Saturday, September 14, 2019

Wltoys 12428b (part 21: rear suspension geometry changes)

While i was very happy with the CVA shock upgrades, the rear mounting (install ball mounts from standard shocks and install i) left a little to be desired, in that:
  1. Snugging up the top screw placed sideways pressure on the shock top, and i could see whiteness (plastic fatigue) where the shock top joins the cap.
  2. The spring required all the spacers I had and even then the ride height could have been higher.  Upon removal, the spring had compressed.
  3. The shock top mount was non-adjustable.
So, i decided to make up new rear top shock mounts.  Unfortunately, I'm still rather devoid of any actual trained understanding of suspension, but I worked on the following approaches:
  1. Suspension components require compliance (movement) in use.
  2. But mounting points themselves (this part) should be solid.
  3. Parts should not be under any static fatigue when installed.
  4. Parts should not be exposed to any avoidable fatigue in use.
  5. The part should allow adjustment for final positioning and tuning.
  6. Anchor points labelled consistently between designs; A and B (see pics)
  7. Incorporate only parts from the 50519 and 50520 kits, specifically use the "hex ball head connector" (ala tamiya 53968)
  8. For maximum rebound effect, the shock should be as vertical as possible.
  9. The shock should be allowed to always retain it's full travel and not be overly compressed when variously adjusted.
  10. The final assembly should not foul the chassis and/or body in any respect.
  11. Where all the above are satisfied, body roll should be decreased by lowering the center of gravity by not using overly stiff springs or via an equivalent adjustment.
So, that setup, I variously started with some drawings, and then mocked up in cardboard, test fit, then onto test parts (1.6mm aluminum sheet).  The process involved several designs:

Designs are numbered, left to right, top to bottom.
V1 through v5: These were produced on a principle of three anchor points being good and strong (A, B and the 3rd not labelled but it's visible in the part marked v4).  The idea here was to have the suspension mount to one of the line of numbered holes along the edge.  This design was abandoned because ultimately the angle precluded use of the hex ball head connector if mounted directly on the part (a stand off of some sort would have worked)

V6 through 9: Worked on the idea that i needed to be actually able to use the hex ball head connector, so it incorporated proper clearance from the start.  It also reduced the number of mounting points down to two, given that 3 was overkill here.  Adjustment in this design was incorporated by removing the mounting bolt A and rotating the part.  This in turn moved the shock top through an arc.  The precise arc being difficult to determine, hence the incorporation of a separate arm used for positioning (v8), then finally built up in aluminum.  This design was abandoned because making the adjustment was fiddly, and the optimum angle was hard to find.. i felt like i needed more science and the fun factor was waning..

and finally V10: Back to basics.  Two solid chassis mounts (A and B), a single non adjustable mount for the hex ball head connector in a good average position which didn't require spacers and allowed the shock its full travel.  The whole assembly installed using long bolts and using the brass stand off pieces (two of which are missing from this picture):

Two brackets machined together, then one reversed for install.
Finally, installed it all into the car.  In this picture it's clear how the brass standoff tubes have been used to give clearance of the bracket from the chassis, and to move the shock into a more vertical position:

The final part as installed on the car.

Verdict: On the road it feels pretty much the same as before :) but I'm happy that the shock top won't fatigue early, and also that I admire my handwork through the quarter window behind the drivers seat :)

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