Seat Reclining Bar

When I originally posted about the seat structure, I left the seat adjustment separate from the main seat structure design. I had the ideas about how to implement the adjuster, but it needed some time to let them mature into a practical design.

The idea is to use the simplest adjustment mechanism possible. It needs to be set-and-forget, so the seat height doesn’t change when you lift it up, but also be easy to adjust, so that means no fiddly mechanism and adjusting it shouldn’t need tools. Low parts count and low weight is important too and there shouldn’t be any extra hardware/fastenings if possible (i.e. I don’t want to use car seat style lever and slide mechanism).

The seat adjuster uses the sloped back of the side support as the control surface for the seat back so, because it is to all be operated by hand, any pinch points needs to be removed or protected. There needs to be no sharp/pointed/serrated edges that could catch loose clothing or skin — especially because opening and closing the seat is to be a normal process for using the luggage space.

The end of the adjuster beam is rounded, as it is a moving structure that sticks out. The radius on the beam where the support locates to minimise the risk of pinching anything when the seat settles back. The horizontal gap between the seat and support means it shouldn’t be able to trap anything between the two.

The adjuster bar clips/unclips to the seat back which holds it in place, while allowing easy adjustment (the removable bar is simpler than having a captive, sliding motion).


Like with the previous lattice hinges work, I’ve been doing some calculations for integrated elastic clips for laser cutting — I’ll be publishing more details on these soon.


Having a long distance between the support positions, which means they are only supporting point loads (and no bending moments), this length allows for a natural shock adsorption as part of the seat design. One downside of this layout however, is that it’s not possible to adjust the seat with anyone sitting in it, you’d have to get out to readjust.

Seat Shape

This post is a bit of a development of design ideas. Sometimes it’s good to go through each stage of a design and explain why it has been rejected.

Of course, like any design process there’s a good deal of opinion, approximation and guesswork to it, so be sure to add a comment below if you see something I’ve missed, or you disagree with me.

Flat

The simplest seat is just a flat surface to sit on. This is fine for an upright bike but for a recumbent seating position, the lack of a back rest means it would be uncomfortable to use for more than a few minutes.

Flat Seat
Flat Seat

Straight Backed

Adding a back allows the rider to lean back comfortably whilst cycling and gives support for them to push against while pedalling. Though this is good for an upright seat, as the seat back becomes increasingly reclined there’s an ever increasing likelihood that the action of pedalling will push the rider up the seat-back. And the more the rider moves about in the seat, the less secure they will feel.

Straight Back
Straight Back

Here’s an example of a straight-backed slung-seat for an upright seated recumbent.

Ideal

The best seat shape would be one that fitted the rider exactly. That’s only possible if the seat is made to fit the rider, but a perfectly fitting hard-shell wouldn’t even need padding. The front of the seat comes up slightly to stop submarining (when the occupants hips slip forward off the seat) the middle of the back gives comfortable lumbar support and the top of the seat curves around the occupant’s shoulders to stop them from riding up while pedalling. Continue reading Seat Shape

Cockpit Clearances

Designing a cockpit to accommodate a range of rider sizes is more complex than laying out the controls to fit one rider; especially having a steering wheel control for such a reclined operator instead of underseat/tiller steering or a moveable yoke.

Rider Clearances
Rider Clearances

To account for different height riders while still keeping the rider’s eyeline above the height of the bulkhead, the seat back reclines, pivoting near the hip joint. All of the swept area must be clear of any other objects. Below the body swept area is space usable for luggage, except where the chain passes through.

To cycle, there has to be room for the rider’s legs to move as they pedal, so the swept area of the rider’s thighs has to be kept clear; the rider’s legs mustn’t hit the bulkhead, nor the steering wheel. Position of the steering column is not affected though, as the rider’s legs will pass either side of it.

Having a steering wheel in the cockpit with the rider in such a reclined position presents a trickier packaging requirement then underseat steering. Not only must the wheel be clear in the straight ahead position, it must also not interfere whilst turning, including the protruding brake levers.

As the body area pivots close to the hip position to accommodate different height riders and the wheel has no variation of position, the best position for the wheel is directly above the rider’s hips. Height of the steering column must be set so the wheel doesn’t impact the rider at the bottom and doesn’t hit the screen (with clearance for the rider’s hand) at the top. A lack of steering wheel adjustment means that there will be a variation in hand position depending upon rider height.

The low height of the cockpit area also means that full steering lock (>±90⁰) may bring the bottom of the wheel (and brake lever) close to the rider and the top of the wheel will obscure some of the rider’s forward vision. Though this isn’t too much of a problem, as full steering lock will only be useful at very low speeds (e.g. for parking or turning round) when the speed of reaction for braking is less critical. All normal riding should need much lower steering wheel angles.