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I'm working on a project where part of a unit needs to rotate around a fixed point, and be supported on two fixed (i.e. not castor) wheels.

When experimenting with the wheels, it feels like fixing them at 90 degrees to the unit results in them 'scraping' across the floor somewhat, as the arc of travel is always slightly misaligned with the plane of their rotation. Further, it feels like holding them at a slight camber mitigates or even eliminates this effect. However, I have no way of confirming this.

Is this basic observation correct, and if so, is there a method for calculating the optimal camber (a and b) for wheels different distances (d and e) from the pivot point x?

(Note, if it helps/matters, the wheels have a more rounded profile than those shown in the diagram.)

simple diagram of system

Tom M
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2 Answers2

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The optimum angle would be that of a pair of cones sharing the pivot point and each cone's axis running through the axis of its respective wheel.

enter image description here

Figure 1. The correct camber can be determined by aligning each wheel's axis with the point of rotation.

The calculation is basic trignometry.

enter image description here

Figure 2. These conic rollers have the same end diameters. They will roll perfectly around the shared point but the dark one will have to rotate at a faster rate.

Transistor
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  • But if the OP can't guarantee the wheels are perpendicular to said axes, you'll still get some lateral drag, won't you? I agree otherwise that this calculation minimizes drag – Carl Witthoft Feb 24 '22 at 14:43
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If you are not powering the wheels (the image suggests that you are not), then the wheels should be free to rotate at their own pace.

If you are seeing scraping then (I would):

  • check that the the center of rotation and the contact points of the wheels with the ground are in the same line.

If they are misaligned that would explain additional lateral forces.

  • try to use wheels with a smaller footprint e.g. like the following

enter image description here

The reduced footprint should create less scraping.

NMech
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