Couldn’t sleep last night so I decided to put the time to use and figure out how to improve my motor mounts.
Let’s start from the top. Following multiple revisions based on all of your feedback last year, this was the design I settled on to fit the Caliber III stock profile:
This design seemed pretty solid as a first prototype but it has become apparent there are many issues which need to be addressed.
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Internal sharp corners which are impossible to machine. These had to be converted to small radii during machining which prevents the hanger profile from making full contact with the flat faces.
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There isn’t enough space to get a socket around the nut. I desperately tried and went through about three sockets. The hole was originally threaded but that didn’t last long. A spanner around the nut works, but only when the clamp and arm are disassembled. Once the arm is assembled, it is impossible to make any adjustment to the mount position.
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The relief cut isn’t deep enough to facilitate enough deformation when the clamping screw is tightened.
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Even if it did deform as much as expected, it would cause the two flat faces closest to the opening to create a taper as opposed to converging flat against the hanger profile.
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Caliber III hangers are cast. This means they have a very imprecise profile tolerance. On top of the poor clamping force, internal fillets and tool deflection from my friend’s old decrepit CNC, this clamp stood no chance of holding up to any sort of vibration. After my first 5-10 mins on the board, it fell off. This is what led me to buy a set of BN 220’s.
I was under the assumption that the clamps would fit BN hangers better due to the extra precision. At the time I thought it was just the cast profile which was the problem, but alas I was wrong so I ended up “reworking” it…
On one of the clamps, I drilled, counterbored and tapped a hole then screwed in an M4 socket head screw and loctited it in place. Unfortunately the tightening torque sheared the bolt in half, so now the mount is held on by a couple of threads and some threadlock. That’s not exactly confidence inspiring when you’re going 30mph.
On the other clamp, I got distracted while drilling the counterbore and accidentally went through to the internal pocket. I tried to drill a new hole in a different location but the drill bit snapped inside the clamp and I couldn’t get it out. I went through almost every drill bit I own, trying to remove the stuck bit but had no luck whatsoeber. I didn’t have the right size tap and/or screws to work with the larger hole so I made do by gluing in an M6 screw and capping both ends with epoxy putty. I did warn you, it’s not pretty.
So yeah, my motor mount situation is sort of fucked right now. But I’ve rode about 50 miles or so on this frankenstein’s monster of a drivetrain without any major dramas. However in the last week or so, I’m noticing the mounts are becoming a tad wobbly. That spurred me to stop putting off the redesign and get to work.
I came up with this design which I was confident in getting machined and using for the next prototype:
This version has all of the right fillets and clearances to enable the flat faces to interfwce parallel to the hanger. But after revisiting the design last night, I’m still not happy with this solution for one reason.
In theory if the clamp is tightened to the trucks first and then the arm is fastened onto the clamp, the index hole positions will have moved due to the clamping deformation and therefore produces a risk of stripped threads and difficult assembly. If instead the clamp is tightened once the arm is in place, we still have the issue of not being able to put a socket or spanner in the space and therefore it will be difficult to apply enough clamping force for a secure connection.
Other motor mount designs overcome this in a number of different ways:
Mboards and a few others use a 2 part clamp. I’m not a fan of this as it would severely limit the amount of rotational adjustment, plus the arm is only secured on one side of the mount which could be a risk for heavier motors.
Another common solution is slotted holes. These may take up some the deformation from clamping, but by relying on friction for the rotational constraint on the arm there is a high risk of the fastners vibrating loose.
Using some kind of toothed/notched system where the arm slots over the clamp and is somehow constrained axially could work, but would create a tradeoff between the resolution of rotational adjustment and the risk of the notches slipping. This is because the valleys between each tooth would need to be filleted, and with the smallest possible tool diameter, the smaller the distance between each tooth/notch, the closer to the tip of the tooth the fillet gets.
The grub screw clamping method is hated by many for good reason, it’s shit. It doesn’t provide enough contact between the clamping faces and the hanger, is prone to loosening via vibration, and leaves ugly marks.
So here is the solution I’ve come up with. I’m probably not the first to try this, but in my research I haven’t seen anything similar.
It’s pretty self explanatory. The tool clearance has been carefully considered to ensure the primary clamping surfaces aren’t compromised. There is a spring washer under the head of an M6 bolt to help protect against vibration. Paired with some loctite, I’m confident this will be more than sufficient.
My only concern is stripping the aluminum thread. However unlike the previous clamping method, the load through the threads will be perfectly aligned with the holes. Maybe a helicoil would work?
I’m considering adding a small spring washer inside the “insert” piece instead of under the cap head to serve the additional purpose of creating additional tension between the screw and insert to support the clamping force generated by tightening the screw.
Anywyay I’d love to get some feedback on this. Has anyone tried anything similar? If so, how did it go? Are there any glaring issues that I’ve missed?
