Gear ratios - what matters?

[slothead]

I understand the gear ratio controls torque and rpm at the drive wheels, but I don't fully understand how the pinion and crown gear (thinking inline here) each contribute to that.

All things else being even, how would a 9/27 setup differ from a 10/30 setup. Any why?

[4424ever]

In a 1/32 car application there is no difference other than whether the gears physically fit into the space available in chassis and tire combination
In a 1/24 drag car it’s a whole different story All power to motivate the car is applied through the pinion so at those power levels the smaller the pinion the greater the chance is you will strip a gear

[Kevan]

Gear ratio is a good start but rollout is more important than gear ratio alone, once you've found the sweet spot for the motor take a note of it, then you can play around with gear sizes to keep the same rollout in the knowledge the motor is still in it's sweet spot. As far as gear sizes go then smaller is going to give you less rotating mass which can be advantageous coming out of and going into bends.

[slothead]

What is the 'standard' gearing used on RTR cars? Such as on the new Scalextric inline cars.

I've fiddled with gearing on my own builds, all 1:32 scale inline cars with 1/8" axles and set screw crown gears, but just stick with what comes in RTR cars.

Also, does the numerical equivalent of an inline ratio work the same for sidewinders or anglewinders? I admit I have no idea what the gearing is on all my Scalextric, Pioneer, Fly, Carrera, Monogram, etc. sidewinders.

[4424ever]

In a 1/32 rtr no matter what the configuration MOST cars will come at 3:1 or very close to it

[strangebrew]

I'm not an engineer, but I think you'll find the more teeth on the pinion will give you more torque.
10:30 vs 9:27 is the same overall (top end) gear ratio, but I've been using the bigger pinion
on the new 18K Predator motor with good corner exit results
& have been gearing the Predator 10:25 or 10:26 for better straightaway speeds to
compensate for the lower RPM.
I told Harry I plan to beat some Piranhas with an 18K Predator for the next Showdown.....
............Brew

[slothead]

I saw how your latest build was focused on being lightweight. Is that one reason you didn't (don't) use motor mounts?

[Kevan]

You can't change the torque of a motor by gearing, you can however change the output curve by gearing

[dinglebery]

...All things else being even, how would a 9/27 setup differ from a 10/30 setup. Any why?

Size in diameter of each gear would be the only difference, if any, like previously hinted at by 4424ever. Here's a quick calculator that gives you more info.

[Kevan]

If the gears were the same diameter (variable pitch) there wouldn't be any measurable difference but if they were fixed pitch so the 10/30 were physically larger in diameter they'd take a little longer to spin up and slow down.

[slothead]

I put some different numbers into the calculator - 8/24, 9/27, and 10/30. These are all 3:1 ratios and each produced the same output values for rpm and torque for the same input values. It makes sense that the diameter of a gear is like a lever and influences how power is transferred, but this is not obvious if you only take the number of teeth of a gear into account. A larger gear should have a larger diameter, and a very small difference might be large percentage wise. The mechanics of a pulley system are impressive but not easily understood.

[Kevan]

Slothead, a larger gear should have a larger diameter but they don't always because they keep the diameter the same but change the pitch on some gears so you don't have to move the motor.

[mattb]

Several things.
Brew, do you really think the 10-30 provides more torque than a 9-27 or 8-24?? I tried and wondered a lot about whether the size of the gears makes any difference. I do see a bigger pinion may mesh a bit better than a small one. Anybody else got opinions/testing about this?

I have figured that it should make a little better handling car if an inline crown gear is on the outside of a circle track car or for a road course on the side of the chassis that will be on the outside of most curves. In practice I haven't seen any difference. but there is some sideways momentum involved in this.

A more experienced and knowledgeable guy told me once that a slot car should be geared to use torque and not rpm's. He said that an electric motor has instant torque and gearing for torque is the best way to set up a car. We kind of listen and if a car is wound out about 1/2-2/3 down the straight, then we go a to a smaller crown. I always figured that you should hit top rpm's at the braking point of the longest straight.

What I've found out is that nothing is the same with all the cars I set up!!!!!

[Fathead59]

Not sure if this will be the same but I was going to built a couple of drag cars and was looking at gear ratio for that . I know that round track gearing is going to be a lot different than straight track gearing

[slothead]

I would think the most important thing with a drag car is getting the car moving WITHOUT wheel spin. If power is going to be applied all at once then torgue would be your friend and rear tire rpm your enemy. If power were going to be applied gradually then overall rpm might be most important.

[Fathead59]

I understand that you do not want wheel spin right out the bat , but with the correct gearing you should be able to get the right speed

[chappyman66]

Kevan is right. Torque and RPM depends on the motor, not the gear ratio (more specifically the stack/wind/magnets/air gap).
What gearing does is what Matt said.....changes acceleration and how long it take to get to the top end. In a SW/AW configuration, it also impacts the size of tire that you can run.

Some motor may have slightly more RPM than a Slot.It orange, but without the torque it takes longer to spool up. On a long commercial track the RPM rules....but on smaller home tracks, torque/acceleration rules. Sometimes a 3.5:1 or even 4:1 is better for a short track to give the acceleration along the shorter straights of home tracks. But Matt is right....if you are winding out at halfway down the straight, you need a different ratio.

