Gravel Bikes Introduction
In Part I we reviewed the mechanical characteristics, relative advantages, and disadvantages of the four most common materials used in bike frame contruction: steel, titanium, aluminum, and carbon fiber composite. We learned that a great-riding bike frame can be built from any of these materials, if it is designed well. Which one is the "right" choice depends on what a rider's priorities are such as weight, cost, durability, availability, etc…. In Part 2 we are going to focus on the how frame design and geometry, rather than the materials, can affect bike handling characteristics and intended application of a particular bike/frame. Our focus will be on contemporary drop-bar bikes suitable for riding on various types of terrain. In Part III of this series, we are going to use our knowledge of frame design and materials to review how to decide which bike to actually buy.

Above: Time ADHX gravel bike set up with 1x drivetrain, dropper seatpost, flared bars, and 38mm tubeless tires…somewhere in the Lower Hudson River Valley. In a world of Treks and Specialized's, Time is a small company, but they are a standard-bearer for carbon tube production. A gravel bike allows for fast, efficient travel over a variety of terrain. Between various combinations of frame materials, geometry, and parts options the possibilities are really endless for the gravel category.

So what is the deal with gravel bikes, isn't it just a road bike with big tires? It turns out the short answer is yes…and no…. In broad terms, a gravel bike is a drop-bar bike designed to cover long distances quickly over a variety of terrain. This definition leaves a lot of room for interpretation by the rider, who will use these bikes for a variety of applications, and by the manufacturers who design and build them. Among the drop-bar crowd, these bikes fall somewhere between road bikes and adventure bikes in form and function. Just so that we are on the same page, an example of a pure road bike might be a Specialized Tarmac SL8 or the Trek Madone…very light, very fast, and meant to be ridden on pavement only. The design of these bikes focuses on stiffness and light weight, with some concern for good aerodynamics. Comfort and cost are not considered top priorities at the pointy end of road bike design, although they are certainly factors. On the other end of the spectrum are adventure bikes, meant to cover distances on a variety of off-road terrain while carrying lots of gear, in relative comfort for many hours of riding. Such bikes are not intended to cruise smooth roads at high speeds, nor are they intended to be light and fast uphill. Durability and practicality, with lots of accessory mounts, places for water bottles, and/or full coverage fenders are priorities. An example of a typical adventure bike is the Salsa Fargo or an Otso Fenrir. These bikes come in steel or titanium frame versions, for extreme durability, with frame geometry similar to a crosscountry mountain bike but with drop-style handlebars instead of flat bars. Some adventure frames have a suspension-corrected front end and so can be fitted with either a rigid mountain fork or a front shock fork. There is huge tire clearance and the bottom bracket and hubs often use mountain bike standards. Adventure drivetrains are often cable-actuated and 1x, for reliability and ability to repaired anywhere. These are absolutely fun bikes to ride. They can make great commuter bikes or foul-weather bikes.

Modern gravel bikes exist somewhere between a Tarmac and a Fenrir. Some designs lean more towards the road bike end and some lean more towards adventure bikes. Various subcategories of gravel bikes are referred by various names like all-road, gravel race, gravel, bike packing bikes, etc….which give the rider some idea of where these various bikes are positioned along the spectrum. There is a certain amount of versatility in the design of these bikes, with a lot of overlap in both design and how people ride  them. Of course many gravel bike owners use them for both road and off-road riding…or even exclusively on pavement.

