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  1. #1
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    New question here. "Brake Jack" - Explanation??

    Could please someone layout how exactly brake-jack occurs in a step-by-step fashion?
    I have tried reading on the subject and I kinda think I know what's goin' on. I have a faux-bar link on my bike and I look and try to visualize the phenomenon, but i just can't quite get it.
    Honestly... ahh I give up

  2. #2
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    Here is a picture of mine. Maybe someone might want to draw on it showing angles, rotation, axle-path etc.
    Attached Images Attached Images
    Honestly... ahh I give up

  3. #3
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    I wouldn't worry so much about it.

  4. #4
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  5. #5
    Chillin the Most
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    Watch the video... http://konaworld.com/dope.htm

  6. #6
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    Quote Originally Posted by Jerk_Chicken
    I wouldn't worry so much about it.
    hehe, i'm not worried about it - just more curious about the physics involved. I like understanding WHY stuff happens. I know the design on my bike may not be affected too much by it, but if that's the case... why?
    Honestly... ahh I give up

  7. #7
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    Quote Originally Posted by RED5
    UGGh...wish i could. My MacBook got stolen, so I'm relegated to this ancient pc - Even tho I have high-speed connection... It's wasted on this paperweight
    Honestly... ahh I give up

  8. #8
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    Yeah I don't know the proper explanation for it. But I'm in the process of building my first DH bike and it's a single povot (orange 222).
    When I sit on it with both brakes engaged, there isn't much suspension movement. Rear brake only gives it some effect, front brake only is a lot of brake jack, like with both brakes engaged.

    Now that's all while stationary, so maybe it's not brake jack in it's true sense. But basically when I depress the suspension the rear wheel wants to roll backwards a bit and the front forwards a bit (more so than the back). When the brakes are on the wheels can't move so the suspension can't work, and since the front likes to move more (maybe because my fork spring is too firm and hasn't got much give) it means the front brake affects it more.

    So I guess linkages change the wheel path to more vertical (rather than wanting to roll backwards / forwards) and brake jack is next to eliminated. I think that's a pretty simplistic explanation!

  9. #9
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    I think ducktape has it. Your wheel is a pivot point and with it locked up the rear arm cannot pivot around it. At least that's how I see and understand it.

    Rich

  10. #10
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    sitting in place on the bike with the brakes engaged and bouncing up and down has nothing to do with brake jack. Actually, I think you're probably thinking of brake squat (most people incorrectly confuse the two). Brake squat is the tendency of pretty much every design out there (in varying degrees) to take the force transmitted to the frame from the brake (while the brake is slowing down the moving wheel) and push it in to the suspension, causing the suspension to dig further in its travel and the feeling of it getting stiffer and less active.

    That was a bit convoluted, so this might help you think about it.

    When you're riding, and you grab the brake, the brake grabs the rotor (which is being forced to spin through attachment to the wheel, so in effect the force from the ground is being transmitted to the brake) the brake will take the force transmitted to it, push it in to the frame, which pushes that force into the suspension. In effect, its like going through a constant G out, in which the suspension compresses and sinks in. The feeling is of the back end of the bike getting harsher and less active as it cannot react to the bumps as efficiently.

    Hopefully that explanation helps a bit in understanding what I think you're after.

    Brake Jack is a bit different. Thats the tendency of the brake to take the force of the ground and take it AWAY from the suspension. The result is that the suspension decompresses and stops taking in the bumps. You'll see this on lawill designs (which is why they necessitate the use of a floating brake).

    A floating brake can reduce jack OR squat.

    Most bikes are designed to have some amount of squat.

