All dynos have a few concepts in common. An electric motor simply converts stored energy from a battery into a combination of kinetic energy and heat
energy. Input power (stored energy from battery) will always equal output energy (kinetic energy & heat energy), since energy cannot be created or
destroyed. The power number dynos show you is the
kinetic energy number. The effeciency number is simply the percent of the input energy that was converted into kinetic. For example, a stock motor that
is 70% effecient will convert 70% of the input power into kinetic energy while converting 30% of the input power into heat energy. Keep this in mind
because increasing effeciency also increases the kinetic energy. This concept often gets overlooked. Too many people have a misconception that
effeciency has something to do with fuel economy and can somehow be ignored simply because they use high capacity cells. The truth is that effeciency is
extremely important, because you want to turn as much input energy into kinetic as possible.
It is also very important to keep in mind that stock motors run at certain amp draws during a real race, and you need to tune your motor for the amp steps
the motor will actually be running at. I always assume my touring car will run at 20 amps on the straight and 28-45 amps coming out of corners. These numbers
may be off by a little, but should generally be a good starting point for our tuning purposes. Based on these assumptions, we will want to tune the motor to
perform well at each of the amp steps from 20 to 45 amps. Get the power number as high as possible at each amp step. In general, you can ignore RPM and
torque until you are ready to figure out your gearing. If you focus on getting the power numbers as high as possible at every amp step, the motor will be
fast when geared properly.
In the stock motor tuning article, I showed steps for cutting down friction in the motor. Cutting down friction will increase effeciency. Picking up
even a couple percent more effeciency will noticably improve the amount of power the motor can deliver. And like I already mentioned, more effeciency means
more power.
Certain motors, especially high RPM motors, will benefit from slightly reducing the width of the brushes. Reducing the width of the brushes reduces the
chance of shorting the brushes across the comm segments. Laydown brushes are actually a little bit too wide, and this causing some shorting. Cutting a
sixteenth of an inch off the leading edge of each brush will eliminate this shorting. The new Trinity Monster Stock motors will often gain 4-5% more
effeciency (especially at lower AMP steps like 20-23 AMP) from leading edge brush cuts. This increase in effeciency directly leads to an increase in power (kinetic energy).
One mistake people often make when tuning for dyno results is to look only at max numbers, such as max power, max RPM, and max effeciency. While these
numbers can be used to quickly compare two motors, the amp step numbers will provide a better comparison. If the max numbers occur at amp steps that your
motor never sees on the track, then the max numbers are meaningless. For example, I have seen max effeciency numbers as high at 85% with stock motors, but
the max effeciency was located at 10 amps. During a race, a stock motor is most likely never going to see 10 amps, so the impressive max effeciency number
is basically meaningless. I always try to make sure my stock motors are at least 70% effeciency at 20-25 amps and at least 65% effecient at 28 amps. These
numbers will vary from one dyno to another, but I provide these numbers so you can get the basic idea.
One of the most useful tuning options is the spring tension on the brushes. If you have too much spring tension, then your brushes are acting as disk
brakes on the comm and that converts more energy into heat. If you have too little spring tension, then the brushes will lift away from the comm and
cause arcing. Arcing is another way to waste energy as heat. So you need to find the right amount of spring tension for each brush to keep the brushes on
the comm without increasing friction. What I do is hook my Robi dyno to my PC and look at the friction number. On my Robi, a friction number above ten is
too high and friction below five is too low. I add or subtract spring tension until my friction number is about 6-8. This may vary widely from one dyno to
the next. The key is to get the friction as low as possible when building the motor before you even put the brushes in the motor. That way your friction
number is merely a function of spring tension.
There are quite a few other tips that I could post here, but these tips should get you rolling in the right direction. If you have some good tips that you
want to add to this article, post them in the forum.
