Why are higher-end freewheels so much louder when coasting?
I tend to ride relatively modest gear (Tiagra/Ultegra), but when I'm riding near higher-end bikes, I sometimes notice that the clicking sound that comes from their cassette/free-wheel while coasting seems to be much louder and more distinct than on my bike. Why is this? Intuitively, one would expect that a quieter drive-train would be an indicator of greater efficiency, but that doesn't seem to be the case here. Is it something to do with different construction, or materials? Can you get 'quiet' high-end cassettes, or is being noisy while coasting the price you must pay for being efficient while pedaling?
I don't know of any correlation between the quality of cassettes and the noice they make, but silent ones certainly are available. http://sheldonbrown.com/gloss_ri-z.html#rollerclutch
It's important to note that the freewheel is part of the hub, and while you might have some or mostly Ultegra parts, many bikes are sold with parts that aren't all from the same groupset. When you say the cassette is loud, what you're actually saying is that the freehub is loud, at least when talking about higher end bikes. Low end bikes typically have a freewheel built into the cassette.
Loudness in the freehub/freebody is usually due to the very light oil used to lubricate the inner parts. Thicker oil can be used to lessen the noise and even grease in some cases, but it's high viscosity is pointed at for not being so efficient.
http://www.youtube.com/watch?v=SG0uBDvgXu0 Noisy freehubs are definitely a design thing. I don't get it.
Lound freehub's seem to give the impression of a positive pawl action. I know that the best all-round MTB hubs I've run are Hope Pro's, and are 'BMX' loud. On that note, noisy BMX freewheels are considered desirable :)
Campagnolo wheels are traditionally very noisy. And higher end bikes often use Campagnolo-made wheels, even under different brand.
Cassettes can actually be loud as well. All of my titanium cassettes create noticeably more "chain noise" than my comparable steel cassettes.
Shimano has a patent on how to make it less noisy. Are you sure you are not comparing Shimano to non-Shimano?
Campy hubs have had a reputation of being noisier. I've heard of people choosing them for that exact reason. Gimmick?
Most of the noise comes from pawls on the freewheel hitting against the splines on the engagment surfaces which makes up the racheting unit.
Some reasons for the noise between freewheels?
- Tension on pawls could be higher causing more noise as they glide over the engagment surfaces
- High end freewheels have more pawls and engagement points than lower end freewheels, so there are more ridges in the engagement surface and more pawls hitting the splines on the engagement surface. The reason this is desirable is that more pawls and engagement points means faster engagement when you start pedaling.
- Different grease (or less grease) could also be used inside higher end freewheels that is less viscous and provides less resistance, allowing the spring action of the pawls to cause more noise as they float over the the splines on the engagement surface since they are less restricted by the grease.
Of course there are exceptions to these. Some hubs don't use your standard racheting mechanism and use a 'roller clutch' instead. The roller clutches tend to be very quiet, but are more prone to failure.Here's a good description of how those work. http://pardo.net/bike/pic/mobi/d.winners-hub/index.html
Great answer. I would like to mention that more points of engagement become particularly using with mountain bikes -- especially when ascending a technical section in a very small gear. I personally don't see the need on the road.
There is also probably something to be said for the amount and type of materials involved in the construction of the hub itself. However, I can't find anything definitive on this. I imagine if you had the exact same hub design made from steel, titanium and aluminum, all three would produce different noise levels, but I can't find any sort of transmission or dampening testing done to back that up.