Early advice for a new build

Hi All

I am at the very early stages of planning an e-motorbike build as I am going to be moving in the next year of two which will see me having a 20 mile commute. I have spent the past few days researching and reading through as many threads as I can to get a good grasp of what will be involved and to set my expectations for what will be achievable. I am at the point where I'm looking for feedback on my line of thinking and any advice on how best to start out. My background is in engineering and I would say I have functional (if rudimentrary) metalworking skills, though I have access to skilled machinists/welders through my work. I am a keen cyclist and am mechaincally minded, though this will be my first motorbike!

The bike will be used for a 20 mile journey that is an even mix of hilly country roads, city driving and then a short stretch of 50mph dual carriageway (slightly uphill). I have the following requirements

50-60 mph top speed
50-60 mile 'typical' range (given the terrain I will be covering I want a good amount of margin)
Good torque/uphill accelleration when needed


Donor bike

I'm in the UK so a conversion from a 50cc or 125cc will allow me to ride on my current driving license after a basic test. Registering the conversion with the DVLA seems fairly easy. My preference is for a 125cc as this will leave room for upgrades down the line. As I have barely any direct motorbike experience any tips on specific models that make good donor bikes is welcome. I've found an excellent thread on the conversion of a 50cc Chinese Yahamha clone (Yamasaki YM50-RE) that I am using as inspiration (https://endless-sphere.com/forums/viewtopic.php?f=12&t=105463).

Motor

My initial thoughts are that a 4kW motor should meet my needs. Whether I go for hub or mid-mount will likely depend on the bike (unless my requirements dictate one or the other). I would really appreciate feedback as to whether a hub motor is suitable for hilly terrain, or whether a geared mid-mount is more suitable (at the cost of battery space). I know there is a QS138 70H V3 3kW mid mount with built in reduction gear but I don't know if a 3kW motor would be sufficient.

Battery
Honestly the finer details of the electrics is the part of the build I am struggling to get my head around, but I see this as a good project to finally get a good understanding of the subject. Again, would really appreciate feedback on what would be suitable. Reading around it appear the pandemic has impacted the supply chain somewhat to sourcing an affordable battery pack may be tricky. Given this, I am leaning towards using a 2nd hand EV battery if I can get a good price on one. I have been looking at a BMW i3 battery module which is 44v,96Ah which has 12 cells in series. Providing the donor bike can fit them all in, could this be re-configured to give the speed/range I want? Alternatively if building a custom pack from new cells more suitable I'm happy to be steered in that direction.

Controller/supplimentary electrics
I'm waiting until the other components are specified and then I will most likely be seeking additional advice to souce a suitable controller, but any advice is welcomed

My back of the envelope costing for the project is around £2000 for the donor bike and conversion. Please let me know if I am being wildly overoptimistic (or if this could be scaled back - somehow I doubt it!)

Just a couple of points, the 3kw unit mentioned will fit your needs just fine, I'm using one on a much heavier bike and 60mph with 50mph on steep hills is no problem at all. A hub motor may work out better for you and for the mid, I'd inquire with QS on when they'll have the 4kw available with a reduction gear as the extra power would be nice but the 3kw is plenty capable.

If you're buying the batteries new and the donor bike is costing any more than practically free then £2000 isn't going to cover the costs, realistically you'd probably need to at least double it. It might just be possible to stay within that with a dirt cheap bike and ex-EV car cells but even then I'd still say you need to up that budget to £3000 minimum, once you start adding in cables, connectors, contactor, BMS, etc. the figure ramps up fast

I’m the guy who built the Yamasaki - what I can say about the 4kw hub motor I used is that it’s definitely equivalent of a 125cc using the 72v pack I had built. Acceleration is quicker up to about 25-30mph than a petrol equivalent then it’s about equal and tbh it tails off after 50mph. All on a closed road of course! With my 2.8kw pack I can get 50-55 miles range which is fine for what I need.

I’ve not really noticed the down sides of a hub motor in terms of unsprung mass but it is very very quiet having no chain as you’d have with a mid drive motor. So much so that I have had a few near misses with folks walking into the road in front of me. I’m just finishing off designing a speed/RPM based sound module set up to provide some simulated engine sound.