Again, with SW/AW you have a gear restriction based on tire size. With a given ratio and motor, taller tires make more speed (larger distance per rotation) but that is offset by the change in handling due to raising the center of gravity. If you change pitch, you can change spur size but you also need to change your pinion pitch to match.

I will also say that I have seen cars totally change character with a change of 1 or 2 teeth on the gear, so having a range available can make a significant difference for some cars. They just "wake up" when the ratio is right for the power band of the motor.

It's all experimentation and compromise. The standard 3:1 is a good compromise ratio, and allows inexpensive manufacturing as well as the opportunity to sell spare parts.

[SpeedyNH]

in a lot of plastic slotcar gears, they fudge the teeth away from an actual gear tooth profile. sometimes they end up making them just about triangular, which is not a good thing. gear teeth are supposed to roll against each other at the point of contact, not slide.
(my uncle taught mechanical engineering courses about gears at Rhode Island School of Design, and after reading some of his books, i got to be real anal about it.)

for a given ratio of inline gears, i go by whichever size pair gives the better mesh. and the way these guys make 'em, that's usually the largest pair which will fit in the car. i sacrifice things like small differences in rotating mass every time to get a low-friction mesh; for me, drivetrain efficiency is king.

for sidewinder gears, you will often find that they are grouped by pitch circle diameter, with several different numbers of teeth available in both pinions and spurs for the same given size so that the motor doesn't have to move relative to the rear axle in a plastic chassis in order to change ratios- they stretch out or squish the teeth together so that the distance between the rear axle and the motor shaft remains the same. sometimes a lower-tooth-count spur in a given size will be a real bear to get to mesh up because from the side they look more like a cartoon mountain range than an actual gear, or one which has been worn out or raced and destroyed from being poorly meshed.

try it yourself and see.

[slothead]

Back in the 60's a slot car magazine had an article about how to make a 2 speed 'transmission' in a slot car. The drawing showed an inline chassis with 2 crown gears on the rear axle, each engaged with the pinion gear on different sides. Springs would be used to determine which was driving the rear axle at a time. When the motor was spinning one way the spring on one of the crown gears would tighten and lock that gear to the axle driving the car. To change gears the polarity would be switched, the motor would spin in the opposite direction causing the formerly 'active' crown gear to free wheel as the other one now took over driving the rear axle.

Such a setup might be advantageous for drag racing to switch from a short grown gear (small size and fewer teeth) to a tall crown gear (larger size and more teeth) once the car got going off the line. For either an oval or road course I don't know how a motor would respond to being reversed multiple times a lap.

I was fascinated by this idea as a kid but never knew how to get or attach springs to gears and axles in such a way that they would slip in one direction and tighten in the other direction. I'm sure there are other ways to make gears spin freely in one direction and lock in place in the other direction, but since I've never heard of a 2 speed slot car actually being made I guess it was a one of those things that seems theoretically possible but not in reality.

[SpeedyNH]

i made one of those. took too long for the motor to reverse, and of course the good ones ran better in one direction.

[chrisguyw]

Torque and RPM depends on the motor, not the gear ratio (more specifically the stack/wind/magnets/air gap).
What gearing does is what Matt said.....changes acceleration and how long it take to get to the top end. In a SW/AW configuration, it also impacts the size of tire that you can run.


I will also say that I have seen cars totally change character with a change of 1 or 2 teeth on the gear, so having a range available can make a significant difference for some cars. They just "wake up" when the ratio is right for the power band of the motor.

Chappyman makes some good points to remember..............particularly his last sentence above.

Getting the optimal ratio involves various elements, rear tire dia., car weight, track design, tire grip etc. etc., but,... critically , a specific motor's characteristics, must be the guide to an "ideal" gear ratio.............a high revving/low torque motor, must be geared entirely differently (numerically higher), than a lower revving/higher torque motor.

I have seen tons of cars in the shop that have been incorrectly geared,....their owners trying to make their specific motor do something it was just not designed to do.

Cheers
Chris Walker

[RichD]

There actually were Aurora Speed Shifter HO cars that worked as slothead described. One crown gear would catch when the motor was turning in one direction and slip when it was turning the other direction. The other crown gear worked the opposite way. I believe that the cars were dragsters and a shift lever changed the direction of the motor. You would think that having the motor reverse part way down the strip would not make for better ETs. I believe that the clutches did not last very long.

[docdoom]

raced nitro sedan for many years and those had two speed gear sets in a side winder style set up. I all ways wondered if one could transfer it over to slot cars,,,,

[Kevan]

2 and 3 speed gearbox works well with a nitro mainly because they have such a narrow power band compared to electric motors.

[Gascarnut]

The problem with transferring that type of two speed transmission to a slot car is that you only get brakes while the transmission is in second gear. As soon as it shifts back to first, no brakes, because of the one way clutch. There was a British off-road electric RC buggy years back with a two speed, and that's the issue we had with it.

[docdoom]

Back in the early 2000's HPI rc company came out with two speed conversion set up that bolted onto off road style buggy and trucks. Biggest issue was they could not handle the torque from the electric motors.