Steel was the bike derailleur material of choice for many years, but advanced manufacturing methods, such as cold forging, especially with aluminum, has reduced the cost and weight of the parts while increasing their strength. Given the differences, mountain bike front derailleurs can’t be used on road bikes and vice versa. Additionally, mountain bikes often have three chainrings on the crankset. Mountain bike: On mountain bikes, the large chainring on the front will be in the 40s or 30s instead of the 50s.You must get the correct derailleur for your type of bike. The rear cassette, by contrast, will be smaller than the ones ordinarily found on a mountain bike. Road bikes will seldom have more than two chainrings on the front. Sometimes there will be a 50/34 chainring on it. Road bike: Road bikes normally have high gearing, with 52/39 chainrings on the crankset.Alternately, you can use a bike chain tool to lengthen or shorten your chain.

This will affect the derailleur you choose for your bike. The larger the cassettes on your bike and the greater the number of teeth on the rear cogs and front chainrings, the longer the chain you’ll need to accommodate them all. When you’re selecting a derailleur, make sure you know if it is a front or rear derailleur as they are not interchangeable. The larger cassette demands a different configuration than the smaller cassette on the front.

Rear: The rear derailleur may have to change between eight or nine, and possibly even eleven, cogs on the back wheel.

The front derailleur will only have to change between two or possibly three chainrings at the most.

Front: The derailleur for the front cassette (on the crankshaft) on your bicycle isn’t the same as the derailleur on the rear cassette on the wheel.

When replacing either of them, it’s important to get compatible mechanisms that work with each other as well as with your model of bike. There are generally two derailleur mechanisms for any multi-speed bike’s gear shifting apparatus: one in the front and the other in the rear. Use the B screw - usually found between the high-limit screw and the low-limit screw on the body of the derailleur. A noisy chain is usually the result of the upper derailleur pulley being too close to the cassette.

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