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Old 08-29-2021, 06:30 PM
dremu dremu is offline
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Default Re: 1:10ish Alvis Stalwart 6x6 high mobility carrier, wrongest of the wrong

So the first step was to throw together a CAD mockup to get an idea of size, sort out what tires would work for scale, etc:

Was actually difficult to find tires with the right NDT pattern, didn't want the modern tread. RC4WD has a set for DUKW's that worked out about right.

Totally jumping ahead there in pix, but for whatever reason I really like tires & wheels to be as accurate as possible. The wheel isn't quite right, didn't have enough room to get all of the bolt heads in, but the hub does actually come off to get a a nutdriver down there.

Next up was sorting out what all that crap in the cutaway did. Like the drive, the steering is per-side:

Highlighted is one side only. Steering rack turns a longitudinal shaft which pushes an L-shaped steering arm which has a linkage to the knuckle on the first axle. The arm is bent to clear the suspension, which we'll get to momentarily. That first axle's knuckle has a linkage to an intermediate arm, which in turn has a linkage to the knuckle on the center axle. That intermediate arm doesn't hinge at the center, but is hinged at an offset of about 2:1, so that the center axle turns about 1/2 as far as the front one.

I've seen a coupla RC builds that used straight axles and had separate steering servos for the front and center axles. Where's the fun in that? It's easy and simple and it works! For my build I really wanted to replicate the 1:1 in mechanical function (or lack thereof) =))

One part of the suspension that drove me crazy for quite some time is the torsion bar springs, because they're *also* longitudinal. I did torsion bars for my M1 Abrams, but that's easy; they're located *laterally* and couple to the arms that hold up the road wheels.

Highlighted blue is the spring for the center axle. I finally sorted it, with much arm-waving and looking at pictures. The upper control arm is hard-mounted to an outer tube. Inside that tube is the spring proper. The spring is affixed to the tube at one end, the front/left in this picture. The aft end of the tube is open; the spring passes through and has an adjuster which pushes against the hull. I used 1mm (or maybe 1.2mm, I forget) music wire, also sold as spring steel. It is resistant to being twisted and is, well, springy The outer tube is just your basic K&S brass; they're soldered together at the one end and then the adjuster is a 3D printed piece with a threaded brass insert and a bolt through that.

I've grown to love those inserts, call 'em "threadserts" for short. They come in a couple flavors and a bazillion sizes. I get the ones that are straight knurled on the sides, so I can just 3D print a hole in my part that's slightly smaller than the threadsert, and then draw the thing in with a fender washer and a bolt from the far end.

There's a baggie of the threadserts and a front knuckle, which has I think eight of them (you can see two on the left side.) Back to the suspension insanity, here's an early CAD of the setup, looking from below (front to the left, the two grey boxes being the steering servos)

The torsion bars are somewhat more visible, ie in the upper right with the upper control arm (red) U-shaped around it, the capped end at the left and the adjuster (blue-green) and its bolt at the far right.

Also had difficulty finding shocks to fit. Once again the 1:1 is annoying: the front axle's shocks mount under the cab and go to the upper control arm, but those for the center and rear axle mount to the water drive box and go to the *lower* control arm, passing by the half-shaft. There's also the matter of the 1:1 using 1960's technology (or lack thereof) so as you see in the cutaway there's four shocks (actually, IIRC, two oil shocks and two rubber rebound dampers.) In the interest of actually getting working shocks at scale, I had to limit myself to two, and even then it's a very close fit.
I mean, how hard can it be?

Last edited by dremu; 08-30-2021 at 12:52 PM.
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