Electric Moped Update #6: “Braking” It Down

Brakes, Brakes, Brakes. A wise man once said, “the ability to stop quickly is what allows us to be quick”. The brakes that came with the razor moped are obviously not made to go quick. Then again, they were designed for a 350watt motor and an 80 lb rider, not a 1500 watt motor and two 150 lb riders. Even then the brakes were obviously designed to a price utilizing cheap sheet metal parts and friction pads that I swear are made of wood. These brakes work by tightening a friction pad around a drum to brake the wheel which converts kinetic energy into heat.

Now as much as I love the thrill of not stopping, it does pose some minor safety concerns. Imagine this, it’s early Tuesday morning. You’re on your way to class on the other side of campus with your roommate. You’re riding your hot-rodded child’s moped with him clinging to dear life on the back of the seat. The cold morning air blows through your hair as the bright glow of the headlight illuminates any stray cats in your path, the golden rays of dawn just breaking over the horizon, it’s almost romantic… you crest the last hill, the mopeds motor not blinking under the load of the two “burly men”. Slowly gravity pulls you down the apex gently accelerating you down the hill and to the street below. The moped begins to catch speed, your eyes began to water from the air rushing by. You hear your roommate holler from behind, “should probably start braking!”. You yell back, panic in your voice, knuckles white from the strain,”I’m trying to! It’s not slowing down!”. Like a runaway train, you careen down the hill, the stop sign at the bottom growing larger every second. Terrified, you shoot through the busy intersection coming to a gradual stop at the other side. Both letting out a sigh of relief which turns into a nervous laugh. You realize if it wasn’t for your early class, the bustle of late-morning traffic would have probably caused things to end differently. Moral of the story, I NEEDED better brakes.

When thinking of my options, I decided to use disc brakes because they have strong braking power and are reliable. The rim brakes commonly used on bicycles wouldn’t work in this situation because the wheels are not compatible and it would be next to impossible to modify them to accept the brakes. I picked up a cheap Tektoro brake set from Amazon for only $20 that had some great reviews and I set out to find how to attach it. These brakes come in two parts: the brake caliper and the brake rotors. The rotor is the braking surface of the system and attaches to the wheel, the caliper attaches to the frame or front fork and is what applies the braking force. The moped doesn’t have any mounting points for either so I was going to have to make my own.

Mounting the Rotor

Finding how to mount the rotor was interesting. The rotor needs to be attached firmly due to the immense amount of torque it will be under. It also needs to spin true without any deviation, otherwise the moped will shudder under braking and wear through brake pads faster. To mount it I had to make two parts; a small steel disc with mounting holes that I welded onto the hub of the wheel and an aluminum standoff that I mounted to the steel disc that the rotor will attach to. After designing everything in CAD and importing a rotor as a reference, I set about making the parts.

The steel disc has five holes to align with the five spaces between spokes, this allows bolts to pass through without interference. The standoff has six tapped holes on one side to match up with the holes of the rotor and five on the other side to match up with the holes on the steel disc. Originally, I designed three more holes into the rotor side that would allow a bolt to go through in the opposite direction and press against the back of the rotor. The goal was to use these bolts to straighten the disc out, acting as a three-point leveling mechanism, but in testing it was warping the disc so the idea had to be abandoned.

The disc was water jet out of 1/16th steel and the standoff out of one-inch aluminum. It didn’t need any post-processing besides a little grinding to get rid of the mill scale. Once I had it cleaned up, I welded it onto the front wheel’s hub with a MIG welder running stargon shielding gas. No hiccups yet, fingers crossed!

The standoff needs a bit more post-processing. I need to face it down to .8in (ended up needing to be more like .64in), ream out and tap all the holes, turn down the outside to the right diameter, and do a boring operation to give the disc weld some clearance.

After a few days, a couple lathes (took a while to find one that’s just right for this small part), and FAR too many guess and checks later I had everything put together and bolted to the wheel.

Looking pretty great to me! The hub ended up being pretty difficult to machine due to its size but I got it working in the small chuck of the mini lathe the FIZ has. I wouldn’t know how true it would be until the brake caliper was mounted.

 

Mounting the Brake Caliper

Compared to mounting the rotor, mounting the brake caliper was a lot simpler. To design the caliper mount I tightened the brake caliper onto the disc so that the edge of the brake pad was lined up with the inside edge of the braking surface. Then  I took a picture of the caliper up against the fork to use as a reference. Next, I measured the distance between the caliper’s mounting holes and used this to scale the image to the correct size in CAD.

I laser cut the design out of 1/4″ acrylic to make sure the model fit. A few revisions later I got it perfect. Time to waterjet! (honestly, loving this waterjet machine. It makes producing very robust parts soooo simple). I forgot to take pictures of it being waterjetted, or mounted to the caliper, or welded on (I just got so excited!) soooooooo, here’s what it looks like done!

Before I could weld I ground off a lot of the paint from the fork in the area where the mount was going. I apparently hadn’t removed enough paint because I had to stop every few seconds and blow out the burning flakes… but besides this everything was smooth sailing. The weld has deep penetration and looks alright so I’m happy. I hit the weld with a grinder to take off all the beads before I set about mounting the brake. The tektro brake I used doesn’t auto-center so I had to fiddle with it for way longer than I would have liked before it stopped rubbing. The disc was definitely wobbling but from what I’ve read it’s ok to bend it straight so I spent a while doing this until it was perfect. After testing it for a few days to make sure everything was alright, I took the wheel and brake caliper off to paint the exposed metal to prevent rust. It doesn’t look super great but if you squint your eyes and look at it from a ways away it doesn’t look half bad!

So, I have been riding with the new brake for about a week now, the difference is amazing! I have so much more braking power that when I want to stop, I actually stop! This has to be one of the best upgrades I’ve made and it has made riding the moped so much safer.

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