jambon

Hi ,

I am hoping to put some 8 speed rings on a shimano deore xt fcm760 triple crank which is a 9 speed crank I believe.

Will the chainrings be too close together ? as the spacing for 9 is narrower I believe

I plan to use an 8 speed chain.

I do have in my parts box however a 9 speed chain and 9 speed chainrings if the above will really not work .But the cassette out back is an 8 speed.

Thanks for reading ,

J

Amt0571

I used a bike with a 10sp triple crankset with an 8 speed chain/cassette for 5 years and it worked well. It was slightly noisier, but I'm not sure if it happened because of the mismatch or because the whole drivetrain was built from the cheapest parts available.

Andrew R Stewart 

There's usually a fair amount of overlap between 8 and 9 speed rings and chains. perhaps the more important match is with the ft der and lever (then the rear end). If you were staying completely within the Shimano family I suspect the small tolerances and dimensional differences will work out better then if subbing other brand rings. My first choice would be to use the 9 speed rings assuming the ft der and lever are 9 speed family. If they are 8 speed then the 8 speed rings might be a better choice. Either way some ring spacing dimension changes can be had with adding shims behind a ring or two. This is where a relationship with a service focused shop can be a real aid as this can be a bit of trial and discovery and redo type of project (where spacers might go and how many/thick). Andy

djb

Mr Ham, I have twice changed out inner rings to smaller ones on two diff bikes that happened to have the same FSA triple 9 spd crank. In both cases, the shape of the stock FSA 30t was such that when I put the 26t flat rings on, I had to shim them out a bit or else some rubbing was happening. My local lbs at the time only had some very very thin shims, so I first tried some thin , but thicker, washers I had in a washer kit I had. They worked out perfectly, even almost perfectly matching the circle diameter, and shimming them out just right.
Not a 8 spd to 9 spd story, but more of an shim example using regular old thin washers from a kit with multiple diff washer sizes and thicknesses, most likely bought at any hardware store that we had in the house.

djb

hey Andy, did you ever find that book about the crazy Brits and that Africa trip?

jambon

So the 9 speed crank will put the chainrings closer together so that the 8 speed chain will end up rubbing on the rings on more extreme chainlines

ill need to shim or use a 9 speed chain ?

djb

in my case, everything was 9 spd, but the shape of the diff smaller inner chainrings had a slightly diff shape to them, so they were physically closer to the crank, so yes at certain positions of the chain on the cassette, there was rubbing going on.
Basically I could see that I needed to shim the chainring out a bit, and who knows if you will have this, the only way is to try.
I just wanted to say that shimming may work, although I realize that my example only involved the inner ring.

perhaps the more experienced mechanics here will know right away about your specific question of "So the 9 speed crank will put the chainrings closer together so that the 8 speed chain will end up rubbing on the rings on more extreme chainlines"

and you do realize if you go the 9spd route, chain cassette, you'll have to change your shifters to 9 spd.
good luck

Andrew R Stewart 

Yes. "Three men on a Bike" https://www.amazon.com/Three-Men-Bik.../dp/086241542X Andy

djb

Great, enjoy

Drew Eckhardt 

You might need to shim out the big and small rings to limit chain rub with an 8 speed chain. My FSA "9 and 10 speed compatible" crank had unacceptable rub in the last few cogs until I did that leaving no overlapping gears between big and small rings.

Wheels Manufacturing, LeTour, and Origin 8 all make 0.5mm spacers for speed conversions.