Rear derailleur question

[All uphill]

I have an old bike that I enjoy modifying and testing ideas on.

It currently has an Acera RD and 7 speed cassette on an 8 speed hub, with spacer.

My parts box has nine speed shifters, cassette and chain. I think I should be able to put these on the bike without problems, but I notice the Acera is described as 7/8 speed.

My question is what would prevent it working with a 9 speed set up?
If I need to get hold of a 9 speed RD I'll do that, but am not sure why I would need to.

Thanks in advance for your insights.

 

[PpPete]

Not sure a 9 speed cassette will fit on an 8 speed hub, but if you can get it on, try and see if you can get it indexed across the range of the cassette.

 

[Ajax Bay]

The shift ratio of Shimano 7, 8 and 9 sp RDs are the same (1.7) so should be good to go.
CycleUK Edit: Chris Juden - "widely respected in the bike world."
"The clicks are in the shifter and that is the thing that determines the number of speeds, by pulling or releasing a certain amount of cable (and of course it has a corresponding number of clicks). The mech provides a conversion or shift ratio that is the same, road and mountain alike and regardless of how many speeds it says on the box. All that really indicates is how precisely the mech is constructed. More speeds means a closer pitch and that demands more precision to avoid the chain rattling against the next sprocket. So the difference is mostly just a matter of shifting accuracy and a 9-speed mech can be used in a 7-speed system."
PS (after Pete's comment): Your 8sp hub will be happy with a 9sp cassette and also with a 10sp (with 1mm spacer).

 

[All uphill]

Thanks @PpPete and @Ajax Bay

It makes good sense that my basic Acera RD might not be precise enough to smoothly deal with 9 speeds, unlike something fancier. I'll try it!

 

[avecReynolds531]

We have the Acera (M360) derailleur on our bikes. Tend to think it's highly underrated as a derailleur. It shifts beautifully with 7 & 8-speed cassettes.

Haven't tried with 9-speed indexing. Grant Peterson - widely respected in the bike world - has this to say at the (US) Rivendell website: 'It friction shifts five through nine speeds, probably even ten, but five thru nine for sure, and is so cheap and all metal, and still under 300g and under $30. As long as you don’t have to index 9sp, it’s the best all-metal derailer value in Shimano’s line and the world...' *

*https://www.rivbike.com/products/derailer-rear-shimano-acera-m360

 

[DCBassman]

Thanks @PpPete and @Ajax Bay

It makes good sense that my basic Acera RD might not be precise enough to smoothly deal with 9 speeds, unlike something fancier. I'll try it!

It will be fine. I've used Altus RD-M310 and 410 for 8 and 9 respectively, with zero issues. The Scott roadie currently uses an even older Altus for 8-speed. If you do go looking, Acera RD-M4000 is a good 9-speed.