Big shaft holes in tires, small shaft in motors

Hi everyone
I am working on a project right now that involves four DC motors attached to tires. I recently took apart an old RC car, which had nice, big, knobby tires on it. I figured that they would work excellently. The only problem is that the hole in them which is much larger than the normal motor shaft diameter. Does anybody have any creative solutions to this?


Well, you can fill in the hole and re-drill it (you have a drill press and centering tools, right?), or you can enlarge the shaft (or a combo of both).

Its difficult to recommend a real solution because I don't know what kind of machining skills you have. For the sake of this discussion, I'm going to assume none, and that you have only the minimum of tools available.

In that case, the easiest would be to purchase a piece of wood dowel from a hardware store (Lowes, Home Depot, etc), which has a diameter equal to, or slightly larger than the hole on the wheel. Hopefully, this piece of wood will also be somewhat much larger than the shaft of the gearmotor output.

If the dowel fits the wheel, that is best. Basically what you do then is drill out a hole in one end of the shaft deep enough to receive the length of the gearmotor's output shaft. Then epoxy the dowel to the gearmotor, and epoxy the other end to the wheel. If the dowel is a little long, you'll probably want to pass it thru a "pillow block" style bearing (which could be as simple as a small block of wood with a hole drilled in it thru which the dowel can pass); doing this is best, because it keeps side-loads off the shaft of the gearmotor, which may not be able to take such loads long before wearing out. Remember to lube the pillow block with something like graphite (dry) lube.

If the shaft is larger than the hole in the wheel, then you have some work to do. If you have access to a small lathe, its easy - you essentially chuck the dowel into the lathe, and turn the dowel down a bit with a file, until it fits the hole in the wheel - then follow the steps above. If you don't have a lathe, then you could try to chuck the dowel into a regular electric drill, then clamp the drill to a work surface, and wedge the power button on, then use a file or rasp to turn it down some. Best way to know what size is needed it to use a caliper to measure the original shaft (you did keep it, right?), so you know what diameter you're aiming for. Be sure, whether you use the homebrew lathe method or a real lathe, that you wear goggles and gloves - NO EXCEPTIONS (and stand to the side as you work, because you never know what will break and be ejected from the spinning dervish).

Note that if you have a lathe and know how to use it - you can quite accurately do the end-shaft drilling of the dowel; otherwise you just have to be real careful to center the drill and keep it vertical in both planes as you drill the end of the shaft.

Now - if the end of your gearmotor's shaft isn't much smaller than the dowel, what you should do is use a larger diameter doweling, and turn down the end for the wheel, but leave it larger for the gearmotor shaft. Note also that this same technique (ie, building a coupling shaft) can be used for plastic or metal, although in the case of metal shafting, you would need to use a metal lathe to do the work, of course.

Another possibility - if you saved the original shafting - is to measure it, then go to the hardware store (or online) and purchase a piece of shafting (metal or plastic) that same size, but then use what are called "shaft couplers" to mate the gearmotor shaft to the wheel shaft. Here are some sites to look into for this:

Note that going this route may or may not be cheap, of course...

You'd probably be better off digging through a hobby site like Tower Hobbies to find a good coupler.

Chagrin: You'd probably be better off digging through a hobby site like Tower Hobbies to find a good coupler.

He might, at that - but he'd do best to still take some measurements; he didn't give any indication as to what kind of "old R/C car" it was, but I suspect it was some kind of toy, and not anything you'd purchase from a hobby shop. As such, the parts used might not be standard in any manner (and thus some light machining may still have to be done).

What are the diameters we are talking about? it is from a 2mm shaft to a 10mm hole, or is it almost the same?

Good suggestions. I'll take some measurements.

andrewf: Good suggestions. I'll take some measurements.

A few photos might be good too, maybe I can make something for you to do the job...

Hi all. I was digging around in the tool chest and found a couple of wooden dowels. I'll probably have to grind them down a little bit. I'll glue them inside the wheel to keep them secure. Just for interest, the hole in the wheel measured some 9/32 in.

If what you're looking for is a cheap and permanent mounting solution, then the epoxy route is a viable and probably ideal solution. If you want to have some sort of serviceability or re-usability of the setup then you should look at finding a hub that will fit those motors.

For example, something like this: (not necessarily that particular hub, depends on your motors shaft diameter)

could be mounted to the motor, and then your RC wheels could be modified to bolt to that hub. Then in the future you could easily swap to different wheels/tire, or reuse the motors for some other project, or swap out to more powerful motors for that particular project.

I'm not saying that is the route to go, it entirely depends on your goals with the project. I've built several projects using epoxy, hot glue, CA, etc because the goal was to quickly, cheaply, and permanently put something together. Just presenting it as another option that provides more flexibility for future modification (especially if/when you start using more expensive components).