Thorn Cycles Forum
Technical => Transmission => Topic started by: vinalopo on March 27, 2011, 12:22:21 PM
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Good morning,
I am new here. Please apologize the long post.
I am willing to add a custom Thorn Audax to my bicycle family and I need advice in putting together the specifications. To be said that I am a DIY guy and I wish to put the bicycle together by myself also sourcing several places / re-using parts.
The bicycle will have bar end shifters and 27 speeds (if 30 would be possible even better) and would be used to train and cycle with friends and take part to audax (mainly 200-300 km) events. I would possibly try to take part to longer audax events up to Paris - Brest - Paris or Mille Miglia.
Front:
I wish to install a Stronglight impact triple chainset, it has a 45 mm chainline, the manufacturer advice using a 115 mm JIS bottom bracket. To me this chainset make sense for the combination quality, price, weight, availability in several chain rings sizes.
I am not such a pro to appreciate external bearing / plenty old square taper bb stiffness difference but have to admit I didn't have make huge use of external bearing cranksets to appreciate the difference, advice is welcomed.
Rear:
I wish to use an MTB rear derailleur to have the possibility of using MTB cassettes. I was thinking in a Shimano LX or XT to be used with Shimano bar end shifters (cheaper than SRAM and proven excellent quality)
Shimano MTB rear derailleurs have 50 mm chainline.
Wheels:
I would use both my current road mavic aksium wheels, both custom MTB hub / dynohub equipped wheels. The latter not yet put together so I could change opinion and use road hub if it make sense. Advice welcomed.
Issue:
Chainline:
does it make sense put a 10 mm wider spindle to match front and rear chainline? UN54 Shimano BB is not available in 125 (115 spindle bb would give me 45mm chianline according to manufacturer spec): better a 122 or 127 mm spindle BB?
Front derailleur:
What derailleur to use? MTB one? I see that thorn sjs use tiagra front derailleur, why?
Rear derailler: are you aware of any 9 or 10 speed road (or otherwise 45 mm chianline) long cage rear derailleur that could be used in combination with 11-32(4) cassettes and would save me chain line issues?
Wheels:
would mixing road and mtb hub wheels create chainline issues or is fine?
Thank you very much!
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Using the Stronglight crank with a road wheel should be fine.
As you say, the chainline for the crank is 45mm, which is the same as Shimano road triple cranks.
The rear derailleur doesn't have its own chainline, it hangs off the drop-out, so its distance from the centre-line depends on the wheel width. The chainline is determined by the hub and cassette.
For a road wheel (130mm) it will be about 43-45mm, depending on the exact spacing between the smallest sprocket and the locknut face. A mountain bike wheel is 5mm wider, so the chainline will be 2.5mm larger.
There is a small difference between 9 and 10 speed cassettes, but it's in the noise.
On a road bike, I don't use the smallest chainring much, so a couple of mm smaller chainline on the crank is actually an advantage, as it means the biggest ring will have marginally better alignment.
Chainline differences of a few mm don't matter really, if you think about the misalignment when you are using either extreme of the cassette. The cassettes are over 30mm wide, so you will be seeing 10mm+ misalignment, even if you avoid cross-chaining.
Other questions:
Road and MTB front derailleurs have different pull ratios, so you must use a road derailleurs with road STI levers. Doesn't matter with bar-ends, as they are not indexed on the left.
You can use 9-speed MTB cassettes with road shifters. I'm not sure about 10-speed, I think they may be incompatible.
The latest Shimano 10-speed groups have an 11-28 cassette, so their road derailleurs can cope with that. There is also a MTB 9-speed 11-28 cassette, which I find works well with a 48-36-26 crankset.
Otherwise you can use MTB rear derailleur. The latest "Shadow" models don't have a barrel adjuster, though, so you would have to improvise something.
Bar-end shifters are very forgiving, and you can always turn off indexing if it plays up.
I'd be a bit more inclined to go for a 9-speed setup. The tolerances are a bit fine on the 10-speed, and the chains are more finicky. I've never broken a chain on 9-speed, but broke my new plastic bike's chain the first time I went up a real hill. Probably a shoddy job by the bike shop, but I still had to call for help.
Some references on chainline:
http://www.sheldonbrown.com/chainline.html (http://www.sheldonbrown.com/chainline.html)
http://www.sheldonbrown.com/bbsize.html (http://www.sheldonbrown.com/bbsize.html)
http://www.parktool.com/blog/repair-help/chainline-concepts (http://www.parktool.com/blog/repair-help/chainline-concepts)
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Thank you very much for your detailed explanation, I really appreciate your help, you've been able to clarify me all the doubts.
I was convinced bar end shifter were indexed also for the front derailler.
I think I will go for a 9 speed MTB rear mech, it seems to me too more reliable and having ridden some thousands km also fully loaded with a 9 speed LX without braking chains or suffering issues make me confident.
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Have been running a mix of 9 speed Shimano road/mtb drivetrain on my Thorn Audax since January and it has worked together perfectly so far with no fine-tuning needed after lots of wet/muddy rides, and shifts well under load.
Had been running with Campag 9 speed spares I already had for my other road bike (bought the Mk3 frame and built it up last August), but wanted to put on a tougher mtb-hubbed rear wheel to cope with touring with panniers and wet weather (front and rear bearings on the Zenith hubbed wheels it had seized up and needed replacing after doing the Dartmoor Devil audax last year). As the Thorn brochure says, "We have long favoured the advantages of 135mm…you get a wheel with less dish and MTB hubs have better seals than road hubs…there are no team cars in audax events!"
Changed the Ambrosio Zenith hubbed rear wheel to a LX FHT660 hubbed Sputnik rimmed wheel (http://www.spacycles.co.uk/products.php?plid=m2b0s176p0) with an 11-32 Deore HG50 cassette, HG53 Chain, Deore M530 low normal (http://www.sheldonbrown.com/gloss_l.html) rear mech, Tiagra FD4503 Triple Front Mech, Dura Ace 9 speed downtube shifters (http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Dura-Ace/ShiftLever/6J50B-SL-7800_7700-EN_v1_m56577569830605247.pdf), kept the 53/42/30 Campagnolo Chorus Triple Chainset with 111m Veloce square-taper BB.
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Dont know the chainline or anything, I have a deore chainset on my thorn audax. I have an ultegra 9sp cassette and (now, not in this pic) deore r.mech. The f.mech is some ancient shimano. Bar-end shifters, still all work well. The chain is pretty straight on the "big" ring to 4th (largest) sprocket. This is my usual gear so I wanted straight chainline for this. The wheel is 135 OLN
(http://farm4.static.flickr.com/3484/3295249966_e980a55be8.jpg)
http://<a href="http://www.flickr.com/photos/24775321@N02/3295249966/in/set-72157614151142918/ (http://<a href="http://www.flickr.com/photos/24775321@N02/3295249966/in/set-72157614151142918/)