Day 5: throttle, batteries, sensors

Proof that everything works:

I started yesterday trying to make a throttle. I could have used one of those handlebar hall effect throttles, like what’s on picoFahrrad, but the steering wheel on this one would make mounting such a throttle messy. Also, its just nice to make things yourself.

I dug through some bins of spare parts and found a constant force spring and a linear pot.  I would use the spring as the basis of a mechanical spring return slide, and simple couple the pot to it. My first idea was to just use two pins that slide in the slots of the aluminum channel. It worked a few times, but then the rod dug in slightly in some parts of the slot, and it was over. It just took too much force to slide, and often got stuck.

first (failed) throttle attempt

assembled (failed) throttle

The next idea was a little bulkier, but much less likely to have a sticking problem. It used 2 ball bearings that ride on the inside of a square tube.  Also, I decided to reduce the spring force by cutting the spring in half. Don’t ever resort to this unless you absolutely have to. It takes a long time, you have to deburr everything, and who knows what awful un-tempering things you do to the spring.

improved throttle design (will be held on by zipties)

the pot is simply double sided taped to the square tube.

jagged edges on the spring from grinding it in half

Next came the battery packs, which Charles was nice enough to pretty much build for me. The kart runs off of two 4s6P for a total of 8S6P. With these LiFe cells, that’s a nominal 26V and 13.8Ah. Safe to say, this will have a much longer run time than picofahrrad.

one 4S 6P pack, 2 of these packs in series power the kart

huge soldering iron in action

balancing leads + battery terminals

Its nice to pad the cells with something, and I knew this would be important for me, because right now, the rider pretty much rests their feet on the packs. We were out of foam, so the leg of an old wetsuit did the job. Finally, everything was wrapped in extra wide Kapton tape.

foam battery pads from an old wetsuit


Next I installed sensors on the motor. A while ago, Charles made this nifty ring that just presses gently onto the motor stator, and has slots that hold digital hall sensors.

3D printed sensor holder

closeul of sensor wiring

The sensor ring is aligned initially by aligning the middle sensor with a space between two stator teeth. The idea behind this was explored a bit in my recent hubmotor build. The positioning of this ring is really quite sensitive. We found it extremely hard to adjust it finely enough so that the motor would run without skipping through the entire throttle range.

positioned on the motor

I am using a Kelly Controller KBS36051,20A,24-36V, so it has a reverse switch, and can take the analog input from the slide pot.

throttle and reverse switch

double charging with Charles's and Shane's chargers

I would have placed the sensors on the other side of the motor, closer to the controller so that the wires are shorter (less noise), but the seat post is in the way.

final sensor orientation

zip tied sensor wires

next to big brother Lolriokart



19 thoughts on “Day 5: throttle, batteries, sensors

  1. Pingback: Go-cart made in 5 days… « adafruit industries blog

    • The fastest I’ve gone so far is about 15 mph. It might be able to go faster, but I am having some trouble tuning the sensor position well enough to go full throttle.

  2. Pingback: Blog » Blog Archive » Go-cart made in 5 days…

  3. AmyQ – you’re incredible! From concept to compete in 5 days?! Seems like people are being awfully productive this summer @ ERS. Why’d I go out to LA?!

  4. i wanna ask, your motor….how much heavy/load that motor can carry out while operation….
    the motor rpm and torque…
    and, the types, quantity battery U use…..
    thank, i maybe i wanna buy this motor…

    • Hi Jangkang, thanks for reading!
      I am using this motor. It drives a 14 tooth pulley which drives a 72 tooth pulley on the wheel. I am over 60kg and it can just barely pull my up a 10, maybe 15, degree slope. I use 6 parallel, 8 series of an older version of these that are 2.2A-h. On a full charge, my friend an I estimate a top speed of 15 miles/hr (6.7 m/s). The wheel diameter is 8 in (203mm). Hope this helps!

  5. How does a hubmotor “skipping” manifest? Is it that it momentarily loses power at certain RPM’s? (I have a motor that does this, and Id like to have something to hit them with so they fix it 😉 )

    • hi! Do you have sensors or are you running sensor less? Skipping often has to do with the motor controller not having its phase well aligned with the actual rotation of the rotor. It would be like if someone were pushing you on a swing set, but instead of pushing you when you are at your highest point, they give you a push somewhere in between.

      • Its using sensors, and I see what you mean by the swing analogy. The odd effect is that this skiping only happen in a small span of 5 kph. Above that, and under that, its pulls fine.

      • Hmm, but it has no trouble starting from zero? It still might be interesting to fiddle with the sensor alignment to the magnet poles and see if that has an effect.

      • No, no trouble at all in starting from zero. And while fiddling with the sensors might be fun in general, its not as fun on an expensive in-warranty thing 😉

        So you’re saying that when you had alignment trouble, the symptoms were partly skipping in a certain RPM range, and difficulty starting from zero?

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