Go Back  Bike Forums > Bike Forums > Road Cycling
Reload this Page >

Wider tires, higher air volume, lower pressure. Why?

Notices
Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Wider tires, higher air volume, lower pressure. Why?

Old 03-09-20, 10:50 PM
  #76  
Gyro_T
Senior Member
 
Gyro_T's Avatar
 
Join Date: Apr 2011
Location: Oregon
Posts: 241

Bikes: 1974 Jack Davis, Zeus Competition, Bridgestone MB2, Kona Rove Ti 1 X 11, Salsa Fargo Ti

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 10 Post(s)
Liked 2 Times in 2 Posts
Originally Posted by smashndash View Post
https://flocycling.com/blogs/blog/fl...or-wider-wheel

I think you’d want to keep casing tension constant. This article seems to provide a guide on how to do that.
I read the article and plotted the data for pressure vs. diameter. It is very nearly a linear relationship. I think calculating the casing tension may be a more accurate way of determining the "pressure equivalency" I have been seeking. I had a gut feeling it was not a simple proportionality, e.g. the area of tire A 28mm tire=615.7mm2, area of tire B 44mm tire=1520.5mm2. A simple proportionality would say that a 28mm tire at 75 psi would be 30.4 psi in a 44mm tire: Tire pressure A* area of Tire A= Tire pressure B * area of Tire B, so in this case 75 X 615.7= x X 1520.5. Solve for x which give you the 30.4 psi for the 44mm tire. 1) it seemed too low, and 2) too simplistic. I think the casing tension may be exactly what I have been looking for and I will wade through the formulas with my figures. Thank you kindly.
Gyro_T is offline  
Old 03-09-20, 10:56 PM
  #77  
smashndash
Senior Member
 
Join Date: Feb 2018
Posts: 835

Bikes: 2017 Specialized Allez Sprint Comp

Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 489 Post(s)
Liked 113 Times in 85 Posts
Originally Posted by Sy Reene View Post
oh boy.. this thread is back... we should now discuss why wider tires are more prone to flats, since their contact patch is wider, x% greater chance of running over whatever tiny debris flats a tire.
No need. Looks like the person who revived it found the specific piece of information they were looking for. Nothing left to see here.
smashndash is offline  
Old 03-10-20, 12:27 AM
  #78  
79pmooney
A Roadie Forever
 
79pmooney's Avatar
 
Join Date: Oct 2014
Location: Portland, OR
Posts: 7,962

Bikes: (2) ti TiCycles, 2007 w/ triple and 2011 fixed, 1979 Peter Mooney, ~1983 Trek 420 now fixed and ~1973 Raleigh Carlton Competition gravel grinder

Mentioned: 98 Post(s)
Tagged: 0 Thread(s)
Quoted: 2093 Post(s)
Likes: 0
Liked 461 Times in 338 Posts
Originally Posted by 2manybikes View Post
Every time I start to count the air molecules I lose track and have to start over.
Put em in a bag and weigh 'em. Or just use the perfect gas law. PV = nrT. n = number of molecules, r is a constant.
79pmooney is offline  
Old 03-10-20, 06:32 PM
  #79  
waters60
Senior Member
 
waters60's Avatar
 
Join Date: Sep 2010
Posts: 349
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 81 Post(s)
Likes: 0
Liked 35 Times in 14 Posts
If you have 30 minutes or so this podcast from VeloNews is very in depth: https://www.velonews.com/2020/03/gea...roubaix_506668
waters60 is offline  
Old 03-10-20, 07:41 PM
  #80  
tomato coupe
Senior Member
 
Join Date: Jul 2009
Posts: 900

Bikes: Colnago, Van Dessel, Factor, Cervelo, Ritchey

Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 542 Post(s)
Liked 291 Times in 155 Posts
Originally Posted by 79pmooney View Post
Put em in a bag and weigh 'em. Or just use the perfect gas law.
Ideal Gas Law.
tomato coupe is offline  
Old 03-10-20, 11:08 PM
  #81  
79pmooney
A Roadie Forever
 
