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Author Topic: Which BB axle width: Rohloff 54mm CL: Stronglight Compact/Sugino Cospea/Alpina  (Read 2838 times)
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Andre Jute
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« on: January 15, 2012, 07:57:55 AM »

EDIT 19 MAY 2012: THE CORRECT ANSWER IS 119mm, with 118mm as the nearest correct compromise to give a chain line within 0.5mm of perfect. See my post of 19 May 2012 below for how this was measured and determined.

****

I bought a crankset described as a Stronglight Double and got lucky, receiving a relabelled Sugino Cospea crankset, by weight and the "Cold Forged" stamp on the back apparently the real thing. I will replace the Stronglight rings that came with it by a single 38T Surly stainless ring in the outside position.

Stronglight says that with a 107mm* axle, the chain line (between the chain rings) is 43.5mm.

What length bottom bracket axle do I need to give a Rohloff chain line of 54mm to the outside ring?

Thanks.

Andre Jute

*same as for the "other" Stronglight Impact, based on the Sigino RD2, so if you have one of those and a Rohloff, please tell me the axle width of your bottom bracket.

« Last Edit: May 19, 2012, 10:51:12 PM by Hobbes » Logged

Andre Jute
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« Reply #1 on: January 15, 2012, 09:23:23 AM »

Andre,

Assuming the same amount of left-side spindle extension past the bearings as Stonglight indicates for the 107mm spindle used to achieve the 43.5mm chainline, then you would need a 117.5mm JIS bottom bracket to get the needed Rohloff 54mm -- if it were still a double.  I get that figure thusly:

54 - 43.5 = 10.5 + 107 = 117.5 to a midline double chainline of 54mm.  "Midline" means halfway, widthwise, between the two rings.

You will be running a single ring so you'll want to make an adjustment for the chainline to run dead center through the middle of that one ring.

Do you wish to mount your single ring on the inside land or the spider, or the outside?  If you value the widest possible tread (Q-Factor), then it would pay to mount the 'ring on the inside, in place of the current inner ring and go with a longer BB.  If you do this, then I would consider the 117.5mm to be a starting point, and add half the difference between the chainring centerlines to the right side of the spindle and recalculate the overall length, again checking the left-side extension remains constant.  SJS describe the Stronglight Impact Compact Double Chainset as intended for "9/10spd drivetrains".  Sheldon Brown's site ( http://sheldonbrown.com/cribsheet-spacing.html ) lists 9-sp Shimano cassette spacing at 4.34mm on-center, and 10-sp Shimano spacing is 3.95mm on-center.  Assuming similar clearance between the chainrings, the average spacing of a 9/10-sp crank should be in the neighborhood of 4.145mm.  Take the 117.5mm we got above, and add 1/2 of~4.1mm (about 2mm) to it if you wish to mount the 'ring on the inside land for a spindle length near 'nuff 120mm to get your needed 54mm Rohloff chainline.  

If you wished to mount your single 'ring on the outside spider lands of the crank, then you'd subtract a nominal 2mm from 117.5mm and you'd get about 115.5mm BB spindle to get your needed 54mm Rohloff chainline.

Again, you'll need to check that left-side offset as well as the right, as not all offsets remain constant even within the same model offerings.  With that caution, I think I've got it right...or close enough to get you headed in the right direction.

Is the crank black in color, as you preferred?  The silver finish is lovely, but it wouldn't take much to refinish it in black, if desired.  Powdercoat, epoxy, or catalyzed acrylic would do it.  For that matter, it isn't terribly difficult to remill a forged crank (pic).

All the best,

Dan.
« Last Edit: January 16, 2012, 04:56:41 AM by Danneaux » Logged
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« Reply #2 on: January 15, 2012, 12:36:01 PM »

Dan, thank you for that. The answer you arrive at squares with the 118mm bottom bracket Stronglight recommends for their Rohloff-specific Impact crank based on the Sugino RD2 (also 107mm/43.5mm nominal axle/road chainline). This has at least one chain guard, so we assume the chain ring is on the inside of the spider's 110 ring.

