Advice wanted for DIY Welsh commuter (Trek FX conversion?)

[Bman80]

Hi,
My name is Benny and I want to build (or buy) an ebike for commuting.
I can see Snowdon from my house and the commute (one way) to Anglesey from the hills is about 8miles/12km with ~1000ft/300m of ascent.

I want to get some excercise, as 1yo and3yo at home have stopped me doing my normal kitesurfing and hillwalking activities.
However, I have chickenlegs and need some help with the hills, a number of which are over 10% on my prefered route.
Was thinking about buying an ebike on cycle to work scheme, but I like to tinker and it seems like a lot of people rate a diy build over a cheap ebike.

I have a donor bike which I think could be perfect. Its a Trek FX 7.3 from 2013 that I picked up a few years ago for £140 with only 100miles on the clock.
In my inexperienced eyes it rides very efficiently for a hybrid and is very light - I have seen it used as a base bike with front and rear hubs as well as mid drive.

My inclination is to keep the weight down as much as possible, but I want to get up the hills at more than a crawl.
I was drawn to the simplicity of a front hub kit - such as Cyclotricity. However the more I read on here the more I am drawn to a rear hub, for better weight distribution and more natural ride, plus less stress on the forks(?). Now reading up on the various motors that Woosh have on their rear hub kits and torn between weight (AIKEMA 85SXC)and power.
Thinking that I might be getting carried away with details and should probably put it out there.

What would you do?
Cheers,
Benny

 

[Jonah]

Hi Benny,
Welcome to the forum.
If you’re reasonably fit, not too heavy and want some exercise then something like the Aikema would be very suitable. Unless you really want / need more assist, lighter setups are preferable in my opinion.

 

[Bman80]

Hi Jonah,
I'm only 68kg and reasonably fit and actually plan to start commuting on fair weather days before the conversion. Partly so that I notice the difference but also, I figure the assistance will help get me on the bike in even the Welshest of weather.

I was excited by the Aikema as a lot of rear hub kits seemed quite heavy. However I became concerned that on the steep hills my speed would be very limited by a lower torque motor - that said I'm happy to pedal some. I notice that the Woosh Faro uses the Aikema and is recommended for flatter areas, which this certainly is not.

 

[Jonah]

I think you would find the Aikema fine. If you’re fit and healthy it should be enough. I expect Tony from Woosh will be along in a bit to give some additional guidance. If you want to take it easy, go for 48v SWX option.

 

[Fat Rat]

Welcome to the site

 

[Hightechpete]

Hi Benny croeso, an e-bike virgin, if you'll excuse the phrase, will underestimate the amount of assistance that a 250 watt motor provides. I am of pensionable age and have no problem negotiating the hills of the Brecon Beacons.
Only failed once on an 18% gradient, I'm going to try again with the lowest gear and max assist.
My advice is as always, beg, borrow or steal a ride, front wheel conversion works fine on a hybrid/MTB, but a rear wheel motor is a nicer ride, especially on a sports bike

 

[Bman80]

Diolch Pete, funny you say about the underestimation of 250W. I looked up the power that a cyclist produces and 250W will probably triple my power output, which sounds great to me.

The front hub conversion is still tempting for simplicity, but the Trek FX is a light, responsive and fun bike to ride that it seems a shame to encumber it for the sake of a bit more work. The more fun it is, the more likely that I'll use it rather than driving, that's half the reason for a motor in the first place I guess.

Good on you for hitting the hills, with any luck a motor will inspire me to go up into the mountains for fun as well as the commute.

 

[Kwozzymodo]

The Trek is a great base for conversion. Rear hub motor and PAS, with a 36v 10.4ah battery should set you back around £300-350 and be good for 30 miles on a charge. Once you get the bits, should take a couple of hours to put it together.

I am not sure about the exact weight of the fx3, I think it has a steel fork - all in, you should be able to build a bike weighing around 14kg. My Specialized Vita weighs in at 12.5kg with battery.

I have a 201rpm rear hub motor and can get up to 12mph on the steep hills around me, and up to 20mph on the flat, with a bit of pedalling!

