Gravel bikes are safety bicycles; modern, multi-use, hybrid. Sheldon Brown did not have an entry for gravel bike in the 2008 version of his Bicycle Glossary. (His blog is maintained, and the site is updated, with many modern contributions from John Allen and other friends of cycling). René Herse Cycles (formerly Compass Cycles) and Cycling Quarterly, a student of European randonée rides began to talk about all-road bikes about 2007. Others began to talk about adventure bikes. Races were set up and promoted. Enthusiasts were able to custom build, or adapt an existing bike. Some custom builds gained popularity among riders in gravel grinders – races or endurance events on gravel roads; the term could have described some cross-country mountain bike races. Gravel bikes are associated with leisure, rural roads and terrain, gravel grinders, endurance rides, randonées, and bikepacking (a blend of cycling and minimalist camping). The category includes:
- Road bikes with wider tires at lower pressure;
- Gravel grind;
- Adventure; bikepacking;
- Mountain bikes with drop bars.
Many hybrids are upright handlebar bikes. Mountain bikes tend to have straight (flat) bars, but there are various bends and other configurations. The simple household/commuting single speed bikes with coaster brakes that were common in middle decades of the 20th century had upright bars. Upright bars:
- encourage an upright riding position,
- support balance and control at lower speeds,
- support a front basket or bag,
- provide a surface area where users can bolt on bells, electronics and other accessories, and
- provide several locations to mount a rear view mirror.
Gravel bikes have drop handlebars, resembling road bikes, cyclo-cross bikes and some touring bikes. The bars on gravel bikes are wider, and slightly different than other drop bars. (Russ Roca of Path Less Pedalled has published a this video on shapes and this one on fit.) Drop bars allow the rider to bend and ride with power. Drop bars with modern long hoods (brake lever assemblies) provide a comfortable fairly upright riding position with good steering control and access to brakes and shifters. As the “tops” of the bars are taped, they provide another upright riding position. Drop bars provide less space for carriers and accessories, and less locations to mount a mirror.
Factory production gravel bikes began to be built around 2010. Salsa, a QBP subsidiary (once a California indie shop) released its Warbird in 2012. Production gravel bikes imitate gravel grind race bikes with features of touring bikes.
Production gravel bikes incorporate many technical innovations of the last few decades: disc brakes, threadless headsets, internal frame routing for cables, indexed shifting integrated in the brake levers. Gravel bikes with disc brakes will usually have thru-axles (as opposed to quick release skewers). Some have suspension forks in the front; some manufacturers have some types of rear suspension. Some users are using upright alternative handlebars. Russ Roca has a PLP video about the Velo Orange Granola Bar; and a (long, discursive) review on the new Alternative Cycling Network of the new model flat bar Specialized Diverge. Alt-bars revive old styles like the North Road Bend. Other normal gravel bike design elements:
- frames that accept wider tires;
- gear combinations for moderately fast riding and moderate climbing:
- one or two front chainrings. A single ring or the outer ring on a two ring set will be smaller than the big ring on a road bike – 46 teeth instead of 50 or 52. The inner ring in a two ring set could be 30 teeth;
- 10 or 11 cog rear cassettes. a range from 11-32 would be normal but riders and customize for small increments or larger gears for climbing;
- fittings for frame mounted bike packing bags, and for racks to carry panniers.
A gravel bike might be used a bikepacking or endurance bike, or a touring bike. Production gravel bikes have fittings for bikepacking frame bags, and for fenders and racks. A rear rack has to fit the outside the chainstays,and attach to the chainstays near the rear axle. The stays are widely separated for wide tires and disc brakes. The axle may have to removed to remove the wheel for maintenance and repair. The rack legs will interfere with the rotation of the thru-axle handle. The handle may be removed with a 4 mm Allen wrench, but when that is done, a larger (e.g. 12 mm) fixed wrench (combination or open end) will be needed to release the axle. There are after-market axles (e.g. the Robert Axle Project) that can be removed using a 6 mm Allen key – a tool that a rider may have in a repair kit.
Many web pages and services discuss design, maintenance and tech in the cycling world. For instance Bike Insights discusses “geometry” (frame dimensions/sizing/ fit) – one of the ways to deduce if a bike might work for a given purpose. (Russ Roca of Path Less Pedalled inteviews the site founders in a PLP video).
