Thoughts on wheels, gears, axles

For FVC competition robots, some existing parts seem out of balance.
The plastic wheels appear to be waaay overbuilt and heavy.
The axles are flimsy compared to the wheels.
The gear hub/axle is the weak point of all but the smallest gears.
The knobby tread on the biggest wheels look like dirt-track offroad tires,
but I dont use Vex in the dirt.

It would have been good to design the larger two wheels as wide gears
with slip on tires. They would be much sturdier and larger than the current 84tooth gear. The 12 tooth pinion gear should have stronger teeth,
but it would probably have to be made of metal. Since each gear has
power loss, minimizing the number of gears in a train by having larger
gears saves power.

The gear train on the arm of FVC robots typically requires
multiple gears in parallel, and/or bolting gears directly to each other.
The 36 tooth gear should have had bolt holes to match the 60 tooth gear.
There should be 4 holes, not just 2. There should be an X shaped metal locking plate with built-in spacers to fit around the flanges of the gears
to match the 4 bolt holes, which would also take the strain off the hubs.

We bent several axles on the middle gear of the arm gear train.
One idea would be to drill out the centers of the gears
and use a threaded hex-beam as an axle.
Once the gears are bolted together, the axle is not the primary torque transfer.

I’d love to see a third circle, with 4,5,or 6"radius. When driven by multiple
12-tooth gears in parallel, it would be the only gear train necessary to
lift an arm. The toothed part of a quarter circle will just about fit on a
15x5 hole plate, if I had a CNC or patience to hand cut it.

Good ideas, but I think the reason things are the way they are is because the parts already existed or it’s cheeper…

The kit is fine and excellent. as stated many times on these boards. proper building, tensioning and concepts reduce the risk of failure. There are too many people complaning about chains breaking, gear tooth fractures, and wheels. If it’s for FVC then deal with it, life isn’t fair and not given to you on a silver platter. IF it’s for your house bot, ge tup and build somehting with what’s in the basement. you must know how the materials will handle stress from opposing robots, torque form the motor and even gravity. I have had no problems at all and built some very complex robots for a compeition. Yes i had trouble witht he tracks and support, but hey i over came it with a simple logic of triangles. Use what you have learned in school, the triangle is the strongest shape. If you leave one side of your axle u supported, duh of course it will bend and fail. Support. Support. Support. Tension the chains. Tension. Tension. As goes for your wheels dilema. I can think of more then 10 diffrent wheel types you can make from the standard wheels. be creative and hope for the best…

Perhap my post went off track, in addition to your misconstueing it as a complaint. I’m an enhancement creep, thats all.
Of course to some extent, with FVC, or any competition, part of the fun of competition is to work within the existing constraints to accomplish a task, ie deal with it.
But from an engineering, parts reduction, multi-purpose point of view, if the wheels were made custom for VEX, I think it would have been good to design the hard plastic part to have a gear rim, rather than what it has.

It can be cheaper to have multi-use multi-purpose parts, if it doesn’t compromise the original or primary purpose. If the molds already existed, of course that is a big reason.

Regarding axle bending, I had a 4" axle in the middle gear of a gear train, supported on both ends, and it bent in the middle when trying to hang. Of course there are workarounds, such as triple gearing or 3 or 4 rail supports. Every interesting engineering decision involves tradeoffs, I’m just expressing that my experiences with Vex would lead to exploring different tradeoffs than were chosen here. I suppose it is futile, since the designer isn’t going to come on the boards to explain what other choices they had available, and why they chose these particular tradeoffs.
My comments on extra holes in the gears would cost nothing, and save users from doing it them selves. X brace locking bars may be a cheap stamped steel additional part to bolster the weak point (hubs) in the existing gear system, as well as teach valuable lessons about torque.

Speaking of 10 different wheel types from the standard, I’d be interested to see your enumeration of them. I’ll contribute this one:
1: Another interesting idea for cutting (or original molding) would be 45 degree angled treads. If direct couple 4" wheels can slip on the foam, would angled treads make a mechanum drive? Imagine 4wd tank setup, but moving both joysticks to the right would cause front and back wheels to turn in opposite directions, and the angled tread slipping would pull robot to slide/drift to the right. Front and back wheels have the tread angled in opposite directions.

For the most part I agree with JGraber’s prior statements. In alot of cases many FVC teams are to inexperienced to understand the strenth constraints of the plastic wheels and gears. In my personal experience I found that when I’m going to put something under alot of load I usually double or triple my motors, gears, and hardware. Also a good solution for stipping gears is to get the metal bar lock and bolt it directly to the 60 or 84 tooth gear or 40 or 48 tooth chain sprocket.

Also by the way mecanum drives are very inefficient as is and taking away the rollers would make it impossible to move on foam. But if your trying to achieve omnidirectional motion a holonomic drive may be your best bet. (4 omni wheels placed at 90 degree angles from each other)


I picked up an interesting tidbit today.

On the FVC fields’ foam surface, a heavy robot will push Vex omni wheels deeply enough into the foam to nullify the rollors that make it an omin wheel…

Keep this in mind as you design and build.

Well im guessing that when the makers of VEX made VEX, they made all of the specific parts for a reason. so i wouldn’t challenge their designs because they must have had a reason for building all of the parts the way that they did.

I never noticed that but it makes since. Did it completely nullify the effect of an omni wheel or just make it more inefficient.

That is one difference between us then. I’m an enhancement creep.
And this is the “New Product Suggestion” forum.

I think the omniwheels are an interesting idea, and the existing ones are somewhat useful for squarebot and hard surfaces. I’ve considered lapping multiple wheels down to just the middle third, and rotating them by fractions of 90 degrees, which would improve many of these shortcomings.
So in this new-products suggestions thread, we can add

  • better omni-wheels that work on FVC foam surface.
    • better = larger rollers, two rows of rollers
  • Maybe even “mechanum” wheels?

“impossible” ? That seems unlikely as a universal statement, since negatives are so hard to prove.
Since medium wheels can lose traction on FVC foam, if the treads were angled, there may be a side-force when the wheel slips. But until I try it, I can’t prove it is possible either. I thought my mention of it might inspire a team with deep pockets of time or equipment to try it out.

I tried that on squarebot once, (without a custom program for it), and agree it is interesting, but not particularly efficient. Although it is the easiest bet, I suspect other methods would work better. The question is what tradeoffs of design/implementation time vs improvement are good.

  • Multiple staggered omni wheels would be an improvement for $$.
  • Multiple staggered thinned omniwheels cost both $$ and time, but save space.
  • Cutting angled treads on the rubber or plastic of medium wheels takes time.

On an alternate track:
All mechanum or holomonic drives work better with suspension.
So what about using a servo to create active suspension, balance between perpendicular wheels?