Seatpost bolt

[Edocaster]

Hello all.

With my Mk1 Nomad, I find that when I torque up the seatpost bolt the lugs on the frame 'bend' inwards. I've attached a photo.

Is this normal? I wouldn't be worried if the seatpost clamp wasn't integrated into the frame.

And if I snap the bolt, does anyone know where to obtain spares? Is it just an ordinary stainless steel bolt (the 'shouldered' portion sits within the lug on the frame - I haven't taken measurements of the diameter though)?

Thanks.

[JimK]

Yeah, that is a touchy spot. I torqued my seat post bolt to 9 Nm. My seat isn't slipping. I figured this was a place where a quantitative approach was worth the while. I don't know what the proper spec is. I was actually planning to increase gradually until the seat stopped slipping, but my first data point has worked, so I haven't tightened any more!

Ah, and I greased the seat post bolt itself. My idea is that the torque will then go more into clamping the seat post and less into twisting the bolt itself. Could probably grease the seat that the bolt head sits in, too, but that's more a matter of an accurate torque reading than of damaging the bolt.

[Danneaux]

Hi Edo!

I am a hobbyist builder, and can say that seatpost clamping lugs of this sort are typically tubes milled from steel rod then drilled, milled, and then slotted nearly all the way through so when they are brazed on, both ears are in alignment (the slot is cut after the boss is brazed in place). I've made some of my own from solid steel rod that has been turned.

To see the ears drawn toward each other makes me wonder if perhaps the seatpost is a bit undersized for the frame or if -- as on my Mk2 Nomad -- a sizing shim that would make up the difference may be missing. Dave Whittle or Thorn/SJS Cycles could best advise in this regard.

If the seatpost remains clamped in position as-is, I'd be inclined to leave it. I don't see any cracks in the braze in your close-up photo, nor does it appear the boss has been ovalized -- both things can occur if a too small 'post is used in a too-large seat tube and the clamps then tightened to make up the difference. That doesn't seem to have happened here, thankfully.

The alternative is to remove the 'post and prise the ears apart -- an operation that can work but is fraught with hazards of cracked or distorted ears and damaged paint, which is why I would be inclined to leave it alone if you're getting good clamping. If it slips, then indeed I would fix it.

If the worst should happen and the ears/tube someday break off, then the remaining pieces can be carefully filed off and and a new unit brazed on or replaced with a separate clamp.

The seatpost bolt appears to be a standard M6x1.0 or M5x0.8 stainess socket-head metric bolt, but I can't say which from the photo.

Torque specs for the Zoom seatpost on my Nomad Mk2 are listed a 9-10Nm, so at maximum, we're talking pretty low torque -- 88.51in/lb or 7.28ft-lb or 102kgf-cm. You'd have to exceed that torque for the bolt to part unless the angle of the ears is such that the bolt would shear across the stress riser of the threads. To have the ears drawn non-parallel as they are means the bolt is being stressed at an angle, so be careful and use a torque wrench if you do adjust it, as the bolt has already been stressed sideways a bit.

An excellent suggestion by Jim to grease the seatpost to prevent it galling into the seat tube. Be sure to adjust torque values if you lube the bolt or apply anti-seize, grease, oil, or a thread-locker in liquid form). Most torque figures are given for bolt threads clean and dry unless specified otherwise. If a lubricant is used, dry torque values should be reduced by 10-20% (or more, depending on material, diameter, thread pitch, grade, and lube) to prevent over-torquing the fasteners (the reduced friction allows greater bolt elongation, to the point of elastic deformation or sheer ailure at the threads, which serve as stress risers) unless specified otherwise. For more on threaded bicycle fasteners, see: http://www.parktool.com/blog/repair-help/basic-thread-concepts

Best,

Dan. (...who is reminded that torque readings are not as accurate as measures of angle/torque turning or bolt elongation but if the maker doesn't specify wet or dry in their torque recommendations, it is anyone's guess)

[JimK]

Be sure to adjust torque values if you lube the bolt or apply anti-seize, grease, oil, or a thread-locker in liquid form). Most torque figures are given for bolt threads clean and dry unless specified otherwise. If a lubricant is used, dry torque values should be reduced by 10-20% (or more, depending on material, diameter, thread pitch, grade, and lube) to prevent over-torquing the fasteners (the reduced friction allows greater bolt elongation, to the point of elastic deformation or sheer ailure at the threads, which serve as stress risers) unless specified otherwise.

Thanks for that, Dan! Makes sense but something I have never heard... the ocean of my ignorance is boundless!

[Danneaux]

Hi Jim!

If you have a few moments to while away, Google "dry vs wet torque" and you'll see the issue is far from cut-and-dried! All acknowledge there is a difference, disagree on how much, and agree that manufacturers and companies often fail to specify wet or dry in their torque recommendations.

