Super Bee, front wheel shimmy

[oldandnewbie]

Ladies/Gentlemen,

I have replaced my gyro's front wheel assembly and now have a severe shimmy problem starting at ~20 mph and not stopping until the speed is ~ less than 5 mph.



It is a free castering wheel with ~ 3.5" of trail behind the headset rotation axis. The headset angle is 90 degrees to the keel, and I am guessing this is the problem.



I have researched the web several hours and have not found any headset angle design guidelines/calculations for this 'free castering' configuration.



Directions/links would be greatly appreciated.



Respectfully.

 

[bugflyer]

Ladies/Gentlemen,

I have replaced my gyro's front wheel assembly and now have a severe shimmy problem starting at ~20 mph and not stopping until the speed is ~ less than 5 mph.



It is a free castering wheel with ~ 3.5" of trail behind the headset rotation axis. The headset angle is 90 degrees to the keel, and I am guessing this is the problem.



I have researched the web several hours and have not found any headset angle design guidelines/calculations for this 'free castering' configuration.



Directions/links would be greatly appreciated.



Respectfully.

If you replaced just the tire/wheel most likely it's imbalanced.
Tire pressure too high could contribute.
Ideally the headset top should be tilted slightly aft.

smiles,
Charles

 

[Vance]

I have found it best to start with the basics.

Did your nose wheel shimmy before you changed the nose wheel and tire?

If it worked before it likely does not need a redesign and has something to do with the change.

What is different about the new tire and wheel?

Tire profile may make a difference so if the new tire has a different profile than what you replaced that may be the source of the problem.

Most free castering front wheels have some sort of damper to reduce shimmy and I don’t see one in your drawing.

If the steering head bearings are worn or loose that can create problems.

The Predator has zero rake on its free castering nose gear and as long as the damper (a plastic disk) is greased with dielectric grease it works well.

As Charles mentions tire pressure may make a difference and it is something that is easy to experiment with.

 

[Abid]

Ladies/Gentlemen,

I have replaced my gyro's front wheel assembly and now have a severe shimmy problem starting at ~20 mph and not stopping until the speed is ~ less than 5 mph.



It is a free castering wheel with ~ 3.5" of trail behind the headset rotation axis. The headset angle is 90 degrees to the keel, and I am guessing this is the problem.



I have researched the web several hours and have not found any headset angle design guidelines/calculations for this 'free castering' configuration.



Directions/links would be greatly appreciated.



Respectfully.

Your geometry is wrong. Just because the fork is free catsering does not mean it is good. Like you guessed that headset angle being 90 degrees is the problem. You need to make that angle correct and this will stop. Shimmy dampener can help but it should be additional item with the correct angle or risk of flipping over is still large. If you can't change this angle at all, then definitely install a good shimmy dampener before even trying to go down the runway with it.

 

[rancherman]

2 cents regarding to in my world, is called 'crazy wheels'

Hay rakes, often use these free castering wheel assemblies. Usually not even a second's thought when in 'work mode'
@ 6 mph... it's when they are hooked to a truck, for transport to the next field @ 60 mph that'll really pucker the driver's you-know-what. Probably the driver of anyone in on-coming traffic too!!
So, a neat little 'upgrade' was added years ago; the 'disk brake' damper on top. Simple 6 inch diameter plate fastened to the 'shin' of the spindle (the vertical part) which is then sandwiched between a hand adjustable 'caliper' comprised of HDPE friction pads. It's tightened just enough to eliminate the shimmy, but still allowing self tracking turns. It doesn't take much.

This is on a machine that weighs 8-10k #'s. --- each tire/spindle assembly weighs around 100#'s..(lot's o' momentum!) so scaling it back for lightweight use should be a piece of cake. I'd take a WAG and say a 3 inch aluminum 'disk' and a 'caliper' made up of some aluminum and some plastic pads would weight less than an additional 1/4 pound... maybe even less as only a SLIGHT drag is really only needed to stop the shimmy..
Dealing with these all summer long, I can testify that too much air pressure (often done in the morning on slow-leak tires) will amplify/reduce the speed at which this shimmy commences)
Here you can see the angle on the far side, (front edge of tire is slightly ahead of vertical pivot point) and the 'damper' on the near side. These fella's been building these rakes for 60 yrs, and have tried every configuration/angle/length to make the 'perfect' self centering castering system.
They went with a damper.

 

[AirCommandPilot]

Your geometry is wrong. Just because the fork is free catsering does not mean it is good. Like you guessed that headset angle being 90 degrees is the problem. You need to make that angle correct and this will stop. Shimmy dampener can help but it should be additional item with the correct angle or risk of flipping over is still large. If you can't change this angle at all, then definitely install a good shimmy dampener before even trying to go down the runway with it.

Vance's machine and all Dominators are 90 degrees. I'm sure several hundred more custom builds are also 90 degrees. I do agree with you on adding a shimmy damper, but I think the issue in this case may be the tire pressure or tire profile. That is unless the fork has been bent or the bearings are sloppy. If the axle is not parallel to the ground it will cause the wheel to constantly try to push to the side.

 

[Abid]

Vance's machine and all Dominators are 90 degrees. I'm sure several hundred more custom builds are also 90 degrees. I do agree with you on adding a shimmy damper, but I think the issue in this case may be the tire pressure or tire profile. That is unless the fork has been bent or the bearings are sloppy. If the axle is not parallel to the ground it will cause the wheel to constantly try to push to the side.

Hi
Yes I agree if bearings are bad or axle bent (which may be possible because he did say he changed it and usually you don't change it till something is fubar to begin with).
90 degree though is not what I would recommend for front fork. A slight forward angle with rear trail is better and creates a little damped response. In absence of that if you look at airplanes that use anything like this, they all use shimmy dampeners. I found this out the hard way when a Ukrainian trike (my first new trike) flipped me over because there was no dampener and there was not enough forward angle. I changed the forward angle when we fixed it and added a Shindy steering dampener from a motorcycle and did not have the problem again.