Fail Of The Week: Car Starter Motors Aren’t The Best Fit For EBikes

A lot of what real engineering is all about is designing to the limits of your materials, with a healthy margin for error. On the other hand, seat-of-the-pants engineering often takes the opposite tack — working with the materials you have and finding their limits after the fact. While the former is more rigorous and better suited to anything where life and limb are on the line, there’s something to be said for the flexibility that informal engineering offers.

[Austin Blake]’s latest eBike is a case study in informal engineering. [Austin] started out wondering if a starter motor from a car engine would make a decent electric bike motor. Our first instinct before watching the video below was to answer that question with a resounding “No!” Yes, starter motors seem like a natural for the job, delivering high torque in a compact package. But starting a car engine is the very definition of a low-duty-cycle application, since it should only take a second or two of cranking to get an engine started. Pressing a motor designed for such a task into continuous duty seems like, well, a non-starter.

And to be fair, [Austin] fully acknowledges this from the start. He even retrofits the motor, wisely replacing the shaft bushings with proper bearings in an attempt to get a better duty cycle. And it works, at least for a while — with the motor, a homebrew battery, and an ESC mounted to a bike frame, the bike was actually pretty peppy. But bearings aren’t the only thing limiting a starter motor to intermittent duty operation. The short drive really heated up the motor, and even with a few ventilation holes knocked in the motor housing, it eventually released the Magic Smoke. The video has all the gory details.

As always, we like to stress that “Fail of the Week” is not necessarily a badge of shame. We appreciate it whenever someone shows us the way not to go, as [Austin] did here. And let’s keep in mind that he’s had success with this approach before, albeit with a much, much bigger starter motor.

40 thoughts on “Fail Of The Week: Car Starter Motors Aren’t The Best Fit For EBikes

  1. Now while its still absolutely not fit for purpose, if he had started (hahahahaha) with a gear reduction starter like a denso unit from a 90s toyota chances are he would fair better, they seem to do much better with excessive cranking. still, poor choice, as most folks have discovered, an alternator is a better base for modification as there rated for 100% duty.

    1. I came to say just that. I had a gocart that ran great with one, on 6 volts: drilled a hole in the middle of the top of a car battery to put a lag bolt into the midpoint, and had two 6V batteries: one to go out, and one guaranteed to get me home.

  2. A “non-starter” Ha ha, Ho ho. It doesn’t get much better than that.

    I was skeptical on the sheer basis of weight, but also had suspicions about the ability to be used on a continual basis without failure. What about cooling? Add a water jacket, water pump, and radiator. You might nieed a gas engine to run all of that, but you could always explain that it was just for cooling.

    1. Most of the heat is generated in the armature, which has poor thermal coupling to the outer housing. Your best bet is to add holes to the end bells, and use a blower to force air in one end and out the other.

      Another problem is that starter motors often have copper (not carbon) brushes. These wear down the commutator very quickly.

      When we were building kid-vehicles, the motor of choice were auto radiator fan or heater blower motors. They were continuous duty, and already had carbon brushes and tolerable bearings.

  3. Why 36V? The magnetic field of the coils is proportional to current. But the dissipated power is proportional to the product of V and I. The number of turns, resistance, and current are designed to not melt the motor too quickly at 12V I would think. Triple the voltage and you triple the heat I think?

    Is that BMS pulse width modulating?

  4. This is definitely interesting, and the video fully acknowledges it’s not a suitable motor – but unsuitable is still an understatement; there’s a lot of better options. I believe a cheap or free angle grinder, with either a throttleable boost converter or a cheap inverter and other speed control mechanism would be a surprisingly good alternative. People generally tend to skip that and go straight to alternators or other things that can be driven as three phase or brushless dc motors, but for cheap and dirty it’d be pretty neat. You wouldn’t have to permanently modify any of the main parts, and it’d be an excuse to put a cable throttle on a variac if you went that route for speed control. (You could still use 21st century methods if you’d rather be boring)

    1. If you are going to go the angle grinder route, get one that works from a battery pack. And also ask yourself how many RPM you want (and consider that an angle grinder isn’t intended for continual duty either.) Some other battery power tool might be a better choice.

      I do have to point out that a good engineer can foresee certain kinds of problems and doesn’t actually need to try things like using a car starter as a bike motor. I mean, even I could have pretty much predicted this would not have worked out well. On the other hand, if plenty of people watch the video, that IS the idea is it not?

      1. Angle grinders might not last a high total number of hours, but they still seem to survive a potentially pretty long duty cycle with only brief interruptions, when there’s a lot of grinding to do in a shop. It’s not perfect, and with all of these you’ve got to forsee the problems from variable speed continuous use instead of fixed speed with limited duty. I suspect it’d work okay if you try to keep the rpm up and the torque low. Agreed you’ll need to adjust rpm a LOT if it’s for a bike; bike wheels are huge and I think the usual velocity of a grinding wheel under load is still fairly high. I’ve seen someone use a friction drive before.

