SkylarG

I'm familiar with the ETRTO chart on what tires are recommended for rims, but I'm curious if anyone has experience moving outside this for larger rims with narrower tires.


Now road cycling is quite happy outside of this, with upwards of 18mm internal width rims and 23c tires (1.3x tire to rim). The idea being a reduced "lightbulb" effect, lower air pressure comfort, traction, etc. What I'm curious about is why we aren't seeing similar trends in some mountain/cyclocross/commuter rims.

I'm building a 650b porteur style commuter/touring/grocery mule bike and plan to run anywhere between 650x35-42b tires on this bike. I'm looking at DT Swiss rims, the XM 401 rim with a 22.5mm internal width. The XM 401 is "recommended" for 1.9"-2.4" (48-61mm) tires. I want to know if I can pop a 650x35b tire on it (1.55x tire to rim) and get the same benefits seen on narrower rim/tire combinations.

Anyone with any experience on this matter?

FBinNY 

Narrow tires on wide rims assume an inverted U profile, vs. the traditional Omega or "lightbulb". Some say this increases steering precision, and maybe it does, but it worsens radial compliance and increases road friction, and worsens the ride. It may be fine on glass smooth roads, but riders on poorer roads probably won't become fans.

Since mtn bike trails aren't exactly lie glass smooth roads, I don't expect this trend to catch on there, but, after decades in the bike world, I know enough to never say never.

Also two other point to consider. Mtn bikes already have been going beyond the ERTRO guidlines but it's to the wide tire on narrow rim side, probably because they handle low pressure better in DH and the like. Also consider that the guidelines aren't arbitrary edicts. They've based on 100 years of tire and rim experience and evolution. Trends narrower or wider will come and go, but it will always come back to the guidline ratios.

JonathanGennick 

I didn't know that about increasing road friction. I'll have to rethink what I'm running on one of my bikes. I've a set of 1.9" micro-knobbies on a what is a fairly wide mountain-bike rim and I was thinking yesterday that it was kind of a miserable ride on the pavement portion of where I had been riding.

There has actually been a trend in mountain-biking toward wider rims. On another bike I've a set of 35mm-wide Salsa Gordo rims with 2.2" tires mounted on them. Not sure about hitting the 1.55 ratio the OP mentions. They are wide rims though.

Rims on fat-tire bikes are sometimes 100mm wide, and that's where you maybe would find the highest ratio of rim width to tire width.

SkylarG

My understanding is somewhat different. When it comes to road rims and tires, the idea of wider rims is the ability to run the same size tire at a slightly slower pressure, resulting in both a more comfortable and faster ride (less rolling resistance + more aerodynamic profile) with better cornering. My own switch from 15mm inner width rims to 17mm and keeping the same 23c tires made a noticeable difference in comfort and cornering.

Sheldon Brown notes "A general guideline is that the tire width should be between 1.45/2.0 x the inner rim width."

My question here is probably more relevant to commuter/touring setups riding mostly on pavement and sometimes gravel/dirt roads. Many touring bikes come with 35-40c tires, but mounted on rims with the same 17mm internal width as road bikes running 23c tires. Why not use wider rims for these uses?

FBinNY 

I can't see any reason not to. I suspect that the reason we see disproportionately wide tires on narrow road rims is more about marketing trend and product availability than anything else. The OEMs like to spec the most popular light rims, which are going to be narrow. Then either they or the consumer, later spec wider tires, but not change the wheels out. So the process leads to the wide tire on narrow rim combination.

I've posted on this a number of times, and the general rule is that tires narrower than ideal have the side walls straighter and more vertical, so they then to be a bit stiffer. Wide tires on narrow rims lack lateral stability, especially at low pressure and can wallow in turns. But those are only rough guidelines, and other factors lie the tire's construction can have greater affects.

My personal rule is ride what you think work best for you, then apply the guideline as a reference tool if you have performance issues. For my part, I commute on 1.9" tires on comparatively narrow mtn rims. They're fine at full pressure, and serve me well on poor roads, especially at night when I can't see as well. However, they're not without issue, and have twice put me on pavement in turns, because a slow leak in the front tire went unnoticed.