Author Topic: Tyre pressure  (Read 745 times)

David Simpson

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Re: Tyre pressure
« Reply #15 on: September 17, 2018, 06:14:43 PM »
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

Here are some of the references from the Mega Brochure:

- DaveS
« Last Edit: September 17, 2018, 07:04:18 PM by David Simpson »

rafiki

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Re: Tyre pressure
« Reply #16 on: September 17, 2018, 06:36:33 PM »
Thanks, Dave. Much appreciated.

macspud

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Re: Tyre pressure
« Reply #17 on: September 18, 2018, 06:10:21 AM »
Good reading, though Andy Blance's calculations of contact area seem to be incorrect.

Mike Ayling

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Re: Tyre pressure
« Reply #18 on: September 18, 2018, 07:29:05 AM »
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

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

mickeg

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Re: Tyre pressure
« Reply #19 on: September 18, 2018, 03:44:35 PM »
...

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.

Matt2matt2002

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Re: Tyre pressure
« Reply #20 on: September 18, 2018, 11:44:40 PM »
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?
Never drink and drive. You may hit a bump  and spill your drink

mickeg

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Re: Tyre pressure
« Reply #21 on: September 19, 2018, 12:31:09 AM »
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.  ...

martinf

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Re: Tyre pressure
« Reply #22 on: September 19, 2018, 08:03:30 AM »
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.

Danneaux

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Re: Tyre pressure
« Reply #23 on: September 20, 2018, 08:32:52 PM »

David Simpson

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Re: Tyre pressure
« Reply #24 on: September 21, 2018, 10:31:09 PM »