Tim Gobber, Mountain Biker, BMX Track Designer,
and Owner of Ozark Bicycles
More Career Connections
Bicycles are generally described
in terms of metric units. However, Americans generally
think in terms of pounds, ounces, and inches. So I always
do a lot of rough conversions in my head. A pound is
about 450 grams. I generally use 100 grams for a quarter
of a pound, even though it's a little more. One of the
most common ways I use math is when I am working with
someone to lighten their bike. People want to make their
bikes faster. According to physics, you want to lighten
the rolling mass. That means you need to lighten the
tires or wheels. That could mean going with a lighter
hub, rim, tube, or even a lighter tire. A full steel
bead tire, full knob, weighs about 800 grams. A hitech
kevlar bead, semislick tire, is about 520 grams. So
that is almost a halfpound difference per tire. That
only lightens the bike by a pound, but lightening the
rolling mass by a pound makes you a lot faster. The
wheels act sort of like fly wheels. The heavier they
are, the harder it is to get them spinning. There is
an analogy with weight lifting. The lighter the weight,
the more times you can lift it. Well the lighter the
bike, the more reps your legs can do, and so you can
ride farther, climb hills easier, and go down hills
faster.
Gear
ratios are something that 99% of all people don't consider.
But there is a real difference. On a mountain bike,
the rear gear is either a six, seven, or eight speed.
Each of those gears has a number of teeth that ranges
from eleven to thirtyfour. With a thirtyfour, you
will be able to climb any hill. It's a very low gear,
or "granny gear." The eleventooth is a faster gear.
You go faster with a slower pedaling gear.
On
a BMX bike, you have only one speed, and the rear gear
always stays the same. However, you do have some choices
for the pedal gear. You can set your bike up with a
pedal gear with fortytwo teeth or with thirtyeight
teeth. Since these are the gears on the pedals, for
the BMX bike, the lower the number the quicker it is
off the start. The higher the number the slower starting
it is, but you have a faster top speed. Our track uses
gravity a lot, so you want the higher top speed. But
if you were riding on a flat course where a quicker
start is needed, you would want to use the lower gear.
Also, shorter crank arms can allow you to pedal faster.
The crank arms connect the pedals to the bike. The longer
they are, the bigger the circle that your feet have
to turn, and the longer it takes to make a full rotation.
The trade off is the longer the crank arms, the easier
it is to pedal.
Cadence
is another important factor. Cadence is the revolutions
per minute that you pedal. These generally range from
70, which is pretty low, to racers that have a cadence
around 110. The important thing for a rider is to find
the cadence with which they are comfortable and to then
have the right set of gears that work for that cadence
and the type of riding they are doing. The beauty of
the gears is that you shift as you ride so that you
can continually keep the same cadence.
Another
thing you will notice about different types of bikes
is the size of the wheels. Physics would say that the
bigger the wheel, the faster the bike. Road bikes have
about a 27inch wheel, where mountain bikes have 26inch
wheels, and BMX bikes have a 20inch wheel. It makes
sense that one turn of a 27inch wheel will move you
a lot farther than one turn of a 20inch wheel.
We're
building a singletrack mountain bike trail and have
learned quite a bit about slopes through the process.
I have a tool that allows me to measure the slope of
a hill. From experience, I have found if you have a
20% slope, which is way too steep to ride across it,
you are at such an angle with the hill that very little
of your tire, just the side of the knobs, is actually
in contact with the trail. The problem then becomes
to find a path around this area, or to resort to digging
out a small path so you're not sliding down the hill.
In general, you are looking to give the rider a range
of experiences and some challenging areas without making
it is impossible to ride.
On
the BMX trail, there is a lot of geometry in the dirt
jumps. The way the jump curves determines whether it
is a good jump or not. The jumps need to be shaped so
they throw you up in the air. A straight slope or constant
incline, like a ramp, won't do the trick. With a bulldozer,
you can create a steady incline. Then you have to dig
it out in the middle and place the dirt on the top so
that the jump really comes when riders hit the top.
The landing ramp needs to be more of a straight incline.
The spacing between the jumps also needs to be just
right and is related to the riders, speed. Riders can
miss jumps that are too close together. On the other
hand, if the jumps are too far apart, riders can't carry
their speed from one jump to the next.
As
a kid, I was always on a bike. I had a Stingray. As
I got a little older, it was just the easiest, cheapest,
and most convenient way to travel. Today, a lot of people
young and old are getting back into different types
of riding. There is really quite a bit of mathematics
and science involved in bikes. A lot of people don't
understand the gears and the ratios, but if you work
at it, think about, and watch how the bike works, it
really does make sense.
