Saturday, June 25, 2011

Tolerance

Many people think of bikes as being precision machines, akin to Swiss watches or fine German automobiles.  In reality, even the finest bicycle is a compendium of ill-fitting parts, loosely assembled into a rolling mish-mash of frame and components.

Yeah, I may have a bit of a skewed view of things.  But, you would, too, if you had worked in a bike shop service department as long as I did.

The thing is, the individual bits and bobs are nicely made.  STI shifters are watch-like in their complexity and precision.  (They are more Timex than Rolex but, still, watch-like.)  Frames are casrefully assembled, for the most part, and are even sometimes correctly aligned right out of the box.  As a matter of fact, the carbon frames had better be, because there is nothing you do to fix them, if they aren't.

Brake rotors are precision-milled.  Forged cranks can be beautifully finished.

Where the whole thing falls apart, sometimes literally, is in the interface between a component and other components, or between the frame and components.  The bolt-holes in that  beautifully finished crank, and the chainrings that go with it, don't  always line up.  In fact, they rarely do, and the chainrings run out of round because of it.  Both pieces are manufactured to a certain tolerance, but if one is on the plus side of that tolerance, and the other is on the minus side, it becomes noticeable.

Try turning your Dura-Ace or Record crank backward, and watch your rear derailleur as the chain runs through it.  It swings back and forth, slightly, taking up the slack that the out of round chainring produces as it spins.

One great example of the loose tolerances rampant in the bike industry came to light in the mid-1990s.  For a couple of years, TREK matrix rims were apparently being produced on the high side of the diameter tolerance, while Continental tires were on the low side of that same tolerance.  Both were acceptable, and the the TREK rims worked fine with most tires.  The Conti tires worked well with most rims.  But, when fitting Conti tires of that age to Matrix rims of that age, you were more likely to break every tire iron you owned than you were to get the tire onto the rim.

I remember spending over an hour installing Town and Country tires on Val's wheels, one day.  I ended up using the redneck tire iron (8" screwdriver) to finally get the damn things on.  At least I didn't have to worry about the tire rolling off of the rim!  I told her that if she had a flat, she would just have to throw the wheel away, because I really didn't want to try to get those things back off.  (I later did take  the tires off, at a trailhead in Moab, to install knobbies.  The T&C's never went back on that bike, however.) 

I think that kind of stuff is one reason why, perversely, I enjoy working on bikes.  It usually seems closer to blacksmith work than engineering.

Sing along!  "If I had a hammer..."

x

3 comments:

  1. I love my Contis, but I don't have them mounted on old Trek rims...

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  2. I never owned that brand of Conti or the Trek Matrix rim, but I do have my own "redneck" tire levers --- forks (silverware). I have to try the "screwdriver-lever" now LOL!

    Peace :)

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  3. This post reminds me a recent one on the Off the Beaten Path blog on the lack of integrated design in bicycles. The response I thought of to that post also involved Trek Matrix rims, along with the rest of my Trek 950 mountain bike, which from its odd proprietary seat bag attachment system down to its Trek Big Kahuna tires, appears to have been attempting to address the lack of integrated design issue, as well as the mismatched precision problem you mention. But "System 2" did not catch on, neither did System 3 or System 4. Something about market forces and the whims of consumers, I'm certain.

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As always, sorry about the word verification. It's a necessary evil, unfortunately.