Thursday, 14 February 2019

BMX Ebike MK2

BMX Ebike MK2 - Intro

I doubt you have seen my original DIY BMX EBike build, as it is the worse thing ever, and although a "good laugh" was pretty much useless. The main problems were lack of wait on the front wheel, battery in the way of knees, and rubbish brakes. If you would rather watch on Youtube, then there is a video.

MK1 and . . . . MK2

BMX Ebike MK2 - Improvements

Planned improvements were to mount the battery to the rear of the saddle, this required lengthening the rear stays, otherwise there would have been heel strike. Lengthening the rear stays also moves the centre of gravity forwards. I also needed to reduce the top speed, add some mudgaurds, lighting and improve the brakes.

BMX Ebike MK2 - Drive System

The drive system although weird is as good as I can make it for the components used. I did not change this.

You fit a rear wheel (110mm wide) in to a front fork, by cutting a re-welding the  left-hand side drop out. This is easy to do as you can use the axle and nut to hold the drop out in place as you weld.

The motor is then mounted so the chain line is straight. The motor need to be geared other wise you bike will be all speed and no torque which is not very good. An ungeared motor may go really fast on perfectly flat ground, but even a shallow hill will result is doom. i.e. slowness. 

The key is to add a plate of steel on which to mount the motor.


If the steel is quite thin it will flex when you are using the motor and the chain will not stay tensioned. Add in a brace as above or use thicker steel. The holes which are used to bolt the motor, should be elongated, this allows you to slide the motor up and down, and thus tighten the chain.

BMX Ebike MK2 - Frame 

I needed to make room for batteries, and alter the front rear centre of gravity.

I added in this lattice structure between the new rear sat tubing, I cut the stays and then bought tube from eBay that would fit in side, I bought stuff with quite thick walls as I was not worried about weight.

Next up was to add in the battery holder frame. This was cut off the MK1 BMX Ebike and welded on. The tubing is thin so you can also hang a pannier bag on the sides too, which is handy. If the battery fit down in side without poking out of the top, then perhaps you could make a seat at the rear. 

But is does poke out of the top, so no seat.  The battery is held in place using a zip tie. A big zip tie.

Here is a close up of the area of frame were it has been extended. The tubing was placed in a vice and bent by hand until it all slotted together, no measuring or anything, just bend a bit and then try it out. 

BMX Ebike MK2 - Electrics & Sundrys

A few extra bits were added to make the bike a bit more practical.  To recap the the bike is powered by a MY1020z, with a YiYun Controller with 48v of headway LiFePO4 at 8ah. I had to work out how to slow this thing down, as I had reduced gearing as far as possible, and still it went 30mph, which is just silly and against the law in the UK. So I figured out how to reduce the speed of the ebike.

A "kill" switch for the controller (ebay: engine stop switch). This is wired in to the "ignition" loop of the controller (yiyun) where the key would normally be. It allows me to kill the system if anything is going wrong. Sticky throttle for example. Next to that is a small toggle switch which is used to switch the lights on and off.

In this picture you can see mudguards have been added, and a front head lamp, this is a 10W LED floodlight, it is a poor choice for a headlamp and the beam is wide a shallow, and does not reach very far up the road.  The lights are power by a Shido Lithium battery at 12v which is taped to the handle bars. It is quite light.

 Lastly the controller and all the spare wiring are tucked inside this webbing pouch to keep then tidy, and a little dry.

BMX Ebike MK2 - Video

If you would like to here my annoying voice, talk about this for 7 minutes then there is a video below.

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