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

Strange question re tyre pressures.

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.

Strange question re tyre pressures.

Old 01-09-12, 11:03 AM
  #1  
Senior Member
Thread Starter
 
CaptCarrot's Avatar
 
Join Date: Feb 2009
Location: Dorset, SW England, United Kingdom
Posts: 425

Bikes: Heavily modded Cannondale Hooligan 1 (2009) and an upgraded Raleigh Max Zero-G

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Strange question re tyre pressures.

OK, It doesn't really matter what the answer to this is, but it is kinda bugging me.

I was on Schwalbe's website, reading there article on tyre pressures. (see page 32 of this pdf)

What is the correct pressure for my tire?

It is impossible to make a general recommendation on inflation pressure for a specific bicycle or a particular tire. The “right” inflation pressure depends mainly on the load exerted on the tire. This weight is mainly influenced by the weight of the rider and any luggage. Contrary to a car, the vehicle weight is only a minor part of the total weight. In addition there is a great diversity of individual preferences with regards to low rolling resistance or suspension comfort.

The permitted inflation pressure range is marked on the tire sidewall. The higher the inflation pressure, the lower the rolling resistance, the tire wear and the less likelihood of a puncture. The lower the inflation pressure, the higher are the comfort and grip that the tires provide. The list of inflation pressure recommendations on the right can only provide a very general guide. The recommendations are for an “average rider” weighing about 75 kg.

If the rider is heavier or carries luggage, a higher inflation pressure should be used. For each additional kilogram that the tire must carry (bike, rider, luggage), the inflation pressure should be increased by approx. 1%. It is recommended that higher inflation pressures are used on very small diameter tires such as recumbants and folding bikes.

Lighter weight riders or riders who prefer a smooth or more comfortable ride can accordingly choose a lower inflation pressure, but the actual tire pressures should never be higher or lower than the maximum and minimum inflation pressures marked on the tire sidewall.

Code:
+-------+--------------------+
| Tire  | Inflation pressure |
| width | Recommendation:    |
+-------+--------------------+
| 20 mm | 9,0 bar   130 psi  |
| 23 mm | 8,0 bar   115 psi  |
| 25 mm | 7,0 bar   100 psi  |
| 28 mm | 6,0 bar    85 psi  |
| 30 mm | 5,5 bar    80 psi  |
| 32 mm | 5,0 bar    70 psi  |
| 35 mm | 4,5 bar    65 psi  |
| 37 mm | 4,5 bar    65 psi  |
| 40 mm | 4,0 bar    55 psi  |
| 42 mm | 4,0 bar    55 psi  |
| 44 mm | 3,5 bar    50 psi  |
| 47 mm | 3,5 bar    50 psi  |
| 50 mm | 3,0 bar    45 psi  |
| 54 mm | 2,5 bar    35 psi  |
| 57 mm | 2,2 bar    32 psi  |
| 60 mm | 2,0 bar    30 psi  |
+-------+--------------------+
Now, I am running 20" Schwalbe Crazy Bob's (54-406).

According to the above chart, for 54mm wide tyres I should be looking at 35psi.

But then it also says "It is recommended that higher inflation pressures are used on very small diameter tires such as recumbants and folding bikes.", but it doesn't say how much higher. I presumed knocking 10mm off the tire width might compensate for moving from 26" to 20". So that starts me at 50psi.

The next part of my calculation is based on "The recommendations are for an “average rider” weighing about 75 kg." and "For each additional kilogram that the tire must carry (bike, rider, luggage), the inflation pressure should be increased by approx. 1%." Well I weigh ~13 stone which is ~83kg. And my work bag weighs about 10kg, thats 93kg, which is 18kg over 75kg. 50psi + 18% = 59psi.

So in theory, I should be inflating the tyres to 59psi. All very well and good, but according to the side wall of said tyre - that is too low!!!

The acceptable range for inflation is apparently 65-100psi (which interestingly is also the pressures for the 47mm wide tyres - where as the 60mm tyres are 30-65psi)


Come the finish, I am inflating to ~65psi and the adjusting by visual appearance and comfort.

Its just I find their tyre markings a little strange based on the above pdf.