79pmooney's Avatar
 
Join Date: Oct 2014
Location: Portland, OR
Posts: 7,962

Bikes: (2) ti TiCycles, 2007 w/ triple and 2011 fixed, 1979 Peter Mooney, ~1983 Trek 420 now fixed and ~1973 Raleigh Carlton Competition gravel grinder

Mentioned: 98 Post(s)
Tagged: 0 Thread(s)
Quoted: 2093 Post(s)
Likes: 0
Liked 461 Times in 338 Posts
Originally Posted by tomato coupe View Post
Ideal Gas Law.
It's been a very long time since I needed to count those molecules. Perfect? Ideal? My memory is neither.
79pmooney is offline  
Old 03-11-20, 07:47 AM
  #82  
eduskator
Senior Member
 
eduskator's Avatar
 
Join Date: Aug 2019
Location: Québec, Canada
Posts: 655

Bikes: TCR Advanced Pro 0 Disc

Mentioned: 4 Post(s)
Tagged: 0 Thread(s)
Quoted: 297 Post(s)
Liked 130 Times in 104 Posts
Fat bikes are definitively the way to go! You can run them at 5PSI!
eduskator is offline  
Likes For eduskator:
Old 03-16-20, 10:03 AM
  #83  
Gyro_T
Senior Member
 
Gyro_T's Avatar
 
Join Date: Apr 2011
Location: Oregon
Posts: 241

Bikes: 1974 Jack Davis, Zeus Competition, Bridgestone MB2, Kona Rove Ti 1 X 11, Salsa Fargo Ti

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 10 Post(s)
Liked 2 Times in 2 Posts
As was pointed out above, the "ideal gas law" or Boyle's law do not apply unless the moles of gas are constant.

Last edited by Gyro_T; 03-16-20 at 12:31 PM.
Gyro_T is offline  
Old 03-16-20, 10:16 AM
  #84  
Rides4Beer
Senior Member
 
Join Date: Oct 2018
Location: SC
Posts: 981

Bikes: Defy | Revolt

Mentioned: 12 Post(s)
Tagged: 0 Thread(s)
Quoted: 516 Post(s)
Liked 407 Times in 223 Posts
I don't really care about the science, all I know is that my 28mm tires (that measure to about 30mm) are more comfortable and corner better than previous 23 or 25mm tires, without sacrificing any noticeable speed. That's all I need to know.
Rides4Beer is offline  
Likes For Rides4Beer:
Old 03-16-20, 01:48 PM
  #85  
RedBullFiXX
Senior Member
 
RedBullFiXX's Avatar
 
Join Date: Jul 2017
Location: SoCal USA
Posts: 169
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 69 Post(s)
Liked 38 Times in 30 Posts
imo
23c to 25c noticable difference
25c to 28c, barely discernible
25c to 30c+, big step

I'll take 25c TL on a road bike for the ideal compromise
ymmv
RedBullFiXX is offline  
Old 03-16-20, 02:07 PM
  #86  
SethAZ 
Senior Member
 
SethAZ's Avatar
 
Join Date: Oct 2013
Posts: 1,373

Bikes: 2018 Lynskey R260, 2005 Diamondback 29er, 2003 Trek 2300

Mentioned: 18 Post(s)
Tagged: 0 Thread(s)
Quoted: 554 Post(s)
Liked 305 Times in 171 Posts
Originally Posted by Rides4Beer View Post
I don't really care about the science, all I know is that my 28mm tires (that measure to about 30mm) are more comfortable and corner better than previous 23 or 25mm tires, without sacrificing any noticeable speed. That's all I need to know.
Why stop there? On my old bike I had 23mm at first, loved the upgrade to 25mm, and could run 28s depending on the brand, but with GP4K I could only fit 25mm up front and 28mm rear. With my new bike I'm rocking 32mm and loving it so much I'm contemplating shelling out for their 35mm model, just to see if it keeps getting better or 32mm was the peak goodness. I had personally applicable Strava data on routes I rode a lot with the 25mm and 28mm tires and then with the 32mm tires, and not only didn't see any drop in performance, but if anything, saw a slight gain. Since my frame and fork support these sizes, I won't ever go back.
SethAZ is offline  
Old 03-16-20, 02:23 PM
  #87  
Racing Dan
Senior Member
 
