[mickey thumbs]

Even the best of my newly acquired GT3 cars have a tendency to “dance” or chatter on acceleration out of corners. chrisguyw has been a big advocate of bracing the axle holder uprights on SW and AW cars, recommending the Slotting Plus combi bushing/brace or aiding a wire U brace. So, the Slotting Plus product is $10 and currently out of stock at my usual supplier. My chances of accurately bending wire so that it fits snug but not so tight it distorts the uprights are nil. So I looked around the workbench and came up with some 3/32” square brass tubing that even I could cut and file to fit. Secured in place with some IC2000 adhesive it works great. The chatter is significantly decreased.
   

If building a car from parts or for a Proxy, the Slotting Plus setup is worth tracking down. But for a quick, easy and cheap solution I am happy with this.

[Kevan]

Make your own axle tube, get a length of 1/8" brass tube the right length to fit inside both bushings (put the recesses on the inside), when you've got it the right length take the bearings and brass tube out, slide them onto an old axle smeared in oil, put axle collars up to the bearings or slide a length of silicone tubing over each end to push it all together then solder the brass tube into each bearing. Smearing the axle in oil will prevent solder sticking to it. 

[Nonfractal]

A main source of shuddering is flexibility in the motor to axle alignment.

I see you have put a short can motor into a long can motor mount.
If the motor is only anchored on one end , this is asking for the kind of behaviours you describe.

Shims are available on thingiverse for 3d printing.
The shims will anchor the motor on both ends and also help to centre the weight of the motor in the centreline of the chassis.
A small dab of epoxy at both ends will ensure that the shims stay in place.

Below is a pair of shims anchoring a Slot.IT MX16 S-Can motor into a Black Arrow motor mount 

   

[rallyhub]

So I looked around the workbench and came up with some 3/32” square brass tubing that even I could cut and file to fit. Secured in place with some IC2000 adhesive it works great. The chatter is significantly decreased.

Maybe next time you can think of a vertical strip instead of square. It gives even more rigidity.
For example: Instead of square 3/32" you can take a 1/32"x1/4" brass strip, which is about 10% lighter and its moment of inertia is more than six times greater;
which means six times stiffer.
Ix=1/12bh³
Square 3/32" Ix=2.673mm4
Strip 1/32"x1/4" Ix=16.93mm4

Hub

[mickey thumbs]

Interesting. I’ll try a strip on my next one.

[Anthony B]

Guys - very interesting - this may be a dumb question - but I assume this for non mag cars ?

Mag cars this should not be a problem?

[Kevan]

Can't answer that for myself as I've never raced with magnets.

[mickey thumbs]

I run on a wood track, so non-mag here. Don’t know about mag cars. As I understand this problem was most notable in old Ninco cars and was known as Ninco hop. The Slotting Plus axle tube-bearing combo was developed to address the chassis flex in Ninco.cars. Just located a supplier today  and ordered a couple to give them a try.