Ride UK Tech Columns

#68 Fits

(This article first appeared in Ride UK issue No. 68 and is reproduced here by kind permission of Ride UK.)

Suppose you went out tomorrow and bought a brand new car, suppose for some crazy reason you bought a Ford. On the front and back it would say "Ford" and even the middle of the wheels would probably proclaim their Fordiness. If you looked under the bonnet you would probably see a big "Ford" cast into the rocker cover, and if you took the air filter out you might see some long ass product code and the word Ford. With the possible exception of the tyres every part of the car would have been specified and designed by Ford for that car and as a result it will all fit nicely together first go.
You don’t see footage of car production line workers asking their mate to pass "the big sledge" so they can get the wheels on, the guys barely break a sweat.
Now look at a few bikes, chances are you will see at least 3 or 4 different companies' parts on there. Gyro, tyres, freewheel, chain, pedals, headset, cranks, hubs all might well have a different brand name on them to that shiny sticker on the downtube of the frame.
With a bike we expect a good level of interchange-ability. We expect, want and NEED the pedals off one bike to fit another in a way that we would never expect of a car. If you tried to take the engine from an Escort and stick it in a Astra you wouldn’t expect it to be straightforward. Yet you would expect to be able to take your mates old cranks and stick them on your bike.

To have any chance of making all this work we need standards, somebody somewhere has to agree a standard size (or sizes) for all these things so that we all can fit one bit on to another. This situation isn't unique to BMXers. People in all branches of engineering need to be able to use standard size nuts and bolts and bearings so that spares are easy to find and fit. Going back to the car, the Ford has all been specified by Ford, but they will still have used standard size nuts and bolts and lots of common off the shelf bearings and seals.

Because of this common need for compatibility things like nuts and bolts have been sorted out for well over a hundred years. A dude from Stockport called Joseph Whitworth made a name for himself as a kick ass engineer and started making standard "Whitworth" sized nuts and bolts which soon became popular. Nowadays we have all kinds of standards groups that produce reams of paperwork to specify things like metric thread sizes and tolerances. Things like bearings and seals are made to incredibly accurate sizes so that fits can be almost perfect.

Unfortunately bikes seem to be exempt, as anyone who has ever built their own bike from parts will have noticed. Try ordering a load of parts for a new bike and putting them together yourself and the chances are you will have at least one big problem fitting the bits together.

Lets start with the obvious nightmare, bottom bracket cups. I would be willing to bet that we have nearly all at some point struggled with bottom bracket cups on a normal USA bottom bracket. Some people have even gone as far as to go back to the weaker euro bottom bracket just to escape it, but why should it be such a problem?
The simple answer is that it shouldn’t. At some point a standard should have been drawn up and published. We should all KNOW that a bottom bracket shell should have an inside diameter of exactly "X" millimetres, plus or minus "Y" hundredths of a millimetre. This should be child's play to achieve with modern machining technology and bottom bracket cups should be gently tapped home with a copper mallet. The shell should be maybe 0.2 of a millimetre smaller than the bearing assembly to get a nice tight fit, and that should be that.

0.2 of a millimetre "interference" is about right on that size of hole for a reasonable fit, but if we measure some typical bits we find enormous variations in size. Bottom bracket shells that are nearly a whole millimetre too small, one piece cups that are nearly a millimetre too big and lots of sizes in between. Now a millimetre may not seem a lot to you but it will when you come to try to put one into the other! That millimetre is the difference between tapping the cup in with a mallet and ringing round garages to ask if you can use their hydraulic press! What's more that millimetre has to go somewhere, so it ends up squashing your bearings so tight that they barely turn and wear out super fast.
Why cant the frame and crank manufacturers get together and sort this out?! Its not rocket science, this is more the sort of thing Sesame Street used to cover in their "cooperation" sketches!

Headsets have got a lot better in recent years thanks to the Aheadset. Before we used to have similar problems with headtubes and it still goes on a bit, but because the Aheadset was patented and licenced, manufacturers had specifications to work to and they have. But here's a subtle one, stems.
Have you ever had a stem close up on you? The back slowly pinches together until the stem just doesn’t seem to grip right? Chances are that it's because of incompatibility between the stem and the forks.
Measure an inch and an eighth fork and the steerer tube should be 1,1/8" right? Well it might not be. Since the steerer tube is made from just that, a tube, the size depends on the production standards of the tube. A nice high quality tube will be almost spot on 1,1/8" in diameter, but as prices drop so does quality and consistancy, a cheap tube might be several thousands of an inch under-size. This is no problem for the aheadset, which automatically adjusts for quite a range of sizes, but for a great thick aluminium stem to bend round it is a bigger deal.
Stick a well made stem with a highly toleranced reamed hole on a high quality fork and you should have no problems, the slight gap (under 0.1mm) will be within the range of the stem and it will fit and lock nicely.
Stick the same stem on a fork that is 5 thousandths of an inch undersize and suddenly it has a gap of over 0.2mm to cope with, to get there the stem has to bend right in and instead of the clamping power going to hold on to the steerer most of it is wasted just bending the stem round to the smaller size. The inevitable result is that it slips.

A lot of these sorts of problems can be explained by a need to make things simple. The original USA bottom bracket shell size was determined by a standard sized tube, the same with the stem problem above and seatposts can have similar problems.

But some problems are just down to bloody-mindedness and ignorance. Some manufacturers wont even consider that they are wrong. Even where there is an existing engineering standard laid down by international committee they wont even try to meet it! They just expect everyone else to fit in with them, don’t you Keith? Not that people like Keith would bother to read an article like this mind you.
Designing and making parts for bikes is serious engineering, just as serious as making cars is. It deserves the same levels of expertise and professionalism.

So what can we do about it? How can you adapt these parts to fit perfectly?
Well there are two basic solutions. The chances are that when you get some new bits you want to get that bike built up as fast as possible, you don’t want to wait around, so your only option is to bodge it. You can sit and file excess material off the outside of your cups, and you can cut up an old pop-can to make a shim for the stem.
The other route, and your only option when things are really bad is to complain. Don’t make your local shopkeepers life hell but get him to measure everything and work out which part is to blame, then get him to moan to the distributor, who can moan at the manufacturer and maybe one day, things will change…… though I doubt it.


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