Like a lot of people I am concerned for the future of this planet, so it is only natural I would be interested in electric vehicles. I once did a survey at a company I worked for concerning workers travel arrangements. It turned out that over 60% of them lived within a radius of two and a half miles from the company and also less than two miles from the center of town. Later I found that the average car journey in UK is less than five miles !. This started me thinking about the sensibility of the modern car.
Now it would be impossible to talk about electric vehicles without mentioning Clive Sinclair's C5 !. Clive has been one of my hero's right from the very early days and was a marvelous innovator and inventor. So I was very pleased when I first heard that he was working on an electric vehicle, thinking it would revolutionize local transport !. It didn't, but it wasn't really Clive's fault. The C5 was designed by the British Ministry of Transport .... so it was doomed even before it was born !.
In 1983 the British Ministry of Transport introduced the Road Traffic act no 1168, entitled "The electrically Assisted Pedal cycle Regulations 1983". It also introduced changes to the Road Traffic act number 1176 entitled "The Pedal cycles (Construction and Use) Regulations 1983" to accommodate the electrically driven cycle. Briefly the specifications allow for ....
a. Two, three, four or more wheels.
b. An electric motor not larger than 200 watts for a bicycle or 250 watts for a tricycle, quad cycle etc..
c. A maximum kerbside weight of 40 Kg for a bicycle and 60 Kg for a three or more wheeled vehicle.
d. The regulations require brakes and refers to BS6102, Part1, 1992. This basically says that the brakes must be able to bring the vehicle to a smooth safe stop within a distance of 5.5 m from a speed of 16 km/h.
e. The vehicle has to be fitted with a name plate giving details of the manufacturer, nominal motor voltage, and the continuous rated output of the motor.
f. The maximum speed of the vehicle is 15 mph.
It was interesting to note that the law requires a vehicle traveling on the pavement to have a much stricter specification than one traveling on the highway !. Since 1983 various groups have argued that this specification should be changed to enable a practical short journey vehicle to be designed. The two main limitations being the low kerbside weight and the size of the electric motor allowed under these regulations. Until such time as the government gets it's act in order, there seems little chance that anyone will try and produce a commercial design after the disastrous failure of the Sinclair C5. So if you want such a vehicle then you will have to build it yourself. That is exactly what I decided to do !.
Probably the major limitation is motor size. A 250 watt motor produces something less than a third of a horse power. Horses are not exactly relevant here so lets have a look at comparable man power. The following statement will put things into perspective. "A cyclist generating an output of 100 watts (0.13 HP) on a level road, no wind, can travel at 30 kph (about 20 mph). On an 8% slope he can travel at 10 kph (about 6.5 mph). Of course if he was maintaining a constant power output he would have to use a lower gear to give the increased torque required to get up the slope. Clive's C5 did not have a gear box and so when it came to a hill the motor began to stall, greatly increasing the motor current. This caused the motor to overheat and the internal over temperature sensor to cut power to the motor. So the first lesson we learn from the C5 is that we need some form of gear box !.
The idea of a gearbox is that when we step down speed, we step up the torque available from the motor, by the inverse of the same ratio. So if we step down the speed 50% this has the effect of now having twice the available torque available from the output of the gearbox.
Due to the kerbside weight limit one has to err towards bicycle technology when designing this type of vehicle, rather than car technology. The two main types of cycle gear systems are the Sturmley Archer geared hub and the derailleur system. The Sturmley Archer geared hubs come in various forms including three and five geared versions. The progressions are ......
So the three speed version gives a maximum ratio of 1.78:1 and the five speed version a 2.24:1 ratio. You are now equipped with all the theory you need to hop on your bike and decide on what ratio's you want in your design.
The formula for calculating the current drawn by our motor is I = E^2/R, so a 12 volt, 250 watt motor would be expected to drawn about 20.8 amps. At this level normal car batteries would have a relatively short life. This is because heavy currents place stress on the construction of the battery plates, strip active material from them etc. The type of batteries we need to use are often called 'deep cycle' and are intended for our particular use. Now there are conventional lead acid batteries that are capable of supplying continuous current of this order but they are very large. Of course the larger the battery, the greater the range, but there is a point of diminishing returns. Consider the case where we have a large battery that could give us a range of 50 miles. If our normal daily average journey is only ten miles then it means we are wasting power carrying around a very heavy 80% of our battery that never gets used !. So the first thing to decide is what range you actually want and to do this you have to forget about internal combustion cars and start thinking electric !. For example if you use your buggy to drive to work, then simply plug the charger in when you get there. If you are going to use it for shopping in town then the battery must have sufficient capacity to drive the double distance, before we can re-charge at home. Using practical sized batteries we can easily obtain more than enough range for local use. Normal car type lead acid batteries For example if we decide to use two 12 ah batteries in parallel that would give us a theoretical range of 2.4 hours. I say theoretically because a number of factors affect range. When you come to the start of a hill, you will have to step down the drive speed in order to increase the drive torque, this obviously slows the vehicle down. On the other hand when going down a hill we don't need to apply power to the motor and this increases the range. This assumes that the driven wheel can free-wheel. This leads us onto the question of Free-wheel vs. fixed drive.
