DAN57
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| posted on 1/1/06 at 10:54 PM |
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Maybe a really stupid castor question.
My partner spent a short time in hospital a few weeks ago and after reading all the posters and lables on all the machines I took up staring and the
beds. (I was a little bored  .) Anyway back to the point. I started examining the castor wheels with great interest and noticed that they are great
at self centering. However the locost suspension appears to me to be back to front. with the castor wheels the top mount is further forward than the
wheel center whereas our suspension has the top mount rearward of the wheel center. I am not a suspension expert by any stretch of the imagination but
if the top wishbone was mounted forward of the lower one would this not give better self centering or am I missing something.
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JoelP
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| posted on 1/1/06 at 10:59 PM |
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you have the correct theory, it doesnt appear to fit practice but if you extend the steering axis to ground level, it will always be as you would
expect, ie the contact patch behind the steering centre.
Beware! Bourettes is binfectious.
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emsfactory
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| posted on 2/1/06 at 12:01 AM |
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Ok. Youll get self centering when the top mount is behind the bottom one. If you draw an imaginary line through these points and extend it to the
ground it will be in front of the point of contact between the tyre and the ground.
This is how you get the drag that helps with the self centering.
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ufe777
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| posted on 2/1/06 at 08:23 AM |
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emsfactory is correct about the self centering.
I believe the correct term is mechanical trail. As far as I'm aware hospital beds and shopping trolleys have too much of it or conversly not
enough damping so you end up getting wheel wobble if you go too fast.
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DAN57
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| posted on 2/1/06 at 07:55 PM |
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Just about to answer myself now. 
From what I understand all cars trikes etc have a positive caster whereas shopping trolleys have a negative caster. Having a negative caster becomes
unstable at high speed hence the wheel wobble when rushing around the supermarket. Somebody far cleverer than me has obviously decided each is best
for its own application so I think I'll just accept it as is. Thats just the way it is.
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JoelP
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| posted on 2/1/06 at 08:00 PM |
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shopping trolley has no castor, but lots of trail. Cars have a bit of castor, and a bit of trail. The selfcentering arises from the trail, not the
castor, which is merely an ends to a means
Beware! Bourettes is binfectious.
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