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Leaning trike single-shock suspension revisited

October 26, 2023 — BarryK

The previous blog post considered a design with two shock absorbers:

And before that, a single-shock-absorber design was considered:

The single-shock design is simple, so decided to revisit it. But, a major rethink of the dimensions.

I found this video useful:

...he rejected the single-shock design, but I have figured out the coordinates such that it looks like the best option considered so far. I do not need the extreme lean that he wants for his velomobile; my trike will be trundling along at around 25km/h maximum speed, no fast cornering.

Here is the SolveSpace design:


The SolveSpace file, with false ".gz" appended, is here.

I decided on a 165mm shock absorber. Reason is, there is a vendor on Aliexpress that sells 165mm shocks with a range of spring strengths. From memory, I think from 350 pounds to 1500 pounds. They are just coils, without any air or oil dampening; don't know if that will cause the front of the trike to bounce around excessively. Have no idea what spring strength will be best; most of the weight of the trike will be toward the back, so a lighter spring is probably going to be the best choice.

As before, a graph showing scrubbing when both wheels hit a ripple on the road:


...I optimized for small ripples. A 20mm vertical deflection of the wheels has negligible scrubbing. 30mm has 2mm scrubbing, which is 1mm per wheel.

This next image shows how much the rider can lean into a corner. The limit is that the inner-mounting of the lower-wishbone will touch the side of the shock absorber. But, 64 degrees, that is, 26 degrees off vertical, seems like plenty:


...see those 60mm struts supporting the shock absorber; the length could be increased if more lean is wanted. The above diagram is probably not the same as the true situation when cornering; haven't taken into account the extra weight of the trike while cornering causing the shocker to compress, which should increase the ground distance between the wheels a little bit.

One thing we do not want, is to fall sideways when the trike is stationary. It will be hard-limited anyway, to 64 degrees. But, we really want for the trike to stay vertical. Nominal vertical stability while stationary is achieved due to the front tyres being a fixed distance apart. In this situation, this diagram shows what happens if there is a slight lean:


...the slight lean causes the shock absorber to compress. In the above diagram, it has compressed about 2mm. Thus, the trike should stay upright; "touch wood".

As stated, suspension has been optimised for small bumps. This also applies to one wheel hitting a bump, for example, a 25mm (1 inch) bump. I simulated it by increasing the radius of the right-side wheel:

img5 scrubbing. Test a 2 inch bump:


...about 2mm scrubbing. Now hit a massive 4 inch rock:


...the shock absorber is getting close to it's compression limit.

I can see why guys spend years playing with suspension designs in their garage! Have to decide on a final design, well, what will hopefully be the final design. That 200x255 rectangle is an aluminium plate (there will be two of them); can drill holes elsewhere if need to experiment further.      

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