On the question of vibration...I often noticed when I put a new chain on a not too worn cassette, I got a mechanical kind of whirr for a maybe 50 / 100 miles, then it goes. Any known reason ?...just intrigued .
Changes to the components in the drivetrain, be it chain, cassette or chainrings, often produce vibrations and noises. The problem is trying to pinpoint the source of the noise. But, there are some known causes.
1) A vibration in high gear (the small sprocket on the cassette) is usually perfectly normal and only picked up by people really attuned to their bike’s sound. On 11-tooth (and nowadays even 10-tooth) sprockets, a phenomena known as chordal action is pronounced. Chordal action is a small vertical oscillation of the chain as it exits a small sprocket which in turn causes a vibration along the chain’s tension cycle. This can often be felt when pedalling hard on a small rear sprocket. The only way to stop it is to up the number of teeth on a sprocket and since that’s not an option, just live with it. To visualise cordal action, you have to think of the last link (half link actually) as it leaves the driven sprocket (cassette) and makes its way straight down the tension line towards the chainring. If the cassette sprocket looked like a Caterpillar (with capital C) track, you'll see that the last link to leave the sprocket is already in the horizontal position and simply steps off the cassette and moves forward. However, if the sprocket is small 11 or 10 teeth, then the last link to leave the sprocket is not in the horizontal position but still pointing slightly upwards. Since it is in tension in that position, it's next move is forceful - it is pulled horizontally and does so with a snap, that induces a vibration into the tensioned chain. A tensioned chain is no different from a tensioned guitar string and emits a noise if vibrated. The length of that piece of chain determines the harmonics but usually it is an audible buzz and and, even if inaudible because of ambient noise, certainly perceivable to the mechanically sensitive rider.
2) A new chain is not yet at its normal length. A brand new chain is slightly shorter than 100%. In other words, its pitch is still less than 1/2 inch. The reason for this is that pins in holes never fit perfectly and under tension pins will always settle to one side of the hole. Also, pins are not parallel in a new chain. Imagine it as a ladder with skew rungs. Under tension in the first few miles it elongates a bit. This is not wear, but settlement. If you were to monitor a chain's length it will start under 100%, grow to 100% very quickly and then slowly and evenly elongate to 101% or whatever point you decide to change it. The cassette sprocket pitch remains 1/2 inch though no matter how worn the cassette is. This shorter chain then clashes with incoming sprocket teeth and hits them and vibrates as it is forced onto the cassette.
3) A worn chainring still has a 1/2 inch pitch but the pitch has moved off-centre from the centre of the teeth. This happens because the leading edge of the tooth develops a pocket (shark fin).This means that the chain runs slightly behind the place where it is supposed to run. All's well except for the next incoming tooth in the sprocket that now hits the roller. Only chain tension forces it down into the root of the teeth and this causes a vibration. This vibration is very typical on single-speed bikes where the lack of a tensioner at the back gives the chain no option to escape by riding over the cassette teeth. It forces it in at both sides because it enters in tension in front and back. This vibration is quite strong.
