Gearing

[kenji]

If you have a car or truck that isn't stock and is either mechanically
modified or built from pieces how do you determine how to gear the thing?

A manufacturer will give gearing starting points in their manuals and
instructions.

Do you just experiment with various spurs and pinions making sure the
turn of your motor doesn't exceed your ESC?

How best to determine a ratio?

[Peter J]

On Thu, 05 Jun 2003 13:47:34 GMT, kenji <yobubba@ameritech.net> wrote:

>If you have a car or truck that isn't stock and is either mechanically
>modified or built from pieces how do you determine how to gear the thing?
>
>A manufacturer will give gearing starting points in their manuals and
>instructions.
>
>Do you just experiment with various spurs and pinions making sure the
>turn of your motor doesn't exceed your ESC?
>
>How best to determine a ratio?


This was posted a while back - it may give you some ideas....



In article <3E9AB87C.71840106@bellsouth.net>,
rokman <rokman@bellsouth.net> wrote:

> I was going by what the book had in the back...is Associated's
> manual wrong?

Yes, it's very wrong :-(

In general, you want to use a 15-22 tooth pinion and an 80-87 tooth
spur.

For stock motors, aim for a final tranmission ratio(*) of 9-12. 9 will
get you moderate acceleration and high top speed, but the motor will
get freaky hot(**). 12 will give you good acceleration and low top
speed, but the motor will thank you for it. If you're a new driver,
you might want to gear for an even higher ratio.

For modified, you want a final transmission ratio of 10-15 (depending
a lot on the type of modified motor, and the desired acceleration
characteristics).

Your pinion/spur combo gives you a final transmission ratio of 8.5,
which is really too low unless you're running in a velodrome :-)

FYI, the final transmission ratio = (spur/pinion)*(basic transmission
ratio). For the T3 and your choice of spur/pinion, that's (85/24) *
2.4 = 8.5.

(*) The final tranmission ratio is the number of rotations of the
motor required to turn the wheels exactly once. So if the FTR is 12.5,
then the motor armature must rotate 12.5 times to turn the wheels
once.

(**) I once tried to explain to a kid at the track that running his T3
with a tiny spur and a giant pinion was a bad idea. He countered that
it was perfectly normal for the label on the motor to curl up and turn
brown, and for the batteries to make a fizzing noise. But he was
frustrated that his motor brushes seemed to wear out after only a
couple of runs.

Rick R.

[Rick Russell]

I wrote the original article. I should qualify it -- the advice was
for off-road stadium trucks with 2.2" diameter wheels. If you've got a
mystery car, you need to compute tire rollout to pick your
gearing. Rollout is the distance that your car travels when the motor
turns exactly once.

Rollout = (1/(Final Transmission Ratio)) * Tire Circumference

Or

Rollout = ( Spur * Tire Circumference) / (Pinion * Transmission Ratio)

In general, you want rollout to be in the 1 - 2 inch range. Low
rollout is for heavy vehicles, high rollout is for light vehicles.

If you don't know your car's transmission ratio, it's easy to find
out. Put a mark on one of the wheels and turn your spur gear. Count
the number of turns of the spur until the wheel has rotated one full
turn -- you should be able to get a very precise number if you watch
carefully. The number of turns of the spur required to turn the wheels
exactly once is the basic transmission ratio for the car.

Tire circumference is easy to measure, of course. Either use a
flexible measuring tape and measure around the tire, or measure the
diameter and multiply by pi (approx. 3.14).

Rick R.


> Yes, it's very wrong :-(
>
> In general, you want to use a 15-22 tooth pinion and an 80-87 tooth
> spur.
>
> For stock motors, aim for a final tranmission ratio(*) of 9-12. 9 will
> get you moderate acceleration and high top speed, but the motor will
> get freaky hot(**). 12 will give you good acceleration and low top
> speed, but the motor will thank you for it. If you're a new driver,
> you might want to gear for an even higher ratio.
>
> For modified, you want a final transmission ratio of 10-15 (depending
> a lot on the type of modified motor, and the desired acceleration
> characteristics).
>
> Your pinion/spur combo gives you a final transmission ratio of 8.5,
> which is really too low unless you're running in a velodrome :-)
>
> FYI, the final transmission ratio = (spur/pinion)*(basic transmission
> ratio). For the T3 and your choice of spur/pinion, that's (85/24) *
> 2.4 = 8.5.
>
> (*) The final tranmission ratio is the number of rotations of the
> motor required to turn the wheels exactly once. So if the FTR is 12.5,
> then the motor armature must rotate 12.5 times to turn the wheels
> once.
>
> (**) I once tried to explain to a kid at the track that running his T3
> with a tiny spur and a giant pinion was a bad idea. He countered that
> it was perfectly normal for the label on the motor to curl up and turn
> brown, and for the batteries to make a fizzing noise. But he was
> frustrated that his motor brushes seemed to wear out after only a
> couple of runs.
>
> Rick R.