Stock Thorn Sherpa seat bolt breaks after 7000 miles

[SuffolkPunch]

I have just had the disconcerting experience of the seat (San Marco Rolls Classic) suddenly fall off my Thorn Sherpa whilst I was riding it (fortunately at low speed on an empty country road).

It's a stock Thorn Sherpa Trekking (straight handlebar) model bought from Thorn Cycles in Feb 2009. It has a San Marco Rolls Classic saddle and a rigid seat post in silver with the Thorn logo in black. Since new, I've only done 7000 miles. After 5000 miles the saddle failed (one of the 2 rails snapped when I moved the bike by gripping the saddle). I replaced it with the same San Marco model. After another 2000 miles the single bolt securing the seat to the post snapped today as explained. And, of course, it's out of warranty.

I have yet to contact Thorn Cycles (it's a Saturday and they're closed until Monday). Does anyone have a similar experience and/or advice?

My initial response it to look for seat posts  which use two bolts to secure the saddle. Does anyone know if there are any such that would work with the San Marco saddle?

Also, is it possible that I had the saddle positioned too far back leading to excessive bending moment being exerted on the bolt?

I do hope someone can help, because I have no wish to repeat this experience.

[triaesthete]

Hello
are you very heavy?  The saddle should work in any position allowed by sliding the rails through the clamp, but an extreme adjustment position will raise more stress than a central one, other things being equal. The Thorn post also has a relatively short rail clamping length which leads to higher rail stress whilst also allowing slightly more setback than say a Thomson.

Were you using the original bolt? These have coded heads that indicate the specification, replacing it  with stainless steel would be unwise.

The Sherpa frame is very rigid at the back so it will transfer a lot of bump force into the seat pin, saddle and you, particularly if you run hard tyres.

These are very good in every way http://bikethomson.com/seatposts/elite/

The San Marco has standard 7mm rails I think so most conventional seat posts will work with it and the Thomson certainly will.

Best wishes
Ian

[SuffolkPunch]

Thanks Ian,

are you very heavy? 

No - 10 st and a bit.

Were you using the original bolt?

Yes.

The Sherpa frame is very rigid at the back so it will transfer a lot of bump force into the seat pin, saddle and you, particularly if you run hard tyres.

I run Panaracer Pasela TourGuard rigid bead tyres (with Kevlar/Aramid belt) inflated to 60psi. Does that sound problematic?

I'll look into the Thompson Elite. thanks for the suggestions.

[triaesthete]

Hi
So you are well within the design parameters weight wise 
What width are the tyres? And is the bike comfortable to ride or harsh?

As an aside, all components will have a finite life and 7000miles is quite a lot of hammering on one bolt. I know people here clock up large mileages but I suspect they are statistically relatively unusual. Note how many "hardly used" Thorns appear on ebay. I suspect a lot of components are built for the typical/average user and the further you are away from this (about 5000miles?  ) the more components will fail in your longer run experience unless you pay a lot more for them. That said I think Thorn come as near as just about anyone on the market to building for serious use.

Does that justify spending lots of dosh on a posh seat pin?
Cheers
Ian

[Danneaux]

Hi SP, and welcome to the Forum!

Ian's suggestions, observations, and advice are surely sound, and it is definitely worth looking at a Thomson post; they have a stellar reputation.

I will add a possible and less-costly alternative to consider; the Thorn/Zoom large layback seatpost at £25.52, shown here:
http://www.sjscycles.co.uk/thorn-micro-adjust-alloy-seat-post-with-large-lay-back-400mm-x-272mm-prod23725/

I found I needed this post on my Sherpa and now on the Nomad in order to get the right relationship between saddle-setback and bottom bracket. It has the added advantage of clamping midway in the available rail travel on my Brooks B.17 saddle, so stress on the saddle rails is reduced, along with stress on the seatpost clamp and bolts. It is something to consider, anyway, and the matte-black anodized, bead-blasted finish goes nicely with the rest of the bikes' components. I don't yet has as much use on the post as you did with yours, but all signs are hopeful at this point. I'm glad you weren't injured; a saddle-connection failure is surely startling. I've been there, thanks to some broken saddle rails. No fun!
See: http://www.thorncycles.co.uk/forums/index.php?topic=2294.msg16209#msg16209

Is it possible to replace the bolt with a new one (perhaps rated for higher strength) and so continue to use your present 'post?

Hope this helps.

Best,

Dan.

[SuffolkPunch]

Hi Ian

The tyres are 1.75in wide, and give a comfortable ride on Suffolk lanes.

I guess the choice is between buying a well-built post (the Elite does get good reviews) with dual bolts which should last me (say) 10 years, versus playing safer by getting a cheaper post and replacing it every 4000-5000 miles. On the whole, if an Elite costing £60-70 saves me from being dumped in the road without any warning, then it's well worth it!

Thanks again!

[SuffolkPunch]

Hi Dan,

Thanks for your suggestions!

Re your suggestion of the large layback seat post: I have observed that I naturally tend to sit further back on the saddle when I'm powering up a hill (which is what I was doing when the seat bolt snapped today!).

Re getting the right relationship between saddle setback and bottom bracket: what exactly is the theory on that?

Re: getting another bolt: Unfortunately the bottom half of the seat clamp vanished in the verge - and I couldn't find it despite searching for some time!

Regards,

Martin

[julk]

Martin,
Sorry to hear about your 'incident'.
I use a Nitto S84 which is all steel with a big layback, suits my Brooks saddle just fine.
The longest S84 is 300mm, not so good if you need a lot of seatpin showing.
They are also around £100ish now and hard to find in stock.

