RC Tank Warfare community hobby forum

If any of you out there are experts on this subject or just highly knowledgeable, perhaps you can guide me thru this. I'm attempting to determine what gear ratio I need to design to move a model vehicle at scale speeds.

The parameters are:
Max motor RPMs are 2000
Powered by a 7.2 v DC battery
The prototype vehicle ran at a max speed of 56 MPH
This is 3.5 MPH if in 1/16th sxale.....=18,480 ft/hr....=308 ft/min...=3,696 inches/min

There are 6 driving wheels on the model…..each with a circumference of 7.59”….meaning the model vehicle covers 7.59” for every one revolution turned.
Thus in 1/16th scale, it would take 486 revolutions/min to travel the 3,696 inches.... to be at a scale 56 MPH speed.

However, all of this is at “no load” on the motor. The model vehicle weighs 2.44 pounds……=39 ounces.

The pinion gear has 8 teeth. Thus the questions are:

How many gears and what teeth per each gear is needed?

At “no load” , using an 8 tooth pinion (#1 gear), a 14 tooth #2 gear, with a second 8 tooth gear attached to it, driving a 19 tooth #3 gear, works out to 481.2 revs/mins…..which is close enough !! (I used Impact’s Gear Ratio Simulator for this calculation)…. http://jbwid.com/art74j.htm

But how do I factor in the 39 ozs of load that the motor will be pulling?….and what about effectts of friction from the moving parts....(drive shafts, U-joints, gear teeth, axels, etc....maybe these can be ignored on a model)
Certainly this extra load requires more revolutions/min to move everything !!
Is there a formula for this calculation?

And lastly…….does anyone know where to order plastic gears with the required number of teeth……like those used in Tamiya gear boxes?

Thanks for any assist. PP

Hi ,
There is a gearbox calculator on the site which might help.....

The gearbox calculator is in the Download section....here is the link to the section its the 7th one down posted by MatDragon...
downloads/


It is downloadable and actually enables you to change the number of teeth on a gear and this will automatically change the results for the whole gearbox including the scale speed......
....and I think the real speed...
.
Here is a screen shot of what you will see when you down load it.... click on this image from my computer to make the image larger....and to give you a better idea.

thank god for you dietrich!
that gave me a migraine just trying to take it all in

Thank You......that is a lot of info. But it looks like it applies to already designed gearboxes. What i need toi do is design one from scratch................to fit the parameters first described above. Do you know of any calculators that would help with that process. Thanks, PP

Hi PP,
Sorry I have no idea..... but there must be programs out there.....

But I have to ask HOW are you going to make the gearbox from scratch ?... Especially if you do not have a source of gears... The spacings of the side pieces of the gearboxes will be determined by the width of the gears.... and the positioning of the individual gear shafts will be determined by the actual size of the gears....

The Robotic groups would be my thought for taylor-made or very flexible gearboxes.... BUT... the mere mention of the word "Robotics" usually doubles or triples the price of things....

In all honesty I think that you would find it easier to adapt an existing gear box to fulfill the roll that you need....How about a photo or image of what you are trying to do ?
.
Its a "Wheeled Vehicle".... so how about all the 1/10th scale racing, rock crawling and off road cars.... surely they must have gearboxes that would suit you ?.. and they will come WITH 4x4 drive systems that you could utilise ?

Alb.

Hi Alb: The model project is a 1/16th scale US M20 Greyhound with 6 wheel drive. Since its to scale, the transfer case (TC) needs to be fairly accurate....which is quite small. Once I figure the number of gears and teeth per gear that I need, I will locate the gears....which will be similar to those found in a Tamiya gearbox. Then I will design the actual TC frane size and a friend of mine will make it out of derin hard plastic using his 3D printer. So I probably need to talk to someone who designs this type thing for a living or maybe those Robot guys might be a help.....thanks for suggesting them. Here are some pics of the model so far. The open area between the U-joints in the last picture is where the TC will come thru the bottom hull......PP

Hi PP,
Thanks for the photo ..it looks really good....

Just to make things a bit more simple.. ( and I know that its not 'Correct' )... how about just going with 4 wheel drive to the rear wheels ??

It will be much easier and will make VERY little difference to the way that the vehicle goes.... Especially since you have put so much time into building it you are hardly likely to be crashing it through bushes and rolling it over...The front prop shaft will still turn.. you just tie it up out of the way...
Getting a 4x4 gearbox will be so much easier and you will have a better space to put it in.....

The Robotics guys probably have a gearbox calculator or sorts and also numerous links to gears.... just be ready for the 'Robotics' price tag..... Its like the word 'Yacht'...

When lived in the UK in Plymouth you could buy a spark plug for say £1 in a motor factors..... (This is now word of a lie)...up the road about 1/4 mile, in a marina.. a ships chandler sold the SAME spark plug for £5 ! ...... Add the words Motorbike, Yacht or Robotics in front of the item and the price multiplies like Rabbits...