godxilla

I recently switched out my 17in wheels for a set of porsche 18in wheels and tires. Since installing the 18s, it feels like my braking power is now reduced. I know 18s are heavier wheels and tires, does this effect the braking ability?

thanks

ppressle

I once filled a friends bike tires with water as a gag... That baby had braking problems!

The 18 wheels may have more rotational intertia, but that small difference should not be perceivable.
The overall diameter should be about the same between the two wheels. That is all that matters.

Pete

RallyJon

Given that an 18" tire will have a wider and shorter contact patch, that makes sense.

But likely it's what Pete said--you probably picked up 5lbs of unsprung, rotating mass per corner and you're feeling it.

Martin S.

18" gave you a wider tire patch...better for breaking. Breaking is a lot about tires. If the tires you ran on the 17" and the 18" were the same, breaking should be better with the 18"...assuming the same break pads.

Greg Fishman

Martin, I think you mean "brake" not break....

I agree with you, it is probably a difference in tire compound or age. Everything else being the same the 18's should be better.

Quadcammer

weight further from hub=harder to stop.

shouldn't be a big difference, but the 18s are typically heavier.

H20NOO

Braking power is the same but, you are now braking more weight and that weight is pushed out further from the axis. So, you will need more pedal pressure to stop the car. That doesn't mean the brakes aren't up to the task. And, if you have a larger or stickier contact patch with the new tires, your braking distances could actually be shorter but - you'll have to push harder on the center pedal than you used to.

Larger wheels almost always weigh more and they push that weight further from the axis (center) where it is more difficult to stop. These rules (of physics) also apply to acceleration so don't be surprised if your car feels like it's not as quick as it used to be.

MC

WHB Porsche

Would larger wheels affect turn-in, as the added weight gives them more gyroscopic stability?

H20NOO

Yes, larger, heavier wheels increase the gyroscopic effect which makes them harder to turn, especially as speed increases. Heavier wheels may also necessitate the use of stiffer shocks because once they react to road imperfections, there is more mass to slow.

In short - there is virtually no performance advantage to running larger wheels unless they allow a larger tire contact patch. Even then, is it worth losing acceleration, braking performance and steering feel to run a larger wheel/tire combo? Only, if the looks are more important than performance.

I've written extensively about this on the 997 forum. I took the 19" wheels off my 997 and put 18" carbon fiber/magnesium wheels on it. The effect is very noticeable, feeling very similar to a lightweight flywheel. The car accelerates more quickly, brakes shorter with less pedal effort and has lighter steering feel.

https://rennlist.com/forums/showthre...ighlight=dymag


MC

WHB Porsche

Very interesting. I wasn't sure if the lateral rotation (vertical axis) of the wheels would be affected by the forward/backward rotation of the wheel. The side to side (about the lateral axis) would definitely be affected. Thanks for the info, those Dymag wheels are gorgeous!

Martin S.

Greg writes, "Martin, I think you mean "brake" not break...."


Spelling is not my forte (No accent on the te), pronounced like fort.

chris walrod

What are the tires you have on the new wheels and the old? Are their treadwear ratings similar? Condition similar?

I feel rubber to be the most significant variable here.

Is the braking differences you feel on-track? On street?

jimbo3

Martin, you are only the third person I've run across who knows how to correctly pronounce "forte". So, you are forgiven for mis-spelling "brake", but don't ever let it happen again, mister....

-Jim

godxilla

The tires i have on my 18s are a little bigger in overall diameter than my old setup on 17s. I am using the tires that came with the 18s take-offs, they are 235/40/18 fronts, and 265/40/18 in back. that could be playing a role in my problem, i assume. I am also feeling that it has effected my low speed steering some too. the steering seems a little vague at low speeds. i really dig the way my 18s look, but i am not fond of all these sacrifrices; braking, steering and bumpiness...

Amfab

Hmm, I pronounce forte: "Forty" but, yo, its easier to rhyme with when Im spittin' out some dope hip hop lyrics homey...

I been seein dat different tires -even da same size
can affect my steering turn-in wize,
But, yo, on suspensions, I ain't no master
can you fix 'dis sh*t by f*ckin' wid da caster?