And before anyone mentions it, I am well aware that they said "It is impossible to make a general recommendation on inflation pressure for a specific bicycle or a particular tire."
CaptCarrot is offline  
Old 01-09-12, 12:11 PM
  #2  
Senior Member
 
Join Date: Oct 2007
Location: West Yorkshire, United Kingdom
Posts: 5,773
Mentioned: 5 Post(s)
Tagged: 0 Thread(s)
Quoted: 453 Post(s)
Liked 104 Times in 87 Posts
That looks to be based on a 700c tire, and you are using a 20" so the inflation data applicable for a 700c will not match that of what you are using.

Would base your inflation on what the sidewall of your tire says, as this will give you the info you need, you will also be able to work this out when riding, does it feel good or not.

The info is on page 31, not 32 as noted.
jimc101 is offline  
Old 01-09-12, 12:29 PM
  #3  
working on my sandal tan
 
ThermionicScott's Avatar
 
Join Date: Aug 2011
Location: CID
Posts: 22,620

Bikes: 1991 Bianchi Eros, 1964 Armstrong, 1988 Diamondback Ascent, 1988 Bianchi Premio, 1987 Bianchi Sport SX, 1980s Raleigh mixte (hers), All-City Space Horse (hers)

Mentioned: 98 Post(s)
Tagged: 0 Thread(s)
Quoted: 3868 Post(s)
Liked 2,559 Times in 1,574 Posts
I think the idea here is that with smaller-diameter wheels, the "bottom" part of the tire makes up more of the total volume, so the rest of the air volume in the tire is less able to resist the deflection at the bottom. So for the same tire width, you'd need a little more pressure to guard against pinch flats. That said, you might need to experiment to find out what pressure achieves the right balance of comfort and flat-protection for you. I was able to run my 700x30 tires in the mid-40s yesterday on a gravel ride without flatting, at a rider weight of ~155 lbs. I often use pressures at or below the recommended minimum on the front of my other bikes, too. I may have luck on my side, though.
__________________
Originally Posted by chandltp
There's no such thing as too far.. just lack of time
Originally Posted by noglider
People in this forum are not typical.
RUSA #7498
ThermionicScott is offline  
Old 01-09-12, 12:38 PM
  #4  
AEO
Senior Member
 
AEO's Avatar
 
Join Date: Jun 2007
Location: A Coffin Called Earth. or Toronto, ON
Posts: 12,258

Bikes: Bianchi, Miyata, Dahon, Rossin

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 3 Post(s)
Likes: 0
Liked 5 Times in 5 Posts
I've never felt a difference between different tire diameters at the same width and inflation pressure.

I go with the "around 15% depression of tire" when fully loaded method.
__________________
Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)
https://sanfrancisco.ibtimes.com/arti...ger-photos.htm
AEO is offline  
Old 01-09-12, 01:15 PM
  #5  
Banned
 
Join Date: Jun 2010
Location: NW,Oregon Coast
Posts: 43,598

Bikes: 8

Mentioned: 197 Post(s)
Tagged: 0 Thread(s)
Quoted: 7607 Post(s)
Liked 1,354 Times in 861 Posts
With a good solid bead hook in the rim. I go with the thumb test.

when my bike feels sluggish I add air.

349-35 & 406-47. 559-47, 622 32 Or 40

On a bike tour I brought a separate gage.. pannier load quickly indicates when the
rolling resistance increases...

reading the imprint on the tire is the basic thing..
fietsbob is offline  
Old 01-09-12, 05:10 PM
  #6  
Senior Member
 
rydabent's Avatar
 
Join Date: Apr 2010
Location: Lincoln Ne
Posts: 9,920

Bikes: RANS Stratus TerraTrike Tour II

Mentioned: 46 Post(s)
Tagged: 0 Thread(s)
Quoted: 3345 Post(s)
Liked 1,054 Times in 634 Posts
I might add a warning tho. In the heat of the summer, when you bike is in the garage or in the shade be carful with hight inflation. If you go to say the stated 100 pounds in the cool of the morning, think what it will be in the heat of the day on 150 degree blacktop highways.
rydabent is offline  
Old 01-09-12, 07:18 PM
  #7  
Real Men Ride Ordinaries
 
fuzz2050's Avatar
 
Join Date: Sep 2008
Posts: 3,723
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 17 Post(s)
Likes: 0
Liked 3 Times in 3 Posts
Originally Posted by rydabent
I might add a warning tho. In the heat of the summer, when you bike is in the garage or in the shade be carful with hight inflation. If you go to say the stated 100 pounds in the cool of the morning, think what it will be in the heat of the day on 150 degree blacktop highways.
I did the math (one very slow day at the bike co-op with a bunch of engineers) and it turns out that you really can't expect a change of more than 5 psi for a high pressure road tire, and even less for a higher volume, lower pressure tire.
fuzz2050 is offline  
Old 01-09-12, 07:23 PM
  #8  
Senior Member
 
