Tyre pressure

[neil_p]

I've found various articles advising tyre pressures based on tyre width and rider weight. But what about wheel circumference? Does that matter? For instance I've found recommended psi for 28mm tyres on a road bike 700 wheels... Would that also apply to 28mm tyres on 26" wheels?

[Andre Jute]

I've found various articles advising tyre pressures based on tyre width and rider weight. But what about wheel circumference? Does that matter? For instance I've found recommended psi for 28mm tyres on a road bike 700 wheels... Would that also apply to 28mm tyres on 26" wheels?

Yes. Think, instead of Bars of pressure, of pounds per square in, PSI. That's a rating that holds regardless of how many inches (of inner carcass) there is, and thus is the same for tyres of any diameter.

Bicyclists are lucky that bicycle tyres generally have a square format of height exposed above the rim being the same as tyre width. In cars the whole business is bedeviled by the standard practice of reducing the profile of wider tires so that we speak of tyre radius by a fractional profile height, say 225x55 which is the width of the tyre's (nominal) footprint and the fraction of the width which is exposed above the edge of the rim, 55% or .55 of 225. A further complicating factor is that these low-profile tyre sidewalls are generally made stiffer the narrower they become, so the sidewall height and its stiffness determines the design tyre pressure.

One of the great pleasures of my fave tyres, Schalbe's Big Apples 622x60 (which on a bicycle is the diameter of the rim's bead retaining edge and the width and, unspoken, sidewall height of the tyre) is how soft that 60mm height permits the sidewall to be. I inflate to just over 2 Bar, and I'm no weight weenie, and besides me, my bike carries my painting equipment or sometimes substantial shopping.

[JimK]

I did a bit of theorizing a while back and came up with "Pressure should be increased inversely proportional to the square root of the wheel diameter. So 26 inch pressures should be about 5% higher" than 700C. See: http://interdependentscience.blogspot.com/2013/06/bicycle-tire-pressure.html. Jan Heine is a fount of insight on these matters. See e.g. https://janheine.wordpress.com/2018/06/11/myth-11-rear-tires-should-run-at-higher-pressure/.

My theorizing was also based on Berto's goal of a 15% drop.  I have to admit though, the more I think about how tire shape changes to support weight... it is a lot more complicated than my poor brain can handle! For sure, the product of the pressure with the area of the contact patch, that will give the weight supported. But that's just the starting point of all the fun!

[mickeg]

I never think about wheel diameter for tire pressure.  I rely only on tire width, what kind of road surface I am riding (rougher means lower pressure), and what my load on the bike is (bike unladen, or loaded down with camping gear).  Front tire I usually inflate to about 70 to 80 percent of the pressure that I put in the rear tire.  Although on a trip last spring where the paved surface was really rough on my hands I lowered the front tire pressure even more than that, dropped it to roughly 55 percent of the rear pressure to give better cushioning.

My bikes range from 24 inch (507mm) to 700c (622mm) for wheel diameter.

If you are not aware, Thorn has made some recommendations for maximum tire pressure for different width tires, but I am not sure where those recommendations would be located at this time.  Those recommendations are based on the risk of damage to a rim, but they only list one set of numbers and not different numbers for different rims.

[David Simpson]

Thorn's recommendations for tyre pressure are in the Thorn Mega Brochure, on page 36 of the latest edition (Summer 2018, Edition 4.00).

http://www.sjscycles.com/thornpdf/thorn_mega_brochure.pdf

I have tried to cut-and-paste them here:

Recommended and absolute min/max for solo (non-tandem) bikes

700c TYRE SIZE
    | Recommended | Absolute
    | FRONT REAR  | MAX  MIN
23c | 108   118   | 130  85
25c | 100   110   | 120  75
28c | 88    95    | 105  65
32c | 70    75    | 90   55
35c | 60    65    | 80   45
38c | 53    58    | 75   40
40c | 48    53    | 70   35

26" TYRE SIZE
      | Recommended | Absolute
      | FRONT REAR  | MAX  MIN
1.35" | 60    65    | 80   45
1.60" | 48    53    | 70   35
1.75" | 45    50    | 65   32
2.00" | 40    45    | 58   30
2.10" | 38    43    | 54   28
2.25" | 35    40    | 50   26
2.35" | 32    37    | 48   24


- DaveS

[Mike Ayling]

If you are not aware, Thorn has made some recommendations for maximum tire pressure for different width tires, but I am not sure where those recommendations would be located at this time.  Those recommendations are based on the risk of damage to a rim, but they only list one set of numbers and not different numbers for different rims.

There is usually a recommended tyre pressures chart in the bike brochure. (see David Simpson's post above.)

I have noticed that since we bought our tandem in 2012 Thorn has increased the maximum pressures by maybe 10%

Mike

[John Saxby]

Thanks, Dave, that's helpful info -- I seem to have missed it in the Mega Brochure. 

On my Raven (with 26 x 1.60 Marathon Supremes) I usually use marginally higher pressures unloaded (50 PSI front, 55-50 rear) and a few more with a touring load. Sometimes, on rough tarmac (e.g., W Qué), I ease off the pressure a bit, following George's practice.

On my derailleur bike, ridden almost always unloaded and on tarmac, I use slightly lower pressures than those recommended for my 700 x35 tires.

Cheers,  John

[rafiki]

Goodness! I must pay more attention! The last Mega B I downloaded was 2015 and the recommended pressures were significantly different viz:

26" TYRE SIZE SOLO
Recommended Pressure  ABSOLUTE Pressures
         FRONT REAR         MAX MIN
1.35" 66       72             85    45
1.60" 60       65             75    35
1.75" 57       62             70    32
2.00" 53       58             66    30
2.10" 48       52             62    28
2.25" 45       50             55    26
2.35" 40       45             52    24

Much higher. I wonder what changed.

[Matt2matt2002]

Global warming?

[rafiki]

Global warming?

No, too simplistic. Must be Brexit!

[energyman]

With or without altitude correction ?

[Matt2matt2002]

With or without altitude correction ?

Funny you should have mentioned this.
I recently rode in Ethiopia and although Addis Ababa sits at 2355m above sea level, I rode up into the mountains at 3,000.
I wasn't carrying a pressure guage with me but should I have experienced a change of tire pressure?
Should I have taken any action if this is the case?

[mickeg]

I have noticed that SJS has changed recommended max pressures a few times, I no longer try to keep track of what they are and how they have changed.  I wrote the numbers onto a slip of paper years ago and taped them to my pump. 

...
I recently rode in Ethiopia and although Addis Ababa sits at 2355m above sea level, I rode up into the mountains at 3,000.
I wasn't carrying a pressure guage with me but should I have experienced a change of tire pressure?
Should I have taken any action if this is the case?

Yes, your pressure would have gone up by a bit less than 2 psi if you went from MSL up to 3000m of elevation.  That is a smaller number than the level of accuracy of my gauges, don't worry about it.

[rafiki]

I'd still be interested to know the reason for the substantial change in recommended pressures.

[jags]

Global warming?

very good matt