brron

Hey Guys,

So I was shopping for a new rear derailleur for my 8 speed bike and noticed that Shimano 105 10-speed rear derailleur is said to be 9 and 10 speed compatible. But it's bigger brother, Ultegra SL, is showing 8/9/10 speed compatible?

Is this a typo or is it true? Or is there anyway I can get the shimano 105 10s rear derailleur to work with my 8s cassette?

Thanks for ur help.

txvintage

Yes.

AngryScientist 

will work fine

Psimet2001 

It'll be shifty, but I take it that's what you're going for.

JTGraphics

YOJiMBO20

I see what you did there.

NickDavid

No, you will over shift, thus pushing the derailleur directly into your spokes, destroying the wheel and derailleur.

Psimet2001 

Careful throwing stuff like that around....someone might think you're serious.....and if you are serious.....sorry but that would be incorrect.

NickDavid

....

Psimet2001 

....so the cat is incorrect too?

NickDavid

no, i know it's fine.. he didn't say anything about the shifters so it should be fine.

c0urt

it is fine, just make sure you are using friction shifters.
if you are trying to use sis, and you get it calibrated, I think you will make the bike forums hall of fame.

caloso

No problem. Just get a dork disk. NB: It must be an 8-speed dork disk. Otherwise you'll really screw things up.

akansaskid

OP: Try the Bicycle Mechanics forum if you want fewer snarky replies.

The shifter determines how much cable to pull in or let out between shifts. It needs to match the cassette. (8speed to 8 speed). The derailleur will move correctly from cog to cog in accordance to the cable change commanded by the shifter. Be sure to adjust the RD limit screws so it aligns correctly at the largest and smallest cogs. Also ensure the derailleur has the capacity for the high-low spread your cassette has - e.g., the SS vs. GS derailleurs in the Shimano line.

Psimet2001 

Maybe my sarcasm alarm isn't set up today but again....this doesn't matter.

The OP mentions that a new RD is needed for his 8spd setup. Asked if a 10spd rd would work. The RD throw is the same on 8, 9 or 10spd Shimano RD's except for DA 8 spd 7600 and DA 7900. Throw being the ratio of movement of the shift lever to the distance traveled by the derail pulleys. The absolute width of the lateral movement is the same for an 8spd setup as a 10 speed setup. 10 just squeezes 2 more cogs into the same amount of space - reducing the spacing between cogs and requiring the use of a narrower chain. In essence then there is complete interchangeability between different RD versions as long as they have the same throw. 8spd, 9spd, and 10spd just become monikers.

Th OP can put the 10spd derail on and connect the cable, set the limit screws, and adjust the system using the same technique as if it was an "8spd" RD. it will then shift and perform just like it should. If there are any issues the OP can always change the chain out to a 10spd chain.....but why would you want to run a weaker chain?

scorpio516

More than likely not the case, especially with a 105, but a 8 speed chain has an outside width of 7.1-7.2mm
A 9 speed chain is 6.8mm wide
A 10 speed chain is 6.2mm wide

If the RD was built with such crazy tolerances (less than 0.3mm from a chain), then you'd have a problem. BUT if someone designed a mass market RD to have at most 0.15mm on each side of a spinning chain... Thats thinner than a 29lb sheet of paper (fairly standard "heavyweight" office paper)

Psimet2001 

Correct. You can usually get away with it without a problem, but i have had some issues with chain rub when I utilzed certain 9spd chainrings on 8 spd cranks using 8spd chains, etc.....stepping up the chain to the next narrower size will almost always alleviate the issue and can be used on any "wider" drivetrain.

Epicus07

I have shimano 105 STI Shifters 8 speed with an 8 speed cassette and ultegra 6600 (10 speed) rear derailleur. My shifts are crisp and definitely smoother than the NOS 8 speed 105 R.D.

Go for it!

JTGraphics

I didn't know that made 105 8 speed STI levers are they not Shimano R500 STI levers?

DieselDan

I must be really screwed using a 10-speed 105 dérailleur with quasi-vintage 7-speed and RSX brifter.

Epicus07

They are just like these. its hard to make out the 105 on the plates but i assure you its there. 8 speed

https://cgi.ebay.com/SHIMANO-105-BRAK...item255823f267

brron

thanks for the help guys, i can always count on bf

psimet,

you're saying there's a chance i need to change my 8 speed chain to 9 speed chain to avoid rubbing? is that correct?

Spinz

It will work just fine. Set your hi and low limits as you normally would and your shifters will do the rest. As previously stated you may need a narrower chain. Lp