Mark
You know when someone asks you something that you should know but you don't?
It's happened today, on another forum.
I was explaining caster to someone and they asked if it would work the other way, that is with the top ball joint IN FRONT of the bottom one?
His logic being that supermarket trollies work in both directions (which was the analogy I was using)
I must admit I don't know.
Anyone?
cheers
Chris
Asda ones work better than Tesco if that is any help...apart from that I have brain fade and can't think...
Mark
I think the difference is that your wheels will try to toe out when the castor is backwards and therefore give better grip.
If you do it the other way then although the car will drive straight, the wheels will try to toe in and so give up a bit of grip.
Also the tendency to toe out will probably assist in cornering
quote:
His logic being that supermarket trollies work in both directions (which was the analogy I was using)
castor makes the car go in a straight line,without it it would wibble around like a nissan micra or a tesco shopping trolley.....i went to college to
learn these things
You don't need to think to hard about it, just try driving your car backwards! My van, if given the tiniest hint of steering, will soon wind
itself up to full lock. Same thing would happen with positive castor if going forwards. Or just ask anyone else that has an MK indy! 
With the top ball joint ahead of the bottom the contact patch of the tyre would be ahead of the point where the line through the kingpin hits the
road, that would mean the wheel would always be looking to swing around to put the tyre contact patch behind wouldnt it?
So to confuse things further should a front wheel drive car not be setup that way as the wheels are pulling the car rather than the car pushing the
wheels? I've never noticed them being like that, they seem to have raked back castor like a RWD.
Is it just me (and this polish vodka) or is that not quite right???
As on a supermarket trolley, the wheel completely swivels all the way round and is therefore still giving the negative castor whichever way it travels
and therefore negates the supermarket trolley analogy from the start?
If you reverse your car and it gets "twitchy" as stated by diy si, then the wheels remain in an inverted position to how they were when the
negative camber was introduced going forward...
Please forgive me if I am TOTALLY off the mark as I am a tad under the (vodka) weather - I was dragged round prospecticve Wedding Destinations
tonight, so I had a drink in all of them, and still going now!!!
Stve
quote:
Originally posted by omega 24 v6
quote:
His logic being that supermarket trollies work in both directions (which was the analogy I was using)
yes but only as long as you keep the wheel totally straight. At the slightest side move ment the wheel will turn through 180 degrees.
Try this, drive forwards in your tintop and if you let go of your steering it will centre and the feedback through the wheel will be good.
Now do it while reversing.
You should feel the difference in the handling in that the steering will feel a lot lighter and very skittish.
The reason that shopping trollies have, what would appear, rear set caster angle, has nothing to do with the finer points of steering. If they had no rear set, and were straight below the pivot, then the wheels could turn in any direction, independant of each other, and they don't have foreword set simply because they would, as soon as you pushed the trolly, go to the rear set position anyway. Hope that clears the air, on shopping trollies anyway. Cheers Ray
wikipedia covers some of it - apart from the driving backwards thing.
seem the supermarket trolley is not a good analogy - they have a lot of trail but no caster angle.
quote:
Originally posted by Mark.
Asda ones work better than Tesco
i assumed that castor causes self centering because the contact patch is dragging behind the centre of the vertical steering axis, if you follow.
Scrub radius left and right cancels out.
Trail on a shopping trolley is a similar concept but obviously different.