How did gravel bikes evolve?
Back in the 90's, when I grew up cycling, steel was real and aluminum was expensive. One could choose between a road racing bike, a touring bike, or a cyclocross bike. Carbon and titanium where rare and exotic…something to stare at in the bike shop window. That was pretty much it. If you wanted really big tires there were mountain bikes, which had 26" wheels, flat bars, and were still fairly new. Around the late 1990's, hydraulic disc brakes became mainstream for mountain bikes. Some manufacturers brought mechanical disc brakes into the road market in the early 2000's, but in order to fully realize the potential of disc brakes for drop bar bikes several things needed to happen. A transition from QR skewers to through-axles for drop-bar bikes had to occur, due to the higher loads at the hub with disc brakes. There were (and are) some very good cable-actuated mechanical disc brakes available, however drivetrain manufacturers needed to start offering hydraulic brake levers for the road market to make the best use of the technology. Lastly, the federations that govern bike racing (UCI,for example) had to make disc brakes legal for road racing. After some amount of controversy, this occurred in 2018. Since then disc brakes have more or less taken over the market for all types of dropbar bikes, even time trial bikes. Some component manufacturers don't even make their top level drivetrains for rim brakes anymore. Placing disc brakes on road bikes made it possible for frames to accommodate larger tires without being limited by the pad clearance and reach of the rim brakes. But forget the tire….you can even use different size wheels on the same disc brake bike! Disc brakes also free the wheel & rim manufacturers from certain constraints in design that are needed to accommodate rim brakes. Not only that, but disc brakes are actually better brakes. Stopping distances are reduced and they work much better in the rain / snow than rim brakes. In essence, it is really disc brakes have allowed the market to develop into what is today, with all the subtle variations in drop bar bikes that we have become accustomed to.

Gravel Bike Geometry
Classic road bikes have traditionally been marketed for road racing. Indeed, the UCI requires the pros to ride bikes that are actually sold to regular riders. As the mantra goes, "race on Sunday, sell on Monday". While it is true that the very fastest bike on smooth pavement is a pure road bike, few of us actually road race. Since many gravel bikes are specifically not marketed to a road race crowd, manufacturers are free to acknowledge that most of us…or more likely none of us….want to be doubled-over in an aggressive riding position with a slammed stem and our chin on the handlebar. So across the various gravel bike categories, with a few exceptions, the riding positions have gotten a little more user friendly- that is with a little less reach and more stack height. In other words the major bike producers have been providing more relaxed geometry that actually will be comfortable for most riders. With the advent of 11, and now 12 speed, drivetrains, we have also seen the inclusion of lower and lower gearing available across the board. Higher numbers of cogs have been accompanied by the availability of compact (50/34) and sub-compact/gravel (48/32, 46/30) cranksets. So not only do we have lots of gears to choose from, we are actually using them. Indeed, 46 x 10 is the new 53 x 11.

The handling of these bike is more relaxed as well. Instead of 73.5 or 73 degrees, head tube angles of 72 or less are more common and this is often combined with a fork that has more rake, such as 45 or 50mm. The shallower head angle will increase the bike's trail for a given fork rake, which tends to make the bike feel more stable (although perhaps a shade less nimble). Trail, by the way, is not a reference to the South County Trail…a bike's trail is the  horizontal distance between the point where the front steering axis intersects the ground and the point where the front tire contacts the ground. It's called "trail" because the front tire contact patch trails behind the steering axis. Trail numbers on gravel bikes vary greatly, but are certainly longer than road racing bikes, so tend to be somewhere in range of 60-80mm. Manufacturers use different combinations of head angle and fork rake (or offset) to strike a balance between handling characteristics and FC measurement. Front-center, or FC, is the distance from the BB (bottom bracket) to the front axle. Lower FC was OK when road tires were 23mm, but since even pure road bikes are using bigger tires, we need longer FC to avoid excess toe-front wheel overlap especially in smaller frame sizes. Fork rake needs to be higher in these bikes with relatively-slack head angles because otherwise the trail will be very high and steering characteristics will not be what we expect for an all-road or gravel bike. Along with longer chainstays the longer FC results in a longer wheelbase (the distance between the front and rear axles). The longer wheelbase is desirable for increased stability, comfort, and balance on uneven terrain and also provides extra room for frame bags and such. These seemingly-small changes to fit, geometry, gearing, and handling all add up, so we can begin to see how gravel-oriented bikes are really a significant departure from conventional road bike design. A gravel bike and road bike may strongly resemble each other at first glance, but they are quite different.

Left Panel: This graph indicates typical trail numbers for bikes in various categories. Trail is not the only characteristic that determines handling, but it is an important one. It is especially useful when shopping for bikes, since you can predict how the steering on one bike might feel versus another. Right Panel: Trail is determined by the horizontal  distance between where the steering axis through the head tube intersects the ground and where the front tire contact patch hits the ground. The contact patch "trails" the steering axis, hence the name.

