Welcome to my latest mini-project – building an electric mountain bike… Why would I want to do that….?

  • I’m surrounded by loads of great off road trails, ideal mountain biking territory
  • Unfortunately there are a lot of steep and rocky climbs which are just hard work
  • I’m a “petrol head” so already have plenty of motorbikes for riding these trails, but I’m interested in exploring the potential of electric power

I do a reasonable amount of cycling on the road on my Sirrus X, but that is no use off road. I could just go out and buy an electric mountain bike, but they are very expensive for anything of decent quality (thousands of Euros) and are constrained by the EU regulations. The regulations restrict the motor power to 250W, are pedal-assist only and the motor cuts out at speeds over 19 km/h. These are all sensible safety measures and even a 250W motor is the equivalent of having a second person pedalling. And where is the fun in buying something ready made when you can build your own ?

I spent some time researching the possibilities and options and quickly concluded that I didn’t want to be restricted by the regulations so would be building an “illegal” bike – but reasoned that for mostly off road use this would be OK. There are many options regarding motor type and location, drive system, battery capacity and mounting that I won’t go into here. Having looked at the choices I decided to start by finding a second hand full suspension mountain bike with disc brakes and then buy the individual components for the electric conversion (motor, controller, battery). I hadn’t finally decided between a mid-drive (mounted on the bottom bracket) or a hub drive (mounted inside the rear wheel), but did want a large capacity battery which I planned to carry in a rucksack. Mounting a large battery on a full suspension bike is not easy and needs to be very secure to survive the off road abuse. My goal was to be able to cycle from my house into Cahors and back on the trails that I have used on the motorbikes…

Having come up with a plan I went to look at some bikes… As I was looking for something a bit specific and France is a big place this involved a few 3h round trips and I didn’t find what I was looking for – one of the issues being frame size. The bikes I looked at were apparently size L but were too small for me. Eventually I found a suitable machine – a Trek Remedy. This is a quality full suspension bike with disc brakes and an ideal base for the project. What made it even more perfect was that it came with an electric conversion kit from French supplier OZO. The kit was a year old and had previously been fitted to the bike but the owner had removed the kit to try to sell the two separately. The complete package met my requirements and made my job a lot easier (assuming I could get it all back together). As a bonus the electric kit was still under warranty…

Trek Remedy

This is the naked bike (the main picture at the top of the post shows the finished conversion). The bike seemed in well used but pretty good overall condition. There are a few things I need to sort out:

  • Pedals are awful and need to be changed
  • Brakes need swapping to get the front brake on the right (which means bleeding the system)
  • Dropper seat post doesn’t lock – it’s hydraulic so may need bleeding/servicing
  • Has a front derailleur with two cogs – this can probably be binned as with the electric assistance 1 cog should be enough. The current big cog (alloy) looks a bit worn.

Having given the bike a check over I did a bit of riding around on it and found it very hard work getting up the hills. I used to do a bit of mountain biking in the UK around the South Downs and brought my bike over to France with me when we moved but soon changed it to road tyres and eventually sold it – now I remember why. I have to admit to being a bit disappointed at this stage… Maybe the electric kit will make a big difference ?

Just need to fit this lot…

The kit has a 1000W hub motor, a hand throttle and a 1KWh battery. It all went together OK but cable routing to my satisfaction took a long time (and used lots of tie wraps). Did some test riding and hill climbing and the transformation was absolutely amazing. What a fantastic piece of kit – overcoming all of the disadvantages of the naked bike. I tend to pedal all the time and just use the motor as a top up to get up hills and in terms of range can already tell that used in this way the battery will last longer than me – 80Km or more should be achieveable.

Disadvantages ?
Well there’s no denying these things cost money – the OZO kit is 1750 Euros new ! There is a lot of added weight – maybe 25Kg +, the motor alone is 10Kg. The battery in the rucksack does start to feel heavy after a while and doesn’t get any lighter as it loses charge ! The heavy rear hub motor does compromise the bike’s downhill performance, overwhelming the rear suspension and then bouncing out of control. There is some resistance from the motor when pedalling but it seems reasonable to me – pedalling on the flat is hard work anyway due to the fat tyres and overall weight, but is manageable.

All looking pretty good, but after a few rides the hub motor started making a funny noise… I’ve contacted OZO and they think it is a hub bearing so it will have to go back to them for a replacement…