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Step 2Mechanical
There are several considerations to be made about the mechanics of your ride.
1. Can the stock power transmission system be used, or, if not, what type will it be replaced with?
Most vehicle systems are specifically designed and calibrated to work well a power level dictated by the manufacturer, and can only handle a certain amount of overhead before something detonates. Depending on how severely you decide to upgrade, you might have to rebuild the power transmission - stronger gears, harder axles, wider belts, etc.
Usually, it is easiest to upgrade in the same family of technologies. For instance, upgrading from #25 chain drive to #35 for more strength, or 5mm HTD timing belt from 3mm timing belt. This is due to the similar form factors for similar technologies - a sprocket has much the same profile as another sprocket, and you can get a new timing pulley in the same width and material as the old, but with bigger teeth. Often, less modification of the structure, wheel hub, etc. is required if the method is kept the same.
However, this all depends on how much you want to change or how much you want to build or buy. A more powerful motor could easily accomplish the same task using a one-stage pulley system in the same space where a previous, weaker motor had to use a large gearbox. Many electric car conversions retain the stock transmission since the electric motor develops similar torque to the engine being replaced, while others using high-torque motors often do away with the transmission and only use the gear reduction of the rear differential.
In the case of this scooter build, the stock 150-200 watt motor was well-suited to the all-plastic powertrain it was attached to. However, I knew that the giant outrunner would shear the tiny belt teeth off instantly.
I went to Stock Drive Products and got myself a set of 5mm HTD timing belts and pulleys in 15mm width, which is both a 5mm upgrade in width and a 2mm upgrade in tooth pitch.
HTD type belts have round teeth which are wide at the base, enabling them to transmit significantly more torque (HTD = High Torque Drive) than a standard timing belt (usually of the "GT" series) of similar pitch.
These pulleys required some creative wrangling to fit into the slightly narrow space, however...
2. Does the chassis require modification in order to fit the upgraded battery, motor, controller, etc.?
There were two problems to be solved. Number one was attaching the big pulley to the wheel, and number two was attaching the new motor pulley to the motor.
In the general case, there is a tradeoff between how well your components would work in an optimal situation and how much machining you're willing to do to reach it. If using your desired motor requires rebuilding the entire back end of your vehicle, it might be wise to consider another vehicle, or to save your motor for another project. If you're going totally balls-out anyways, you're probably not the target audience for this article.
Often, this entails picking components which are either similar in size or even smaller than stock. Considering the vast increases in power-to-weight ratio of a BLDC motor to a similar ferrite-magnet, standard-issue run-of-the-mill PMDC motor, this is often a viable solution. Again, realize that an equivalent DC motor that can produce 10 kilowatts of power like the Plettenberg Predator is usually about 8 inches in diameter, a foot long, and weighs over 100 pounds. *
In this build, adapting the new pulleys and belts was not too hard, but did require access to a lathe, which a good friend of mine was able to help with. I planned out a stronger hub attachment than the stock double-d mount for the wheel (which would explode instantly on application of power from the new motor). Also, I enlarged the bore of the motor pulley to fit on the motor, and secured it in place with a pin, which both transmits torque and locates the pulley axially so it does not slide around.
A series of holes were drilled at a measured diameter around the large aluminum wheel pulley's hub such that dowel pins, when pressed in these holes, were able to grip the wheel spokes. This greatly increased the strength of the attachment over a plastic double-D shaft. The hub of the wheel pulley was machined to fit snugly into the bore of a 5" scooter wheel, and a bearing cavity was also machined into it at the right location such at the entire new assembly was able to just drop into the space where the previous wheel-pulley assembly was without its axle spacer. I took up the space which that spacer used in order to add a larger part.
I did need to cut away a bit of the chassis to clear the new wide pulley, but this was just a few minutes with a file.
Next, I needed to find a way to mount the new motor.
3. What is the most effective way to mount new components if they cannot be simply dropped in?
This sort of goes in concert with question #2. While add-on mounting systems aren't as hard to re-engineer as the entire chassis, you still have to make sure you can actually put your new part on. Usually it's the transition from "small" to "large" which is an issue, and not the reverse, but one thing that smaller parts require is adaptor plates, shaft couplers, bore spacers, etc. in order to interface to the rest of the system.