Tires, Wheel Size, and Trail
Every bike is – or ought to be – designed to be optimal for a specific wheel & tire combination. For a modern race-oriented road bike this might be for 25 or 28mm tires. For a  gravel bike the manufacturer may anticipate 40mm tires, and an adventure bike might be optimized for 650b x 60mm tires. Actual clearances might be more, but the bike will be optimum for a specific size overall wheel+tire diameter. Of course we might elect to make our bikes more versatile by mounting 700c x 35mm slick tires on an adventure bike or riding our gravel bike on roads most of the time with 30mm tires, instead of 45mm knobbies. Why not just make all drop-bar bikes with frame clearance for massive tires and tons of mounts? As mentioned above, there are some trade-offs between tire size and frame design, weight, and handling. As tires get wider, the total diameter of the wheel + tire system also gets larger. When there are extra mounts added to the frame and fork, extra frame material (especially with aluminum or carbon fiber) is added to allow for the extra frame stresses. To accommodate all this the frame ends up heavier and with longer wheelbase, wider forks, slackened head tube, and generally increased trail (see above). In this scenario some nimbleness and lightness of handling will be sacrificed, compared to a road bike. Can you ride such a bike on roads and paved bike paths? Absolutely! Would you purchase this bike specifically for spirited group rides, tight high-speed pacelines, criterium races, and curvy mountain road decents? Probably not. So how can you asses how a bike might handle before riding it? Wheelbase, head tube angle, and amount trail can often tell the tale - at least part of it. High trail can make the bike feel more stable, but often sacrificing that feeling of nimbleness one has on a fast road bike. A lot road bikes have trail around 58mm or so. Too much trail and it can feel harder to initiate a turn. Too little and the bike can feel less stable at high speed in a straight line. Personally, I like the way a road bike handles with slightly more trail, around 65-68mm, with a head tube angle around 71-71.5 degrees. Trail numbers in the 70-80mm range with a really slack head tube angle make a bike's steering feel really sluggish to me…like the bike needs extra effort to enter and exit turns at high speed on roads. There is sometimes a sensation of the front wheel "flopping" into and out of turns. However, such a bike might be optimal for technical trail riding. Complicating all this, different size wheel & tire combinations may change the amount trail on a particular frame. For factory-made bikes, the head tube angle and trail numbers are generally published in their sizing chart; looking at both of these is a good way to predict how the steering on the bike will behave. This is especially true if you know the trail and various other measurements of your current bike, so you can make direct comparisons. I  recommend also looking at the BB drop and overall wheelbase to help predict how the bike will handle in general. Longer wheelbases tend to improve stability, at the expense of tight handling. Less BB drop will increase the height of the rider's center of gravity, which will also affect handling. These aspects of frame geometry may help decide among bikes that otherwise look similar.

Top Row: A titanium No 22 Bicycles Drifter gravel bike, geometry and the final build as ridden. This drifter has accommodation for full fenders (titanium in this case, anodized to match the frame), rack mounts, clearance for large tires of about 700c x 50mm, extra bottle mounts, frame couplers for easier travel on airplanes, set up with a 2x drivetrain. In this instance the bike has a small frame bag and rear racks while touring along the Empire State Trail. Bottom Row: No 22 Bicycles Great Divide All Road bike; frame design (left) and the bike out in the wild (right). At first glance these bikes seem like they might be the same - same manufacturer, same material, even similar anodized color scheme. However there are quite a few differences. The Drifter has less BB drop so the bottom bracket clears more obstacles; the Great Divide has a lower BB (more drop) to make the frame more stable at high speed and more nimble since the center of gravity is a little lower. The Great Divide only accommodates 32 or 34mm tires, and so has lower FC and shorter overall wheelbase, again making the bike more nimble for high speed turns on the road and group riding. With shorter trail and steeper head tube angle the Great Divide steering feels quicker. There are also some differences in wall thickness, shaping, and butting of the frame tubing that make the Great Divide a little lighter, the frame also lacks rack and fender mounts. Lastly, the riding position of the gravel bike is little higher, and so the rider is slightly more upright.

