Thorn Cycles Forum
Community => Tandem Talk => Topic started by: bouced on September 11, 2026, 11:36:42 AM
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Hello
I would like to adjust my front chain tension but I think there is a problem with the screws.
I took two pictures from below.
Shouldn't there be a head on the screw so that I can loosen it to adjust the eccentric?
Bests
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Gosh that's new to me.
How's about a few details, bike, age, is this the first time you have looked at this?
Matt.
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Gosh that's new to me.
How's about a few details, bike, age, is this the first time you have looked at this?
Matt.
I bought the bike second-hand about two years ago. Sorry I don't know its age.
The frame is a thorn voyager like this one:https://www.sjscycles.co.uk/frames/medium-xx-small-thorn-voyager-tandem-frame-yellow/?geoc=FR (https://www.sjscycles.co.uk/frames/medium-xx-small-thorn-voyager-tandem-frame-yellow/?geoc=FR).
It's the first time I look at this part of the bike.
Cedric
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It appears the machine screws have sheared flush from the application of too much torque. If so, then a time tested means for extraction involves drilling and using a screw extractor ("EZ Out").
Best, Dan.
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From the photo I can't see what kind of eccentric you have in it.
On my Thorn Nomad Mk II, I have a gold colored eccentric, some photos of mine and also info on how I adjust it at this link:
https://thorncyclesforum.co.uk/index.php?topic=14475
If your eccentric looks different than mine, then I have no idea if my discussion at that link is applicable.
I sort of agree with Dan's comments, but I can't imagine how the eccentric pinch bolts could shear off, I suspect someone cut them off with a saw. If that is the case, I agree with Dan, drilling and a screw extractor. Perhaps start with a center punch so that the drill bit does not wander about as you start drilling
If they are the same ones I use, they are a very fine thread.
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I can't imagine how the eccentric pinch bolts could shear off...
George, I wondered the same thing, so I tried to play forensic analyst...
I went to the linked profile of the frame and note Thorn is (currently) selling the frame alone and says, "These frames are made to a price, finish is not perfect but completely functional". This indicates the frames weren't made to the same spec as Thorn's higher priced offerings. The rear dropouts support this, as they differ from the other tandems Thorn produced.
In Cedric's photos, the sockets for the machine screws aren't milled from billet and circumferentially welded as on our Nomads. Rather, they are nuts tack-welded to the eccentric BB shell. I'm not sure the screws are the same diameter or fine pitch as ours. If the pitch is more coarse and the diameter smaller, then any shearing might have been the result of the previous owner's attempts to get the eccentric screws tight enough to hold against the force of two people's pedaling torque.
It is odd they'd shear at about the same point, but I suppose the nuts could have served as a collar to isolate the threaded shaft until it emerged and twisting forces concentrated on the unsupported shaft. A bit hard to tell, but I think the break doesn't look clean enough to be sawn through, and my first thought -- that they'd somehow hit something and sheared from impact -- isn't supported by the photos.
Just some thoughts on a Friday morning that saw me awakening to reddened leaves on the trees -- strong hints of Fall!
Best, Dan.
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Try drilling them out (after centre punching) with a LH drill. Sometimes using a LH drill loosens bolts like this and they unwind.
Once out you need to see what thread pitch they are.
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The welds holding the nuts on look to be crudely executed. It occurs to me that an alternative approach would be to carefully grind through them using a thin cutting disc and remove the nuts with the sheared setscrews in situ. Either new stainless nuts could be welded on, with minimal damage to the paintwork, or an expanding eccentric BB shell could be fitted, if one of a suitable diameter can be sourced.
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Does look like a job for an EZ Out as Dan suggests, but they're not as easy as the name suggests and what is easy is making it worse! If you have a small mechanic or engineering works near you, it might be worth a visit, a workshop used to dealing with older motorbikes would be ideal, they'll be very familiar with removing sheared bolts.
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Thanks a lot for all yours answers. I'll try to contact the man who sold me the bike and ask him what happened.
Some colleagues of mine are mechanic, I'll have some help of that topic.
Do you think that the threading is to be metric?
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Do you think that the threading is to be metric
I think it is almost certain to be, but it would pay to check after the broken studs are removed.
