A friend just got a new tri bike with Zipp wheels and they are loud when she is coasting. What causes that?
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There is a part on the hub of the wheel called the Freehub. This is a ratchet mechanism which allows your bike to freewheel, or move forward, even when you are not pedaling. This is different from Fixed gear bikes, which have no freewheel mechanism, and will force your pedals around, as long as the wheels are moving.
The noise you are hearing is the pawls, which are spring loaded teeth. These teeth allow the hub to move forward when you stop pedaling, by moving out of the way. But thy will spring back up and grab the inside of the freehub shell when force is applied from the chain. Meaning when you start pedaling.
This photo is of an old style freewheel, but it illustrates the concept well.
The noise is caused by the spring action of the pawl clicking back into place. Because your friend is riding Zipps, which are large hollow constructs of carbon fiber, that noise is amplified particularly loudly. The rims act kind of like speaker boxes, and the noise sounds loud.
There is unlikely to be any problem with her setup, it's just different than yours.
Most bikes have 'freehubs', a design Shimano came up with that is brilliant because it places the wheel bearing outboard of the sprockets rather than inboard, as per the 'freewheel' design that preceded it. Inboard bearings place a shear force on the spindle when going over bumps that can break the spindle.
Anyway, Shimano patented the freehub, and rightly so. Their freehubs run reasonably quiet, (with a lower 'note' on the ones with cheap steel sprockets attached to them).
Other manufacturers are not able to licence the freehub design or are unwilling to pay whatever licence fees Shimano charge. As a result they have their own technical workarounds.
The pawls of a Shimano freehub are placed deep inside the freehub body, the noise they make when freewheeling is therefore insulated, i.e. quiet. Other makes place the pawls inside the hub (Campagnolo) or else they use ratchets instead of pawls inside the freehub body (DT/Swiss). Hence the different noise characteristics of these up-market, high-end freehubs.
Here is the freehub body from a typical, recent Zipp wheel. As you can see they have not used the original (and totally excellent) Shimano design, the pawls are placed in the wheel, not the freehub-body. Coupled with the aforementioned 'guitar case' of the disc wheel a loud racket is made.