One of the areas in which I think BMX riders excel over their
peers in other cycling sports is possession of the ability to
perform basic bike maintenance. I have done a lot of road riding
over the years, and I probably can't count the number of times
I've come upon a stranded rider who had no idea how to fix his
bike. Most of the time these unfortunate souls were suffering
from nothing more serious than a broken derailleur cable or something
of similar import, but they were stuck in their tracks for the
lack of knowledge to solve what should have been a simple problem.
Roadies have a traditional image of being very bike-savvy people,
but for every shaved-leg Bernard Hinault who polishes the insides
of his STI cable tunnels to assure smooth ratchet engagement,
there are ten guys who burst into tears when their cranks begin
squeaking. A lot of these riders actually hate bicycles and are
only into long-distance cycling because you can't carry three
water bottles while running. You can't expect them to be too interested
in what makes their bikes tick.
Those of us who leave the ground during our bike races, however,
are quite different. We like bikes and like to fiddle with them,
particularly if it will make our scoots look cooler or more hardcore.
And most of us have some time on our hands, so we can afford to
spend time figuring out how a bike works. This is usually done
by disassembling the bike and then putting it back together wrong,
several times.
There are a couple of areas of bike maintenance where BMX riders
do kind of look like Neanderthals to our contemporaries, though.
I don't know a rider out there who has taken the time to put his
frame on a frame jig to check its alignment. Nor do a great many
racers take the time to clean their brake shoes or grease their
brake cable. But where we really drop the ball is in wheel maintenance.
Those of you who understand how a bicycle wheel works may leave
the room now. Hey! You in the back! Sit back down! Almost no one
in BMX knows how a wheel works. So our purpose this month, after
what has proven to be a rather dragging introduction, will be
to learn to understand a BMX wheel, so that we may extend the
life of ours.
The spoked wheel as we currently use it is one of the great inventions
in cycling. Let's compare it to a wooden spoked wheel. A wooden
spoked wheel, say from an old horse-drawn carriage, had three
important parts: the hub, the spokes, and the rim. The spokes
went between the hub and the rim. The rim sat on the ground and
held the weight of the carriage. As the wheel rolled around, the
spoke that was on the bottom would be compressed for
the full weight of the carriage. Let's say that was 1000 pounds.
When the wheel continued rolling, the next spoke would take the
load. If any of those spokes weren't good for 1000 pounds, each
and every time, over and over again, the wheel would break. This
happened all the time.
This is also the way the wheels on a car work. If you've got a
Porsche 911 with Antera five-spokers, each spoke has to support
the weight of the car as it rolls around. If it can't, it breaks.
This is a pretty straightforward way to build wheels. It also
results in wheels that are very heavy. How could you expect a
ten-pound wheel to hold up a two-ton car?
Those of you who have an interest in frames and framebuilding
may recall that steel has a lot of tensile strength, which means
you can pull on it a lot without breaking it. Steel is not real
big on compressile strength, however, so you can't put a lot of
weight on it without being in for trouble. A regular steel wheel
is a pain because it's constantly being squeezed, which it isn't
very good at. How could we use steel's tensile strength, instead
of its compressile strength, to make a wheel?
Here's the answer. A spoked bicycle wheel is created by putting
more than a hundred pounds of tension on each spoke. When you
look at a bicycle wheel hanging up at the bike shop. each spoke
on that wheel is pulling the rim like crazy towards the hub. The
rim is designed to resist this pressure, but it is still under
tremendous tension.
When you sit on your bicycle, the tire and rim bend slightly where
they touch the ground. This decreases the tension on the spokes
holding that section of rim under pressure. So if your rim is
under five hundred pounds of tension from the spokes near the
ground, and you are putting one hundred and seventy-five pounds
on that back wheel, you are not loading the wheel at all! You're
simply making it easier on the rim. Now it only has three hundred
and twenty-five pounds of worry to deal with. When you take your
weight off that section of the rim, it returns to round form because
that will equalize the pressure from all the spokes. Equal pressure
from all spokes will almost always result in a perfectly true
wheel.
As your wheels roll during a race, pressure is reduced and replaced
on the spokes thousands of times. They don't mind. It's what they
were designed to do. A well-built spoked wheel will last nearly
forever. As a matter of fact , the Wolber rims on my Cannondale
are about to celebrate their tenth birthday of fairly frequent
use (Too bad the rest of the bike is a piece of junk--JB).
The difficulty with BMX is that a hard landing can momentarily
put so much pressure on the rim that it exceeds the spoke tension.
When you all of a sudden give the spokes nothing to do, they unwind
in their nipples, which gives the rim uneven tension when the
load is removed. With uneven tension pulling on it, the rim will
form an uneven shape. This is an out-of-true wheel. We can true
it by returning tension to the spokes that have been loosened,
as long as we do it within a couple of weeks. Most of the damaged
wheels out there could have been fixed without any aftereffects
if their riders had known what to do at the time.
Occasionally a rider will land hard enough to wipe all the tension
off the spokes at the bottom and the top of the wheel. When this
occurs, the hub will "hang" from the top side of the
rim by its spokes. At this point, the rim will be under no tension
whatsoever. The force of the impact will also bend the rim, since
the rim was never designed to be put under that kind of pressure
without nice, even spoke tension to keep it round. A large "clunk"
will be heard as the rim bends. When this happens, it's usually
time to replace the rim. You may have to replace the spokes, too,
because once a spoke loses tension it will bend very easily. Remember,
a spoke is designed to pull, not push. It won't take more than
a little bit of pushing without giving up and bending. A bent
spoke can be retensioned, but it will never do its job very well
again.
A wheel builder creates a wheel by laying out the spokes, rim,
and hub, assembling them, and very carefully adding tension a
little bit at a time. As he does so, the spokes will start to
rub against each other and build up stress, so he will "stress-relieve"
the wheel to prevent the spokes from pulling improperly. A truly
great wheel builder will put the precise maximum amount of pressure
on each spoke without breaking it, so that the wheel can withstand
tremendous impacts. A master wheel builder will recognize the
individual personality of each rim and work to strengthen it.
When the wheel is returned to him for re-spoking, he will choose
different spokes in consideration of the wheel's age and usage
patterns.
This is not how your wheels were probably made. Chances are that
a machine assembled your wheels using a giant, many-headed device
to tighten all the spokes at the same time. This results in a
decent wheel, but also fails to remove the little stresses that
are built inside the wheel as the spokes are tightened.
Or, some kid at a bike shop could have misassembled your wheel
by tightening it too fast to save time, failing to use a spoke
gauge to assure maximum tension, and loosening some of the spokes
to make the thing appear perfectly true as you spin it between
your hands at the shop. Wheels built in this manner will not survive
long without bending.
Those of you who have a true interest in how to build a great
wheel would do well to read The Bicycle Wheel by
a fellow named Jobst Brandt. In 1990 and 1991, I participated
in a series of Netnews discussions with Mr. Brandt and had the
chance to benefit from his wisdom. There is nothing worth knowing
about bicycle wheels that he does not know. His book puts pictures
and words together to create a truly easy-to-learn environment
for the student of wheels. He discusses the failures of radial
lacing, the nature of different varieties of spokes, and lays
out a set of instructions for building a proper wheel.
Not too many people are really interested in wheelbuilding, unfortunately
for technophiles like me. They just want to have straight wheels
that don't weigh a ton and don't cost five hundred bucks. For
that reason, we'll spent next month talking about how to true
a wheel properly, how to save a damaged wheel, and what to look
for when shopping for new ones. Keep an eye out for me in the
meantime. I'll be by the side of the road, helping roadies straighten
their bars.