Contradiction between manufacturer's pressure recommendations and Thorn

[jzdparis]

I have Schwalbe Marathon Dureme 26 x 2.0 tyres and see that there is a huge discrepancy between the recommended pressure marked on the sidewall, and that contained in the Thorn Owner's Guide which says that the recommended rear pressure is 58 and the absolute MAX is 66.  This is a lot lower.  Why is there such a difference?  Is there a reasonable way of assessing the pressure if one does not have a gauge?

[Danneaux]

Why is there such a difference?

The manufacturer's ratings are typically the maximum considered safe (with a generous margin) before the tire blows off the rim.

The Thorn recommendations are considerably lower because a wide tire mounted on a relatively narrow rim exerts tremendous lateral jacking forces on the rim bead when inflated to the maker's maximum and then loaded with a rider and touring load. This can result in rim or sidewall failures, where the aluminum extrusion actually splits.

As a side benefit, fatter tires run at lower pressures provide better shock absorption and a more comfortable ride without increased rolling resistance.

Is there a reasonable way of assessing the pressure if one does not have a gauge?

As a general rule, no. The human thumb is not so finely calibrated as a gauge. I have had good luck with both Schwalbe's and SKS' digital tire pressure gauges, both of which have proven much more accurate than the analog gauges on a number of pumps I have used.

Best,

Dan.

[Danneaux]

For more on Thorn designer Andy Blance's take on the matter of maximum tire pressures, see the Mega Brochure:
http://www.sjscycles.com/thornpdf/THORN_MEGA_BROCHURE.pdf
Pages 14, 15, 30...and the chart and text on page 31.

Best,

Dan.

[mickeg]

The Thorn numbers are based on rim strength.  I suspect that they had a lot of rims fail and started using that criteria to try to prevent that.  I do not know where Thorn got their numbers and their numbers have changed at least once.  And Thorn uses the same numbers for all brands of rims, so they are generic.

My Mavic A719 rims that I bought in 2004 are the only rims I have seen that have a pressure rating on the rim, photo attached. 

I have exceeded the Thorn numbers on my Salsa Gordo rims (Thorn did not sell me those rims) when I use 40mm wide Schwalbe Marathons (with Greengard) on my Sherpa, but otherwise I try to use the Thorn numbers as maximums, or in the case of my Mavic A719 use the numbers on that rim.

I keep a note on my pump for home use with the max pressures.

A gauge is the only good way to measure pressure.  The gauges on small pumps are not very good, I usually assume at least a 5 psi error on those gauges.  Even when I worked in a bike shop and aired up tires several times a day every day, I could never get my thumb to be "calibrated" for accurate pressure.

EDIT:  I wrote the above before I saw Dan's comments.

At home my Toppeak Joe Blow pump has a good gauge on it.  My Silca pump has a good gauge, but I need to use a conversion chart for it as nobody uses kg/cm^2 pressures any more. I have a Meiser Accu-Gauge which is my favorite gauge for Presta, other than the gauges on pumps.  My Toppeak digital gauge is pretty good for Schrader valves, but it looses too much pressure when I try to use it on Presta so I rarely use it.

[martinf]

The Thorn recommendations are considerably lower because a wide tire mounted on a relatively narrow rim exerts tremendous lateral jacking forces on the rim bead when inflated to the maker's maximum and then loaded with a rider and touring load. This can result in rim or sidewall failures, where the aluminum extrusion actually splits.

A number of years ago, in the British CTC mag Chris Juden recommended testing rim wear by overinflating the tyre, IIRC by about 20 psi. If the rim splits, it is worn out. He recommended eye protection in case the rim failed explosively.

I run my 2.0 tyres at quite low pressures, but often use higher pressures to "seat" the tyre when fitting. I once destroyed a new rim doing this (it folded outwards along the wear indicator groove), so I now take Thorn's recommendations seriously.

[jzdparis]

Thank you very much for all these helpful comments.  Do you think the brand of rim makes any difference?

[mickeg]

... Do you think the brand of rim makes any difference?

