# Help me choose the right gearbox

1. ### 9988776655

35
Im building a model car with mass 0.3kg. Im thinking of buying a fancy 6 speed gearbox. But before I spend my money, I need to know if any of the gear ratios are going to move my little car.

Here is the motor:

the gearbox has ratios:
11.6:1
29.8:1
76.5:1
196.7:1
505.9:1
1300.9:1

If I wanted maximum speed, but I want my car to run, which gear ratio should I use?

I have four wheels, each 0.0028m in radius. Im using a 3V battery. The gearbox drives an axle which runs through two wheels. The torque applied by the axle is through the centre of rotation. The coefficient of rolling friction between the ground and the tire is about 0.01, maybe a bit less since the tires are so small.

I know that F = umg where u is the coefficient of friction but thats it. Please teach me how to work out a similar problem for next time. I have a feeling that I will be building alot of model cars in the future.

2. ### Danger

9,879
I must admit that I'm a bit baffled by the question. The one serious advantage of an electric motor over an IC one is that you don't need a transmission.
Your ratios look way out of whack to me, as well. Are you trying to get this thing supersonic or pull stumps with it?

3. ### 9988776655

35
So any gear ratio will work? well my car is also remote controlled and it has steering which will be pretty impressive. Ill put a picture up when its done.

I dont see how they gear ratios are out of whak. All I want is a fast little car to race my friend's one. Here is the product specification:

http://www.tamiyausa.com/product/item.php?product-id=72005

If my vehicle weighs 0.3kg and all the weight is in the middle and it is 0.2m long, the required torque from my axle is:

Mo = mgL
Mo = 0.1m*0.3kg*9.8m/s
Mo = Required torque = 0.294Nm

But I have 4 wheels but i dont know how this effects my torque calculation.

working torque = stall torque / 2 <------ should i rather use torque at maximum efficiency here as given in the motor spec in previous post?
= 0.00686N / 2
= 0.00843N

Gear ratio = required torque / working torque
Gear ratio = 0.294N / 0.00843N
= 34.9:1

But my spur gears arnt that efficient
Would this calculation be correct??

Last edited: Feb 26, 2008
4. ### Danger

9,879
I think that I see the difficulty here. (There are a lot of other people here who know far more about it, though, so don't take me at my word.) That gearbox appears to be intended for robotic applications, where you might need incredible torque increases over what the motor itself could provide directly.
Have you even considered checking the math to see how fast your little car would go with a 9,400 rpm motor geared down 1,300:1? As I said, it would be a stump-puller, except you wouldn't have any traction to make use of the torque. If you had the tranny installed backwards, as I originally thought that you meant, you wouldn't have enough torque to push a feather out of your way, but you'd have the sucker moving like greased bacon through a goose's ass once it managed to accelerate.

5. ### 9988776655

35
So it would go pretty swiftly but will it be able to accelerate with 1300:1?

6. ### Mech_Engineer

2,299
This "R/C" car you're planning on building sounds like its WAY smaller than any standard racing R/C car I've ever heard of... we're talking even smaller than those little Zip-Zaps miniature R/C cars you can buy at Radio Shack. The tires are only 5.6mm in diameter?! This thing is going to be way smaller than that transmission by itself, and like Danger said, it doesn't look like it is made for a car application that is shift-on-the fly.

I the same vein, it sounds like that motor you have specced out doesn't have much power at all... 1/10th scale race-quality motors for competition typically put out at least 50-100times as much torque, and 50 times as much total power using up to a 7.2V battery; and they rev up to over 20,000rpm.

I would recommend browsing around www.towerhobbies.com for some ideas. They have lots of kits and race/hobbby specific products for R/C vehicles.

Last edited: Feb 26, 2008
7. ### 9988776655

35
they are 5.6cm diameter maybe i put the decimal point in the wrong place oops. I thought the motor had a bit more power than that. Maybe i need buy a more powerful gearbox/motor combination. I thought since my car was only 20cm long and weighs 300g all up a weak motor would be okey

Last edited: Feb 26, 2008
8. ### Mech_Engineer

2,299
That kind of reduction would give you an output shaft speed of only 5.8rpm, useless for anything that wants to go faster than a slug.

