Tilting trike with free-to-castor (FTC) steering

Front view

This trike started life as a rowing trike, as seen below, with a 2F1T configuration (two front wheels, one tilting rear wheel).

Rowing trike

Seating position

Tilt control

Parallel levers with a fulcrum under the seat operate via a mechanical linkage directly to tilt the entire frame relative to the rear axle. Only the rear axle and wheels remain at a fixed angle (the electric motor is also non-tilting, appended to the rear axle).

Low speed control

FTC means there is no direct connection to the front wheel. To steer around tight corners at low speed without having to tilt far over, each tilt control lever is connected with an elastic cord (of the kind used by surfers as a "leash" for their board - it is very strong, but stretches much less than rubber) to the steering. Tension in the cords can be adjusted, thereby allowing a progressive freeing of the linkage between tilt and steering. Tension is increased by pressing with the ankles down on the footrest, which is shown below.

Footrest

When the cords are tight there is an almost fixed tilt-steering relation. The actual ratio is separately adjustable, but while riding the amount of constraint on the front wheel can be varied. When the feet are extended (as in all the pictures seen here) to release tension, the front wheel is indeed free to castor.

Dimensions

Road tests

  1. The picture below shows the trike before the electric motor was fitted and still with only 50cm track width. Like this, the trike would fall over flat onto its side just like a bike when unsupported. Running it downhill, I never reached more than a walking speed, because very soon I simply fell over sideways!

    Rear view (Click to see full size)

    Failure at this stage was due to difficult low speed control. At walking speed it must be possible to steer effectively with very delicate tilt changes. I adjusted the elastic connection so that when pulled tight it gives a 2:1 ratio between tilt and steering (i.e. for 1 degree of tilt there is 2 degrees of steering).

  2. To be able to concentrate on perfecting the low speed control, I decided to change the width to 80cm, and I also lowered the front by fitting a smaller wheel. The electric motor was added, so I could test-drive the trike under more varied conditions than only going downhill. Below are two views of the trike as it is now.

    Trike side view Trike detail view

    So far tests have only been done on a large level parking area, at speeds up to 18km/h (which is the maximum the motor is capable of). Generally performance and handling were satisfactory under these conditions. This can be seen in the videos provided below. The "bump sequence" clip shows what it is like to go over a drain cover with the front wheel. No difficulty was experienced at this speed.
    See also note (2).

    Short slow-motion clips from the above movies, suitable for single-frame viewing, are provided below (all in Windows media video format)

Electric motor

The motor is actually a hub motor, but was adapted here to sit on a short sub-frame and drive a small (12") fourth wheel mounted on the rear of the trike. The weight of the motor (3.5 kg) keeps the fourth wheel in contact with the road. It is rated at 250 watts, and working through a Sturmy Archer 3-speed gear hub, it can just get the trike up to 18 km/h.

(Click to see full size)

The fourth wheel is not situated directly between the two rear wheels, so the tyre scrubs a little when steering, but this is not serious. As can be seen in the photo, the battery (4 kg weight) is mounted on the main tilting frame, just behind the rider. With the motor and battery, the weight distribution is biased towards the rear.

History

What you see here was the latest in a number of variations on the TTW, mostly concerned with all-wheel steering, that were attempted between 2001-2005.

The general aim was: To build a human-powered trike that is narrow, preferably not much wider than the shoulders of the person riding it and certainly no wider than a normal doorway, lightweight because it must be carried up and down two flights of stairs to get it out of my house onto the road, and suitable for commuting in normal traffic because I wanted to sit with my head at the same height as most other road users.

Experiments with all-wheel steering and FTC were in line with this aim, particularly in seeking lightweight and simple control for a TTW. Previously, I had tried a single steered rear wheel with fixed-ratio tilt steering on the two front wheels, but that proved unsatisfactory. It seems very difficult or perhaps impossible to get proper feedback through a rear-wheel steering control. Mine was neutral, i.e. giving no feedback. One can learn to use such a control, but in my opinion it is sub-optimal, if not downright unsafe. For that reason I decided simply to reverse the whole setup: To have a single steered front wheel plus two rear wheels that give fixed-ratio tilt steering. Phillip James (in message #5263) describes a similar system as used in the GM Lean Machine (see US patent 4423785):

... It has power tilt [feet] and a fixed tilt steer ratio due to rear wheel steer. The driver has handlebars [connected to steer the front wheel], as a "modifier" of the fixed tilt steer ratio.... and he must restrain any impulse to do anything dramatic with the steer control of the front wheel [at speed].

My trike, being human-powered, has the tilt controlled manually by two levers at the side (seen in yellow in the pictures above). One option was to include a dual-control system, similar to the controls on Al Fonda's HyPhy trike, so that steering input to the front wheel could act as the "modifier" that Phillip mentions above.

However, free to castor (FTC) steering is superior because it uses only a single driver control input, but can that be made to work on a lightweight human-powered vehicle without hydraulics? That was the question I wanted to answer in the tests reported above.


Footnotes

The following details were added in 2008.

Note 1:

The van den Brink Carver also has a small amount rear wheel steering built into the tilt mechanism. See their 2001 patent WO0187689.

Note 2:

This was first published in October 2005. In the same year, I replaced the 250w motor with a 36v 500w motor by Heinzmann, and was able to test drive it more extensively, even going on public roads at very early hours on Sunday mornings. On a very bumpy parking lot much larger than the one seen in the pictures here, I experienced some serious shimmy on the front wheel, but only at low speed. The cause was identified as a legacy of the donor rowing trike: Too little torsional stiffnes in the frame where it supports the front fork. It was not possible to make the frame more rigid at that point, since it was not designed to support a front fork in the first place.

A maximum speed of 25km/h on a straight level road was possible with this motor, and this was achieved without encountering any control problems.

The trike was de-commisioned in 2006 after the NiCd battery pack was stolen!

Conclusion

After several months and many kilometers of experimentation it became clear the biggest problem was the lack of sufficient power to control the vehicle tilt at low speed. At moderate speeds this problem disappears entirely, and if anything one needs assistance getting the chassis to tilt far enough. In the video clips shown above, the driver is often seen leaning his body with the turn, which shows the need for additional tilt force. Interestingly, I soon discovered that one only needs to accelerate for the required tilt force to become available.

The problem was acute only at very low speed, particularly when the road is not level. I did fall over several times, being unable to hold the chassis upright by muscle power alone. Within the constraints imposed by an HPV design, I decided there was no way to solve this without exceeding the criterion that it should be possible for one person to lift and carry the vehicle. Without that restriction, I do believe power-assisted tilt control could be developed for an HPV.


Frank Bokhorst,
Created: October 2005. Last modified: 25 November 2008.

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