Front wheels wobble on a street fox help

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This is not the first trike owner to come on here saying they have a wobble , there have also been other models affected ?

I don't think this problem is in anyway confined to Brad's design's as they follow ' commercial trike ' practice as far as I am aware ? [ not being a tadpole builder ]

However what I don't understand is why look to toe in/out for the answer ?

My gut feeling is whether you have toe in/out if the trike runs forward without steering input it would self centre so there was an equal amount of toe in/out either side of the centre line and so would be stable ?

There does seems to be 2 contributory factors ?
Hitting a bump
Speed increasing

Very puzzling ?

The trike if deflected from a straight line should have a natural stability which maybe after a left/right weave of less and less deviation from the straight line should resume a straight path ?

Maybe there is something wrong with the geometry [ of a particular build ] that causes the opposite and the weave becomes more pronounced till it runs out of control ?

Anyone any thoughts ?

A steering damper or ' sticktion ' is only a sticking plaster for a broken limb , not a cure !

Paul
 
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My warrior build did not suffer the from the "wobblies". It tracked straight when being pushed, etc., but ...

Most of my riding is off road, encountering many bumps and camber changes, ruling out and "Look Ma, no hands!" riding.

One day, after a few years of riding, without any sign of the "wobblies", I found myself with both hands off the steering for some reason and got a hell of a surprise - it shimmied remarkably. It still continued straight but it gave me an unexpected fright. No hint of it before. Not a problem with at least one hand steering.

My reasoning is that the "wobblies" are a result of self-centreing trying to correct bump steer. It may be exacerbated by too-sensitive steering - too much turn with little input. This willalso cause increasing deviation of straight line steering at speed.

The easy fix? Keep at least one hand on the steering and ride at sensible speeds.
 
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I notice the same on my Azub Tricon. As long I have one hand on a "stick", it's tracking straight without any effort worth mentioning.
Pushing or pulling it, it tracks straight as an arrow.

But handless, above 3mph. Not a good plan. My solution, wristrests.
 
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So

The easy fix? Keep at least one hand on the steering and ride at sensible speeds.

As long I have one hand on a "stick", it's tracking straight without any effort worth mentioning.


Human steering dampers.

It would seem the initial bump adds energy to an unstable system that starts the wobble.

The speed then adds more energy to the system making it divergent [ unstable ] and not convergent [stable ]

That might imply that the system has al;most no self centering to start with and the self centering force is easily overcome by external events ?

So what creates self centering in a two wheel steering system ?

Paul
 
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My vote goes to wheels not running true and vertical. Toe-in/out not so much effect other than to make it harder to pedal and sapping forward motion and scrub tyres.
.
See at 19minutes the "twitch", but watch the whole thing because it is really well delivered and fascinating.

This is my naïve level of understanding of the process.

The faster a bike wheel turns the more gyroscopic force tries to straighten it (which allows you to let go of a rolling 2 wheeler and it carries on going, or assists you in riding "no-hands" on a 2 wheeler DF bike).

In a tadpole with 2 linked front-wheels the wheels are in harmony/opposition and these forces are trying to maintain this status-quo.
Knock them out of kilter by hitting a rock/bump with one wheel and as the yare linked by a tie-rod both of them "twitch" and the gyro force tries to straighten the wheels and this destabilises the entire system and it has to try to re-stabilise. The level of force in the gyro is directly linked to the speed of rotation. If one wheel gets out of synch with its partner its attempts to re-establish its own smooth running destabilises the partner wheel which then tries its own re-stabilising which then...(rinse-repeat) and off we go..."Death-Wobble".
I would check the vertical alignment as well as toe-in/out.
 
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The faster a bike wheel turns the more gyroscopic force tries to straighten it (which allows you to let go of a rolling 2 wheeler and it carries on going, or assists you in riding "no-hands" on a 2 wheeler DF bike).
Some of this makes sense up till the point it is simply wrong ? [ well for 2 wheelers ]


This disproves gyroscopic action and a couple of other myths , and claims it is one of the worlds unexplained mysteries
 
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No mystery. It's a combination of weight ahead of the steering axis and trail. They demonstrated the reasons themselves. No example demonstrated of weight behind was stable, just a maths table claiming so. The weight pulls the steering in the direction of fall and centripetal forces pull the bike back up wherein the weight no longer pulls the steering. As the bike gets close to upright the trail then acts to centralise the vertical steering preventing it from falling the other way. The trail only adds a small stabilising effect so uneven ground can set the bike falling again at which point the weight again steers the bike into the fall and centripetal forces once again pull it up etc etc. On a very smooth surface and/or at high speed gyro forces will act as a slowing mechanism to this process, meaning trail has sufficient time to exert it's force to prevent the steering moving enough for gravity to really take over and start the process of fall, turn, stabilise.
There's a big difference between vertical stability and a shimmy though with a two wheeled tank slapper having much more to do with rapid steering input than gravity. That rapid steering input can be from a number of sources. On a trike it could be bump steer or bearing failure. It could be differing caster angles each side or differing tyre pressures.
 
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Some of this makes sense up till the point it is simply wrong ? [ well for 2 wheelers ]
Sorry. My wife also says I am wrong....about so many things. :whistle:
But, I think the free-running 2-wheeler part is a red-herring and the precession, snatching element of a wheel "disturbed" (as shown in the lecture) and the linkage of 2-wheels where one can directly influence the behaviour of its sibling is still very much relevant. .....although you may still find evidence to tell me I am still "WRONG". ;)
 
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All interesting discussion.

