What are the benefits of external bottom brackets?

Question

Pretty soon, I plan to replace the bottom bracket on my early 90's Sakae Litage road bike. It takes a standard, English-threaded bottom bracket. Right now I think it contains loose bearings--I was planning to replace it with a decent-quality cartidge.

I see a lot of external bottom brackets when I am looking at my LBS's website. I have replaced many sets of loose BB bearings and several cartridge BB's. I know nothing about these external things. Why do there seem to be more of them for sale than cartridge bottom brackets? Do they have larger, longer-lasting bearings? Would it benefit me to consider one as opposed to the cartidge BB's that I have been using recently?

I ride pretty seriously for fun, commuting, and errands... I built this bike specifically for doing really hilly weekend rides. But I'm not a racer.

Curious. Many thanks.

Answer 1

External bottom brackets allow you to have both large bearings and a large, hollow bottom bracket spindle. A large hollow spindle can be designed to be as stiff as a small solid spindle for less weight. Smaller bearings reduce the longevity of a bottom bracket, so the typical bottom bracket design allows a narrow range of suitable spindle sizes and bearing sizes - to get the weight savings from a larger spindle you need to sacrifice durability. Moving the bearings to the outside effectively removes this tradeoff.

There is an additional performance factor in situations where high stiffness is required in that the distance between the cranks and the bearings is reduced, lessening the torque on the axle.

For commuting, errands, recreational riding and other "normal" purposes a cartridge bottom bracket is almost always going to be more than sufficient. If you are particularly concerned about durability, the SKF bottom brackets are engineered for 10 years / 100,000km of riding, but for the price a $25 Shimano cartridge will usually last what feels like forever.

Answer 2

The main benefit is weight - because the axle is now a pipe it gains strength from diameter and hence can use less metal. Bicycle frames are made of tubing rather than solid rod for the same reason.

One disadvantage is that the balls in the bearings have to be smaller because there's less space to fit them in, so all things being equal they will wear out faster. But things are not equal, and what's made it practical to have external bottom brackets is improved metallurgy and precision manufacturing. So an external BB should last as long as a similarly priced internal one. (edit) And as lantius pointed out in the comments, the balls in an external BB will be close to the same size as the ones in a cartridge BB anyway, the "smaller" comment is mostly relevant to a comparison with open bottom brackets.

Shimano had problems with the seals in their external bottom brackets that was causing a lot of premature failure but I believe those have been solved now. We've stopped seeing them coming in for early replacement, anyway.

You will also need to change your cranks to fit an external bottom bracket. Rather than a square taper external BB's all use a splined system. Which is an improvement over the square taper but there are several types. Make sure you can get a BB to match whatever cranks you buy.

They have also changed the sizing, from memory there are only a couple of lengths of external BB available rather than 10 or more. This may mean that you end up with your chainrings slightly offset from their current positions, but with a derailleur system that shouldn't be a problem. On a singlespeed it will give you a bent chainline which is bad.

Personally I don't see the point in upgrading an existing bike. If you needed new cranks and were focussed on weight it might make sense, but it's an expensive risk (things might not work the first time) for a very small gain. For the same price you could get a Phil Wood BB that will last forever, or save money by buying cheap cartridge BB's as you need them.