just tumbled this article in the net. hope it helps to more understand what brake jack is...
Instant centre theory:
In a true 4-bar linkage (this includes FSR, VPP, DW-link, Lawwill etc, as well as floating brake linkages), ie not linkage-driven singlepivots, the instant centre (IC) of zero velocity is a point about which everything in the “isolated” link (the rear triangle/seatstays) is moving tangent to. A good explanation can be found here
www.mtbcomprador.com/pa/engli...#InstantCenter, I highly recommend reading it. Please note the mention of a “constant centre” also.
With respect to braking performance, forces/reactions on a 4-bar linkage bike can be calculated at any instant in the same way as a singlepivot (as demonstrated above) by using the distance from axle to IC as the length of the swingarm when trying to determine the effect of the couple moment. If the linkage is a parallelogram (as many floating brakes are) or the links are parallel at that point, then the IC will be at an infinite distance away, and as such the couple moment will have no (direct) effect on the suspension’s state (it will not attempt to compress or extend the suspension). If the linkage is a parallelogram, then you will note that the axle’s tangent path is parallel to the tangent path of each of the two “arms” of the linkage. From this we can see that it must have a centre of curvature that is fixed, which can be placed relative to the forward pivots, identically to how the axle is placed relative to the two rear pivots. (Fig. 7)
In order to calculate the moment acting on the suspension due to the horizontal force on the axle, the vertical displacement between axle and main pivot (on a singlepivot) can be substituted for the vertical distance between the axle and the centre of curvature here. Calculating the reaction forces at the shock and pivots however, is more complex due to the linkage setup. If you are proficient with basic vector calculations and trigonometry (or instant centre theory in its entirety), you should be able to calculate these reactions. However, the calculations are too variable and numerous to demonstrate here.
A few notes:
- The vast majority of bikes have some amount of brake squat. Very few bikes actually have brake “jack”, but apparently that doesn’t stop every man and his dog misusing the term “brake jack”.
- Generally the instant centre of any bike is in front of the axle. Some bikes have ICs that are behind the axle (such as Lawwills) which due to the brake’s couple moment effect may tend to develop a next extension force under braking – but, and this is a big but, having an IC behind the axle does not automatically necessitate that a bike will “jack” (extend), due to the horizontal axle force giving an equal or greater moment trying to create squat.
- All demonstrations of calculations above are for conceptual explanation only – they make assumptions for the sake of simplicity which make them mathematically inaccurate (by which I mean not exact, not that they give a “wrong” answer/idea). They are used only to give the reader some basic understanding of how the main calculations are performed. The inertia of the wheel, brake, linkage/swingarm etc have been ignored thus far, and all calculations have been assumed to be at equilibrium – this is not precisely the case in the real world, but it will suffice purely for the purpose of conceptual demonstration.
- It is possible, although not necessarily desirable, to have a suspension setup which does not have a squatting or extending tendency under brakes (at a given instant or instances). This, I believe, should be kept separate in terminology, from suspension extension due to weight shift. It is fairly easy to understand that if the brake torque/axle reaction force doesn’t exhibit a compressive or extensive force/moment on the suspension AT ALL, then under any braking the suspension will extend due to weight shifting forwards. For this reason, it may be useful to have some amount of pro-squat (tendency to compress). I know of no situations where it is helpful to have anti-squat (a net extension force, in other words actual “brake jack”) under brakes, as this only exaggerates the forwards weight transfer.
- The “couple moment” and “horizontal force on axle” effects can be treated as independently as above in all situations – note that in some cases, one can be negative whilst the other is positive, reducing either the pro- or anti-squat reaction under braking (depending which one has a greater effect).
- All calculations above work for all situations that I know of. That is, to my understanding, this explanation is universally applicable.
- There are other factors which I have not bothered to mention due to their relative consistency between bicycles, such as centre of mass, shock rate etc. These do have an effect on braking, believe it or not, but their contribution is not significant enough (as well as being much more complex) to be worth mentioning in this article.
- Parallelogram linkages and floating brakes do remove the couple moment component of BISI, but they do not (necessarily) make any change to the effect of the horizontal force on the axle. On many (singlepivot) bikes a floating brake makes a considerable difference to the braking characteristics because of said removal of couple moment component, however this does not completely eliminate all brake-induced effects on the suspension (for better or worse).
- There are other ways to explain braking characteristics, as far as I know these are all in agreement with what is written above. A common way is, with acceptance of the internal forces within the wheel/swingarm, to simply consider the rear wheel contact patch (and the frictional force acting on the wheel at that point) with relation to the IC, and nothing else. This is a perfectly viable method of calculation/comprehension, but I have chosen to explain it differently because separation of the two main components of brake interaction shows more clearly how the forces internal to the suspension can separately affect the suspension and how they can be manipulated to give the desired effect.
- The claim that bikes will “brake jack” and/or “lock up/out under brakes” should always be treated with suspicion. This is nearly always untrue or inaccurate – braking does not “lock out” any bikes, and in no circumstances that I am aware of is braking capable of bottoming out a properly set-up bike. True brake “jack” (extension) can give the rider the impression that the bike has stiffened suddenly, but unlike a rigid/locked out bike, the braking extension tendency is not a reaction to a vertical (bump) force, and as such will not be immovable or rigid, as those forces can be overcome.
The end result: what does it mean?
Most bikes squat to some degree. This is not necessarily a bad thing, as mentioned above. This includes most FSR bikes, despite Specialized’s “fully active in all circumstances” claim (with the understanding that "fully active" means "shock absorption completely unaffected by braking"). Some bikes do actually “jack” but these are few and far between, and it’s not always bad enough to even be noticeable, let alone a problem. There is a definite placebo effect surrounding brake systems, and it is not unlikely that this is due mainly to lack of education/understanding on the subject. Some people will swear black and blue that singlepivots are nearly unrideable due to perceived “brake jack”, others will simply state that they’ve never even noticed it. From this we can make a logical conclusion: BISI does exist, and that it is not necessarily a problem – in fact in some forms and to some degrees it can even be useful. However it is hard to believe that any common amount of brake squat can make a bike unrideable, or anything to that end. Notably, the bike on which Fabien Barel won the 2004 world DH championships on had a brake linkage designed specifically to increase the level of (pro-)squat far beyond what normal bikes generate. Riding the production version of this bike, you can feel a huge tendency for the rear end to dive when the rear brake is applied. Given that no owners of those bikes seem to have any problem with the extreme brake setup, one might logically assume that it’s not actually that bad, and that other bikes with considerably less brake induced squat can hardly be any worse off, and thus are perfectly fine to ride – although not necessarily as comfortable as they could be.
The moral of the story is that almost any pro-squatting braking setup is usable. That is not to say that there is no reason to dislike certain degrees of brake interaction – that varies with riding style, terrain and personal preferences. Another important point to note is that true “neutrality” under any acceleration (positive [pedalling] or negative [braking]) is not necessarily an optimum setup – certain reaction forces under braking/pedalling can help stabilise the bike as well as offer greater comfort and traction. It is also useful to know that it’s not hard to make stuff perform worse, so be wary of playing with your bike’s braking characteristics unless you know what you’re doing – that incorporates more than is written in this article.
All content remains copyright of myself (and Kenneth Sasaki, to whose work I linked regarding instant centres). Feel free to reproduce it as long as full credit is given.