In addition to repair and suspension queries, we receive a good number of head-scratchers concerning mountain bike frame geometry. Folks want to know how important each measurement is, how they affect ride character, and how they interact with other elements of a bike’s geometry and suspension layout. We will go through some of the most important geometry measurements with a broad brush to demystify things a bit for newer riders — beginning with the bottom bracket. It’s nearly impossible to include every aspect of how one frame measurement will affect the way a bike rides, so we will do our best to hit the key points that affect a majority of bikes.
Bottom bracket height is a vertical measurement from the ground to the center of a bike’s BB when the suspension is fully extended. Another measurement, BB drop, is a vertical measurement from a horizontal line through the center of the bike’s hubs to a parallel line at the center of the BB. These two measurements are valuable in different ways when looking at a bike and determining how it might ride.
The BB drop is often what riders look at to get an idea of how “in” versus “on” a bike they might feel. Additional BB drop often results in a more planted and confident ride character where the rider has the sensation that they are sitting down in the frame rather than riding on top of it. A BB that’s slung low between the axles often feels better than a higher one when pushing through turns and over messy earth. This measurement is generally fixed, and not affected by different tire or wheel sizes. It is, however, one of the geometry shifts that a flip-chip will typically alter. A lot of frames with flip-chips can have their BB raised or lowered by 5-6mm in combination with the other angles and measurements that the chip affects. Depending on your trails and preferences this can change a bike enough that one setting works for a particular trail center while the other is better suited to different locations.
The height of the BB from the forest floor is a more varied variable, moving up and down with flip-chips, tire width shifts, fork axle-to-crown length changes, wheel mixing, and any other factor that moves one or both of your axles in relation to the dirt. Preferences around BB height are often personal, with some riders preferring to scrape their pedals against stones in the name of a planted ride feel, while others enjoy a higher transmission that’s safely out of harm’s way.
Small things can shift BB height, creating meaningful changes in the way a bike handles. For example, a 170mm x 29″ Fox 38 fork has an axle-to-crown measurement of 583.7mm whereas a RockShox ZEB in the same size measures a longer 586mm. Every other fork on the market measures differently and will give the bike a tad different flavor on the trail.
For any gravity bike, the placement of your feet and hands is particularly important since those are your only touch points while descending. When comparing the BB height and drop of two different frames it can be helpful to look at the stack height in relation to those numbers. Stack is a vertical measurement between one horizontal line through the BB and another that passes through the center of the upper head tube opening. While stack can be adjusted with spacers above and below the stem, it’s a good idea to take a peek at this number before buying a frame to make sure you will be able to achieve the desired handlebar height, compared to the BB drop, that works well for your needs.
Bottom bracket height and drop of some top-results EWS frames
|Bike||BB Height||BB Drop||Tire sizes R / F|
|Canyon Strive||32mm (in low)||2.4″ / 2.5″|
|Commencal Meta AM 29||21mm||2.4″ / 2.4″|
|Giant Reign||27mm||2.4″ / 2.5″|
|GT Force||347mm||30mm||2.4″ / 2.5″|
|Ibis Ripmo||341mm||30mm||2.5″ / 2.5″|
|Kona Process X||345mm||30mm||2.4″ / 2.5″|
|Lapierre Spicy||21mm||2.4″ / 2.5″|
|Norco Range||355mm||20mm||2.4″ / 2.5″|
|Nukeproof Mega||345mm (in low)||30mm (in low)||2.4″ / 2.4″|
|Orbea Rallon||336mm||35mm||2.4″ / 2.5″|
|Pivor Firebird||350mm (in low)||20mm (in low)||2.4″ / 2.5″|
|Rocky Mountain Altitude||34mm (slackest)||2.4″ / 2.5″|
|Santa Cruz Megatower||340mm (in low)||33mm (in low)||2.4″ / 2.5″|
|Scott Ransom||353.5mm||22mm (in low)||2.6″ / 2.6″|
|Specialized Enduro||347mm (in low)||28mm (in low)||2.3″ / 2.6″|
|Sunn Kern||25mm||2.3″ / 2.5″|
|Trek Slash||345mm (in low)||29mm (in low)||2.4″ / 2.6″|
|Yeti SB 150||347.8mm||2.4″ / 2.5″|
Shorter crank arms and bash guards create a little extra space and security for lower BBs, but you will want to watch out for your toes when pedaling across tall rocks. For riders with shorter legs, added BB drop also requires a shorter seat tube length in order to accommodate the desired dropper post travel. For example, the size large Raaw Madonna that I’m currently riding has a wallowing 35mm BB drop that makes the bike feel fantastic at jankier speeds. With 165mm cranks mounted up I’m barely able to squeak a 170mm dropper post into the frame’s 445mm long seat tube. There are about 4mm between the seat post collar and the base of the dropper collar, so a lower BB, resulting longer seat tube, or longer crank arms would force me to reduce my dropper travel or ride a smaller size frame; neither of which sounds appealing. On the flip, taller riders will receive a touch more seat post insertion thanks to additional BB drop and a smidge more seat tube, giving their post more purchase inside the frame.
Tire size is an easy way to play with BB height and make slight changes to your bike’s head-tube angle that don’t require any major surgery. If your bike came with a set of 2.4″ tires, and you mount a 2.35″ out back and a 2.6″ in the fork, the following pedal will undoubtedly feel different. Note that your bike’s geometry table was measured with the stock tires in mind, leaving you with the power to try different combinations that might improve the ride.
These are some of the many factors that BB height affects, and that affect BB height. Do you have others to share that we can all benefit from? Please write them in the comments below.