 

[Benjahmin]

Greetings from Ceredigion. The 250w rating of the motor is merely the continuous rating, i.e. the power it can take ad infinitum without overheating. A 17A max rated controller will give you a nominal 612W in maximum setting, enough to give you the boost you need for almost any hill.
My own front hub is backed by a 20A controller, there have only been two hills that have defeated me. Expect to work, but you can get up stuff you'd think twice about walking up. My motor has never felt even mildly warm, the controller, however, does get too hot to touch at times.
Front hub can be a bit scrubby on steep and slimey narrow Welsh lanes, standing up with weight over the bars generally sorts it. it's OK so long as you're not caught unawares. Rear hub would negate this.
Either way I would definitely fit a torque arm, the counter rotational force that must be present when pounding up a long climb must be immense.
As to the weight, pedelecs are heavy - that's why they're electric! I would favour a high torque motor over some weight saving. Speak to Woosh, tell them the terrain you're in, they'll advise accordingly. I have an Ezee motor (same size/weight as a Bafang bpm) and I carry two batteries. All up, with water bottle,panniers etc, we are talking 26/28Kg. Still manage to cycle out of Llangrannog, if you know it, overtaking sports cyclists on the way.

 

[Nealh]

The small 6 mosfet KT's suffer with extreme heat on climbs and high assist/current use with faster wound RPM rated hubs, the controller might fair a bit better with a 201rpm hub. For hilly areas a 201rpm hub will far out weigh the use of a faster wound hub.

 

[Bman80]

Hi Benjahmin, I don't know Llangrannog, it looks lovely, and I see why you'd want some assistance cycling up from the cove. My Trek has alloy forks, so would need a torque arm to be safe by the sounds of things. I take your point about having enough torque. I definitely want enough help to make myself take the bike rather than drive, even when I'm knackered and wouldn't bother. More importantly I guess, is the reliability factor, and therefore not overworking the smaller motor by running it at too low revs.

 

[Bman80]

I'm still a bit hazy about the rpm thing with motors. Have just had a quick read and drawn the following conclusions: Do I have this right?

1, quoted rpm is the maximum they are designed (at specified voltage) to run at?

2, Faster rpm motors are used with smaller wheels to achieve efficient running at legal max speed.

3, Efficiency decreases as operating rpm decreases.

4, Inefficient running produces waste heat which drains battery and potentially damages motor.

For the following reasons its important use the correct rpm motor.
So for a 700c wheeled bike like my Trek using a 201rpm motor would give 2.15m x 201rpm x 60min/hour /1.6km/mi = 16.2mi/h at max revs.

So the Aikema that I mentioned above runs at 265 rpm, meaning that it is more likely to be be lower in its rev range, and therefore inefficient.... but I'm guessing there is more to it than that as the Aikema uses a larger reduction ratio, and therefore spins faster... what are the implications in that?

 

[Woosh]

1, quoted rpm is the maximum they are designed (at specified voltage) to run at?

quoted RPM is where the motor delivers the maximum power.
eg:
201RPM: perfect for 700C wheels.
235RPM: perfect for 26" wheels
260RPM: perfect for 20" wheels
328RPM: perfect for 16" wheels

No load RPM: is where the motor spins on throttle when lifted off the ground. The yield is zero, power near zero but gives you an idea how this motor will behave. For example, if I don't want customers abusing my kits, I would want a noload speed of 300RPM for 26" wheels.

2, Faster rpm motors are used with smaller wheels to achieve efficient running at legal max speed.

That's correct.

3, Efficiency decreases as operating rpm decreases.

Efficiency decreases either side of the quoted RPM.
Efficiency decreases faster at above the quoted RPM, more gradual below the quoted RPM.
Efficiency follows the shape of a sawtooth.
At very low RPM, the motor is effectively in a state of magnetic saturation, the yield drops off a cliff when its magnets are saturated by the magnetic field. This is the main reason why heavy riders need a larger motor.

4, Inefficient running produces waste heat which drains battery and potentially damages motor.

