Author Topic: Thorn stem problems  (Read 970 times)

Andyb1

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Thorn stem problems
« on: June 02, 2024, 08:14:05 pm »
I have a shiny silver Thorn stem on my Sherpa.  I am currently using one which is 130mm / 14 degrees to give me the riding position I want (previously the bike had a shorter one) - but both have the same problem - the handlebar can creak.   This is with quite narrow straight handlebars.

The Thorn design has the clamps around the handlebar held together with two M5 bolts while later MTB clamps use 4 x spaced out bolts - which I think would hold the handlebar much firmer.

Has anyone else had the problem?   I guess I could use a locktite flange adhesive to bond the clamp to the handlebars, but otherwise the only solution I can see is to fit an MTB stem (which will probably be black and look ugly).
« Last Edit: June 02, 2024, 08:18:44 pm by Andyb1 »

Danneaux

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Re: Thorn stem problems
« Reply #1 on: June 03, 2024, 01:07:03 am »
I have solved past handlebar creaks in hatchplate stems though several methods...
1) Applying adequate/recommended torque to the fasteners.
2) Greasing the 'bar/stem interface.
3) Applying a thin strip of cellotape/packing tape to half or all of the interface.

I have had much better and longer term success using four-bolt stems rather than two-bolt designs. The clamping force is better distributed and remains creak-free longer but can still occur, particularly if the bike is used or stored in rain for a long period of time.

Last, check to make sure the stem-to-steerer clamp bolts are also tight and the headset is properly adjusted. If the bars are exposed, creaking can be an early sign of cracks so an inspection would be good insurance.

Best, Dan.

PH

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Re: Thorn stem problems
« Reply #2 on: June 03, 2024, 11:21:33 am »
If the bars are exposed, creaking can be an early sign of cracks so an inspection would be good insurance.

Best, Dan.
Having had a handlebar snap*, no creaking beforehand, any creaking would give me serious cause for concern.  Creaking has to indicate some movement and there shouldn't be any. 
All my stem faceplates are four bolt, I'm not sure it makes much difference, but it can't do any harm. I think you might struggle to find a replacement in that size, most makes only go up to 120mm and 14 degrees is a bit unusual.  If you do go down the replacement route, you could calculate how different components would place the bar in the same position.

* I thought my bars snapping was a unique experience, but since found plenty of other examples. 

mickeg

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Re: Thorn stem problems
« Reply #3 on: June 03, 2024, 12:23:07 pm »
I had a two bolt stem years ago, had trouble getting it tight enough so that my handlebars would not rotate in it, I used drop bars.  With my hands on the hoods, hitting a nasty bump could cause the bars to rotate in the stem.  Replaced it with a four bolt model.

Stems are available in silver too, you may have to pay a bit more for a polished silver one.  I have a silver one on my Sherpa.




Andyb1

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Re: Thorn stem problems
« Reply #4 on: June 03, 2024, 02:00:44 pm »
You are right Phil, my 130mm stem has a 17 degree angle, not 14 degrees.
It also has M6 clamping bolts, not M5, so I went up to 10NM - but there is still a slight rock and creak.
I think a stem with a 4 x bolt clamp is the way to go, even if it has to be black - polished alloy stems in 25mm seem rare.
My handlebars just rock a fraction in the clamp, I would be very worried if they could rotate as there could be scratches cut into the handlebar that would later crack.
Did your handlebar fail around the edge of the clamp Phil?

PH

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Re: Thorn stem problems
« Reply #5 on: June 03, 2024, 04:10:28 pm »
My handlebars just rock a fraction in the clamp, I would be very worried if they could rotate as there could be scratches cut into the handlebar that would later crack.
That doesn't sound right, are you sure you haven't got a 25.4mm bar in a 26mm stem?  If you have a matching pair, you would, with enough force, be able to crush the bar, there's always a gap between faceplate and the rest of the stem. If they're mismatched, the curvature remains wrong however much you tighten it.
Quote
Did your handlebar fail around the edge of the clamp Phil?
Close too, the area where where the diameter changes for the clamp.  On examination, I could see that the crack had been developing for some time, about 30% was obviously not new, the rest looked very fresh, the last 10% jagged.  It would have been hidden by the bar bag bracket, not that I'd been examining it anyway. I've been through all the scenarios that could have caused it without reaching a conclusion. 

BTW - It's Paul not Phil, though I've been called worse.

