Monday, February 04, 2019

Rim Width And Gravel Tires

Internal rim width- How wide is too wide?
It is no secret that rims have been getting wider. It also is no secret that we have pretty much quit focusing on external rim width and now pretty much speak in terms of internal rim widths. Why? Also, you may wonder why any of this matters. Wide rims are......well, wider, and that's that, right?

Well, not so much. Actually, wider rims do some beneficial things for riders of any bicycle, but I am going to keep the focus on gravel travel here. That said, much of what I am writing about today can be transferred to other types of bicycles and other genres of riding.

First, let's get into why rims were, for the most part, narrow. One must remember it wasn't always the case that rims were narrow. Way back they were wider because tires were bigger and more material was necessary to keep rims strong. Old tolerances were not as tight either, so width helped support the tires that didn't necessarily fit the greatest.

Note the difference in outer rim width versus inner rim width. Image courtesy of Velocity USA.
Then tolerances got tighter and materials technology advanced enough that things swung the other way. To cut weight, both the rim and the tires went on a diet. Rim widths got ever narrower until the 80's/90's when internal rim widths on some XC MTB rims was sub 20mm! That's for 2" plus wide tires!

Road rim widths, internally, were 17mm-19mm wide and stayed that way for a few decades. But then something happened which, in my opinion, opened the door wide for experimentation on rim width. During the late 90's, early "oughties", the down hill mtb scene started to push rim widths out to better support tires in corners and hard landings. Other MTB disciplines started looking at this and with a helping hand from materials technologies, once again, rims started getting wider for mountain bikes.

29"ers were something that also started companies walking down that wider rim lane. I recall when Salsa Cycles announced their Semi (30mm outer) and Gordo (35mm outer) rims, Velocity USA announced the Blunt  (28mm outer) and then the Blunt 35, (Obvious, innit?), and we all started thinking harder about wider rims. Keep in mind that about 12 years ago internal rim widths for most 29"ers was still around 20mm, give or take a millimeter. These new rims were mind blowingly wide.

Tubeless stuff also made wider rims more popular. The whole idea with tubeless was that one could lower air pressures, get more traction, and have similar or lower rolling resistance as with tubes. But, without a wide, stiff foundation for the tire to work from without a tube in it, the problems of burping tires and rolling tires off rims was commonplace. This moved companies to start taking a look at wider rims. Weights went up, for sure, but with new technologies, that weight wasn't as great as it would have been in the decades previous. The ubiquitous usage of disc brakes and the problem solving that came with wider rims brought a new focus on what mattered- inner rim width. 

With tubeless tires, inner rim width is important to note.
 With no regard for cantilever brake's limitations on rim widths, designers of mountain bikes were free to widen the inner rim widths to better support tubeless tires. No longer were riders considering rims only for the lightest weight, like back in the 90's. Now try and find a MTB rim with an inner width below 20mm. Not gonna happen!

In fact, try and find a current, tubeless compatible gravel/all road wheel with an inner rim width of less than 20mm. Yep! Pretty much a unicorn at this point nowadays. So, with the bonus of a good foundation for tires to work off of, and weight penalties being minimized, is there a point where tires and rims don't work as well when inner rim widths get really wide? In fact- yes. There is a point where rims can be too wide.

Now, as I've stated, I'm not going to go all in on mountain bike, fat bike, or road rim widths and tire matching to those. That's a HUGE subject which could fill a book. I'll just keep the focus on my experiences with gravel tires and rims meant for that pursuit.

I've used very narrow, 20-ish millimeter wide rims all the way up to 25mm internal rim width rims with tires ranging from 32mm through 50mm. Basically the gamut of tire widths used for gravel travel. In my opinion, some matching of tire to inner rim widths makes sense here. So, to put a 32mm CX tire on a rim with a 25mm inner width isn't a great idea. the tire casing flattens out, the overall height/volume of the tire suffers, and some elements of "self-steer" will rear their ugly heads on set ups like that. Self-steer is where your wheel kind of has a mind of its own and goes not where you want to point it. I know......that's a pretty technical explanation. Sorry! I would take the time to break that down, but this post is getting long as it is!

Of course, you can put a 50mm tire on a very skinny rim, and I've done that too. You end up having to use a higher pressure to mitigate tire squirm and possibility for roll-offs, and your ride quality will suck. Again- technical speak. But I think you get the picture here. Extreme match-ups are to be avoided. Try to use a wider tire with a wider internal rim width and vice-versa.

A good, all around internal rim width for almost any gravel tire, in my opinion, is 24-25mm. It would seem that most manufacturers agree with that. A few years ago I built up a set of wheels I dubbed the "Project Wide Gravel Wheels". I used a WTB KOM i25 rim set. Yes......that's 25mm inner rim width. I figured I was pushing the limits of "good" and maybe went over the line. Let's keep in mind that even three years ago 22-23mm was considered "really wide" for gravel. But I found that i25 rims were actually ideal for the 40mm-45mm tires I was using. That size even works well for up to 50mm tires, as I have found by using a set of 24mm inner width rims with 650B X 47mm tires.

So, would something even wider work? Maybe for the wider gravel tires, yes, it might. I am thinking about pushing a wheel set out and using internal 30mm width rims, like the Blunt SS shown above. It might prove to be interesting to try that out. You may say, "Well, that's a mountain bike rim!", and it is, technically. But take a look at what mountain bikers are using now and much of what you will find makes an internal 30mm wide rim look "skinny".

There is more to this rim width subject, but this post is already at "novel" length, and either your coffee is gone already, or it's cold. Sorry about that...... I'll update this subject down the road if there seems to be enough interest. I could even upgrade my "Project Wide Gravel Wheels" to "Project Even Wider Gravel Wheels" and maybe that is what I should be doing here. Real world test that idea!


Bob said...

Timely article. This weekend I mounted Extraterrestrials on Mulefut 50s for rail-trail touring this summer.

Barturtle said...

The Velocity rim was introduced as the P35 by Kirk Pacenti.

Guitar Ted said...

@Barturtle- Correct, but that is not how people will remember that rim, so I used the modern equivalent name as a reference.

Gravel Boat said...

I've been running GK 43's tubeless on Blunt SS rims for 18 months or so with no problems, but I wouldn't want to try any tires smaller than the GK43 on this rim.
I haven't found many rims as light or lighter than the SS in the 24mm range.

baric said...

I mated up a pair of 29er Extraterrestrials on Velocity Dually's on my Fargo. I think they're pretty much a perfect matchup, but they do make the bike almost nose-bleed tall and sometimes hard to get on board.