broken setpost bolt
Re: broken setpost bolt
Some friends and I were discussing the incident today, a couple of observations, our friend carried a small torque wrench with him on tour, so over tightening was probably not a prime cause, however, the wrench was part of the full load of a large carradice saddle bag, a contributory factor maybe?
Cheers.
Rob.
Cheers.
Rob.
Re: broken setpost bolt
I have always tried to use a strong two-bolt seat pin for carrying a saddlebag and the like.
IME overloading of single-bolt seatpins usually manifests itself as stripped serrations rather than broken bolts, but 25K miles is long enough for fatigue to have set in even if the bolt was good quality, and the saddlebag would not have helped.
cheers
IME overloading of single-bolt seatpins usually manifests itself as stripped serrations rather than broken bolts, but 25K miles is long enough for fatigue to have set in even if the bolt was good quality, and the saddlebag would not have helped.
cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: broken setpost bolt
IME (but not on any of my own bikes) failure of the single-bolt on those designs of seatpost is one of the most common just-riding-along failures. So common was this on day rides and tours, that my companions soon learnt how road-signs are also held together by M8 bolts, that could be 'borrowed' to get-you-home!
I am well aware of how some of the fittings on bicycles subject bolts to cyclic bending described by Brucey, and will give some examples below. No single-bolt saddle clamp that I've ever seen however, exhibits this problem. They fail simply because the bolt is over-stressed by the leverage imposed by the rider's weight, bouncing on the back of the saddle. (And I doubt that saddlebags ever weigh enough, despite their more rearward location, to compare with the effect of a heavy rider.) Substitution of a high-tensile bolt - easily sourced for less than a quid at any industrial fastener supplier - invariably solves the problem for good.
Other designs of clamp however, certainly are prone to the cyclic bending problem. Here is a good-bad example from our friends at USE.
In the photo each saddle rail is represented by a short section of transparent plastic tubing. Two M3 bolts draw the front and back 'under' sections of the clamp together along a curved rail, pushing the saddle rails up against the central 'top' clamp. Relative movement along the curve as the bolts are tightened, continuously alters the angle between the front and rear sections of the clamp. The bolts will be straight only if the saddle rails are exactly the diameter and only at the clamping force anticipated by the clamp designer. At all other sizes and states of tightening the bolts must bend at the point where they enter the front threaded clamp; and as they are turned they effectively bend to and fro. It was no surprise to me when I read complaints of these bolts snapping.
USE have since changed the design, but only to substitute a single, bigger, central bolt. This bolt still screws directly into the clamp and appears just as likely to suffer from cyclic bending as it is tightened and the alignment of the front and rear clamps changes. Properly to improve this design the bolt needs to screw into a self-aligning nut, like the bolt in the seat-tube clamp illustrated on the right below. Or alternatively a cheaper and just-as-good solution would be to make the (hexagonal) head of the bolt captive in the front part of the clamp, so the bolt does not turn but simply passes through clearance holes in both parts (slotted vertically so it can self-align), and instead simply tighten a nut where its end emerges from the rear part of the clamp.
Bad design, bolt bends as clamp tightens .... Good design, bolts screws into cylindrical self-aligning nut
I hope readers find these observations useful in avoiding failure-prone designs.
I am well aware of how some of the fittings on bicycles subject bolts to cyclic bending described by Brucey, and will give some examples below. No single-bolt saddle clamp that I've ever seen however, exhibits this problem. They fail simply because the bolt is over-stressed by the leverage imposed by the rider's weight, bouncing on the back of the saddle. (And I doubt that saddlebags ever weigh enough, despite their more rearward location, to compare with the effect of a heavy rider.) Substitution of a high-tensile bolt - easily sourced for less than a quid at any industrial fastener supplier - invariably solves the problem for good.
Other designs of clamp however, certainly are prone to the cyclic bending problem. Here is a good-bad example from our friends at USE.
