Custom Generative Designed Truck Project

[gavisgon]

I got frustrated with the cheap hobbyking motor mount that I bought and fitted to a Paris RKP truck that I modified. It worked ok, and I did nearly 300km on the board before I shredded the first belt, but I realised that the wear was caused by misalignment due to the motor mount moving on the hanger over time. The shaft of the motor didn’t stay parallel to the axle. It was because of this that I decided to design my own truck.

Initially I considered machining the entire hanger and motor mount out of a single billet, but it would be awkward and a waste of material, so I instead opted to machine a flange into the hanger, and bolt on a motor mount plate. This would ensure precise alignment of the belt.

I work in the CAD industry, and one of the exciting technologies experiencing rapid growth at the moment is Generative Design. If you’re not familiar with this tech, it basically involves defining a mechanical problem in the form of loads and constraints, and allowing a computer algorithm to iteratively generate a structure that fulfills the criteria of the design scenario. The latest generation of this technology takes into account materials and manufacturing methods, to ensure the generated result can be manufactured.

I have access to a 3-axis CNC milling machine, so that was the method I used for the first design. I chose to use 6061 grade aluminium.
Here is the shape that the software came up with.

I then generated the toolpaths for machining it (I use Autdoesk Fusion 360) and machined it in a few stages. First on the CNC mill, and then turning the ends on my Dad’s manual lathe.

I soon realised this design had some pretty serious flaws. There wasn’t enough ground clearance, and I wasn’t happy with the load cases. I refined these, and came up wit a 5-axis version which ticked more boxes. A friend with a 5-axis machine offered to make it for me, which will be happening soon. Here are some pictures from the CAD.

This design has much better ground clearance, uses less material, and I think it looks more interesting.

I also prototyped the flange and kingpin design here. I’ll make the motor mount from 6061 plate, and the stub axles from 4140 steel.

I’ll post more pics as this progresses.

Thanks to @pixelsilva for giving me the kick in the ass I needed to get this thread started.

[Zach]

Now this is awesome! Really wish I understood generative design more, since there seems to be so much promise behind it. Definitely following this and excited to see what you come up with!

[ShutterShock]

Really cool idea. As a mechanical engineer who has done years of CAD work, I would love to see how you set up the load cases.

Fusion360 is alright but I had some weird issues with the simulation portion in the past. I also do remember them adding a generative design module, did you use that, or one in a more professional software like ANSYS?

Again super cool, should be fun to see where this goes!

[gavisgon]

There’s a course on it which gives a pretty good intro here, with an Autodesk certification at the end of it:

Generative Design for Additive Manufacturing | Coursera

[professor_shartsis]

the design looks great… bugatti chose to use generative design on their latest brake calipers…

[rusins]

Before I criticize you, I have to say this is really cool and I love what you are doing.

With that out of the way:

1. Why not force the truck to be symmetric?
2. Seems like you are letting the rake of the truck be whatever the computer wants…
3. I doubt you’ll achieve something groundbreaking with your current constraints. You’re having it design an RKP truck hanger, and everyone knows what those look like. Making it more complex with all these additional holes / beams has no practical purpose from the looks of it. (Edit: I recall airplane manufacturers used generative design to decrease weight while remaining the same loads. I guess your truck might be a few grams lighter than other trucks, so that is actually useful.)

I think it would be better if you had the constraints of: mounts to board like this, has 2 ends for wheels, when you apply sideways pressure, it must turn. Then see what the computer comes up with! Maybe it will make a TKP truck, maybe a DKP one, maybe something that doesn’t exist! Now that would be cool!

[gavisgon]

Cheers. I’m happy to share the Fusion design with you if you want to check out the loads and constraints. We are a bit limited with the constraint and load types in Fusion at the moment, but there are definitely ways to represent the scenario accurately enough to get meaningful results. I used the Fusion Generative Design environment. Surprisingly, it’s some of the most sophisticated Generative tech out at the moment, due to the advanced solves for manufacturing method and materials etc. You should have a look at what VW have been working on with Autodesk.

Autodesk Collaborates With Volkswagen Group on Generative Design in Electric...

[gavisgon]

Good points.

1. It’s assymetric because the loads are assymetric. I have a motor on one side only, so there is a torque on that side that is not present on the other side.
2. The rake is set by the preserved regions at the connection points, so it’s as-designed.
3. I did it mainly because I think it looks cool, but generative design will put the material in the most efficient places, so if I get the loads and constraints right, I can make the lightest possible truck, that will be strong enough, in theory.

[ShutterShock]

That’s interesting, I guess it has developed more since I saw it last. I know ANSYS has been working on one for quite some time too.

I’d love to hop on that file, want me to PM you my Autodesk email?

[gavisgon]

Epic! I haven’t seen that. Thanks!

[gavisgon]

Sounds good. I’ll invite you into the project.

[Zach]

Will definitely be checking that out. Out of curiosity, mind if I hop on that fusion file aswell? Would love to see the workflow of someone more experienced than me

[gavisgon]

Here are some examples of different results generated from different loading conditions.

1st, the standard hanger for reference

And here are some of the generative outcomes

[gavisgon]

Sure, no problem. PM me your email address. Would be great to get some discussion going on the loading conditions, and get you guys to critique my assumptions.

[professor_shartsis]

if they’re for hub motors remember to factor the torque

Hub Motor / Direct Drive Axle Stress Safety Discussion

Hub motor and direct drives place additional stress on axles and trucks compared to gear drives or unpowered skateboards due to the torque the motor applies directly to the axle (a force not present in belt/gear drive systems). There have unfortunately been various instances and reports in the past of wheel separations (wheels falling off the board while riding) with some direct drive systems. Over time some manufacturers have attempted to varying degrees to address the issue by designing custom…

Hub Motor / Direct Drive Axle Stress Safety Discussion

assumptions: roughly 75kv 120a motor current, 8mm diameter axle

axle radius: 4mm

torque per amp: 0.127nm/a = 60/(2 * pi * 75kv)

total torque: 15.24nm = 120a * 0.127nm/a

total force at axle: 3817.5 newtons = 15.27nm / 0.004 meters axle radius

total force at axle (pounds force): 855lbs force = 3817.5n * 0.224 pound force per newton

^ the force applied to the axle from torque from the motor was a factor of 17 times greater than the normal force caused by someone standing on the board.

[gavisgon]

Great point! I mentioned an assymetric loading due to the torque above, but you’re absolutely correct, it’s not actually an additional torque on that side, but instead a force from the belt tension.

[professor_shartsis]

with a belt you’ll get much less torque but the motor mount itself still acts as a lever, applying some torque to the truck, though i haven’t calculated how much

[gavisgon]

Hmm… ok, that makes sense. I’ll go back and look at what I did there. I can’t actually remember how I loaded that flange.

[ShutterShock]

That last one looks pretty visually appealing. Have you considered mirroring the motor mount for 2 motors? Most of us on here find one motor to be annoying to ride with a single motor because of torque steer, loss of traction, weak power, etc lol

I have to wonder how much that would change the generation too, having uneven loading conditions would obviously make it run differently

[rusins]

Wouldn’t the force of hitting a bump with a heavy motor attached to a long mount be way more than the torque could ever be?