        I have a grudge against the battery ones; the one I used at the school shop always seemed underpowered and the battery was always five minutes from empty. It’s self contained, but I think not very likely to be found for ten bucks lying around. Which is what I look for in these “what can I do with this piece of salvage” videos – you started with something cheap and only partially suitable; don’t throw so much resources at it you could have just bought the right thing in the first place. But yeah, the point is to watch the video, you’re right.

        1. Ah well, I haven’t had any personal experience with battery angle grinders. The two I have are corded (albeit from Harbor Freight and they threaten to last forever!) If I’m angle grinding I am not too far from my welder so I have AC power handy — but I was just trying to avoid the need for an inverter in an already doomed project.

          As far as a bicycle, I am over 65 and still find pedaling the best option, and I’m not looking for silly projects, even if they score big on Youtube.

          1. Yep, I’ve not actually done it for similar reasons, even though I could use all the parts elsewhere once I was done. If I did do anything, it’d probably have more wheels and let me power tools or lights off of it out in a field away from a plug.

  5. A brushless power tool motor with its planetary gear reduction output might deliver longer lasting compact power in a minimal space. Over a kilowatt of output power with the weight of a potato.

  6. The starter motor died because he got it way too hot. In a car, as you said, it is a low duty cycle motor and it runs right off the starter battery. I have no question that I could work out a system with a temperature sensor and pwm system that would not let the smoke out. At the same time, it may not cart your butt up steep hills or go as fast as you might like on the level. I would expect it to have really great acceleration when started cold though. If you live in a flat place it might work out well. Normal running would have to be a small fraction of full power though, and the whole thing would need to be monitored for how hot it is getting, and how fast it is getting hot. Better bearings and better magnets might help too, though better magnets tend to be more heat sensitive.

  7. I don’t actually like to watch videos of people destroying things for no good reason. However, he can make up for it by repairing the starter like Lin Guoer does in her videos. (This is assuming it’s toast; the video didn’t quite make it clear.)

  8. Depending on how fast you want to go get a Ionger chain that would suit a much bigger fixed gear on rear wheel ie motorbike chain. Maybe a wider wheel on the rear. See how you go with that

  9. I still remember the smell of our ship’s starter motor after having run continuously for 20-ish minutes. That was some heavy magic smoke that had definitively escaped.

    Reason for running it 20 mn was the starter relay was too weak and had welded itself closed, and you don’t hear the starter over the engine. The new system had a normal automotive relay to close a beefy truck starter relay to close the solenoid.

  10. Well, a starter motor would do well on a drag bike run in 30 second bursts! Years ago I purchased a 24 volt York motor designed to run a refrigeration compressor on long haul trucks. The application was for an Electrathon race car that had to run for one hour races. Other than weight something like this would do well.
    Bill in Boulder

  11. I’ve been thinking about trying a brushless DC motor meant for a model car.

    Some of them come in quite high wattages for well under $100 USD. Use an inrunner, perhaps wind some copper tubing around it and epoxy it in place to run water cooling.

    Then use the ESC motor controller meant for it, and a nice, large LiIon power pack to power it.

    1. Dude, for $100 you can make a really nice pulsejet!

      The only slightly tricky part is the reed valve. Spring steel, etching primer, scratches, salt water and a battery charger and you’re golden.

  12. I had a similar experience years ago, building an electric motorcycle. I began with a modified starter motor and a 12v car battery. Results were unsatisfactory, to say the least. Then I found a 24v electric truck motor (the old man at the repair shop said “it’s rated 2 hp, but it’ll make a lot more than that.”) With a pair of 12v/100AH deep cycle batteries I could make the 5 miles to school easily. And it was really fast! My transistorized PWM controller blew up the first time I tried it—not enough current limiting—but I had a “wide open throttle” bypass via a micro switch and a ford starter relay.

    I learned a lot about physics, materials, electronics, motors, batteries and chemistry—my high school had a fully equipped chemistry lab and the teacher allowed me to mix concentrated H2SO4 down to make up for spilled electrolyte.

  13. I wonder if he tried it again using heatsinks on the outside of the starter housing and a fan to pull air thru them. The cases on starters with the windings are pretty dense and seem to really insulate well. My dad used to tractor pull with 3 chevy 454 cid engines. these engines were very high compression and starters only lasted a short time, there was literally a pile of them waiting to be returned for core charge. We never salvaged the pile as kids, for lack of batteries. But I had a thought experiment of attaching aluminum angles around the starter case, and attaching a fan (like an electric motor has) to the thing to blow air thru it. Never seen anyone try, but a 1.5 version might use that idea? Hey, it works on CPUs!

  14. I actually think it looks like this is a fine e-bike motor… Any motor will end up in smoke if you push it too hard. Driving an e-bike at 58 kph is unrealistic and (most likely) illegal in most developed countries anyway. Put a current limiter and thermometer on it and it will run fine.

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