Where would the front castor be measured from?

Tri gear, front wheel

Working on a friends CZech Sports Cruiser, he has it completed but for substantial body shake/shimmy upon applying brakes.
This is a known problem with an ad to install two belleville washers in the front vertical spindle (item# 26) and limiting tire pressure (done).
Rear brake rotors dial true.

Improper castor comes to mind as a frequent cause in non aircraft related builds but I cannot figure where this measurement would take place (in degrees) here.

Would this be measured in the vertical spindle or the trailing Fork assembly (item #9)
In virtually all pictures seen of this assembly this fork is quite angled as depicted in the attached pic, his is all but horizontal.

opinions? open for any.

Thanks
Ken

How much effort does it take to swivel the wheel by hand with the nose wheel off the ground? I would start with the pivot tight enough so that 6-8 lbs of force is required to make is swivel.

Marty,
That is part of the CZ AD mentioned, 4-6lb it is set at 6lb as well as max tire pressure of 16 pounds, you're close.

Thanks
Ken

Next suggestion is to make sure the swivel axis is perpendicular to the ground with normal weight on nose wheel. If the angle increases as the front suspension compresses, that could be the source of your shimmy. Item #6 might have to be bent or arched to correct this. The angle of the fork you refer to is insignificant.

That tube had been bent 5deg negative in an accident, it was straightened to make the swivel axis straight with no change, we then added 3deg positive castor, no change.
I know I am stuck on castor hence my question on where it would be measured, can't quite wrap my thinking around the trailing fork with respect to castor.
I do know castor can cause all sorts of shimmy in street rods if it is much below 6-8 degrees.
If that fork, now virtually horizontal needs to be as shown in the pic then that is likely the problem, that said, it does not appear to be bent.

For laughs, that #6 tube assembly with no fork, just the tube is $5k plus shipping.

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Originally Posted by Blue Chips

I do know castor can cause all sorts of shimmy in street rods if it is much below 6-8 degrees.
If that fork, now virtually horizontal needs to be as shown in the pic then that is likely the problem, that said, it does not appear to be bent.

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I don't think that is your problem. Trail, or the horizontal distance from where the steering (swivel) axis intersects the ground to the point where the tire intersects the ground is the same whether the fork is inclined or horizontal. In both cases, caster is positive because of trail. I would still set the steering axis vertical with normal weight applied to the nose gear vs inclining or castering it 3 deg.

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For laughs, that #6 tube assembly with no fork, just the tube is $5k plus shipping.

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Thank God it's not a certificated aircraft part.

The caster would normally be measured under load at the vertical spindle. Belleville washers seem to be a common way to address this issue. That's what you see on the Cirrus machines, the Velocities, and various other free castering nose wheels. The Belleville washers do wear out, so often times are replaced at annual and do need frequent adjustment. They are there to set a preload to resist turning, which should dampen out shimmy. But first, you need to make sure the geometry is correct.

-CubBuilder

Unfortunately, correct geometry is an unknown, manufacturer is no help, same with State side dealer, personally my thinking is they don't know and only want to sell and/or install parts so it's has been trial and error.

As mentioned, that vertical spindle has been -5º, 0º and +3º castor, none of which made a difference, at least a noticeable one, one would think it would be but that geometry may just not be that critical.
that said,
I've looked at at least a hundred (on-line) front castoring wheels with trailing fork, virtually all have shown to be at a distinct positive angle from horizontal, that can't just be a coincidence.

Thanks for the input, it's all good input.