In addition to maintenance and suspension issues, we also received a ton of head-scratching questions about mountain bike frame geometry.One wonders how important each measurement is, how they affect ride characteristics, and how they interact with other elements of bike geometry and suspension layout.We’ll take a rough look at some of the most important geometric measurements to demystify new riders—starting with the bottom bracket.It’s nearly impossible to cover every aspect of how one frame measurement affects how a bike rides, so we’ll do our best to get to the key points that affect most bikes.
Bottom bracket height is the vertical measurement from the ground to the center of the 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 bicycle hub 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 rides.
BB descents are often what riders use to see how they might feel “in” and “use” the bike.The extra BB drop generally results in a more grounded and confident rider who feels like they are sitting on the frame rather than riding it.A BB that sags between the axles generally feels better than a taller BB when driving through turns and messy dirt.This measurement is usually fixed and not affected by different tire or wheel sizes.However, flip chips usually change one of the geometry changes.Many frames with flip chip can raise or lower their BB by 5-6mm, combined with other angles and measurements of chip influence.Depending on your route and preferences, this can change the bike so that one setting works for a particular center of the route, while another is better suited for a different location.
The height of the BB from the forest floor is a more varied variable, with the flip chip moving up and down, tire width changes, fork axle-to-crown length changes, wheel mix, and any other movement of one or both of these. Factor in your axle’s relationship to the dirt.BB height preference is often personal, with some riders preferring to scrape the pedals on the rocks in the name of a planted ride feel, while others prefer a higher transmission, safely out of harm’s way.
Small things can change BB height, making meaningful changes to how the bike handles.For example, the 170mm x 29in Fox 38 fork has a crown measurement of 583.7mm, while the same size measures 586mm in length.All other forks on the market are different sizes and will give the bike a little different flavor.
With any gravity bike, the position of your feet and hands is especially important because they are your only point of contact when descending.When comparing the BB height and drop of two different frames, it can be helpful to see the stack height in relation to these numbers.Stack is the vertical measurement between one horizontal line through the BB and another horizontal line through the center of the upper head tube opening.While the stack can be adjusted using spacers above and below the stem, it’s a good idea to look at this number before buying a frame to make sure you can achieve the desired handlebar height, compared to BB drop Effective is very suitable for your needs.
Shorter crank arms and bash guards create a little extra space and safety for a lower BB, but you need to watch your toes when pedaling tall rocks.For riders with shorter legs, the increased BB drop also requires a shorter seat tube length to accommodate the desired dropper travel.For example, the big  I currently ride has a 35mm BB drop which makes the bike feel great at slower speeds.With the 165mm crank installed, I could barely get the 170mm dropper post into the frame’s 445mm long seatpost.There is about 4mm between the seatpost collar and the bottom of the dropper collar so a lower BB, resulting in a longer seat tube or longer crank arms will force me to reduce my dropper travel or ride a smaller size Frame; neither of those sound appealing.On the other hand, taller riders will get more seatpost insertion thanks to the extra BB drop and more seat tube, giving their stems more buying power within the frame.
Tire size is an easy way to adjust BB height and make fine adjustments to the bike’s head tube angle without any major surgery.If your bike comes with a set of 2.4-inch tires and you install a 2.35-inch rear and 2.6-inch front forks, the pedals underneath will no doubt feel different.Note that your bike geometry chart is measured with the spare tire in mind, so you can try different combinations to improve your riding experience.
These are some of the many factors that influence BB height and can affect BB height.Do you have anyone else to share that we can all benefit from?Please write them down in the comments below.
I would like to offer a different perspective.What if many people prefer a low BB bike, but it’s actually due to the handlebars being too low?Because the height difference between the BB and the handlebar is really important for handling, and in my opinion most bikes have a head tube that’s too short (at least for the big size) and is usually sold under the stem when the bike is sold Not so many spacers.
What about the pole?A longer steerer tube in a shorter head tube causes more flex.Changing the handlebar height increases the “stack” without affecting the bend in the steerer tube.
Well, yes I have a 35mm stem with 35mm spacers and a stem…but my review is not about how to have a taller handlebar.This is because the handlebars of the bike may be too low, people like a low BB because it increases the height difference between the handlebar and the BB.
BB changes during suspension setup.The rider sets the sag, which can change the BB height and drop.The BB height changes as the suspension cycles through compression and rebound as the suspension rides, but typically rides at the set height during sag setup.I think sag settings have a bigger impact (height, drop) than tires or flip chips.
You make a solid point that sag has a significant effect on both measurements.We have to use fixed points when comparing bikes, and everyone’s sag is different, which is why I use pre-sag numbers.It would be great if all companies also shared a geometry table with 20% and 30% sag, although there may be some riders who don’t have balanced front and rear sag.
The difference is caused by the bb height in relation to the ground and wheel contact surface, not the wheel’s center of rotation.
Any value of bb drop number is a well-maintained myth that is easy to understand for anyone with experience with small wheel bikes such as bmx, brompton or moulton.
A lower BB doesn’t mean a longer seat tube.It doesn’t make sense at all.Especially if you’re talking about adjusting BB height using tires and forks etc.The seat tube is a fixed length on a given frame, and no adjustments will stretch or shrink that seat tube.Yes, if you shorten the fork a lot, the seat tube will steepen and the effective upper barrel will shrink a bit, it may be necessary to move the saddle back on the track, and then the saddle needs to be lowered a little, but it still doesn’t really Change seat tube length.
Great idea, thanks .My explanation might be clearer in that section.What I want to convey is that if the frame engineer drops the BB while keeping the height of the top of the seat tube/opening the same, the seat tube will get longer, which can cause problems with dropper post fit.
fair enough.Although I’m not sure why it is necessary to keep the exact position of the top of the seat tube.
Trial bikes in particular, their typical uses range from +25 to +120mm BB.
Frankly, my is a custom one with a +25 intended to go to zero with the rider in place.This is done to satisfy requirements, because there’s nothing worse than spending your hard-earned money on a suspension that buries the pedals into the earth if it’s taken off the piste.
For the next custom hardtail, I’ve finished the CAD file, including the “Shall” page.That’s the terms on BB.
I’d love to see some real drop measurements from cyclists on sag.My rigid with  is between -65 and -75 depending on the position of the eccentric.I run mine lower and it holds the line better in the corners and I feel more planted in the long grass.
Wrong, both are true.BB drop is measured relative to dropout, wheel size doesn’t change this, although fork length does.BB height is measured from the ground and will rise or fall as tire size changes.This is why larger wheeled bikes often have more BB drop, so their BB height is similar to smaller wheeled bikes.
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