Bicycle tire sizing: Tires are described by the diameter of the tire, mounted and inflated. A tire is always marked with a tire size in one of several systems , and a rim size described in the ISO or ETRTO system.
- Tires – A 700c tire is 700 mm (70 cm) in outer circumference, in principle. Not all 700c tires are 700 mm in circumference. 700c tires may be purchased from 700c x 18 up to 700c x 47 or more. The second number indicates the width of the tire. Road tires typically are 700c x 23 to 700c x 28 mm (narrower tires were popular and have a following). All 700c tires use 622 mm rims;
- Rims – The ISO system reports inner rim width x bead seat diameter, in mm. A 23-622 rim will support 700c tires from 700c x 32 to 700c x 46. Rim and tire manufacturers, bike shops and users may try to extend the range. A production gravel bike will have an n-622 rim, typically 23-622, and tires from 700c x 35 up to 700c x 40 or more. A bike with 650b tires will have an n-584 rim.
Tire size also depends on frame clearance. Some gravel bikes will take up to 700c x 40; others e.g. Salsa Warbird take up to 700c x 45. Some gravel enthusiasts like to use a wide 650b tire on an n-584 rim. Salsa has an FAQ (in 2020, for the Warbird) indicating that it can run fenders on 700c x 38 or 650b x 47 tires. Cannondale’s Topstone alloy models shipped with 700c x 40 (WTB Nano) tires in 2019, but changed to 700c x 37 (WTB Riddler) in 2020.
Many road and gravel riders don’t use fenders. Fenders are useful in keeping water from spraying into lubed components and onto the rider and anyone near the rider. Fenders are useful for road riding and touring, but not for muddy conditions. With tires wider than 700c x 38, fitting fenders is a problem. Most fenders are a special order. The largest fenders available may be 62 mm, which should fit 700c x 44, if that fender will in the frame. Mountain bike mudguards are suitable replacements for fenders for some uses.
Most new gravel bikes have bead clincher tires and tubes, but are tubeless ready. For instance, the Cannondale Topstone alloy models shipped with WTB ST i23 TCS tubeless ready rims. The i23 is a metal alloy 23-n rim. WTB has versions of i23 including a “26 inch” mountain bike rim, and 23-622 for 700c tires. Wheel manufacturers have refined the bead hook at the outer edge of the rim into a channel – e.g. WTB describes its system as a beadlock – to make the bead fit tightly. Removing a tire from such rims can be a challenge. Bike mechanics know how to break the bead out of from this kind of rim without using tools that might mar the bead or the rim. (Bicycle tires should not require a hydraulic press bead breaker).
The conventional advice is to inflate tires to the pressure as stamped on the tire. The marked pressure is a maximum and a safety warning – it is half the pressure at which a tire will fail predictably, such as by the bead of the tire not holding to the rim of the wheel. It is not a recommended pressure to reduce the risk of pinch flats or other damage to tires, inner tubes or wheels. It is not a recommended optimal pressure. Optimal pressure depends on road, rider, and load. Sheldon Brown’s approach to tire pressure was nuanced. (The link to a Bicycling Quarterly on the Sheldon Brown site page has gone stale – the useful equivalent is “Tire Pressure Take Home” (2016)). Russ Roca of Path Less Pedalled published an interview with Jan Heine of René Herse and Bicycling Quarterly about wide tires and low (i.e. less than max.) pressure, and an interview of Josh Poertner “Your Tires are Lying to You“.
An illustration of the effects of tire pressure. I put a Cateye Mity 8, an old fashioned cycling computer on my gravel bike. It needs to be programmed with the circumference of the wheel to calculate how far the bike moves forward each time the wheel goes through a full revolution. The tire will flex under load; the distance travelled is a little less than the circumference of the inflated tire measured unloaded. The difference between running 700c x 38 tires at 45, 60 or the maximum 75 psi affects how this kind computer records of distance. Where the distance travelled on the ground is about 50 km, the effect is several hundred meters. The circumference of a tire on a wheel is affected by the actual pressure by up to 1-2 centimeters (on 170-225 cm, depending on the tire). Thisiinaccuracy is only about .5-1%, which should not affect navigation or trip planning. Pressure affects performance on different surfaces.