I've measured on the crank I have. To move the chainring on there from the inside to the outside will cause a displacement of 6.5mm. So, if we work back from the 118mm, we're at 111.5mm

What I don't understand is why we're adding and subtracting these displacements only once, rather than twice, once to each side.


****

About your photograph, if you suddenly see an Irishman with a French name pedalling a highly polished crank reading "Dan Wood", you'll cry in vain for its return. As always, the detailing of your bikes is exquisite.

Yes, mine is polished silver too. Black isn't offered in that model in the Stronglight line. Nobody has had the Sugino one in black in stock since 2008. I will use it in silver for the time being -- there are some ali and stainless details on my bike, and the spokes and hubs are silver, and I wasn't planning on painting the stainless chainring, so it could look good. We'll see. If it looks terrible, I'll have the cranks and chain wheel painted mat or velvet black, not shiny.

Thanks for the help.

Andre Jute
« Last Edit: January 15, 2012, 01:14:47 PM by Hobbes » Logged

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« Reply #3 on: January 15, 2012, 05:28:20 PM »

Quote
What I don't understand is why we're adding and subtracting these displacements only once, rather than twice, once to each side.

A very good point, Andre, and one with direct Impact (sorry) on your application.

I made the adjustments only once -- because I assumed for calculations' sake the bottom bracket would be conventionally asymmetrical, with the left-side extension holding fairly constant and the length adjustments being made on just the right.  If the left side is held constant, then with an asymmetrical BB, any length adjustments must take place on only the one (right) side to get the needed chainline.

However...

If you value symmetry in your tread -- or, as in your case, your frame requires additional offset on the left so that arm and your left heel will clear the wide chainstays needed to accommodate the wider balloon tires you're running -- then a second adjustment must be made for the other side (I was in a similar situation with the full-sus Folder I'm building from scratch; the folded rear triangle rests lying forward across the BB, requiring a spindle wider on both sides to clear it).

In that case, to get everything to clear properly, then -- yes! -- two adjustments will need to be made, and you will have to calculate for the left side as well. Given all components are JIS-dimensioned, I would measure the left offset of your present BB, take the difference between it and the one Stonglight is using for reference, and add that figure to the overall length we came up with, making sure the right side stays where needed (it will likely be listed as a "symmetrical" BB). That should do it.  To recap: I think you'll need ~120mm to get the single inside-mounted chainring centerline mounted to a 54mm chainline with an asymmetrical BB.  Add to that whatever length you need to get the left arm out far enough to miss the chainstay (which implies a symmetrical BB, most likely a 124mm*) and you'll be fine.  You once mentioned you wanted a wide "tread" or Q-factor, so it might be worth punching out the left side for that reason alone.

Or, you could just use a chart.  Wink  Sheldon Brown's site has one here that shows symmetrical equivalents ( http://sheldonbrown.com/bbsize.html ), but please note: Sheldon's chart lists interval measurements for traditional cup and cone BBs.  Those interval measurements will be off for cartridge units but the overall length recommendations and distinctions between symmetrical and asymmetrical types should still hold.   Sheldon also rightly cautions,
Quote
Substituting the 'symmetrical equivalent' bottom bracket will preserve the position of the right crank and chainwheel, but the left side will often wind up farther out...These spindles are getting harder and harder to find.
I suggest, if the proper symmetrical BB cannot be found, go with a longer right-offset and then shift the cartridge leftward using Phil Wood rings or place spacers under the left retaining cup.  It is a bit of a bodge, but works if the needed difference is <4mm or so.

[Later edit] *Confirmed!  Looking at Sheldon's chart, Andre, he lists a 120mm asymmetrical in the "D-Length Overall" column.  The symmetrical equivalent is 124mm, as I suggested above.  Remember, Sheldon's measurements are for cup 'n' cone units, but the overall length and proportions should be okay.  You can still identify a symmetrical cartridge BB; just ensure the spindle is symmetrical by checking that the "A" and "C" measurements past the cartridge bearings are equal (or nearly so). 'Might pay to first measure what you already have.  By happy accident, you might achieve the same same chainline with the new crank on the old BB.