In the photo each saddle rail is represented by a short section of transparent plastic tubing. Two M3 bolts draw the front and back 'under' sections of the clamp together along a curved rail, pushing the saddle rails up against the central 'top' clamp. Relative movement along the curve as the bolts are tightened, continuously alters the angle between the front and rear sections of the clamp. The bolts will be straight only if the saddle rails are exactly the diameter and only at the clamping force anticipated by the clamp designer. At all other sizes and states of tightening the bolts must bend at the point where they enter the front threaded clamp; and as they are turned they effectively bend to and fro. It was no surprise to me when I read complaints of these bolts snapping.
USE have since changed the design, but only to substitute a single, bigger, central bolt. This bolt still screws directly into the clamp and appears just as likely to suffer from cyclic bending as it is tightened and the alignment of the front and rear clamps changes. Properly to improve this design the bolt needs to screw into a self-aligning nut, like the bolt in the seat-tube clamp illustrated on the right below. Or alternatively a cheaper and just-as-good solution would be to make the (hexagonal) head of the bolt captive in the front part of the clamp, so the bolt does not turn but simply passes through clearance holes in both parts (slotted vertically so it can self-align), and instead simply tighten a nut where its end emerges from the rear part of the clamp.
Bad design, bolt bends as clamp tightens .... Good design, bolts screws into cylindrical self-aligning nut
I hope readers find these observations useful in avoiding failure-prone designs.
Chris Juden
One lady owner, never raced or jumped.
One lady owner, never raced or jumped.
Re: broken setpost bolt
Brucey wrote:I would far sooner have a ductile, work-hardening bolt material than one that is less ductile, even if the latter is stronger.
Lack of ductility, brittleness if you will, does not become a matter of concern until one enters the realm of even higher strength bolts than 12.9 grade.
And I don't think it helps to toss around such terms as 'work-hardening' and 'inclusion population' or debate the merits of rolled versus cut threads, yield versus UTS, etc. when you and I both know that none of the people reading this (including ourselves) can make good use of such information, even if we had it. Fact is: any good quality bolt we are likely to be able to buy from a reputable supplier, in any of these materials, will have rolled threads, will be work-hardened, will not have too many inclusions and will have a yield strength that's near enough the same percentage of UTS to rank in near enough the same order.
Chris Juden
One lady owner, never raced or jumped.
One lady owner, never raced or jumped.
Re: broken setpost bolt
Dual bolt seatposts, as well as stems with 4 bolt faceplates and 2 bolt clamps are safer. Chrome plated bolts can suffer from hydrogen embrittlement. Small size bolts are less fatigue resistant than large ones. I had an American Classic seatpost that had a small single bolt clamp that broke on me.
- NATURAL ANKLING
- Posts: 13780
- Joined: 24 Oct 2012, 10:43pm
- Location: English Riviera
Re: broken setpost bolt
Hi,
Mmm, I have taken all that CJ and Brucey says.
Don't like any of the designs that CJ posted above either, apart from the self aligning nut tube clamp one.
I don't have a problem with single bolt, but there again despite three out of five of the bikes I ride having forward overhang of the saddle (large frames, top tubes), another half so and one central on the rails, they all have a self aligning nut or head washer to eliminate any bending in torqueing nut.
A mixture of eighties shimano and cheap even pressed tin rail clamps of conventional single bolt design.
Have a few two bolt seat posts but have not used an of them as yet.
Also with this self centring / aligning washer / nut, any bending / cyclic loads through riding will be transferred better to the bolt in the form of aligned pull, not bending of the shank of bolt ?
M8 has a 36 mm core cross section and two M6 bolts would be about 40 mm, but they work differently and the M8 single vs fore and aft M6 is probably more stressed (M8) per mm on bending due to rider weight.
Does depend on saddle position fore / aft and also what terrain, off road means more time and bounce in saddle as your pedalling is not so uniform, rider style?
The multi bolt designs could be split into several different designs which have a different loading on the clamp bolts.
Boggling the vastly different designs and attempts at something not already patented.
Some of the multi bolt designs may work ok , but would draw the line at some of the ones that use inline (with rails) bolt through clearance holes to force the clamp as said by CJ.
