Square-taper Shimano compact-double chainsets – which one?

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Samuel D
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Square-taper Shimano compact-double chainsets – which one?

Postby Samuel D » 1 Jun 2015, 12:31pm

Back story: I converted a triple-equipped, 8-speed Dawes Clubman to a compact double. I used a Stronglight Impact (110 mm bolt-circle diameter) with 46T and 34T chainrings.

Despite trying to deny it for a while, I have two problems with this chainset:

  • I can’t seem to get the perfect bottom-bracket spindle length. Using a 110 mm spindle, the chainline is farther outboard (about 45.5 mm) than I want. I have also tried 107 mm (won’t fit) and 113 mm (worse, of course)
  • there is a distinct wobble (close to 1 mm) at the chainring that has persisted through different bottom brackets. I am convinced this wobble is in the chainset as opposed to the rings, because the rings are finely machined while the cranks are roughly forged.
The wobble isn’t terrible, but it means I am always fiddling with the trim, both adjusting it at home and flicking between the two settings while riding.

Since I’m sick of badly documented Stronglight products, I’m thinking of getting a cheap Shimano chainset, ideally one that would take my existing 110 mm square-taper bottom bracket and my 34T and 46T chainrings (110 mm BCD). One such option seems to be the discontinued FC-2350, if I can find it.

Should I consider anything else?

Is a certain degree of wobble inevitable with square-taper and avoidable with (for example) Octalink? The Shimano FC-2450 (Claris) uses Octalink.

Thanks for your advice.

beardy
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Re: Square-taper Shimano compact-double chainsets – which on

Postby beardy » 1 Jun 2015, 12:37pm

I have also tried 107 mm (won’t fit)


Is that because the chainrings are touching the frame, the dérailleur will not work or the body of the crank touching the frame/bottom bracket?

Samuel D
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Re: Square-taper Shimano compact-double chainsets – which on

Postby Samuel D » 1 Jun 2015, 12:48pm

beardy wrote:
I have also tried 107 mm (won’t fit)


Is that because the chainrings are touching the frame, the dérailleur will not work or the body of the crank touching the frame/bottom bracket?

The right crank contacts the bottom-bracket ‘cup’ when I tighten the crank bolt (though not before tightening, so it’s a close thing). That’s with a Stronglight JP400 (aluminium-cup model) bottom bracket sold as 107 mm and physically labelled “107.5”.

Interestingly, there’s still miles of space between the chainrings and the frame with the 107 mm bottom bracket. And the derailleur would have worked okay, if I recall correctly.

I would try a Shimano 107 mm bottom bracket (which has slightly different outer ‘cup’ shapes than the JP400 and might avoid contact with the right crank) if there wasn’t the additional wobble problem. How much wobble is normal at the outer chainring?

Samuel D
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Re: Square-taper Shimano compact-double chainsets – which on

Postby Samuel D » 1 Jun 2015, 12:53pm

By the way, I just measured the wobble more accurately: it’s more than 0.75 mm but less than 1 mm at the outer chainring. At the inner chainring it’s considerably less. Am I being too fussy?

beardy
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Re: Square-taper Shimano compact-double chainsets – which on

Postby beardy » 1 Jun 2015, 1:09pm

In order to get the closer chainline that you desire you could look for a chainset that uses a larger axle, like a 118mm or 122mm and then use that with a shorter bottom bracket. Or just look at them with your eye to compare ring to square taper position.

Stronglights newer versions of the impact use a longer axle so would reduce the chainline a few mm if fitted on the same axle.

I had a Shimano crankset with such a wobble, you can confirm it is the crankset by laying the rings on a flat surface to check them and fitting the cranks in all four positions on the square taper and seeing how that effects the wobble.

Yes 1mm is probably too fussy.

stewartpratt
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Re: Square-taper Shimano compact-double chainsets – which on

Postby stewartpratt » 1 Jun 2015, 2:23pm

Samuel D wrote:I am convinced this wobble is in the chainset as opposed to the rings, because the rings are finely machined while the cranks are roughly forged.


They should both be accurately made, whether forged or machined. For what it's worth, you can test to see which is bent by taking the rings off and rotating them round by one or two bolt positions: note the crank's position as the wobble passes the front mech and if it's different before and after this change then it's the rings; if it's the same it's the cranks.

A wobble is neither inevitable with square taper nor impossible with any other system. The usual culprit is the rings, the crank spider is less likely and the spindle less likely still—though all theoretically possible.

Spa's silly-value-for-money XD2 double cranks are designed for a 110mm spindle. I haven't bent mine yet :)

beardy
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Re: Square-taper Shimano compact-double chainsets – which on

Postby beardy » 1 Jun 2015, 2:39pm

The 110mm axle is for the triple XD2 with 107 for the double.

Interestingly Stronglight specify 107 for their double, so it should have fitted the OPs 107.
The old double specifies a 103mm axle so that should have been even more spacious on a 107mm.

The TD2 double specifies a 103mm axle (as it is almost the old stronglight).

If an Xd2 will clear the BB with a 103mm axle then you would be 2mm shorter in your chainline, just need somebody with one to see if there is that sort of spare space.

stewartpratt
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Re: Square-taper Shimano compact-double chainsets – which on

Postby stewartpratt » 1 Jun 2015, 2:41pm

beardy wrote:The 110mm axle is for the triple XD2 with 107 for the double.


Ah yes. Whoops. *rubs eyes*

Samuel D
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Re: Square-taper Shimano compact-double chainsets – which on

Postby Samuel D » 1 Jun 2015, 2:46pm

Thanks for your comments, both.

This is the problem with Stronglight products. Five websites give you about five different answers for any Stronglight question, perhaps because the products have undocumented changes during production.

