New Derailleur

[Jimmy Welch]

Hi Guys
Fitting a new derailleur for my pal , his cassette is as 105 28 11 going to fit a Ultegra what size cage do I need ?

 

[Dogtrousers]

Hi Guys
Fitting a new derailleur for my pal , his cassette is as 105 28 11 going to fit a Ultegra what size cage do I need ?

What chainrings does he have?

Work out (Big ring + big sprocket) minus (little ring + little sprocket). That's the required capacity of the mech.

Look up possible mechs, they must have a capacity of at least that.

 

[Jimmy Welch]

Front 50/34
Rear cassette 28/11

 

[Big John]

I won't argue with Dogtrousers but if you Google how to work out the rear mech capacity you'll discover it's different to how they've suggested. It's (large chainring-small chainring) + (large sprocket-small sprocket). In your case that'll be (50-34) + (28-11) = 33. If you go with what has previously been suggested the number would be (50-28) + (34-11) = 45. A bit of a difference.

 

[Cycleops]

This should be the one:

https://bike.shimano.com/en-EU/product/component/ultegra-r8000/RD-R8000-GS.html

 

[All uphill]

I won't argue with Dogtrousers but if you Google how to work out the rear mech capacity you'll discover it's different to how they've suggested. It's (large chainring-small chainring) + (large sprocket-small sprocket). In your case that'll be (50-34) + (28-11) = 33. If you go with what has previously been suggested the number would be (50-28) + (34-11) = 45. A bit of a difference.

You might want to check your reading of the two methods; both give the same answer. 33.

 

[T4tomo]

This should be the one:

https://bike.shimano.com/en-EU/product/component/ultegra-r8000/RD-R8000-GS.html

the correct answer is either the SS (short cage) or the GS (medium cage) linked above. (assuming its 11speed)

The SS will do a recommend max diff of 35T and up to 30T on the rear. The GS will give you the option to fit a 34T rear sprocket if needs be and up to 39T diff.

 

[rrarider]

You need a medium cage to give you that 33 capacity

 

[rivers]

Short cage will be fine. That's the set up I have on both my road and TT bike and both have a short cage derailleur. My gravel bike also had a 50/34 and an 11-30 on the rear originally. It no longer does, and I have a spare 105 R7000 short cage derailleur going spare if you're not set on ultegra.

 

[Jimmy Welch]

Short cage will be fine. That's the set up I have on both my road and TT bike and both have a short cage derailleur. My gravel bike also had a 50/34 and an 11-30 on the rear originally. It no longer does, and I have a spare 105 R7000 short cage derailleur going spare if you're not set on ultegra.

Cheers mate , it's a pals bike I'm doing it on and he's set on an Ultegra as what he's always had on it , what are the advantages of the shorter cage

 

[rivers]

Cheers mate , it's a pals bike I'm doing it on and he's set on an Ultegra as what he's always had on it , what are the advantages of the shorter cage

There's no real advantage or disadvantage to short or medium cage derailleur in your friend's case. Both will work with an 11-28 cassette. a short cage will officially take up to a 30 tooth big cog (although I was running a 32 with mine) and a medium will officially take a 34 tooth big cog. So in the current climate with parts still potentially hard to get a hold of, go with what you can get.

 

[Gunk]

Why not just fit like for like, much less hassle

 

[Jimmy Welch]

Why not just fit like for like, much less hassle

Because I've got a feeling it needs to be a bigger mech when it's in both smaller front and back (I know it shouldn't be ) the chain drags on the top of the chainstais

 

[figbat]

Because I've got a feeling it needs to be a bigger mech when it's in both smaller front and back (I know it shouldn't be ) the chain drags on the top of the chainstais

Maybe the chain’s too long?

 

[Ming the Merciless]

Because I've got a feeling it needs to be a bigger mech when it's in both smaller front and back (I know it shouldn't be ) the chain drags on the top of the chainstais

Shorter chain will sort that.