We maybe spitting hairs here with all the bolt tech by CJ and Brucey, but heh this is for discussion and opinions.
On threads in aluminium-
The stem maybe, common design threadless.
Seat post different type of thing (bending loads and any cyclic), which is why we don't see bolts into aluminium with a gap between the clamps on seat posts
When there is no gap and you thread with a clamp into alu then the parts are mated as one so there is little or no movement / bending the bolt thread in the threaded nut part.
On motorcycle exhaust clamps to the head, some makers used M8 bolts threaded into the head, some M6 studs with nuts, guess which kept coming undone and stripping....
If you get thread depth to allow the bolt to stretch beyond yield then that might well happen in the thread and change the pitch........any pick up of thread or lube to combat normally ends with thread failure in the aluminium.
Very common in engine casings where old oil has got onto threads, and parts are removed often.
Bottle racks, mudguard stays, rear rack, all m5 St Stl. Aesthetics, more than man enough for the job, parts are clamped as one and only shear stress mainly on rear racks with some bending.
Stainless fasteners into steel do not need thread lock with bolts that go all thru the frame lugs I have found as long as there is no gap between mating parts, would be the same with steel. But care full that good thread depth on alu frames.
Stainless nuts on stainless bolts need thread lock or nylocs, other wise you will lose them
Aluminium bottle rack fasteners..............imagine giving those to your diy mechanic might as well be made of plastic.
Would not use st stl fasteners any where but those above, with exception of accessory mounts.
Critical parts do remain steel on all vehicles as far as I know.
Single seatpost bolt failure might well be as described, but I still hedge bets that there was something else going on with OP there, apart from pure old age use, we have no other details to examine?
Mmm, I have taken all that CJ and Brucey says.
Don't like any of the designs that CJ posted above either, apart from the self aligning nut tube clamp one.
I don't have a problem with single bolt, but there again despite three out of five of the bikes I ride having forward overhang of the saddle (large frames, top tubes), another half so and one central on the rails, they all have a self aligning nut or head washer to eliminate any bending in torqueing nut.
A mixture of eighties shimano and cheap even pressed tin rail clamps of conventional single bolt design.
Have a few two bolt seat posts but have not used an of them as yet.
Also with this self centring / aligning washer / nut, any bending / cyclic loads through riding will be transferred better to the bolt in the form of aligned pull, not bending of the shank of bolt ?
M8 has a 36 mm core cross section and two M6 bolts would be about 40 mm, but they work differently and the M8 single vs fore and aft M6 is probably more stressed (M8) per mm on bending due to rider weight.
Does depend on saddle position fore / aft and also what terrain, off road means more time and bounce in saddle as your pedalling is not so uniform, rider style?
The multi bolt designs could be split into several different designs which have a different loading on the clamp bolts.
Boggling the vastly different designs and attempts at something not already patented.
Some of the multi bolt designs may work ok , but would draw the line at some of the ones that use inline (with rails) bolt through clearance holes to force the clamp as said by CJ.
We maybe spitting hairs here with all the bolt tech by CJ and Brucey, but heh this is for discussion and opinions.
On threads in aluminium-
The stem maybe, common design threadless.
Seat post different type of thing (bending loads and any cyclic), which is why we don't see bolts into aluminium with a gap between the clamps on seat posts
When there is no gap and you thread with a clamp into alu then the parts are mated as one so there is little or no movement / bending the bolt thread in the threaded nut part.
On motorcycle exhaust clamps to the head, some makers used M8 bolts threaded into the head, some M6 studs with nuts, guess which kept coming undone and stripping....
If you get thread depth to allow the bolt to stretch beyond yield then that might well happen in the thread and change the pitch........any pick up of thread or lube to combat normally ends with thread failure in the aluminium.
Very common in engine casings where old oil has got onto threads, and parts are removed often.
Bottle racks, mudguard stays, rear rack, all m5 St Stl. Aesthetics, more than man enough for the job, parts are clamped as one and only shear stress mainly on rear racks with some bending.
