Building a Cargo Bike

For discussions about bikes and equipment.
iandusud
Posts: 116
Joined: 26 Mar 2018, 1:35pm

Building a Cargo Bike

Postby iandusud » 13 Oct 2018, 8:37pm

I quite fancy having a go at making a two wheeled front load cargo bike. There are many examples on the web. Usual set up is using a MTB for the rear and 20" bmx/mtb head tube, fork and front wheel for the front end and then weld up the frame work in between from mid steel sections/tubes.

Something like this one: https://www.instructables.com/id/build- ... argo-bike/

I was just wondering if anyone here has had a go at this? I'd be very interested to hear how you got on.

Cheers, Ian

Brucey
Posts: 31568
Joined: 4 Jan 2012, 6:25pm

Re: Building a Cargo Bike

Postby Brucey » 13 Oct 2018, 9:33pm

the first thing I would say is that you should definitely not follow that instructable plan. The net result looks like a cargo bike but certainly won't be adequate for any serious use. In fact it may not even be safe to ride at all, leave alone with a load in.

The reason is that the design is structurally unsound and the materials used are not adequate for the task. It is as if the bloke who built it had only ever seen some fuzzy photographs of the real thing and knows nothing about designing anything that has more structural integrity than you might achieve by using crumpled tinfoil.

The main backbone appears to be built from 34mm dia x 2mm wall (~1-3/8" x 14SWG) tubing. This is just not stiff/strong enough for carrying any load. There are circumferential welds, loading in bending, in places where the stresses will be very high indeed. There is a single triangulating brace added to the most loaded area (near the base of the steerer) but really it needs more than this; as it is the frame (basically) has 'a hinge' in the middle. The whole front section is not only of inadequate stiffness in bending, but will also be lacking in torsional stiffness, making the steering vague or worse still promoting shimmy.

Commercially designed cargo bikes use a much fatter tube in the spine. Because the stiffness and strength go (roughly depending on how the tube wall thickness is scaled) as the cube of the tube diameter then a 50mm (~2") tube could be x3 stiffer/stronger and a 63mm (~2-1/2") tube could be a whopping x6 stiffer/stronger. Even with hydraulically formed tubes of the correct type, poor welds in the main spine result in breakages in commercial machines; I have repaired several of them. In the same place as the instructables machine has 'a hinge' a commercial machine has some beefy reinforcing plates that are ~5mm thickness and they are welded in a specific fashion so that the repeated bending stresses don't (often) result in cracks forming.

If you ride a proper cargo bike with a load in, the chassis is clearly quite springy despite being many times stiffer than the joke version. If you have a load on you can be toting 1/4 tonne (or more) total and steering/stopping that on hills is no small undertaking. Tandem rated parts are often not strong enough; wheels and tyres from kid's bikes..??.. :shock: :shock: ..... forget it.

If you want to build your own, and actually use it for carrying anything heavy and/or precious then you should

a) make sure your fabricating skills are up to snuff (if the welds are not good enough then the thing will fall apart) and
b) make sure your design is adequate. Take a long hard look at commercial machines and copy those, for goodness sake don't follow some half-a***d joke of a design like the one you have linked to.
c) use parts that are up to the task in hand. This means rims, tyres and brakes that are heavy duty ones with adequate load ratings, not stuff from kid's bikes.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

iandusud
Posts: 116
Joined: 26 Mar 2018, 1:35pm

Re: Building a Cargo Bike

Postby iandusud » 13 Oct 2018, 10:34pm

Hi Brucey,

Thanks for your rapid and detailed reply. I was myself very sceptical about the strength of the main spar and the bracing around the head tube on that particular example. I only linked to it as an example of the idea of such a conversion. Others I have seen use a box section for the main spar. I will certainly take your advice and look at commercially available cargo bikes for pointers.

I would very much like to hear from anyone who has attempted such a conversion to learn from their experience.

Cheers, Ian

P.S. I don't know what your French is like but this is interesting: https://fixme.ch/wiki/Galerie_Long_Andr%C3%A9

le.voyageur
Posts: 62
Joined: 10 Jan 2017, 8:30am

Re: Building a Cargo Bike

Postby le.voyageur » 13 Oct 2018, 10:52pm

have you seen this one?

Brucey
Posts: 31568
Joined: 4 Jan 2012, 6:25pm

Re: Building a Cargo Bike

Postby Brucey » 13 Oct 2018, 11:17pm

iandusud wrote:
P.S. I don't know what your French is like but this is interesting: https://fixme.ch/wiki/Galerie_Long_Andr%C3%A9


that looks a bit more like it. However the end result is probably a bit heavy (OK as it has electric power), and they put a lot of work into the steering swivel parts, where ball joints would have been a lot easier.

BTW there are fundamentally different ways of doing the steering rod; if you use ball joints both ends the rod hangs under its own weight and may collide with the front wheel when you turn if you don't allow for it. The French one uses a rod that articulates differently and can have a side bend in it that stays put.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

iandusud
Posts: 116
Joined: 26 Mar 2018, 1:35pm

Re: Building a Cargo Bike

Postby iandusud » 14 Oct 2018, 5:52am

le.voyageur wrote:have you seen this one?


I think you may have forgotten to insert a link. :shock:

iandusud
Posts: 116
Joined: 26 Mar 2018, 1:35pm

Re: Building a Cargo Bike

Postby iandusud » 14 Oct 2018, 6:12am

Here's another interesting project. However I think the head tube joint and the joint where the front downtube meets the main spar look very vulnerable and need some fillets. https://imgur.com/a/DAijq

BTW if I do go ahead with this project the idea is, if successful to add electric assistance.

le.voyageur
Posts: 62
Joined: 10 Jan 2017, 8:30am

Re: Building a Cargo Bike

Postby le.voyageur » 14 Oct 2018, 7:57am

doh!
https://imgur.com/gallery/DAijq#Vr8GkQD
good luck

Edit: just looked at your link and it's the same as the one I itended to post up thread.

Brucey
Posts: 31568
Joined: 4 Jan 2012, 6:25pm

Re: Building a Cargo Bike

Postby Brucey » 14 Oct 2018, 8:52am

this one

Image
https://hackaday.com/2011/10/01/incredibly-cheap-upcycled-cargo-bike/

has some attempt to make a rigid chassis by virtue of triangulation of the load platform area, effectively making a spaceframe. I'm not at all sure it is 100% successful in this case but it may give you ideas. There is a lot of work in making a spaceframe but it can be a more efficient structure. There is a basic issue in having a chassis that is strong, stiff and light enough, yet having some flex in the vertical plane isn't all bad; when you go over bumps the flex takes the sting out of the bumps. Most commercial 'spine' designs (Bakfiets, 8-freight etc) flex noticeably and it isn't all bad provided the spine doesn't then fatigue.

Fundamentally a low platform is useful if you plan to carry individual heavy objects about the place (eg washing machines, beer barrels etc), or you just want it as versatile as possible. However if you plan on always running with a box, or carrying sprogs or whatever, there is something to be said for making a spaceframe that comes up high enough that it is flush with the sides of the box, a bit like the bullit design.

However if you go down this route, a lot of (different) extra thought needs to go into the steering tube area; it is easy to end up with a floppy quadrilateral where the spaceframe joins the steering tube. A good example of this is the Nihola tricycle design; these don't see the quite the same stresses as a bicycle obviously but I have seen quite a few Niholas which have cracked up in this area.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

iandusud
Posts: 116
Joined: 26 Mar 2018, 1:35pm

Re: Building a Cargo Bike

Postby iandusud » 14 Oct 2018, 3:22pm

Hi Brucey,

That look very interesting but all that mitre-ing of tubes is a bit beyond my skill set. I like the simplicity of a central beam as used on most commercial designs and can see the value in its ability to absorb shocks. Std sections for RHS in appropriate sizes seem to mostly be square. 60 x 40mm is available but only in 3mm, which strikes me as possibly a bit OTT. 50 x 50 in 2.5mm is readily available but I wonder if that would be up to the job. At the end of the day I'd rather it be a bit OTT than not strong enough. It stikes me that the key areas for strength are going to be around the front headtube and the join of the front down tube to the main spar, where there will be braking forces and road shock to deal with. I reckon these areas will benefit from being beefed up.

I had a look at this video showing the assembly of a Milano Bakfiets and very much liked the way the design used the base of the box to supply the support for the box itself, rather than building an elaborate frame.

https://www.youtube.com/watch?time_cont ... XBKUx5HukY

All food for thought.

Brucey
Posts: 31568
Joined: 4 Jan 2012, 6:25pm

Re: Building a Cargo Bike

Postby Brucey » 14 Oct 2018, 3:44pm

I agree the tube mitres can be awkward, but in fairness

a) they don't have to be that good if you are MIG welding; on crucial joints you might want to do root and filler passes anyway to give a decent fillet, and
b) you can download a nifty piece of software that allows you to print out (on paper) a 'mitre template' for specific tube ODs and intercept angles. You cut this out, then wrap it around the tube, then mark through and mitre to the edge of the pattern. You can quite easily make mitres accurately enough for bronze-welding this way.

I agree that available RHS types are possibly limiting. One mad idea I had was that since the stiffness of the section doesn't need to be constant along its length, you could start with 2x2" and, near high stress locations, slit it, flare it and put in reinforcements so that you have turned 2x2" into 2x3" or even 2x4" locally (increasing the bending stiffness by ~x3.4 and x8 respectively). (A bit like 'darts' in flared trouser legs if you like...) The reinforcement welds would be near the centre line of the tube, so that there are not going to be high loads on them when bending loads are applied. This way you could locally reinforce high stress areas (e.g. at the base of the steering) without having lots of welds in high stress areas or heavy material throughout.
In a similar vein I don't think the main spine needs to be full size where it joins the BB shell; the loads local to here don't justify it. You could reduce the depth of a RHS at this point again using a cut and shut technique.

Another idea I had was to take round tube and squash it into an oval profile. This might give increased bending stiffness where it is required.

More than one way to skin a cat and all that...

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

iandusud
Posts: 116
Joined: 26 Mar 2018, 1:35pm

Re: Building a Cargo Bike

Postby iandusud » 14 Oct 2018, 5:55pm

All good ideas but keeping it as simple as possible for a first time project is my main concern.

Cheers, Ian

Brucey
Posts: 31568
Joined: 4 Jan 2012, 6:25pm

Re: Building a Cargo Bike

Postby Brucey » 14 Oct 2018, 7:49pm

simple isn't always the same for everyone. There are things other folk find 'simple' that fox me... it certainly pays to weigh up your options when embarking on a project like this. To add to an old adage;

think thrice, measure twice, cut/weld once


...perhaps?

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Brucey
Posts: 31568
Joined: 4 Jan 2012, 6:25pm

Re: Building a Cargo Bike

Postby Brucey » 14 Oct 2018, 8:05pm

this one is interesting

https://makezine.com/2010/07/13/diy-cargo-bike/

Image

it uses a fabricated truss instead of a large tube section for the spine (good bending stiffness, not such good torsional stiffness) and the steering rod is on ball joints; note that the front wheel swings beneath the steering rod, so the lock is not restricted by the rod. A lot of commercial Bakfiets designs work the same way.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Brucey
Posts: 31568
Joined: 4 Jan 2012, 6:25pm

Re: Building a Cargo Bike

Postby Brucey » 14 Oct 2018, 8:18pm

this photo shows a commercial bakfiets design at the crucial point

Image

even with the oversized tubes in use, it is necessary to add plate braces at this point. From the point of view of reducing fatigue when the spine tube sees bending loads, it is best if the attachment welds for the braces and for the box brackets lie on the horizontal centreline of the spine, not on the top or bottom of the spine.

This website has lot of interesting stuff

https://www.bakfiets-family.net/category/car-free-living/

and this site https://www.mancave.nl/maandagmissers-de-meest-bizarre-diy-fietsen/ has loads of weird things including

Image

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~