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 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.
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.
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.
- The track length is approx. 110cm; track width 80cm; seat height
- 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.
- 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).
- 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
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
- The bump sequence, 205 kilobytes:
- Tight turn, 915 kilobytes:
- Slalom turns, 856 kilobytes:
- More slalom turns, 1.16 megabytes:
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.
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
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.
The following details were added in 2008.
The van den Brink Carver also has a small amount rear wheel steering built
into the tilt mechanism. See their 2001 patent WO0187689.
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!
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.
Created: October 2005. Last modified: 25 November 2008.