Tech FAQ: Rims and Pressure to Fail

Dear Lennard,

A friend of mine who works in a bike shop says that a bunch of lightweight mountain bike rims from different manufacturers have broken in the shop when they inflated the tires when seating them initially. They never got above the rated maximum pressure imprinted on the tire, but still the rims failed. How can companies sell rims like that?
-David

Dear David,

A number of superlight mountain bike rims will not hold the pressure rated on many tires. Disc brakes have removed the need to make the sidewalls as strong, so in the interest of bringing the wheel weight and wheel inertia down, some superlight rims, particularly some meant for tubeless tires, now have such thin sidewalls that they will not hold the pressure that many a tire is rated to.

While that probably seems horrible on the face of it, the fact is that the outward stress from the air pressure is a completely different kind of stress than encountered when rolling over rough terrain with low air pressure in the tire. The rim may be plenty durable, at least for cross-country applications under a rider light enough to care about having incredibly lightweight wheels.

The problem occurs generally upon tire mounting. The mechanic will air the tire up hard to seat it. This is particularly common among mechanics seating tubeless tires. We all have seen a tire pop into place better as the air pressure increases. But if the tire sidewall says it’s rated to 65psi and the mechanic airs it up to that (or higher), he or she may be going way higher than the rim can handle.

Really, the rim, at least a disc-brake rim, should have a pressure rating imprinted on it by the valve stem. This is much more useful than what is printed on the tire. In fact, there are plenty of small slick tires that fit 26-inch MTB rims, and even more, of course that fit 29-inch (700C) rims. These tires may very well have 90psi or higher imprinted on them, and that certainly would fail many superlight MTB-specific rim sidewalls.

So, word to the wise: If you’re buying superlight disc-brake wheels, DO NOT inflate to the rated pressure on the side of the tire. In general, don’t go over 35psi with rims like that.
-Lennard

Regarding the Sept. 10, 2010 Tech FAQ question on bottom brackets for a 1991 Fat Chance Wicked :

Dear Lennard,

Phil Wood makes an external BB that presses into BB of older mountain bikes like Kleins. They require a 35mm I.D. shell.  Obviously, I don’t know if they’ll fit the old Fat Chance Jason was asking about, but a good caliper would give him the info he needed.
-Chris

Dear Chris,

The old Merlin and Fat Chance press-in bottom bracket size is smaller. I measure the bearing OD at 30mm, ID at 17mm. So, no, that would not fit Jason’s Fat Chance.
-Lennard

Readers can send brief technical questions directly to Lennard Zinn.

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

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