Saturday, May 24, 2008

Slightly lower bottom bracket height...

Anonymous recently asked "What would you define as "slightly lower" BB height for mountain and road bikes?" I started to answer in the comments section, but think it deserves its own post because it became quite lengthy.

Considering this is a road frame I'm mentioning, I'll stick with that, but the same would apply to a mountain bike frame. Short answer: "slightly lower" equates to a few millimeters. "A lot lower" is more than a few millimeters.

Seriously, it's all relative. Bottom bracket height is really a term that doesn't mean much because tire size/diameter is more of a determining factor in "bottom bracket height." For example, say you've got a mountain bike that has a bottom bracket height of 12" with 2.1 sized knobby tires and you want to run 1.5" road tires. As you're riding, you notice that the bike feels awfully low and you are scraping pedals going around corners. Well, by going from that 2.1" tire to a 1.5" tire, you've successfully lowered your bottom bracket height by about 10-12mm (based on my database of tire size diameters/radii). So bottom bracket "height" is really only constant if you use one size of tire. But, if you design a frame that is capable of running various sizes of tires, how do you call out bottom bracket height? One height for each tire size? Or do you use the more appropriate term of bottom bracket "drop?"

Bottom bracket drop allows you to clearly define where the bottom bracket is in relation to the center-line axis of the wheels. How far below (or above in the case of some mountain bikes) the wheel axis helps determine how stable a bike may ride. Road bikes usually have a BB drop of between 65-75mm. Most road bikes designed for racing and 23c tires fall between 70-68mm. Increasing BB drop just a few millimeters can substantially increase the stability of a bike by positioning the rider lower within the confines of the wheel's center axis. Bikes like Rivendell's Atlantis are known as very stable bikes with BB drops in the 77mm range.

But, that 77mm BB drop on an Atlantis doesn't mean that the pedals are now 7mm closer to the ground compared to a bike with a BB drop of 70mm. Because riders of an Atlantis will use tires in the 32-35c range, compared to a 23c on a road racing bike with 68mm drop, the relative bottom bracket height could actually be higher on an Atlantis than a road bike. But, the bottom bracket position relative to the wheels' centerline axis will still be lower creating that feeling of stability. I believe this is one of the main reasons why 29" wheel mountain bikes has won over so many fans. Twenty-nine inch wheel mountain bikes have a BB drop that is quite a bit more than 26" wheels putting the rider within the wheels much more so than 26" wheels where the rider sits on top of the wheels. Just a discussion on 29" wheel mountain bike geometry could take volumes and make the head spin.

So, instead of saying "Slightly lower BB height - check," I should have said "Slightly more bottom bracket drop - check." And by slightly more bottom bracket drop, I mean a few millimeters ;-) .

(What's playing: KWMR Morning Glory show and a very nice rendition of Amazing Grace by Hayley Westenra from New Zealand)


Badger said...

Stumbled here through Foresta Bikes recommendation and I can say that I will be back. Great article, I love the technical aspect of the bike but it seems that the engineers are not that forth coming with their information.

blackmountaincycles said...

Thanks, Badger.

evan said...

found this on a google search for "BB drop and 29er". I'm shopping for a 29er frame, (I just bent my current 29er frame's disc brake mount, and having trouble correcting it.)

My current frame specs @ 65mm BB Drop. What I am looking for: BB height taller than a 26er and BB drop lower(greater) than a 26er. With 1.5" of extra axle height, I should be able to have BOTH! jeeper lift kit clearance and rally car stability, like the Baja truck. (And should fate deal 38mm touring tires, the pedals should still have adequate cornering clearance.)

I can access a Surly 29er frame inexpensively, but this frame has a greater drop spec @ 2.7" (69mm). This is only 4mm (a few mm difference)-maybe I'm being too picky and I should just go with it. Could try 170mm cranks, another rear cog tooth, and taller saddle if I'm dragging too much. Surly also allows room for 2.7" tires, whatever that really means.

I am now curious to read some discussion on wheelbase and chainstay length though.

blackmountaincycles said...

My only advice is to not get caught up in the numbers too much and to take what you read with a grain of salt - even here.