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.

Answer 3

This is a mighty old posting that I am responding to almost 5 years later, but I thought it would be helpful for those who are looking to overhaul their BB and thinking of moving to the external.

For the most part that is captured here, External hubs are at least 30% lighter than the internal hubs. Because the bearing cups are outside the BB shell, it provides more leverage on riders who ride hard on the pedals, hammering away. The bearings are indeed LARGER than the internal hubs which means longevity of the BB. When they do wear, the bearings can be replaced without chucking the entire unit, saving you time and money. However you will need to purchase a BB Exit Tool ($15) to make it happen.

Replaced my internal hub twice and then tried the external hub, so far it has lasted me more than double the life expectancy of my two internal hubs.

So in a nutshell, external hubs are 30% lighter and 40-45% percent longer lasting than your internal hub.

Hope that helps.

Answer 4

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?

There are two things that last longer with external bottom brackets.

Firstly, the bearings. The bearings can be made larger and thus they last longer. This is mainly fixing an annoyance as broken bearings won't hurt you.

Secondly, the crank spindle. It can be made hollow, larger and thus lasts longer. This is a very important safety consideration as a broken crank spindle can really hurt you.

Long time ago, square taper spindles were used. They allowed the bearings to be made okayish-sized but the square taper spindle broke often especially for heavy and strong riders. Then Shimano attempted to prevent spindle breakages by introducing Octalink spindles. They didn't break as often, but they had a design flaw in the left crank attachment that if you rode on uneven ground freewheeling with the right crank forwards and left crank backwards, the reverse torque caused the left crank bolt to loosen. The reason for this design flaw is that Octalink lacked press fit.

Also, Octalink bottom brackets have very small bearings because the big spindle takes so much space. Thus the bearings don't last long.

Shimano fixed once and for all the Octalink flaws by introducing Hollowtech II external bottom brackets. There is no crank bolt loosening anymore, and with external bottom brackets, you can have at the same time lightweight, stiff and durable crank spindle (due to its large diameter), and big bearings (due to external bearings).

Answer 5

If you have a Fuji, and its got a double chainring, you have what is called a loose ball bottom bracket that is square taper.

Yes the ball bearings are caged in a little cage, but it is loose ball. All Japanese bicycles from the 80s have these horrible bottom brackets except the very top of the line, because most of them used the funky external nut tightened, poor quality, bottom bracket.

The best thing is to get rid of that junk.

Third, your bicycle is too low of a model to dump money in. There is no need to change to a hollow type bottom bracket or a super new bottom bracket and cranks because you run into chain issues (width) and gear spacing and crank spacing. The strength of the new cranks is great, but square taper is more than adequate for any person who has ever used these forums and its been used by the best just fine (Le Mond) or (Merckx) etc.

What you do need is the right spacing. Generally you can go quite a bit more narrow than the giant size you picked. I recommend a 110 for double and 113-114 for triple.

The all in one units like Phil Wood are all about the same really for your purposes. You can get a great bottom bracket for $25 these days in a sealed unit square taper.

The average bicycle poster seems to have large amounts of money I find and considering your bicycles other parts, its not worth dumping anything modern or excessive into, except the great 700 x 28c wire bead (non tour guard) Pasela tires and a nice solid 7 speed chain. with a traditional bicycles you are not missing much like the geeks would have you think. The old bicycles were wonderfully tough and rode well with little investment. They also didn't have racing dimensions for bicycles that get used by people who want to just roll around and enjoy a nice ride.

Ok details...

The new cartridge units are a narrow stance on the bearings and that Does make a difference and loads are not only horizontal or vertical on bicycles you must remember. This flex is not enough to out weigh the ease of installation and good quality seal they all have.

Tighten your cranks tight, then come back after your first ride and tighten again. Also go a tad bit tighter on your bottom bracket than the recommendation and you will not have to go back later and adj. again. Oh and also...re bearings...smaller bearings are better.. The closer the inner ID and the OD of the races, the less sliding the bearing has to do. Bearings always slide some when surfaces are different diameter. Think about it. There is so much false information out there. Look at roller bearings ... they are tiny bearings ... Good luck. $25 is all you need.