MATTER OF FACT: Increasing Stability on Gravel

Welcome to the first of what will become an ongoing stream of posts that will share questions I receive from members of the riding community about items of interest to the collective. These posts will primarily be about MATTER – the technologies we engage in the act of cycling – but anything and everything related to cycling is fair game, including the stuff I’m definitely not a real expert on, like cognitive science!

Holler at me, I’ll run with questions that spark my interest and are likely to do the same for others. Consider the ‘branding,’ MATTER of Fact, a tongue in cheek thing. There are/no facts. Fire away to: teknecycling@gmail.com

“I hate it when my mtb back wheel (knobbly tire 26″ wheel) fishtails in sand or gravel, it makes me nervous. I’m looking for a gravel bike to take on camping trips so I only need take one bike which I can also use for road riding. I’m thinking of getting a gravel bike with 700c wheels for road use, and a separate set of 27.5 with wider tires for off-road use.

Some say the frame is designed for one size wheel and not to change it. Is this true, or true for certain bikes/frames only? – Julia”

First, you’re right to think 26″ wheels are less stable on sand and loose surfaces. The bigger wheels have more inertia, which means they ‘want’ to go straight more. Think of a flywheel.

Another thing to keep in mind is that for loose surfaces like sand, and loose-over-hard surfaces like gravel, there is such a thing as a tire that is too wide to provide grip when leaning through turns at speed. Rather than either cut into sand or through gravel to firmer material (or simply functioning like a rudder), the tire floats over the surface, which is itself shifting underneath. So, for example, I find 650b x 48mm tires unstable on loose-over-hard roads in the turns compared to a 700 x 38mm tire. On the other hand, the bigger tire floats on top of sand a lot better, so sometimes it’s a matter of compromise and choosing a setup that works for 90% of a route, and for the other 10%, you adapt (for me, descending at 85% speed versus 95% while on the 650b x 48s).

The big thing going on with a 26″ bike will tend to be the wheelbase is short compared to a 700c gravel bike or a newer MTB. On top of that, body positioning that might be good for trails isn’t necessarily also stable on gravel roads. In general, fishtailing around might mean your weight is biased toward the front of the bike too much; this something to keep in mind on whatever bike you’re using. 

So yeah, the plan you have in mind for a new bike to run 700c for road, 650b for offroad, is great. You probably know this is what I do with my drop bar bikes. You might have looked at the Lauf True Grit frameset, which is definitely on the more stable end of the spectrum in terms of geometry. Of all the dropbar bikes I’ve ridden, it’s definitely the most stable on loose surfaces; this comes down to its slack head angle and suspension fork (which works fantastically well). I believe the version I rode had 430mm chainstays, while the current iteration has 425mm, which means a little less stable (assuming I’m right about 430). My bikes use 425mm chainstays, and I find it great overall.

Lauf designed that bike expressly NOT to ft 650b wheels. I spoke with Benedikt, one of the owners and designers, about this, and the reason was that they wanted to fully optimize the bike for 700c wheels, which they feel are most efficient for gravel. I believe they are right when it comes to fairly fast gravel like Dirty Kanza stuff, where you’re mostly maintain momentum (and inertia) and you sort of want your wheels to roll like juggernauts over the terrain (again, more like a flywheel). For rough gravel this will be the smoothest ride, which will likely outweigh an aerodynamic loss compared to a smaller 650b.

So yes, some companies have made their bikes for 700c only, and it’s true that 650b really doesn’t fit them, even in 42mm wide. I’ve had such a bike, and the issue was the shaping of the chainstays: bent inward for crank clearance. However, since riders like me have been telling companies for a few years now that we want 650b clearance, they are doing it more and more, so many bikes will fit both well. And you don’t have to do anything special when swapping between them.

There is a new crop of gravel bikes out that are finally meeting the needs of riders like us who want to ride dropbar MTB style AND other stuff on one bike. People have been having these custom made for a while, but Nukeproof is the first I know of to go to production with one. If I was to buy a bike off the shelf, this would be it, and my buddies I ride with are of the same mind. The Digger is closer to a modern MTB in its geo approach than a classic road bike. It uses 425 chainstays combined with a long front centre, which provides a long wheelbase (just shy of the Lauf’s), lending lots of stability. It is meant to be ridden with a stem shorter than usual for a CX or gravel bike. It uses a 70mm bottom-bracket drop, which I find ideal for use with 650b x48ish and 700c wheels (it’s too low for trails with 650b x 42, and too high for 700 x 42+). It fits proper MTB size tires: 650b x 2.3″; I’d tend to recommend a 2.1 like the Schwalbe Thunder Burt.

Here are my recommendations for things to look for in a new bike and your wheel set-ups

Look for a bike that is designed with a longer than usual front end (like the Digger), and a shorter stem, so you have a long wheelbase for stability

• Head tube angles lower than 72 degrees will give you more stability. Don’t exceed 72 degrees. Something around 71 will work best.

• 425 – 430 chainstays will give you the most stability, including while climbing. The OPEN U.P. uses a 420 to provide more of a ‘road bike feel,’ which is not what you want.

• 70mm – 80mm bottom bracket drop will serve you well. Closer to 80 will provide the most stability possible, but reduce pedal clearance a bit on trails. If the bike fits tires around 650b x 2.1″ and you run them offroad, your pedal clearance would be fine at 80mm drop, no problem. If you max out at 48mm, I’d suggest staying closer to 70mm.

• The sweet spot when it comes to swapping between wheel formats and maintaining a very similar handling feel would be 700 x 33mm-ish tires for road riding, and 650b x 48mm for offroad. Similarly, 35mm tires for road, 2.1″ tires for off-road. The external diameters of these pairs are extremely close, maintaining a very similar ride height between them. This helps stay comfortable and feeling like you ‘know your bike’ when you swap wheels.

• For tires, I suggest the ‘roadiest’ you go be something like the Compass Stampede Pass in 32mm, Extralight, paired with something like the Panaracer Gravel King SK in 650b x 48. My preference for road (I’m not talking road racing here, I do that on an actual road bike) is the Compass Bon Jon in 35mm, Extralight and I will pair that with the Schwalbe Thunder Bert in 650b x 2.1″ for trail, and Compass Switchback Hill in 48, Terrene Elwood in 48, for less gnarly stuff.

• If you normally use a zero-offset seatpost, consider whether setback is an option for you for a slightly more rear-biased position on the bike. This helps with climbing traction and floating the front over rough stuff. If you vie toward a shorter than usual crank this is easier to do, with the caveat that a shorter crank also means your seat moves vertically proportionally, increasing distance from foot to ground when trying to plant it in the event of a slide or tip-over. A little detail….

Same bike as above, with 700c x 33mm tires instead of 650b x 48mm, for cyclocross. Note the proportionally short stem on this bike. It’s geometry is custom, based on the principles discussed above.

I imagine this is more than you were expecting, but I feel this is pretty much everything you need to know to arm yourself with what you need to make a decision that leads to great times on the bike. If you’re in Canada or have access to Brodie, check out their Elan Vital, which takes what I’ve been talking about a little further into the bikepacking/camping end of the spectrum really nicely. It’d be even more stable than the geometries I’ve been talking about, and would give you lots of options for fitting gear. Also, not a bank-breaker!

UPDATE: See MATTER: Columbus Futura Cross, Adjustable Rake, and Steering Stability for more detail and advice on achieving steering stability across various wheel diameters and tire volumes.