The Angle: Bottom Bracket Drop

The Angle is a new series of posts here at TCI that will delve into the geometry of bikes. From road to mountain to gravel, we will look at how the dimensions of a bike affect how it handles and how it fits. The payoff is that you’ll better understand just why your bike handles the way it does, what you might expect from a new bike, as well as why your bike might not be as comfortable as possible.

We’re going to smart small, with one dimension, the one that may be the single most important in determining the character of a bike: bottom bracket drop.

A working definition
So what is bottom bracket drop? Broadly, it is a way to express a bicycle’s center of gravity. That’s the answer without the math, the simple answer.

The more complicated answer, the technical answer is it’s the difference between the height of the bicycle’s axles above the ground and the height of the bottom bracket itself. This is the dimension that builders set to determine the bike’s center of gravity. An easy way to understand it is to think of it as how high the pedals are above the ground. It not only establishes how stable the bike is, but it also determines how far a rider can lean into a turn while pedaling without scraping a pedal on the ground.

More bottom bracket drop means a bicycle is more stable. Less BB drop means a bike is more maneuverable—but also less stable.

Think about it like this: When you crouch down and plant your feet, you can be agile and yet also stable. Now consider standing on stilts: One not particularly aggressive push can send you to the ground. The stability of a bicycle tracks in direct proportion to the height of a bicycle’s bottom bracket.

BB drop is, however, not something as easy to measure as a bicycle’s wheelbase, which is the distance between the bicycle’s axles. That’s easy to measure: just stretch a measuring tape from the center of one quick release skewer to the other and as my French friends say, “Et voila!”

To measure BB drop, the first step is to draw a line from one axle to the other—the bicycle’s wheelbase. Step two is to draw a line that runs at a right angle from the wheelbase to the center of the bottom bracket. That distance is measured in millimeters. That is BB drop.

How it relates
So now that I’ve defined what it is, let’s talk about what those numbers mean. With road bikes, the great majority of what’s on the market comes in at 68 or 70mm of drop. Builders settled on 70mm because it struck a balance between stability on fast descents and the ability to pedal through corners.

European frames like Colnagos, DeRosas and the like were usually built around 75 to 78mm of drop. This was often referred to as “stage race geometry” because these bikes tended to be calm at speed, and were easy to handle on descents. Two weeks into a grand tour, tired riders benefitted from bikes that were predisposed to rolling in a straight line. That’s still true, even if fewer bikes are made to handle like this these days.

Cyclocross bikes, traditionally, have had less drop than road frames, often on the order of 50 or 55mm because when cyclocross first began, remounts on traditional road bikes meant that the toe clips would often scrape the ground or catch on grass until the rider got the pedals flipped and their feet in the clips. A higher BB lifted the pedals so they didn’t hit the ground, and added the benefit of making it easier to pedal through corners. Since the advent of clipless pedals bottom brackets have come down some—60mm of drop is fairly common. However, a high bottom bracket makes a bike more nervous at speed which is why a cyclocross bike doesn’t make a good gravel bike if there are any significant descents on the course. Gravel bikes can make for very serviceable ‘cross bikes though they can’t be pedaled through fast, sweeping turns where the bike is likely to be leaned a fair amount. They do fine on all the slow, tight turns because the bike isn’t leaned much.

You’ll see that were talking about a fairly narrow range—from 50mm at the highest to 80mm at the lowest. You wouldn’t think that 30mm could make that much difference in how a bike handles, but it fundamentally shapes the bike’s behavior. Think about how one chocolate bar can change the personality of a five year old.

Size matters
Because smaller bikes are usually spec’d with shorter cranks and big bikes are spec’d with really long ones, you’ll tend to see drop change through a size range, with small bikes having the most.

Somatic specifics
So what’s it all mean? Ultimately the numbers are just a way to express something your body feels when you ride a bike. Most riders I know have only ridden road bikes with 68 or 70mm of drop. To ride a bike with 75 or 80mm of drop means looking at what the industry typically refers to as endurance road bikes. The Specialized Roubaix and Trek Domane handle fundamentally differently than the Tarmac or Emonda precisely because they both have 5 to 10mm more drop.

A bike with 75 to 80mm of drop just feels good. There is a naturalness to how the bike handles. I’ve never encountered a rider who didn’t prefer riding a bike with more drop. My custom steel bikes have drops of 78 and 80mm. Bikes with that much drop tend to feel calmer and more assured on fast descents; I’ve gone as far as to describe the experience as making the bike feel like it was moving slower, so that 50 felt more like 40, which is terrific for confidence, kinda like if the person you’re crushing on winks at you.

So how important is BB drop? To me, it’s central to purchasing a bike, no different than asking yourself if you want an SUV or a sedan.

Notes:
1. In the early 2000s, after roughly 20 years of building bikes with 70mm of drop for racers, Serotta Competition Cycles looked at the fact that they were (as Ben Serotta put it to me in an interview in 2003) “selling bikes to doctors and lawyers who weren’t pedaling through corners,” and decided to lower their BBs by 10mm, giving their bikes 80mm of drop and cornering so assured that a cat on carpet could admire.
2. Not all bikes have BB drop. BMX bikes have bottom bracket rise, that is, the bottom bracket sits higher than the axles. Between the short wheelbase and the high BB BMX bikes feature the quickest handling of any bike being raced. Nothing else can turn like a light switch.
3. The SpeedX Leopard, which made such a big splash on Kickstarter as the world’s first “smart bike,” had 60mm of BB drop. That was one of the reasons I criticized the bike as yet another example where bike industry neophytes misunderstood a central aspect of bike design. The only reason I could come up with for deliberately designing a road bike with so little drop was that they anticipated that most purchasers would be riding them with platform pedals which will scrape the ground rather easily if the rider pedals through a turn.

Image: Velo Orange Bicycles