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Technical FAQ: Gravel riding with rim brakes

Lennard Zinn /
A 28C tire can fit snugly under a rim brake. Photo: Spencer Powlison | VeloNews.com

Tires and brakes for gravel

Dear Lennard,
I’m a long-time road cyclist who’s been off the bike for a couple of years. I’m hoping to get back in the saddle, but I’m finding many of the roads in my area intimidating in terms of heavy traffic. After spending a bunch of time chatting with the folks at my local bike shop, it’s been recommended I consider adjusting my bike setup to be a bit more gravel-friendly. Apparently there are ways to get to good quiet roads if I’m willing to mix in some gravel sections. The tech I spoke with suggested upping my tire size to something in the 28C range to make this feasible. My bike has Dura-Ace 9-speed. What I’d like to know is what the actual tire size capacity of the brake calipers are.
— Jason

Dear Jason,
Generally, a 700 x 28C tire is about as big as you can fit under a Dura-Ace road brake caliper. I am running 700 x 28C tires on my road bike that has an Ultegra group on it, and I have plenty of tire clearance. Whether that will fit in or not also depends on the specific tire as well as on your frame and fork.

Some tire models of the same imprinted tire size will be significantly bigger than others when inflated. And sometimes, for instance, the brake bridge is fat enough in the center that it rubs when the brake caliper does not. Also, when going up in tire size on a tight road-racing frame, you may have clearance issues at the chainstays behind the bottom bracket or with the back of the seat tube or the front derailleur band clamp. Finally, how much clearance the caliper has under it is also dependent on how tight the cable is, since the clearance under the caliper increases when the arms are pulled by the cable.
― Lennard

Dear Lennard,
What’s your feeling on 28mm tires vs. 25mm, for general riding — mostly asphalt, with a few miles here and there in the dirt and gravel? We all switched to 25mm, largely on your recommendation, and I can say with certainty that none of us will ever go back to 23mm tires. But I’m not sure what to think of going up one more size. One friend did make the attempt, but the 28s would not fit in his Domane. I never really considered it before, but the rear brake bridge for short-reach racing calipers seems to be the limiting factor for most folks. Interestingly, another riding buddy’s cheaper Domane, with the Taiwanese made frame, does look like it will fit a 28mm tire.
— Steve

Dear Steve,
Unless you’re a stickler about weight, I recommend a 28mm tire over a 25mm tire for general road riding that includes some dirt. I rode a lot of dirt for decades on 700 x 23C tires without knowing any better and was quite happy with them, especially once I went to tubeless ones and didn’t need to worry about pinch flats hitting rocks or thorns on rough dirt. I noticed a nice improvement when I went up to 700 x 25C, and I noticed even more improvement going to a 700 x 28C.

Now, especially since I no longer care about Strava KOMs nor would I race another hill climb, I only ride 700 x 28C on the bike I take all over the world. Other than a weight cost for climbing and accelerating and a bit of extra aerodynamic drag, I’m sure it is a net advantage, whether on pavement or dirt. Cornering is better, rolling resistance is reduced, comfort is enhanced, and traction is improved.
― Lennard

Tire pressure vs. tire size

Dear Lennard,
Could you publish a definitive list of maximum safe tire pressure by rim size? It seems as if I have been riding my 32mm tires at too high a pressure, 95psi. I think this issue has safety implications across the industry.
— Ian

Dear Ian,
Indeed it does. As I wrote in this piece, analyzing hoop stress shows why the same pressure in a bigger tire puts more stress on the big tire’s casing (and on the rim constraining its beads). The letter about fat bike tires exploding at the bottom of this post is a particularly good read on that subject.

To answer your question, what I would say is that for a given tire model, if you go up in size, your pressure in the bigger tire should not exceed the maximum pressure you run in the smaller tire, multiplied by the ratio of widths of the small tire to that of the big tire (i.e., multiply the pressure in the smaller tire by the width of the smaller tire and divide by the width of the larger tire). So, if the maximum safe pressure is 100psi in a given 23mm-wide tire, if you go up to a 35mm width in the same tire, the maximum safe pressure is 100 X 23/35 = 66psi.
― Lennard

Converting to SRAM 1 x 11

Dear Lennard,
I am planning to convert my Cannondale CAADX gravel commuter bike to SRAM Apex 1 x 11 and will purchase a new cassette, derailleur, chain, and SRAM X-Sync 40T 11-speed Chain Ring. I was wondering if I can utilize the SRAM 10-speed shifters for this — Apex, Rival, or Force — or if a SRAM 11-speed shifter is required.
— Kreig

Dear Kreig,
You need a SRAM 11-speed shifter.
― Lennard

Following up on breaking Campy Eurus spokes

Dear Lennard,
I ended up breaking two spokes after visiting two bike shops. The second bike shop was kind enough to replace a broken spoke for free after they installed one on the wheel. Both of these spokes broke on rides that lasted about a mile. These rides occurred after the replacement was installed. After the second spoke was replaced for free, I thought the breaking spokes might be caused by my riding, so for the first 50 miles after this spoke replacement I rode the bike gently to the point where I dismounted and crossed rough sections of road by foot. I don’t know if this made a difference, but I haven’t experienced any more broken spokes. I’m thankful for that because the spokes are expensive and difficult to obtain.
— Douglas

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