Fork column failure on thread less forks.

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rjb
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Fork column failure on thread less forks.

Postby rjb » 5 Dec 2016, 5:01pm

Recent threads about fork column failure has prompted me to make a secondary retention strap to hold it all together and give me time to make a controlled stop unlike poor George Hincapie in the 2006 Paris Roubaix. :( https://m.youtube.com/watch?v=UZg1vrvGbdE
Here's my idea on the bench against a fork column to give an idea. It involves an M6 connector nut with a 6mm hole drilled through which is located over the caliper brake bolt. A length of M6 threaded rod is then screwed down through the star fangled nut into the connector nut. A bit of thread lock to hold it firm and then the top cap is replaced. You could make a neat job by drilling the top cap to take a recessed alan key brake bolt, but I am going to use a M6 nut and some washers initially. Cut the threaded rod to size and there you go worry free. An additional advantage is the connector nut makes a simple mudguard retainer by screwing a short bolt from below. :D
You could leave out the star fangled nut altogether on a new build but on a retrofit its easier to leave it in situ.
Total cost was £2.35 and I have enough hardware for half a dozen bikes depending upon fork column length.
I can now ride my 16 year old 1" ITM Big One carbon forks with aluminium steerer without worry of imminent failure. :D
image.jpg
Fork retention strap
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin, Raleigh 20, Falcon K2 MTB dropped bar tourer, Longstaff trike conversion on a Falcon corsa. :D

Keezx
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Re: Fork column failure on thread less forks.

Postby Keezx » 5 Dec 2016, 10:19pm

This thing will not prevent you for falling on the street unfortunately.... :roll:
Why not whack a matching thinwalled pipe in the bottom part of the steerer, prevents the steerer breaking.

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andrew_s
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Re: Fork column failure on thread less forks.

Postby andrew_s » 5 Dec 2016, 10:27pm

They are commercially available, though usually aimed at MTBs with disc/V brakes, so an inverted stem cap or similar at the bottom rather than an eye to thread a caliper brake bolt through.
http://www.chainreactioncycles.com/guss ... -prod17766

I've also heard of people whacking a length of broom handle down their steerer

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CREPELLO
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Re: Fork column failure on thread less forks.

Postby CREPELLO » 6 Dec 2016, 8:43am

Well that looks like a good hack to me :)

Bmblbzzz
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Re: Fork column failure on thread less forks.

Postby Bmblbzzz » 6 Dec 2016, 10:20am

andrew_s wrote:They are commercially available, though usually aimed at MTBs with disc/V brakes, so an inverted stem cap or similar at the bottom rather than an eye to thread a caliper brake bolt through.
http://www.chainreactioncycles.com/guss ... -prod17766

I've also heard of people whacking a length of broom handle down their steerer

I know a guy who raced with a Belgian club in the '80s. They were a bit concerned about his British steel frame (a Longstaff, I think) on the local cobbles and sent him to a local workshop, which did just this. That was a threaded steerer, of course.

rjb
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Re: Fork column failure on thread less forks.

Postby rjb » 7 Dec 2016, 9:16am

Last thing I would do is whack a broom handle in a pair of carbon forks. :shock:
Most of the commercially available kits don't have provision for using caliper brakes.

If anyone else is doing this mod here's some feedback from my experience. Check the internal size of the fork column at the base. Mine was 10mm and my hexagonal connecting nuts needed releaving to fit. I see you can get tubular ones of 10mm diameter so these would have saved some effort. The Allen brake bolt needed shortening by a couple of mm. I used a 10mm nylock nut at the top cap and used a couple of presta valve retaining collers (opened out to 6 mm use a drill or file) to fill the relieved hole in the cap. :D
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin, Raleigh 20, Falcon K2 MTB dropped bar tourer, Longstaff trike conversion on a Falcon corsa. :D

pwa
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Re: Fork column failure on thread less forks.

Postby pwa » 7 Dec 2016, 9:54am

Bmblbzzz wrote:
andrew_s wrote:They are commercially available, though usually aimed at MTBs with disc/V brakes, so an inverted stem cap or similar at the bottom rather than an eye to thread a caliper brake bolt through.
http://www.chainreactioncycles.com/guss ... -prod17766

I've also heard of people whacking a length of broom handle down their steerer

I know a guy who raced with a Belgian club in the '80s. They were a bit concerned about his British steel frame (a Longstaff, I think) on the local cobbles and sent him to a local workshop, which did just this. That was a threaded steerer, of course.


I too would avoid doing that with carbon. But it could work with steel. You would want a snug but not rammed fit. Wood swells with dampness and shrinks as it dries. So you would want extremely dry wood that would just about slide in with a bit of grease and not much more than a few moderate taps with a mallet. After that the humidity in the air would probably make it swell slightly, ensuring a tight fit. But it would make a good bung to prevent water ever draining out so might lead to rusting. That would be my main concern with this.

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anniesboy
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Re: Fork column failure on thread less forks.

Postby anniesboy » 7 Dec 2016, 10:36am

Why not use a suitable sized seat post (cut down of course) instead of broom handle.

Threevok
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Re: Fork column failure on thread less forks.

Postby Threevok » 7 Dec 2016, 11:52am

I fail to see how, putting an additional tube down the centre of a steerer on a thread-less set of forks, would have any benefit.

The two most likely places (if at all) of a steel steerer breaking - would be either at the point where the star nut is fitted, or where the steerer meets the crown - the two most likely places for them to rust through.

In either case, it is not possible to get sufficient overlap to prevent (or delay) any catastrophic failure - and such a device would be of no more use - than the additional tube you already have around the outside of the steerer - your head tube.

In my opinion, the only way to prevent such an (unlikely) occurrence, is to remove the fork, check it regularly, and treat or discard as appropriate.

