Technical Research: Steering and Anti-Bump Steering

 

Handlebars

Lever
Drag Arm
Pivots

 

The drag arm connected to the forks must be parallel to the control arms and the same length. If the drag arm isn't the same length, the handlebars will turn side to side when the suspension moves up and down.

Imagine the handlebars fighting back: that's called bump steering. Not all designers understood the geometry and as a result a lot of old cars suffered from bump steering. The Model-T had plenty of it.

Ideally the drag arm pivot would be aligned with the control arm pivots, but it doesn't have to, as shown in the next photo.

 

 

 

As long as the drag arm pivots are the same length apart as the control arms pivot, and parallel to the control arms, everything will move without binding or bump steering. When the piece on the right side is moved up or down, it remains exactly parallel with the piece on the left side.

Once again a cheap cardboard experiment proves an idea works.

 

 

 

Forks

 

 

 

 

Before building the new design on the Test Mule, I decided to try something different. I printed out the CAD drawing at full size and glued it to a sheet of plywood. Next I used drawings and cardboard to simulate the triple-clamps, the handlebars and pivots, and connected it all together with thumbtacks. The result was a 1:1 mockup which took less than an hour to make and cost pennies. Cheap and easy.

Turn the "handlebars" in any direction and the drag arms pull the center pivot back and forth. The center pivot pulls and pushes the other drag arm to turn the forks. The handlebar and the forks each have reference lines to measure how much each of them turned. As you can see, both pieces are pointing to the same 45-degree mark.

 

For the ProjectVF steering I turned to small bench-top simulations and CAD versions of the designs to figure out what worked, and just as importantly, what didn't. Getting the steering to work with so many constraints was the toughest puzzle of all. I'm still not completely sure it'll work but after the research shown below, it better.

 

 

 

 

 

 

 

 

The basic design is two pairs of levers which are the same length. A centerline (yellow) is drawn from the center of one pivot, such as the handlebars, to the center of the next pivot.

Two short lines (red) are drawn at 90 degrees to the centerline. They represent the levers. The ends of the levers establish where the drag arm (also red) connects.

The length of the drag arm is the same length as the yellow centerline.

If the levers and drag arms aren't designed this way, one drag arm will move a shorter or longer distance, creating steering that doesn't match the way the handlebars are turned.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Here's a spindly version on the Test Mule. It worked just as good as I'd hoped. The photo gives a pretty clear example of how the levers work. Note the 90-degree angle of the drag arms, and how the levers are all the same length. It's the best design so far, but getting to forks to turn is only half of the puzzle.

 

 

 

Here's a short video (with an older steering mechanism) which shows how the drag arms can be out of line with the control arm pivots without causing any bump steering. Click on the pic to watch.

The vertical levers were intended to push/pull the drag arms but the design failed. However, for the purpose of this video they provide a reference point. Watch the tops of the vertical levers and see how little they move as the forks go up and down and side to side. Good geometry is how bump steering is eliminated.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Here's a side view of ProjectVF with a lightweight steering mechanism just like the one on the test mule. The pivots of the drag arm aren't aligned with the control arm pivots, but the offset is the same at the front and the back. Compare it to the cardboard version above.

This worked very well. There was no bump steering and the forks turned the same amount as the handlebars just like a standard motorcycle. Now I need to fine-tune the design, move it inboard for aesthetics and make it much more heavy-duty.

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