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

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

• It was transformed to one-front one-tilting configuration (1F1T) because I wanted to try some experiments with a tilting front wheel. Given the existing frame, there was minimal work involved in converting it to a 1F1T configuration, which would make it possible to try both manual countersteer with dual controls and "free to castor" steering with tilt control only (more background details are given at the end).

• The rowing mechanism was removed, and the ground-clearance was increased to give a seat height of aprox. 50cm, the roll cage was improved, and a five-point safety harness fitted. As a legacy of the original rowing trike frame, the legs and feet extend forward inside a subframe formerly part of the rowing mechanism, and so the feet cannot be placed on the ground while seated.

• Rear track width is rather wide at 80cm until low speed steering control has been perfected. It will finally be reduced to 55cm.

• An electric motor was fitted to the rear, so the trike could be test-driven (motor details given below).

• Initially it had 20" wheels all round. To lower the center of gravity (CG) a 16" front wheel was later fitted. The composite photo below shows how the seating position and the approximate location of the CG changed.

Tilt control

Low speed control

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

• The track length is approx. 110cm; track width 80cm; seat height approx. 50cm.
• The tilt pivot is approx. 15cm above the road surface, and the angle is adjustable. When that angle is slightly negative, as in the GM Lean Machine, the axis when projected intersects the road surface at a point behind the rear wheels - so that it will steer the rear wheels slightly in the opposite direction to the angle of tilt. That would facilitate the force required to tilt when moving at speed. When the axis intersects the ground at the front wheel contact patch, then there is no rear steering effect.
  See also note ⁽¹⁾.
• Max. tilt is only approx. 30deg. each way. The tilt control levers have to be moved approx. 40cm fore-aft to achieve that. There is sufficient leverage to be able to balance while stationary, but it is not possible to recover from extreme tilt angles. That only becomes possible when the vehicle is moving.
• The 16" front wheel has a trail of approx. 2cm but the angle of the fork is adjustable to give more or less trail.

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!

   (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.

   

   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⁾.

   • Shorter version (Windows media video 6.6 megabytes):
     
   • Longer version (Windows media video 27 megabytes):
     

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

   • The bump sequence, 205 kilobytes:
     
   • Tight turn, 915 kilobytes:
     
   • Slalom turns, 856 kilobytes:
     
   • More slalom turns, 1.16 megabytes:
     

Electric motor

(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

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

Note 1:

Note 2:

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

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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