Weakest point of a MTB frame?

[Stephan]

A friend of mine recently jumped on his road bike and heard a strange noise. Turns out his frame was cracked. This got me thinking, which part of a bike frame takes the most stress. I assume the weakest point will be the weld points, but of all the weld points on a bike frame, which ones are most under stress? I am guessing the top-tube to seat tube weld point, and down tube to BB weld point? 

[117]

50,

I think the weld points are stronger than we expect - unless it shaddy workmanship. The point where the BB sits is probably the most integral part of the frame as it has to deal with forces from all around, plus a hefty gimp pedalling like crazy on various surfaces. 

To me it'll be where the material is compromised, like a bend or a reduction in diameter/wall thickness where the material is not supported. An example would be where the chainstay meets up with the seat tube and has to curve around the fat tyres. Another failure point is the top of the seat tube where compression slots are cut out to allow the seat post to be squeezed. I also presume the top tube meeting the seat tube is another issue area as standing up while pedalling and pulling on the bars offers a twist force in the frame which will meet at this point of the junction

Carbon frames are slightly different in my book as the loads are distributed in a more thought out fashion (being designed via autocad type programs that offer structural guidance and model dynamics). Yet carbon frames dont like a knock or a bump in places not designed to recieve them and could crack rather easily in places least expected. 

I'm interested to hear what others in the know can add to your question

 

 

[Mojoman]

2 x Ghosts failed on the chainstay where a cable guide was welded (same spot, both frames)

1 x Momsen on the area where the seat stays, seat tube and down tube all come together...was welded...failed again..

So for me I would say its anywhere there is a weld?

[TheoG]

Agree, a weld is a crack waiting to happen or a metal fatigue crack initiation location if you will ....  

The specific weakest point will differ from bike to bike, model to model.  Only way to tell with more certainty is to do a FEA on the specific frame in question, or based on experience as per @Mojoman, again this only apply to the specific frame.

It becomes even more complicated on a carbon frame.

[The Ouzo]

I'd say the weakest point is the fleshy bit ontop of the bike

[love2fly]

I had s Trigon carbon dual suss MTB crack on the top tube where it meets the seatpost tube as well as where chainstays meet BB. Replaced under warranty. Many MTB's have a stiffener added to this seatpost/toptube intersection. The Trigon didn't and I think the stresses from the seatpost need this unless the frame is better engineered.

[gerriemtb]

19 minutes ago, ouzo said:

I'd say the weakest point is the fleshy bit ontop of the bike

100% The weakest point is the nut holding the handlebars

[Steady Spin]

Always cracked frames around welds. 

Will be interesting to see where the carbon bike cracks. If ever. I hope never!

[DieselnDust]

4 hours ago, Stephan said:

A friend of mine recently jumped on his road bike and heard a strange noise. Turns out his frame was cracked. This got me thinking, which part of a bike frame takes the most stress. I assume the weakest point will be the weld points, but of all the weld points on a bike frame, which ones are most under stress? I am guessing the top-tube to seat tube weld point, and down tube to BB weld point? 

The areas of the frame that takes the most load is the headtube junctions to the top and down tubes. This area has to resist bending buckling, twisting from cornering suspension movement, impacts, braking and acceleration loads (including those due to braking)

The BB area is the next highest loaded area and also an area where deflection is most limited by design.

The seat top tube junction is quite lightly loaded in comparison, but often also an area where weight is saved so tbe sections are thin, sometimes too thin.

 

What makes a frame weak?

poor fabrication, including welding or moulding. 99% of sports bicycle frames are by design more than strong enough. Where they lack is in the consistency of fabrication.

This lack of consistency can be huge variance in fibre lay up, resin mix age (never good to have a frame where the last drops of resin where applied to the layup), resin impregnation where resin rich is bad and will lead to wrinkling of the fibre sheets against the inflatable bladders. In alloy frame, weld temperature, tube thickness consistency, weld gap at the joint before and during welding, etc. Sometimes the engineers do get the design wrong especially when the accountants department gets involved in project meetings

Edited January 5, 2022 by DieselnDust

[Trashy]

1 hour ago, DieselnDust said:

The ares of the frame that takes the most load is  the headtube junctions to the top and dwon tubes. This area has to resist bending buckling, twisting from cornering suspension movement, impacts, braking and acceleration loads (including those due to braking)

The BB area is the next highest loaded area and also an aea where deflection is most limited by design.

The seat top tube junction is quite lightly loaded in comparision.

 

What makes a frame weak?

poor fabrication, including welding or moulding. ((% of sports bicycle frames are by design more than strong enough. Where they lack is in the consistency of fabrication

Best answer. Having worked in frame manufacturing for 5 years, the most common point of failure was at the head tube/down tube junction, underneath the down tube. Top tube onto seat tube was next.

With other brands, it was always at a weld, often due to poor design and insufficient material for the joint in question.

With aluminium, tube consistency is always the issue, often tubes would have too much material removed in the sanding process rendering them way to thin in places. Bends suffering the worst.

 

Carbon reduces many of these problems due to the molding process and with any reputable brand you can be fairly certain that good R&D process has been followed as well as adequate stress testing before release to the public 

[porqui]

It seems to differ when it is stress over time or sudden impact too.

[DieselnDust]

Stress overtime is fatigue. Welded bicycles can also fail due to excessive inherant strain in the heat affected zone on the weld joint and thermal elastoc-plastic strain. This could be the result of several factors starting with a poorly designed welded joint to incorrect welding execution. Material selection and condition also steps in t play a role, hence thermal control during cooling of the weld joint and frame overall is important.

[Trashy]

18 minutes ago, copperhead said:

That's why I bought a titanium bike 🤣

We can't all be so lucky!

[DieselnDust]

56 minutes ago, copperhead said:

That's why I bought a titanium bike 🤣

eeerrr thats a welded bike.....

[TheoG]

8 hours ago, copperhead said:

That's why I bought a titanium bike 🤣

Sorry to burst your bubble but like @DieselnDust said its still welded with the same inherent issues.  Ti does have an advantage over Al besides the higher tensile strength, like steel it also seem to have an endurance limit where Al don't.

Meaning:

1. Both steel and Ti can have an infinite fatigue life if the stresses are kept well below the fatigue/endurance limit.

2. Aluminum will fail eventually even at lower stress levels, it might just take a very long time.

3. Any kind of welding reduce the fatigue/endurance limit drastically.

Edited January 6, 2022 by TheoG

[Chadvdw67]

This is welding art 😍😍😍

 

Granted, the last 2 don't have anything to do with bicycles, but they a BEYAYOOTIFULL