Many motors are face-mounted, which means they have bolts sticking out of the same side as the output shaft, or perhaps threaded holes. They are intended for use in applications where they're directly stuck into the side of something.
Some other motors have base mounts, which are usually welded steel brackets that have bolt holes, for mounting the motor shaft parallel to a surface. Other motors yet have absolutely no mounts - they're designed to be clamped by a circular ring, such as as a hose clamp, into the frame!
The biggest challenge is usually converting between these types of mounts. Large metal angle brackets, with the bolt circle of the motor drilled on one leg and a mounting pattern on the other, are often used to convert base-mount to face-mount and vice versa. Here's one example, which is also often used in large industrial motors, since they make for versatile mounting arrangements.
Base-mount and clamp-mount conversions are also doable, but usually require some more engineering. They can be as simple as a circular cutout of the motor's diameter in a piece of material and a strap or clamp over the motor that holds it sturdily to that base material. Mounting holes are in turn drilled into the base. To convert clamp-mount to face-mount often entails drilling or machining mounting holes into one end of the motor.
Here's a good example of a clamp mount for a high performance DC motor (which also happens to be a great form of EV motor!)
Luckily, in my case, the bolt circle of the large outrunner was just a hair over that of the stock motor. I did not have to do much machining at all, since I only had to cut a bit of material off one mounting hole which was close to the back end of the mount, as seen in the pictures. Effectively, I have a bolt-and-slot configuration which also allows for convenient belt tension adjustment.
The new motor did not have mounting studs, so I had to simulate it using some long allen-head cap screws, washers, and aluminum spacers. This was handy since I had to increase the mounting offset (from face of motor to face of mount) to clear the wider belt pulley. A gallon of red Loctite later and the assembly was bulletproof.
One thing that you have to remember is that most of the time, electric motors do not directly drive their loads. Prime movers in any situation - internal combustion or otherwise - tend to move too quickly and with too little force to be directly applied to their load efficiently, which is why cars come with a pile of gears attached to the engine. The same is true for electric motors. A high performance brushless DC motor will hit speeds of over 20,000 RPM without incident, and the most extreme ones regularly exceed 60,000 RPM. On an 8 inch pneumatic wheel, that's a little bit over half the speed of sound at mean sea level. Fun, right? But it will never get there, and chances are it will not even start you moving. Even the power required to keep a steady speed against wind resistance will overwhelm it at a certain speed.
Most electric motors have an interesting property that their torque-speed curves are linear, with maximum torque at 0RPM and minimum (zero net) torque at maximum rotational speed. The motor will find a happy balancing point somewhere in the middle, the exactly location of which depends on your gearing, wheel size, electrical system capability, transmission efficiency, and a bunch of other stochastic processes. Overall, power does not automatically mean torque, which is what actually gets you moving. So that begs one last question to be asked:
4. Does the gearing need to be changed? A motor that runs far faster than stock might warrant additional gearing to operate efficiently. The stock reduction on the 200 watt motor was 4:1. I decided that since the giant outrunner was an order of magnitude more powerful, that a slightly lower reduction was not going to hurt performance. Also, the largest pulley I could fit in the dimensions given and which SDP had in stock was 3.75:1.
*Note that this DC motor is most likely designed to produce 10KW reliably with minimal heating, while the BLDC motor will require active cooling due to its reduced size. This is a rough comparison with maximum power figures.
1. Can the stock power transmission system be used, or, if not, what type will it be replaced with?
Most vehicle systems are specifically designed and calibrated to work well a power level dictated by the manufacturer, and can only handle a certain amount of overhead before something detonates. Depending on how severely you decide to upgrade, you might have to rebuild the power transmission - stronger gears, harder axles, wider belts, etc.
