Modifications on three wheels

Posted on May 7th, 2006 in Opinion by Julian Edgar


You’ll have to indulge me, to pander to my obsession. A few months ago I wrote about my newly purchased Greenspeed pedal trike (see Driving Emotion ) and in the time since, I been both pedalling it a lot – and modifying it. True, the modifications aren’t fundamental, but they’ve added to the riding enjoyment.

But first, if you haven’t read that previous column, what’s this about a trike? The Greenspeeds are recumbent machines, ones where you lie back at a steep angle on a hammock-like seat. The pedals are way out front and the trike uses two front steerable wheels and a single rear chain-driven wheel. My trike has 63 gears, front drum brakes and zero scrub radius steering.

Changing Front Toe

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And in fact it was the steering that I changed first. I’d like to give the front-end more castor, as the strength of the self-centring is not as great as I think it should be. The steering – very direct and sensitive via two levers and rose-jointed steering rods – is beautifully precise but at speed, can become a bit twitchy. (More sporting Greenspeed models than my touring GTR have greater castor built-in.) The castor is determined by the angle of the kingpins, and on these machines they are part of the welded frame, with no facility for adjustment. So what else could I change to reduce the high-speed twitchiness? (And high speed on this trike means 70 or 80 km/h down hills….)

After mulling it over, I decided to adjust the toe. I made up a simple toe measurer that consisted of a piece of timber with nails in it. By applying it first to the leading edge of the front wheels and then to the trailing edge of the front wheels, a quick indication of toe could be gained. This appeared to show about 3mm of toe-out, which on a car at least would cause a tendency to rather suddenly turn into corners and to promote straightline instability. I adjusted the steering rods to give a fraction – say 1mm – of toe-in and went for a ride. The stability was better – but not by a huge margin. I then dropped front tyre pressures a little, to find that (again like on a car!) too low a pressure gave soggy steering and cornering understeer, but with the required less responsive steering. Some juggling of values gave the best outcome – less steering twitchiness but still good cornering grip.

(Incidentally, I put a lateral accelerometer on the trike to find that I can pull 0.6g in cornering acceleration before I start to lift a front wheel!)

Building a Unique Headlight

But the modifications about which I am happiest are the lights. What’s that, you say? What’s there to get excited about with bike lights? Let’s take the front light first. If you want the greatest brightness and greatest range (not the same thing), lightest mass and lowest power consumption, it becomes a really difficult design exercise. Said quickly it’s a case of “yeah, whatever”, but when you start thinking about it, it is bloody complicated.

White flashing LED headlights are useless (they are not intended to illuminate the path ahead, just to make you visible to other road users), while traditional incandescent filament light bike headlights have poor range and spread, lousy colour temperature and high power consumption. Off-road cycling lights (eg miniature halogens) have excellent range and spread but ultra-high power consumption that necessitates a huge battery – normally a Sealed Lead Acid design – or a very short duration between charges. Small headlights taken from other machines – eg tractors or motorcycles – have waaay too high a power consumption and are also usually too heavy.

So what do you do?

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Well, I’ve made my own headlight. Developed over two years (I was also working on it when I ran an electric bicycle – see Building an Electric Bike, Part 1 ) the final design consists of a housing made from a stainless steel drinking cup. A hole is cut in the base of the cup and protruding through this hole is a 5-watt Luxeon LED mounted on a heatsink. (If you don’t know anything about Luxeons, they’re the brightest LEDs in the world and have amongst the highest efficiencies – ie lumens output for watts input – of any light source available.) The heatsink is required as LEDs with this much power get quite hot.

In front of the Luxeon is a narrow beam collimator, a special plastic lens which uses total internal reflection to gather up as much of the light from the LED as possible. However, the headlight design also uses a second lens – across the front of the cup is a huge 75mm glass lens. The combination of the two lenses creates a 15 degree focused beam. Holding the front lens in place is a flanged ring.

The design has two spreads of light – the first narrow beam is created by the collimator and the glass lens, and the second broader spread by the reflections within the stainless steel cup acting at an angle through the glass lens.

The headlight design is light, durable, good looking (well, I think so anyway), has unbelievable range and good spread, and consumes only 5W of electrical power. In fact, despite scouring the web and looking at dozens of bike headlight designs, I think that this combination might make it the best bike headlight in the world.

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On the road (and the roads I ride on have no street lights and are rural and semi-rural) reflective signs can be seen at well over 500 metres, on-coming cars dip their high beams as soon as they see me coming, you can see drivers that have passed setting their internal mirrors to cancel the glare, and as this pic shows, objects 35 metres away are brightly illuminated. It’s like riding down a tunnel of light – light sufficient to avoid pythons crossing the road (truly – we get pythons on local roads!) and easily enough illumination to allow high speed downhill blasts through the darkness.

I’ve run out of space to tell you about the rear lights – four high-speed flashing 1W Luxeons using a combination of narrow and broad beam collimators – and the very special tyre-driven alternator that has sufficient output to run the whole lot. Not to mention the alternative rear light design (installed on my wife’s trike) that uses a convex lens out of a ‘projector’ car headlight…

Footnote: I’ve decided to build my own recumbent trike from scratch, using aluminium for the frame and running full suspension front and rear. Stay tuned…

3 Responses to 'Modifications on three wheels'

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  1. on November 15th, 2007 at 1:27 pm

    You’re right–that light does look good. Any chance of a bit more info about it? Maybe even some plans? 🙂

  2. Julian Edgar said,

    on November 15th, 2007 at 2:38 pm

    I am thinking of covering a new version in a full AutoSpeed article.

  3. on November 19th, 2007 at 1:48 pm

    Great–I’ll keep my eyes open for that.