Mid-drive conversation kit for pulling a heavy cargo trailer

[Simonh82]

First time poster on here, looking for some advice about mid drive conversation kits.

I want to convert an old hybrid bike to an ebike. I really want to cut down or eliminate short car journeys that I make regularly. These are generally to the big supermarket and to a localish park which is well enclosed and where our daft whippet who loves to run away can safely be let off the lead.

Having been inspired by some YouTube videos I'm in the process of building a large foldable cargo trailer that I hope will hold a full load of shopping or a dog (carefully and fully secured) and child. To add to the fun my local Asda has a largish hill with about 40m of climb and about 30m of decent between the shop and my house. I think the trailer plus shopping could be around 80kg on top of my 75kg and whatever the bike weighs.

I would like some advice about what motor/kit to get to allow me to get me and my shopping over the hill without killing me. I cycle to work regularly so have a base level of fitness but it's a flat commute and I don't think I'm ever going to win a polka dot jersey. Basically I need a good bit of assistance to get me home with a heavy trailer.

I've been thinking about a 500W Bafang mid-drive kit. I am aware that they aren't road legal but I would only use it at 500W for the hill and would set it back to 250W at other times. I know this may not cut it with the police if challenged but I'm hoping that a middle aged rider, not going too fast and riding sensibly would hopefully not attract too much attention. I'm also going to be upgrading the brakes to make sure I have enough stopping power as I don't want to be dangerous to other road users.

Does this sound like a reasonable choice of motor for my purpose? I'm assuming that it's possible to put the kit back to 250W output after it has been derestricted. Is that the case?

Thanks for any advice.

[stodd]

The poster has also sensibly posted this on the pedalecs site.
https://www.pedelecs.co.uk/forum/thread ... ike.48348/
I think he will get more responses there, so I suggest to anyone who finds this thread here and is interested in such conversions also looks there.

[Paulatic]

Does it need to be multi lingual?

[Carlton green]

The poster has also sensibly posted this on the pedalecs site.
https://www.pedelecs.co.uk/forum/thread ... ike.48348/
I think he will get more responses there, so I suggest to anyone who finds this thread here and is interested in such conversions also looks there.

Yep, a few sensible answers there already. As helpfully pointed out 250 Watts is a continuously rated motor output ‘limit’ and higher outputs for short periods are common (it all depends on what electrical power the controller and battery can supply).

I am reminded that what’s important (in making the objectives achievable) is gearing of the motor and then the onward transmission from the bottom bracket, and that whilst getting caught with more than 250 Watts (as a rated motor unit) might be unlikely it could have painful consequences - consequences I certainly wouldn’t risk and wouldn’t encourage others to risk.

[gaz]

.

[Biospace]

You're going to be making a very positive contribution to your local environment, great stuff! Just make sure the braking is more than adequate for damp road surfaces - braking with a heavy trailer brings unusual challenges, especially while steering to avoid an obstacle.

[Simonh82]

The poster has also sensibly posted this on the pedalecs site.
https://www.pedelecs.co.uk/forum/thread ... ike.48348/
I think he will get more responses there, so I suggest to anyone who finds this thread here and is interested in such conversions also looks there.

Thanks for pointing to the other thread on Pedelecs. I waited a while for this post to be approved but thought the post might have been rejected for talking about non-road legal motors. So posted there too.

[Simonh82]

I think the trailer plus shopping could be around 80kg on top of my 75kg and whatever the bike weighs.
...
Thanks for any advice.

First bit of advice would be to weigh your typical shopping load.

My no longer available low-budget Avenir Mule trailer comes in at under 10kg and is rated for a max 40kg payload. A Surly Bill weighs around 20kg with a 62kg limit.

The Mule suited me well over many years for hauling home the weekly shop for a family of three, although I've since upgraded to a lighter Burley. Over that time I can count the number of occasions I've exceeded the 40kg with household groceries on one finger.

I've no experience from which to comment on the proposed e-conversion.

I will check the typical weight of my shop. When I do a big shop I'm also shopping for my elderly father in law and he is partial to his daily can of ale so I've often got quite a bit of weight from that alone. Maybe I'm over estimating it but it's often a full large shopping trolly.

[Simonh82]

You're going to be making a very positive contribution to your local environment, great stuff! Just make sure the braking is more than adequate for damp road surfaces - braking with a heavy trailer brings unusual challenges, especially while steering to avoid an obstacle.

