07 June, 2016

Bottom Bracket Basics Revisited

By Chris,

We get an e-mail with bottom bracket questions almost every day; that's the way it's been since I started VO. I wrote the original BB Basics post in 2009 in an effort to rein in the confusion and to relieve Scott's e-mail load. Here it is again, with a couple of minor updates. This post covers only square taper BBs that fit our frames. Clint will write about some of the post retro-grouch BB standards later this week:

Lately my in-box has been overflowing with questions about bottom brackets. So I thought I'd write this simple guide that I can later forward to those seeking advice. Of course, this is only about square taper cranks and BBs.

What spindle length?

I often get questions such as, "What is the proper BB spindle length for a 1954 Urago mixte?" Come on; I'm not Sheldon Brown! In any case, the length of your BB spindle is determined largely by the model of crank you use, not by the type of bike. Every crank's manufacturer recommends a BB type and length. Of course, if the crank is long out of production, a lengthy conversation with Google may be required to find the specification.

As with anything, it's not quite that simple. There are a couple of other factors that can influence spindle length. If your bike has particularly wide chain stays, such as those on a mountain bike or a loaded tourer, a longer spindle may be require in order to ensure that the crank arms clear the chain stays. Often, the only way to be sure is to install the crank and see if the  BB works. If not, measure the interference or gap and buy a second BB. Unfortunately BBs that have been installed are not returnable, but there is always E-bay. The other factor that might influence spindle length is BB spindle taper.

Symmetrical VS non-symmetrical spindles
Some BB are non-symmetrical: the spindle is longer on the drive side. Today it can be hard to find the proper non-symmetrical spindle BB for old cranks. One solution is to use BB spacers to push the BB over a few mm, effectively making a non-symmetrical BB. Or you can just get a slightly wider symmetrical BB and not worry about it.

Spindle taper
The vast majority of square taper cranks uses either an ISO or a JIS taper. Basically, the angle of these two types is almost identical. It's just that a different portion of the taper is used, as I've tried to show in my sketch.

For the most part, only Italian frames use ISO today. The rest of the world uses JIS. But there are exceptions, such as the top-of-the-line Sugino track cranks; all other Sugino cranks are JIS. Some older cranks from Stronglight, TA, and other European companies also used ISO tapers. Only Ofmega and Avocet, of the major manufacturers, used a proprietary taper.

It's usually possible to fit an ISO crank on a JIS BB. The only caveat is that the crank will be 3-4mm wider than if an ISO BB was used, so pick a slightly narrower spindle and tighten the crank bolts securely.

I don't recommend using a JIS crank on an ISO spindle because the crank may bottom out, thus permanently ruining the taper. If, however, you are careful, it will work with some combinations.

One of the problems with this whole idea is that manufacturers sometimes take a casual attitude toward  following one standard or the other.  I have seen TA cranks, for example, that appear to be ISO and an identical crank that's JIS, or perhaps something in between.

BB threading
Let us begin with the French standard because this is, after all, Velo Orange. The French decided that BB shells should be 68mm wide, threading should be35mm x 1mm, and both cups should tighten to the right. Simple and effective! Of course VO makes French BBs.

The Swiss, improved the French standard by reversing the thread on the drive side. This became the Swiss standard and was adopted not only by Swiss manufacturers but also by a few French companies, notably Motobecane. (Motobecane also used French threading on occasion and later switched to British threading; good luck.)  The BB shell is 68mm wide and also threaded 35mm X 1mm; but the left cup tightens to the right and the right to the left.

The reason for this improved standard was that the spinning of the crank was thought to loosen the right-hand thread non-drive BB cups. After a few decades, the Swiss noticed that French cyclists, in fact, were not stopping every few miles to tighten their cups. And so the standard was quietly abandoned. The only Swiss threaded BBs made today are the super expensive Phil Wood units. But VO does have a threadless BB that is a less expensive alternative.

The French standard was also eventually abandoned and, to the everlasting annoyance of the French and of francophiles everywhere, the British (or ISO) standard became the world standard. If your classic frame was made in America, Japan, Taiwan, Germany, or Britain, it almost certainly accepts a British threaded BB. This standard is 1.375" X 24 tpi with the left cup tightening to the right and the right to the left; the BB shell is 68mm wide. Most VO BBs are British thread.

Not leaving well enough alone, some British manufacturers, particularly Raleigh, came up with other standards that are outside of my understanding, as did some old American manufacturers.

Finally the Italians, who we reluctantly acknowledge do know something about bicycles, if only the racing sort, blithely ignored everyone else and stuck to their own standard. The Italian BB shell is 70mm wide and has 36 mm X 24 tpi threading; both cups tighten to the right. VO makes Italian threaded BBs in sizes to fit our cranks.

