Strong Frames Custom Bicycles

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What is a Gravel Bike?

The “gravel bike” is all the rage these days. It reminds me of the late 1980s and early 1990s during the MTB explosion when NORBA races were everywhere and hundreds, even thousands of participants turned out to race.

So what are gravel bikes? We hear all sorts of terms thrown around: gravel bike, gravel grinder, all-road, adventure bike, etc. Are they all the same thing with different names, or are they different? And how do they differ from a road bike or a cyclocross (CX) bike, or even a MTB for that matter?

_DSC4853Pascal Beauvais info@pascalbeauvais.com 406-599-1472_.jpg
Photo by: Pascal Beauvais info@pascalbeauvais.com

Why They’re So Popular

If I had to guess why they are so popular, I would boil it down to three things: flexibility, safety, and adventure.

Flexibility – All-road and gravel road bikes offer little to no compromise. You can run smaller road tires and they are just about as capable as a true road bike. But they don’t limit you to the smaller tires and allow you to install larger tires to run gravel or use them with fenders for spring and fall training on the road. They are also great for commuting.

In the same vein, the adventure bike is to the MTB what the gravel bike is to the road bike. The adventure bike opens up riding that you just wouldn’t want to do on a heavy, fully suspended MTB. 

Safety – With more and more cars on the road and drivers being so distracted, many riders don’t want to risk being hit. The all-road and gravel bikes are perfect for those riders because they get you off the busy streets, out where you can relax and enjoy yourself—places where you can ride side-by-side and chat without worrying if someone is going to buzz you or confront you.

Adventure – You are no longer tied to the pavement. Take that turn and go down that road you always wondered about. You can add dirt, gravel, or trail to road loops. Or if you want a big adventure, you can do the Tour Divide. You can now find the bike that will serve the adventure you desire, and you can optimize it for the ride you envision.

Basic Description

Most simply, a gravel bike is a road-style bike that can be ridden comfortably on gravel roads. But what about dirt roads, jeep roads, logging roads, trails, or for that matter, paved roads? This is where things get interesting.

I like to think of all bikes fitting along a continuum. On the left side, let’s say we have an edgy road race bike. On the right side, we have a cross-country style MTB. Of course, there are lots of other bikes, but for this conversation, we’ll limit ourselves to this range.

Below are the key areas that are taken into consideration when designing any bike. Let’s take a look at each of these areas of bike design and discuss how we design for gravel bike use as well as all others along the spectrum.

Sub-Categories

The most common general term we hear referring to these non-road bikes is gravel bike. But because there is such a range within the category, you’ll hear other terms like all-road or adventure bike. The basic difference goes back to our continuum. An all-road bike is typically closer to the left end and the adventure closer to the right. In practical terms, that means the all-road will be designed for faster riding and the adventure bike will be designed to carry more weight for camping. The gravel bike fits somewhere in between.

Design Considerations

Fit

Gravel bikes fit much like road bikes. More often than not, they have drop-bars (although we are starting to see more and more with flat bars, blurring the line between MTB and gravel road). 

The most common difference with a gravel bike fit is the saddle to bar drop. That’s the difference in height of the saddle from the ground to the top of the bars to the ground. Most riders prefer their handlebar to be higher on a gravel bike than a road bike. This takes a little weight off the hands which helps with comfort on rough surfaces. It also gives the rider the ability to unweight the front wheel faster to avoid potholes and obstacles.

Tire Size/Forks

I think changing opinions about tire size are largely responsible for this revolution in bike design. Until recently, it was thought that smaller tires with higher air pressure were the fastest. We’ve recently been learning that that’s not actually the case. Larger tires, if inflated to the proper pressure for the rider, can be faster, more comfortable, and grip better. The key is figuring out just how big you want to go—and that will depend on the terrain you ride.

Rather than restating others’ work, here are a couple of interesting links about tire size:

Path Less Pedaled – Your Tires Are Lying to You

The Tire Pressure Revolution 

Tire size and the fork your bike is designed around will have more to do with where it falls on the continuum than anything else. The fork is going to dictate the max tire size the bike will fit as well as the steering geometry to a certain degree. Forks generally fall into five categories:

  • Road Fork – Tires up to 30 mm
  • All-Road Fork – Tires up to  37-40 mm
  • Gravel Fork – Tires up to 55 mm and even a bit larger
  • MTB Rigid Fork – Tires up to 3 inches
  • MTB Suspension – Tires up to 3 inches

For any of these forks, you’ll have a fork rake, also known as “offset,” which when combined with your headtube angle creates a figure called “trail.” Trail is a lever arm. The longer the lever, the slower the bike will steer. On the left side of the continuum, trail can be as low as 55 mm, sometimes even lower. On the right side, it can be as high as 100 mm. For reference, the sweet spot for road bikes is about 59 mm and most all-road bikes try to stay in this ballpark. 

