shopping cart wheel
shopping cart wheel
(OP)
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
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





RE: shopping cart wheel
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RE: shopping cart wheel
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!
RE: shopping cart wheel
Norm
RE: shopping cart wheel
-Jon
RE: shopping cart wheel
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: shopping cart wheel
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.
RE: shopping cart wheel
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.