Stainless fasteners into steel do not need thread lock with bolts that go all thru the frame lugs I have found as long as there is no gap between mating parts, would be the same with steel. But care full that good thread depth on alu frames.
Stainless nuts on stainless bolts need thread lock or nylocs, other wise you will lose them
Aluminium bottle rack fasteners..............imagine giving those to your diy mechanic might as well be made of plastic.
Would not use st stl fasteners any where but those above, with exception of accessory mounts.
Critical parts do remain steel on all vehicles as far as I know.
Single seatpost bolt failure might well be as described, but I still hedge bets that there was something else going on with OP there, apart from pure old age use, we have no other details to examine?
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
- NATURAL ANKLING
- Posts: 13780
- Joined: 24 Oct 2012, 10:43pm
- Location: English Riviera
Re: broken setpost bolt
Hi,
Something interesting for comfort / seat post defection? -
http://www.bikeradar.com/road/gear/arti ... vel-46208/
Something interesting for comfort / seat post defection? -
http://www.bikeradar.com/road/gear/arti ... vel-46208/
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
Re: broken setpost bolt
CJ wrote:Brucey wrote:I would far sooner have a ductile, work-hardening bolt material than one that is less ductile, even if the latter is stronger.
Lack of ductility, brittleness if you will, does not become a matter of concern until one enters the realm of even higher strength bolts than 12.9 grade.
you would expect so, but practical experience suggests otherwise. If you repeatedly tighten a 12.9 bolt such that it yields even slightly in bending, it may well have a substantially shorter life than a notionally weaker (but more ductile) bolt.
.... Fact is: any good quality bolt we are likely to be able to buy from a reputable supplier, in any of these materials, will have rolled threads, will be work-hardened....
my understanding is that is not quite the case; 12.9 bolts with rolled threads (which is not all 12.9 bolts even) may be made by rolling then heat treating or by heat treating then rolling. The resultant bolts differ little in tensile strength but considerably in terms of fatigue resistance. The former route is preferred by bolt manufacturers for cost reasons.
I mentioned some of the other factors because the best method of specifying and manufacturing a bolt for certain conditions is still a matter of live debate; you might think it has all been done to death but folk are still doing research and finding anomalies in expected behaviour.
cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: broken setpost bolt
Does anyone know of a supplier for replacement bolts? I don't want to replace a seat post for the sake of snapped bolt, but haven't found anywhere yet supplying bolts! (And all the super cheap seat post offers, seem invariably to be in "bespoke" sizes, 31m, 30.6mm, 32mm et al
- NATURAL ANKLING
- Posts: 13780
- Joined: 24 Oct 2012, 10:43pm
- Location: English Riviera
Re: broken setpost bolt
Hi,
Heres a start-
https://www.hibike.com/procraft-screw-f ... 371f9a4b3a
gbnz wrote:Does anyone know of a supplier for replacement bolts? I don't want to replace a seat post for the sake of snapped bolt, but haven't found anywhere yet supplying bolts! (And all the super cheap seat post offers, seem invariably to be in "bespoke" sizes, 31m, 30.6mm, 32mm et al
Heres a start-
https://www.hibike.com/procraft-screw-f ... 371f9a4b3a
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
- NATURAL ANKLING
- Posts: 13780
- Joined: 24 Oct 2012, 10:43pm
- Location: English Riviera
Re: broken setpost bolt
NATURAL ANKLING wrote:Hi,gbnz wrote:Does anyone know of a supplier for replacement bolts? I don't want to replace a seat post for the sake of snapped bolt, but haven't found anywhere yet supplying bolts! (And all the super cheap seat post offers, seem invariably to be in "bespoke" sizes, 31m, 30.6mm, 32mm et al
Heres a start-
https://www.hibike.com/procraft-screw-f ... 371f9a4b3a
This is M8 -?
https://www.ebay.co.uk/itm/Mountain-Bik ... SwLgNaZ9yF
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
Re: broken setpost bolt
Many thanks, have ordered .
Having already had to replace a near new lock/brand new glasses/2 No. tyres in the space of 4-5 weeks, forking out for a complete new seat post was too much!