Figure 3.12)
We first say a word on floating disc brakes.
Floating disc brakes are rear disc brake mechanisms wherein the brake is mounted on its own linkage arms, which are not part of the load bearing, rear suspension components. Figures 3.12 A and B) show simple diagrams of this type of mechanism, pictured in red, both at extension and compression.
Many claims are made for floating brakes (see the “False Claims for Floating Brakes.” section for an expose on some widely believed things that floating brakes do not do). However, the only thing that these mechanisms do is to give a bike the braking character of the brake mechanism's linkage system. For example, these mechanisms can give a mono-pivot the braking character of a 4-bar with suspension geometry identical to the brake mechanism's linkage geometry. This means that mono-pivots with floating brakes may develop some propensity to extend under braking, as is the case with typical 4-bars. 4-bars may or may not develop a change in character under braking, depending on whether or not their suspension geometry is significantly different from that of the floating brake.
Below we will do some analysis involving 4-bar suspensions. Since there is no real distinction under braking between a 4-bar suspension linkage and a floating brake linkage with the same geometry, all statements below regarding 4-bars will also apply to bikes equipped with floating rear brakes.
The biggest question regarding braking in dual suspension bikes is whether or not 4-bars rear-brake better then mono-pivots, in general. We will take up specific theories regarding this question in the “‘Brake Induced Shock Lockout' (BISL).” section of Chapter V. Here we will examine how the performance of possible 4-bar link configurations compare to those of mono-pivots with identical main pivot locations.
Figure 3.13), depicts a 4-bar suspension frame with various possible locations for the upper, forward pivot, giving an IC at the main pivot. Path Analysis tells us that this frame will rear-brake identically to a mono-pivot with identical main pivot location, at the depicted point in travel, because the path tangents of the rear brakes will be the same in both mechanisms.
Figure 3.13)
This is most clearly seen when considering the two suspensions as part of a mirror bike. In a small neighborhood around the depicted point in travel, the paths of the components will be essentially the same, the 4-bar swingarm and rear link moving just as the mirrored segments of the mono-pivot rear triangle. There is essentially no relative movement about the 4-bar lower rear pivot in the small neighborhood about this position in travel, so it does not matter whether or not the lower rear pivot is even there.
We have drawn Figure 3.13) with a 90 deg. angle between the rear and upper links in order to produce the most visually convincing physical situation. But any 4-bar configuration with IC at the main pivot will brake equivalently to a mono-pivot. To see this, consider Figure 3.14). Here we depict a 4-bar suspension with multiple possibilities for an upper link configuration, attached to a base, and oriented horizontally. All forces in both mono-pivots and 4-bars are identical under braking with the exception of the interplay between the rear wheel, rear brake, and suspension components. Mounting the frame horizontally allows us to consider this interplay in isolation.
Figure 3.14)
Under braking, a force F is induced from the rear wheel through the brake to the rear link. Neglecting the mass of the upper link, which is very small, we see that this force will in turn be transmitted to the upper link and ultimately to the main triangle, directly down the axis of the upper link.
To see this, it may help to consider the forces involved between the main triangle and suspension components of the 4-bar as we did the force in Figure 3.6. Decompose the force through the upper rear pivot, from the rear link to the upper link, into forces parallel and perpendicular to the upper link. Do the same for the force between the upper link and the frame.
We see that the torque balance about the main pivot in a 4-bar with IC at the main pivot will be the same as in an equivalently main pivoted mono-pivot. We also see that an IC in front of the main pivot will create a suspension more extending then a mono-pivot under braking (this is sometimes called “brake-jack”), since the rear brake path ascends through suspension travel more then it would in a mono-pivot with identical main pivot location. An IC behind the main pivot will create a suspension that is more compressing.
Note: It is a very common misconception [see the braking analysis of Ellsworth's “Instant Center Tracking” (ICT)] to believe that the angle between the rear and upper links is what determines the brake's effect on the suspension. But it is the force transferred to the rider/main triangle that ultimately determines whether or not the suspension will react.
Imagine varying the angle between the rear and upper links, while holding the axes of the upper and lower links constant, producing a constant IC location under variation. The components of the force on the upper and lower links, from the rear link, are changing, but so too are the lever arms. In the end, this variation in angle will not change the brake's effect on the suspension.
We have done numerous experiments on mono-pivots that show them to be generally neutral (neither extending nor compressing) under braking [see the “‘Brake Induced Shock Lockout' (BISL).” section]. For both mono-pivots and IC/main pivot coaxial 4-bars then, the effects on the main triangle will remain largely the same throughout a smooth-surface braking process.
Most 4-bars have an IC in front of the main pivot, causing them to extend relative to most mono-pivots, under smooth-surface braking. Extension has been confirmed by experiments on an Intense Tracer, a very typical 4-bar design. Interestingly, this extension has the potential to cause the suspension to press against the top-out bumper in very short travel designs meant to be run with little or no sag. This would be especially true in designs such as the Giant NRS. A bump force would have to overcome the extending brake force before the suspension would compress.
Some 4-bars, such as the Jamis Dakars and the Psycle Werks Wild Hare, with ICs just about at the main pivot, will brake equivalently to mono-pivots on a smooth surface.
The Yeti AS-R, which has an IC behind the main pivot, will be compressive under smooth-surface braking relative to mono-pivots.
When a 4-bar hits a bump and compresses, the instant center will move, thus changing the geometrically inherent suspension rate under braking. If the upper link in Figure 3.13) points up from the main triangle (rotates clockwise under compression), then the contribution to the effective suspension rate from the wheel force on the brake will become less extending/more compressing as the suspension compresses over a bump. Similarly, if the upper link in Figure 3.13) points down (rotates counterclockwise under compression), then the opposite will be true.
This may offset to some degree the tendency of most 4-bars to become more extensive with application of the rear brake.
The effect will be smallest for bikes with upper and rear links starting out at 90 deg. to each other, such as in the Dakars and Wild Hare. So these bikes should still brake almost exactly like mono-pivots, regardless of the ground conditions.
I hope that you all have found this work useful and enjoyable. I wish you happy trails.