[SEVEN CYCLES A-Frame Gravel+Road 743] One-Point Geometry Comparison / BB Drop

Did you get a chance to read yesterday's blog post on the SEVEN CYCLES A-Frame Gravel+Road 743?

In it, I talked about how I came to ride a SEVEN. Going forward, I'll of course be covering the details of the bike, but alongside that I'd also like to mix in some slightly more technical talk about how "geometry" affects the way a bike rides.

Do you remember this line from yesterday's post?

"On that first on-road ride, comparing it with my road bike, I came away with a slightly negative impression: 'The tires are wider and the geometry is different, so it's only natural that it feels a little mellow.' But that was the only time I felt that way."

Today, I'd like to unpack what that ride feel actually was,
looking at it from the angle of BB drop.

So what is BB drop?

The effect of tire width is easy enough to picture, but the influence of geometry can be a bit harder to grasp. This time I'll explain it by comparing the numbers of the bikes I own.

First of all, BB drop is
"the vertical distance by which the BB (the center of the crank axle) sits below the horizontal line connecting the front and rear axles."
The larger this number, the lower the BB sits relative to the axles.

Geometry comparison of my bikes

The CIELO is 68mm, while the SEVEN is 78mm.
A difference of 10mm.
You might think it's just 1cm, but in the bike world a 10mm difference is big enough to fundamentally change a bike's character.

So what actually changes when this number is different?
I've put it into a table to make it easy to follow.

Seeing them side by side makes it easy to understand.

Of course, BB drop isn't the only thing that affects ride feel, but it's a fact that changing just that one number is enough to make it a different bike.

What that "mellow feel" really is, and its close link to tire diameter

The "mellow feel" I noticed the moment I started riding the SEVEN. The reason for it turned out to be this deep 78mm BB drop.

In fact, on modern bikes, the BB drop setting is closely tied to the "tire size" the bike is designed around.
Tires on modern bikes keep getting wider. As tires get wider, the overall diameter of the tire grows, which raises the axles themselves higher off the ground.

• Narrow tires (25C, etc.): The axles sit low, so if the BB drop is too deep, the BB ends up too close to the ground.
• Wide tires (40C, etc.): The axles sit higher, so unless the BB drop is set deep, the rider's center of gravity ends up too high, giving an unstable, perched-up feeling.

In other words, the SEVEN's 78mm design is the builder's calculation for keeping a stable, road-bike-like center of gravity even when running wide gravel tires.

The odd feeling I had on the road came from my body being used to the "sharpness" of the CIELO (68mm), a road racer. But once I took it out onto rough surfaces and long gravel rides, I realized that the "reassurance of a low center of gravity" created by this deep BB drop is a fantastic asset, keeping the bike from wobbling over bumps and reducing fatigue.

Incidentally, recent road bikes are also trending toward deeper BB drops (roughly 70–80mm) in pursuit of stability and a lower center of gravity. Some bikes have a 74mm BB drop designed around 700c x 28mm tires, and some endurance models have an 80mm BB drop designed around 700c x 32mm.

One more thing: the Rambler's 68mm BB drop can be seen as a choice made for tackling rough surfaces and obstacle-strewn terrain.

Geometry is a message from the builder

Hidden in every single number is the builder's intent: "where, and how, I want you to ride this bike." Once you start reading your own sensations through geometry, choosing and setting up bikes gets a lot more fun.

Next time, I'd like to dig into an important factor directly tied to the "sharpness" and "straight-line stability" of handling: "head angle and trail."
Stay tuned.