Mixing 8 and 9 speed components

[Rouxenator]

Until a week ago I had an 8 speed setup (cassette and shifters) but with everything else 9 speed. So its Deore RD-M591 at the rear, XT FD-M770 at the front and a FC-M590 crank. All worked like a dream until the crank fell apart and I went back to a 8 speed Alivio FC-M411 crank.

 

The problem is that now the front derailleur needs the limiters set to their extremes to even reach the granny and highest chain ring. Apart from that the chain scrapes against it when in the top and lowest two gears at the back.

 

Could this be because the chain ring spacing on the 8 speed crank is a bit wider since it is for 8 speed chains and therefor the 9 speed derailleur cannot handle it? It did work fine with the 8 speed chain on the 9 speed crank...

[David Marshall]

My understanding is that you would have to have replaced the BB as well. This could mean that the chainline has changed significantly. You may need a longer or shorter BB. The chain ring sizes may also be different on the two cranks so make sure that the FD is positioned correctly ie the correct distance above the large ring and rotated optimally on the seat tube. This can be tricky to set as any adjustment means resetting the limit screws. Hope this helps

[Rouxenator]

You are 100% spot on about the chain line. I Googled around a bit and found that for most triple blade MTB the chainline should be at 47.5-50 mm at the front. I measured mine and its 57mm.

 

Any idea on how I can change that? can you simply have them make the BB shorter?

Edited September 11, 2013 by Rouxenator

[David Marshall]

The BB shaft length varies from (If I remember correctly) 110mm (for road two blade cranks) to about 128mm for the longest on a MTB, going up in 3 to 5mm increments. You need to specify the length when purchasing the BB.

[Rouxenator]

Seems they fitted the wrong one when I reverted back to an Alivio crank. I'll call up the LBS to see if they can get a shorter one.