11. Gearing to climb steep hills
Lots of cyclists intuitively know that -
(i) after climbing a hill, when they want to accelerate down the other side, they shift to their biggest chainring on the front, and the smallest on the back (both on the outside); and
(ii) when confronted by a steep climb, they do the opposite, by shifting to the smallest crank on the front and the largest on the back (both on the inside).
However, many of these cyclists have never thought through the simple arithmetic of "division" which prompted those gear selections.
Gearing to go fast
If your bike has a conventional double front chainring of 53/39 (ie. 53 large front chain ring and 39 low front chain ring) and an average range rear cassette of say 12 / 25 on your 9 speed or 10 speed, highest gear is 53 on the front and 12 on the rear. If you divide 53 by 12 = 4.416.
Therefore for every rotation of your pedals, you are rotating the front cassette 53 spacings of your chain which similarly passes thru 53 teeth of your 12 teeth smallest rear cog which turns your rear wheel 4.416 times. On a steep downhill, this can generate considerable wheel speed.
Gearing to climb steep hills
On the same bike, your lowest gear is 39 on the front and 25 on the rear. If you divide 39 by 25 = 1.56. Therefore for every rotation of your pedals, you are rotating the front cassette 39 spacings of your chain which similarly passes thru 39 teeth of your 25 rear cog which turns your rear wheel 1.56 times. On a steep hill, that can place a lot of strain on the quads, knees and the cardiovascular system of many riders.
Climbing very steep hills is no more onerous than cycling rigorously on the flats provided you have low gearing ie. your lowest gear ratio is no more than say 1 : 1.2 - thereby for every pedal revolution, the rear wheel only revolves 1.2 revs, enabling most adult cyclists to climb steep hills without bursting their quads or blowing a gasket.
Gearing set-ups to assist climbing very steep hills includes:
(a) Triple front chain ring 50/39/30; or
(b) Compact front crank 50/34, 50/36 or 48/34; or
(c) Long arm rear derailleur to accommodate a 11/32T or 11/34T rear cassette; or
(d) Double front chainring 53 with a Full Speed Ahead 38T small chainring (in lieu of a Shimano 39T); or
(e) Rear cassette 10 speed 11/28 (Shimano) or 9 speed 11/28 on a short arm rear derailleur.
If you have a traditional double front chainring of 53/39 and say a ten speed 12/25T rear cassette, when in your lowest gearing of 39/25 (in Shimano Ultegra or 105 grade short arm rear derailleur), for every rotation of your pedals your rear wheel turns 1.56 revs. If you want to achieve lower gearing to climb hills under less strain or steeper hills such as some >15o climbs around Kangaroo Valley and Kiama, the -
(i) cheapest way to achieve lower gearing is (d) above by replacing your 39T front small chainring with a 38T front small chainring (cost $44 approx) to achieve a 2.56% reduction in rear wheel revs (mindful that 2.56% is small, best to wait until your chain rings are about 5 years oldf)
(ii) next cheapest way is (e) above @ -
* $100 for a 10 speed 11/28 Shimano Ultegra cassette = 1 : 1.39 is a 10.71% improvement; or
* $80 for a 9 speed 11/28 Shimano Ultegra cassette = 1 : 1.39 is a 10.71% improvement.
(ii) next cheapest way is to replace your existing 53/39T front crank with (b) above @ $175 for a FSA 48/34 = 1 : 1.36 is a 12.82% improvement.
(iii) next cheapest way is to replace your short arm rear derailleur with (c) above @ $90 for an Shimano Deore XT RD M751 SGS long arm rear derailleur and $89 for a 11/34 Shimano Deore XT CS-M760 rear cassette so a 39/34 = 1 : 1.147 is a 26.47% improvement.
Further information e-mail ScribePJ@bigpond.com