Tuesday, September 29, 2020

When More Is Too Much

Are inner rim widths getting out of hand?
 Wheels are an interesting subject. We've been doing a lot with regard to wheels on RidingGravel.com this year which has prompted me to take a closer look at trends for 'gravel' wheels and I have noticed something that concerns me. Are we getting to the point where our wheels have unnecessarily wide internal rim widths? My opinion is that the answer to that question is 'yes', and here is why........

Ten to twelve years ago, a decently wide trail bike rim for a 700c based mountain bike was about 28mm-30mm overall width. Note- we didn't ever think about rims in terms of inner rim width. That came about when carbon rims had to have wider rim bead/wall dimensions to withstand the beating the rims might undergo in use on mountain bikes. That made outer rim dimensions meaningless. So, we flipped to talking about inner rim dimensions. So, back in the day, how wide internally were those old trail rims? That is what we need to look at first. Here are some actual measurements of some older popular trail and XC 29"er rims.

  • Bontrager Race Lite wheels- 18.5mm
  • Bontrager Rhythm Wheels - 21mm
  • Stans Flow rims - 21.5mm

Are you surprised? I was. These old mountain bike rim designs would be shot down in a heartbeat on a forum dedicated to gravel wheels. For further context, keep in mind that any good suspended trail 29'er back then was running Stans Flow rims. "Wide" mountain bike rims were maybe 30-35mm in overall dimensions back then, which in aluminum would have yielded an inner width of about 26mm-30mm maximum. 

Also keep in mind that things in the MTB world haven't changed all that much. Look up any decent XC hard tail and check the rim spec. 23mm or so internal rim widths are common, and for a full on trail bike? You are looking at around 30mm internal rim widths. Okay? What about current trends in gravel wheels?

This is interesting to note. Four years ago, I did a wheel build as a project for RidingGravel.com called "Project Wide Gravel Wheels", and for those wheels I used, what I thought of as then being, a ridiculously wide inner rim width of 25mm. Now given all of the above information, you might see why I was thinking that way. In 2016, a twenty-five millimeter inner rim width for gravel was unheard of. 

Project Wide Gravel Wheels used a 25mm inner rim width.

Since then, inner rim widths for 'gravel' category bikes has been creeping upward. 24mm is probably average now with some (FLO Cycling G700, which I recently tested) being 25mm and now we're seeing 26mm inner rim width being pushed as a 'gravel' wheel idea. Granted, materials technology advancements have allowed this to occur without the negatives of weight being an issue, but things are beginning to get out of hand, in my opinion. 

First off, when do we start calling these wheels mountain bike wheels? I'd say we're pretty much at that point. The Boyd wheels linked here are even marketed as 29" XC rated wheels. And then you've got to wonder how tires are being affected at some point. Width ratios are getting a bit out of whack when you look at the differentials between tire overall width vs. inner rim width. Consider that the Bontrager Kovee Comp rims, with an inner width of almost 23mm, has a 2.4" maximum tire width rating. Think about that for a minute in comparison to average widths of tires for gravel uses. 

But let's say the Kovee Comp is on the anemic side for XC use. Okay, maybe what? Go to 28mm inner rim width? 30mm? That's still only 3-5mm wider than where we are with current average gravel wheels which are using tires much smaller than an XC MTB will be using. 

Look, I'm not against experimentation and I happen to think about 24mm inner rim width is about 'right' for a gravel tire in the 38mm-45mm width range. I think 22mm is okay for a racing wheel, for sure, where lighter weight and all is paramount. But when we are talking 26mm inner rim widths? Hmm..... Do we really need that, and I have serious doubts that our tires are going to work correctly over that wide a rim if we are talking the sweet spot for gravel travel, which in my mind is still 42-45mm wide. But if you are in the 38mm-40mm tire crowd, that 26mm inner rim width or wider? Uh.....no. I think you've gone past the point of 'good' and your returns are diminished. But maybe that's just me......


MuddyMatt said...

These are timely thoughts Mark. My hardtail run 2.3 Vigilantes on WTB i25 rims (i.e. 25mm internal) and they seem just fine.

At the same time I'm in the market for some gravel wheels for my Topstone which currently runs on 38mm Specialized Pathfinders although I plan to move to 42mm tyres next time round, of some form or other.

The wheels I'm looking at are Hunt 4 season gravel disc wheels which are a sensible price BUT also come with 25mm internal rims. They are as wide as my MTB!!

Now I have some 30mm Hunts for my 29er full sus and have been very happy with them, so I'm not concerned about the quality or performance. I just can't get my head round 25mm rims for gravel, it feels like overkill for me but the reality is the OE rims on the Topstone are 23mm internal anyway. Not such a big jump then, but a very weird world we're in!

Stud Beefpile said...

I agree it's a pretty fine line. I do think that gravel cyclists in general are appreciating the wider rim widths, and based on just my anecdotal experience, gravel tires (in general) hold up surprisingly well to being stretched out on these wider rims.

I've had a set of Gravelking SK's (non-plus 43's) mounted to Stan's Arch MK3's (26mm internal), and I've really liked the tiny bit of extra cushion it affords (versus 21mm internal Arch's of the past). The tires have held up well (no flats, cuts, etc.) on these rims to around a 50/50 ratio of urban commute/gravel riding.

Just as you said, though, I don't want to go with a rim any wider, and I wouldn't feel comfortable riding a tire under 40mm (actual width).

I've got a 38mm SK+ on a 24 or 25mm internal (I don't have calipers) and it's at the limit of what I consider acceptable. It's held up fine on gravel roads, but I won't be underbiking it on our local XC trails.

Exhausted_Auk said...

My understanding years ago for road wheels was that the tire should be 1.5-2X the rim internal width. At the time, we were running rims of 13-15mm internal width with 20-23mm tires, which makes me shudder to think of now! Granted, MTB tires sometimes go well beyond 2X, but if we apply this same rule of thumb for a 40mm gravel tire, it would make the acceptable internal rim width 20-26.7mm. It looks like most gravel wheels are in this range at the moment, but I agree we probably neither need nor want to go even further.

Nebo said...


S.Fuller said...

I wonder how much of this is being driven by manufacturing costs and profits? Extrude one rim profile and then label/market it separately as an XC and a gravel rim?

Guitar Ted said...

@S. Fuller - Happens more than we think. This was the case when 29"ers first came about. You will remember the Delgado Cross, a rim designed for CX, but was suddenly the darling of early 29"er builds. The list goes on, but yes- many early 29"er rims were, in fact, rebadged touring rims without machined side walls so they could be "disc specific".

You can bet that many disc MTB rims are being sold similarly.

And when we think about carbon, this is even more appealing to manufacturers. The Boyd example in the post being a blatant example.