Photo by: Loretta Strong/Strong Frames Inc.

Geometry

Geometry is where we start to see a divergence in design from a race-oriented frame to an adventure-type frame. 

Steering Geometry

As we move from left to right, steering geometry slows down. On the left, steering will be as close to a road bike as it can be, although the larger tires of a gravel bike will have some influence over steering, slowing it some, which is beneficial. As we move right, both the larger tire and slacker steering geometry will slow the steering more and more. As you move right, terrain will become more technical and the slowed steering is a desirable trait.  

Wheelbase

Again, as we move from left to right, the wheelbase of the bike will grow. The reason for lengthening the wheelbase is primarily for tire clearance. As tires grow larger, it requires a longer frame to house them. Another consideration, especially on bikes closer to the left, is avoiding any toe overlap, which is when your toe hits the front tire while turning sharply, typically at very low speeds. Overlap is more of a problem on smaller bikes, but can be on most sizes when the tires are very large.

A longer wheelbase creates a larger steering radius and less maneuverability—or more stability, depending on how you want to look at it. The longer wheelbase also softens the platform. Think about a 2”x4” on two boxes. The farther apart the boxes are, the more the 2”x4” will flex when a force is applied.

Chainstay Length

As tires get larger, the larger radius of the wheel requires longer chainstays. Some designers go out of their way to keep the chainstays short in order to keep the handling similar to a road bike. As the tires get larger, keeping the chainstays short becomes more challenging. In the most extreme case, designers will use dropped or raised chainstays in order to provide clearance for the large tire and chainrings.

Bottom Bracket (BB) Drop/Height

The BB height is usually discussed as drop by bike designers. The BB drop is a measure from the center of BB down to the center of the axles. Designers prefer to use this measure instead of height because it isolates the frame design from the tire size. It also describes the leverage that is produced when a rider is standing on the pedals. The drop plus the crank length is a lever. That lever wants to stand the bike upright. The longer that lever is, the smaller the effort that’s required to stand the bike up. More drop also allows you to get your weight further over the tires contact patch when cornering fast, which improves grip.

BB drop is what separates a gravel bike from a CX bike. Most designers use a BB drop that is very similar to a road bike while CX bikes have higher bottom brackets for more pedal clearance. The value will vary depending on the designer’s philosophy, but you can typically expect BB drop to decrease (or BB height to increase) as the design goals of the bike focus more on trail riding in order to produce more pedal clearance on trails and in rocks.

Weight Distribution

Weight distribution is hard to discuss in absolute terms because it changes so much with small movements of the rider. I find it’s easiest to think of it in relative terms. So going back to our continuum, let’s use a road bike as a baseline at the left end of the continuum. As the tires get bigger, the wheelbase gets longer. Lengthening the wheelbase itself does NOT require any change in the weight distribution. But what we are starting to see is modern MTB geometry finding its way into the gravel and adventure bikes on the right side of the continuum. 

In recent years, MTB geometry has shifted to really short chainstays, a super long top tube, short stems, and wide bars. In doing so, the weight of the rider has moved rearward…and quite a bit. This geometry creates great, confidence-inspiring handling on steeper terrain. But on flatter terrain like roads, it can have less precise steering. So if you will be doing a lot of trail riding, you might like it. If you are going to spend more time on the road, I wouldn’t recommend it.

Materials

The material conversation for gravel bikes is the same as it is for any other. You can find more detailed info on frame material HERE. Below are the bullet points. Please keep in mind that the comments below are for high-end frames. The characteristics of all materials merge to heavy and durable at the low end.

  • Steel – Inexpensive/Prone to denting/Heavy
  • Aluminum – Inexpensive/Lighter than steel but fragile when made really light
  • Titanium – More expensive than aluminum or steel but more durable and lighter
  • Carbon Fiber – The most expensive and lightest/Ideal for racing

Trends

So with all the hype and evolution that’s been happening, what can we expect in the future? For a lot of roadies, the true road race bike will probably become a specialty bike (like a TT or CX bike) and an all-road or gravel bike will be your main steed for training. For the MTBrs, especially those who live in the mountains, the adventure bike will make a great tool for riding before the snow melts and also for long bikepacking trips where you have a lot of road sections. 

We will continue to see more separation of gravel bikes into race and adventure categories. Offerings on the right side of the continuum will continue becoming more and more extreme, with the drop-bar adventure bikes moving closer to MTBs and some even starting to sport flat bars. 
It’s a great time to be a cyclist. Wherever you fit on the continuum you should be able to find exactly what you need. Hopefully, we cleared things up a bit and you can use this info to narrow your choices. If you have any questions, feel free to email me, I’d be glad to answer them.