Advice on (limits to) changing cassette sought

Hi,

I've currently got a double chainring setup, with a 9-speed 12-25
cassette at the back, everything's 105.

I like the double, and most of the time this is a good setup for me.
However, I'm going to be taking some serious climbs soon, and - London
being as flat it is - feel I may need some more room for maneouvre on
the gears.

So I'm wondering if there are any views from the group on how far I
can go in changing that cassette to cope: Shimano sell 11-32 and
12-34T at the top end. The latter is a little extreme, but the former
would certainly give me confidence that I could get up anything. I'd
envisage it being a temporary switch, so I'm trying to minimise the
expense.

The question is: will my current setup cope with a switch like this,
and if not, how much could they cope with? From reading around I see
'chain length' is one of the things to check: I've got an unfancy HG73
chain, and not much of an idea how to go about measuring it or
identifying what alternative size is needed (if one is indeed
needed)...

Any advice gratefully received,
John

"John L" wrote in message
m...

The question is: will my current setup cope with a switch like this,
and if not, how much could they cope with? From reading around I see
'chain length' is one of the things to check: I've got an unfancy HG73
chain, and not much of an idea how to go about measuring it or
identifying what alternative size is needed (if one is indeed
needed)...


The only things really to worry about are chain length and rear derailleur
capacity (capacity is the difference in teeth between the front rings plus
the difference in teeth between the biggest and smallest cog). . If its a
short cage rear derailleur it may struggle to cope with the full variation
of cog and ring sizes. The chain may not be long enough for the big ring
and the bigger cogs. I would be inclined first to just switch the cassette.
Try going carefully through the range at the rear with the bike on a stand.
If it looks like the derailleur is pulled fully forward and the chain tight
across the bottom run, don't go on to the next bigger cog! Once you know
the limits you can either decide to invest in a new chain/splice in a bit of
extra chain or live without a few of the gears in the big ring (gears that
are almost certainly closely replicated by the upper range of the small
ring). On derailleur capacity, they will usually handle a lot more than
the manufacturers specify so again its worth trying it and seeing first.
Again you can get away with not using the big/big and small/small
combinations as an alternative to a new larger capacity derailleur.

If you are going to go for the not using some gears approach just be
carefull not to use the big ring/big cog forbiddens as they are likely to
damage the derailleur/frame if you try to shift the chain onto them.

Tony

John L wrote:
Hi,

I've currently got a double chainring setup, with a 9-speed 12-25
cassette at the back, everything's 105.

I like the double, and most of the time this is a good setup for me.
However, I'm going to be taking some serious climbs soon, and - London
being as flat it is - feel I may need some more room for maneouvre on
the gears.

So I'm wondering if there are any views from the group on how far I
can go in changing that cassette to cope: Shimano sell 11-32 and
12-34T at the top end. The latter is a little extreme, but the former
would certainly give me confidence that I could get up anything. I'd
envisage it being a temporary switch, so I'm trying to minimise the
expense.


The limits are usually set by:

a) the rear changer capacity. This will differ between changers, long-arm
types are usually much larger capacity than short arm types. One can get
away with a little more capacity than stated on the changer because its
usually unwise to operate the inner ring onto the smallest 2 sprockets, or
the outer ring onto the largest 2 sprockets, so you can ignore those from
the calculations.
b) the chain length. You may find that the chain will be too short having
put a larger cassette, even though the changer will cope. In which case
you'll need some more links for the chain (which effectively means a new
chain unless you can find someone with a few spare links). The chain needs
to be long enough to reach around the worst gear combination (2nd or 3rd
largest rear cog to largest front is my rule).


Things you could change;
larger rear cassette - will give you lower gearing, but with coarser steps.
chain rings - smaller chain rings will lower the overall ratios, though most
double rings limit you to a 39T as the smallest front. If already running a
42 front, then swapping to 39 isn't a huge change.


Get out a gear calculator (sheldon brown website has one) and play around
with the numbers.


I would expect the 12-34 to fit, though you may need a longer chain, and may
have to be careful to avoid the extreme gears (inner ring front to smallest
rear cogs, outer ring front to larger rear cogs).




--
NC - Webmaster for http://www.2mm.org.uk/
Replies to newsgroup postings to the newsgroup please.

John L wrote:

Any advice gratefully received,
John

Its impossible to say what lowest gear you want. I would try switching to a
39 inner chainring first, you can go as small as 38 on a Shimano road
double. The front mech ought to work OK, I have 53/39 which is a 14 tooth
difference as would be 52/38. The you could go with a 12-27 cassette if you
need to go lower. A normal 105 short cage rear mech is happy with this
combination. I have not checked 105 but Ultegra with is almost identical is
not recommended for sprockets bigger that 27 teeth though it will probably
work with 30. Chain length is not critical, not to slack and not too tight
in the most used gears, some tension in every combination will be OK

John L wrote:
The question is: will my current setup cope with a switch like this,
and if not, how much could they cope with? From reading around I see
'chain length' is one of the things to check: I've got an unfancy HG73
chain, and not much of an idea how to go about measuring it or
identifying what alternative size is needed (if one is indeed
needed)...