I hope this helps, Andre.  I love this stuff, and am happy to talk about it further if you'd like.

Quote
if you suddenly see an Irishman with a French name pedalling a highly polished crank reading "Dan Wood"...
That means I'd finally see Lovely Ireland in some way!   Cheesy  Thanks for the kind words, Andre!

All the best,

Dan.
« Last Edit: January 16, 2012, 05:13:29 AM by Danneaux » Logged
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« Reply #4 on: January 31, 2012, 02:32:21 AM »

Andre,

If you're still working on the bottom bracket sizing for your Kranich and the new Stronglight crank, there is a more direct approach I have sometimes used...

Assuming your existing and replacement cranks and bottom brackets are all made to the same taper (in your case, JIS)...

1) Try the new crank on your old BB spindle and measure the offset.  By happy chance, it may be correct or nearly so.  Be careful it doesn't drive on too far and cause paint damage to the stays.

2) If the offset is not correct, measure in the usual fashion (to center of seat tube) to see how far it is off.  Add or subtract from the existing right-hand measurement of your spindle, note the workable existing left-hand measurement, and use the results to select a replacement BB spindle.  It is extremely difficult to measure to the center of the seat tube with any precision, so what I do is take the distance from the crank to the side of the tube and add half the tube diameter to get a precise on-center measurement.

Hope this helps,

Dan.
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« Reply #5 on: January 31, 2012, 02:40:35 AM »

It does indeed help, Dan, in that it confirms what I've been doing. I reckon the BB will have to be either 122.5 or 126mm, assuming it is symmetric, so that I get the widest possible tread and, not incidentally, clear the chain stays. But I'm happy to wait until the actual chain wheel I will use arrives, at which point I'll fit it all up again, measure one last time, and order a BB overnighted. The chain wheel has now been en route over two weeks -- British Post is wretched!  -- Andre
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« Reply #6 on: May 19, 2012, 10:48:27 PM »

Eventually I had all the bits and this is how I determined the bottom bracket axle width to put the chain wheel centre line at 54mm to suit my Rohloff HGB:

The previous set up was an Amar crankset with Kinex BB w/ 131mm axle. The new crank was a Stronglight Impact Compact aka Sugino Cospea, and it would run as a single on the outside mounting with a 2mm thick Surly stainless steel chainring. The desired chain line for a Rohloff installation is 54mm, to the centre of the Surly CR as mounted. Assume a symmetrical installation.

What not to do: Don't bugger around trying to calculate from the manufacturer's supplied figures. They measure chain line to thin air between the two chainrings! Don't try to measure in one hit from some notional eyeballed middle of the seat post; you'll get it wrong. Don't try to calculate from the old installation even if you know exactly how many spacers were used behind the lockring of the bottom bracket in making it work; you're just multiplying unavoidable errors and slippages that can easily add up to 2mm and more. For a Rohloff installation, according to people who have Bernd Rohloff's private number on speed dial, you want to be within 1mm max, or try again.

The right way: Measure with the components you want to use. Fit the crank and chain wheel you want to use to the BB you already have. Torque it up to spec. Measure the thickness of the seat post. Divide by two. Measure from the side of the seat post to the centre of the chain ring. Add half the seat post thickness and the distance from the seat post side to the centre of the chain ring. If short of the chain line you want, add twice the shortfall to the axle length of the BB you've in the bike, and that is the axle length you want. If further out than you want, subtract twice the shortfall.



Measuring with an analogue vernier calliper just for Dan, and measuring on the custom-cast trapeze tube lug to the seat tube on my Utopia Kranich because that's the place where we find the straightest measurement, we discover the lug diameter is 33mm, of which half is 16.5mm.



Second data point, measured from side of lug to centre of chain or chain ring, 43.5mm.

