Go Back  Bike Forums > Bike Forums > Bicycle Mechanics
Reload this Page >

Quick tire and wheel math check

Notices
Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

Quick tire and wheel math check

Old 03-22-16, 06:27 AM
  #1  
Senior Member
Thread Starter
 
Podagrower's Avatar
 
Join Date: Nov 2012
Location: Central Florida
Posts: 678

Bikes: Giant Defy Composite, Spechialized Ruby (hers), Niner RLT9, Miyata 712, Condor ??

Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 82 Post(s)
Liked 31 Times in 19 Posts
Quick tire and wheel math check

2012 Giant Defy Composite needs a new rear wheel. The factory wheel is 22mm outside width, and I am running 25mm tires. I have a few mm clearance on the sides, but only about 2mm clearance to the seatpost. One of the rims I am looking at is 25mm wide, which in my mind will result in a slightly wider and slightly less tall wheel.
Podagrower is offline  
Old 03-22-16, 07:07 AM
  #2  
Senior Member
 
Kopsis's Avatar
 
Join Date: Apr 2008
Location: St. Pete, Florida
Posts: 1,258
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 83 Post(s)
Likes: 0
Liked 1 Time in 1 Post
Not necessarily. HED has a diagram (which I can't find at the moment) that shows actual profiles for a GP4000 on 19, 23, and 25mm (external) width rims. Going from 19 to 23 on both 23 and 25mm tires resulted in a significant height increase. Going from 23 to 25mm width rims, the height of the same tires remained the same.

In a simplified "theoretical" model, the tire assumes a circular shape. Adding width to the rim is effectively increasing the tire cross-section circumference. In this model, the tire diameter will increase by (rim width increase)/pi. So a 2mm width change would be a 0.64mm increase in both width and height. However, real tires aren't uniform in cross-section and don't assume a perfectly circular shape. Under pressure the casings tend to flatten at the sidewall so more of the rim width increase goes into increased tire height than width (up to a point).

tl;dr; your tires probably won't get shorter on wider rims and may even get taller. If you're tight on clearance, make sure you can test fit before committing to purchase.
Kopsis is offline  
Old 03-22-16, 08:36 AM
  #3  
Full Member
 
Join Date: Dec 2014
Location: Retired to Penang Malaysia originally from UK
Posts: 346

Bikes: My 1978 Raleigh from new, 1995 Trek, & constant changing & rebuilding of other bike projects.

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 41 Post(s)
Likes: 0
Liked 3 Times in 3 Posts
As the tyre size gets bigger so naturally its circumference will increase, I’ve just had this problem on rebuild where I had to drop the trye size to fit between the chain stays.

Without a photo I’m trying to understand what exactly you mean maybe I’m missing something :-

You say the distance to the seat post do you mean the seat post half way down to the BB when looking at the wheel at 9.0’ clock ? or are you referring to the seat post bridge where the brake caliper is ?.

Either way it seems a very small distance,

I’m asking a stupid question – is the rear wheel fully back in the drop out slots ?
Bike tinker man is offline  
Old 03-22-16, 12:29 PM
  #4  
Really Old Senior Member
 
Bill Kapaun's Avatar
 
Join Date: Feb 2007
Location: Mid Willamette Valley, Orygun
Posts: 13,824

Bikes: 87 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds.

Mentioned: 20 Post(s)
Tagged: 0 Thread(s)
Quoted: 1774 Post(s)
Liked 1,231 Times in 852 Posts
Visualize an incandescent light bulb.
Without changing the diameter of the "globe", imagine installing it on a larger diameter base.
It doesn't have to "curve in" as far.

Also keep in mind that tire sizing isn't always consistent.
Bill Kapaun is offline  
Old 03-22-16, 12:39 PM
  #5  
Senior Member
 
Retro Grouch's Avatar
 
Join Date: Feb 2004
Location: St Peters, Missouri
Posts: 30,225

Bikes: Catrike 559 I own some others but they don't get ridden very much.

Mentioned: 16 Post(s)
Tagged: 0 Thread(s)
Quoted: 1572 Post(s)
Liked 642 Times in 363 Posts
If my heart was set on the 25 mm rim, I'd buy it, test fit my existing tire on it and see if it works. That's knowing for sure. Everything else is just guessing.

If it was my bike, however, and my tire clearance was already tight, I'd be looking for a 22 mm wide rim to match the one that I already know just barely works.
__________________
My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.
Retro Grouch is offline  
Old 03-22-16, 12:47 PM
  #6  
Senior Member
 
RoadLight's Avatar
 
Join Date: Oct 2014
Posts: 195
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 54 Post(s)
Likes: 0
Liked 4 Times in 4 Posts
Originally Posted by Podagrower
2012 Giant Defy Composite needs a new rear wheel. The factory wheel is 22mm outside width, and I am running 25mm tires. I have a few mm clearance on the sides, but only about 2mm clearance to the seatpost. One of the rims I am looking at is 25mm wide, which in my mind will result in a slightly wider and slightly less tall wheel.
Hi Podagrower,

I agree with the previous responses that you probably won't see a significant decrease in tire height with the larger 25 mm wide rims.

Here's something else to consider: For best aerodynamics it is best to use a tire that is slightly wider than the rim. A 25 mm tire would be ideal for a rim whose outside width is 22 mm. Since your frame won't accommodate a larger tire, I would hesitate to go to a 25 mm rim---I would stay close to the 22 mm rim width. Unfortunately for those of us with older frames, the current trend is toward wider rims and larger tire widths. There are some good advantages to this. Unfortunately, there are many, many road frames in use (like yours and ours) that don't have the clearance for the bigger tires. This may limit your rim selection if you are looking for the "latest and greatest" on the market.

My Team Fuji and my wife's Specialized Ruby frames (both carbon frames) max out at a 25 mm tire size. So we're in a similar boat as you. Where we ride, wind is frequently a major factor and crosswinds are very common. For training, we've been running Shimano Ultegra wheels (WH-6800 and WH-6700). Their outside width is 21.4 mm. I'm running 25 mm wide tires and she's running 23 mm wide tires.

Kind regards, RoadLight
RoadLight is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
dvai
Road Cycling
4
07-16-19 07:03 PM
FlashBazbo
Road Cycling
27
05-14-18 07:46 AM
JoeO
Road Cycling
6
02-15-18 01:37 PM
part_robot
Road Cycling
3
05-05-15 07:14 AM
cooper34
Classic & Vintage
6
12-13-11 03:05 PM

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

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

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.