For safety, chain needs to be long enough to go round the largest
chainring and largest rear sprocket; doesn't really need to be longer than
that. It's difficult to know what length will be required in advance,
although there are formulae - see:
http://www.parktool.com/repair_help/...inlength.shtml

Keeping the chain on the short side, however, helps a rear derailleur work
with larger sprockets and a higher capacity than it was designed for by
pulling the mech down away from the sprocket.

I suggest getting the 12-34 cassette (or something more like 14-32 if such
a thing could be found or made up), try it with existing derailleur and
chain, alter chain length* if necessary, fit a cheap MTB rear derailleur**
if bottom gear not work - these are compatible with road STIs.

A smaller inner chainring would help as well if you have a 40+T but I
doubt it's worth changing for a one-off as they're not cheap and the gear
difference won't be all that big.

* Links could be added from a brand new chain if current chain hardly
worn. Whole new chain will be required anyway if current chain is well
worn. If current cassette goes up to 28T, you might be lucky and have a
chain that's already longer than strictly necessary for the current
cassette (so will wor with larger sprockets).

** Quite likely needed for 32T, highly likely for 34T; maybe not for 30T,
probably not for 28T.

~PB

Pete Biggs wrote:
pulling the mech down away from the sprocket.

I suggest getting the 12-34 cassette (or something more like 14-32 if
such a thing could be found or made up), try it with existing
derailleur and chain, alter chain length* if necessary, fit a cheap
MTB rear derailleur** if bottom gear not work - these are compatible
with road STIs.


14-32 in 9 are not available from Shimano, I am making one for my tourer by
butchering a 11-32 and a 14-25 (when I get a job). Actually I am making an 8
speed on with 9 speed spacings

"John L" wrote in message
m...
So I'm wondering if there are any views from the group on how far I
can go in changing that cassette to cope: Shimano sell 11-32 and
12-34T at the top end.

I borrow the 11-32 cassette, rear mech and chain from my mtb for very hilly
routes (being cheap an' all that).
Reasons a
11-32 to get up steep hills.
MTB rear mech because the road mechs top jockey engages the 32 sprocket
whereas the mtb mechs longer body (main pivot to cage pivot) to overcome
this.
The road mech could handle the increased capacity if extremes are avoided,
as they should be anyway, but the longer cage allows for errors.
Longer chain required for the longer cage and bigger cogs.
(The mtb chain is long enough to fit the bigger chainrings of a road bike as
the road bikes chainstays are shorter).

If you fit a new cassette you ought to fit a new chain anyway. A budget
mtb mech will work as well as a dearer one but will be slightly heavier and
although less robust should be up to years of road use
The mtb mech will work fine with roadie shifters.

--
Regards,
Pete

Many thanks to all who have posted in response here. There's a lot of
genuinely useful information that helps the decision...it seems that
for a one-off there are some good inexpensive options to try out. A
quick glance at ebay (and in my garage, where I have a couple of bikes
that could be cannibalised) would suggest I should be able to get a
configuration that does the trick.

Another naive (but quick) question - if I change the rear mech will
the existing (105) shifters still work ok?

Thanks again to all,
John.

John L wrote:
Many thanks to all who have posted in response here. There's a lot of
genuinely useful information that helps the decision...it seems that
for a one-off there are some good inexpensive options to try out. A
quick glance at ebay (and in my garage, where I have a couple of bikes
that could be cannibalised) would suggest I should be able to get a
configuration that does the trick.

Another naive (but quick) question - if I change the rear mech will
the existing (105) shifters still work ok?

Thanks again to all,
John.

Yes providing you have the same number of 'speeds'. Shifters, whether bar
end, downtube or STi dictate how far the mech moves per click[1]. This
distance of course must be the same as the spacing of the sprockets. The
mech itself will work with just about any combination of levers and
cassette. THemech must be able to work with the required largest sprocket
though.

[1]The mech has a multiplier which translates cable pull into sideways
movement. This value is the same for all Shimano mechs. and has been for
years, except for Dura Ace 8 speeds apparantly. As an example I have a 2003
9 Speed XT rear mech in use with 1988 Shimano 107 6 speed dwontube levers
and cassette. From what Iearn on this NG and rec.bicycles.tech Campagnolo
is not as tolerant to mixing and matching old and new parts

"MSeries" wrote in message
...
John L wrote:
Many thanks to all who have posted in response here. There's a lot of
genuinely useful information that helps the decision...it seems that
for a one-off there are some good inexpensive options to try out. A
quick glance at ebay (and in my garage, where I have a couple of bikes
that could be cannibalised) would suggest I should be able to get a
configuration that does the trick.

Another naive (but quick) question - if I change the rear mech will
the existing (105) shifters still work ok?

Thanks again to all,
John.

Yes providing you have the same number of 'speeds'. Shifters, whether bar
end, downtube or STi dictate how far the mech moves per click[1]. This
distance of course must be the same as the spacing of the sprockets. The
mech itself will work with just about any combination of levers and
cassette. THemech must be able to work with the required largest sprocket
though.


Yeah, I even have a long cage XTR 'rapid rise' rear mech with 105 STI
shifters - works fine. I'm saving it for the day when I put an 11-32 MTB
rear cassette on with my 52/42/30 105 triple front for use on some
luggage-laden big hills!

Rich