Now we can calculate 16.5 + 43.5 = a chain line of 60mm, 6mm more than the 54mm we want. We know the axle of the BB used for these measurements is 131mm, so 131 - (2x6) = 119mm. (Remember, this is measured to the centre of the actual, single chainring in my installation. So Dan was pretty close when he suggested 117.5mm measured to the halfway between the two chainrings.)

Only instantly available (cost-free delivery) 118mm BB is Shimano UN55 from CRC, so that's what I fitted. Normally I fit Kinex at a third the price and all the quality of a Shimano UN54 (and near relatives like the UN55), but postage on a Kinex from SJS or Petra to Ireland would kill the pleasure of knowing about a really good BB at a stunning bargain price.

That brings us to within 0.5mm of the perfect chain line. The thinnest spacer I have is 1mm, which takes us to 0.5mm the other side of perfect. Half a millimetre on such a long wheelbase is plenty close enough.

118mm is also the BB Stronglight recommends for Rohloff installations on their Rohloff-dedicated Impact (nee Sugino RD2) crankset. Ironically, the Stronglight BB is a relabelled Kinex, like the one I took out... What goes round comes round.

Andre Jute
Finished installation:



« Last Edit: May 19, 2012, 11:28:33 PM by Hobbes » Logged

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« Reply #7 on: May 19, 2012, 11:41:19 PM »

Oh! I'm just delighted this all worked out for you, Andre, and I really appreciate the followup. You've made the task a lot easier for those who follow; a really nice contribution and a corker of a photo-essay.  Well done! Nice job on the installation, too.

All the best,

Dan. (Kranich's looking more handsome than ever with the new crankarms and Surly stainless 'ring)
« Last Edit: May 19, 2012, 11:50:39 PM by Danneaux » Logged
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« Reply #8 on: May 20, 2012, 11:02:08 AM »

Andre,
I know it is mostly out of sight, but with such a magnificent machine surely you should be treating it to one of these Grin
http://www.sjscycles.co.uk/royce-titanium-bottom-bracket-with-73-mm-shell-red-all-weather-seals-and-stainless-royce-crank-bolts-119-mm-prod13261/
Julian.
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« Reply #9 on: May 20, 2012, 12:11:38 PM »



Second data point, measured from side of lug to centre of chain or chain ring, 43.5mm.

Now we can calculate 16.5 + 43.5 = a chain line of 60mm, 6mm more than the 54mm we want.
That looks like 34mm to me  Huh This would give you a chain-line of 50.5 which is 3.5mm short of the Rohloff chain-line.


Andre, a seperate question.

What are the two cable housings running up your rear mudguard for??
« Last Edit: May 20, 2012, 01:52:51 PM by il padrone » Logged
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« Reply #10 on: May 20, 2012, 11:03:24 PM »

That looks like 34mm to me  Huh This would give you a chain-line of 50.5 which is 3.5mm short of the Rohloff chain-line.

Well spotted. That is a photo of the wrong way to do it, with the original Amar crankset still on the bike. I'll see if I can find the photograph of the right way to do it, with the new crank and chainring, and substitute it, but it'll have to wait a couple of days as I'm tied up this week.

The numbers in the text are correct: Stronglight Compact (or Sugino Cospea) crank /Surly stainless chainring on outer mount requires 119mm BB to give 54mm Rohloff chainline.

Andre, a seperate question.

What are the two cable housings running up your rear mudguard for??

One is for the BUMM Toplight Line Plus, the other for the Magura hydraulic rim brake. The bike has no seat tube, so all the cables are brought under the bottom bracket

Andre Jute
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« Reply #11 on: May 20, 2012, 11:07:59 PM »

Andre,
I know it is mostly out of sight, but with such a magnificent machine surely you should be treating it to one of these Grin
http://www.sjscycles.co.uk/royce-titanium-bottom-bracket-with-73-mm-shell-red-all-weather-seals-and-stainless-royce-crank-bolts-119-mm-prod13261/
Julian.

Tasty! If only we had a friend at the Lottery Commission, we could claim our Royce BB installation was a work of art. "Where engineering function meets discreet form; the new British Bauhaus."
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