HOT: Active* forum members generally gain 5% discount at starbike.com store!
Weight Weenies
* FAQ    * Search    * Trending Topics
* Login   * Register
HOME Listings Articles FAQ Contact About




Post new topic Reply to topic  [ 6 posts ] 
Author Message
PostPosted: Sun Feb 24, 2013 8:17 pm 
Offline
User avatar

Joined: Tue Nov 18, 2008 9:57 pm
Posts: 678
Location: Canada
I'm going full nerd mode at this point and trying to figure out the best rim for a wheel build, looking at weight on the rim versus weight on the tires for a given tire volume.

For example, I know from my road wheel building that going from a rim with a 15mm internal width to one with a 17.5mm internal width allowed my tire to increase from 26mm width to 28.4mm width - quite a nice increase given that the rims used in this particular instance weighed the same.

For mountain biking, I'm interested in comparing something like a Stan's Crest rim (380g & 21mm internal width) versus a Pacenti TL28 (450g & 23mm), and I'm curious to know what the effective increase in tire volume is. It seems that for most high-end tires, a 0.2 inch increase in tire size (eg from a 2.0" model to 2.2") usually corresponds to an increase of around 50g.

So, I'm very curious to know if anyone has access to different rims like these (or any other rims where the internal widths are similarly measured and slightly different) so that they could mount the same tire to each and see what the resulting difference in mounted tire width ends up being between the two.

It would seem that if the difference is 0.3" or more, then it's a no brainer to go with the heavier rim. If it's less then that, then it's back to balancing out all of the other attributes of each rim, but at least we'll have a better idea of how to consider the effective system-weight that each rim would offer.


Top
 Profile  
 
Posted: Sun Feb 24, 2013 8:17 pm 


Top
  
 
PostPosted: Mon Feb 25, 2013 7:28 pm 
Offline
User avatar

Joined: Thu Oct 26, 2006 8:54 pm
Posts: 377
Location: USA
If you make a simple model where the rim is width W, the tire's "flattened-out" width is L (ie distance from bead to bead), its inflated/installed width is D, and you assume that the tire takes the shape of a circle who's bottom is cutoff by the rim... then there are two interesting regimes:

Fat rim (where D ~ W.... tire looks like a half circle with rim width roughly the diameter):
* dD / dW ~= 1

Narrow rim (where D >> W):
* Rate of change of width of tire, D, with rim width, W, is:
dD / dW ~= 1 / Pi < 1

With your road bike rim I think you're pretty close to the first regime: the widths of the rims are fairly comparable to the inflated width of the tire, and for every 1mm increase of rim width you therefore get an increase of roughly 1mm of inflated tire width all else being equal.

Unfortunately, with the mountain bike, I think you're really part way between these two regimes, and so adding 1mm in rim width will get you somewhere between 1/Pi ~ 0.3mm and 1mm of increased tire width. Given that 0.2 inches is 5mm, getting 5mm from the tire change costs you 50g, whereas with the rim change for 70g the 2mm of rim width increase will probably only get you something like 1mm in tire width. So there's really no comparison here at all in terms of the two.
Of course, as you said, that's not to say that there aren't other variables that might make the wider rim worthwhile. But weight per tire width is definitely not one of them.

_________________
Enjoying my Funk


Last edited by dwaharvey on Mon Feb 25, 2013 9:17 pm, edited 1 time in total.

Top
 Profile  
 
PostPosted: Mon Feb 25, 2013 8:00 pm 
Offline
User avatar

Joined: Wed May 13, 2009 5:23 pm
Posts: 596
Location: Denmark
I must've missed something in your post, dwaharvey. First off, what's D (did you mean L as you defined in the beginning)?

Second, is dD / dW supposed to be a derivative? What's the reasoning behind this?

_________________
Cannondale F29 Carbon 1 2013


Top
 Profile  
 
PostPosted: Mon Feb 25, 2013 9:19 pm 
Offline
User avatar

Joined: Thu Oct 26, 2006 8:54 pm
Posts: 377
Location: USA
Yeah sorry, forgot to define D... the inflated width of the tire. I edited the post to define it explicitly.

Yeah the dD/dW is a derivative: the rate of the change of the inflated width with the rim width. The reason to do this is simply because it makes the math easier and we're not interested so much in what the exact width might be, but how much it will change for a given change in the rim width.

_________________
Enjoying my Funk


Top
 Profile  
 
PostPosted: Mon Feb 25, 2013 10:12 pm 
Offline
User avatar

Joined: Tue Nov 18, 2008 9:57 pm
Posts: 678
Location: Canada
Thanks dwaharvey - I was working the numbers around in my head a bit on today's ride and realized that there would definitely be diminishing returns, but it's very useful to have it expressed as a formula.

I guess the increased stiffness is going to be the big decider now... but that sure is a lot of weight!


Top
 Profile  
 
Posted: Mon Feb 25, 2013 10:12 pm 


Top
  
 
PostPosted: Mon Feb 25, 2013 10:20 pm 
Offline
in the industry

Joined: Sat May 12, 2012 7:25 pm
Posts: 1226
Location: Glermsford, Suffolk U.K
There is the velocity P35 if you realy want wide.

_________________
www.thecycleclinic.co.uk


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 6 posts ] 


Who is online

Users browsing this forum: Google [Bot] and 16 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  

   Similar Topics   Author   Replies   Views   Last post 
There are no new unread posts for this topic. 20.5 or 23 mm rim width for 23c tubular tire?

in Road

Boss93

6

583

Wed May 14, 2014 7:00 am

tinozee View the latest post

There are no new unread posts for this topic. Roubaix SL4 max tire width

in Road

dha

3

225

Fri Jul 11, 2014 7:23 pm

dunbar42 View the latest post

There are no new unread posts for this topic. Tire "Volume"

in Everything wheels

mellowJohnny

3

208

Fri Feb 21, 2014 10:34 pm

bm0p700f View the latest post

There are no new unread posts for this topic. Tires: Volume vs weight

in MTB

jazclrint

6

876

Fri Oct 18, 2013 3:01 am

jazclrint View the latest post

There are no new unread posts for this topic. Is it more cost effective to be your own mechanic?

[ Go to page: 1, 2, 3, 4, 5 ]

in Cycle Chat

carbon2329

74

3967

Sun May 11, 2014 3:42 am

bombertodd View the latest post


It is currently Thu Jul 31, 2014 9:13 pm

All times are UTC + 1 hour




Advertising   –  FAQ   –  Contact   –  Convert   –  About

© Weight Weenies 2000-2013
hosted by starbike.com


How to get rid of these ads? Just register!


Powered by phpBB