Hello, I have been challenged by Our head mentor to convert one of his motorcycles to electric FRC parts(vexpro, andymark, etc.) He said he would like a blend between acceleration and speed but would like to lean to the side of acceleration, his goal is to do a burnout. It is a shaft driven chopper. We where also brainstorming and thought if we had say 6-8 motors, the battery would need to be charged over and over, so we where thinking maybe have a motor on a planetary gearbox turn a alternator? I have not had a lot of experience, so forgive me if my ideas are outrageous.
Thanks again, Logan madden
Frc team 6341
Your biggest problem is going to be the power source. You can stick as many motors as you want on the motorcycle, but if you’re drawing power from only one battery it won’t be going anywhere fast. If you use multiple batteries, you now have to take into account all of the extra weight. Each battery weighs 14 pounds and can just about power one FRC robot’s worth of motors. If you want to have enough motors to move both a person and the motorcycle itself, you’ll probably need at least two batteries to keep it running for any more than a minute.
Beyond that, this is a great application for the JVN calculator. I’m on mobile so I can’t find the link right now, but if you Google it it should come up. There are plenty of threads on here about how to use it to design your own custom drivetrains. I highly suggest you CAD the gearboxes and whole assembly before you spend any money ordering parts or time assembling it.
Converting a conventional motorcycle to electric sounds like a great project, but one that will be very difficult/impossible to achieve using FRC parts if you want normal motorcycle performance(ie doing a burnout).
A quick google search brings up that a typical Harley Davidson motorcycle has an engine power of between 30-60(or more)HP. If the common powerful FRC CIM is to be used, 100 of them will be needed to be able to generate 45ish HP. That number( or anywhere close to it) is going to be unfeasible due to the high cost, complexity and weight( thats 280lbs of motor)
A very large brushless outrunner, or a number of smaller DC motors such as ampflow/magmotors( you’ll obviously need big speed controllers, there is one made by VEX Robotics called the Victor BB( capable of 15kw cont), so its sort of made by an FRC supplier.)
You will also need a lot of either heavy or easy to upset batteries.
Similarly normal FRC mechanical parts are going to be unsuitable for this, due the high stress. The components which normally work fine for our purposes, such as a 1/2" hex shaft will be instantly destroyed when asked to drive a motorcycle sized wheel with 30hp.
In regards to the alternator, unless you have invented free energy( in which case please PM me:)) it won’t be effective, as you are essentially applying more load to the electric motors which are fed from your battery in order to run the alternator( you are using more electricity than the amount you generate).
This works for a combustion engine because the primary source of energy for moving the bike comes from a chemical combustion reaction. The alternator replenishes the catalyst (effectively).
For an all-electric bike, this is equivalent to regenerative braking. Coasting to a stop, or coasting down a hill would recharge the battery some. Yet the efficiency of mechanical to electrical, then back to mechanical energy would render it less practical for use as a primary energy source. In other words, the bike is still only as good as its battery ;).
The burn-out can be easily done using a single 775Pro and a single FRC battery - especially considering that this thing doesn’t have to be street-legal. How badly does the mentor want it to look like a typical chopper? But before I give the answer - think about why JVN’s old calculators used to have a “% weight on driven wheels”. In a drive train with rear-driven wheels and a caster in front with equal weight distribution, what tends to happen? The concepts are very similar.
Physically slipping the rear wheel is possible with anything( even a servo motor) if you have enough reduction. In terms of spinning the rear wheel of a heavy motorcycle(choppers are heavy) with a person on it, the 775pro isn’t going to be able to spin it fast enough to create an actual burning rubber burn out.
Horsepower is really a less than optimal metric to use for electric motors. Instead, comparing watts at desired RPM, or even better: compare the entire motor curve to what your needed power range is. Watts, Kv, and torque at desired RPM is very important for electric vehicle design.
Another aspect to consider: Do you want to keep the transmission, or have a large enough motor to drive the Primary? Electric motors can reach far higher safe speeds than IC motors (pesky piston rings and wrist bearings…), so for certain power ratings, a transmission isn’t actually needed. It sure does make interfacing with the motorcycle easy, especially since Harley’s use chain driven primary drive sections.
114 has a retired mentor that builds electric motorcycles for fun. If he’s still even interested in the internet, I’ll see if I can get him to weigh in here.
This sounds like a lot of fun, and you can definitely convert a motorcycle from gas to electric at a reasonably low cost. However I think you will be very limited by a decision to use FRC components. The components available to us through the FRC, or combat robotics suppliers are great, but they are not really the right scale for an Electric Vehicle project. Here are a couple of projects I have book marked that did similar. Here
You can do the math and figure out the power needed based on the power of similar gas engines, I suspect if you added up the number of FRC motors needed to get that much power it would not be feasible.
You cannot turn an alternator with an electric motor and end up with more battery power. The motor is turning Electricity into a kinetic energy, the alternator is turning that kinetic energy back into electricity. Because the energy cannot be created, the system will slowly lose energy due to inefficiencies. You would end up killing the battery faster.
A common way to slow battery drain in electric vehicles is to use regenerative braking. It will however make your system more complicated.
We have a running joke on our team about putting swerve drive on everything because our CEO doesn’t want us to do it yet bacause we are a rookie team, I was telling our head programmer about the project and how I wanted to anodize the frame.
Me: Dan you know what I want to do to this
Me: no dan no swerve this time
That sounds like the right idea. I was looking into building an electric skateboard, and those outrunner motors are insanely powerful! Nothing in FIRST is designed for such an application though, so you’re right that it would be good to look elsewhere.