Join Date: Feb 2009
Posts: 6,660
Mentioned: 16 Post(s)
Tagged: 0 Thread(s)
Quoted: 582 Post(s)
Liked 171 Times in 138 Posts
I don't think the diameter is that important. https://docs.google.com/spreadsheet/...hl=en_US#gid=0
davidad is offline  
Old 01-09-12, 07:25 PM
  #9  
Constant tinkerer
 
FastJake's Avatar
 
Join Date: May 2010
Location: Wisconsin
Posts: 7,954
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 184 Post(s)
Liked 92 Times in 75 Posts
100psi seems like way too much for a 54mm tire, but I don't run 406 rims either... My 54mm tires on 26" MTB rims see anywhere from 20-50psi depending on what they're being used for. I've never had a pinch flat but I only weigh 130lbs. I do frequently ride down stairs and other stupid stuff.

+1 Adjust per your personal preference based on comfort and if you get any flats although I seriously doubt you'll see a pinch flat at anything greater than 50psi unless you're hitting really bad potholes.
FastJake is offline  
Old 01-09-12, 07:52 PM
  #10  
Certified Bike Brat
 
Burton's Avatar
 
Join Date: Jan 2011
Location: Montreal, Quebec
Posts: 4,251
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Likes: 0
Liked 6 Times in 6 Posts
I'd also agree with the "around 15% depression of tire when fully loaded" method.

Those tires have a load rating of 85kg each and you're so far within that you could run them on the minimum recommended pressure marked on the sidewall if you wanted to. Respecting the minimum will avoid damaging the sidewall structure and increase the expected tire life. Any difference in tire pressure front and back should be related to the actual weight distribution.
Burton is offline  
Old 01-09-12, 09:08 PM
  #11  
Don from Austin Texas
 
Join Date: Jul 2009
Location: Austin, Texas
Posts: 1,211

Bikes: Schwinn S25 "department store crap" FS MTB, home-made CF 26" hybrid, CF road bike with straight bar, various wierd frankenbikes

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 11 Post(s)
Likes: 0
Liked 1 Time in 1 Post
I disagree with the Schwalbe advice, in that, on poor surfaces maximum inflation can actually be counterproductive to lessened rolling resistance and speed.

Don in Austin
Don in Austin is offline  
Old 01-09-12, 09:53 PM
  #12  
Banned
 
Join Date: Jun 2009
Posts: 4,788
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 2 Times in 2 Posts
OK, as a dedicated "KISSer" (Keep It Simple, Stupid), I have to ask: did you look at the sidewall of the tire first, or did you just default to the website?

Tire inflation is a simple science; inflate within limits embossed on the sidewall, adjusting for personal comfort. Once you find that "sweet spot", then you CHECK psi and remember it. It may take a few test rides, but it's worth it, as every rider is different.

On my MTB, I run different tires during different times of year; right now, I'm running some real aggressive treads to handle winter conditions. These work best for me at about 44 psi (I'm 230+ lb). My other, 3-season tires roll best at 55-60 psi. Both are rated at 65 psi max.
DX-MAN is offline  
Old 01-10-12, 02:19 PM
  #13  
Senior Member
Thread Starter
 
CaptCarrot's Avatar
 
Join Date: Feb 2009
Location: Dorset, SW England, United Kingdom
Posts: 425

Bikes: Heavily modded Cannondale Hooligan 1 (2009) and an upgraded Raleigh Max Zero-G

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Yes I did read the sidewall and I do inflate to personal preference, however I was reading schwalbe's site and decided to do the math.

Thanks for all the input.
CaptCarrot is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
lindafranc
General Cycling Discussion
50
03-15-18 08:28 PM
IanS
Tandem Cycling
24
08-28-17 03:34 AM
volosong
Fifty Plus (50+)
35
07-25-14 09:25 AM
Roadie607
Road Cycling
38
02-25-14 01:28 PM
hobkirk
Clydesdales/Athenas (200+ lb / 91+ kg)
31
06-26-10 12:11 AM

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.