Re: Question for Jobst and other individuals the feel so inclined.
Jobst Brandt 27 October 2005 02:15:05
Nate Llatikcuf writes:
A couple questions regarding the "Ouch" thread I posted last night:
1. Don't threads act as stress risers?
Yes, but there are no significant bending loads at the upper head bearing above which the steertube extends only slightly. Why this one broke is not apparent, but it cannot be related mainly to threads on the steertube.
2. In your opinion would a non-threaded tube be less likely to fail> catastrophically in an event like this?
Yes, but then I didn't see where the tube failed. There has got to be more to this than what was visible in the crash picture. Quill stems are noted for their wobbly nature at the exit from the steertube. Just the same, that motion only outs a lateral force on the end of the steertube while the stem bends or swivels about its expander end.
Llatikcuf wrote:> A couple questions regarding the "Ouch" thread I posted last nignt:>
1. Don't threads act as stress risers?
yes. and if the quill stem is too high and on the same region of the stem as the threads, the threads are stressed not only with road loads, but the pre-load of the quill expander.
2. In your opinion would a non-threaded tube be less likely to fail> catastrophically in an event like this?
yes, much. no risers. no discontinuity in stress transfer until you got to the necessarily much larger external threadless stem clamp.
On 26 Oct 2005 15:51:55 -0700, "Llatikcuf" <Llatikcuf@gmail.com> wrote:
A couple questions regarding the "Ouch" thread I posted last nignt:>
1. Don't threads act as stress risers?
Yes.
2. In your opinion would a non-threaded tube be less likely to fail>catastrophically in an event like this?
Appleas and oranges unless you specify more than just threaded vs non-threaded; examples of both bulletproof and dainty-fragile extremes exist for both types. This particular steerer failed; that doesn't necessarily make it a predictor for dissimilar units. -- Typoes are a feature, not a bug. Some gardening required to reply via email. Words processed in a facility that contains nuts.
A couple questions regarding the "Ouch" thread I posted last nignt:>
1. Don't threads act as stress risers?
I believe this steer tube probably failed due to the stem's expander wedge being inserted into the threaded portion of the fork's steer tube. This causes failure fairly often (or did when most steer tubes were threaded). I'll bet that the steer tube was ruptured nearly all the way before the jump, and that the impact (obviously a "bad one") finished it off.
2. In your opinion would a non-threaded tube be less likely to fail>catastrophically in an event like this?
Both would be equally reliable, had they both been installed correctly. A comparison to a threadless stem installed 1cm too high might be a better one.
On Thu, 27 Oct 2005 07:00:50 GMT, Werehatrack <rault00@earthWEEDSlink.net> wrote:
On 26 Oct 2005 15:51:55 -0700, "Llatikcuf" <Llatikcuf@gmail.com>>wrote:>
A couple questions regarding the "Ouch" thread I posted last nignt:>>
1. Don't threads act as stress risers?>
Yes.>
2. In your opinion would a non-threaded tube be less likely to fail>>catastrophically in an event like this?>
Appleas and oranges unless you specify more than just threaded vs>non-threaded; examples of both bulletproof and dainty-fragile extremes>exist for both types. This particular steerer failed; that doesn't>necessarily make it a predictor for dissimilar units.
Further considerations: Although it is difficult to tell from the photo, it appears that there is a chance that the quill stem was inserted only shallowly into the steerer. If that is the case, and particularly if the steerer was threaded to a point below the insertion level of the quill, I would hazard that it would have been the primary reason for the failure. -- Typoes are a feature, not a bug. Some gardening required to reply via email. Words processed in a facility that contains nuts.
Jobst Brandt 28 October 2005 05:23:24 [ permanent link ]
Sheldon Brown writes:
A couple questions regarding the "Ouch" thread I posted last night:
1. Don't threads act as stress risers?
If they're cut with dies, yes. This is why it's a bad idea to>> re-thread steerers. But steerers are made with rolled, not cut,>> threads so the breakages are typically caused by quill expander>> wedges.
I have never seen a steerer with rolled threads, and don't believe> such a thing exists.
Neither have I but then as I said, there isn't much bending force at that portion of the steertube. So my question to you is being in the bicycle repair business, how many steertubes have you seen broken in the threads and what were the circumstances.
Rolled threads wind up with a maximum diameter greater than the base> diameter of the shaft/tube. To make normal dimension rolled threads> on a steerer, you would need to start out with a thinner than normal> steerer, which would in itself be weaker.