Racing Dan's Avatar
 
Join Date: Aug 2015
Posts: 1,515
Mentioned: 9 Post(s)
Tagged: 0 Thread(s)
Quoted: 881 Post(s)
Liked 62 Times in 50 Posts
Originally Posted by Gyro_T View Post
I read the article and plotted the data for pressure vs. diameter. It is very nearly a linear relationship. I think calculating the casing tension may be a more accurate way of determining the "pressure equivalency" I have been seeking. I had a gut feeling it was not a simple proportionality, e.g. the area of tire A 28mm tire=615.7mm2, area of tire B 44mm tire=1520.5mm2. A simple proportionality would say that a 28mm tire at 75 psi would be 30.4 psi in a 44mm tire: Tire pressure A* area of Tire A= Tire pressure B * area of Tire B, so in this case 75 X 615.7= x X 1520.5. Solve for x which give you the 30.4 psi for the 44mm tire. 1) it seemed too low, and 2) too simplistic. I think the casing tension may be exactly what I have been looking for and I will wade through the formulas with my figures. Thank you kindly.
The size of the contact patch is determined by the pressure, not width or anything else. For that reason i tend to believe "pressure equivalency" IS pressure.
Racing Dan is offline  
Old 03-16-20, 02:29 PM
  #88  
Seattle Forrest
Senior Member
 
Seattle Forrest's Avatar
 
Join Date: Mar 2010
Location: Seattle, WA
Posts: 20,973
Mentioned: 63 Post(s)
Tagged: 0 Thread(s)
Quoted: 11533 Post(s)
Liked 2,739 Times in 1,582 Posts
Wut? A cyclocross bike with tubulars at 32 psi has the same size contact patch as your car?
Seattle Forrest is offline  
Old 03-16-20, 03:18 PM
  #89  
Gyro_T
Senior Member
 
Gyro_T's Avatar
 
Join Date: Apr 2011
Location: Oregon
Posts: 241

Bikes: 1974 Jack Davis, Zeus Competition, Bridgestone MB2, Kona Rove Ti 1 X 11, Salsa Fargo Ti

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 10 Post(s)
Liked 2 Times in 2 Posts
Originally Posted by Racing Dan View Post
The size of the contact patch is determined by the pressure, not width or anything else. For that reason i tend to believe "pressure equivalency" IS pressure.
I suggest you take a look at the articles smashanddash referenced above. To get comparable "drop" the pressure will vary with diameter. Yes, that is all tied in with contact patch.
Gyro_T is offline  
Old 03-16-20, 03:33 PM
  #90  
Racing Dan
Senior Member
 
Racing Dan's Avatar
 
Join Date: Aug 2015
Posts: 1,515
Mentioned: 9 Post(s)
Tagged: 0 Thread(s)
Quoted: 881 Post(s)
Liked 62 Times in 50 Posts
Originally Posted by Seattle Forrest View Post
Wut? A cyclocross bike with tubulars at 32 psi has the same size contact patch as your car?
What im saying is it doesnt matter if the tyre is 23 or 32 mm, the area of the contact patch is the same if the tyre pressure and the load on the wheel is the same. If the load is 100 lbs and the pressure is 100 psi the area is 1 sq inch no matter and there is no reason 100 psi should feel any softer or harder just because you put it in a wider tyre or narrower tyre.

Last edited by Racing Dan; 03-16-20 at 03:38 PM.
Racing Dan is offline  
Old 03-16-20, 06:33 PM
  #91  
RedBullFiXX
Senior Member
 
RedBullFiXX's Avatar
 
Join Date: Jul 2017
Location: SoCal USA
Posts: 169
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 69 Post(s)
Liked 38 Times in 30 Posts
Originally Posted by Racing Dan View Post
What im saying is it doesnt matter if the tyre is 23 or 32 mm, the area of the contact patch is the same if the tyre pressure and the load on the wheel is the same. If the load is 100 lbs and the pressure is 100 psi the area is 1 sq inch no matter and there is no reason 100 psi should feel any softer or harder just because you put it in a wider tyre or narrower tyre.
hmm, something not right with this statement ?
Understand you are using a base number for clarity, but if both a 23c and a 40c on appropriate rim size, no way 30psi "feels the same"
23c tire would be folded over the rim, i.e pinch flat
RedBullFiXX is offline  
Old 03-16-20, 07:03 PM
  #92  
DeadGrandpa
Senior Member
 
