Nice Design, which is different to the usually seen spinner, so i like it.
But there is one thing you might consider: Your upper Wheel mounting looks fragile.
So i suggest to support the wheel on both side or to use a really solid axle.
For example, i use a toolsteel axle with nearly 12mm Ø, which is supported on both ends in my feather.
The second point, it seems you have a slotted hole for the upper tire axle, what is the function?
It is good to use a lot screws, but don`t you think the wall thickness between the srew holes is at certain points a little bit to thin?

Thx.
Yeah mounting on both sides would be stronger.

The slot serves two purposes. One is to adjust the pressure between the upper wheel and the main wheel. The other is that when the upper wheel is under pressure, it will have a tendency to press up against the main wheel so it will have even more contact. Also, a part of the upper wheel's load is actually taken by the main wheel.

The big amount of screws are actually there to compensate the lack of connecting lugs in the chassis (my early designs actually had lugs, but the machinist said those would be hard to manufacture even with rounded corners.). The actual strength of the parts is something I still haven't tested or simulated. Backup plan is to either switch to fewer bigger (probably m8) bolts or to add lugs to the design.

Update:

Updated top wheel design.



Ran a few simulations in SW. For all the aluminum pieces the bolt holes didn't seem to be a problem maybe because of the pocketing in the plates weakening the rest of the plate down.

However it's a bit different for the steel parts (wedges), which have no pocketing. Now I have an F1 front spoiler.

Well the forces which actualy lead to tensile forces on the screws are mostly not the biggest challenge.
In your case the walls arround the screws are thin an if you simulate lateral forces, there will be a complete different result.

Sorry not so sure what you meant by lateral forces... Did you mean these?





In these cases the screw holes do seem to have very high stress concentration.

I then experimented with fewer, larger screw holes, but that created even higher stresses.

Well it`s not easy to communicate about such things only with writing, but i think you made it.
Your first simulation looks pretty close to what i was thinking about.
Can you simulate the forces ont the small top surface instead of the front wedge?

Well the deformation looks... umm... postmodern





With deformed result off the red is a bit concerning even at 1000N.

How do you reckon can this stress concentration be reliefed?

EDIT: btw I do also have a question about fixturing: should the tapered portion of a countersunk hole also be a part of the fixture? (the sims were done without those as fixtures)

Hello

The most important thing about FEM-Simulation is, it only works if you stay under the yield strenght from the used material.
It is also Important to select a appropriate mesh size, due the fact it will affect your simulation.
And of corse the forces and the fix point has to be set accurately.
So without nowing the used material and the picture diagramm, it`s, as my engineering teacher says, producing colroed pictures

To answer your question, i would use the countersunk area only.

Thx for the advice.