Drift Control

We took some more steps moving forward to control the drifting and make the trike more reliable in cases of weaving in and out of traffic.  First we just needed a way to keep the gate lever up or down.   The weight is mostly on the back side of the pivot point in the lever such that the gates want to stay down over the wheels.  Originally, we solved this by just having a bungee cord hold up the gate lever when driving, and when they were flipped down, the gates gripped the wheel brackets firmly enough that they'd stay down despite the tension from the bungee cord.  However, when driving over bumps and over long distances, the cord would eventually still work the gates back up.  So I wanted to have the lever freely pivot without the tension, and just have a clip on the trike frame to hold down the lever handle when the rider wants the gates up for drifting.  I started by just bending some sheet metal strips around the same sized steel tube.

It was easy to form by hand, and I ended up with something that clipped onto the tube pretty firmly.  Then Jack showed me that he had a bunch of commercial clips of the same type, meant for holding brooms and stuff, and I liked those better than the one I threw together.  

Mine vs. Commercial Clip

I decided to hold the clip to the bike frame with rivets, which are quickly becoming my favorite fastener.  Again, it's quick and easy and did the trick just fine.

The final configuration held the lever in place to a sufficient degree:

Gates Up

Gates Down

Further Wheel Confinement 

The final adjustment was very simple, but made an incredible difference in the steering dynamics.  I just put a big bolt through the caster mount that served as a hard stop for the wheel bracket and restricted the range of motion of the casters.  Now, they were both limited to just under 180 degrees of motion as opposed to the original full 360.

Using my new handmade GoPro mini, I took a video of the rear, left caster wheel while I'm driving to show the full range of motion in the new configuration.  I start off just going straight, disengage the gates, and make some tight turns during the bumpy part where I go over a set of railroad tracks before straightening the wheels and flopping the gates back down again.  The video shows that the gates align over the wheel brackets a bit more easily than expected.  Sorry that the strap from my backpack keeps flying around in front of the camera!

If you saw some of the earlier videos of driving in the hall, it was difficult to control because during turns, the caster wheels would continue to rotate until the trike completely spun out and the casters reached a stable position with the trike rolling in reverse.  This left the rider with no control over the trike, and the motor didn't have enough power to get the vehicle back to moving forward with all its momentum rolling backward.  The stable state of the caster wheels refers to its natural position where the contact patch of the wheel with the ground is behind the pivot/rotation point as it moves forward, or in other words, the wheel has positive trail. This is the case for the front wheels of a shopping cart.

Since the hard stop of the bolt hitting the wheel bracket prevents the casters from over-rotating, the trike can still drift, but the rider is in a position to steer out of a turn and keep going instead of spinning out and flying backward.  Additionally, when pushing the bike directly backward, the casters can't reach a stable position, but rather rotate as much as they can with the constraints until they reach a symmetric configuration: 

Outside

Inside

On surfaces that give the wheels sufficient traction to not slide, these configurations stop the bike from rolling backward, just like the way to stopping in introductory skiing.  When rolling the trike in and out of the elevator, it's nice to be able to engage the gates, and then it can still be pushed forward and backward in a controllable manner with the wheels staying parallel.

Also, if you notice in the pictures above, the restricting bolt is not quite centered over the wheel such that the wheels can rotate more to the outside than to the inside.  This adds a nice effect where the caster on the inside of the turn can make a sharper angle than the one on the outside, and really helps to not only drift, but also smoothly move the trike around and through the turns. I suppose this is an unsophisticated but roughly comparable form of Ackermann steering.