DeadGrandpa's Avatar
 
Join Date: Sep 2014
Location: Carolina
Posts: 817

Bikes: Trek 1120, Santa Cruz Tallboy 3CC, Fandango DC-9 MTB tandem, Jamis Renegade Expert, Bike Friday Pocket Llama, Santana Arriva tandem, Bridgestone RB-1

Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 263 Post(s)
Likes: 0
Liked 47 Times in 38 Posts
I'm a little confused. Am I closer to perfect on a 700 x 40c 55psi tire than on a 29 x 3 inch 17 psi plus tire? You want me to go 700 x 25? That bike rattles my bones.
DeadGrandpa is offline  
Old 03-16-20, 10:35 PM
  #93  
smashndash
Senior Member
 
Join Date: Feb 2018
Posts: 835

Bikes: 2017 Specialized Allez Sprint Comp

Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 489 Post(s)
Liked 113 Times in 85 Posts
Originally Posted by Racing Dan View Post
The size of the contact patch is determined by the pressure, not width or anything else. For that reason i tend to believe "pressure equivalency" IS pressure.
Boy I hate to pile in on this zombie thread...

but why should a 23mm bike tire and a 50mm bike tire have the same contact patch size? Shouldn’t your contact patch size be proportional to the size of the tire?

You are certainly wrong that a 100psi tire feels just as hard regardless of tire size.

Last edited by smashndash; 03-16-20 at 10:40 PM.
smashndash is offline  
Old 03-17-20, 06:33 AM
  #94  
Rides4Beer
Senior Member
 
Join Date: Oct 2018
Location: SC
Posts: 981

Bikes: Defy | Revolt

Mentioned: 12 Post(s)
Tagged: 0 Thread(s)
Quoted: 516 Post(s)
Liked 407 Times in 223 Posts
Originally Posted by SethAZ View Post
Why stop there? On my old bike I had 23mm at first, loved the upgrade to 25mm, and could run 28s depending on the brand, but with GP4K I could only fit 25mm up front and 28mm rear. With my new bike I'm rocking 32mm and loving it so much I'm contemplating shelling out for their 35mm model, just to see if it keeps getting better or 32mm was the peak goodness. I had personally applicable Strava data on routes I rode a lot with the 25mm and 28mm tires and then with the 32mm tires, and not only didn't see any drop in performance, but if anything, saw a slight gain. Since my frame and fork support these sizes, I won't ever go back.
I'm thinking about it, the new Defy can handle up to 35mm. So I could run 32mm GP5Ks that would prob measure out to 33-34mm and be good.
Rides4Beer is offline  
Likes For Rides4Beer:
Old 03-17-20, 04:43 PM
  #95  
SethAZ 
Senior Member
 
SethAZ's Avatar
 
Join Date: Oct 2013
Posts: 1,373

Bikes: 2018 Lynskey R260, 2005 Diamondback 29er, 2003 Trek 2300

Mentioned: 18 Post(s)
Tagged: 0 Thread(s)
Quoted: 554 Post(s)
Liked 305 Times in 171 Posts
Originally Posted by Rides4Beer View Post
I'm thinking about it, the new Defy can handle up to 35mm. So I could run 32mm GP5Ks that would prob measure out to 33-34mm and be good.
This isn't a hard prediction to make: if you try it, I think you'll love it. If you've been riding 25mm the step up to 32mm isn't subtle.

