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Old 2013-01-22, 04:49 AM   #11
mad
Unicyclist
 
Join Date: Jul 2012
Location: SoCal
Posts: 86
Quote:
Originally Posted by Ewok View Post
Normally I'd say: just look up what is the minimum recommended width as according to ETRTO, whose table Schwalbe cite on their website, but sadly it doesn't extend far enough into the realms of special mining applications for rims that wide.
Your inner rim width should be about 36mm or "36c" (42-6, rule of thumb).
The table goes only to 29.
Extrapolating with a quadratical model (and adapting the model to fit all data points), I calculated a minimum width of 75mm or 3". A ridiculous result.
Maybe the original table in the ETRTO standard is more complete. You could buy it for a mere 114 Euros to look up the recommendation, a real bargain.

Or more easily: just make use of nlouis1's experience and try tyre width=inner rim width+6 -> 42-622, maybe Vittoria Randonneur.
While that ETRTO rim/tire chart doesn't go high enough to show their recommended minimum tire size for a 36c rim it does show a maximum recommended rim size, of 27c, for a 50mm tire. ETRTO recommendations are certainly going to be conservative, but we are still talking about a rim that is 9mm, or 33%, wider than recommended for a 50mm tire.

Tire pressure is probably a big part of the equation. They have to consider what is the largest rim a given tire will stay on, reliably, even at maximum (or higher) pressure and under load.

Quote:
Originally Posted by Killian
Thought I'd throw this in just to clarify my fears. Top of second page, tire blowing off 42mm rim
That blow off was at 65psi. At the end of that same thread pstrick says he is now running at 55psi and is doing ok so far. Later, again in that same thread, pcuni says he has put hundreds of miles on a 45c tire, mounted on a 38mm KH XC rim (about 32c), at 45-50psi without issue. This rim/tire combo is a little less out of spec but the rim is still 23% wider than it should be, by ETRTO standards, for a 45mm tire.

Klaas_Bil's blow off was with the narrower KH XC rim and a 2.0 tire, even closer to spec but still over by about 18%, but that was at 70psi.

Seems like it mostly boils down to tire pressure. Anecdotal evidence on this forum suggests that the combination of a 50mm tire and a 42mm (36c) rim will probably be ok at 45-55psi, and will probably fail at 60-70psi. But, to ensure compatibility, a narrower rim or wider tire would be advisable.

When you were running the SB8 2.1 on that rim what was the tire pressure?

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Maybe we should start a thread asking all the experienced 29er commuters to chime in with their rim/tire/psi stats. Especially if they are running on wider rims.

Of course that's if there are any left that haven't moved on to 36" commuting.
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