Wheels

The wheel size is determined by the required ground clearance and the size of the drive sprockets. Initially, I was planning to use six off-road style 'balloon tyre' wheels approximately 200mm (approx 8") in diameter and with about 50mm tyre width but, I went for some larger 10½"/260mm diameter wheels in the end, to give improved ground clearance.

I'm using six off-road style 'balloon tyre' wheels with a 50mm tyre width, three on each side and all are driven. These are driven in tank-drive format, which means one motor drives all the left wheels and the other all the right wheels.

Reading many of the Robots Wars competitor sites, reveals some interesting issues with this layout and drive format. It provides great manoeuvrability when used with sticky tyres but, it is best suited to a short wheel base and a wide track. This is less of any issue for our robot, as high speed direction changes are not required and it will be mainly travelling in straight lines. A lot of the use will be 'off-road' on grass and other loose surfaces.

Driven Wheels

The wheels are not meant to be driven and have bearings to allow them to free-wheel smoothly. These bearings have 5/8" or 15.875mm bore. There are various options to enable mounting:

Weld Wheel To Axle

I don't like this as it makes removing the wheel really hard. With in-built bearings on my wheels, this is also going to be quite difficult to achieve.

Dowel Pin Through Wheel Hub

This is nice and simple approach, using a 6mm dowel pin through the protruding part of the wheel hub. It's not as strong as the keyed approach though.

Welded Flange

This approach uses a flange which is bolted to the wheel, using the four through-bolts, with the flange being welded to the axle. I like this approach because it is easy and strong. It would most likely require a 5/16" diameter axle, which is fairly easy to get hold of and a flange with a 5/16" hole in it but, this appraoch basically enables me to use any size axle I want. Given that 16mm axle and bearings seem easier to get hold of, this is my preferred approach.

Remove Bearing Mounts

This is quite easy to acheive as these are held in palce by four 8mm bolts that hold the two sides of the wheel together.

Attach Gear Direct To Wheel

This is a possible option as the wheel has four through-hub bolts to hold it together and using some spacers it would be possible to bolt my gear to it. The downside of this approach is that it results in the gear (and hence chain) being outside of the chassis and prone to dirt and water.

Chosen Solution

I bought six 260 x 80mm sack truck wheels from Tools By Post . These come with a hub mounted flange and each has a pair of 5/8" bearings.

Having removed the wheel hub bolts, it is plain that the tyre is effectively clamped in place by two halves of the hub, which are simple and cheap, dished discs of metal.

The outer half is different in that it has a valve hole. I'm going to leave the valves on the outside for ease of access. On a Robot Wars competitive robot, you would probably want these in-board for protection.

The inner half has a flange clamped to it and two pairs of 5/8" ID / 35mm OD bearings press-fitted into it. These bearing are of low quality.

I knocked these bearing out with a punch.

This still leaves the flange attached to the hub, via some press-folded edging and this will be grinded off, to allow my new axle flange to be bolted using the through-hub bolts.

Since the drive gears mount to a 14mm axle, it makes sense to use a 14mm diameter axle and flange to fix the wheels but, 15mm axles and bearings are much more readily available so I'm going down this route.

It was quite a quick job to take an angle grinder to the inner half of the hub to separate the flange and the hub itself. I can now create a new flange and axle mount to fix the wheel to the driven axle.

Having taken the wheels apart, it is clear they are just stamped out of cheap steel and will corrode quite quickly if exposed to any damp, so I've taken the precaution of rubbing them down, priming them and painting them.

Now I've got this far, I can design the wheel hub/boss (MS Powerpoint) which will fix the wheels to the axle. I'm going to pay someone to fabricate these for me.