Ride UK Tech Columns

#75 Three Piece Crank Fitting

(This article first appeared in Ride UK issue No. 75 and is reproduced here by kind permission of Ride UK.)

Once again I find myself sitting in front of the computer wondering how to start my tech column. Every few seconds I turn my head to look out the window for inspiration… or is it distraction.
How the hell do you write a thousand words in such a way that people want to read them? How do you compete with the other hundred odd pages of pure quality journalism in this magazine? How do you write on the subject of a small thin tube and not loose the readers interest after just one paragraph?
Well this month’s literary gambit will be much like previous months, I will swear and rant and hope you see the truth behind my rambling bollocks. Chances are you wont, and you will carry on in the same old way, but eventually things will catch up with you and when it next breaks you will remember my words and do it right next time.
So once again I return to the heart of any bike; the cranks.
Specifically three-piece cranks with sealed bearings.
There are several problems people have when working on their cranks and most stem from the same thing.
The bastard manufacturers don’t give you any shagging instructions with the fuckers! A hundred plus quid on some twating cranks and the cunts are too damn cheap or lazy to put a black and white A4 sheet of instructions in with them! Or, if they do then they don’t actually help. How hard would it be for them to put a detailed photographic guide on their website?!? Maybe they could actually use flash for something useful for once…
Deep breath.
So to make up for this shortfall, this month I will try to cover some basic concepts.

First off, the dreaded “spacer-tube”. The questions on everybody’s lips seem to be; “Do you need to use the spacer tube?”,” Will it work without it?” and “which tube do I use?”
The short answers are “YES, for sphincter’s-sake use the bloody spacer tube.”, “Probably not for long” and “The right one.”
To understand how, why, what and which, let’s review how it all goes together.

Whether you run a USA or euro or some other new and wacky bottom-bracket they all work pretty much the same way. Determining how it all works is the width of the bottom bracket shell.

In the first cut away view you can see that everything on the axle should butt up to everything else with no gaps. Let me emphasise that; NO GAPS.

This is so that when you tighten, for example, the right-crank-axle-bolt; it pushes the crank arm against the sprocket, and the sprocket (with top-hat spacer) against any extra spacers on that side. These spacers in turn press against the inner race of the bearing, which presses on the infamous spacer-tube, which presses against the far bearing’s inner race. The inner race then presses on the other side spacers which press on the left crank arm which is held in place by its own end bolt.
The force in the bolt can therefore be transmitted through all the parts on the axle to the far bolt which is screwed into the axle too and the “circuit” is completed.

IF however the spacer tube was removed the force from the axle bolts has only one way to get from one side of the bottom bracket to the other. Through the bearings! Without the spacer tube the force has to press the inner race against the balls, then the balls have to press against the outer race, then the force can work its way through the BB shell and back through the other side bearings.
As discussed in the hidden-headset article two issues back, this “axial” load is not something a normal bearing can cope with very well. With the bolts done up tight this force could be well over a ton; add to that the impact forces from tailwhips and 360s or even just dropping your bike and it’s no surprise that the bearings can die very fast, especially on the tiny bearings in a euro bottom bracket.

So how do you pick the right spacer tube? Well the easiest way is to put the likely candidate on the crank axle and put the bearings on each end. You can then put this assembly up against the BB shell itself and have a look if it seems right.

The lips of the two bearing cups must be WIDER apart than the BB shell. Ideally this should be just half a millimetre or so but 2 or 3 millimetres is fine. DO NOT be tempted to go with a width that is just under the width of the shell. If the spacer tube is not compressed then it is doing nothing and the bearings will suffer. On a USA BB it is fine to run with one cup sticking out slightly, although it looks dodgy this is absolutely correct.

With a euro BB you will need to adjust the “lock ring” to fit and hopefully “lock” everything in place, if this lock-ring slips later on then the loads can build up on the bearings and kill them, so keep an eye on it.
If you don’t seem to have a suitable length spacer-tube in your set, then you can use the shorter one and one or more of the smaller external spacer washers to make up a custom length.

For cranks that don’t have pinch-bolts this set-up is essential and the only way that will give decent bearing life.
But some people want a little stiffness in their crank bearings, they want the crank arms to stay still in the air, and taking the spacer tube out will give this. Although it isn’t recommended you can get away with this on cranks that use pinch bolts (like Primos). Because the end bolts aren’t needed to keep the crank arms on, you CAN run these cranks without the spacer-tube. In this situation you only want to tighten the end bolts enough to install the cranks and set the “resistance” in the bearings; then lock the arms in place with the pinch bolts. On a USA bottom bracket this should work fine and let you keep a little stiffness in the bearings without risking killing the bearings too fast. On a euro BB you will have to be a lot more careful, the bearings (especially the smaller ones needed for cranks like Primos which have pinch bolts) will still be at risk of axial overload. Any impact on the end of the cranks will now only be transmitted through one bearing (since the other is free to slide on the axle).

While we are on the subject let me emphasise a few other points.

With splined non-pinch-bolt cranks (like Profiles) use as few spacers as possible so that there is as much axle inserted into the arm as possible. The less arm you have inserted the more chance there is that the cranks will wear the spline and become loose. Also over time it is easy to only ever tighten one side of the axle. Without knowing it, you can be pulling the axle over to one side leaving the other arm with very little axle inserted.

Most cranks are designed with the intention that NOTHING goes between the sprocket and the crank arm, this includes the “top-hat” spacer. If you put a spacer in this gap then you must put one of the same thickness on the sprocket bolt. This is the most common cause of this bolt always coming loose.

Remember that left pedals and right-hand euro BB cups have a reverse thread.

Well that about wraps it up for this month, I hope this twaddle was of some use to someone, and if not… well at least you had something to read during that long session on the shitter after a questionable curry… (difficult with the internet version though I admit...)

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