Usually, it is easiest to upgrade in the same family of technologies. For instance, upgrading from #25 chain drive to #35 for more strength, or 5mm HTD timing belt from 3mm timing belt. This is due to the similar form factors for similar technologies - a sprocket has much the same profile as another sprocket, and you can get a new timing pulley in the same width and material as the old, but with bigger teeth. Often, less modification of the structure, wheel hub, etc. is required if the method is kept the same.
However, this all depends on how much you want to change or how much you want to build or buy. A more powerful motor could easily accomplish the same task using a one-stage pulley system in the same space where a previous, weaker motor had to use a large gearbox. Many electric car conversions retain the stock transmission since the electric motor develops similar torque to the engine being replaced, while others using high-torque motors often do away with the transmission and only use the gear reduction of the rear differential.
In the case of this scooter build, the stock 150-200 watt motor was well-suited to the all-plastic powertrain it was attached to. However, I knew that the giant outrunner would shear the tiny belt teeth off instantly.
I went to Stock Drive Products and got myself a set of 5mm HTD timing belts and pulleys in 15mm width, which is both a 5mm upgrade in width and a 2mm upgrade in tooth pitch.
HTD type belts have round teeth which are wide at the base, enabling them to transmit significantly more torque (HTD = High Torque Drive) than a standard timing belt (usually of the "GT" series) of similar pitch.
These pulleys required some creative wrangling to fit into the slightly narrow space, however...
2. Does the chassis require modification in order to fit the upgraded battery, motor, controller, etc.?
There were two problems to be solved. Number one was attaching the big pulley to the wheel, and number two was attaching the new motor pulley to the motor.
In the general case, there is a tradeoff between how well your components would work in an optimal situation and how much machining you're willing to do to reach it. If using your desired motor requires rebuilding the entire back end of your vehicle, it might be wise to consider another vehicle, or to save your motor for another project. If you're going totally balls-out anyways, you're probably not the target audience for this article.
Often, this entails picking components which are either similar in size or even smaller than stock. Considering the vast increases in power-to-weight ratio of a BLDC motor to a similar ferrite-magnet, standard-issue run-of-the-mill PMDC motor, this is often a viable solution. Again, realize that an equivalent DC motor that can produce 10 kilowatts of power like the Plettenberg Predator is usually about 8 inches in diameter, a foot long, and weighs over 100 pounds. *
In this build, adapting the new pulleys and belts was not too hard, but did require access to a lathe, which a good friend of mine was able to help with. I planned out a stronger hub attachment than the stock double-d mount for the wheel (which would explode instantly on application of power from the new motor). Also, I enlarged the bore of the motor pulley to fit on the motor, and secured it in place with a pin, which both transmits torque and locates the pulley axially so it does not slide around.
A series of holes were drilled at a measured diameter around the large aluminum wheel pulley's hub such that dowel pins, when pressed in these holes, were able to grip the wheel spokes. This greatly increased the strength of the attachment over a plastic double-D shaft. The hub of the wheel pulley was machined to fit snugly into the bore of a 5" scooter wheel, and a bearing cavity was also machined into it at the right location such at the entire new assembly was able to just drop into the space where the previous wheel-pulley assembly was without its axle spacer. I took up the space which that spacer used in order to add a larger part.
I did need to cut away a bit of the chassis to clear the new wide pulley, but this was just a few minutes with a file.
Next, I needed to find a way to mount the new motor.
3. What is the most effective way to mount new components if they cannot be simply dropped in?
This sort of goes in concert with question #2. While add-on mounting systems aren't as hard to re-engineer as the entire chassis, you still have to make sure you can actually put your new part on. Usually it's the transition from "small" to "large" which is an issue, and not the reverse, but one thing that smaller parts require is adaptor plates, shaft couplers, bore spacers, etc. in order to interface to the rest of the system.
Many motors are face-mounted, which means they have bolts sticking out of the same side as the output shaft, or perhaps threaded holes. They are intended for use in applications where they're directly stuck into the side of something.
Some other motors have base mounts, which are usually welded steel brackets that have bolt holes, for mounting the motor shaft parallel to a surface. Other motors yet have absolutely no mounts - they're designed to be clamped by a circular ring, such as as a hose clamp, into the frame!