Engineering wise to avoid cutting the swingarm you still have work to do even with a hub motor compared to say having to make the engine mounts for a mid drive motor but I still think hub motors are an easier build all round. That said I do fancy another build using a 4kw version of QS mid drive motor with the built in gear reduction gearbox.

Anyway keep us up to date with progress and where you get to on design decisions.

Sorry forgot to say …. Factoring in battery pack, motor/controller and all the other bits such as contactor, display if you want to monitor battery in real time, cables, crimping tools etc you are looking at £2,500 - £3,000 on top of your donor bike …. based on my experience when I built mine.

stan.distortion said:

Just a couple of points, the 3kw unit mentioned will fit your needs just fine, I'm using one on a much heavier bike and 60mph with 50mph on steep hills is no problem at all. A hub motor may work out better for you and for the mid, I'd inquire with QS on when they'll have the 4kw available with a reduction gear as the extra power would be nice but the 3kw is plenty capable.

If you're buying the batteries new and the donor bike is costing any more than practically free then £2000 isn't going to cover the costs, realistically you'd probably need to at least double it. It might just be possible to stay within that with a dirt cheap bike and ex-EV car cells but even then I'd still say you need to up that budget to £3000 minimum, once you start adding in cables, connectors, contactor, BMS, etc. the figure ramps up fast

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Thanks for the input, it's really appreciated.

I did feel I was being overly ambitious regarding the budget. As I have a fair amount of time before I need to start the build I'll keep an eye out to see if I can pick up any of the components at a decent price.

I contacted QS and got a very fast response. They said that they don't have any plans for a 4kW geared mid drive as the 3kW gives a simliar performance as a 4kW motor.

swiftyds said:

I’m the guy who built the Yamasaki - what I can say about the 4kw hub motor I used is that it’s definitely equivalent of a 125cc using the 72v pack I had built. Acceleration is quicker up to about 25-30mph than a petrol equivalent then it’s about equal and tbh it tails off after 50mph. All on a closed road of course! With my 2.8kw pack I can get 50-55 miles range which is fine for what I need.

I’ve not really noticed the down sides of a hub motor in terms of unsprung mass but it is very very quiet having no chain as you’d have with a mid drive motor. So much so that I have had a few near misses with folks walking into the road in front of me. I’m just finishing off designing a speed/RPM based sound module set up to provide some simulated engine sound.

Engineering wise to avoid cutting the swingarm you still have work to do even with a hub motor compared to say having to make the engine mounts for a mid drive motor but I still think hub motors are an easier build all round. That said I do fancy another build using a 4kw version of QS mid drive motor with the built in gear reduction gearbox.

Anyway keep us up to date with progress and where you get to on design decisions.

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Really appreciate the direct experiece regarding the hub motor. I hope you don't mind but I'd like to pick your brains on a few specific points on your build.

Looking at that Yamasaki it does look like an ideal donor bike, would you use that frame again and/or are there any other bikes I should keep an eye out for?

Also have you got any tips on good places to source components within the UK? I take it ordering the motor and controller is best ordering via QS/aliexpress (though the shipping on the motor is a little painful!) but I'm struggling to find anywhere to get a battery made for the build if I go down that route, or an idea of how much that would cost?

It's good to know that 3kWh should meet my needs for the battery, it looks like if I do go down the EV route I could get what I need from an eGolf for ~£700 plus a new BMS. My main concern is how much space they will take. What was the dimensions of the battery box you had fabricated? and what do you think the largest size of battery that frame could accomodate? (for reference I could configure the eGolf batteries to be 350x300x100mm or 350x200x150mm plus a bit extra for the box and BMS)

Yes I would definitely use the Yamasaki frame again given low cost of donor bike and the fact it is almost a 600cc large size bike which comes with a lot of space inside the frame for battery box, and under the tank where I mounted the controller.

It’s not a Yamaha or Kawasaki so you need to accept the quality of components isn’t the same but I’ve seen other production electric motorbikes including Zero and it’s not too far behind those. Just really depends how well used the donor bike is and how well it’s been looked after.