In general, see:
http://tinyurl.com/l8au99p
...or...
https://www.google.com/search?q=dry+vs+wet+torque&oq=dry+vs+wet+torque&aqs=chrome.0.57j59.1953016j0&sourceid=chrome&ie=UTF-8

For a more specific search, do "Dry vs wet torque +bicycle", as here:
http://tinyurl.com/mmblw5v
...or...
https://www.google.com/search?q=Dry+vs+wet+torque+%2Bbicycle&oq=Dry+vs+wet+torque+%2Bbicycle&aqs=chrome.0.57j62l2.2809j0&sourceid=chrome&ie=UTF-8

All the best,

Dan. (...who gets all wound-up about such things)

[triaesthete]

And all of the above assuming the torque wrench is accurate, and calibrated, and not used at the extreme ends of it's torque range, and that the operator is not mixing up his in-lb ft-lb and Nm torque units!

I've always done, and been taught, to do it by feel and understanding of the materials ad design involved.

Torquing nonsense
Ian

[Relayer]

Hi Edo

The lugs bend inwards on my RST as well, and I broke the original seat post bolt - I bought a stock of 10 in case it happens again!  (I can't remember if they are M5 or M6.)

However, I didn't tightened up the new bolt nearly as much, i.e. only until the first hint of movement in the lugs, and have never had a problem since.  This was done by pure guesswork, I have never owned a torque wrench and wouldn't know how to use one.

Jim  (sailing along happily on his ocean of tecchie ignorance)

[Edocaster]

Thanks for the replies everyone. The post doesn't slip, so I'll definitely remember not to overtighten. I don't have a torque wrench, but will just be careful with a short hex key.

The post is 27.0mm, which I understand is Thorn's standard. I doubt the seat tube was reamed out for a thicker post. Everything seems to have a generous coating of copperslip too.

I'll probably just get some spare bolts to be safe.

Speaking of safety, a completely different issue is the way the mudguards/fenders attach - half way up the forks/rear rack. I understand this is a special Thorn thing. But I've never seen a Thorn with these attachments AND SKS secu clips (the ones designed to release the front mudguard if something gets caught in it). Are they compatible? Is it less of a safety issue than with standard mudguards?

[mickeg]

My Nomad Mk2 has a spacer or shim inside the frame, the 27.2mm seatpost goes inside that spacer.  In your photo I do not see a spacer.  Did the Mk1 not use a spacer?

The bolt is a common one.  I have never busted one, but I was afraid if I did that it would be hard to extract without shop tools.  I put a small slot in the end of the bolt so that if I busted it, a small screwdriver might be sufficient to remove it.  I have heard that one of the best ways to mess up a bike tour is to bust your seatpost bolt and not have a spare.

[Danneaux]

Speaking of safety, a completely different issue is the way the mudguards/fenders attach - half way up the forks/rear rack. I understand this is a special Thorn thing. But I've never seen a Thorn with these attachments AND SKS secu clips (the ones designed to release the front mudguard if something gets caught in it). Are they compatible? Is it less of a safety issue than with standard mudguards?

Hi Edo'!

I adopted this same approach some years before I got my first Thorn bicycle, and wrote a friend about the reasons why:

Mike, I got to looking at my bikes this weekend, and realized there was a hidden safety hazard that is often overlooked.

As you know, most front mudguards attach to the dropout. This is fine, unless some debris (like wet, rotting leaves from the gutter) get sucked up and jam between the tire and the fender. Attaching the stays to the dropout, if the fender gets jammed, momentum pulls the 'guard ahead toward the fork crown and as it does, the stays shorten, drawing the fender harder and harder into the tire, stopping it immediately.  A surprising number of people have been hurt, paralyzed, or killed by this process when they rotate around the suddenly locked front wheel.  It is especially a problem with plastic fenders and metal stays, like I use.

I decided to shorten the stays -- making them more rigid and therefore less likely to vibrate -- and attach them to the lowrider bosses in the middle of the fork.  As a result, if the fender gets jammed, the result will be much less severe.  Looking at the geometry of it, the stays will get pulled *away* from the tire in a widening arc, and not into it.  As a side benefit, the fenders are now completely silent and don't rattle or vibrate even a tiny bit.

Making things worse, my ESGE/SKF fenders came with "Secu-Clips".  I don't know if you're familiar with them, Mike, but they are designed to allow the bottom of the fender to "break away" if it gets jammed.  A noble idea, but it has a *big* problem in my experience -- Secu-Clips just press onto a loop in the stays (they use one stay that is bent into an "L" shape to serve as two stays) and with use and vibration, the stays get looser and looser in the Secu-Clip. If the Secu-Clip lets go, the end of the stays can unship, causing almost as big a problem as if the whole works got jammed with wet leaves. Unfortunately, I've seen several bikes parked in the rack with their Secu-Clipped stays loose and rattling. Going to this new mount allowed me to get rid of the Secu-Clips, achieving the safety benefits without the problems.

I'm happy with the setup.  Hopefully, this will all make sense if you look at the pictures.

Sorry for the low-res photos, as I pulled them from my archived email of some years ago.

