Steering Wheel | Deferred Procrastination

Recumbent tricycle steering is normally one of four types:

Direct steering - essentially a half-handlebar, directly connected to each front wheel pivot, with a bar connecting the wheels to keep the steering angle consistent side-to-side.
Yoke steering - a thinner version of bicycle handlebars. Often the bar position is not fixed, so they can be moved out of the way for mounting/dismounting.
Underseat Tiller - A bar mounted under the seat is pivoted around a vertical axis to steer. Often the bar ends are vertical to raise the rider’s hands to a comfortable position.
Sidestick - A pair of linked, vertical bars are pushed forwards/backwards to pull the steering arms on the wheels. I don’t have an image I can embed, but an example is available here.

Direct Steering on a Terracycle Cargo Monster Yoke control in Pjotr320’s Mango Sport Tiller Steering on the Pterosail trike

The design of a steering system has to balance two different needs:

Steering should be smooth and easy to control around the straight ahead position. So a small movement at high speed should not unsettle the vehicle
The steering movement to reach full lock should not be excessively large. A long wheelbase vehicle with a small turning circle must turn the front wheels by a large angle.

The Atomic Duck will use an alternative steering mechanism to those shown above. Taking influences from both bicycles and racing cars, Atomic Duck will use a steering wheel with all the controls mounted on it.

Atomic Duck cockpit concept

Using hand controls, with the rider’s fingers resting over the brakes, will be very familiar to every bicycle rider; and a steering wheel should be just as familiar to car drivers. The similarity of the controls to two vehicles that most riders will already be comfortable using means it should take a minimum of time for a new rider feel in control of an Atomic Duck.

The resting, vertical hand position means normal steering motions can use most of a 160-170 degree range of motion, this means that the rider’s hands will never normally leave the brake levers, while still allowing good high speed stability. For tight, very low speed, turning maneuvers; riders can use a greater range of motion and more than a half-turn of the wheel because braking reactions need not be as quick.

The large steering motion allowed by a steering wheel should allow very tight turning circles in the Atomic Duck, despite its long wheelbase. One of the reasons the wheels are so far from the body (see the concept design) is to allow them enough room to move to steer round in very tight circles.

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Comments

Opus the Poet wrote, “OK so roughly a 1:2 steering ratio? Turn the steering wheel 90 to get 45 degrees of turn at the front wheels? That would solve the sensitivity straight ahead and the turning circle issues. The trick on this one would be getting the ackerman correct with such a large rotation of the steering arm at the center of the vehicle.. But if one used a chain and sprocket to make a poor man's rack and pinion then that would solve the funky motions at the steering arm issues. You would have to either attach the chain to something to make it rigid, like use a drag link under tension with the chain attached to the drag link. Or a sliding block with the chain attached with the Morse cables connected to the sliding block. But if you're going to go to that much trouble you might as well crib a rack from a quad bike and just be done with it.”

Patrick Fenner wrote, “I'm not sure of the exact ratio (I haven't calculated it yet) but I think I'll be higher that 1:2 even. I'm intending to use cable steering, with a pair of wound pulleys behind the dash (cable pull only), with a link cable between each wheel. That way, steering ratio is dictated by the radius of the pulley and the length change of the cable is linear with steering wheel rotation. The akermann is then taking care of at the other end of the cable an the steering geometry, after it has passed down the centre of the front axle. If full lock at the wheel is 1.2 turns (~430 degrees, 215 degrees each way from the centre) , the greatest steering angle I would want is about 42 degrees (84 degree sweep, taken at the axle centreline), giving around a 1:5 ratio and a minimum turning circle of 8m curb-to-curb. This means being able to turn in one go on all but the very tightest roads (it's the same spec as a London Taxi). That would give steering wheel angles for the following corner radii: r= 10m; steering wheel angle= 171deg r= 20m; steering angle= 77deg r= 30m; steering angle= 50deg r= 40m; steering angle= 37.2deg r= 50m; steering wheel angel= 29.5deg r= 100m; steering wheel angle= 14.5deg r= 1000m; steering wheel angle= 1.4deg r= 10000m; steering wheel angle= 0.14deg From that, a really tight turn would go beyond 90 degrees at the wheel, but a long turn (100m radius is the tightest for a motorway) still needs positive steering input. and you shouldn't need to take your hands of the controls for a tight road corner (10m radius). this is using [latex]\theta = 90-cos^{-1}\left(\frac{w}{r-t}\right)[/latex] where θ is steering angle (of the tyre/wheel) w = wheelbase, r = turn radius and t = with of half track. ”

Opus the Poet wrote, “OK for that to work you need to have the cables under tension somehow and that implies a drag link on the opposite side of the steering to keep the cables taut, or a second cable that does the same thing as a drag link but can stay hidden in the dropped axle.”

Patrick Fenner wrote, “Yes. So it has a left turn cable and a right turn cable - each used in tension only. Then a cable that connects each each upright (on the opposite side to the steering cables) that acts in tension to link between the wheels - effectively a cable drag link.”

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