I find on my Thorn bike that the Brooks name stamped into the saddle is centered on the line of the seatpin when it has the best fore and aft position for me.
I hope this helps.
Julian.

[triaesthete]

Hi Martin
Your tyre pressures sound high to me, I run 1.6 Marathon Supremes at about 44 and 48 psi.  Not the fastest, but not really draggy either and gives a very comfortable ride so the components between me and the road must be getting an easier life.

When failure cannot be allowed to happen the only way to achieve this is to retire all components before their anticipated or tested life expectancy to failure. Classic examples are car cam belts and aviation components as failure in use is usually catastrophic.

Insurance on a practical level really. Lots of fun thinking it through if you're a bike geek, and a bit like Russian roulette if you are too "careful" with your cash.

I wonder if the rail failure you had previously overstressed the bolt. It must have applied some unusual loadings as all the weight went onto one rail.

Enjoy shopping
Ian

[triaesthete]

Julian you are a cruel man. Those Nitto posts are beautiful and now I can't un-know about them.

Cheers
Ian

[Danneaux]

Re getting the right relationship between saddle setback and bottom bracket: what exactly is the theory on that?

Hi Martin!  

I'm glad you asked; answer right here...discussed and thrashed at length:
http://sheldonbrown.com/kops.html

I started with KOPS (Knee-Over-Pedal-Spindle) and did pretty well, for me then refined it to my own needs, which worked even better.

I have to accommodate my knees. When I was 16, they went on a tear to see if it was possible to remove a window crank, bend a shift knob, and break a steering column cover. Unfortunatey, they needed the help of a whole car accident to do so and brought me along for the ride ("bad knees", indeed!). That's what got me into 18 months of physiotherapy and stumping through my senior year of high school with a cane. Turns out it was a Good Thing 'cos it also got me cycling "with intent" (sounds less pretentious than "cycling seriously") and really "into" cyclng with the purchase in 1977 of my first "good" 10-speed, a Nishiki Olympic. I've been riding ever since. Everything goes well for me so long as the bike is setup correctly and I keep my cadence fast (a hummingbird-like 110-120RPM) and light. I do *not* do well with lower cadences (<80RPM) or high loads, and the saddle height and setback from the BB has to be correct for me. Otherwise, Things Go Bad. Quickly.

Bottom line: The long-layback seatpost allows me to get into a position I find comfortable, efficient, and pain-free, while easing strain on my (Brooks) saddle rails. Not everyone is the same (or has rogue knees), but it sure works for me. Long-layback seatposts used to be thought of as "ordinary" seatposts, but a trend developed to reduce setback -- sometimes to the point where there was zero setback (clamp concentric with post). Brooks saddles (the kind I prefer) were designed in the days when even racing bicycles had very laid-back geometry -- often with far slacker angles than today's touring bikes. On the majority of today's bikes with upright geometry, it is hard to get a good position with a Brooks saddle (and some others with relatively short rails) unless the saddle is pushed well back, increasing leverage on the rails, clamp, and clamping hardware. Long-layback seatposts correct for this, but everyone is different and most people do fine with the standard variety.

All the best,

Dan.

[SuffolkPunch]

Ok, I've removed the seat post using the bolt and clamp from another seat post.

It came out easily and (naturally) was thoroughly greased when assembled at Thorn Cycles.

My question is what sort of grease is suitable for steel alloy and aluminium posts? I have a scarcely-used tub of Finish Line Anti-Seize assembly lube which I have used to grease pedal threads. Would that do? It claims on the tin that it's suitable for press fit assemblies of aluminium, stainless steel and alloys. The Sherpa tube the seat post fits into is steel.

My current plan is to get a post with layback (+ 2 bolts), and reduce the tyre pressures from 60psi to 48psi.

Martin

[triaesthete]

Hi Martin
it sounds like the lube you have is ok.

Please update us on your lower tyre pressure experiences.

If you are thinking of a Thomson layback post beware that  the restricted setback, because of the long length of the rail clamp, is one of their few limitations. However they do have a very nice engineering drawing with all the dimensions at the bottom of this page:  http://bikethomson.com/seatposts/elite/  You can work it all out properly before you buy. Not many manufacturers do this.

Happy fiddling
Ian

[peter jenkins]

FWIW, I have had 2 bolts fail on Thorn seat posts in the past. (I'm talking 6 and 7 years ago)

Both were original bolts supplied with the seat posts and both were fitted correctly. The seat posts were branded Thorn and boasted "extra lay back".

I advised Thorn of the problem at the time of the second failure in case it was a case of a faulty batch, and Robin Thorn responded to the effect that he tested a sample in stock and couldn't find any problem so I put it down to bad luck.

The second incident occurred about 6 miles north of Inverness on LEJOG so I had to ride back to find a bikeshop (why didn't I think to take a spare seatpost bolt??!?) with panniers and no saddle! I'm still using the replacement seatpost, which is branded "Outlander".

I have 3 other bikes, which have not suffered the same problem. One has a Thompson Elite seatpost with a two bolt arrangement and this would be my preferred option if purchasing seat posts in future.

Cheers,

pj

[Danneaux]

Fellows,

Can you recall...did the seatpost clamp bolts break at the threads, or midway on the shaft somewhere? I would guess the threads, since those comprise stress-risers, but I've seen some break mid-shaft.

I have managed to strip the threads on the alloy centering nut on occasion, but it was due to excessive porosity in the casting making for bad threads. I simply machined a replacement out of 7075 billet, threaded it, and was good to go. Fortunately, I had some warning -- the sadde and seatpost cradle started to rock, so I knew something was "off". When I tightened it, the nut threads stripped right out with gentle finger pressure.

Thanks.

Best,

Dan. (Curious!)