I made this golf trolley about four years back as a winter
project. I was beginning to find that carrying my golf bag for 18 holes was a
bit tough on my back and knees.
I bought the motor/gearbox and 2 back wheels for about £90 from Standel Dawman, a company I found
on the Internet.
The wheels have a built-in freewheel mechanism so the
trolley coasts downhill and is easy to turn, too. The axle is 12-mm steel rod, drilled
to take spring pins that hold the wheels. Later, I modified it to a tubular
axle with solid stubs in each end to save weight (and just for the fun of it!).
The gearbox provides the left side bearing. The right side has a ball race
(steel balls in a plastic housing), wrapped in polythene to keep the dirt out.
Cardboard Aided Design (CAD)
The basic plan was sketched and pretty much done by guesswork,
taking some dimensions from a friend's Powakaddy
golf trolley. The folding mechanism was worked out by CAD (cardboard-aided design),
and was the trickiest bit. The central arm folds, but broke a couple of times.
So, I gradually increased the dimensions. I welded-up rods on plates for the
main pivots, where the A-frame at the back joins onto the chassis tray and the
sliding loop, which locks the two parts of the central spine when in use.
It is all made of plywood bent with the aid of a hot-air gun. Where the radius
was too tight, it was laminated out of thinner sheets of ply. The pieces were
cut, bent and then glued together with a bit of bracing added in an ad-hoc
manner to hold the battery and support the bag. I had to modify it slightly
when I bought a new golf bag, but that's easy when you've built it yourself.
Speed Controller Specs
The speed controller should have been easy since I'm an electronics
designer; however, I learned a few new things. The basic control is pulse width
modulated. The actual variation of pulse width between a gentle crawl (when
loaded) and full speed is a relatively small. The controller is based on a hex
inverter CMOS chip with a few Rs, Cs, and a pot. This drives a pair of decent
power FETs in parallel. I have an 0.15R resistor in series with each one just
to "calm things down a tad" as the stall current at switch-on can be rather
high, and I wanted to ensure reliability.
The first version flattened batteries rather quickly. I then added a "flywheel
diode" (4qdtech.com had some excellent
stuff which taught/reminded me of stuff I should have known) that greatly
enhanced the battery life.
The other electronic problem was the charger for the 20 Ah gel
battery. Many trolleys use bigger batteries, but they are heavy (which is self
defeating). My old car battery charger ruined the first gel cell, so I built a
two-stage charger, 3 amps up to a certain voltage and then a constant voltage
trickle. Voltages and currents were obtained from the battery manufacturer's
website, which is the only really reliable source of information.
Reliability and Rubbish
The trolley works very well and has proved more reliable
than some of the cheap ones which appeared on the market a couple of years ago.
Oh yes ... the front wheel? It came from a child's ride on toy I found at the
council rubbish tip!
Now it's time to go golfing!

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