On simple electric bikes a free wheel is used and the power control is a simple on off switch. We call this BANG BANG control, because every time the power is applied, the backlash in the chain drive is taken up suddenly and you can feel the shock. It is a fairly simple matter to introduce a 'soft start' circuit to avoid this. A better approach is to use a more normal proportional type speed control, such as a pedal or twist grip. Using this method power is applied in a much more controlled way and system shock is avoided. Again we can use a freewheel with proportional control.
Another approach is to have a fixed drive between motor and the driven wheels in the same way as a car has. When you take your foot off the accelerator, the motor slows down and has a braking effect on the driven wheels. One advantage of this is that reverse becomes possible if one dispenses with the free wheel.. Also when going downhill etc., the motor acts as a generator putting power back into the battery. This works well with heavy vehicles with lots of kinetic energy to drive the generator when traveling downhill and also when using reactive braking.. On large vehicles this can extend range up to 10%. However, on a lightweight buggy gains may be small or even negative.
If you ride a bike then you will know how much vibration is transmitted through the tires and seat to your body. If you can't remember that means that you didn't notice the vibration and that it was acceptable. We can approach the subject of vibration in several different ways. First the effects of vibration on the body are time dependant. The longer we are subjected to vibration, the greater the effect it has on us. Our theoretic buggy is intended only for journeys of short duration and so we can accept a higher level of 'driving experience'.
Minimizing wheel vibration to the driver can be achieved by decoupling vibration at the wheels, decoupling at the seat interface or a combination of both. Personally I went for decoupling at the seat interface which makes the design of the suspension much simpler. This is the usual approach employed in fixed and rotary wing aircraft.
After a false start, I decided to have two bike wheels at the front and one at the rear. This decision meant that somehow I had to fit brakes to the two front wheels. If I had chosen to use a single front wheel I could have used a normal caliper brake on it. My solution to this problem was to buy a pair of wheel chair brake hubs from a company called Urathon. I intend to get these built into wheels using 16 inch rims. I have the blank rims but have not had time to build a jig to drill the spoke holes yet. The brake drums actually have 36 spoke holes and I failed to find any UK sourced 36 hole rims.
The single driven rear wheel is also a 16" with a Sturmley Archer hub and the brake is a heavy duty caliper, cable operated, fitted with a free wheel sprocket.
If I wanted to use my buggy on a pavement it would have to be a maximum of 800 mm wide !. For use on the road there are no practical limits (I think the law says 2.5 meters maximum width). Now I didn't like the C5 because it was so low and apart from weight there is little advantage in making a buggy really small. So I envisage the final height to be about the same as a mini and about two thirds the width. The actual chassis is "T" shaped made of two lengths of 40 x 20 x 1mm steel tube. The front axle is a single 50 x 25 mm steel tube. On the ends of this axle will be two standard bike headstocks, which will act as the steering bearings. Fixed to the lower end of the headstocks are two 12 mm stub axles for the front wheels.
I think on reflection I would use am 'A' frame chassis for a Mark II three wheeler and go for a shorter wheel base.
Unfortunately the law says that we have to have them and that they have to be capable of driving the vehicle. One company tried to circumvent this on an electric scooter by fitting token pedals and lost in court over it. Originally I felt that rotating pedals took up too much space to operate so I 'designed' linear pedals, to meet the requirement of the specification in the traffic act. They are the two "L" shaped tube assemblies running on Delrin rollers. The rear ends are connected via a short lengths of cable to a length of chain that fits onto two free wheel sprockets on the lay shaft. The two ends of the chain are cable connected round a pulley thus completing a continuous loop. So when one pedal goes forward, the other comes back under it's own steam. In normal use the two pedals will be centered and not used. There will be two conventional pedals, one for brakes and one for proportional speed control.
Under the motor can be seen a chain driven lay shaft. This has three inputs, two from my linear pedal system, one from the motor via a freewheel and the third is the sprocket that drives the chain to the rear wheels. I am hoping that I have chosen sprocket sizes that will limit the maximum driven speed to the legal 15 mph. An advantage of doing this is that we gain in torque.
I decided that since my buggy may be the first encountered by the police and traffic wardens in this area, that I would be confronted with the problem of people questioning the legality this class of vehicle. For that reason I decided that the design had to strictly meet the specifications laid down in the traffic act!. Hopefully when it is finished with a lightweight fiber glass body on it, it will look like a small car. Now imagine what is going to happen when I park it without a parking ticket !. Remember it is classed as a bike. What happens if I drive it into an area where cars are not allowed ?. I will have no tax disk, no MOT, no insurance (I may do something about the latter as there is no telling how much damage I could do if a 40 ton truck runs over me!), no crash helmet, no registration plates and 'illegally parked', the traffic wardens are going to have a field day .... up to a point !.
Parkside Electronics email@example.com
CVT design www.idbike.com
C5 article www.sinclair5.co.uk/technical/c5paper.htm