Even assuming direct drive to the axle after the transmission, 5.4rpm would equate to about 10cm/min. The "minimum" reduction in that transmission would equate to about 20cm/s (7 in/s); not very fast in my opinion, but maybe fast for such a small vehicle.

Again, it sounds to me like this "transmission" will be larger than the rest of your car combined.

Last edited: Feb 26, 2008
9. ### Mech_Engineer

2,299
What kind of R/C racing are we talking about here? The real deal, or just for fun with home made stuff? How big is the car supposed to be?

Last edited: Feb 26, 2008
10. ### 9988776655

35
thats way too slow but at least it can accelerate. I tried to work it out before and I got that 34.9:1 would be a good ratio so I would pick the only one close to that 76.5:1 which the gearbox offers. Thats 100 rpm. Would that be able to drive the car or do you think I need a more powerful gearbox/motor? Its for fun. It can be whatever size but of course i need it big enough to fit a receiver pcb, batteries and the gearbox itself. Im using plywood as a chassis. Im in second year uni doing mechatronic engineering but all they teach me is programming, maths and statics. What is the use of that?

Last edited: Feb 26, 2008
11. ### Mech_Engineer

2,299
No, it can't. That's maximum linear speed with the motor running at its maximum speed with no load.

How did you choose that "good" ratio? Based solely on friction (which you have calculated incorrectly)? You're going to want a motor and gear combo that gives you WAY more power and torque than would be required to overcome friction, so that your car actually accelerates and can maintain a top speed.

That's 2.9 cm/s based on the numbers you've posted thus far. Slow. Real slow.

There's no way you're gonig to keep your vehicle weight under 0.3kg including batteries and electronics while building the vehicle out of plywood. I would guess at least triple that weight depending on overall size.

It sounds to me like your estimates and calculated numbers are all WAY off right now. My recommendation is to look at some cars that already exist, at a hobby shop for example, to get an idea of what size you're looking for, what driveline components you'll want, and how you plan to accomplish it.

Last edited: Feb 26, 2008
12. ### 9988776655

35
I feel a little hopeless after your post

13. ### 9988776655

35
My wheel is 5.6cm diameter, circumference 17.6cm.
My revolutions per minute is 100
My revolutions per second is 1.67
Every revolution, My wheel turns 17.6cm
My distance per second is 1.67r/s * 17.6cm = 29.5cm/s
But you got 2.9 cm/s? am i missing something?

You are right about my estimates being way off. It would me nice to actually know how to do the "torque required" calculation correctly but I dont know how.
I cant find a more powerful gearbox/motor combo

Im looking at another gearbox which may be just as bad. You can do
1:125
Does that mean for every 1 motor revolution, the axle turns 125 times?

Last edited: Feb 26, 2008
14. ### Danger

9,879
Yes. So figure what that means with your 9,400 rpm motor.

15. ### 9988776655

35
I think ill pass on that gearbox then. I dont think 29cm/s is slow. But maybe i didnt work it out properly. To overcome friction initially I need:

torque > Us*m*g
torque > 0.4*0.5kg*9.8m/s
torque > 1.96N

(I used an estimate for the friction of rubber on wood)
Rolling friction has to be less than this
But I have 4 wheels so I dont know how that effects friction

Even with the highest reduction, my gearbox/motor still wont even move

Last edited: Feb 26, 2008
16. ### Mech_Engineer

2,299
No need to feel hopeless, we are here to help. I suspect you'll be able to build a very capable vehicle once we get a few basics ironed out.

Well there's the problem, your original post said the wheel radius was 2.8mm (0.0028m), so there was an extra zero in there.

Well, I think your problem is that you're not really grasping what the friction calculation is useful for.

