XT 739 running on 9 spd?

I just caught a forum post elsewhere suggesting someone was using 739 shifter ( 8 speed) with 9 speed derailleur. I realise this was actually because the derailleur has no control, so to speak, but HAS anyone found a cunning ruse for running 8 speed Shimano shifters with a 9 cassette on the rear?

I'm presuming also that the type of chain (8 or 9 speed) is dictated by whatever the front derailleur spec is.

might be wrong, but...

Run an 8sp shifter and 8sp cassette with a 9sp mech, no worries, 8sp all the way

run a 9sp cassette with an 8sp shifter, you get 8 speeds, could work ok but might need careful adjustment due to cable pull between indexes.

8sp or 9sp chain is close but just a bit different and its more the gap between the cassette than the chainrings that will cause a problem, ie 8sp chain is slightly 'wider' so wont engage well with 9sp cassette.

An 8spd shifter from any manufacturer will not work on a 9spd cassette - the cable pull is wrong and the chain will jump all over the place. you may get about 4 or 5 out of the 9 to 'index'. Just fit an 8spd cassette??

As for 9spd front mechs, they also have a different cable pull and reach, sometimes within the same groupset (2005 XT to 2011 XT etc etc blah de blah) and have to be matched carefully to the shifter otherwise again, indexing will be all over the place.

The chainset doesnt matter so much as the spacing isnt too bad from 8 to 9 but 9spd front mechs require 9spd chains unless you like the clattering of the wider 8spd chain on the mech cage.

A good solution is to use 9spd chains on EVERYTHING. They work perfectly well on 6/7/8/9spd systems allowing the use of 9spd front mechs (with suitable thumb shifter or correct 9spd shifter) and 9spd rear mechs on 5/6/7/8/9spd setups (I'm running 9spd Ultegra on a 5spd freewheel, its works marvellously).

The narrower 9spd chain exterior indexes just that bit smoother (the interior is the same as 7/8spd chains) and even helps a bit up front with front shifting - depending on what you use.

You can fanny around with the cassette and spacers but then the 9spd cogs are that wee bit narrower than 8 and it all goes mad again...

Just buy an 8spd cassette

or run friction thumb shifters

Or a brand new bike.

I used an 8 speed shifter on a 9 speed cassette by accident (always check what you ordered is what gets delivered)

I spent a good hour and a half trying to figure out why it'd go two gears during one of the shifts. Even rode it about 10 miles. Other than going two sprockets for one click once in the block it was surprisingly good.

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Understand it not quite the question but I had a 9 speed shifters, 9speed cassette and m739 rear mech set up. Worked well

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No it wouldn’t work, the 8 and 9 speed cassettes are the same overall width, so the spacing between sprockets is smaller and the cable pull will need to match that spacing. No reason that the derailleurs and chainset wouldn’t work though, would just need Shifters, Cassette and Chain

thanks all - glad to have that confirmed!

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A wildcard. Old 8 speed Suntour thumbies work well with a shimano mech on a 9 speed block. Less cable pull per shift. You can get all the gears too.

So will DX and XT and any thumbie set to friction. But there'd be no indexing - thats fine for non ramped cassettes but with hyperglide its not the best.