Search sheldonbrown.com and sheldonbrown.org
When you close the skewer, your hand strength is accomplishing two things: It's applying a clamping force to the skewer, and it's overcoming the mechanical friction of the mechanism.
Quick release skewers come in two distinct types: Enclosed cam and Exposed cam designs.
The original type of quick release skewer, invented by Tullio Campagnolo, features a steel cam surrounded by a solid metal body. The body is the part that moves back and forth as you flip the lever, and usually has teeth to press against the left dropout.
The cam is well-shielded against contamination, and can be lubricated by applying a couple of drops of oil every couple of years.
Sometime in the '80s, a variant type of skewer was introduced, one that is less expensive to manufacture, and is sometimes a bit lighter. This type uses a split external cam that straddles the end of the skewer. External-cam skewers use a curved plastic washer between the cam and the toothed metal washer that presses against the dropout.
This type was originally marketed as an "upgrade" because it could be made a little bit lighter.
Despite the marketing hype associated with these "boutique" skewers, they are actually considerably inferior in functionality to the traditional type. They are often seen under rather prestigious names, as was the one photographed here. (I Photoshopped the logo off, so as not to pick on one particular brand.)
The exposed cam can not be kept as clean and well-lubricated as the shielded one can.
In addition, the exposed cam has a larger diameter, (typically 16 mm vs 7 mm for an enclosed cam) so the friction is acting on a longer moment arm (the radius of the cam.)
The result is that the exposed-cam type provides very much less clamping force for a given amount of hand force on the lever.
The exposed-cam skewers are generally OK for vertical dropouts in back, and for forks with "lawyer lips", but should not be relied on with horizontal dropouts or plain forks.
General practice is to install the skewer so the handle is on the bicycle's left side. Thus, the right side is secured by the acorn nut that threads onto the opposite end of the skewer.
Good-quality skewers have acorn nuts with steel serrations that can bite into the face of the dropout, so the wheel won't slip forward. Good skewers have a serrated steel surface to bear against the outside surface of the frame, but most of the "boutique" skewers have soft aluminum parts in this position, presumably to save weight.
The aluminum "teeth" are too soft to get a good grip on the dropout. Since the chain pulls on the right side of the hub, where the acorn nut commonly resides, this type of skewer is almost always unsatisfactory for use with a frame that has horizontal dropouts. In addition, "boutique" skewers generally have (yuck!) aluminum threads, vs the steel threads of the two-piece acorn nuts. These are much easier to strip.
|One-piece aluminum acorn nut.
Aluminum threads and teeth.
|Aluminum acorn nut
with steel insert.
As a result of these design flaws, "boutique" skewers should not be used on frames with horizontal dropouts, nor on older forks that don't have "lawyer lips."
There have been reports of a particularly nasty failure mode with the external-cam type skewer, related to the radiused plastic washer. If the user isn't careful, sometimes this washer can get rotated 90 degrees, so that the cam presses on the points, rather than sitting in the valley of the curved side of the washer. This creates a likelihood that the skewer can rotate in use, so that the cam falls into the "valley." When this happens, the skewer will suddenly open up, possibly causing the wheel to fall out.
James Annan has recently created something of a storm in the industry by pointing out a serious safety risk to users of front disc brakes with traditional forks. He has identified a mechanism whereby the forces generated by the disc brake can cause the skewer to loosen up in use!
This failure mode is possible even with the better skewer designs, but it's more likely with the external-cam type.
|Articles by Sheldon Brown and others|
Last Updated: by John Allen