Mid-drive conversation kit for pulling heavy cargo bike

[matthewslack]

V- brakes are not going to be right for a cargo bike with small wheels. You'll wear through aluminium rims very quickly, and the braking force will be unreliable. If you going to spend all that money, you should do it properly.

Another thing to think about is the gearing. Crank motors are designed for large wheels. When you use them on bikes with small wheels, the gearing can be too low, especially if the bike has a freewheel rear hub (typically 7-speed or less).

Agree on the brakes: commercial cargo bikes often specify 4 piston disc calipers, with at least 180mm rotors and there are now from some makers, calipers that take much thicker brake pads so they last much longer. Magura MT5 as used on the Terns, for example.

Gearing can be sorted. The mid-drive kits force you to typically at least 42T chainwheel, and you can fit larger, either directly or as I have done on my 20" wheel trike project, by drilling and bolting a big chainwheel to the existing one. If you have a spider this is not necessary; mine does not.

I aimed for the same top gear as my 700c Ridgeback, which has 38T front 11T rear. The more or less 500mm diameter instead of 700mm wheel overall diameter means I need 7/5 x 38T. I've started with 52T, and I'll see how I get on. Larger ones are available.

But you might be more bothered about lowest gear, in which case much less to worry about: my 38T example only needs 36T at the back to match the 51T bottom gear on the 700c bike. That gear climbs anything. A 42T chainwheel would do the same with a 32T cassette, so bog standard sorts the low gears.

 

[AndyBike]

For brakes I'd recommend Hope tech4 V4 or maybe E4. They have similar power to Trickstuff maxima, but nowhere near that price, and on Solid 2.3mm rotors

 

[lenny]

I rode an e-bike up Snowdon in winter, and it was so cold that the brake fluid froze!

I rode an e-bike up Snowdon in winter, and it was so cold that the brake fluid froze! - MBR

It might have a railway up the side and a cafe at the top, but Snowdon is a proper mountain. As I found out recently when I attempted to ride up this magnificent Welsh mountain on an e-bike. Snowdon, or Yr Wyddfa to use its correct Welsh name (say Uhr With-va) rises to a prominence of 1,085m...

www.mbr.co.uk

 

[saneagle]

Agree on the brakes: commercial cargo bikes often specify 4 piston disc calipers, with at least 180mm rotors and there are now from some makers, calipers that take much thicker brake pads so they last much longer. Magura MT5 as used on the Terns, for example.

Gearing can be sorted. The mid-drive kits force you to typically at least 42T chainwheel, and you can fit larger, either directly or as I have done on my 20" wheel trike project, by drilling and bolting a big chainwheel to the existing one. If you have a spider this is not necessary; mine does not.

I aimed for the same top gear as my 700c Ridgeback, which has 38T front 11T rear. The more or less 500mm diameter instead of 700mm wheel overall diameter means I need 7/5 x 38T. I've started with 52T, and I'll see how I get on. Larger ones are available.

But you might be more bothered about lowest gear, in which case much less to worry about: my 38T example only needs 36T at the back to match the 51T bottom gear on the 700c bike. That gear climbs anything. A 42T chainwheel would do the same with a 32T cassette, so bog standard sorts the low gears.

You have a 11T top gear. If OP has a freewheel, it'll be a 14T top gear, so he won't get anywhere near. 14T top gear, 20" wheels and a crank-drive would need a cadence like you see in the BMX World Championships.

There are various ways to fit a 52T chainring. One way is to use one of these adapters. I'm not sure where else you can get them from:

https://ebikestuff.eu/en/tuning-elements/282-tsdz8-chainring-adapter-130bcd-11mm-offset.html?fbclid=IwZXh0bgNhZW0CMTAAAR0ZmxPBsLkWTMbC-sN-4o-d6oKDWRXsupBdtvYkq5df9Qp1fyhkeGlmGDw_aem_m15odZ1T5UilCmmGF6YPyA

 

[Simonh82]

V- brakes are not going to be right for a cargo bike with small wheels. You'll wear through aluminium rims very quickly, and the braking force will be unreliable. If you going to spend all that money, you should do it properly.

Another thing to think about is the gearing. Crank motors are designed for large wheels. When you use them on bikes with small wheels, the gearing can be too low, especially if the bike has a freewheel rear hub (typically 7-speed or less).

The bike is a fairly standard hybrid bike with 26" wheels and an 8 gear cassette. I made a mistake in the thread title it's a cargo trailer not a proper cargo bike with small wheels.

Before I attach the motor I'm going to test the trailer fully loaded with new brake pads. If I'm not happy in any way I'll go for a different bike with disc brakes.

 

[Simonh82]

I'd like more details about the trailer, sketches, photos. Will the hitch be secured via a QR axle? 115.22kg combined weight of trailer+cargo, was deffo too much for the QT axle on mine, got yanked out of the rear wheel by the hitch. Fortunately I was only transporting water, nothing valuable or particularly alive.

https://www.pedelecs.co.uk/forum/threads/heaviest-haul-with-the-homcom-trailer-so-far.43921/post-661817

I really don't want that happening again, especially on a steep hill of course because the Homcom folding trailer has no brakes, and it's a busy hill. 115.22kg of trailer and cargo bouncing downhill and through a windshield doesn't bear thinking about...

https://www.amazon.co.uk/HOMCOM-Folding-Trailer-Bicycle-Storage/dp/B078W39W37

Thanks for the advice. This is really helpful I hadn't fully considered the strength of the axle.

The trailer I'm building is based on a modified base I have adapted from a double kids trailer. The base will be 61X98cm plywood with foldable sides that will lock in place when up and fold down flat on the base for storage.

I don't have the sketches to hand. I've paused the build till it gets a bit warmer/dryer as I'm building it outside.

I'll have a proper look at the thread you linked to. A brief glance looks like it will be useful.

 

[saneagle]

The bike is a fairly standard hybrid bike with 26" wheels and an 8 gear cassette. I made a mistake in the thread title it's a cargo trailer not a proper cargo bike with small wheels.

Before I attach the motor I'm going to test the trailer fully loaded with new brake pads. If I'm not happy in any way I'll go for a different bike with disc brakes.

When you have a non-electric bike with a trailer loaded up and having a lot of hills. , your average speed would be around 8 mph When you have an electric motor, your average speed would be about 12 mph. Your moving energy is 1/2 MVsquared, so the energy will increase by 225% when you electrify it. If you find that the brakes are about right for towing the trailer without the electric stuff, you'll need to add a second set of brakes plus a bit more to get the same braking. That's how physics works.

Considering the total cost of the project, why do you want to use a substandard donor bike?

 

[Simonh82]

When you have a non-electric bike with a trailer loaded up and having a lot of hills. , your average speed would be around 8 mph When you have an electric motor, your average speed would be about 12 mph. Your moving energy is 1/2 MVsquared, so the energy will increase by 225% when you electrify it. If you find that the brakes are about right for towing the trailer without the electric stuff, you'll need to add a second set of brakes plus a bit more to get the same braking. That's how physics works.

Considering the total cost of the project, why do you want to use a substandard donor bike?

Have ditched the original plan and picked up a second hand bike today on Facebook marketplace that has decent disc brakes. Was a good bargain and actually thinking that I may keep it as my unpowered commuting bike and convert my existing bike to the ebike. I know the breaks on that are very good.