Trail, Geometry and "The Wobbles"

[Chris Westland]

Just thinking out loud, but wheel wobble in EUC’s can be explained in the same way that we think of bicycle instability.  In my metaphor, mechanical trail (normal trail) is the perpendicular distance between the projection of the CoG onto the ground and the point of contact between the front wheel and the ground.  This latter point is actually a patch (a small ellipse) whose size varies with the inflation pressure of the tire.   Trail in bicycles or motorcycles is always designed into the geometry behind the contact patch (in the direction of motion) because when it is forward of the patch, the bike is highly unstable.  EUCs don’t have a fixed steering geometry like bicycles, and therefore mechanical trail effects are entirely determined by the projection of the CoG on the ground (forward or back of the contact patch).  Mechanical trail is one of the most important variables in determining the handling characteristics of motorcycles, bicycles and (in my estimation) EUCs.   

In bicycles and motorcycles, larger trails force the bikes in into stable, straight-line motion, because the rear wheel drags the rear part of the front wheel along the axis defined by the front and rear contact patches.  For that reason, larger trails are favored by inexperienced riders because they are more “stable” (at least in a straight line).  Think Harley-D.

In theory, stability increases as the computed deviation from the ideal path during steering action is reduced, and thus a trail of zero is ideal because it eliminates:

1.     influence of the position of the center of pressure of wind forces coming from the side, and

2.     wheel flop effect.

What I think riders on this forum are experiencing as “wobble” is wheel flop by another name – a steering behavior in which a bicycle or EUC tends to turn more than expected due to the front wheel "flopping" over with any steering input right or left. Wheel flop is caused by the lowering of the front or back end of a EUC as the pegs are rotated away from the "straight ahead" position.  In wheel flop, the force due to gravity will tend to cause rotation to continue with increasing rotational velocity and without additional rider input on the pegs. Once the pegs are turned, the rider needs to apply torque to the handlebars to bring them back to the straight-ahead position and bring the front end back up to the original height.  The rotational inertia of the wheel will lessen the severity of the wheel flop effect because it results in opposing torque being required to initiate or accelerate changing the direction of the wheel. Increasing wheel flop increases the torque required to bring the EUC back to the straight-ahead position and increase the EUCs tendency to veer suddenly off the line of a curve. Also, increasing the weight borne by the front wheel of the EUC, either by increasing the mass of the EUC, rider and cargo or by changing the weight ratio to shift the center of mass forward, will increase the severity of the wheel flop effect. Increasing the rotational inertia of the front wheel by increasing the speed of the EUC and the rotational speed of the wheel will tend to counter the wheel flop effect.  

I suspect, more fundamentally, the contrasts we see in EUC performance between say a Gotway Monster (22” diameter & 70 lbs) and a Gotway Luffy (10” diameter & 16 lbs) are basically the result wheel flop.  A certain amount of wheel flop would generally be desirable, but an EUC with too little wheel flop would be sluggish in its reactions to turning inputs on the pegs; one with too much wheel flop would tend to veer off its line at low and moderate speeds.