Shimano 9/10spd rear mech on an 8spd cassette?

[Landslide]

I want a new rear mech. According to this article, all current Shimano mechs have the same shift ratio, however it notes that whilst 9spd mechs will accept an 8spd chain, it makes no mention of running an 8spd chain with a 10spd mech.
Is this because the jockey wheels and cage are thinner?
Could I fit 8spd jockey wheels to a 10spd mech, thus making it compatible with 8spd chain?

 

[fossyant]

Just run a 9sp chain - works fine with 8 speed. The only mech that is different on cable pull was the Dura Ace 7400 mech.

I run a 9 speed chain on my 8 speed set up. I'd get a 10 mech and run a 9 speed chain.

 

[jayce]

I want a new rear mech. According to this article, all current Shimano mechs have the same shift ratio, however it notes that whilst 9spd mechs will accept an 8spd chain, it makes no mention of running an 8spd chain with a 10spd mech.
Is this because the jockey wheels and cage are thinner?
Could I fit 8spd jockey wheels to a 10spd mech, thus making it compatible with 8spd chain?

i ran a ten mech ultegra on a seven speed that worked with a 8 speed chain

 

[Radius]

i ran a ten mech ultegra on a seven speed that worked with a 8 speed chain

 

[andy_wrx]

8sp, 9sp 10sp chains are practically the same internal width, just got externally narrower by changes to the rivets and sideplates, so there's no worry with them fitting on the jockeywheels, the issue would be them rubbing on the outside cage.

But I run a 10sp 105 short-cage rear mech, 9sp Ultegra front mech, 9sp 105 chainset, 8sp SRAM chain, 8sp cassette and bar-end shifters on my TT bike.

The front mech needs a bit of trimming as I shift up and down the block, but the rear mech is fine.

As Fossy says, a 9sp chain would work fine too, although I suspect a 10sp chain might be a bit narrow and not shift so cleanly at the block.
- can't say I've ever tried though, as a 10sp chain is dearer and being thinner won't last as long, not that that'd be an issue on my TT bike...