1. Steering Geometry - crunchysteve/TriceratopsTwo GitHub Wiki

Basic Principles

While I've mentioned that this trike lean steers, like a bicycle, it's not quite that simple. There are 2 wheels, placed about 650mm apart at the tracklines. This means that, the vehicle doesn't actually lean "like a bicycle." The secret to this machine is a completely counterintuitive steering head angle. Most bicycles have a steerer angle of between 71 and 73 degrees, placing the tyre contact patch out in front of the bike.

A lean steering trike frame would want to turn opposite the lean with positive head angles, so we set up a negative head angle. The higher parts of the steerer point forwards, the lower parts point backwards. The "virtual ground contact point" of the steerer is as far behind the front axles as the steering center's height above ground. Even with vertically fixed wheels, this negative steering angle creates a geometry that centers the cross member when the rider is upright but, rotates the crossmember to steer left if you lean left, right if you lean right.

Here's that animation of the steering schematic from the repo front page. Look very closely at the top of the steerer, it always stays centered in the middle of the head tube. It's also a "scrubless geometry" because both front wheels always remain on axis, unlike conventional Ackerman steering trikes, where the front wheel axes don't perfectly align on the rear wheel axis at turning extremes.

Real World Cornering

This schematic is "cornering" exceedingly aggressively, in real life, a few degrees of lean will pivot enough to make a wheelbase of 900mm turn quite easily. In an urban environment, turning an "inside" street corner (left in UK, Aus/NZ, right in the US and Europe) is about a 3 metre radius at it's tightest, even on a bike lane and more typically 4 metres. A 3m radius is a few arc-minutes more than 16° lean and while that corner would feel aggressive as you read this, 16° is not a long way over.

You're probably leaning more on a bicycle. 3.1m is 10 feet in US "money" so, even pulling a 20° lean at low speeds is nothing. You're on a trike, it won't fall over if your balance is off. The schematic examples at the bottom of the page are approaching 30° of lean, which would probably result in your backend getting loose and spinning out at anything other than low speed maneuvering in tight spaces. Under 20° also never results in more the a few centimetres lengthening of the outer wheelbase, 25mm to 30mm give-or-take and, with your body weight leaned inwards, the trike will corner like it's on rails.

Leaning Mechanism

In the animation, the wheels are also leaning, and Triceratops will do the same, thanks to horizontal kingpins, leaning brackets and 2 tierods. As I say above, even if the front wheels don't tilt, the crossmember will pivot and steering will be effected. However, if the front wheels stay vertical, there will be less traction in corners on softer or wetter road surfaces, and the inside wheel will come closer to the rider's hip and forearms. Without leaning, there would also only be tyre wear in the centre. Leaning spreads the wear over the arc of the tread.

The kingpins allow the leaning brackets, to which the wheels are mounted, to pivot left and right from the vertical. Tie rods will keep the wheels leaned, relative to the main, centre frame, where the rider sits. By adjusting the position of these forward or backwards at the frame, we can ensure that the inside wheel leans slightly more than the outside wheel, giving a slight camber to improve stability and keeping the angles in line with cornering forces.

When you compare this geometry to the more conventional trike's ackerman pattern steering, lean steering never finds itself out of alignment at the cornering extremes and always maintains a more appropriate contact point for cornering, based on bicycle tyres often having more aggressive grip away from the centre line.

This is a completed schematic of Triceratops, showing an impression of all the main parts. Note that there aren't handlebars on the frame yet, nor points for mounting the tierods. Also not the front spar and "bottom bracket" (front bracket?) for adjusting pedal position to the rider before fitting the chain.

And, a static, schematic view of how lean steering works