I am very aware of this. At the least I will upgrade the brakes to the best rim brakes I can get. If I'm not satisfied I'll likely buy a second hand bike with hydraulic disc brakes to give me plenty of stopping power.

[Biospace]

I am very aware of this. At the least I will upgrade the brakes to the best rim brakes I can get. If I'm not satisfied I'll likely buy a second hand bike with hydraulic disc brakes to give me plenty of stopping power.

Great. Downhill stopping is something those who go out and buy a heavy four wheel drive car become aware of - at first they expect steeper downhill performance to be as much of an improvement as that uphill!

Both hydraulic and disc brakes bring advantages as well as disadvantages, imo it's the hydraulic aspect which brings most advantage.

[Vantage]

Hi Simon. I've read this post and the other on pedelecs and I'm astounded at the amount of rubbish people have blabbed out.
How much power you need depends on the terrain you're riding, how strong you personally are and the bikes gearing. You'll also need to consider wind speed and direction.
Some time ago my dad and I set off for a tour from Wales to Anglesey. Fully loaded touring bikes and lots of climbing and I can tell you that my 250watt perfectly legal bafang motor did not have the muscle to get me up one particular climb.
After that, I upgraded to the 750w motor. That thing will climb anything. It does come supplied with a handy 250w sticker incase the dibble check your bike. You'll be wanting to remove the throttle because if they check that (and they will) you're stuffed. Those throttles cannot conform to UK regs without some software tweaks.
Braking. What people have said about this is beyond stupid. It matters not one bit about how powerful they are for the simple reason that bike trailers do not have stabiliser bars. If you brake hard enough, the trailer will either jackknife or it will force the rear wheel into the air and that just ends badly. Braking long and slow is your only way here. Also, you don't need brake sensors or the adapted brake levers. How often do you brake whilst pedalling? Soon as you stop pedalling, the motor cuts out. Simples.
The Shimano cues cassettes are a little bit more hard wearing than standard ones but you may need to grind the rivets out to remove one of the spacers or it might not fit the freehub. Also check your derailleur can cope with the bigger sprockets if you do decide to get bigger gears. The bafangs like to be spun quickly. I personally use the stock chainring and an 11-46t cassette which gets me up some silly long climbs. You'll be wanting a longer ebike specific chain too. Kmc ebike chains are quite good.

[Biospace]

What people have said is clearly not "rubbish" or "beyond stupid". Suggesting that a particular motor/battery and a steep hill means that the OP will struggle unless they break the law seems rather odd given the wide choice of motors, batteries, combinations of them and their location together with no indication of the incline. 40 metres vertical averaged over a mile and a half is around a 1 in 60 gradient.

Long and slow braking is ideal with a heavy load but clearly thought needs to be given to what happens in an unforeseen situation. Brake trials with a loaded trailer, well away from other traffic, might offer some indication of what speeds are safe. Nobody has mentioned the type of connection to the trailer or trailer brakes, possibly because both of which are worthy of their own topics.

Were I towing a heavy trailer much above jogging speed I'd likely organise a stable coupling for a braked trailer.

[Carlton green]

With regard to trailers and braking I would suggest that the devil is in the details and that some designs are much safer than others. Separate threads exist to discuss trailer merits.

With regard to motors I’ve yet to find comprehensive comparative data on their characteristics so it’s very difficult to usefully compare them beyond 750 Watt is more powerful than 250 Watt. To gain the best crankshaft torque out of a 250 Watt ‘motor’ it needs to be very low geared (within its attached gearbox) and that gearing data does not appear to be available. Similarly output power at crankshaft speed is seemingly not available. My legs typically drive my chain-set around at 60 rpm, I need a motor / gearbox unit that pushes out its maximum power at the same speed, ie. to match the cadence of the rider.

A recent post in the ‘how do you carry big shopping’ thread shows an e-bike used in what used to be called ‘Snowdonia’ with a large trailer attached. If I’m correct that bike (Cube Reaction Touring One e-bike?) has a Bosch Drive Unit, Performance Generation 3 (75Nm) Cruise (250Watt), Smart System. That unit is coupled to a 40T crankset and a 11 - 46 nine speed cassette.

The owner seems very happy and where he lives is renowned for its challenging terrain, perhaps that example demonstrates something.

https://www.leisurelakesbikes.com/bikes ... me__419982