Originally Posted by
mtseymour
The statement that "disk braking on a tandem places much more demanding forces on the fork than it does on a single" ignore the extensive use of disc brakes in mtn biking under grueling conditions. First, no one has actually provided any measurements to show that tandem braking is more demanding than a mtn bike. Secondly, mtn bikes with disc brakes are routinely expected to handle steep, bumpy terrain. By steep, I mean 30-50 deg (not % gradient) slopes. Although some descents are very bumpy (with 3-5' drops), I have not heard of a QR wheel being twisted off by harsh impacts or heavy braking.
The forces on a bike from bumpy terrain, jumps, etc, are all directed upward so do not play a role in wheel ejection like those from disc brakes, which are directed downward at the front wheel (assuming the caliper is directly behind the axle). Also, the braking force attainable is limited by the friction force between the tire and surface, which is much less on dirt. The greatest braking forces are attained when you combine high coefficients of friction (grippy tires, asphalt, grippy brake pads) with powerful brakes, and heavy bike/rider combination (for example, a tandem). Steepness of terrain would play a role only to the extent that skidding does not occur. This is simple F=ma stuff. Where these forces are applied, and their magnitude depends on the brake design. A little beyond simple F=ma. The only reason mountain bikes came up in the discussion is that they were the first bikes to use disc brakes, so they were the ones to experience front wheel ejection.
A search on "front wheel ejection" turned up a 2003 discussion involving none other than Jobst Brandt, who gives an example, and concluded that "this is not a safe condition [braking] when repeated often". The other main participant in that discussion is scientist James Annan, who brought one or more lawsuits in the early 2000's as a result of crashes that resulted from wheel ejection as a result of "...huge force on the front wheel, in the direction of the open dropouts." The simple example/comparison I gave in my earlier post is consistent with Annan's description and Jobst’s more quantitative analysis. (Am pretty sure Annan had a detailed analysis somewhere on the internet also but links in the thread do not work.) Since that time QR dropouts for disc forks have been angled forward, and it seems that most people are fine with that.
The surprising fact is that this whole phenomenon was missed by not one, but probably a fair number of design engineers. It must have been a real "Doh!" moment for them. Of course, they had to argue that the design was fine due to the liability.
Disk brakes and quick-release unscrewing (James Annan; Jobst Brandt)