Andyb1

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Re: Thorn stem problems
« Reply #6 on: June 03, 2024, 09:54:11 pm »
Sorry Paul, I usually get the first letter right……

VERY good point about the handlebar diameters, I will have to measure the bars that came with the bike and the MTB straights that I later fitted.  The clamp faces do not meet, but they could still be the  wrong size - the rock is very slight.  What diameter bars did Thorn fit as standard on the Sherpa?

It amazes me why handlebars of both 25.4mm (1”) diameter and 26mm exist.   I can understand if much larger diameter bars are needed, but 25.4mm and 26mm are just too close.

mickeg

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Re: Thorn stem problems
« Reply #7 on: June 04, 2024, 01:33:52 am »
You can shim a 25.4 mm bar in a 26mm stem.  I am quite sure that I shimmed one at one time but I do not recall which bike.

I think Dan commented that he cracked a 26mm stem that was used on a 25.4mm bar.


Danneaux

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Re: Thorn stem problems
« Reply #8 on: June 04, 2024, 07:26:27 am »
Quote
I think Dan commented that he cracked a 26mm stem that was used on a 25.4mm bar.
Close, but not correct. I milled out a couple 25.4 stems to successfully fit 26.0 handlebars. I have also pantographed/engraved my name in a number of stems to personalize my bikes and make them easier to identify. Some care must be used to avoid creating stress risers, so I use a ball-headed cutter. Stove enamel makes for nice contrast.

Nitto, among others, offer shims to fit 25.4mm handlebars in 26.0 stems. You can make suitable, serviceable shims easily by cutting up aluminum drinks cans with a pair of sharp scissors or shears.

For years, 26.0 was considered the European road standard, probably based on Cinelli's popularity. 25.4 was more popular in Japan, later Taiwan and Asia starting in the Bike Boom days when Japanese "10-speed" road/campus bikes were imported to America and elsewhere to meet demand. It later became the MTB standard, carried over from 1in threaded steerers and 22.2mm quill stems to 1-1/8in threadless, based on Dia-Compe's (later Cane Creek's) original Aheadset standard. Both types of stems are available in each handlebar clamp diameter and in black anodized or powder coat or silver anodized or polished if you look for them, though some combinations can be difficult to source. Other, more exotic colors are available sometimes also.

In a different but related topic, I have replaced a number Brooks B.17 saddle rails that cracked in two. These were chrome plated and likely broke as a result of hydrogen embrittlement during the plating process, a known problem at the time.

Best, Dan.
« Last Edit: June 04, 2024, 01:07:41 pm by Danneaux »

Andyb1

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Re: Thorn stem problems
« Reply #9 on: June 04, 2024, 08:57:12 am »
I have measured the original silver handlebars that were on my Sherpa when I bought it and they are 25.4mm.  As are a set of Thorn Touring bars I bought secondhand - it looks like Thorn stuck with 25.4 diameter handlebars…….so my Thorn stems should be sized 25.4mm.

There is a great discussion in Zen and the Art of Motorcycle Maintenance about using alloy shims from drinks cans - on one level they are the perfect material……but shunned by some as they are not O/E parts.


mickeg

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Re: Thorn stem problems
« Reply #10 on: June 04, 2024, 04:51:57 pm »
I did the drink can shim on one stem, but I do not recall how many layers I used.  I have a really accurate caliper that only goes up to 1 inch, quantified in thousandths of an inch, veneer scale for a ten thousandth of an inch.  Used that to measure the aluminum can thickness, then calculated from there.

I also used a steel food can as a shim for a seatpost on the bike on my trainer.  Only one layer was needed.

JohnR

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Re: Thorn stem problems
« Reply #11 on: June 04, 2024, 09:53:56 pm »
I would add enough layers of an improvised shim such that there is a small gap between the stem and the clamp when the bolts are fully tightened. However, before trying a shim first look to see if that gap exists. If it does it means that inability to fully tighten the clamp isn't the cause of the problem but there could be imperfect machining resulting in the clamp not making full contact.

Andyb1

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Re: Thorn stem problems
« Reply #12 on: June 05, 2024, 09:55:35 pm »
Hi JohnR
The two halves of the clamp have a gap between them, but I reversed the front half of the clamp (even though it looked symmetrical) and that seems to have worked.

I have a stem with 4 x clamping bolts on order so I will still fit that as I think 4 x wider spaced bolts will be inherently better than 2 bolts in line.

Thanks for all your help.