I haven't inspected any, but mine are definitely lathe turned because they don't make steerers in that length.
Actually, this is an area where the old English Whitworth system was > superior, because the thread peaks and valleys had a round radius rather > than a sharp point. Thus, Whitworth pattern threads were less of a risk > for stress risers than normal threads.
Well, in fact US threads are also round in the minor diameter but the specification isn't that stringent. I have looked at a lot of threads and never found one with a sharp root. A threading tool would not do well with a sharp point.
On Fri, 28 Oct 2005 02:23:24 GMT, jobst.brandt@stanfordalumni.org wrote:
Well, in fact US threads are also round in the minor diameter but the>specification isn't that stringent. I have looked at a lot of threads>and never found one with a sharp root. A threading tool would not do>well with a sharp point.
True, and even a rolled thread still presents a stress riser inasmuch as it results from a process that creates a reduction in the cross-section of the threaded part. If the threads don't go below the wedge position, however, I doubt that this would be an issue, and those threads *shouldn't* go that far. (This does not guarantee that they won't, however...but no hard information is available about whether this potential factor was present on the bike in question in any event.)
Now that I think about it, I can't recall a bolt failure due to tension which didn't occur either at the head on in the threads...not that such failures have been common in my experience. -- Typoes are a feature, not a bug. Some gardening required to reply via email. Words processed in a facility that contains nuts.
Jasper Janssen 28 October 2005 20:05:01 [ permanent link ]
On Fri, 28 Oct 2005 04:16:52 GMT, Werehatrack <rault00@earthWEEDSlink.net> wrote:
Now that I think about it, I can't recall a bolt failure due to>tension which didn't occur either at the head on in the threads...not>that such failures have been common in my experience.
Seatpost clampbolts have regularly failed for me while (over)tightening. They didn't look like they failed from the threads, though: more like the middle stretched, ie, tightened past yield.
Interestingly, I currently have a regular M5 or M6 or so (same thread as the bike-specific part) nut & bolt on this bike rather than the specific seatpost bolts with a key at the head to prevent rotation (so tightening requires 2 wrenches), and it seems to be much, much stronger.
Jobst Brandt 29 October 2005 02:03:16 [ permanent link ]
Jasper Janssen <jasper@jjanssen.org> writes:
Now that I think about it, I can't recall a bolt failure due to>> tension which didn't occur either at the head on in the>> threads... not that such failures have been common in my>> experience.
Seatpost clamp bolts have regularly failed for me while> (over)tightening. They didn't look like they failed from the> threads, though: more like the middle stretched, ie, tightened past> yield.
These failures are from bending because the faces of the seat post clamp do not remain parallel when tightened, especially if there is any clearance to the inserted post. This problem can be solved by having a spherical washer under the head of the bolt that allows it to swivel.
Interestingly, I currently have a regular M5 or M6 or so (same> thread as the bike-specific part) nut & bolt on this bike rather> than the specific seatpost bolts with a key at the head to prevent> rotation (so tightening requires 2 wrenches), and it seems to be> much, much stronger.
As along as the bolt is not tightened so that it tries to conform to the non-parallel faces of the clamp the bolt remains at low stress.
Jasper Janssen 29 October 2005 16:00:01 [ permanent link ]
On Fri, 28 Oct 2005 23:03:16 GMT, jobst.brandt@stanfordalumni.org wrote:>Jasper Janssen <jasper@jjanssen.org> writes:>
Seatpost clamp bolts have regularly failed for me while>> (over)tightening. They didn't look like they failed from the>> threads, though: more like the middle stretched, ie, tightened past>> yield.>
These failures are from bending because the faces of the seat post>clamp do not remain parallel when tightened, especially if there is>any clearance to the inserted post. This problem can be solved by>having a spherical washer under the head of the bolt that allows it to>swivel.>
Interestingly, I currently have a regular M5 or M6 or so (same>> thread as the bike-specific part) nut & bolt on this bike rather>> than the specific seatpost bolts with a key at the head to prevent>> rotation (so tightening requires 2 wrenches), and it seems to be>> much, much stronger.>
As along as the bolt is not tightened so that it tries to conform to>the non-parallel faces of the clamp the bolt remains at low stress.
This regular bolt pulls the faces of the clamp parallel, regardless of what *it* wants, or so it seems.
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