The website I bought my cranks from (a French site) recommended a 107 mm bottom bracket, so I got Stronglight’s own JP-400. It didn’t work. I even tried it again months later, thinking I must have been imagining things. No dice.

I almost wonder if I have some oddball chainset that isn’t what it claims to be. It came with the 46T/34T rings (ideal for me), which I haven’t seen available anywhere else (e.g. Spa Cycles offer 46T/36T or 48T/34T, but not 46T/34T).

Samuel D
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Re: Square-taper Shimano compact-double chainsets – which on

Postby Samuel D » 2 Jun 2015, 5:40pm

I’ve now read that 107 mm and 110 mm bottom brackets have the same (or very similar) chainline, yet my 107 mm Stronglight puts the right crank closer to the centreline than my 110 mm Shimano.

I’m afraid to order a 107 mm Shimano bottom bracket to try it, since the 110 mm Shimano already puts the left crank about as close as it should go. If the difference is all on the non-drive side, the 107 mm Shimano would neither narrow my chainline nor fit on the non-drive side.

So unless someone has a better idea, I think I’ll have to order another chainset. Preferably a Shimano so I know what I’m getting.

Any idea how to source an FC-2350? Nothing on eBay.

Do any other Shimano compact doubles, past or present, take a 110 mm square-taper bottom bracket?

Samuel D
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Re: Square-taper Shimano compact-double chainsets – which on

Postby Samuel D » 2 Jun 2015, 6:34pm

Sorry to be such a pain in the neck, but here’s a picture of my Stronglight Impact double before I installed it:

Image

Does that not look a bit weird? It almost looks like a triple except there are no holes drilled for the inner chainring.

There are two pictures of Spa’s very similar TD-2 cranks here. The black one looks like a triple and has the bosses seen on my own crank. The silver one looks like a double and is missing these bosses!

If my crank is really a triple pretending to be a double, no wonder it takes a 110 mm bottom bracket and has a chainline a bit far out! I feel like Sherlock Holmes at the moment. Anyone agree with my reasoning?

stewartpratt
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Re: Square-taper Shimano compact-double chainsets – which on

Postby stewartpratt » 2 Jun 2015, 9:23pm

Yeah, some of the old Suginos (and their numerous close relations) were like that. Both types were made from the same casting, then the inner mounts were drilled and threaded on the triple, with additional spacers under the third ring. The triple naturally used a 5mm longer spindle.

Brucey
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Re: Square-taper Shimano compact-double chainsets – which on

Postby Brucey » 2 Jun 2015, 9:32pm

quite a few chainsets will only give a narrow 'road' chainline if you fit them to a cartridge BB which has a recess in the RH cup; when so fitted part of the crank disappears inside the recess.

So to this end I'd suggest that you try a shimano BB or perhaps one made by 'token'; they can have a suitable recess.

if you fit a crankset with a slight mismatch to the tapers (through machining tolerances) then the chainring can run out slightly. My belief is that the shimano method (some grease on the taper, high torque on the fixing bolt) is meant to allow for this possibility.

I'd expect a Sugino crank to fit nicely (and to run out less than 1mm) onto a good quality JIS spec BB; whether the JP400 can be called that I don't know.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Samuel D
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Re: Square-taper Shimano compact-double chainsets – which on

Postby Samuel D » 2 Jun 2015, 10:27pm

stewartpratt wrote:Yeah, some of the old Suginos (and their numerous close relations) were like that. Both types were made from the same casting, then the inner mounts were drilled and threaded on the triple, with additional spacers under the third ring. The triple naturally used a 5mm longer spindle.

Hmm. So not a smoking gun in my case, then.

Brucey: I’ve read a 107 mm Shimano would produce the same chainline as my current 110 mm Shimano (BB-UN55). The cheap BB-UN26 isn’t available in 107 mm, so I’d risk putting good money after bad by trying this – something I seem to be good at!

Then again, my 107 mm Stronglight produces a very different chainline than my Shimano 110 mm, so I don’t know what to believe.

The Spa cranks are just £20 – barely more than a 110 mm BB-UN55. Might be worth the risk? But they seem to require either a 103 mm or 107 mm bottom bracket. I don’t have a 103 mm at home and my 107 mm is the possibly dodgy Stronglight. The XD-2 also needs special chainring bolts (another £5 please).

I’ll sleep on it anyway.

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recordacefromnew
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Re: Square-taper Shimano compact-double chainsets – which on

Postby recordacefromnew » 2 Jun 2015, 11:27pm

Samuel D wrote:I’ve now read that 107 mm and 110 mm bottom brackets have the same (or very similar) chainline, yet my 107 mm Stronglight puts the right crank closer to the centreline than my 110 mm Shimano.


AFAIK you can't always assume two JIS bb's of notionally the same spindle length from two different manufacturers to have exactly the same spindle asymmetry/symmetry, and certainly not the same taper manufacturing tolerance so delivering the same chainline. FWIW JIS only specifies the dimension of the taper, I am not aware of any relevant length/symmetry specification in the code.

Samuel D wrote:I’m afraid to order a 107 mm Shimano bottom bracket to try it, since the 110 mm Shimano already puts the left crank about as close as it should go. If the difference is all on the non-drive side, the 107 mm Shimano would neither narrow my chainline nor fit on the non-drive side.


I agree 107mm and 110mm UNs should deliver the same chainline in theory (i.e. manufacturing tolerance permitting), therefore you are right that if the 110mm UN is already a close call on the left crank you will most likely be wasting time and money trying a 107mm UN.

I think something similar to your experience was previously reported here viewtopic.php?f=5&t=77436&start=15