What you want is this thing to be fast, so you're going to want to accelerate at a decent rate, which means the majority of your motor's power will go towards accelerating the vehicle's mass. As long as all of the parts are chosen properly, friction should be a negligeble effect when compared to the required acceleration force.

That is the reverse of what you would use a gearbox for. Since you're going to be using a small, fast-revving motor, the gearbox needs to be used to REDUCE the rotational speed, not increase it. So if you were using a gearbox with a 1:125 ratio, the output shaft of the gearbox will turn once for every 125 turns of the motor.

Are you going to be using a differential for the drive wheels, or will the output of the gearbox directly drive the rear wheels?

It's all a matter of perspective. I have a "stock class" 1/10 scale electric off-road race buggy that easily goes 25 mi/hr, which works out to about 11 m/s (1100 cm/s). A 1/8th scale nitro-powered monster truck I have can hit a top speed of about 40 mi/hr (17 m/s, 1700 cm/s), even faster with different gearing.

What kind of a race are we talking about? On-road, off-road, parking lot, table top, etc?

YOU'RE NOT CALCULATING THE RIGHT KIND OF FRICTION. Static friction (what the above calculation you did is for) would be if the wheels were locked up, not rolling, and you were dragging the vehicle along the ground.

For your purposes, I would first assume friction to be negligible, and instead decide how fast you want your vehicle to accelerate to its final top speed. Then, using kinematics, you can figure out how much force would be required to accelerate it that quickly. From there, a motor can be chosen and a gear reduction can be decided on based on a required final top speed and acceleration.

17. ### Mech_Engineer

2,299
To give you a better idea of what you should be looking for, start here:

http://www3.towerhobbies.com/listings/cat-c2.html

That is a basic listing of R/C car parts at tower hobbies, which should be very useful. They have tires, motors, bodies, controllers, servos, everything. In my opinion, it sounds to me like you wil want a "stock" 17 turn motor and regular speed controller, since they will be cheap, and far more powerful than the motor you currently have specced.

Motor Examples:
Team Associated 18-turn Stock Motor Replacement, $10.99 Team Associated 17-turn motor,$14.79

18. ### 9988776655

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Im thinking of using the gearbox that I was originally looking at but replacing the motor with of the ones you suggested. The gearbox is meant for a 2mm shaft but the 18-turn motor is 3mm. I do not know the diameter of the 17-turn motor. Do these motors also run on 3V?

The car is going to be on-road.

I thought to make the output shaft of the gearbox run the wheels.

I dont really need a speed controller. Im happy with max speed all time.

I worked out that the small components+wood+batteries+gearbox is 527g. If we change the motor it could be a bit heavier.

19. ### Mech_Engineer

2,299
I don't think that gearbox will be a very good application for what you're trying to do. As said before, it looks to be more of an automation/robotics application for low speeds and forces.

While it's theoretically possible to run on 3V, why not just buy a single 6.4-7.2V battery pack? They're cheap, and specifically designed to work with the high current draw of this kind of motor.

Commonly the gear reduction used with these specific motors is a spur and pinion gear combination that drives a transaxle differential (combined transmission/differential), which in turn drives the wheels. Common pinion/spur ratios are around 1/3 to 1/4 (for example, a 78 tooth spur gear and a 24 tooth pinion gear, you can select different options to suit your driving style) and the transmission might be about 1/2.6, making the output speed to the wheels about 1/7 to 1/10 that of the motor's output speed. That setup (primarily off-road) would normally be used with 8.89 cm diameter drive tires, that are 6.1cm wide. On-road applications would be faster (less total gear reduction) with slightly smaller diameter tires.

But how? You're going to need some sort of transaxle at very least to make the single output into dual axle shafts...

Basic speed contrtollers are cheap, and these motors cannot be run without some sort of speed control. It is the throttle for the motor, so you can slow down in turns, and accelerate on straight aways. You have to remember it's going to have a pretty fast top speed and running with an on/off switch to the battery isn't really an option.

My guess without any calculation at all is you should aim for about double that, at least.

20. ### 9988776655

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