bumpier it will feel. and on louisiana roads, its a good way to beat the crap out of your car and have everything rattling in no time.

Okay increasing diameter of the wheels will allow for bigger brakes, increase turn-in response, reduce acceleration but increase top speed.

Also well a 17inch rims and 18 inch rim may have the same rolling diameter (difference in sidewall height) and mass. Having more of the mass near the outside of the wheel will hurt performance compared to having more the mass near the center of the wheel. So going with a lighter weight tire helps a lot.

it wont increase top speed....

some of you guys are making generalizations without understanding how it works.

the overall diameter of the wheel + tire is the item your looking for + weight.

If the wheel weights more, even if its the same size as your OEM, it would slow you down. Whether or not you will feel it, would be determined by how much weight is added.

If you upsize your wheels, you can downsize the side wall ratio to keep the same overall OEM diameter and therefor your TOP speed / acceleration would not be altered DUE to the size. it would, most likely be altered due to the weight and the shifting of the weight load to the outer radius where it would sap the most power.

I went to a 20x12 wheel, with a 325/25/20.

Run the calculations on the overall diameter compared toa 275/35/19. You will find my overall diameter is shorter by about .17" which technically means i should accelerate faster. but since its a 20x12 wheel with massive rubber, i know it weighs more than the OEM and therefore realistically would slow me down. Can i feel it? not really. I dont go full throttle everytime i drive my car all day long and so i wasnt attuned to the oem vs these. the car is still quick as hell.

In my case, when I dropped from 19 to 18, combined with my tire changes I did reduce the overall circumference. And yes, mass matters more the further it is from the center of the wheel.

The "top speed" factor isn't worth mentioning since the gearing on our cars goes way past the point of drag-limit anyways. You'll never reach the top speed of the gears+wheels regardless.

"Increased turn-in response" isn't a function of wheel size AFAIK. It's a function of running shorter (as in thinner) tires, which you can do on any reasonable wheel size.

The only good "performance" argument for increasing your wheel size is to fit larger brake rotors. Otherwise the ideal is a smaller wheel with a low profile tire.

found this yesterday . motordyne comparing different wheel size and there effects . Motordyne Test of Wheel Diameter Vs HP

I meant increasing overall diameter will increase top speed.