Bicycle Mechanics - Metric/Imperial measurements.
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I'm a metric man. I can't think in terms of inches, but I see when it comes to chain wear for example I have no choice! :)
Measurement for wear is based on the end rivet offset measured at 12" (304.8mm). Less than 1/16" (1.59mm) is ok. Less than 1/8" (3.18mm) is questionable, and anything over is bad news. As you can see, no steel metric ruler is going to be graduated to those metric values. I suppose you get steel rulers that have both metric/imperial graduations, but the problem does not end there. I also see that headset sizes and other measurements seem to be quoted in inches. Am I too far in the future, or should I tool up for my own maintanence exclusively using inch-based tools? Elsewhere, technical descriptions use a hybrid of both standards and that makes it even more confusing - do I need two of this tool, two of that tool, etc - one for inches, one for metric.
Yikes! What do you guys do?
BTW. I'm 1.8m tall and weigh 89Kg! Darned proud of it too!
I am a metric man, too. But, I don't think that you need to worry about it. Certain things like headsets deserve special tools. So, buy a headset wrench that fits your headset. You won't be able to use it for much else, but you'll have the one you need. Sure it seems stupid to fork over the money, but it is just a one-time expense.
As to the chain, well, just take a close approximation, like 3mm, as you measure for what is too much slack in the chain.
Most other things on bikes are metric, so you'll be okay. You are certainly better off than American drivers with Japanese or European cars were in the 1970s. Stories abound about having a breakdown somewhere in the rural U.S. and the local mechanic neither having the tools nor a clue what to do.
And all this despite the fact that the U.S. Congress passed a (non-binding) resolution in the 1930s declaring metric the official measurement system of the U.S.
All bolts on bicycles are now metric. Unless you are working on older machines with Imperial standards you won't need 2 sets of tools.
07-23-02, 05:41 AM
But the BB thread is still Imperial on most bikes.
FWIW, the maximum chain elongation should be .5%. This means that, over, say, 100cm, you should have an elongation of no more than 5mm.
Of course, since chains are 1" to the link, this may be difficult to measure quickly.......
07-23-02, 08:34 AM
Just get a Park chain measuring tool and it does't matter which system you use. And they're cheap!
07-23-02, 08:38 AM
You can Metric/imperial steel rules on market stalls for a quid generally.
I find it troubling and ironic that the two surviving BB cup standards have 24 threads per inch. The somewhat rare Swiss BB standard (35mm x 1 thread/mm, with LH thread on the fixed cup) should have prevailed. Most pedals shafts are 9/16" x 24TPI, although one generally affixes them with a 15mm wrench (go figure).
I have never seen an all-metric bicycle, although some of the 1970s Peugeots came fairly close.
As for the chain, if the nominal length of 24 half-links is 304.8mm, then the chain should be replaced when 24 half-links exceed 306mm.
Of course, these questions of threads per inch or centimeter generally only matter to the guy building the bike. The average bear only needs to have the parts match, i.e. BB tube being the same as the BB cups. Ditto with pedal shafts. Standards are standards and as such don't matter. But, the manufacturers know that metric is the way to go, so that's why you get a 15mm bolt.
Think about the fact that the bicycle drive train is on the right. It's there because they were adapting to a horse world. Mounting from the horse's left meant you mounted a bike from its left and so they put all the greasy stuff on the right. Nowadays, G.B. and a few others aside, we have our bikes on the right side of the road, which would imply that the drive trains should go on the left. But, that will change about as fast as the world gives up the incredibly inefficient QWERTY keyboard.
P.S. This message happily typed at 70 wpm with a Dvorak keyboard.
07-25-02, 11:03 AM
The issues of fractional (Imperial) sizing versus metric is quite complex. If the entire world were metric, it would be of course much easier. As it stands in the USA, metric taping and machining is more expensive and difficult to come by. Steel stock in the USA comes in fractional sizing, which means that fastener length in Imperial sizing matches. If a steel plate is 1/2-inch thick, a 1/2-inch long bolt fits perfectly, but a a metric bolt may be 10mm, or 15mm. Getting a metric threaded bolt in 12.7mm length means a custom bolt.
The bicycle industry does use some odd threading, unknown to most industrial engineers. The bicycle legacy is as much out of the barn and garage of early manufacturers/builders as out of R&D labs. If we could start over knowing what we know now, bottom brackets would be bigger, and cranks may not even have pedal threads.
For more on threading see http://www.parktool.com/repair_help/thread.shtml
Regarding chains, the agreed nominal chain pitch is 1/2-inch. However, manufacturers vary in their recommendations on replacement, and some even state they have no recommendations at all.
07-25-02, 11:18 AM
I have always been a bit amused and confused by the fact that bicycle hubs have metric cones and lock-nuts and imperial ball bearings.
07-25-02, 11:44 AM
Calvin, you work for Park Tool, aye? :) (if so, thanks for taking the trouble to make me a no-groove PRS-2-compatible 100-8C clamp :)) Or are you another of the same name?
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