If you are new to building electric drives for small vehicles you might
find our technical article "Electric Drives for Small Homebuilt
Vehicles" useful. It was first published in the web publication
"Engines & Wheels TM
" Issue #41.
The Engine & Wheels website is now defunct unfortunately.
I've also made the article available here for our visitors -
Tech Article. (200Kb)
I also have two articles about the development of an antique style EV
- also originally published in Engine & Wheels. You can download
them here - Part1
Part2.
A third and final article was also available in issue #53 of
Engine & Wheels.
We have number of
information pages for some of the most commonly used components
in the vehicles - bearings,
spur gears,
chain & sprockets,
shaft steel,
drive motors,
motor controllers,
batteries. Have a
look........
A simple vehicle
power
calculator is also available now.
Here are some commonly asked questions & some answers -
-
Who are the plans intended for?
-
What tools are needed?
-
What size of rider?
-
How much do they cost to build?
-
Are they metric or imperial designs - can I get the parts?
-
How do the motors run - should they get hot?
-
What affects the power consumption and range between battery charges?
Who are the plans intended for?
The plans are
intended for anyone more interested in making a kid's kart than
simply going out and buying one and who is a competent DIY-er, or who
is willing to learn the necessary builder's skills.
You will find they also make great alternative projects for those of
you who already have the skills to build a robot or a kit car and for
those with model engineering skills.
The buggy designs have been developed to allow manufacture using a range of fairly standard
DIY tools and materials plus some commonly available stock engineering components - however they are
not
click-together "Lego" kits. Plans are in the form of engineering
drawings which describe in detail each part and assembly so buyers also
need to be happy to take the time to study and, with the help of the
many explanatory notes on them, understand the drawings to figure
out how to make the parts.
Those with an
interest in mechanical drive transmission design for small vehicles
including home-built robots, RC controlled tanks etc may also find the
plans useful as will those with a general interest in how things work
and in mechanical engineering design.
What tools are needed?
The best place to
find the answer to this is in the introductory notes that come with the
plan sets - you can read a sample copy
here
.
The usual range of DIY tools are required for both wood and metal
working with probably the most exotic being a stand mounted drill or
bench drill. The Electric Voiturette is a bit more complicated than the
kids' buggies and a small metal working lathe will be useful in
its build.
Read our "What
Tools?" page here for more
information on tools and equipment used to build the karts and
buggies.
What size of rider is accommodated?
The kids' designs were made to accommodate riders up to 1.6m (5' 3") tall
(this is the
approximate
height of a 95th percentile 11 year old UK boy) and of average build
and weight. You can check for yourself however if you have a particular
user in mind - the GA's (General Arrangement Drawings) for each vehicle
can be viewed from the web pages for the particular vehicle. Download
the drawing of interest to you and print it out using the "No Scaling"
option in Acrobat Reader's print dialogue. The drawing print should
then be at 1:10th scale and can be measured-up for the dimensions of
the rider you have in mind using a scale rule with a 1:10 scale. The Voiturette
is intended for averaged sized adults.
Many builders will
find it quite possible to determine from the drawings how to modify a
design to accommodate a rider with, for example, longer leg reach etc.
How much do they cost to build?
This is difficult to
answer precisely because it depends so much on where you get the parts,
whether they are new or second-hand, what country you are in etc. The
most expensive parts bought
new
are usually the main electrical components - the motors, batteries and
controller. For the single motor vehicles and sourced in the UK these
will cost,
new
,
about £200 rising to about £250 for the double motor
versions. It seems that many components are cheaper in the US than in
the UK.
In addition to this
you need gears, sprockets, bearings, steel, timber, nut & bolts,
screws, adhesive, dill bits etc - all in all a working budget of about
£500 in the UK would be appropriate for a build using only new
parts. Remember that many of the main components can be re-used in
subsequent projects; eg the whole drive gearbox, motor controller,
batteries and wheels could be reused. The cost could be reduced by
using some salvaged or used parts. The more complicated vehicles will cost a bit
more.
Metric or Imperial?
Those of you who
know a bit about science and engineering will know that different areas
of the world often use different systems of measurement. The designs
have been developed in Scotland where, in common with the rest of
Europe, metric or S.I systems are now generally in use in most
engineering industries. This means that the majority of parts and
components for the vehicles are defined in millimeter sizes. However,
in order to achieve important fits between some parts (without recourse
to specialist lathe turning, milling etc) and because several electric
scooter and other parts are used, a combination of both metric and
imperial (inch) sized components are actually used. The UK's "imperial"
past (no reference to the Empire - honest!) still
makes it quite easy to obtain the required inch sized
components here though and many of the scooter spare parts used are
available all over the world including the UK.
In the US, where
inch sized components are more commonly used, it is still quite
possible to obtain metric components and the electric scooter parts
such as the motors and brakes are also commonly available. Many
builders will be able to use their knowledge to substitute inch sized
timber, steel flat, bright bar etc for many of the non-critical
millimeter sized parts in the drawings. You need to check the
drawings.
Electrical component
sourcing including batteries and controllers is more at the discretion
of the builder and shouldn't be a problem.
Look over our Links pages to get a feel for the range of on-line suppliers of components and materials.
How do the motors run - should they get hot?
The short answer is
that you can expect the motor(s) to heat up. The amount a particular
electric motor heats up is related to how much current it draws.
In common with most DC PM motors, those used in our vehicles will
draw current roughly in-proportion to the torque demand at their output
shaft - to put it in plain English the harder you work the motor the
more current it draws and the more it heats up. The plans include
recommendations about the gearing used in the vehicles - using the
correct gearing limits the torque demand on the motor and allows the
motor to be used in a sustained way without over-heating. However the
plans allow for discretion on the part of the builder - you can usually
overload motors for short periods of time - but in the end you must
stop and allow any hot motor to cool back down before the temperatures
reach damaging levels.
How hot? A difficult
question. The particular motors used are MY1016 250W scooter motors and
the manufacturers data for them can be found
here
.
The data shows that at their rated output of 250W they are about 80%
efficient. This means that of the 310W (24V x 13 Amps) electrical power
that goes into the motor about 60W is lost and goes into internal
heating. So to operate continuously at 250W power output the motor
needs to loose 60 Joules of heat per second (ie 60W). The motor (any
motor) can't loose heat unless its external surface temperature is
higher than the surrounding ambient temperature and a simple heat
transfer analysis of the motor would suggest that, to loose 60W of
heat, motor surface temperatures higher than can be comfortably
touched will be very likely. Once the motors are too hot to touch
though it's difficult for a builder to tell just how hot they are!
When building and
testing our vehicles our general policy has been to stop and park-up a
buggy once the motors get to hot to comfortably touch We have yet to
have a burned-out motor.
Be very careful however -
this is not a recommendation to touch hot motors
-
do not
let children touch hot motors and
do not do so yourself
unless you are
absolutely sure
you will not be harmed!
What affects the power consumption and the range between battery charges?
Battery charge drops
as current is drawn. As stated above the harder the vehicle's motors
are worked the more current they draw and so the faster the battery
charge drops. A key parameter in this is the torque demanded from the
drive wheels as this transfers directly, through the drive
transmission, to torque demanded from the drive motors and thence drawn
current. For our low speed buggies the factors most affecting motor
torque (assuming the vehicle is properly made and there is no internal
binding of shafts, bearings etc) are rider weight, drive surface
condition and operating gradient.
These are related.
Some drive torque is required to accelerate the vehicle and this is
proportional to the mass being accelerated - the heavier the driver the
more energy is needed to get the vehicle moving. Once it is moving at a
constant speed the acceleration is zero, however energy is still
required to keep the vehicle going. Here wheel rolling resistance and
vehicle gradient forces come into play and both are affected by rider
and vehicle weight.
Rolling resistance
is usually expressed as the force required to roll the loaded wheel
over a surface (this relates directly to wheel drive torque) and is
given as a multiplier of the weight acting down on the wheel - so
vehicle and driver weight are directly involved. For example, for a
pneumatic tyre on new asphalt or concrete surfaces the coefficient of
rolling resistance is
about
0.01. Ie the force required to roll the wheel is 0.01 x the downwards
load on the wheel (assuming frictionless bearings) . On loose gravel
it's about 0.04 - 4 times that on concrete and, on sand it's anywhere
between 0.1 and 0.3 - ie 100 to 300 times that on new concrete. Good
(hard and smooth) drive surfaces take much less drive energy than
gravel, grass, dirt or sand surfaces and consequently drain batteries
much slower. (For more info on
rolling resistance see here.)
Gradient forces
occur when the buggy is driven up a slope by the drive wheels. This
demands drive torque in addition to the acceleration and rolling
resistance effects. The gradient force is related to the angle of the
slope and the weight of the vehicle and rider. For example, for a 5
degree slope (about a 1 in 11) the required drive force is approx 0.09
x the weight of the vehicle/rider (that's about nine times the rolling
resistance on new concrete) for a 10 degree slope it's about 0.17 x the
weight - forces similar to driving on the flat in sand!
The up-shot is if
you want your battery charge to last as long as possible don't let Dad
test drive the buggy up steep soft sand dunes on the beach! Best
to use it on good drive surfaces on the flat and keep heavy adults off
it (this is much better for the motors too!) . With the correct gearing
the motors can handle poorer surfaces (gravel, mown grass
etc) nevertheless, they will still demand more energy in
these driving conditions than driving on smooth asphalt on the flat and
so will run the batteries down faster.
I have made available a
simple vehicle
power
calculator here which can be used to see the effect on power
consumption of different design parameters.
I
will add further questions and answers as they are asked.
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