The biggest challenge is usually converting between these types of mounts. Large metal angle brackets, with the bolt circle of the motor drilled on one leg and a mounting pattern on the other, are often used to convert base-mount to face-mount and vice versa. Here's one example, which is also often used in large industrial motors, since they make for versatile mounting arrangements.
Base-mount and clamp-mount conversions are also doable, but usually require some more engineering. They can be as simple as a circular cutout of the motor's diameter in a piece of material and a strap or clamp over the motor that holds it sturdily to that base material. Mounting holes are in turn drilled into the base. To convert clamp-mount to face-mount often entails drilling or machining mounting holes into one end of the motor.
Here's a good example of a clamp mount for a high performance DC motor (which also happens to be a great form of EV motor!)
Luckily, in my case, the bolt circle of the large outrunner was just a hair over that of the stock motor. I did not have to do much machining at all, since I only had to cut a bit of material off one mounting hole which was close to the back end of the mount, as seen in the pictures. Effectively, I have a bolt-and-slot configuration which also allows for convenient belt tension adjustment.
The new motor did not have mounting studs, so I had to simulate it using some long allen-head cap screws, washers, and aluminum spacers. This was handy since I had to increase the mounting offset (from face of motor to face of mount) to clear the wider belt pulley. A gallon of red Loctite later and the assembly was bulletproof.
One thing that you have to remember is that most of the time, electric motors do not directly drive their loads. Prime movers in any situation - internal combustion or otherwise - tend to move too quickly and with too little force to be directly applied to their load efficiently, which is why cars come with a pile of gears attached to the engine. The same is true for electric motors. A high performance brushless DC motor will hit speeds of over 20,000 RPM without incident, and the most extreme ones regularly exceed 60,000 RPM. On an 8 inch pneumatic wheel, that's a little bit over half the speed of sound at mean sea level. Fun, right? But it will never get there, and chances are it will not even start you moving. Even the power required to keep a steady speed against wind resistance will overwhelm it at a certain speed.
Most electric motors have an interesting property that their torque-speed curves are linear, with maximum torque at 0RPM and minimum (zero net) torque at maximum rotational speed. The motor will find a happy balancing point somewhere in the middle, the exactly location of which depends on your gearing, wheel size, electrical system capability, transmission efficiency, and a bunch of other stochastic processes. Overall, power does not automatically mean torque, which is what actually gets you moving. So that begs one last question to be asked:
4. Does the gearing need to be changed? A motor that runs far faster than stock might warrant additional gearing to operate efficiently. The stock reduction on the 200 watt motor was 4:1. I decided that since the giant outrunner was an order of magnitude more powerful, that a slightly lower reduction was not going to hurt performance. Also, the largest pulley I could fit in the dimensions given and which SDP had in stock was 3.75:1.
*Note that this DC motor is most likely designed to produce 10KW reliably with minimal heating, while the BLDC motor will require active cooling due to its reduced size. This is a rough comparison with maximum power figures.
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Nov 24, 2008. 6:09 PMswbluto
says:
"Also, I enlarged the bore of the motor pulley to fit on the motor, and secured it in place with a pin, which both transmits torque and locates the pulley axially so it does not slide around." When you secured it in place with a pin, what type of pin did you use? Also, did the motor's shaft already have a hole for the pin to fit through or did you have to machine/drill it in? My build currently is set-up such that the grub-screw is holding the timing pulley to the motor's shaft for all its worth but I have a feeling that's not going to withstand the ~2000 newtons it's going to experience.(The grub screw/set-screw is pressed against the motor's shaft as tightly as possible. Would drilling a small depress/hole-wannabe and then screwing the grub screw in as tightly as possible work just as effectively as a pin?)
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Nov 24, 2008. 9:52 PM
teamtestbot (author)
says:
teamtestbot (author)
says:
Absolutely. Set screws need to be used with a spot drill in the shaft or a flat. Otherwise they are extremely ineffective at transmitting anything except the lightest torque (And if they slip, they make a nice round gouge around the OD of your shaft, making it impossible to remove anything ever again. I used a spring (roll) pin, and drilled through both the motor shaft and the pulley.
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