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.
BB changes during suspension setup. The rider sets sag, which can alter the BB height and drop. BB height changes as the suspension cycles through compression and rebound when riding, but generally rides at the height set during sag setup. I think the sag setting has a larger effect (height, drop) than tires or a flip chip ever could.
You make a solid point that sag has a significant effect on those two measurements. When comparing bikes, we have to use fixed points and sag is different for everyone, which is why I use the pre-sag numbers. It would be sweet if all companies also shared geometry tables with 20% and 30% sag, though it’s likely some riders don’t set balanced sag front and rear.
it is the relation of bb height vs ground and contact patches of the wheels that makes a difference, not the relation vs center of rotation of the wheels.
any value of the bb drop number is a well maintained myth, easy to comprehend for anyone with experience from small-wheeled bicycles like bmx, brompton or moulton.
A lower BB does NOT mean a longer seat tube. This make no sense at all. Especially if you’re talking about tweaking BB height with tires and forks and such. The seat tube is a fixed length on a given frame, no amount of tweaking is going to stretch or shrink that tube. Yes, if you shrunk the fork length by a large amount, the seat tube would steepen by a lot and the effective top tub will shrink a bit, perhaps necessitating moving the saddle back on the rails, which then necessitate lowering the saddle slightly, but it’s still not actually changing the seat tube length.
Good point, and thanks for catching that, Justin.
My explanation could be more clear in that section.
What I had hoped to convey is that if the frame engineers drop the BB while maintaining a consistent height for the top/opening of the seat tube, that tube would become longer and which could cause fitment issues with the dropper.
Fair enough. Though I’m not sure why there would be a need to maintain the exact location of the top of the seat tube.
Hi Gerow. I would like to offer a different point of view. What if many prefer low BB bikes but this is actually due to the fact that the handlebars are too low?
Because the difference of height between BB and handlebars sure is important for handling, and in my opinion the majority of the bikes has too short headtubes (at least for large sizes) and bikes are normally sold with with not so many spacers under the stem.
What about riser bars? A longer steerer tube in a shorter head tube will induce more flex. Changing the bar height adds “stack” without compromising steerer tube flex.
well, yes, and I’ve got a 35mm riser bar with 35mm spacers and a DMR Defy stem…but my comment isn’t really about how to have a higher handlebar.
It’s about the fact that maybe bikes have too low handlebars and people likes low BBs because it adds to the difference in height between handlebars and BBs.
Okay, I understand. The “low” trend won’t go much further!
Have a look into trials bikes in particular, they range from +25 to +120mm BB for their typical usage.
Frankly, my Turner Sixpack was a custom one off that has a +25 intended to become zero with rider in place. This was done for the Jeep requirement since there is nothing worse than paying hard earned money for a suspension to have it burry a pedal into the planet if it is taken off piste.
For the next custom hardtail, I already have the CAD file completed, including the “Shall” page. On that is the clause on BB.
BB shall not be lower than -25, nor higher than -/+0 based on A/C of 550mm.
I require a trialsy dirt ripper since trials is part of riding at the wholesale level.
I’d be interested to see some real drop measurements with the rider on the bike, into the sag. My rigid SS with an EBB is between -65 and -75 depending on where the eccentric is. I run mine in the lower position, it holds the line better in corners and I feel more planted over the rough.
“This measurement is generally fixed, and not affected by different tire or wheel sizes.”
“Tire size is an easy way to play with BB height”
Which is it? (Hint: it’s the latter, the former is wrong.)
Wrong, both are true. BB drop is measured relative to the dropouts and wheel size won’t change that, though fork length could do to . BB height is measured from the ground and that will go up or down with a change in tire size. This is why larger wheeled bikes frequently have more BB drop, so their BB height is similar to smaller wheeled bikes.