Gravel Bike Suspension?
Most gravel bikes rely on their longer wheelbases, relaxed geometry, and the "suspension" provided by larger tires to add comfort in challenging terrain. Some "extreme" gravel bikes have an elastomer or air/oil front shock fork that you might see on a mountain bike. Many standard mountain bike suspensions are not optimized for the kind of terrain a  gravel bike is intended for. However, some suspension forks are available that have been scaled-down for gravel use. Also there are suspension stems and suspension seatposts available which are great at damping out small bumps, ruts, sticks, etc.… A suspension seatpost or stem can really add some comfort, but does not improve the tire  contact with the surface. A front suspension really helps keep the tire in contact with the terrain. Based on my personal experience, a suspension stem or seatpost suspends the rider, providing comfort, but does not help the handling of the bike as much.

Above: A custom carbon fiber Calfee Manta RS. The frame is optimized with 650b wheels with tires up to 42mm. 700c x 28mm can be run as well with similar handling. The carbon fiber construction is light, of course, very comfortable. In addition, a small suspension unit incorporated into the rear mono-seatstay gives about 2cm of movement. There is no pivot point or bushing at the bottom bracket, like with most mountain bikes that have rear suspension. The chain stays have a very flat profile and are simply allowed to flex with the suspension. This makes for a very smooth riding experience and really helps to keep the tires in contact with the surface.

Above, Top Row: Gravel bikes can you anywhere, anytime, in any conditions. This titanium Litespeed Ultimate G2 gravel bike has been set up for winter riding. A flared handlebar helps the rider get extra leverage if the bike is bogged down in snow or mud. An easy 1x drivetrain with some very low gearing (38T chainring, 9 x 52 eThirteen rear cassette) provides simplicity and (hopefully) enough "oomph" to get up and over icy ruts and built up snow on the sides of the road. Studded 30mm winter tubeless tires provide grip on ice. A front dynamo drives the headlight which is very reliable; there is no need to worry about a headlight battery freezing up on the coldest of night rides. Full fenders, of course, and a suspension seatpost smooths the way. Below 20 degrees, I take along a thermos with hot tea or coffee, and an extra puffy parka tucked away in the handlebar bag in case there is a need to stop for some reason.

Above, Bottom Row: This is a carbon fiber Lauf Siegla gravel race bike. It is intended for spirited gravel rides and racing, thus it does not have rack or fender mounts. The front fork features a rather unique suspension in which the front axle is installed into a floating "carrier" which is permanently attached to the front fork by 6 fiberglass leaf springs on either side. The leaf springs can flex up and down. On a rigid fork, one can experience the front tire getting some "air time" over bumps and such at high speeds. With a front suspension like this, the tire stays in much more consistent contact with the ground. Consequently, one can really attack the terrain. This bike is very fast and stable on gravel. Obstacles that warrant extra caution on a rigid fork-equipped bike are no problem with a good front suspension. This does not really offer much benefit on the road however, and I would not suggest such a bike for someone looking to spend a substantial amount of time riding on pavement.

Above: This custom Alliance stainless steel all-road bike looks rather exotic, since stainless steel in rarely seen in a sea of carbon and aluminum bikes. Is at a gravel bike? Not really. This bike has a relatively short wheelbase and lower bottom bracket than is typically seen in gravel bikes. Tires up to 34mm can be fit…although with little extra clearance. I typically run this bike with 28mm tires and some narrow fenders. The stack and reach are intended to provide a slightly more aggressive, lower riding position. I consider this to be a prototypical "all-road" bike, I think that with room for slightly larger tires than would typically be run on a pure road bike, the rust and corrosion-resistant stainless steel, fender mounts, and lack of anything "aero" about the frame…it lies between pure road and gravel on the drop-bar pantheon.

Conclusion
I think we have demonstrated here the incredible variety and types of frames and bikes that can be had in the spectrum along the gravel continuum. With subtle changes in frame geometry and component choices, the possibilities are indeed endless. In the next and final chapter in the series we'll attempt to put this all together and identify some ways to figure this all out when shopping for a new bike. Plus we'll review when it might be a good idea to look to smaller manufacturers and some ways to customize a bike, whether its just exchanging a few stock components or a completely custom-built frame.

Stay tuned!