Pursang

My front wheels have arrived. Chinese dirt bike, nice quality.....and the price... around $54 ea. with free shipping. Whoo Hooo!
These were on Aliexpress, you need to spend a fair bit of time sussing out the bargains and pay particular attention to the shipping cost.
Bit of tension while monitoring the tracking, they were held up at Chinese airport for a couple of weeks to get on special "oversize'" freight flights.
I actually got some discount vouchers because of the delays.

My original plan was to build my own wheels around some nice 1970's Yamaha MX hubs I have. Then sourcing the 17" rims and custom spokes, tyres, tubes and rim bands, add the time to assemble and true and we are looking at hundreds & hundreds of $$$$$ for 2 wheels.

Now that I have the wheels, I can finish the pillar spacing and the stub axle design to meet the track width spec.

Original plan for driving the trike was with a rear mounted engine and a swing arm. (Simple)
In order to fit it into the available space, I was planning to extend the motor drive past the front of the wheel and put the Torque Converter on the other side of the wheel. This was not going to be a problem as one of my 'donor' engines is from a portable generator. These have a tapered drive into the alternator rotor, press the rotor apart and away we go, extended shaft, (Simple). Next step was a short swing-arm pivoting in-line with the output sprocket of the TC. (Simple... sketch attached). For proper alignment with the wheel sprocket, I would need to place the drive sprocket on the inside end of the TC. (Simple) Apparently, this is possible with a genuine Comet TC. My Chinese clone, TAV20, on the other hand, has a 5/8" shaft through the rear pulley and drive sprocket, expanding to 17mm through the alloy bearing support. (Not so simple??) New Plan in development.

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.