Could there actually be a performance loss from using larger rims and tires? I went from stock 14"s on my SE with 185/70/14 size tires to 205/55/16 on 16x7 19lb wheels. I just wanted to know if I lost performance (and I know I gained weight).

 

16x7 19lbs (plus additional tire weight) is pretty heavy, but it only matters if you notice it yourself.

 

yeah, i noticed a power loss on my wheels, thats why im switching to 17's.

 

If the overall diameter of the new tire and wheel package is larger, your gearing is going to become longer effectively, slowing down acceleration. Also, the increased weight has a large effect on the inertia the rotating wheel has. So, you aren't just increasing your car's weight by X amount of pounds, you're giving the engine more weight to turn in order to accelerate. This is noticeable enough to be a big impact on a chassis dyno, depending on the gear.

I hope the looks and added grip is worth it!

 

Welcome to rotating mass! You have more mass that you not only have to accelerate forward, but in rotation as well. It's like putting a heavier flywheel on each corner of the car. I have 17s, but I'm going to be putting down over 300hp so I need even more grip than I'll be getting from the 255/40R17s I have out back.

 

Yeah, the 18's on my FC are hurting the acceleration and the braking a bit, so im planning a down-sizing as well. The performance upsides of big rims are that they allow for wider tires, the more rubber on the road the better control. Big rims also increase the topspeed of a car, this is because the bigger the rim, the fewer rotations it takes to go the same distance as a smaller rim/tire combo.

 

Taller tires will only help top speed if your car is RPM limited rather than drag limited. If you drive a NA FC you are drag limited.

 

been wondering this for a while...wouldn't a larger tire throw off your speedometer? I'm assuming the speedometer is based on axle rotations...

 

Yes, your car will be going faster than before at the same speedo reading. If you can find a nice flat straight stretch of highway with mile markers and time one at 60, you may find that like a fair number of 7's your speedo reads high already.

 

you should also feel some slowness from getting a wider tire, more traction means a larger contact patch, which means more friction... but you prob wouldnt notice that from 185 to 205. But you might if you went from 185 to 225.

 

Its not just the difference in wheel weight, its where that weight is located. On a 14" wheel the mass of the wheel is much closer to the hub, and that means that it takes less force to accelerate that mass inward (i.e. rotate it). On a 16 or 17 inch wheel/tire combo, the majority of the mass is located on the extreme ends, since the wheel rim and the tire are closer together (assuming that both the 14" and 17" setups have a similar overall diameter with tires mounted). This requires more force to rotate simply because it is farther from the hub. This is what is slowing you down.

Brief analogy from physics I: when a figure skater does a spin and with her arms out and then pulls them into her body, the speed of the spin increases. This shows how much energy is being used to keep the outside mass of her arms spinning at the same speed as the rest of her body.

(hope I got that all correct, I haven't looked at physics stuff in a few months)

 

You got that all correct... it's why yo-yo's are outside-mass heavy

 

In reality with the power of the car and relativley low driver skill, for most people the rotational mass principle won't come in to play.

What you need to worry about is the overall diameter or the new tires on the car.

If it's different from stock, it WILL change your effective gear ratio which will affect your performance.

 

Actually, a wider tire has the same size contact patch. 185-315 all have the same contact patch on the same weight car.

 

won't come into play?

it's a pretty obvious difference, even when you're only increasing the weight of the wheel by a few lbs.

the car will have more sluggish acceleration and the steering will feel less responsive.

it doesn't take a trained driver or a certain amount of power to notice the effect.

 

Actually, a larger wheel with a lower profile tire should make steering more responsive. Less sidewall flex.

 

not in terms of performance

in terms of feel

i went from a 15" wheel to a crappy heavy 17" wheel and turning the wheel was harder simply because the wheels were just alot heavier.

 

Were the tires on the 17 inch wider than what was on the 15 inchers?

 

Wow, thanks for all the help. I removed about 30lbs of weight and hope that will get the car to what it was before (weight wise). But there isn't anything I can about the rotational mass . Thanks!

 

1.7 mph offset produced by my rim and tire package. According to a online calculator (thanks dDub!).