Speaking of Italian BBs, I should note again that at least one company, Ofmega, used there own propitiatory axle taper. They also made Avocet cranks which use the same bizarre standard. A regular taper won't work. I know this since I destroyed an Ofmega crank on my old race bike by trying it. Find a BB on E-bay if you have one of these.

Damaged BB shells
If the threads on your frame's BB shell are cross threaded, stripped, or otherwise damaged you can use the VO Threadless BB that I mentioned earlier. Taper is JIS. We have used these on British, French, and Swiss BB shells and they work perfectly. Customers report that they also work in Raleigh frames with a 71mm wide BB shell. These BB's won't work on mountain bikes, as their standard width (73mm) is too wide for the shell to expand. They also do not work on Italian threaded BB shells because those have a wider ID.

So there you have it, a gross oversimplification of BB standards that should, nonetheless, provide sufficient information to fit 99% of traditional frames with BBs.

If you need help installing BBs, please read the excellent BB section at the Park Tools site.


Al said...

Is there a contradiction in the second paragraph of "BB threading"? "The Swiss...by reversing the thread on the non-drive side." Is the non-drive side also the left side? "...the left cup tightens to the right and the right to the left."

S. Molnar said...

"For the most part, only Italian frames use ISO today." You mean cranks, of course, and by Italian you probably mean Campagnolo, but maybe there's another.

Also, my own experience confirms that right-hand threaded fixed cups, at least of the Italian variety, do indeed come loose with time unless aided by Loctite, whereas left-hand threaded fixed cups stay fixed.

Dave Feldman said...

Hope I don't get moderated for this, but IRD supplies Swiss threaded cups to go with their Tange-manufactured cartridge BB's. Phil isn't the only one who sings this song.

Dave Feldman said...

Oh, and Schwinn's freewheel bottom bracket was the Shimano FFS front freewheeling system which combined with their first attempt at indexed shifting system. It was a system designed to allow shifting a bike with derailleurs while coasting. Used a dual, solid core derailleur cable as the cable had to both push and pull. It might have been successful if introduced in the early-70's bike boom, but it came on the scene when derailleurs had improved substantially and mountain bikes were about to hit the maintstream. It was expensive, heavy, and other than cables there were never a lot of spare parts available.

James Biffin said...

I wouldn't call it a "British" bottom bracket when in fact it uses a 60 deg. thread angle which is much more strongly associated with the US as opposed to the 55deg. Whitworth thread angle used on every other type of British bottom bracket. In continental Europe what you call a British bottom bracket is called BSA. Britons sometimes think of it as an American standard because it is not whitworth. BSA took the 24tpi whitworth (55deg) bb dimensions (used on Phillips bikes, or anything built in Birmingham) and altered it in terms of thread angle bringing it in line the Unified Thread Standard that never really caught on in the UK which, in many industries, went straight from Whiworth to Metric. I've long wondered if BSA motorcycles used Whitworth or UTS threaded components probably Whitworth.

The BSA bottom bracket caught on probably because of the popularity of BSA's components (see early Schwinn Paramount) and was adopted by the Japanese for whatever reason. I have seen early Toei with BSA cranks; perhaps BSA components were commonly used on early post-war Japanese high end bikes setting the standard for the rest of the industry. BSA components were sold over a large price range and many were fairly ordinary steel cottered cranks but probably had some snob value association with professional racing and track bikes. Other companies like TA and Chater Lea didn't sell much in the way of cheaper parts as far as I know.
So here we have a company, BSA, which used a North American thread standard to produce what became a world wide standard that is considered British even though every other British manufacturer didn't use it. BSA was far from the largest bicycle manufacturer in the UK. They were bought by Raleigh and in the end wound up being nothing more than a rebranded Raleigh with Raleigh's 26tpi fine whitworth bottombrackets.

Raleigh bicycles used a fine whitworth thread of 26pti as opposed to the Phillips 24pti whitworth bb. I believe the Raleigh bb is techically a BSF thread and Phillips is a BSC thread.
There was no British standard. Other companies used the fine whitworth 55deg thread of 26tpi in bb shells of various sizes: Chater Lea, Bayliss Wiley etc. Essentially what you had was a headache inducing variety of proprietary bb shell sizes all using standard and fine whitworth threads.

People often mistakenly assume that Phillips built bicycles have a BSA or "english" bb because it is 24tpi and works well enough with BSA threaded cups. The 5deg difference in thread angle produces what the Sutherland's manual calls a class B fit.