I woke up in the middle of the night thinking about it. I've never tested for hands-off shimmy on a sealed surface and I will try and remember to do that when I get an opportunity. However, using the centreing test mentioned in the Warrior construction manual proved that mine centred as required on concrete and sandy surfaces when pushed forward. My two or three shimmy experiences have only been on hard sandy surfaces with various camber changes and other small variations from flat and level.

I tend to think that self-centreing is due to more than toe-in. I would expect that caster has a greater impact and this will also be affected by the mass of both the trike and the rider. What causes shopping trolley wheel shimmer? Their wheels aren't cross connected. What causes the rear wheel steering shimmy for those who have tried it?

All things being considered, is wheel shimmy mainly caused by insufficient caster?
 
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Wheel shimmy as far as I know (my knowledge is based on what I know about automobiles, not trikes) is due to insufficient caster or loose linkage. I'd imagine that toe-out would also cause it, but I've never dealt with too much toe-out.

Perhaps you could temporarily run a smaller wheel in the back as a test. This will increase your positive caster which should stabilize things.
 
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But, I think the free-running 2-wheeler part is a red-herring and the precession, snatching element of a wheel "disturbed" (as shown in the lecture) and the linkage of 2-wheels where one can directly influence the behaviour of its sibling is still very much relevant. .....although you may still find evidence to tell me I am still "WRONG". ;)
Can't find what I am looking for will this do ?

gyrobike

or this ?

Gyro effect to small to balance a bike

There used to be some stuff on the net of a normal bicycle fitted with 2 extra wheels one mounted above each of the normal wheels , they were close enough that the tread on one tyre touched the other.
So as you rode the bike the lower wheels went in the normal direction and the upper wheels spun at the same speed but in the opposite direction .....

It was ridden quite easily and showed no bad manners ?

Very true :-What causes shopping trolley wheel shimmer? Their wheels aren't cross connected.

castor of course is also very hard to measure , especially once the trike is built ?

Paul
 
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??? Caster is easy to measure. Simply place a smartphone against the kingpin, vertical when looked at from the front. Any relevant app will tell you the angle.
 
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??? Caster is easy to measure. Simply place a smartphone against the kingpin, vertical when looked at from the front. Any relevant app will tell you the angle.
If only it was that simple grass hopper !

There is only 1 place you can measure castor and that is a line on the kingpin that is at the furthest point of the cylinder [ either front or back ] from the frame before the king pin curves away from the frame ?

Usually a king pin has cups for bearings that reduce it's height to less than a smart phone

Smart phones do not have square case edges , they are rounded so cannot lie flat and perpendicular ?

I gave up trying to use one to determine prop shaft angles and bought one of these instead:-



Now this will be hard to use as it has a flat base and a king pin is generally round not flat ?

Paul
 
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If only it was that simple grass hopper !

There is only 1 place you can measure castor and that is a line on the kingpin that is at the furthest point of the cylinder [ either front or back ] from the frame before the king pin curves away from the frame ?

Usually a king pin has cups for bearings that reduce it's height to less than a smart phone

Smart phones do not have square case edges , they are rounded so cannot lie flat and perpendicular ?

I gave up trying to use one to determine prop shaft angles and bought one of these instead:-

Now this will be hard to use as it has a flat base and a king pin is generally round not flat ?

Paul
If your kingpin curves you should straighten it Paul! The kingpin is a straight line and as such can be measured at any point along it. I assume you mean the curve of the headtube? Written English is often an opportunity to fail to express your meaning. I've been misunderstood many times when I thought I was clear.

So what if the kingpin itself is shorter than your phone. It doesn't stop you measuring it but can actually make it easier. Cups are always very uniform and almost always the same top and bottom on cheap donors so can be used as points to measure across perfectly well. The angle measured across the cups should be exactly the same as the angle of the headtube itself. If there is any variation in the two then the angle across the cups will be the more accurate as they contain the bearings that hold the pivot. The headtube is just the means of holding the cups. The most important thing is that both sides are near enough the same. The actual amount of caster can be anything within a fairly large range. The nearest degree will do for our purposes.

Almost every smartphone I've ever seen in the last 10 years is rectangular with rounded corners. The bits between the corners are usually straight as a die so can lie flat to one edge perfectly well.

The gauge you have almost certainly works on the same principles as a phone and app with exactly the same sensors and accuracy but is more limited than an app by it's software and display being fixed and basic respectively. The shorter edge over a smartphone will equal more guestimation when trying to hold it against a shape such as an AZ kingpin just as a short spirit level is more prone to errors than a long one. That shortness may prevent you measuring across the cups too. Get a good app on a phone and it'll be perfectly tolerant of being less than vertical in one direction to measure another. ie it's perfectly possible to lay the phone directly up the front face of the headtube at it's lean in angle across the two cups and still measure just the laid back aspect of it and not the lean in of it. One angle will be ignored to measure the other.

I used a smartphone and app to weld the two tubes that form the important part of the uprights on the Drypod2 - the kingpin and axle mounting tubes. This joint is critical both structurally and in terms of accuracy. The angles were set to the nearest half degree by holding the phone to the edge of the cylindrical steel in exactly the same manner as I would to measure caster on a headtube. For certain holding it in exactly the right place is difficult against such shapes but not so difficult that taking several measurements to assure myself of the accuracy was onerous. Without using this method I'd have had to weld the two tubes in situ on the trike meaning a big effort in mocking and holding everything in place whilst welding it solid. A few minutes measuring saved hours of jig making.
 
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