Our series of ‘Myths in Cycling’ continues with a look at rim width. It seems to make sense that wider tires should run on wider rims.
The idea is this: A wider rim makes a tire more U-shaped (left), rather than O-shaped (right) on a narrower rim. The sidewalls are more vertical, so they can better support the weight of the rider. This is said to make the tire flex less, so it corners more predictably.
When we first started experimenting with wider tires more than a decade ago, I was concerned: There weren’t any really wide rims available back then. I mentioned this to framebuilder Peter Weigle (above, in the center). His response surprised me: “I don’t think rim width matters. We used to race mountain bikes on narrow Mavic MA-2 road racing rims. I actually preferred how the bike handled with the narrow rims.” This came from the guy who won the cyclocross national championships on a mountain bike!
When I thought about what Peter said in the context of all-road bikes we were developing, it started to make sense. The extreme case of an O-shaped tire is a tubular: It’s perfectly round, and it touches the rim only at its very bottom.
And yet tubular tires are known for descending very well. Almost all pro racers in the Tour de France (above) ride on tubulars. If vertical tire sidewalls were essential for good handling, tubulars would have fallen out of favor long ago.
I realized that with supple tires, the sidewalls don’t really hold up the weight of bike and rider. It’s the air that supports the weight. And thus rim width doesn’t really matter. On my Firefly, I run 54 mm-wide Rene Herse Rat Trap Pass tires on 20 mm-wide (internal) rims without any problems (above). And the Firefly corners even better than a racing bike, because it has so much more rubber on the road. Like the pros, I never notice any squirm caused by the O-shaped tires.
When you look at the ETRTO chart for tire and rim compatibility, you can see that 54 mm tires on 20 mm rims are no problem. I could even go as low as 19 mm. (The values are internal widths in millimeters, and the ‘C’ stands for ‘crotchet’ rims.)
The chart shows that the real concern is at the opposite end of the spectrum: If the rim is too wide, tire pressure no longer pushes the bead strongly against the hook of the rim. This can cause the tire to blow off the rim. The ETRTO chart is quite conservative in this respect and recommends rims that are significantly narrower than the tires.
Together with other tire and rim manufacturers, Rene Herse Cycles is working on expanding the chart to include wider rims. In our experience, it’s fine to use wider rims as long as the internal width is at least 20% narrower than the tire. For my 54 mm Rat Trap Pass tires, this means that I can run them on rims somewhere between 19 and 40 mm wide. On wider rims, the tires will get a little wider, but even that doesn’t make a huge difference – we are talking maybe 2-3 mm.
What if you run stiffer tires? Would you want a ‘better’ match of tire and rim widths to take advantage of the sidewall stiffness? When I was at Paul Camp a few years ago, I got to ride a wonderful Steve Rex monstercross bike, shod with mountain bike tires (above). When I first rode the bike, the tires felt very harsh. I let out air until the bike began to float over the rough gravel in Paul’s parking lot. I loved riding the Steve Rex, and I pushed it harder and harder.
When we reached some really technical terrain, the front tire’s sidewall collapsed as the tire hit some rocks. It was very sudden, and more extreme than I had experienced when running supple tires at too-low pressures.
When Paul’s mechanic saw this, he checked my tires and shook his head: “You need to run about 30 psi in those tires!” Now 30 psi (2 bar) is more than I run in my Rat Trap Pass Extralights. Perhaps the mechanic was overestimating my weight, or adding a factor of safety, but it appears that when you ride really hard, tires with stiff sidewalls may need almost as much air as supple tires.
Relying on the tire sidewall for support has a disadvantage: Once the tire starts to flex, it bows outward (right). It goes from U-shaped to O-shaped. The more it bows, the easier it becomes to flex. Once it starts to collapse, it becomes less and less stiff, and there is little to stop it. This is called a ‘regressive’ spring rate. It explains why the collapse of the tire was so extreme.
Using air to support the bike results in a ‘linear’ spring rate. No matter how hard you push on the tire, the air pressure doesn’t change: It continues to push back with the same force. (In theory, the pressure goes up slightly as the tire deforms, but the tire’s volume is so large that this isn’t significant.)
Mountain bikers have found that the best shocks have ‘linear rate’ coil springs. ‘Progressive rate’ air springs are a second option for those who want to save weight (air weighs less than a coil spring). Nobody would ever suggest a regressive spring rate, yet that is what you get when you use the tire’s sidewall as a spring.
This suggests that you might even want to run stiff tires on narrow rims: The O-shaped tire will be easier to flex and thus more comfortable. Without the tire ‘standing’ on its sidewall, you’ll have to run a little higher tire pressure, but then you don’t have to worry as much about the tire collapsing. When Peter Weigle mentioned that he preferred the ride of wide mountain bike tires on narrow rims, I believe he was talking about this.
With a supple tire, this effect is magnified: With mostly air holding up the bike, the spring rate is very linear, and you can run a lower total spring rate – less stiffness of tire and air combined. That means you get a tire that absorbs shocks better and is more comfortable. And because the supple casing absorbs less energy as it flexes, it’s faster, too.
What this means in the real world:
- Rim width doesn’t matter for supple tires. You can run our widest Rene Herse tires on relatively narrow rims or on wide rims. There will be little or no discernible difference in how the tires feel and corner.
- Don’t use a rim that is too wide for your tires. The tire should be at least 20% wider than the rim.
- Using the tire sidewalls to hold up the rider results in a regressive spring rate. This can result in the tire collapsing suddenly.