Edo', to answer your questions, "Yes" you could combine Secu-Clips with Thorn's mounting, but it would be sort of redundant and -- at least in my experience with them -- the result won't be a rigid as bolting the shorter stays directly to a higher mounting point. Thorn's mounting method and boss location addresses the safety issue in a different way compared to conventional placements or Secu-Clips.

Best,

Dan.

[sg37409]

Snapped my 2nd s/post bolt the other day. 1st time on the stoker s/post on the raven tandem, 2nd on my venerable XTC. The flanges bend slightly and cause misalignment for the bolt. I bought a pack of 10 longer bolts when the tandem one snapped and I was lucky enough to get some moles on the other side and unscrew the stub from the opposite side. Thats what prompted me to get longer bolts, and when the XTC bolt snapped (was also lucky to use same extract method) I've use one of the longer bolts.

I dont use a torque wrench, though I'm pretty sure I'm not over torquing these.

I'll probably just get some spare bolts to be safe.

[Danneaux]

I dont use a torque wrench, though I'm pretty sure I'm not over torquing these.

...and...

The flanges bend slightly and cause misalignment for the bolt.

Stainless steel work-hardens pretty easily and can become brittle. If the flanges aren't aligned, the bolt is twisted laterally as it is screwed in and tightened, and the threads act as a stress-riser, initiating a crack and subsequent failure.

Just as a general caution (not in any way implying you did, but in general), I think it is real important to avoid over-torquing those seat-clamp ears, especially early on. Once distorted, the chance of bolt failure increases due to misalignment. If worse came to worst, the brazed-on fitting could someday be replaced with a clamp-on version. I once repaired an old Mizutani Super Seraph whose pretty, lugged seat cluster included stamped clamping ears. I got the bike frame as a shop discard and milled out the damaged portion and replaced it with a brazed-in milled steel tube. Those original stamped ears were so fragile on frames built in the 1970s, they caught many people out and it didn't take much to distort or even crack them.

My Nomad Mk2's brazed-on wraparound *collar* is well-aligned, but as a precaution, I've tossed a second longish bolt in with my tool kit, as I do on all my touring bikes. Beats trying to improvise on the road in the event of failure. I do see Thorn have included the same collar on the new, redesigned Sherpa Mk3 and its near twin, the new Raven.

Your black RST is still one of the prettiest bikes I've ever seen: http://www.thorncycles.co.uk/forums/index.php?topic=2831.msg13255#msg13255 It inspired me to choose Matte Stealth Black for Sherpa and to repeat it on the Nomad. I'm glad I did.

Best,

Dan.

[Edocaster]

Thanks for the explanation of the mudguard stays. It makes a lot of sense - hopefully one less thing to worry about!

[CWM]

Hi.  I have read a few of the discussions about seat post bolts and wondered if anyone could offer me some advice.  I have managed to break the seat post bolt on two different Thorns, an Audax and a RST. Both were extracted with relative ease but as one of these events was while on tour it could have turned out a lot worse. I am keen not to let it happen again.  I tend to have the seat post out quite a lot. I don't have quite as many saddles as I have bikes and so I sometimes swap the post and saddle from bike to bike. I read of someone removing the braze ons and fitting a free standing collar.  Is that too extreme? See photos below of the RST.  This is my commuting bike so not as clean as it might be at the end of a long winter  - it has been cleaned, just not in the last two weeks, honest. And I know that the head of the proper bolt is meant to fit into the seat, but this is the only one I could find when the last one broke. I am pretty sure that the seat post is the right size, all my bikes are steel and all seem to take a "standard" 27.2mm(?) post.

Is a titanium bolt a good idea? Though I do rather like the idea of removing the problem and installing a standard issue collar. Got that on a Salsa and it seems to work just fine. 

Thanks in advance.  Clive

[Danneaux]

I don't have quite as many saddles as I have bikes and so I sometimes swap the post and saddle from bike to bike.

This might be an excellent solution for you, Clive:
http://www.sjscycles.co.uk/thorn-quick-release-lever-m6-to-replace-allen-key-seat-bolt-of-most-thorn-bikes-prod15533/?geoc=us

Looking carefully at your photos, it could be a matter of photographic perspective, but it appears the seatpost bolt lugs ("ears") are not parallel anymore. This can happen sometimes on a bicycle when the seat bolt is overtightened or if the seatpost itself is a bit undersized (I'm amazed at how often this is the case for even reputable "name" seatpost makers). If the lugs are indeed skewed toward each other, then this can be a factor in side-loading the bold and can lead to failure (I have seen it happen many times on various brands other than Thorns). Just a thought.

It is also possible if you swap posts between bikes, the one in the Thorn might not be the exact size leasing to the converging seatbolt lugs.

Yes, removing lugs of this type and fitting a clamping collar is a pretty straightforward process provided one uses care with hacksaw and file and touches up the pain afterward. Do be aware, however, not all removable clamps are created equal, and many can bind similarly if the design is not made with an eye toward avoiding side-loads on the bolt. I tend to favor this type:
http://www.sjscycles.co.uk/m:part-threadsaver-seat-clamp-black-prod16562/?geoc=us

All the best,

Dan.