I did start the build with an Aprilia RS50 but I just couldn’t get the hub motor mounted without radical mods to the swingarm. Also the battery box would have been tight in the frame. I did look at much smaller bikes like the Derbi GPR50 and Yamaha TZR50 but it’s difficult to find a decent one of those now given 50cc prices have skyrocketed, plus packaging the controller and battery was going to be difficult. In the end I bought a decent bike that was less than 12 months old from the importers direct (CheapBikesRUs).

As for parts I think you would have to go direct to QSMotors for motor and controller - Robert Chen was very helpful and back in 2019 I got mine shipped with DHL and had it in about 3 days but it did cost £145 plus custom charges on the goods themselves. Robert was very helpful and answered questions about spec of controller and what I was trying to get to with performance and range etc.

Subsequently I found a guy in Ireland who could supply the 3kw mid mount motor and various controllers but at a higher price than from QSMotor - looks like he was using them for karts he was building for a series. Can dig his details out if it helps.

As for battery pack, my pack was built in the UK by a specialist I trust and cost me best part of £1300 including a 150A BMS with android app. I think they’ve now had to stop builds because of shortage of cells (mine uses Samsung 40T cells). The box for my pack measures approx 250mm x 230mm x 200mm. Could easily go to 350mm x 280mm x 250mm I reckon. You should be ok but I’m about to take the side panels off on mine as I need to replace the throttle and want to make a few wiring updates so I can double check if it helps

Something maybe worth considering are older aircooled twinshock bikes. It's way easier to make a fully faired bike look right but twinshock means there's extra space that's usually taken up by a monoshock, swinging arms are often shorter which can add up to a bit more space and air cooled engines usually take up more room too. Often the engine is unstressed or only lightly stressed too, adding bracing where the engine was an integral part of the frame can be a lot of hassle. Doesn't always apply of course, most 125 commuter engines aren't particularly big but there are quite a few 250s with potential (often sleeved down 400s).

Thanks again for the information.

Funnily enough while I thought I'd be just browsing and speccing parts for a long while I've just came across a 1999 Aprilia RS125 with a bad engine going for practically nothing so I am very tempted to go have a look.

Looking on the forum there seems to have been a couple of successful mid-mount conversions and the mounting plate doesn't look too tricky (famous last words). I take it the RS125 is slightly wider than the RS50 so hopefully there would be more room for a decent battery pack, even if I have to get a bit clever on how it is packaged. Looking at what was tested with a plywood template on one of the other builds it certainly looks feasible


Any thoughts on this as a donor bike? I have to say I like the look of it, or would something this old be more hassle than its worth (even if it is only £100).

DeadParrot said:

Thanks again for the information.

Funnily enough while I thought I'd be just browsing and speccing parts for a long while I've just came across a 1999 Aprilia RS125 with a bad engine going for practically nothing so I am very tempted to go have a look.

Looking on the forum there seems to have been a couple of successful mid-mount conversions and the mounting plate doesn't look too tricky (famous last words). I take it the RS125 is slightly wider than the RS50 so hopefully there would be more room for a decent battery pack, even if I have to get a bit clever on how it is packaged. Looking at what was tested with a plywood template on one of the other builds it certainly looks feasible

Any thoughts on this as a donor bike? I have to say I like the look of it, or would something this old be more hassle than its worth (even if it is only £100).

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If I was in your shoes I'd already have it bought, especially if the fairing's still in one piece! There's quite a bit of space in the frame (enough to squeeze in a honda xr650 engine, certainly enough for the 3kw/hr or so minimum you'll need for 20 miles), the engine's not a highly stressed member (but uses the swinging arm pivot as a mount iirc, a plus with the 3kw) and they're just about the most fun you can have on 2 wheels!

On the downside, finish quality is crap, be ready for a fork and shock/linkage rebuild but you might be lucky, they often blew up long before the cycle parts wore out In that light, it might be worth considering getting a second controller when you order that and the motor. That combo can be pushed really hard, close to 30kw and it would almost be a sin not to with that bike. Folks have seen 80mph on enduro bikes with them and on paper it's enough for 100mph with a fully faired bike, pretty much the same as the original engine but with much more torque at lower speeds. Should be a real blast to ride but it's pushing the controller too hard for day-to-day use imo, far too much temptation for me but not a problem if you can keep it turned down for the sake of reliability.