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Cranksets, Bottom Brackets and their Specs

A bicycle bottom bracket (often called the "BB" by bicycle mechanics) is that bearing and axle assembly that the crank arms are attached to. The bottom bracket shell is the part of the bicycle frame that houses the bottom bracket. Usually the chainstays, downtube and seat tube are attached the the BB shell.

We'll explain the BB shell first and then deal with the types of bottom brackets or as they were called in older times, hanger sets.

Bottom bracket shells and their specs

If you have an older frame, odds are, your frame has an "English" BB shell unless it is an exotic Italian machine. Then it might (but this is no sure thing) have an Italian BB shell because some Italian builders as well as nearly all modern French makers use English threading on their export production.

It was not always this simple. 100 years ago tariffs and the high cost of transport limited European trade. Italian bike makers sold most of their goods in Italy; French makers concentrated on French customers and British makers made bikes for their home market. The result was a hodgepodge of design specifications that can confound the consumer trying to figure out what replacement bottom bracket he needs, especially if it is an older bike.

Chart of Bottom bracket shell specs
An explanation for all these bb types is just below the chart.

The right-hand cup is usually referred to as the fixed cup and the left side is the adjustable cup side. This will be explained in the bottom bracket section.

BB Name Threads Left or
adjustable cup threading
Right or
fixed cup threading
BB shell
width

Cup outside
diameter (O.D.)

Shell
inside
diameter
(I.D.)
English 1.370" x 24 TPI right-hand left-hand

68mm,
73mm oversize,
83mm free ride

34.6mm - 34.9mm 33.6mm - 33.9mm
ISO 1.375" x 24 TPI right-hand left-hand same as above    
Italian 36mm x 24 TPI right-hand right-hand 70mm 35.6mm - 35.9mm 34.6mm - 34.9mm
French 35mm x 1mm right-hand right-hand 68mm 34.6mm - 34.9mm 33.6mm - 33.9mm
Swiss 35mm x 1mm right-hand left-hand 68mm 34.6mm-34.9mm 33.6 - 33.9mm
Raleigh 1 3/8" x 26TPI right-hand right or left-hand 66-68mm    
Chater-Lea 1.450" x 26 TPI right-hand left-hand      

For an explanation of what the above chart means let's start with the classic picture of a bottom bracket shell from an old Campagnolo catalog:

This is a view of the bottom bracket shell from underneath the bike, It is for what is now an older style 3-piece crankset (the crank arms slide onto a steel tapered axle) but it will do perfectly well to explain all but BB30 bottom bracket shells.

BB width:

First of all, note the center measurement, the width of the bottom bracket shell. This usually varies from 65mm to 70mm but in some cases can go to 72mm. Generally Italian road frames have 70mm wide BB shells and British and French use 68mm shells. Track frames can use 65mm shell to pull the arms in a little closer to the center line of the frame to give the pedals a little more clearance on a banked track.

BB threading

There are 3 common basic BB thread types:

English: 1.370" x 24 TPI. This means the hole in the bottom bracket shell is about 1.370 inches in diameter (it's actually the diameter of the cup that is screwed in) and that the threads are spaced 24 to an inch. The right-hand side has left-handed threads. The bearings on the left side travel counter-clockwise as the crank is turned by the traveling bike rider. This has the beneficial effect of putting a constant tightening force on the right hand cup. This is the most common bottom bracket specification in the world. The Japanese, Taiwanese and Chinese manufacturers chose to use English bottom brackets. Even some Italian makers build with english bb shells now.

Italian: 36mm x 24 TPI. Both sides of the shell have right-hand threads. The 36 mm diameter is slightly larger than the English 1.370". If you try to insert an English bb set into an Italian shell, it will just fall in.

French: 35mm x 1.0mm. The bottom bracket diameter is slightly smaller than the Italian and has 1 thread per mm. I don't know that any bikes are made with French bottom bracket any more, but there are millions of them out there because a lot of the bike boom bikes of the late 60s and early 70s were French.

There are other bottom bracket shells out there:

ISO: 1.375 x 24 TPI. For all intents and purposes, the same as English and often sold as English. It's 5 thousandths of an inch bigger in diameter and is reasonably compatible with standard English-threaded parts. It can cause problems when a bottom bracket is tapped with a set of English taps and an ISO cup that is just a little oversize (products usually vary plus or minus a couple of thousandths) is fitted. Sometimes the cups just won't go in and the shell has to be tapped with an ISO tap.

Swiss: Also 35mm x 1.0mm like the French BB, but with a right cup with left-handed threads. In the 1970s riders would come back with Mondia or other Swiss-made frames and go crazy looking for bottom bracket parts. This spec can also show up on old Motobecanes

Raleigh (and other Raleigh brands like Rudge): 1 3/8" x 26 TPI with left handed fixed cup. Unless it has 24 TPI. Generally a Raleigh with a bottom bracket wider than 70mm has 26 TPI and a narrower one is 24 TPI. If you are restoring an old bike you may need a thread gauge.

