Front Mech Chainline Curiosity

jedi_master

I have a question out of curiosity. While looking through some tech docs on Shimanos site I was looking at some of the front mechs and they would say applicable chain line 50mm and some had applicable front chainwheel 44T/48T or just 44T, is the applicable chain line what the mech works best with or is it the maximum chainline you can have for the mech to work?? Same with the crankset?

Thanks

supersonic

Have a read of Sheldon and chainline.

nicklouse

it is what chain line it works with.

and what size chin ring it can work upto.

so if you have a 52mm chain line and a 48T big ring you dont want a 50mm 44T front mech.

jedi_master

nicklouse wrote:

it is what chain line it works with.

and what size chin ring it can work upto.

so if you have a 52mm chain line and a 48T big ring you dont want a 50mm 44T front mech.

I see, so 44T/48T it will work up to both I take it. With the chainline, if its 50mm will it work with a chainline under 50mm? or only 50mm?.

SS, going to read the other chainline info on Sheldons later.

nicklouse

wellllll

new and old design.

old chainline and mech thoughts were 48.5 and 50mm and now a lot are 50 to 52mm

bike design determines what you need.

jedi_master

I see. My brothers bike has a 48mm chainline I believe, so if he was to change his front mech to a newer one that states 50mm would he need to change the bottom bracket with a wider axle to get 50mm.

Thanks for the info.

nicklouse

it should work with no problems.

which mech are you looking at?

jedi_master

Not actually changing the front mech it was more of a hypothetical query more than anything. The tech docs I looked at though was the SLX M660 and XT M770.

DaveyL

I think I've just encountered a chainline problem. I have a 2002 Specialized Rockhopper, which was fitted with a Deore LX groupset.

I've just bought an SLX crankset and rear mech as the bike was starting to need some upgrades, but have kept the Deore front mech.

I fitted (as per the Shimano SLX crankset tech doc) 2 x 2.5 mm spacers on the drive side and one 2.5 mm spacer on the non-drive side when fitting the Hollowtech II bottom bracket (it is a band-type mech and a 68 mm bottom bracket).

Now, the from mech doesn't shift up to the big chainring, and I reckon the SLX is designed for a 50 mm chainline while my old Deore was 48 mm (it looks like 48 from a rough measurement after reassembling the square taper BB and Deore cranks).

I guess I should just be able to take the cranks off again and remove a space from either side. Or buy a nice new SLX front mech to complete the set

Just wondering if anyone has any comments as to whether the 2002 vintage Deore was definitelty 48 mm, and whether my new set-up will be OK (just by removing a couple of the spacers) ?

nicklouse

DaveyL
you have wound the stop screw right out?

if you cant get the mech adjusted then disassemble the bb and swap the spacers around.

DaveyL

Not *right* out, no, but I did tinker with it a fair bit. Then the fact the chainrings were in quite a different place compared to the old ones that got me thinking about the chainline.

It won't be too much of a faff to take the cranks off - I need to take a dodgy link out of the chain anyway, so will take the spacers out at the same time.

Can't believe the difference in weight - the old Deore cranks + BB were just over 1.3 kg, and the new cranks + BB are about 700-800 g.

supersonic

Fiddling with the gears should solve it - reset the L stop, set tension, set H stop. Might need to be higher too if the old cset was a 42 toother.

It weighs 889g.

DaveyL

Thanks - I did get it sorted with a bit of fiddling with the front mech position and setting up the gears again (was changing the gear cables over as well).