Mixed-size wheels?


#1

In theory, it’s possible to drive mixed sized wheels by using pulleys or sprockets with identical ratios. So, put a 24 tooth pulley on a 4" wheel and a 36 tooth pulley on a 6" wheel.

In reality, I’m worried about the 6" wheel actually being 5.95" and the 4" wheel actually being 4.03", in which case the wheels would constantly fight each other. But, that problem also exists when you have different wheels of the same nominal diameter (e.g. a 6" traction and a 6" omni), and doesn’t seem to cause problems there.

Has anybody done different wheel sizes? What issues did you run into?


#2

It would be possible (at least from my experience), but I don’t think it would be very practical.


#3

3946 did this for STRONGHOLD, running one 4" wheel and four 8" wheels per side, though the 4" wheels only got any use in getting off the walls of the moat and such. We had several troubles with the drive train that year, but this wasn’t one of them.

Added: to clarify, we direct-drove the 4" wheels, and used 16 tooth sprockets on those axles to drive 32 tooth sprockets mounted on the 8" wheels.


#4

My first thought is the different surface speeds of the wheels. When you use a larger wheel, the outside of the wheel that contacts the floor would be spinning at a very different speed than a smaller wheel, which would introduce a large number of problems.


#5

Like 3946, we used two sizes in stronghold as well. When driving normally, only the larger wheels were on the ground. The smaller ones, while still part of the drive train, were used solely for helping get over the defenses.


#6

Re-read OP: the different sized wheels are geared to spin at different angular speeds, resulting in the same (or very similar) linear speeds.


#7

the big question is what wheels are actually touching the ground. If you have a six-wheel drive chassis and put the small ones on front with a rear weight bias, so they mostly ride around up in the air, there’s probably no problem at all. Or small ones in the middle, they’ll never touch.

If you have a four wheel chassis, it might be. Your OP implies that you’re trying to get them to all turn at the same floor speed…yes?..if that’s the case, mixing wheel size would be no issue at all.

And the small amount of difference in floor speed from omnis to what-ever would just result in more slide and wear. You’re planning for wear anyway.


#8

I thought of another data point. Getting ready for Red Stick Rumble this year, we noticed that the direct-drive wheels on Mini-Koopa had worn down so that there was hardly any tread left, but the other two had minimal wear. All the wheels were 6" hi-grip (KoP) wheels, and the driver hadn’t really noticed any change in performance, though two were about 1/4" (4%) smaller diameter than the other two.


#9

How is the weight distributed on Mini-Koopa? That’s what I would expect if a majority of the weight were over those wheels. If a wheel is being driven, I don’t think it should matter if it’s connected to the gearbox directly off the shaft or via a belt.


#10

You can see a picture here.

Yes, a majority of the weight is over the rear (direct drive) wheels. The motors and gearboxes are back there, the battery is a bit rear of center, and the heavy pneumatic cylinder (Bimba FOS 502 IIRC) and heavier end of the arm and a couple of metal air tanks are all in the back half. The only things forward of the front wheels are chassis, bumpers, and intake wheels.


#11

As long as you reduce angular speed you should be fine