tech question - rear cassette
steve68 Posts: 18
in General Chat
I have a 50/36 compact with a shim 105 rear mech and a 9 speed 12-25 cassette.I would like to drop a 12-27 or similar onto the back to help cope with Lake District hills, particularly in the early season when HR is supposed to be kept low, Is this possible? Or would I get a better result with keeping the cassette and using a 34T inner ring. The front mech is a tiagra.
Get out on your bike, you will be fine using a compact.
If you want to change the cassette do so....get the 27 tooth out.
Stay seated on climbs as much as possible, this will keep your HR as low as it will be for that particular hill and it will improve your climbing. If you stand up then your HR is rocket and youll be wasting energy.
But saying that If the gradient is so steep you have to stand then do so.
According to http://www.sheldonbrown.com/gears/a 36/27 will give the same benefit on a climb as a 34/25.
AFIK the Shim 9 speeds are as follows:
12 - 13 - 14 - 15 - 17 - 19 - 21 - 23 - 25
12 - 13 - 14 - 15 - 17 - 19 - 21 - 24 - 27
So except for the bottom 2 ratios in practical terms by going for a 36/27 you sacrifice nothing on the flat or gentle climbs but gain an advantage in the climbs that is only marginally different to a 34/25.
What you may also want to look at if you do get the 12-27 is to get a 39 then you would have a demon set of ratios you can play around with for flat or hilly courses e.g. 39/25, 39/27, 36/27, 36/25 - just a thought, have a look at the combos on the gear claculator. Oh yes and don't forget you may have to look at the chain length http://sheldonbrown.com/derailer-adjustment.html#chain