Join Date: Mar 2008
Location: The 'Wack, BC, Canada
Bikes: Norco (2), Miyata, Canondale, Soma, Redline
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Even at the best of times bicycle shifting is not "light switch" fast. The higher end components are faster but they often still have a quarter to half turn or so of the cranks before things begin to happen and another half or so before things are done and settle down. During that time you want to keep pedalling but greatly reduce the force used so you minimize the clunking and snapping that occurs when you shift under full load.
First step is to set up the cable adjusters so your shifts both up and down are equally quick. That's done by adjusting the barrel adjuster on the rear derailleur where the cable housing stops. The housing fits into that barrel adjuster. As noted it's all shown at www.parktool.com/repair
. Just hover your mouse over the rear derailleur at the rear of the bike and then click.
If the bike is older it would not be a bad idea to treat it to a set of new cables and housings for all of the brake and shifting systems. Over time the cables cut into the plastic low friction liner of the housing and when that happens it tends to bind and add a lot of resistance to the systems.
Once the new cables and housings are on you can adjust the cable tensioning barrel adjusters found at mountain bike levers and at the rear derailleur. The idea is that you want to even up the speed for up and downshifting by centering the jockey pulley cage of the derailleur with the gear currently selected.
If the rear derailleur has taken a few good thumps or falls over it's life then there's also a good chance that the derailleur hanger on the frame is twisted. To check and correct that will require a trip to a bike shop since the tool used for checking the alignment is not a common shop item.