Daryl's "Vintage Morgan" Inspired 3 Wheeler.

Sliding Hub Suspension - Part 2

Previously, I described how the pillar doesn’t need to be inclined rearward to produce caster.
This time, I will try to explain the ‘drawback’ of pillars that are inclined to the centreline of the trike. (below is a pic of recommendations for a 4 wheeler.)

The hubs slide up the pillars. If the pillars are narrower at the top than the bottom this will produce Toe-in on bump and Toe-out on rebound, (assume steering arms at the rear). Bad enough for stability in a straight line, now consider the Bump Steer produced by only one wheel compressing on a bump. Even worse, the strange alignment caused by the trike rolling in a sharp or high-speed turn. Outside wheel compresses causing Toe-in, inside wheel extends and Toes-out. Even if they somehow manage to ‘balance each other out’ your Ackerman is shot and the vehicle is no longer pointing in the same direction as your input at the steering wheel.

But! But! What about the Scrub Radius?
Scrub Radius is the distance between the steering axis and the centre of the tyre contact patch. It can be +ve , -ve or zero.

Scrub ‘sounds’ Bad… so zero must be Best!!???
If you manage to produce zero, don’t cheer yet. You have now got SQUIRM.
As the tyre tries to turn around the centre point, one half of the contact patch is scrubbing forward and the other half is scrubbing backward. Tyre wear and instability result.

Negative is less than zero….. so -ve must be Best!!??
-ve has some advantages for modern cars, but not so much for Cyclekarts. In FWD cars -ve scrub is less affected by torque steer. Also brake inputs, helpful with ABS and failures in split systems or in the case of a flat front tyre.

So...+ve? Older cars had +ve scrub. It helped parking without power steering because the wheel could roll as it was steered. +ve scrub provides more “feel” to steering inputs because of leverage caused by the radius.

Conclusions: For me, Vertical pillars and some Positive scrub radius.
We have skinny motorcycle tyres, minimal weight, and no front brake issues to consider.
If I want to (or need to) reduce the scrub, I have another option. I can use stub axles that are not perpendicular to the pillars to create positive camber, this will bring the bottom of the wheels closer to the bottom of the pillars. It is a classic Morgan trike look and I might do it just for that!

Cheers, Daryl.