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Jay's "GP-Bot" Robot Project - Do the Tighten Up (Page 4)

Do the Tighten Up. Not the song from the 60's, but physically. The steering system on Power Wheels vehicals are loose as a goose. Not a good situation when you need your 'bot to drive straight and steer controlably. Driving over a small stone or hitting a little bump can send your bot a considerable distance off course. A number of things need to happen to tighten up the steering system for improved stability and accuracy. Much of the looseness in the system can be corrected or improved upon in a number of ways:

Improving the Front-End Geometry

The Front-End Geometry....

The King is being replaced

The King Pin is the pivot point that a wheel steers on. On the Power Wheels vehicals this pin is mearly a 13/32-inch diameter round rod bent at a 90-degree angle. It serves as both the king pin as well as the wheel axle. See Geometry above, for more detail.

The King Pin fits through a drilled hole in the vehicals frame. This hole is considerably larger than the pin and thus allows for excessive sloppiness. The wheels are allowed to wobble about and the ability for the GP-Bot to drive in a straight line is seriously impaired. We need to fix this if we ever expect to win any robot contests, not to mention avoiding lawsuits when your bot runs off coarse and into the spectators... DOH!

Enter our new King Pin Mounting Block.

The new block is designed to provide a better fit for the pin to rotate with less wobble or free-play. On the drawing to the left, I show a 7/16-inch hole for the pin. This may be a bit too tight, allowing only 1/32-inch clearance, so drill/file to your desired fit and freedom of movement. A much better block would have support bearings both at the top and bottom of the block, but hey, we're operating on the cheap here.
The block adds additional support for the pin itself. The frame only allows roughly a 3/4 span for support. The new block provides a 1-3/4 span. This should provide for much less tipping side to side of the pin within the block. This translates to less wobble in the steering system.

Dimensions of the block are not critical. My design was simple and could be manufactured from any old piece of scrap laying around the shop.

Baby needs a new pair of Shoes

The tires used by Power Wheels are simple hollow molded plastic. They support the weight of a small child or two. They even support the weight of an adult! However, the wheels have no bearings. The axle mearly goes through the plastic with little surface area to bear the weight. The wheels often wear at this point and become out of round. The more out of round they become, the more unstable the steering becomes. The newer Power Wheels use a much wider tire, and hence a wider span between where the axle contacts the tire. This helps to take much of the strain off the plastic. But they still wear, badly.

My approach to this problem is to replace the plastic tires with 9-inch split-rim inflatable rubber tires. The rims are manufactured with two roller bearings. The tires are rated at approximatly 300 pounds load, each. Far more than the "GP-bot" should ever see. The bearings are not of high quality but certainly will support more weight and sustain more use (milage) than the old plastic wheels could ever hope for.

To mount these new shoes we will need to use a sleeve to adapt the inside diameter of the new bearings (5/8") to the Power Wheels axel (13/32").


Slacker

The Tie-Rods....

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