Yes. If your motor yield is 40%, 60% of input energy turns into heat.

 

[Bman80]

Thanks, that makes sense to me, but still confused about motor size and gearing, given that most motors quote 250W continous output, but obviously behave differently.

I am keen to keep the weight down as much as I can, without sacrificing reliability/efficiency/rideability. It seems obvious that a small motor generates less power/torque than a big one, but the quoted number are very similar. What aspect of the small, high geared motors like Aikema that makes it less suited to hill climbing?

 

[Woosh]

still confused about motor size and gearing, given that most motors quote 250W continous output, but obviously behave differently.

The 250W is a legal requirement. It guarantees the minimum standard, that is your motor will maintain 15mph on a flat road without the casing temperature exceeds 25 degrees C. You only need 200W for this, so all EAPCs pass the test with ease.
How your motor helps on hill is not prescribed, as long as your bike does not exceed 15mph and you pedal. How much you need to pedal is also not prescribed. That's why manufacturers can offer a wide range of products to suit different individual needs. Look at the range of Bosch motors.
If you weigh under 15st, I would recommend a 40NM hub motor (85SX) or a 80NM crank motor (BBS01B).
If you weigh under 17st, I would recommend a 45NM hub motor (XF08C) or a 90NM crank motor.
Over 17st, I would recommend a 50NM hub motor (DWG22C or 48V SWX02) or a 100NM crank motor (48V TSDZ2).
The reason that the need for torque seems higher for crank motors is because they are specified for 1:1 front to rear ratio, if you have 44T front, then it's given for 44T rear. So, assuming you have 11T-28T cassette on your bike, 100NM on 44T front = 64NM on your 1st gear 28T rear sprocket, 25NM on 8th gear 11T rear sprocket.

 

[Bman80]

OK, I only weigh 11st and my bike is probably only ~12kg, and I'm not that unfit, I've just got chickenlegs and get put off by the hills and the weather round here.

Which Woosh kit would you recommend?

Given that I prefer the idea of a rear hub, for the reasons outlined at the top, it looks like a choice between the Aikema 85SXC and the MXUS 08C.

 

[Woosh]

the Aikema is 1kg lighter, but gives 5NM less on steep hills.
On flattish roads or small hills, you don't feel the difference.
for your weight, I would recommend the 85SXC.
I ride the Faro (85SX) for a year now and I weigh 12st. It feels just right.

 

[Bman80]

Yeah, thats my inclination too. I guess with lower energy demands, the range could be better than the bigger motor so further weight savings by using the smaller battery. I read your Faro review and it sounded like just the kind of bike I'm after. Hopefully my Trek conversion should feel very similar. I would prefer a small battery, I see you do 13AH and 17AH, whereas the Faro has 8AH.

I guess the 13AH would do ~40-50miles?
Roughly what weight would this kit be?

 

[Woosh]

the Faro has an inframe battery, it weighs less than 1.5kgs because its plastic case is very thin, about 3mm, and it is built with just 30 cells.

Faro's battery:

https://wooshbikes.co.uk/2019/faro/battery.jpg

https://wooshbikes.co.uk/2020/faro/charging-port.jpg

I suppose you could fabricate a sleeve to attach it to the downtube.

I guess the 13AH would do ~40-50miles?

Yes, but when you ride on the last 20% of the battery, the power is less so the ride is unpleasant. 40-45 miles is more realistic.

The 13AH battery (samsung cells) weighs 3kgs, same as the 17AH battery (panasonic cells). They have different cells.

 

[Bman80]

I've got a Nissan Leaf, so know how unpleasant the last 20% can feel, though that's range anxiety rather than power limitation (never hit turtle mode yet thankfully).

Still, the Faro battery would be fine for me, but haven't the time to start with fabrications really.

Seems a shame to go for the 13Ah when the extra range of the 17AH carries no weight penalty, but 70mile range seems excessive for my 16mile round trip commute, but I guess its good to run them between 80% and 30% anyway and once degradation kicks in I may be thankful for the longer range.... talking myself into spending more money here