Hi Mike, and welcome to the Forum!
Like you, I was initially surprised at some of the figures shown for recommended maximum tire pressures by Andy Blance in the Thorn brochure. They seemed especially low in my eyes, coming new to the world of 26" rims and tires. As a roadie (and someone who toured on 700x32C tires) I was especially surprised.
Doing the math, however, it is easy to see the reasons for the recommendations. Equations aside, here's the reasons --
First, the 26" tires have a lot more air volume -- up to 6 times that of a 700C tire, depending on width comparisons.
Second, while most 26" rims are pretty close in width to those used on 700C touring wheels, the tires are proportionately wider. If filled with the same pressures as ther narrower cousins, these wider tires exert much greater leverage against the rim sidewalls, placing them under greater stress. The sad result is a greater incidence of sidewall cracking...and the appearance of something almost never seen with narrow tires on narrow rims: Rims splitting right down the middle, from spoke well to spoke well.
Third -- at least with trekking tires like the 26x2.0" ones commonly used for touring -- these wheels see a lot of weight compared to the average sport-touring or racing bike with 700C wheels. This loads the rims further, and if tires pressures are already high...well, problems are more likely.
By its very nature, a tandem carries the weight of two people -- even more if you tour with one. When I took my tandem touring with my visiting Duch pal and I hauled a trailer, I was shocked to find the lot (riders, bike, and trailer) weighed 600lb/272kg. I was equally shocked to find we did alright with "only" 85psi/5.9bar of pressure in the 26x1.5 tires. Based on my experience with narrower 700Cs, I would have bet on at least a couple pinch flats, but we rode the bike extensively on poor gravel roads and didn't experience a single problem. In fact, I think tire pressure was likely a bit high, and if I had been running 26x2.0 tires, would likely have gone with a maximum of 51psi/3.5bar as Andy recommended. Going further, I would consider that a maximum under heavy touring loads, and would use even less air pressure for riding with small day loads or just myself on the bicycle. This was borne home to me on a recent day ride on the Nomad where I used the same pressure as I had the day before when trialing different packing schemes for loaded touring. The bike rode like a jackhammer; using 5-10psi less in the tires made a world of difference. The fault was mine, I simply forgot to adjust pressures downward for my solo ride.
By way of illustration, these folks had a perfect storm of rim failures:
http://www.goingslowly.com/gear/, detailed here:
http://journal.goingslowly.com/search/rims/2009/08/velocity-rims/http://journal.goingslowly.com/2009/12/velocity-rims-part-2.htmlThere problems were largely isolated to the rear rims, and likely due to the *very* heavy loads they carried and -- I suspect -- pressures that may have been a bit high, though pressures were not mentioned in their reports. The manufacturer really stood behind their product with continued support and repacement, but their failures were identical to those produced in other brands as well. A quick GoogleImage search for "Bicycle Rim Failure" (
https://www.google.com/search?hl=en&client=firefox-a&hs=kFe&rls=org.mozilla:en-US:unofficial&channel=fflb&q=bicycle+rim+failure&bav=on.2,or.r_gc.r_pw.r_qf.&biw=1173&bih=610&um=1&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi&ei=5NBlUNKQJcSX1AXhgoG4Ag ) shows some spectacular results sometimes due to factors other than wear.
I'm not trying to defend Thorn, but looking at the factors above, I've come on my own to think their pressure recommendations are well-intended to prevent the possibility of harm or failure in touring use. Legal implications aside, that's surely not a bad thing!
Best,
Dan.