Please let us know how you come out. Your colleagues should be able to get things sorted for you.
Best of luck, Dan.
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I agree with Dan, likely metric. But there are fine threaded and course threaded versions, so if you get them out undamaged, measure the diameter and the pitch. Your mechanic friends most certainly should be able to help.
If it is M8 X 0.75, you likely need this one. That is the same one I use on my Nomad Mk II.
https://www.sjscycles.co.uk/bottom-brackets/thorn-m8-extra-fine-conepointed-screw-for-eccentric-bottom-bracket-shell/
One thing that has not been mentioned here, if you get them out and get new bolts, you should install the eccentric so that the thick metal part is the part that the pinch bolts press against. If you set it up so the pinch bolts press against thin metal, it may deform, thus you want to avoid that. See attached photo of mine, the thick part of the eccentric is on the bottom where the pinch bolts are, not the thin part of the eccentric.
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All good informed replies above.
Given the condition of the bolts, I would be inclined to at least take a look at the eccentric. Perhaps the previous owner can shed some light on recent maintenance? Does it have lots of small indentations?
I managed to take a look at the eccentric without taking the cranks off by sliding it half out when the bolts were removed. I think a crank puller is required for that job to be done properly and beyond my limited mechanical abilities.
Looking forward to hearing your progress.
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According to the details in the link Cedric posted earlier
https://www.sjscycles.co.uk/frames/medium-xx-small-thorn-voyager-tandem-frame-yellow/?geoc=FR
the eccentric screws that go with that Voyager frame are these
https://www.sjscycles.co.uk/bottom-brackets/thorn-screw-lock-nut-set-for-new-voyager-frame-eccentric-fitting/
Unfortunately no details are given about the thread etc measurements but if you messsge SJSCYCLES I’m sure they’ll tell you and confirm if these ard in fact the set of screws you’ll need.
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As Matt has said, on our Raven Tours the bolts have pointy ends and are designed to dig into the EBB so will leave an indentation in the soft alloy. If small rotational adjustments of the EBB are made the indentations can become a groove and the eccentric is then not correctly locked.
On mine I can remove the bolts, look down the holes with a torch, and rotate the EBB to see where the next indentation is. I have been lucky as mine had been adjusted properly by the PO and the indentations were well spaced apart - so I have simply reused those indentations rather than make new ones.
I have found that the bolts can loosen off so mine are now drilled and lockwired (others use bands made from innertube). I also found that after 1500 miles in one position that the bolts needed to be wound in a touch further to take out a little looseness between the EBB and frame - the downward loads from cycling had pushed the bolts deeper into the EBB.
If you get new nuts welded on make sure they are fine thread M8, not standard M8 nuts.
Edited to add:
What I have written above is relevant to a Raven, looks like the Voyager may use a slightly different system with flat ended bolts??
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Hello again
The previous owner sent me a link to the eccentric he installed on the bike: https://funecobikes.com/product/boitier-de-pedalier-excentrique-aluminium-pour-tandem.
The part is made of aluminium and my frame is made of steel. Isn't there a risk of corrosion.
I will try to remove the screws next week and tell you.
Thanks again
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Hello again
The previous owner sent me a link to the eccentric he installed on the bike: https://funecobikes.com/product/boitier-de-pedalier-excentrique-aluminium-pour-tandem.
The part is made of aluminium and my frame is made of steel. Isn't there a risk of corrosion.
I will try to remove the screws next week and tell you.
Thanks again
Dissimilar metal corrosion can be a problem with steel and aluminum, but the metals have to be in contact for that to be a problem.
For example, aluminum seatposts can bond to steel frames. But if you grease the seatpost, that not only solves that problem, but the grease reduces the amount of water from rain that gets inside the frame as the grease acts as a sealant.
My Nomad Mk II has paint (or powder coat or something like that) on the steel shell around the anodized aluminum eccentric, the anodizing and the paint do a good job of preventing dissimilar metal corrosion.
I do not read French, I do not know if that is anodized or not. But if the shell on the frame has a good layer of paint, it is unlikely that it would be a problem.
My Thorn Sherpa has an aluminum seatpost that I make sure is greased, but most of my other bikes have painted (or some other material) seatposts that prevent the metal to metal contact.