Crack in aluminium recumbent frame

[StephenW]

Hello.

I was adjusting the seat angle on my Metabike recumbent on Sunday, when I noticed a substantial crack in the seat bracket, where it is welded onto the frame. Uh oh!

The crack starts on the left and extends about halfway across, along the upper edge of the weld.

Is the frame now scrap, or would it be worth repairing?

Would the crack have been caused by the heat of the welding changing the properties of the aluminium, or is it due to the bumpy shape causing a high stress in this location, making a crack initiation point?

I bought the bike second-hand, so obviously I don't know how it was treated before. I wouldn't say that I have done anything unreasonable to it, and I'm tall but not especially heavy (12 stone).

Rob, who has a blog called "Darkerside", mentions that there have been other reports of Metabikes breaking in this area, but I haven't come across these reports from a quick google.

Incidentally the manufacturer has closed down, but a bike shop in the USA bought the rights to the design, and it is now being produced in Taiwan.

[cyclemad]

try giving these people a call / email ; they are tandem specialists ;-

https://www.tandems.co.uk/products.php

A good welder who specialises in alloys / aluminium may be able to sort that out.

[Brucey]

there would have been the 100% evil trifecta of

- a very bad stress concentration
- a zone of rubbish mechanical properties
- a very high level of stress (part residual, part service)

at that weld start. A good welder ought to be able to sort that out, but there are a couple of things you would need to attend to

1) the materials used. It would be almost essential for the welder to know the grades of material used (*), and
2) that it may be a very good idea to revise the design so that the worst stresses at that weld are no longer seen.

On the latter point the bracket may have seen very high bending stresses in service (imagine it trying to 'hinge' along the weld line) if it sees a lot of load from a flexible seat base. One way of reducing such stresses would be to remake the bracket with an additional support onto the tube, i.e. so it is less like a two legged stool and more like a three-legged stool if you like.

(*) note also that the frame may have been heat treated during original manufacture. A repair weld is unlikely to be heat treated in the same way, and risks that the main frame spar ends up with reduced properties vs the original manufacture. If that were to break then you would be right in the poop. A less risky strategy than a weld repair could be to have a clamp-on bracket to support the seat; for example on a Bachetta the seat is adjustable on such a bracket. It may be possible to obtain a suitable bracket as a spare part from another machine or indeed to make something. If the bracket is to be fixed position, it could be mounted using clamping bolts and adhesive.

edit;


this is the same bracket? If so it isn't too much like a two-legged stool to start with. Also it is clear that tube is curved at this point, which makes a clamp-on bracket/reinforcement to the bracket rather problematic.

hth

cheers

[foxyrider]

Given you are aware of other failures at this point it's clear it's not a random bad weld or due to abuse but as Brucey says, a poor bit of design.

The original Airnimal Chameleons had a similar design issue - many frames failed several years down the line in the same place. The later frames were redesigned with an additional stress riser added. How do I know? I had the former failure and replaced the frame with the latter.

[StephenW]

Hello Brucey

Yes, it is like in that picture from Rob's blog. Mine has four sets of holes, rather than three, which I think means that it is from a later batch than Rob's. He says:

I have heard a report from LaidBack of a more recent MetaBike frame going at the mount under the seat, but that was definitely a manufacturing fault.

I also thought about some kind of clamp-on bracket, such as a Bacchetta one, but as you say the curve in the tube would make that tricky.

I had been considering getting a different recumbent for other reasons, but it would be a shame to scrap it rather than selling it.

As the manufacturer has closed down, it could be hard to find out the exact alloy used. Although the bike is now manufactured by Performer in Taiwan, I guess they could choose to use a different alloy. I also see this comment on Rob's blog from 2016:

Hi,

experience review: after 5 years of intensive use (about 10.000 km per year) the aluminium frame of my Metabike Metaphrastic is broken (right rear tube). Metabikes in Spain doesn’t reply to any request – even on the information about the used aluminium alloy, which is necessary to try successfull welding actions.

Doesn't seem too promising...

Supposing I did manage to find out the exact alloy, roughly how much would you expect a repair to cost?

[Brucey]

thinking about it, you could perhaps repair the part -or at least render it serviceable- by making up a plate (of similar thickness) that covers the original (and is bolted to it) and extends forwards from there to a band clamp of some kind.

cheers

[Cunobelin]

Had this on my (very early - 23rd off the production line) Catrike Expedition between the boom and cross frame

They replaced the frame without any fuss whatsoever

The new frame was strengthened with a gusset at the point where the failure had happened and this was a design change in the model.

[Brucey]

I note that in the OP's case the frame isn't going to actually break wholesale if the crack carries on running as it is, which is in contrast to most other failures. in this case the only thing that will happen is that the seat comes adrift, which is less worrying.

cheers

[StephenW]

the only thing that will happen is that the seat comes adrift

True, although if that happens halfway down a steep hill it might not be a very jolly experience!

Strangely, the top mountings of the seat are only held into the seat stays by friction. There are these aluminium "nodules" which are bolted onto the seat, and these just press into the ends of the seat stays. I think there might be something missing here, but I haven't had a chance to look at another Metabike to check. It was like this when I got it.

This means that if the bottom seat mounting breaks, the seat could come off completely!

[StephenW]

I wonder if the weld at the rear of the bracket will have been unusually stressed whilst the front one has been cracking?