shopping cart wheel

[villon]

Hi.

I'm trying to figure out what causes a shopping cart wheel to 'wobble'. I'm working with electric wheelchairs and they have rear wheels similar to those of a shopping cart. I need to find the important factors and how they affect the wheels stability. So far my only guess is that it has to do with gyroscopic percession and should thus be related to the angular speed of the wheel and the length of the arm holding it.

I'd be most grateful for any tips or ideas

 

[patprimmer]

I would have thought it had to do with out of phase eccentricity of each side of the tyre.

Regards

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[villon]

I thought about that too, but then I can't explain why the wobbling occurs at specific speeds and can be initiated by disturbances.
And also why the length of the arm holding the wheel plays a part in it, a longer arm will dampen the vibrations. Another phenomena is that a heavier wheel, such as a solid wheel instead of one filled with air, will cause more wobbling.
However, you may very well be right, thank you for your answer!

 

[NormPeterson]

A solid wheel is also less compliant, and probably has less damping as well.

Norm

 

[jons999]

It's due to the caster angle of the wheel (the reason wheels like that are called casters) If you look at it from an automobile alignment point of view a small amount of positive caster (steering axis tilted rearward) give easy steering and good directional stability. Too much positive caster will make the steering wheel shimmy, just like the shopping cart wheel wobbles back and forth. Negative caster gives you poor directional stability and will make the wheel wander side to side. If you change the mounting point of the shopping cart wheel bracket in relation to the center of the wheel you can eliminate the wobble but it will be slightly harder to steer.

-Jon

 

[GregLocock]

That's fine as far as it goes, remember to add in the effect of mechanical trail as well. The interaction of trail and castor is (I think) enough to explain this phenomenon.

Cheers

Greg Locock

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[spectreeng]

The wobble comes from large mechanical trail with no castor (angle of pivot line). To simplify, imagine the force on the wheel always pointing towards the rear. Then it acts very much like the classic pendulum with a longer arm leading to slower frequency.

Positive castor (upper pivot behind lower pivot) on cars works to dampen this oscillation because of the jacking effect of having wide flat tires (the side of the tire pushes the car up).

However, I suspect for your application you are using pneumatic tires that have a very round or bulging profile so there would be no jacking effect with positive castor. That leaves us with using negative castor to dampen the oscillation (the wheel would be at it's highest point when pointing straight back - moving from this point would lift the wheelchair up ie. jacking effect).

But castor is directional - I suspect the disabled would want to be able to go backwards as well (much like shopping carts) so we have to rule out castor.

I suggest playing with mechanical trail length and types of tires to minimize the oscillation effect.

 

[jbel]

How many shopping carts have you seen that are in pristine condition???? Usually they are beat down, banged up, or otherwise treated poorly.

This leads to two possible conditions: altered geometry and reduced wheel clearances i.e. the wheel locks up and refuses to turn or altered geometry and increased wheel clearances i.e. the thing wobbles like crazy.

Go the the next grand opening of your local retailer and try the carts out. When they are brand new I would venture to guess that not a single one has wobbly wheels. Then go back in six months and experiment again. I think you could find a wobbly wheel almost immedistely.

Hopefully, the wheelchair villon talks about would be treated with more care than the local shopping cart. If that were the case then this probably wouldn't be an issue.

If the carts do not suffer from the condition when new, then the geometry of the caster, mounting bracket et. al. is probably acceptable. When the thing gets beat up or tweaked from day to day use, you can no longer depend on the original design intent as actual operating conditions. This being the case you cannot perform an analysis and expect to get dependable results.