Thorn Cycles Forum
Technical => General Technical => Topic started by: neil_p on September 13, 2018, 08:48:12 am
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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?
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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.
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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 (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/ (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!
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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.
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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 (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
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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
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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
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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.
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Global warming?
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Global warming?
No, too simplistic. Must be Brexit!
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With or without altitude correction ?
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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?
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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.
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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.
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I'd still be interested to know the reason for the substantial change in recommended pressures.
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Global warming?
very good matt ;D ;D
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I'd still be interested to know the reason for the substantial change in recommended pressures.
I can't say for certain, but I believe it is due to the research of Jan Heine and others on the effects of tire pressure on rolling resistance for bicycle tires. In the past, it was believed that lower pressure always increased rolling resistance, and the advice for cyclists was to keep the tires inflated to a high pressure. However, recent (last 10 years?) research has shown that the situation is not quite that simple. There are many factors affecting rolling resistance, and a high quality wider tire at a lower pressure may have lower rolling resistance than a lower quality narrower tire at a higher pressure.
You can read more of Andy Blance's thoughts on the subject on pages 33-34 and 43 in the Thorn Mega Brochure:
http://www.sjscycles.com/thornpdf/thorn_mega_brochure.pdf (http://www.sjscycles.com/thornpdf/thorn_mega_brochure.pdf)
Here are some of the references from the Mega Brochure:
- https://janheine.wordpress.com/2012/08/12/suspension-losses (https://janheine.wordpress.com/2012/08/12/suspension-losses)
- https://janheine.wordpress.com/2016/06/14/suspension-losses-confirmed/ (https://janheine.wordpress.com/2016/06/14/suspension-losses-confirmed/)
- http://trstriathlon.com/talking-tires-with-joshuapoertner (http://trstriathlon.com/talking-tires-with-joshuapoertner)
- http://www.roadrevolution18.dtswiss.com/endurance/ (http://www.roadrevolution18.dtswiss.com/endurance/) (See the last few pages on that link)
- DaveS
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Thanks, Dave. Much appreciated.
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Good reading, though Andy Blance's calculations of contact area seem to be incorrect.
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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 (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
I have managed to find my paper copy of the Thorn tandem Brochure fro0m when we bought the tandem at the end of 2012.
The table is short:
26"tyre Size Max front Max rear
1.35" 72 80
1.50" 63 70
1.60 55 62
1.75" 50 58
2.10" 40 45
2.25" 38 42
2.35" 32 37
Anyway since day one we have been running our 1.75s at 50psi front and rear as Mary is a petite economy size gal.
Mike
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I have managed to find my paper copy of the Thorn tandem Brochure fro0m when we bought the tandem at the end of 2012.
The table is short:
26"tyre Size Max front Max rear
1.35" 72 80
1.50" 63 70
1.60 55 62
1.75" 50 58
2.10" 40 45
2.25" 38 42
2.35" 32 37
Anyway since day one we have been running our 1.75s at 50psi front and rear as Mary is a petite economy size gal.
Mike
Interesting that they have different numbers for front and rear for max. Since the max was purportedly to prevent (or at least reduce chance of) rim failure, since the front and rear rims are the same about 98 percent of the time you would expect only a max number per size without specifying front vs rear. Based on that, they obviously had other thoughts in mind when they produced that table than rim strength.
WHen I mentioned max pressure recommendations, I did not expect it to take over the thread this much.
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Out of interest....
When sitting on my Raven unloaded is there equal weight on each wheel?
I am guessing that with my panniers and tent on the rear rack the balance will be different.
Could this be a reason for different front and rear pressures?
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Out of interest....
When sitting on my Raven unloaded is there equal weight on each wheel?
I am guessing that with my panniers and tent on the rear rack the balance will be different.
Could this be a reason for different front and rear pressures?
Even sitting on your bike unladen, you will have less pressure on the front than rear. Panniers and tent on the rear exaggerate it even more. Yes, that is why you want more pressure in the rear than the front, but that is not a reason for a "maximum" pressure to prevent component damage.
I mentioned above that ...
... Front tire I usually inflate to about 70 to 80 percent of the pressure that I put in the rear tire. ...
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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?
I don't think 26", 650B and 700C are different enough to have much influence on optimum tyre pressures. But I inflate the tyres on my 16" wheel Bromptons to a higher pressure than I would use on a large wheel bike with the same width of tyres.
When I get a different model or size of tyre I experiment a bit until I find a tyre pressure that suits me, which is generally lower than the maximum recommended. Then I note the pressure and periodically check and reinflate the tyres. I add 5 to 10 psi to the unloaded pressure if I load the bike up heavily.
I mostly have lightweight tyres with flexible sidewalls, which cope well with fairly low pressures. Rear tyres on my two visitor bikes are Marathon Plus with thick sidewalls, these seem to work better at slightly higher pressures.
Examples of pressures I currently use:
Old 700C lightweight 700Cx28mm Schwalbe Ones - 65 psi front, 75 psi rear.
Raven Sport Tour 26"x35 mm Kojaks - 55 psi front, 65 psi rear.
Brompton 16"x35 mm Marathon Racers - 65 psi front, 75 psi rear.
Old 650B utility bike 650Bx50 mm Supremes - Now 32 psi front, 40 psi rear as I often carry a shopping load of 15-30 Kg on this bike, mainly on the rear rack. I used 28 psi front, 32 psi rear when doing survey work on paths and tracks with this bike.
Raven Tour tourer 26"x50 mm Supremes, used lightly loaded - 30 psi front, 35 psi rear, with full load 40 psi front, 45 psi rear.
Wife's Raven Tour Step-Through 26"x50 mm Supremes - 28 psi front, 32 psi rear.
Given that my wife only weighs half as much as myself, the tyre pressures on her bike should theoretically be much lower, but I don't like to drop pressure too much for fear of pinch punctures on rocks or potholes.
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From Jan Heine...
https://janheine.wordpress.com/2018/01/03/12-myths-in-cycling-1-wider-tires-are-slower/
Best,
Dan.
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Also this one:
https://janheine.wordpress.com/2018/06/11/myth-11-rear-tires-should-run-at-higher-pressure/ (https://janheine.wordpress.com/2018/06/11/myth-11-rear-tires-should-run-at-higher-pressure/)
- DaveS