PS: I have never ever heard of a steel (or alloy for that matter) steerer breaking on a fork, under normal operation. Even when they are pushed past their intended use (eg DJ or Freeride on a non-suitable fork) it's usually the wheel, QR/Fork lower, or stanchions that give way first.

It's only carbon ones I hear of breaking - and it's usually due to micro-fractures - due to improper installation, or a manufacturing defect.

Brucey
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Re: Fork column failure on thread less forks.

Postby Brucey » 7 Dec 2016, 1:19pm

Obviously one ought to inspect them regularly, to be sure that there are not cracks starting. But some steerers are constructed in such a way as inspections are very difficult, plus steerers (mostly not steel ones) can and do break more often than makes anyone feel comfortable.

I happen to think that the OP's suggestion has merit; in fact I have proposed something similar in the past myself. If viewed as a backup to the normal inspections etc, that will most likely let you retain enough control to at least stop the bike (using the rear brake) should the steerer fail, I think that would be about right.

Some notes on loading;

Hard on the front brake, the steerer can see bending loads of up to about 1000 Nm.
Just due to rider weight, the steerer can see bending loads of around 80 Nm (although it could be more than this).
On bumpy roads, peak loads are likely to be between the two figures above.

If you work out how much tension a central tie rod would have to withstand to give an 80 Nm bending load (once the steerer has zero strength in tension as is the case when it is cracked most of the way round), it is about one tonne with a 1" steerer, and about 10% less with a 1-1/8" steerer.

In the broadest terms this means that (depending on the strength of the tie rod materials) an M5 tie rod is going to struggle, and a M6 threaded tie rod might work for long enough to get the bike stopped OK if you use the rear brake only and don't go over any big bumps. Commercially available 'headlock' parts use an M8 thread IIRC.

On the subject of ad-hoc tube reinforcements;

yes you can reinforce a steerer but

a) once the reinforcement part is fitted, you can't inspect one surface of the steerer tube any more
b) once the reinforcement is made, there is a crevice that may encourage corrosion
c) the steerer gets about twice as stiff wherever it is reinforced (which may not be what you want)
d) wherever the reinforcement ends, there will be a very bad stress concentration

Since steerers with uninspectable joints in them are one of the 'problem types' anyway, a)/b) are of concern. Needless to say for this method to be useful you would have to know where the weak spot in the steerer was, and that c)/d) are not going to cause problems of their own. d) may just shift the failure point to a different location.

I am told that back in the 1950s it was common practice to use a wooden reinforcement inside a steerer tube. However it appears that the steerer ceased to be a weak spot in lightweight steel frames once suitable butted steerer tube designs became the standard.

cheers
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Gattonero
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Re: Fork column failure on thread less forks.

Postby Gattonero » 11 Dec 2016, 11:15am

I could appreciate one's effort and concern, but a length of M5 threaded bar would not offer any increase in safety in case of fork failure.
Should the steerer separate from the crown, the threaded bar would only hold against axial force, but would do nothing against the -most relevant- momentum that would push the whole wheel+bottom of the forks against the frame.
I can see a crash happening anyway.

After 16 years of service, if the mileage has been done, I wouldn't spend any time fettling old forks. Put them in the bin, buy and install a set of those Tange Infinity forks and that's happy days for a long time. Heavier, but less hassle.
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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meic
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Re: Fork column failure on thread less forks.

Postby meic » 11 Dec 2016, 11:27am

It will not save you from a break near to the crown but it will buy you time for a break near the top bearing (which is where mine failed) or for a CF tube crushed by overtightening the stem bolts.
Yma o Hyd

Brucey
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Re: Fork column failure on thread less forks.

Postby Brucey » 11 Dec 2016, 11:38am

Gattonero wrote:Should the steerer separate from the crown........that would push the whole wheel+bottom of the forks against the frame....


but that isn't what happens if a steerer starts to fail in normal use; anyone who has seen this happen will tell you that the fork will bend in the other direction (i.e. making the bike chopper-like) unless the front brake is applied.

Not that I would advise that you do it, but some folk have continued to ride on a steel fork that has started to fail thus, only packing it in when the fork offset has increased by a couple of inches.... :shock:

Steel forks often fail 'gracefully' as it were; the addition of a 'long bolt' may confer an equally graceful failure mode to forks that would otherwise be somewhat less forgiving.

cheers
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Gattonero
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Re: Fork column failure on thread less forks.

Postby Gattonero » 11 Dec 2016, 1:20pm

Brucey wrote:
Gattonero wrote:Should the steerer separate from the crown........that would push the whole wheel+bottom of the forks against the frame....


but that isn't what happens if a steerer starts to fail in normal use; anyone who has seen this happen will tell you that the fork will bend in the other direction (i.e. making the bike chopper-like) unless the front brake is applied....


If that happens in the direction you suggest, the result is the same: the forks would still move away. An M5 threaded bar is not a failsafe for the mass of cyclist+bike unless we're talking of very low speed; it would need something bigger, and especially something that would prevent the movement of this bar inside the headtube.
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...

Brucey
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Re: Fork column failure on thread less forks.

Postby Brucey » 11 Dec 2016, 1:53pm

I agree an M5 would be marginal, as per my calculations upthread.

Having had a fork break in a similar way (I went over a small bump and the offset instantly increased by about 1" ... :shock: ) I can say without fear of contradiction that when this happens one's instinct is to slow down as quickly as possible without unduly stressing anything, i.e. by using the rear brake whilst keeping one's weight well backwards.

Whilst it is no substitute for forks that oughtn't break in the first place, good maintenance etc., anything that gives you a fighting chance of stopping thusly is surely no bad thing.

cheers
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