I’ve been back and forth on this concept ever since seeing @MoeStooge’s 3-link truck builds and my latest design iteration has proven successful enough to share some progress updates.
The goal of this project is to build an adjustable, compact gear drive and integrated truck for small urethane wheels (76-80mm). I’ll share my progress below in a general overview of the project with links to specific parts if you’d like to delve into the weeds and geek out with me.
The rapid adjustment mechanism is shown in action below. This GIF shows angle adjustment from 50 deg maximum to 30 deg minimum. Adjustment happens via the M5 x 0.8mm “leadscrew” and the 1/4-28 Ball Joint Linkage w/ Integrated Stud locks in the setting when torqued down.
Adjustment Mechanism Details
The current version was cut in 0.100" thick 304 stainless steel via SendCutSend. I designed in relief cuts to make it foldable, but getting the tabs to align properly was a little fiddly and I think I have some better ideas to improve this process on the next version.
This older version has adjustment from 50-20deg, but requires welding to lock the folds in. It is also just 25mm ID where the latest version is 40mm ID and I think the wider mechanism functions a bit better (more leverage over the truck axle).
Boardside, bushings are Tall Fatcones and streetside they are Short Street Cones from @RipTideSports - I could use some insight as to which formula would best suit this type of truck, but I’ve got a workable combo going right now with 97.5 APS on the rear and 95 on the front. Bushing seats are 3D printed and easy to swap/modify.
Gear drive is presently 15:30 1.5 module, straight cut gears for a 2:1 redux on 76mm thane. Motor spur gear is hardened steel and the wheel hub adapter gear is 3DP (and surviving made from PLA+, I’ve got better engineering grade filaments to try as well). Obviously, there’s not much clearance with gear drives on such small wheels, but it survives smooth asphalt well enough.
eSun PLA+ is my favorite material, hands down. No idea how many total miles on the gears, but I’ve got hundreds of miles on printed sprockets (#25) using the same material. It’s been cold and wet out here in western PA, I’ll update on the fate of these gears as the warmer weather gets here and I put them through some trauma.
I am surprised, though… even this open geared experiment from an earlier version is holding up well:
Awesome project dude.
Imagine there could be a clever way to get that angle adjustment bit driven on a servo/worm gear so it could be automated somehow for less steep arm angle at higher speeds?
Love small wheels, this looks amazing
I’ve been thinking about the same thing…wouldn’t be terribly difficult to make it speed dependent using VESC hardware. Way more effective than a steering damper for sure
Great that you got this post for us to drool over.
First thing I was wondering when I first looked at your pictures is weather this design could use a modified RKP hanger. Basically just cut the pivot part off and add the rod attachments. If you were to CNC a hanger for this it would take a smaller piece of alu stock than a normal one, cool stuff.
Swapping BN axles and M1 drives/belt mounts would be awesome.
Onto more crazy ideas in addition to motorizing the steering rod angle adjustment, the vertical bushing kingpin would make it more straightforward to add gearbox/motors that change the bushings preload.
Hmm I guess the near vertical angle on a TKP truck also could have that idea applied but not as cleanly.
I think the construction you’ve got is awesome. Like seeing the 3D printing, love seeing the plate metal.
A traditional bushing seat I think has more potential for more direct bushing engagement, and a better center.
But in terms of geometry there’s a few things going on.
First you’ve got axle offset happening, and with this deign it seems unavoidable. Even if you did line up the axle with the pivot axis, as you adjust the angle the alignment would change and you’d get offset again. Thats kinda neither good nor bad, but something to keep in mind, changing the angle will change the axle offset.
Something which is might be more of an issue is the fact that the offset providing negative rake. Negative rake is more stable, but its super uncommon to see because it dulls the feel of the ride so much. To put it in more traditional terms, its like you’re running flipped hangers. You’ll likely get a much better feel dropping the axle down closer to the pavement so that the axle position is in line with the pivot axis at its highest angle.
Last thing, you what to be mindful of how the angle of your bushings interacts with the pivot angle, how out of phase they are with each other. On a normal RKP truck, the bushing and pivot angle are 90 degrees out of phase, which provides maximum bushing interference and engagement. TKPs sometimes are only like 70 degrees out of phase at the extreme, these trucks looks like they might be able to go as low as 30 degrees out of phase, which means the bushing does more spinning in its seat then compressing in the turn. You may want to explore changing the angle that the kingpin comes off the deck at so its more perpendicular to the pivot axis if you’re not able to get the bushing performance you want.
Last last thing:
The thinner the bushing seat is, the thickness of the material between the top and bottom bushings, the better the bushing response is going to be. The ideal bushing seat would be a zero thickness plane.
What you’re having now is pretty darn chonky. With steel you should be able to get that seat thickness right down to within a few mm.
Thanks, I appreciate the well thought out post! You raise many valid points and after tinkering with these things for nearly two years, I’m still not really sure how comparable the mechanisms of action on this design are to RKP/TKP geometries.
The two linkages (4 pivot points) coupled with the additional motion imparted by having the axle offset (I’ve been calling this trail in my head, but caster may also work) from the bushing/kingpin/pivot allows the axle to move in an additional plane when compared to RKP/TKP trucks. It allows the axle to pitch up and down a tiny amount during the turning process…adds a bit of compliance/suspension to the motion of the axle. I don’t know if this is better, worse or just different at this point- it is definitely more “carvey” feeling.
My main goal of this project was to get the motor as close as possible to the axle to minimize the effect that extra mass has on the turning motion of the truck. This particular steering/linkage mechanism is a result of that goal, so I’m sure there are improvements to be made!
