BB Drop and Height are important factors to consider when looking at the geometry of a bike. We'll be defining, comparing, and explaining how BB Drop and Height effect the ride qualities of a bike.
BB Drop is the vertical distance between the centre of the bottom bracket, and the axles of the front and rear wheels. This is a fixed, static measurement. Once a bike is built up and has no rider on it, this doesn't change. On a full suspension bike, the BB Drop can also be measured at the recommended sag point but is only regularly published as a static measurement.
BB Height is the vertical distance from the ground to the centre of the bottom bracket. This can vary on a bike for numerous reasons. Tyres and fork lengths are the biggest culprits for adjusting the BB Height of a bike.
High Bottom Bracket
- Poppy and playful
- Increased rock clearance
- Easier to lift the front wheel
- Unstable at speed
- More susceptible to external forces
- Harder to tip sideways
- Feels like you're riding on top of the bike
Low Bottom Bracket
- Stable at speed
- Less susceptible to being knocked off line
- Rails corners better
- Feels like you're sitting 'in' the bike
- Less rock clearance
- Harder to move the bike around, manuals, bunny hops, etc
Other Factors to Consider...
Typically, bigger wheels will have a bigger BB Drop. This is because the axles are further from the ground, so if you don't have much BB Drop, you'll have a much higher BB Height than on a smaller wheel size.
Full Suspension vs Hardtail
A full suspension bike is more likely to have a smaller amount of BB Drop, because when the bike is at sag point, the BB will sink further down. For example, if a bike with 100mm travel has a BB Drop of 10mm, and is sagged at 20%, the BB Drop will now be at 30mm.
Whereas a hardtail will be more true to the static BB height, as it is only the fork that is sagging.
Relation to Other Geometry Factors
While a low BB won't be as poppy as a higher one, a short chainstay can counteract that effect, at the cost of more stability.
Low BBs make your bars feel like they're higher, this can be counteracted by dropping your stem down a few spacers, having lower rise bars, or by the bike being made with a shorter headtube.
There are a whole other host of ways that BB Drop can affect a ride in relation to other geometries, but we're focussing on the isolation of the effect of BB Drop in this post.
That's cool and all, but what's better for me?
Like everything in the mountain bike industry, it's always going to be a trade-off. Low is stable, high is playful. Compare the bike you're after with other bikes in the category, and evaluate that against what you want from the bike.
As always, get in touch if you have any more questions, and we'll do our best to help out and answer any more questions regarding BB Drop and Height.