I haven't tried GP5Ks yet. I liked GP4Ks, but haven't yet worn out the Compass (now Rene Herse) Stampede Pass 32mm tires currently on the bike. I'm loving these tires so much I'm not sure I'll buy anything else when these ones are finally done. If anything, the most likely replacement for the 32mm Stampede Pass tire will be the 35mm Bon Jon Pass tire. I may try out the 32mm GP5K just to see what it's like, though. The Compass/Rene Herse tires are spendy, but for things I'll use for hundreds of hours if it brings a noticeable benefit in comfort or safety, I'm down. I remember when I rode Gatorskins back in the day, and then tried the GP4Ks. Sure, I had a few flats with the GP4Ks that I might not have had on Gatorskins, but that's momentary pain that only happens every once in a while, while the harsh, dead ride on Gatorskins is something that's with you hour after hour, day after day, month after month. No thanks.
SethAZ is offline  
Old 03-17-20, 11:01 PM
  #96  
Gyro_T
Senior Member
 
Gyro_T's Avatar
 
Join Date: Apr 2011
Location: Oregon
Posts: 241

Bikes: 1974 Jack Davis, Zeus Competition, Bridgestone MB2, Kona Rove Ti 1 X 11, Salsa Fargo Ti

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 10 Post(s)
Liked 2 Times in 2 Posts
Originally Posted by waters60 View Post
If you have 30 minutes or so this podcast from VeloNews is very in depth: https://www.velonews.com/2020/03/gea...roubaix_506668
It was a well done interview of Neil Shirley by Velo News tech guy Dan Cavallari. It was worth the listen for me. Dan is very savvy. It did a great job pointing to direct experience of racers using high volume tubeless system. It was fun hearing about their teams experience at the Paris-Roubaix cobbles. The advantages of higher volume on any surface that is not fairly pristine tarmac, is pretty much indisputable at this point. Tubeless is great for specific situations. The interview does not delve into the idea of pressure equivalency, which is fine. I have learn a lot from my research and from this forum. I have learned that being a bike geek like my friends on this forum does not give you an intuitive feel for the relationship between tire width, pressure and contact patch. The physics that describes it involves not just width, but casing thickness, compound properties which effect hysteresis, circumference of the "torus", and total volume. The great thing is that we can reduce it down to just two of those variables, inflation pressure and tire diameter , to explain 95% of the factors contributing to the ride. The Berto paper (https://campusebikes.com/documents/Tireinflation.pdf) is a gem because he did his measurement directly with a dial caliper, hydraulic jack, and load. Direct measurement like this are invaluable as they can be compared to casing tension calculations like are covered in the Flocycling paper: https://flocycling.com/blogs/blog/fl...or-wider-wheel. Thanks to all that contributed to this discussion. Much can be translated to wheelchair users, many who suffer from vibration exposure from their wheelchairs. Tailoring tire width and pressure are readily available tools for reducing this exposure.
Gyro_T is offline  
Likes For Gyro_T:
Old 03-23-20, 12:29 PM
  #97  
xroadcharlie
Senior Member
 
Join Date: Apr 2019
Location: Windsor Ontario, Canada
Posts: 257

Bikes: 2018 Giant Sedona

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 86 Post(s)
Liked 39 Times in 35 Posts
Originally Posted by smashndash View Post
Boy I hate to pile in on this zombie thread...

but why should a 23mm bike tire and a 50mm bike tire have the same contact patch size? Shouldn’t your contact patch size be proportional to the size of the tire?

You are certainly wrong that a 100psi tire feels just as hard regardless of tire size.
I was baffled by that claim too. I think that claim is based on using the same pressure in each tire. 30 psi in the 23 mm tire might yield a tire drop of 65% with a 175 lb person on the bike giving a very long, But narrow tire patch. The same pressure in a 50 mm tire of the same design with the same load (bike and rider) might have a drop of 20%, For a short but wide tire patch. End results might be very close.

However, Many real world tire pressure calculators are based on the 15% drop method. So the tire patch would not only longer (15% of a tire height of 23mm vs 50 mm) but wider too, Yielding a much larger contact patch. One reason we use wide tires for beach cruisers and narrow tires (to about 32mm) for road bikes.

Last edited by xroadcharlie; 03-23-20 at 12:34 PM.
xroadcharlie is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
rhm
Bicycle Mechanics
1
09-04-15 09:33 AM
volosong
Fifty Plus (50+)
35
07-25-14 09:25 AM
Myosmith
Bicycle Mechanics
75
12-11-12 03:10 PM
uprightbent
Classic & Vintage
106
03-13-11 11:50 PM
c_dinsmore
Road Cycling
5
04-02-10 11:21 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


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.