Notices
Wheel and Tire Forum
Sponsored by:
Sponsored by:

rolling diameter for dummies

 
Old 11-28-2003, 04:24 AM
  #1  
trebor_quitman
User
Thread Starter
 
Join Date: Jun 2001
Location: AZ
Posts: 1,111
Likes: 0
Received 0 Likes on 0 Posts
Default rolling diameter for dummies

Can someone break down the math for me? I know an inch is ~25mm, But I've been told the width plays a factor also. How much of a role does this play in rolling diameter with a reasonable PSI? Say I move from 16s to 17s and want to keep the rolling diameter ~equal, for obvious speedo issues. Is it as simple as getting a tire with ~12.5mm taller side wall rating? Thanks,
trebor_quitman is offline  
Old 11-28-2003, 11:07 AM
  #2  
maurice97C2S
Addict
Rennlist Member

 
maurice97C2S's Avatar
 
Join Date: Nov 2001
Location: Herefordshire
Posts: 552
Likes: 0
Received 0 Likes on 0 Posts
Default

Theoretical rolling diameter can be calculated quite easily as the section (or profile) approximates the percentage ratio of sidewall height to tread width -

1. convert wheel size to mm
2. sidewall height = tread width x section (225*0.4, 265*0.35 for example)
3. RD = wheel size + 2 x sidewall height

MS Excel is an easy tool for charting different options. I say theoretical because manufacturers vary in their construction, tread width and section % are not always in correct proportion.

Cheers, Maurice
maurice97C2S is offline  
Old 11-28-2003, 02:12 PM
  #3  
Bill Verburg
Addict
Rennlist Member

 
Bill Verburg's Avatar
 
Join Date: May 2001
Posts: 10,966
Likes: 0
Received 8 Likes on 8 Posts
Default

There are lots of factors that will contribute to a given tires rolling radius, not the least of which are the ideosyncasies of each manufacturer. The best thing to do is to go to each individual manufacturers web site for the correct relevant info of the tires you are interested in. Yes, wheel width does affect rolling radius and section width by stretching/compressing the cross section of the tire, a wider rim stretches the tire axially giving a greater section width and at the same time a shorter rolling radius.

At the manufacturers web site look for the following info
  • measured rim
  • section width
  • overall diameter
  • static loaded radius
  • rolling circumference
  • revs/mile
Each gives some info that is useful to know when choosing a particular tire
Bill Verburg is offline  
Old 11-29-2003, 07:43 AM
  #4  
trebor_quitman
User
Thread Starter
 
Join Date: Jun 2001
Location: AZ
Posts: 1,111
Likes: 0
Received 0 Likes on 0 Posts
Default

DOAH! Shows how much I know. All this time I thought that the section number was a mm measurment not a %age of the width... I appreciate the education. Thanks,
trebor_quitman is offline  
Old 11-29-2003, 10:30 AM
  #5  
Bill Verburg
Addict
Rennlist Member

 
Bill Verburg's Avatar
 
Join Date: May 2001
Posts: 10,966
Likes: 0
Received 8 Likes on 8 Posts
Default

How to read a sidewall;
245/45R16
245 is the nominal section width in mm
45 is the aspect ratio = section height/section width
R-radial
16 rim diameter in inches

section height = distace from the bead seat to the outer tread surface of the inflated tire
section width = distance between the outer sidewalls of an inflated tire

here are some examples form some tires i once had on my Carrera;
front nominal size 225/50R16 on an 8x16" rim, 248/36R16 when measured as installed

rear nominal size 245/45R16 on 9x16" rim, 260/34R16 when measured as installed
Bill Verburg is offline  
Old 12-22-2003, 03:12 PM
  #6  
Robby
Addict
Rennlist Member

 
Robby's Avatar
 
Join Date: Jun 2001
Posts: 3,953
Likes: 0
Received 0 Likes on 0 Posts
Default

Yes, the middle # (aspect ratio) IS a percentage of the WIDTH of the tire- in other words, when people refer to 45 "series" tires, or whatever, they are icorrectly implying that a 245/45/16 would be the same as a 315/45/16 & this is WRONG- a 245/45/16 has a RD of 24.68"- the width is 245mm & the profile is 45% OF 245mm- therefore, 110.25mm- so, you convert the 110.25mm to INCHES by dividing 110.25 by 25.4 & get 4.34"- you double this # b/c you have profile on TOP & BOTTOM (dealing w/diamater VS radius) & you get 8.68- add this to 16" & you have 24.68" RD. A 315/45/16 (IF there IS such a tire) would be 141.75mm of profile- MUCH taller profile than a 245/45 & divide by 24.4 & get 5.58"- DBL this & get 11.16" & add to 16" & get a RD of 27.16"....

you need to match the RD whenever going to a larger (or smaller) rim or you change your speedometer AND by extension, gearing... In the 951 world, most people like to go from a 16" to a 17" rim- the standard 944 turbo had 225/50/16 rear tire (24.86" RD) & the Turbo S, a 245/45/16 (24.68" RD) & ususally go to a 255/40/17 (25.03" RD)- this gives a tiny bit more footprint (255mm VS 245mm) AND a slightly taller (yet VERY CLOSE) RD match- it will HURT accel a TINY bit, as it is taller, but, is still well w/in the 2% (+or-) range of error that is considered acceptable...
Robby is offline  
Old 01-02-2004, 02:44 PM
  #7  
944Willie
Addict
Rennlist Member
 
Join Date: Dec 2003
Location: Great Pacific Northwest
Posts: 237
Likes: 0
Received 0 Likes on 0 Posts
Default

Try this site. It's a good one for comparing tire of different sizes. http://www.miata.net/garage/tirecalc.html
944Willie 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: