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Q&A & Some Tech Stuff


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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