Chater Lea: 1.45" x 26 TPI. Another old British spec that will turn up most often on tandems.

Bottom brackets

A traditional bottom bracket has 2 bearing cups that screw into the frame, a right hand, or fixed cup and a left side cup. The fixed cup is the one that goes on the chainring side. It usually has no adjustability. It is installed first and it tightened until the flats on the cup are snug against the bottom bracket shell.

The second cup is the left hand, or adjustable cup. Because component manufacturers have to make bottom brackets that will fit a wide variety of bicycles, built with varying degrees of precision, the left cup is installed last and after making sure that it is optimally tight so that the bearing run freely, without binding, it is in some way locked down.

Bottom brackets, like the rest of the bicycle, have changed and evolved until they would be almost unrecognizable to a mechanic working in 1965. As crank design has changed, bottom brackets have followed. but for years, bottom brackets have been plagued by the fact that early on in bicycle design the diameter of the shell was fixed at about 1 1/3". Engineers wanted to produce thin-wall, larger diameter hollow bottom bracket axles that were light and stiff. But, that left little room for the bearings. Small bearings mated to a large axle is a recipe for short bearing life. Indeed experiments with large hollow BB axles and small bearings were generally unsatisfactory.

Here are the two kinds of three-piece bottom brackets a mechanic is likely to come across today

Cottered steel: Charly Gaul raced with steel cottered cranks long after his competition had switched to aluminum. By the mid 1970s, long after high-performance bike brands and most Japanese makers had switched to aluminum cranks, most European factories continued to equip their bikes with steel cottered cranks. That is a large reason why your current bike is not made in France. Steel cranks are heavy and cottered bottom brackets can be devilishly dificult to work on. A high percentage of the bike boom bikes of the 1970s that so many mechanics are turning into "fixies" came with these cranks.exploded drawng of a cottered cranksetThe The bottom bracket axle of a cottered crank has a pair of notches, one at each end. Through a hole in the crank arm a steel cotter with a tapered flat is placed, passing over the notch of the axle. It is secured with a nut and washer. The tapers (depth and angle) of the cotters must exactly match so that the arms end up exactly 180 degrees apart. Notice that the cotter for the drive side in the picture is facing the opposite direction of the cotter on the non-drive side. This is essential. Otherwise the arms will notline up.

Less important is whether or not the cotters face up or down (this picture shows down-facing cotters). It is generally thought down facing cotters are more reliable and less likely to come loose.

If a rider uses his bike when a cotter has come a bit loose, it can mushroom inside the arm and require drilling out. Even when everything is correct, a cotter can be very difficult to remove.

There are two ways to remove a cotter, with a press or with a hammer. If you use a hammer, something solid and heavy has to be put under the crank to prevent the pounding force from brinelling (pitting the cups) the bottom bracket bearings. Traditional advice is to use a block of wood, but that will not keep the hammer's energy from being transfered to the bearings and cups. Loosen the nut till it is flush with the top of the cotter (do not remove it) and strike the cotter sharply. If you are lucky, one or two blows will suffice to lossen the cotter and the treds will stil be usable.

The French toolmaker VAR made a cotter remover with a huge lever arm that could usually get a stubborn cotter out, but it regularly ended up mashing the soft cotter. You can use a c-clamp to try to press out a cotter. This will prevent damage to the bearings. A socket placed over the bottom of the crank where the cotter will come out will let you squeeze out the cotter, if you are lucky.

If all this fails, get a drill. Crank cotters are soft and in a few minutes you can get the stubborn cotter out.

Mechanics used to keep bins of various cotters with different diameters and tapers. I have spent long hours filing cotters when the right one wasn't at hand to get the crank arms just right.

When re-installing cotters, press or tap them in, don't use the nut to draw the cotter in. Snug the nut, ride the bike around the block and then re-tightn the cotters. they could be fine for some time to come.

Crank cotter specs:

Aluminum cotterless 3-piece crank with tapered flats:

Sometime in the 1930s the French company Stronglight developed a surperior design that did away with the troublesome crank cotters, the aluminum cotterless crank. Both primitive aluminum metalurgy and industry conservatism prevented the design from being widely used until after the war.

Most racers in the early 1950s used the better design but consumer bikes were spec'd with cottered cranks until the Japanese bikes of the 1970s showed up. Previously cotterelss cranks only came with high-end bikes but brands such as Nishiki and Centurion used them on their entry-level models. Soon all bikes had cotterless cranks.

The axle had tapered flats that fit into the crank arm. The arms were secured with bolts that drew the arms up snug. Removing cotterelss cranks is usually a breeze. A crank puller is screwed into threads in the crank arms and the arm is removed with little muss or fuss.