Jose Rizal <_@_._> wrote in message news:<
[email protected]>...
> Carl Fogel:
>
>
> > Most mechanics share Jim's fetish for tension values. They use torque wrenches (whose accuracy
> > is often less than admirable) to tension all sorts of threaded fasteners that undergo far less
> > variation than spokes.
>
> Not true in general, and certainly not most mechanics, auto or bike. In fact, my observation is
> quite the opposite; many mechanics palce a lot of trust in their ability to "feel" the "right"
> amount of tension/tightness in bolts, screws, and such things, especially when many of these are
> involved.
>
> > The more important the tension is, the more likely professional mechanics are to use torque
> > wrenches.
>
> Not convinced. Next time you have your car serviced, see how many mechs at your local garage use
> torque wrenches.
Dear Jose,
You may be confusing routine service folk with professional mechanics.
While the service fellows who change oil, tires, spark plugs, alternators, and even water pumps have
little use for torque wrenches, the mechanics who work on actual engines use them regularly, where
tension matters.
This is analogous to the fellows who tighten seat bolts, quick-release skewers, derailleur parts,
brake pieces, pedals, and even bottom brackets and axle cones, as opposed to people who are
tensioning spokes.
Even if you're just putting the brick-simple cylinder head back on an air-cooled single-cylinder two-
stroke (a finned plate with a dome, a spark plug hole, and some holes for the mounting studs), you
want to use a torque wrench in the prescribed pattern, which usually consists of going around all
the nuts in a criss-cross pattern at a low tension and then again at a higher tension.
The reason is that you want to press the head down as evenly as possible onto the soft metal gasket
and to have the tension close to what the engineers figured out would be a good idea when the
violent explosions underneath the head begin heating everything up and the studs expand along with
everything else. (How much the gasket is compressed also affects engine compression.)
If you get the torque too far wrong, you'll notice nasty explosive blasts where the loose head
gasket is leaking, or else even nastier blasts where the too-tight head broke a stud and the gasket
is leaking even worse.
On more complicated four-stroke engines, with water cooling, and with multiple cylinders (cars),
things become even trickier. It helps to have things over-built so that you can get away with
mistakes, but if the fellow doing actual engine work on your car (as opposed to oil changes) doesn't
keep a torque wrench handy, find another mechanic.
The same sort of logic applies to the go/no-go gauges that professional bike mechanics like Peter
Chisholm, Andrew Muzi, John Dacey, Sheldon Brown, and others use to check threads--they get better
results when they measure. And no engine mechanic wants to set valves or old-fashioned breaker
points without feeler gauges.
Think of tire pressure. We can estimate it by squeezing well enough, but most of us end up measuring
it because it gives better results than the seats of our pants.
For bicycle spokes, the usual idea is to tighten things until you notice local rim yield and then
back off a bit (or maybe a smidgen, which equals 5/4's of a bit). Helps to remember to lubricate
things first, and I forget if it's only 0.5 smidgens if the rim begins to yield when you're grabbing
the spokes to stress relieve them.
But with fewer and fewer spokes and lighter and lighter wheels, more and more people are starting to
think like real mechanics and measuring spoke tension instead of using the seats of their pants.
Since bicycle spokes and nipples are so tiny, torque readings at the nipple are unlikely to say much--
even tiny imperfections in the threads or in how the spoke seats on the rim (even when oiled) make
torque readings next to useless (think of spoke wind-up and its effect on a torque wrench). But a
tensio-meter gives a fair notion of tension and helps people build better bike wheels.
As always, the old guard disdains and even rails at the new-fangled notion. Doctors do this,
computer programmers do it, mechanics do it, even chefs do it. In Mark Twain's day, the riverboat
pilots scorned the notion of getting together in a union and sharing information about the depths,
snags, and marks needed to avoid sinking steamboats. Eventually, the insurance companies forced the
old boys to join the modern world, and fewer steamboats sank.
Any day now, I'll break down and buy a tensio-meter. I don't build wheels, but Jim Beam has
privately hinted in a gracious and tactful fashion that it's a cheap and obvious tool for anyone
interested in this kind of stuff.
Carl Fogel
