A frame for gravel, but what does that mean - really?
Updated: Jan 4, 2021
If you are at all into bicycling and the culture surrounding it, by now you have heard of "gravel biking", and cycle manufacturers are eager to capitalize on this trend - offering a galaxy of options. But what, exactly, makes a bicycle a "gravel bike"?
Personally I started riding gravel a few years ago (2014) on a converted mountain bike, a "monster cross" or locally known as a "glenny bike". Essentially the rider selects a hardtail mountain bike frame one size too small, adds drop bars and a longer stem, a few bags here and there and finally installs a rigid fork - and viola! A gravel bike is (was) born. And what makes that type of setup work - leads into the discussion about what attributes a gravel bike needs to have.
Clearance for mud
The gravel frameset I am working on right now has been designed around a very common gravel tire size, 700 x 42mm. However, clearance for up to a 50mm tire has been given. Not because the rider will ever use a tire that wide - but rather because when sticky, gummy mud starts to accumulate - tire clearance can mean the difference between riding and walking. In the photo below I am using my custom #farr_frameworks tire clearance jig to ensure plenty of room exists between the maximum tire size and the seat stays.
Clearance is key. Some of the early gravel bikes from major manufacturers were simply road bikes with slightly knobby tires and a single front chainring. I was on a ride in 2018 where a friend wore a hole through the carbon chain stay of her carbon gravel bike - after a few hours of riding through mud. Obviously a steel frame wouldn't have had that problem and only paint would have been compromised. That friend now rides a steel bike for gravel.
The latest KRUCH build started life in #BikeCAD as a reproduction of the customer's cyclocross bike. His cross bike fit him well, so it was an obvious place to start the design. That model was input, digitized then changed in the following ways to "gravelize" the geometry:
Slacken the head tube: A gravel bike needs stability at speed, as gravel roads can be steep and the surface is loose. Road bike handling is simply not required.
Lower the bottom bracket: A gravel bike can be loaded up and heavy at times and it needs to be stable. Low = stable. The pedal clearance/criterion racing cornering ability of a road bike is not required. A lower rider position also mean a smaller wind profile, and after grinding into headwind for a few hours this can be a beneficial feature.
Lengthen the wheelbase: A longer wheelbase is both inherently more stable, but also provides a greater degree of adsorption of the vibrations from washboard and pit run type gravel. Longer wheelbase literally equals a smoother ride, and gravel riding doesn't require carving tight switchbacks or navigating a fast paved road course.
Steepen the seat tube angle: This is for power and for climbing. On a long gravel ride, anything that can be done to aid efficiency is desirable. A steeper seat tube provides a more rider-forward experience and a more efficient climbing position.
Lengthen the chain stays: Longer stays mean more rear axle "compliance" or vibration dampening, something will be appreciated three hours into an epic gravel ride. It also allows the wheelbase increase noted above, without the penalty of slackening the head tube to angles more suited to mountain bikes (68 degrees and slacker).
AFTER (subtle - but maybe not so subtle if you compare certain angles and measurements to the above):
Although carbon fibre bicycles are very light, and stiff and offer certain advantages - they simply don't last long in abusive applications. Aluminum, titanium - and my favourite - steel, are far better choices in my opinion than carbon fibre. That being said, carbon wheels, bars, seat posts and other supporting pieces work well to keep overall weight down.
At minimum, two water bottles should be allowed for within the frame triangle. Ideally, three can be fitted (with a third placed on the bottom of the down tube, or elsewhere). A small frame bag is handy, for a windbreaker, snacks and a water purifier/filter on days when the heat is intense and distance long. I've enclosed a photo of my personal gravel bike below - the frame bag was built for me by #porcelainrocket here in Calgary:
The frame itself was built for me by #waltworks out of Salt Lake City, Utah. The bike was pretty forward thinking in my opinion, as it was built at a time when gravel bikes were simply road bikes with slightly more tire clearance. I specced the frame/fork above as follows:
Room for 700 x 47c tires
Room for 180mm rotors
Large frame triangle
Slack head angle (68 degrees)
The truth is that any bike you choose to ride on gravel is a gravel bike. Heck, even an ultra-light XC suspension bike with mild tires will suffice. The point is ultimately to get out there and put some miles on!
I hope the above helps when it comes time for you to select a bike for gravel.