Calculating Toe-in (REPOST)

[cadcoke5]

I posted about calculating a toe-in angle for an electrical pivoting system for a theatrical set. The set is on casters for general pushing-around. To make the set revolve in the view of the audience, we are installing a pivot point that will lower in the center of the set and lock the set in place. Then the electric 12" diameter drive wheel (solid rubber tire) will also be lowered to the floor at around a 8' radius from the center.

All the info I can dig up on this relates to 4-wheel vehicles. Two of you guys replied recommending 0 deg toe-in.

I did more reading about "scrub" in vehicles. This is distortion of the tire tread resulting from the turning forces on the wheel. I did a calculation for the scrub angle from the footprint of my tire (which was a guess), and got 0.4 degrees. I figure I should split that in half for a neutral load and go with 0.2 degrees.

Joe Dunfee

 

[dykpa]

If I may suggest:-
1 Make certain that the axle of the drive wheel is in line with the radius through the centre pivot.
2 Make certain that the castors have some 'Lead' on them so that the wheels follow their pivots.
3 Make certain that drive wheel is not too heavily loaded
4 get the drive wheel as far as possable from the central pivot
if the radius is large enough I do not think scrub is of any real worry
Hope this helps

 

[cadcoke5]

Thanks for the comments (and others whose posts were lost earlier).

Re #1; We may do just a "tad" of toe-in because we feel that will help the pivot point stay put.

Re #2; In our case, because the set must revolve in both directions, caster lead is not possible. But we are using tri-casters (3 casters on one plate fastened to the set with a ball joint) so we are not particularly concerned with this issue.

Joe Dunfee

 

[red300zx99x]

If the set needs to go in both directions then why add any toe at all. Forming a right angle with the line created from wheel center to pivot should work nicely as the wheel is always pointed towards the tangent of the circle.

 

[cadcoke5]

You're right. I hadn't taken into account that the set goes both ways! Any toe-in one direction will hurt us the other way. Should have seen that from the beginning.

Joe Dunfee