Fork column failure on thread less forks.

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Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Fork column failure on thread less forks.

Postby Brucey » 24 Dec 2016, 1:17pm

BTW RS 1-1/8" steerers (and probably other sizes too, with different dimensions) are all made the same way; they are a tight fit (a push-fit into aluminium ) into the fork crown, on (IIRC) a 30.00mm diameter (which conveniently also locates the crown race).

The lip at the base of the steerer is there to

a) prevent the whole affair from falling apart should the fit be lost, and

b) to withstand whatever over-preloaded loads/service loads introduce tension into the steerer; such loads could pull the steerer through the crown otherwise; (the crown race bears against the crown rather than a shoulder on the steerer).

With RS forks, it is normally necessary to remove the crown race in order to inspect the steerer properly.

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

Brucey
Posts: 39475
Joined: 4 Jan 2012, 6:25pm

Re: Fork column failure on thread less forks.

Postby Brucey » 24 Dec 2016, 1:33pm

folk with an engineering bent may like to read this;

https://web.wpi.edu/Pubs/E-project/Available/E-project-042910-084057/unrestricted/Fatigue_Analysis_of_a_Bicycle_Fork.pdf

which is interesting. Amongst other things they saw that (having nailed a disc mount onto a fork blade that I think wasn't really meant for it), without a hub there, one fork leg was twice as stiff as the other one... :shock:

If I am critical, they

a) didn't attempt to measure the residual stresses
b) didn't examine how variations in welding procedure (overlap positions etc) might alter the weldment properties
c) didn't assess the possibility that grain boundary intrusion of braze metal might have nucleated fatigue cracks
d) didn't examine the possibility that the silver-brazing of the crown race support could be carried out at a lower temperature
e) didn't examine the possibility that the crown race support brazing could be used as stress-relief (or normalising) treatment for the TIG welds.
f) didn't address the difference between real world-loading (from the brakes) and the test loading
g) didn't quantify how the stiffness of the fork varied with its design.

But the worrying thing is that -whilst they improved the fatigue life of the fork under the test conditions- they moved the failure location from the fork crown to the steerer.

Arguably the former is going to give you a bad day but the latter might make it your last.

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

Threevok
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Joined: 30 Sep 2016, 3:11pm

Re: Fork column failure on thread less forks.

Postby Threevok » 24 Dec 2016, 3:59pm

Brucey wrote:BTW RS 1-1/8" steerers (and probably other sizes too, with different dimensions) are all made the same way; they are a tight fit (a push-fit into aluminium ) into the fork crown, on (IIRC) a 30.00mm diameter (which conveniently also locates the crown race).

The lip at the base of the steerer is there to

a) prevent the whole affair from falling apart should the fit be lost, and

b) to withstand whatever over-preloaded loads/service loads introduce tension into the steerer; such loads could pull the steerer through the crown otherwise; (the crown race bears against the crown rather than a shoulder on the steerer).


Thanks Brucey, that makes sense

On the SID's - placing my finger inside the steerer itself, at the base - theres a lip (of about 1mm) - 40mm above the overlap. I don't recall there being one on my Tora's though(can't check at the moment as I have A mugard holder fitted).

Brucey
Posts: 39475
Joined: 4 Jan 2012, 6:25pm

Re: Fork column failure on thread less forks.

Postby Brucey » 24 Dec 2016, 10:48pm

RS 1-1/8" steerers are typically swaged to an OD of 30mm near the base. This leaves the wall thickness about constant, and a small change in diameter (hardly a step per se) on both the ID and the OD.

It may be that change in diameter that you are feeling; it will usually occur about 40mm up from the bottom because the 30mm diameter extends above the fork crown, as it is used to locate the crown race.

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

Canuk
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Re: Fork column failure on thread less forks.

Postby Canuk » 26 Dec 2016, 8:15pm

Just my two penneth worth. I've been using steel and also carbon fork frames for over thirty years, on as many as 45 bikes, through very tough Canadian winters which are to say the least brutal, and long lasting.

Fork failures so far:

Steel: 1 (rusted through steerer tube)

Aluminium: 2 (1x de-bond, 1x corrosion)

Carbon: 0

I think this outlines what most of us already know, fork failures are extremely low by comparison to frame defects and failures. Modern composite to aluminium (or composites) bonding is extraordinarily tough and resilient. I rarely check my forks, it's wheel and tyre failures which are my greatest concern.

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Gattonero
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Re: Fork column failure on thread less forks.

Postby Gattonero » 31 Dec 2016, 9:49am

mig wrote:by how much would a fork steerer have to be made thicker to be significantly more resistant to everyday type fatigue?

are all threadless steerers standardised around the same thickness? or is it 'anything goes'?


Alluminium forks are most often a 3-piece design, with steerer, crown and blades bonded together.
Given the massive difference in ID required, I am not sure those steerers are actually cold-drawn, at all.
Fatigue will not incur only in the material itself, but the bonding too.
In normal use, this takes many, many years. Then there is misuse...
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

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Gattonero
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Re: Fork column failure on thread less forks.

Postby Gattonero » 31 Dec 2016, 9:51am

Keezx wrote:Might have something to do with the way many carbon forks with metal steers are built.
The steerer tube is pressed over or pressed in the crown part, i've never seen a steerer and crown in one piece of metal.


Suspension forks have a very solid interference fit that most of the times is not even bonded.
Road forks would mostly use a conical fit that is bonded.
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

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Gattonero
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Joined: 31 Jan 2016, 1:35pm
Location: London

Re: Fork column failure on thread less forks.

Postby Gattonero » 31 Dec 2016, 10:05am

Brucey wrote:well, you often need a different size star nut with an Al steerer vs a steel one....

cheers


Some 1" 1/8 track forks have a really thick steerer and would need to use a 1" star nut.
The others, are fine as the star nut would flex enough.
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...