Notices

Does going from 17in to 18in wheels reduce braking power?

 
Old 10-13-2007, 11:08 PM
  #1  
godxilla
User
Thread Starter
 
godxilla's Avatar
 
Join Date: Feb 2006
Posts: 97
Default Does going from 17in to 18in wheels reduce braking power?

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
godxilla is offline  
Old 10-13-2007, 11:13 PM
  #2  
ppressle
Addict
Rennlist Member
 
ppressle's Avatar
 
Join Date: Aug 2004
Location: Los Altos, CA
Posts: 941
Default

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
ppressle is offline  
Old 10-14-2007, 12:05 AM
  #3  
RallyJon
Weathergirl
Rennlist Member
 
RallyJon's Avatar
 
Join Date: Oct 2002
Location: SE PA
Posts: 4,842
Default

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.
RallyJon is offline  
Old 10-14-2007, 12:07 AM
  #4  
Martin S.
Rennlister Extraordinaire
Rennlist Member

 
Martin S.'s Avatar
 
Join Date: May 2001
Location: Solana Beach, CA
Posts: 7,863
Default

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.
Martin S. is offline  
Old 10-14-2007, 12:10 AM
  #5  
Greg Fishman
Addict
Lifetime Rennlist
Member
 
Greg Fishman's Avatar
 
Join Date: May 2001
Location: Austin TX
Posts: 7,165
Default

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.
Greg Fishman is offline  
Old 10-14-2007, 11:15 AM
  #6  
Quadcammer
Super User
 
Join Date: Nov 2006
Location: Clifton, NJ
Posts: 12,725
Default

weight further from hub=harder to stop.

shouldn't be a big difference, but the 18s are typically heavier.
Quadcammer is offline  
Old 10-14-2007, 12:14 PM
  #7  
H20NOO
Addict
Rennlist Member
 
H20NOO's Avatar
 
Join Date: Nov 2001
Location: CA
Posts: 1,283
Default

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
H20NOO is offline  
Old 10-14-2007, 12:29 PM
  #8  
WHB Porsche
I'm Still Jenny
Rennlist Member
 
WHB Porsche's Avatar
 
Join Date: May 2006
Location: New England
Posts: 5,199
Default

Would larger wheels affect turn-in, as the added weight gives them more gyroscopic stability?
WHB Porsche is offline  
Old 10-14-2007, 12:40 PM
  #9  
H20NOO
Addict
Rennlist Member
 
H20NOO's Avatar
 
Join Date: Nov 2001
Location: CA
Posts: 1,283
Default

Originally Posted by WHB Porsche View Post
Would larger wheels affect turn-in, as the added weight gives them more gyroscopic stability?
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
H20NOO is offline  
Old 10-14-2007, 12:47 PM
  #10  
WHB Porsche
I'm Still Jenny
Rennlist Member
 
WHB Porsche's Avatar
 
Join Date: May 2006
Location: New England
Posts: 5,199
Default

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!
WHB Porsche is offline  
Old 10-14-2007, 12:57 PM
  #11  
Martin S.
Rennlister Extraordinaire
Rennlist Member

 
Martin S.'s Avatar
 
Join Date: May 2001
Location: Solana Beach, CA
Posts: 7,863
Default

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


Spelling is not my forte (No accent on the te), pronounced like fort.
Martin S. is offline  
Old 10-14-2007, 01:07 PM
  #12  
chris walrod
Guru
Lifetime Rennlist
Member


Rennlist Small
Business Sponsor

 
chris walrod's Avatar
 
Join Date: Aug 2001
Location: yorba linda, ca
Posts: 15,608
Default

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?
chris walrod is offline  
Old 10-14-2007, 01:24 PM
  #13  
jimbo3
Licensed to Keel
Rennlist Member
 
jimbo3's Avatar
 
Join Date: Oct 2005
Posts: 11,601
Default

Originally Posted by Martin S. View Post

Spelling is not my forte (No accent on the te), pronounced like fort.
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
jimbo3 is online now  
Old 10-14-2007, 02:32 PM
  #14  
godxilla
User
Thread Starter
 
godxilla's Avatar
 
Join Date: Feb 2006
Posts: 97
Default

Originally Posted by chris walrod View Post
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?
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...
godxilla is offline  
Old 10-14-2007, 02:33 PM
  #15  
Amfab
Addict
Rennlist Member
 
Amfab's Avatar
 
Join Date: Apr 2007
Location: Los Angeles
Posts: 1,201
Default

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?
Amfab is offline  

Thread Tools
Search this Thread

Contact Us - About Us - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: