Need to find strong tube for my rack

[cwah]

Hello all,

I've been using my dahon for carrying stuff:

Unfortunately, the aluminium rod weren't up to the job and started bending...

I'd like to change the tube to have something stronger, and also longer, and requirement would be to ideally be strong enough to carry a passenger (doesn't have to):

After all, all the bike with decent cargo capacity can carry heavy load.. why can't mine?

I just need to find out what tube to use, that is strong, cheap and easy to work without welder or any advance equipment.

Is stainless steel the way to go or maybe bike aluminium T6061 tube? If so, where to get these at decent price?

Thank you

 

[anotherkiwi]

Your tube is strong enough, your design is wrong. In design school we had one interesting challenge: build a bridge from the least amount of 1/16" square balsa with a 30 cm span between two bricks that would support another brick. With the right engineering you could make a bike rack from any kind of tube.

 

[cwah]

it's not the first time the tube fail... I'd like to get something stronger anyway...

I doubled the number of tube on the rear to increase strength....

How can these little aluminium tube that can bend flat with a hammer can ever support a human weight?

 

[anotherkiwi]

OK some math:

You have a 25kg box on your rack which is attached to the non suspended part of your bike. You hit a pothole and the wheel is accelerated to 3gs (not unrealistic) you box breifly weighs 75kg. In fact because of the angle of your tube you are subjecting it to even greater forces but we don't have time for a course in engineering...

You need to create a truss beam that is attached to the suspended part of the bike. With such a beam you could have a 75kg person on it. Will the tyres hold out on the same pothole is a completely different story. Look at racks for completely suspended bikes, they are attached to the seat post.

If you want to stay with the same rack design you need your short tube to be vertical and a linkage between the long tube and the short tube. Look at the Eiffel tower and get inspiration, you need to triangulate like that with short tubes. A flat aluminium and 90° angled aluminium beam is what I would do to remove the problem of weakening tube by flattening it.

In fact you are doing it the hard way, you really need a trailor for loads like that.

 

[cwah]

Yes putting it a bit like the motorbike is the best way to do that.

But I still don't think it can be done only with my soft aluminium tube?

I know a trailer would be simpler but it would add a lot of inconvenience and not as practical to cycle around.

So to some extent a rear rack is required.

Material does also matter shouldn't it? Where can I find decent aluminium beam and rod i can attach somewhere close to the seat tube?

Thank you

 

[anotherkiwi]

I did a quick and nasty paint drawing on your photo.

The weight is supported by the vertical tube. Even your soft aluminium tube is very strong if it takes the strain lengthwise. If you use flat aluminium bar for the bottom "V" it will be stronger than flattened tube where the short reinforcement tubes are joined to it. When you flatten tube you create a weak spot. This rack design is TM me all rights reserved but offered under an open source licence

 

[cwah]

Thank you.

Please see what I intend to do, updated from your very kind advice:

I'd like to increase the length of the rack by about 50% and add some side load area for bigger parcel (eg. a bike wheel).

That configuration would keep my bike compact but still able to carry decent amount of stuff. Currently it's very tight in space.

I'd like to be like this bike to extent rack capacity beyond the rear wheel:

They don't have the 90 degree re-enforcement, I suppose it's because it has very strong material?

Anyway, if I can accomplish this extension for my bike that would be great!

 

[cwah]

Would replacing everything with this 12mm stainless steel save the day?

http://www.metals4u.co.uk/stainless-steel/round/12mm-diameter-303/detail.asp?prd_id=1698

 

[trex]

They don't have the 90 degree re-enforcement, I suppose it's because it has very strong material?

the horizontal beams are load bearing in this case.

 

[Nealh]

Would replacing everything with this 12mm stainless steel save the day?

http://www.metals4u.co.uk/stainless-steel/round/12mm-diameter-303/detail.asp?prd_id=1698

That's solid tubing Cwah and SS not the easiest material to cut,drill and bodge together and will be heavy.

 

[Arash]

There are different stainless steels, some are soft and some are hardened, all usually come annealed (unless you specify differently) and you would have to harden them yourself. You would need an oven capable of taking the steel to its critical temperature and then it needs quenching to make it hard. After that a couple of trips in a tempering oven to make it less brittle. On most stainless it needs to be done in a low oxygen environment, some need cryogenic quenching in a vacuum!!!!

You are best off using a tube rather than a rod, weight for weight tube is stronger than a rod.

You will need a dc tig welder to weld stainless steel and an ac/dc tig welder to weld aluminium. You may be able to use a mig welder with the right wire and gas but its hard work as the wire is soft and gets stuck often.

Make up a design and get a local engineering company to weld it up for you.

 

[Deleted member 4366]

View attachment 12240

The two outer down members would bow and collapse like that. You'd need to tie the two mid-points with a tie bar to stop that.

The whole idea of strengthening the rack and carrying heavy loads on it is a bad one. The light-weight Dahon with suspension is completely the wrong bike. It would be better to start again with a much stronger steel-framed donor bike.

 

[eHomer]

I agree with Neil, Stainless is much heavier than ally, and much more tricky to drill.

I've made quite a few DIY mods to my bikes and trikes, and I've always found ally to be plenty strong enough if you use the appropriate wall thickness.

By coincidence, I'm currently renovating a Linear recumbent bike and modifying it for electric assistance, and I've just been buying various ally sections and tubing off ebay.

Things like the seat backrest stays were half inch ally tube with a wall thickness of 10swg, which is over an eighth of an inch.

They're incredibly strong, and yet weigh practically nothing, even though they're nearly 2 feet long. They take the full backward force of pedaling, and at an angle too.



Here's the ebay link http://www.ebay.co.uk/itm/331279532004?_trksid=p2060353.m1438.l2649&var=540468950432&ssPageName=STRK:MEBIDX:IT

 

[cwah]

The two outer down members would bow and collapse like that. You'd need to tie the two mid-points with a tie bar to stop that.

The whole idea of strengthening the rack and carrying heavy loads on it is a bad one. The light-weight Dahon with suspension is completely the wrong bike. It would be better to start again with a much stronger steel-framed donor bike.

I know I a stronger bike would be better but I live in a tiny place and I don't have space for a bigger bike. I've sold most of my stuff and even my downhill kona stinky, if I could I would get a proper cargo bike but no way I can keep it inside a small flat.

 

[cwah]

Yes you are right, maybe stainless is too hard to work with and maybe I should stick with aluminium. What thickness is best from the ebay shop?

Shall I get the 2 inch ones? Only 2 left!

 

[cwah]

Another possibility is to get the axiom journey rack:
http://www.axiomgear.com/products/gear/racks/journey-series/journey/
http://www.ukbikestore.co.uk/product/329/171339/axiom-journey-rear-rack.html

Manufacturer state the ability to hold 70kg! So I can install it on my bike and extend it to have it carry more stuff?

 

[anotherkiwi]

My "design" requires flat aluminium bar AND tube. And NO it won't work if you extend by 50% as you have drawn.

Yes I think your best choice is the rack above. I hope they supply very strong frame screws...

A folding trailor fits in a tiny appartment, I live in one. Think about doing more journeys rather than carrying more stuff, that is how I move.

 

[anotherkiwi]

Of course they make folding cargo bikes in the USA!

https://www.bikefriday.com/bicycles/haul-a-day_basics they even have a Bionix version!

 

[trex]

this Thule rack is very strong, may be you could use the same idea:

https://roofracks.co.uk/thule-packnpedal-tour-rack

 

[cwah]

Of course they make folding cargo bikes in the USA!

https://www.bikefriday.com/bicycles/haul-a-day_basics they even have a Bionix version!

Yeah I've seen this cargo bike. But bike friday is out of my budget and the frame doesn't seem so strong either...

I'm considering buying the rear rack that would allow up to 70kg but I have to hope it's true... but also I may need a bit of hacking to make it do what I need to do (someone sitting on it)... so maybe a DIY route would be better!

Some good aluminium bar were shown, just need to figure out best way to carry heavy load.

this company does it well:

http://www.thebikeseat.com/

Unfortunately it doesn't work well with suspension bike and it doesn't work for cargo!