Gear Ratios Relative to Motor RPM

[Fast Co.]

Hey guys,

I came across some excellent tutorials on the Austin Slot Car Club website yesterday - http://austinslotcarclub.com.

One of the tutorials mentioned that ideally, rear wheel RPM should be within a range of 7500 to 8500 RPM. I'd never heard that before, but the range seems reasonable to me. I'm assuming those figures are dependent on the track - most likely 7500 RPM for tight, technical tracks and 8500 RPM for faster, less technical tracks. I soon realized that i had never seen a gear ratio chart that factored in motor and rear wheel RPM.

So, I attempted to do that with the spreadsheet attached below. Assuming a rear wheel RPM of 8000 I arrived at ideal gear ratios for motors turning from 14k to 26.5k RPM.

I think this spreadsheet will take the guess work out of setting up gears for novice racers. An inexpensive digital tachometer is required to measure motor RPM at track voltage.

What do you guys think? Useful tool, or not?

Steve

http://homeracingworld.us/132mods/Gears ... Ratios.xls

[HomeRacingWorld]

Email it to me, I'll host it.

[dr fabio]

Id like to know the logic in how they came to those rpm values? How does it take into account the diameter of the wheel? The rpm and torque of the motor etc. ? A 20mm diameter wheel is going to give a very different rollout to a 30mm wheel even if you kept the rpm the same.

[Fast Co.]

The guys that posted the tutorial run 1/32 scale rtr cars such as Slot.Its and NSRs on a home routed wood track with an approximate lap length of 60'. Naturally, these figures would apply then to 1/32 scale cars running on a similarly designed circuit. As I mentioned, this was the first time I'd heard someone comment on ideal rear wheel RPM. When I thought about it the 7500 - 8500 seemed reasonable for a 1/32 scale car running tires of say, 17 - 21 mm diameter.

I put the spreadsheet out there to solicit opinions on whether other racers thought these figures have some validity - not as an authority on the subject. Of course, I realize there are other factors that these numbers do not address such as motor torque and gear mesh as well as the design of the circuit.

In practice, a motor's torque and braking have more to do with performance than maximum motor RPM. How do you convert torque figures, gear ratios and rear wheel performance - accelerating, braking and cornering into usable data?

It's academic. The proof is in the testing.

[Mayberryman]

I always attempt to gear my car according to the track. If the track has two major straights one a little shorter I want my car to top out at the end of the shorter straight. I will sacrifice a little speed on the longest straight for a car that has better brakes, pulls better in the corners and over all is a little easier to drive. For those of you who have driven on a commercial Engleman track you will understand that I always geared for best performance in the straight leading to the drivers panel. As far as RPM is concerned there are many factors in RPM, although I mostly wanted to over rev my motor to a certain extent the actual RPM will be controlled by the magnets and timing of the com. Also as stated tire diameter plays a part.