We've recently had a lot of questions from viewers
asking how they decide whether they need
a short, a medium,
or a long cage rear derailleur
And also, whether they need to change it out
when they change the ratios of the cassette
or the chainrings at the front.
Well, we've been doing some research here at GCN
so hopefully in this video we will answer those questions.
First up, though, let's explain what the derailleur cage is
and also what it does.
So this is the cage, just here
on the lower part of the derailleur, and it
houses these two jockey wheels.
On a short cage derailleur like this one,
the jockey wheels will be quite close together
and on a longer cage one, they will be much
further apart. Now these cages
have a spring on them, and that is to keep
tension in the chain. Without that
spring, there would be a lot of chain
slack down here, which means as you're riding along
this chain could be flapping into
your wheels, it will probably come off the front chainrings
and at the very least, you will
not get crisp changes here
at the rear sprockets. So let's show you
what it does by putting it onto
the small chainring and the
smallest cog at the back.
As you can see, the hanger's gone
right back here as there's not much tension
in the chain itself.
Conversely, if we go right up to the big ring at the front,
and all the way up
to the biggest cog at the back
you can see there's a lot of tension
in the chain, so that cage has had to move
right forward. Now the longer
the cage, the more of a difference in ratio
between the smallest and the biggest sprockets
and the smallest and the biggest chainrings,
your rear derailleur will be able to take.
Now that's why almost every mountain bike
has a long cage rear derailleur because
in general they have wider gear ratios
especially here at the cassette
but also sometimes at the front, still,
when they're running triple chainrings.
So, in order to decipher which is the proper
rear derailleur for your particular
bike and gear ratios, you first
need to decide exactly how much of a ratio
you've got on both the front chainrings
and also at the cassette.
So here at the front we've got 52 and
and 36 chainrings, that's a differential
of 16. At the back we have
11 up through to 28
which is a differential of 17.
So total them up and you've got a differential
total of 33.
Yes, maths has always been my strong point.
Having worked out our total differential,
what we then want to do is find out
the capacity of our rear
derailleur. Now that often is a case of
looking at the instruction manual which came with
your rear derailleur or with your bike when you
purchased it. If you've mislaid
that, then you should be able to find it by looking the model up
online, and some companies
such as SRAM actually mark down
the ratios that each of their rear derailleurs
is compatible with, both in terms of chainring sizes
and also in terms of cassette sizes.
What you want to find is that the
capacity of your rear derailleur is
at least as much as the differential
that you calculated earlier on in this video.
So then, for example,
if you want to change your rear
cassette size or your chainring size, maybe something
monstrous like this, which is 11
up through to 36,
this will then give us a differential at the back
25, plus the 16 which we've
still got at the front, that's 41.
And that probably means that we're going to have to
change our rear derailleur if we want
to be able to use that.
Now a couple more things to take into consideration:
Firstly, chain length is
also important. If it's too short,
and I'll give you an example here,
it pulls the rear derailleur so much
that actually it won't physically be able to get it up
onto the biggest sprocket at all.
Secondly, there's also
a maximum sprocket capacity
on all rear derailleurs, and that means
the maximum it can have in terms of your biggest
sprocket here at the back. And that
is because of the distance that they can get
the top jockey wheel away from the frame.
If it's not enough, then it's again
physically won't be able to get it onto a very
big sprocket here at the back.
Now, lots of your are probably wondering
why you can't just use a long cage
rear derailleur for all gear ratios.
And that is a very good question.
Firstly, it will be slightly heavier.
That might not matter much to many of you.
And secondly, some people say
that the shifting there at the rear cassette
isn't quite crisp and precise
as if you had a shorter cage one.
But in all honesty, it probably won't make too much difference
and yes, you would get away with it if you really wanted to.
Anyway, hopefully that will explain
what you need to do. And if you change over your chainrings
or your cassette, you'll just need to do
that calculation all over again,
and make sure the capacity is at least
as much as the total differential.
Now, a couple more videos that you might
be interested in now, just up there
Si talks you through how to know
when to change your chain, cassette
and chainrings. And just down there,
we show you how to change
your cassette.
Oh, and don't forget to subscribe to the channel, it's free, just click on the globe, which is here.