There is a very well established myth (well-propagated by the magazines) that mono-pivot shocks will lock under rear braking. For example, in Mountain Bike Action magazine, Richard Cunningham states, “Most, if not all, mono-shock rear suspensions lock up under braking.” (Page 76, Mountain Bike Action, May 2001). This is known as “Brake Induced Shock Lockout” (BISL).
Given the suspension dynamics we have covered, a little thought should make it obvious that application of the brake will have no “locking” effect on the shock, whatsoever.
Numerous theories have been put forward trying to establish the existence of BISL.
BISL theory #1:
Perhaps the most amusing was the idea that if one locked the brakes on a mono-pivot and bounced up and down on the bike, then one would find the suspension would also be locked. The first flaw here is that while static friction between the wheels and the ground keeps the wheels at a constant distance if the brakes and wheels are locked (neglecting fork compression), the same is not true while a bike is in motion. Static friction is lost either between the wheels and the brake or the wheels and the ground. Either condition will unlock the suspension. But beyond this, our plots of 4-bar paths in the “Typical Horst Link Designs.” section show us that if mono-pivots are locking, then so too will most 4-bars, none of which are said to lock.
BISL theory #2:
Some (see Richard Cunningham's quotes in the “‘Internal Force' Theories.” section) believe 4-bars to brake better over bumps because braking forces are “isolated” on the rear link by the two rear suspension pivots. (This is an example of an “Internal Force” Theory, which we will further discuss in that section.) However, “Nature Varies Smoothly” (NVS) and the “Coaxial Condition” establish that pivots do not “isolate” forces. This theory is usually taken in combination with theory #3.
BISL theory #3:
Another widely accepted explanation for BISL, related to the last theory, is that application of the rear brake will push the swingarm down, causing the shock to compress, and thus stiffen or even lock against its bottom-out bumper. 4-bars are thought not to do this, since the force of the brake is not directly on the swingarm. Rather then the suspension supporting the main triangle, the main triangle is actually thought to hold up the rear and upper links!
The main thing that this explanation misses is that the front triangle will pitch forward under braking. The ultimate effect is that there is no significant shock compression or extension under braking in any of today's typical mono-pivot designs. This author has done a variety of tests on a number of mono-pivots to demonstrate that the shocks neither lock nor compress under braking. Bikes used in the tests included the now infamous, but really not so bad, Trek Y-bike (a URT, but equivalent under braking to non-URT mono-pivots). This bike was supposed to have one of the worst problems with BISL. We were fortunate in that one of the Y-bike shock pivots had a little squeak in it that was very noticeable under even very small movements (the squeak was not associated with any significant friction). From this, we had both visual and aural confirmation that the Y-bike shock was undergoing no compression or extension under braking.
As a final blow to BISL, note that we can construct a mono-pivot and a 4-bar that are both essentially neutral under smooth-surface braking and with the mono-pivot having, if anything, a slightly advantageous suspension rate under braking. In fact, the two 4-bars that we are about to consider fit the bill precisely.
In addition to the errors we have pointed out in the above BISL theories, there are a number of suspension bike reviews in prominent magazines that indicate BISL to be a sham.
The Jamis Dakar series of bikes and the Psycle Werks Wild Hare are examples of bikes with IC's just about through the main pivots at equilibrium. They will thus be neutral under smooth braking. The upper links on these bikes also hang from the top tube, so they will experience an increased rise in rate under braking, as the suspension is compressed by some obstacle. This effect should be small, however, since the links start out at or near perpendicular. If there is such a thing as BISL in mono-pivots, then these bikes should suffer from BISL to an essentially equal (or perhaps slightly greater) degree.
But we have seen numerous "Dakar and Wild Hare reviews" in Mountain Bike Action, Bicycling, and other industry magazines, with no mention of BISL as a problem. We find it particularly interesting that Mountain Bike Action, that BISL stalwart, would find no problems with these designs under braking.
Although there are small physical differences in typical 4-bars and equivalently main pivoted mono-pivots, we have yet to see any remotely reasonable explanation as to why one should brake better then the other in general.
The biggest consideration is the relation of the rider's body mass to the wheels and what it will do under braking. This author believes that between most of the designs, the differences are just not enough to merit a general statement.
Some people find 4-bars to brake better, but others do not, though we have seen no double-blind tests. In the end, the small difference between some bikes may be significant enough for some people to feel a difference. But in general, we suspect that this is again just a case of the very well established psychosomatic phenomenon. This would not be the first time that people have been told that something is so and many have experienced what they have been told (this is why placebos cure illness). Or perhaps it is again a little of both.
We also have no doubt that the BISL myth has been propagated by some in the interest of selling more expensive 4-bar designs. We see no $ 2,000 mono-pivot frames.
In the near future, we hope to do a double-blind experiment to see once and for all if there is a difference. We will publish any results in subsequent editions of this work.
In any case, our advice here, as always, is for one to make decisions through testing the bikes, if possible.