Designing the Ultimate Bike

No single bike design is the ultimate for all tracks.  
The following section will explain the effects of 
various bike properties on bike performance.  With 
this information and a little experimentation, you 
should be able to design bikes for all types of tracks.  Of course, you then have to learn to ride them.

Power and Weight

This one is simple.  To make the fastest bike possible, 
you always want maximum power and minimum weight.  To 
minimize the weight, minimize the bike weight, the 
front wheel weight, and the rear wheel weight.

Brake power determines how much braking occurs when 
you bring your throttle down to decelerate.  You will 
probably want to set this at it's maximum value so 
that you can slow down as quickly as possible.

Bike Geometry

This requires a little more thought.  The parameters 
that affect overall bike geometry are front and rear 
wheel radius, front and rear distance, front and rear 
height, and rake angle.

In general, you want the center of gravity (the 
location of the weight of the bike) to be as low as 
possible.  A top heavy bike tends to flip over more 
easily (especially when you try to accelerate quickly).
  So front and rear height should be kept as low as 
they go.  The only reason to increase these values 
might be for appearance sake (e.g. creating a tall 
"Monster Bike").

 A large wheel radius tends to roll over small bumps 
easier, but a large radius make the entire bike taller 
and hence raises the center of gravity, which works 
against you.

The front and rear distance determine how far the 
front and rear suspension and wheels are located from 
the center of the bike.  Increasing these values tends 
to make the bike more stable for two reasons.  First, 
increasing the wheel base makes the geometry inherently 
more stable because it moves the points that contact 
the ground farther from the center of gravity.  Second, 
increasing the wheel base increases the moment of 
inertia of the bike which makes the bike more 
difficult to rotate.  Keep in mind that a stable bike 
may not always be what you want.  Increasing the moment 
of inertia decreases the "gyration" effect which is 
explained in "Riding Tips".  A simple and important 
fact to remember is that increasing the rear distance 
allows you to accelerate faster without flipping over 
backwards.

The rake angle is the angle between the frame and the 
front suspension.  A "chopper" usually has a very high 
rake angle, which makes the bike look cool, but doesn't 
help the handling very much.  A lower rake angle allows 
the front suspension to absorb the bumps in a vertical 
direction better (which is the direction that most of 
the bumps affect the suspension), but you also want to 
absorb the bumps in a horizontal direction as you run 
into them, so some rake angle is usually desired.  
You'll have to play with this.

Suspension

This probably the most important aspect to designing 
the ultimate bike (and also the most difficult) is the 
suspension.  The suspension consists of the front and 
rear travel, the front and rear spring rates, the front 
and rear compression damping, and the front and rear 
rebound damping.  The wheel weights also affect the 
suspension, but as mentioned above, it is gererally 
best to have these weights set to their minimum values.

When designing suspension for a particular bike, there 
are some general concepts to keep in mind.  The first 
goal is to keep the rear wheel in contact with the 
ground as much as possible since acceleration is caused 
by the power transfer between the wheel and the ground. 
 Second is that you want to use all of the suspension 
that you have.  In general, you want the suspension to 
"bottom out" (you hear a "clank" when it bottoms out) 
once or twice in a race.  This way, you know you are 
using all of the available stroke.  If it bottoms too 
much, your suspension is too soft and if it doesn't 
bottom at all, it might be too stiff.

The suspension travel is the distance that the wheel 
can travel up and down.  For a motocross bike, you 
generally want this to be as large as possible so the 
suspension can do the best job at absorbing bumps and 
the impact from high jumps.  Increasing the travel 
raises the center of gravity, however, so you will 
want to bring the travel down when designing a drag 
bike,  a hill climb bike, or a trials bike.

The spring rate is the stiffness of the spring that 
the suspension rides on.  A higher spring rate will be 
able to absorb the impact from large bumps and high 
jumps better, but a lower spring rate will allow the 
suspension to absorb small bumps better.  Keep in mind 
that if you lower the suspension travel, you will 
probably have to increase the spring rate since the 
spring has less distance in which to do it's job.

If you have a suspension with springs and no damping, 
the bike will just bounce like a pogo stick.  For fun, 
you can try this by setting all damping to zero.  In a 
real bike, damping is done by the oil in the shock 
absorber.  As the suspension compresses and rebounds, 
the shock resists, or damps, the motion.  If you push 
down on the hood of your car and then let up, the car 
should come back up and stop.  If your shocks are bad, 
it might bounce up and down a few times.  If you had no 
shocks, it would bounce up and down and up and down for 
a long time.

The difference between compression damping and rebound 
damping are when they work.  Compression damping works 
when the suspension is being compressed and rebound 
damping works when the spring is pushing the suspension 
back out.  If your rebound damping is set too high, 
the suspension will compress when you hit a bump, but 
the damping will keep the suspension from recovering 
(the wheel coming back down) before you hit the next 
bump.  In an ideal situation, you would want the wheel 
to be all the way down for each bump.

Compression damping resists the compressing motion of 
the suspension.  It acts to stiffen the suspension, 
like the spring rate does, but since it's not a spring, 
it doesn't help to provide the force to push the 
suspension back out.

The trick to designing the ultimate suspension is in 
finding the proper balance between spring rate, 
compression damping, and rebound damping.  Trial and 
error is often the best teacher.  Keep in mind the goal 
of keeping the rear wheel in contact with the ground as 
much of the time as possible. 

Gear Ratios

Gear ratios determine the relative speeds between the 
engine and the rear wheel when you are in a particular 
gear.  The most important setting to adjust is probably 
the ratio for the highest gear.  The higher you set it, 
the faster your bike can ultimately go on a flat 
surface, but the higher you go, the less torque you can 
generate to accelerate your bike.  This is why lower 
gears have lower ratios.  You use those lower gears to 
get the torque you need to get the bike moving, then 
switch to the next gear when you start to run out of 
engine speed in that gear.  The gear ratios should be 
spaced so that you can increase speed from one gear to 
the next.