Help understanding gears

[CrewChief6]

We're trying to understand gearing a bit better. It's easy to find out how to get to a particular RPM using the ratio chart, but how do you figure out what's the right RPM for a particular track or driving condition without asking someone else?

How do driving habits and track conditions affect what you choose for the gears? Is a larger engine gear better for some conditions? Larger axle gear?

Is there a general rule of thumb for gear selection?

Example: At a race, someone told us that our RPMs were too high for a short flat track, that we were running out of steam coming out of the straightaway. Why would a too-high RPM do that? We went down on the axle gear by 3 teeth and went faster. Why?

Is there a good place to find some documentation on this?

Thanks for your input.

[goffin20]

The larger engine vs larger axle gear is pretty irrelevant, the ratio is the ratio.

http://www.ziggysmotors.com/gear_charts.htm#gears


As for what RPM’s are better, it all depends on your motor.  I have had a 160 that loved to be cranked around 5500 high with a 5200 low and would scream and one that could not catch itself if it cranked over 5100 high with a 4700 low, both obtained the SAME lap times.  Each motor is different, some like to be cranked and some like to be lugged.  Your motor builder will provide a dyno sheet and give a recommended RPM range.

Some also say to look at the high and some say look at the low RPM’s.  I find my peak torque range and keep my low RPM’s about 200 higher than the peak torque RPM, the high is what the high is.

As for driving habits, I found that with a new driver that tends to make mistakes, getting loose and lifting off the throttle, more gear helped recover from those mistakes but if the race stayed green we were pretty much screwed, however, when there was caution after caution it was great.  The better the driver gets the more you can dial the car and gear to perfection without the worry of those mistakes or your compensating of gear hurting you.

As for finding documentation, I don’t know of any other than what I have read on the forums, by talking to others and my own experience.  Practice is the best way to figure out what your motor likes, go to the track with a handful of engine and axle gears and go from the extreme low to the extreme high, you may be surprised as I was with one of my motors.

And remember, just because someone tells you what gear they are running, factors like tire diameter and the engine do make a big difference between the gear on each car.

[CrewChief6]

Thanks Scott. As always, a fountain of knowledge!

Ah. OK, so I think I'm understanding a bit better. The best RPM would be whatever my motor's optimum torque range is. If the RPM is too high (or low) then I don't get the max power the motor puts out, so I lose momentum. In my case, the RPM was too high, so it revved beyond where I needed to sustain the power.  Does that sound about right?

What about the same ratio but different gear combinations? I think I remember someone telling us that a larger engine gear is better for constant speed (less traffic) but smaller engine gear is better for driving through traffic/passing. (Or was it the opposite?)

[goffin20]

It's like driving a stick shift car, you’re in 1st gear running 40mph at about 5000rpms, but you can feel the motor stop pulling as hard around 30mph at around 3500rpms, even though it will run to 5000rpms.  Your QM engine is the same, it will pull starting off and then pull harder and harder and then start dropping off, the goal is to keep it in the range where it is pulling hardest at its lowest point, through the corners.

As for the larger engine vs axle gear, I have tried both with the same results so its hard to say either way, some will swear by one or the other but as the motor builder below stated, ratio is ratio.