Yes, but if the rim manufacturer does not tell you their numbers, it does not really matter how they differ.  And they might not have ever run any tests to determine how strong their rims are.

[leftpoole]

Hello,
All bikes with Continental gatorskins on my cycles are run at around 110 psi which is presumably why I suffer no punctures.
John

[in4]

http://www.conti-tyres.co.uk/commuting-touring/gatorskin

F.I.O.

[JamesWhitby]

My Mavic A719 rims that I bought in 2004 are the only rims I have seen that have a pressure rating on the rim, photo attached. 

To my consternation, my DT Swiss X470 rims from Thorn came with a sticker to the effect that their maximum tyre pressure is 4 bar, not on the face of it a good match for the 700x28C Ultremo tyres (85-115 psi). I haven't dared go much above the 4 bar pressure (58 psi): I've had no problems with the tyres themselves, but have had several punctures, either snakebite or no clear cause and am no thinking of switching to higher volume tyres. 4 bar seems really low, is this meant to be the absolute pressure, or 4 bar above the usual atmospheric pressure?

[JimK]

I don't know why the rims would have a pressure limit like that. But for sure you don't want to be running 28mm tires at 58 psi! Mostly the stress on the rim from tire pressure goes up as the tire width goes up. So the pressure limit should be higher for narrower tires. But there could be some other reason for a pressure limit, I sure don't know!

[martinf]

To my consternation, my DT Swiss X470 rims from Thorn came with a sticker to the effect that their maximum tyre pressure is 4 bar, not on the face of it a good match for the 700x28C Ultremo tyres (85-115 psi). I haven't dared go much above the 4 bar pressure (58 psi): I've had no problems with the tyres themselves, but have had several punctures, either snakebite or no clear cause and am no thinking of switching to higher volume tyres. 4 bar seems really low, is this meant to be the absolute pressure, or 4 bar above the usual atmospheric pressure?

58 psi seems a bit too low for 700x28C, especially when the tyres are a bit smaller than the stated size, as is probably the case for these lightweight Schwalbe tyres.

On my old 700C tourer I have Schwalbe One 700x28C tyres, which are very similar to Ultremo. With a total bike+ luggage + rider weight of about 102 kg I currently run these at about 75 psi front, 85 psi rear. My rims are Mavic Open Pro. Because of the risk of snakebite punctures I wouldn't like to go much below 70 psi with 28C.

If ignoring the 4 bar limit on the DT Swiss rims bothers you, I'd suggest contacting Thorn for advice. In my experience they are very helpful.

[Bill C]

To my consternation, my DT Swiss X470 rims from Thorn came with a sticker to the effect that their maximum tyre pressure is 4 bar, not on the face of it a good match for the 700x28C Ultremo tyres (85-115 psi). I haven't dared go much above the 4 bar pressure (58 psi): I've had no problems with the tyres themselves, but have had several punctures, either snakebite or no clear cause and am no thinking of switching to higher volume tyres. 4 bar seems really low, is this meant to be the absolute pressure, or 4 bar above the usual atmospheric pressure?

58 psi seems a bit too low for 700x28C, especially when the tyres are a bit smaller than the stated size, as is probably the case for these lightweight Schwalbe tyres.

On my old 700C tourer I have Schwalbe One 700x28C tyres, which are very similar to Ultremo. With a total bike+ luggage + rider weight of about 102 kg I currently run these at about 75 psi front, 85 psi rear. My rims are Mavic Open Pro. Because of the risk of snakebite punctures I wouldn't like to go much below 70 psi with 28C.

If ignoring the 4 bar limit on the DT Swiss rims bothers you, I'd suggest contacting Thorn for advice. In my experience they are very helpful.

those dt rims are sold as 29er rims the 4 bar max probably means when running 29er knobblies, being a lot wider they exert more stress on the rims, i very much doubt 28c would have the same affect, if it was me i'd peel the stickers off the rims and bin them and do as your tyres say


edit
found this page it has no answers but you might want to read it anyway  http://bicycles.stackexchange.com/questions/34870/how-seriously-should-i-take-the-maximum-pressure-rating-of-my-cyclocross-rims