    Hopefully that helps

  11. #11
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    Quote Originally Posted by William42
    sitting in place on the bike with the brakes engaged and bouncing up and down has nothing to do with brake jack. Actually, I think you're probably thinking of brake squat (most people incorrectly confuse the two). Brake squat is the tendency of pretty much every design out there (in varying degrees) to take the force transmitted to the frame from the brake (while the brake is slowing down the moving wheel) and push it in to the suspension, causing the suspension to dig further in its travel and the feeling of it getting stiffer and less active.
    Exactly what I'm talkin about
    Honestly... ahh I give up

  12. #12
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    Yes, and to add what William42 understands, “jack” normally means the rear suspension raises the frame or extends the shock. But the “brake jack” term has been commonly used to describe brake stiffening suspension geometry reaction that causes the rear to suddenly compresses momentarily when brake friction rotates the swingarm with the wheel, then suddenly skids from traction reduction from the firmer stiffening reaction and rises to become more compliant in the softer spring range, grabs traction again and compresses and skids and rises or “jacks” repeatedly like a jack hammer.

    Monopivot swingarm designs typically behave with brake stiffening and easy skid and “jack” reactivity, but multi-links often do this also when the IC, instant center, of the swing links (where the swing links would intersect if extended forward) is located where a monopivot swingarm typically is positioned, such as VPP. Delicate rear brake modulation and or exaggerated rearward rider positioning is required to avoid the repeating skid induced “jack” effect.

    More rear brake power is usable if the rear suspension geometry allows some mild extension so that traction is not suddenly lost so easily. Look for multi-link IC’s located in front of the big ring and well behind the front axle at sag for best balance of braking stability without too much forward pitch and dive, also not too much stiffening causing the “jack” reaction.

  13. #13
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    I had exactly the same frame and there was no brake jack. I experienced some mild brake squat though. Generally, brake jack occurs in "true 4-bars", such as HL, VPP, DW-link. The frame on the picture is "seatstay 4-bar" AKA "single pivot with linkage driven shock", those are squatting under hard braking.

  14. #14
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    Lemme see if I got this straight

    Brake Squat - Causes suspension to compress under braking; effectively stiffening and reducing travel?

    Brake Jack - Suspension is still active, but skids more easily due to upswing in the travel?

    and this is due to pivot locations? namely either SS vs. CS locations? Is this because of the rotor torque exerting rotational forces via the caliper mount ?
    So it seems to me the caliper would want to rotate counter-clockwise pulling up? in my picture?
    Honestly... ahh I give up

  15. #15
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    Quote Originally Posted by DaggerMadd
    I had exactly the same frame and there was no brake jack. I experienced some mild brake squat though. Generally, brake jack occurs in "true 4-bars", such as HL, VPP, DW-link. The frame on the picture is "seatstay 4-bar" AKA "single pivot with linkage driven shock", those are squatting under hard braking.
    Not trying to be a jerk, but thats not actually true. floating rear triangles (quad link, dw, vpp, meastro, canfield, cove, etc) have brake squat, same as a 4 bar. They have a bit less suspension interaction with the brake (meaning less effect when you grab the brakes) which is the general purpose, but they all exhibit some amount of brake squat. Very very very few bikes exhibit brake jack (Rotec is the only one I can think of still using a lawill design that exhibits brake jack, but there are probably others. The old yeti DH9 used a lawill linkage).

    At any rate, most people agree some amount of brake squat is good. Contrary to what most marketing geniuses will tell you, most bikes are designed to have some squat (dw himself explained that most riders prefer some, and that none is bad). Think about it this way, usually when you're braking its because the trail as gotten particularly steep or rocky. At this point, having the bike settle back a little bit and have the front end higher then the rear is a good thing, there is less issues with being pitched forward and going over the bars.

    With both brake jack and brake squat, you'll lose traction - with brake squat the suspension will lose its ability to react to the ground, effectively losing traction. the traction issue really isn't a huge deal if you think about it, since most of your braking force should be coming from the front end anyway. There is a benefit to having the bike settle back under braking and effectively becoming higher and slacker in the front, but to what degree is dependent on the rider.

    Will

  16. #16
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    Quote Originally Posted by William42
    At any rate, most people agree some amount of brake squat is good. Contrary to what most marketing geniuses will tell you, most bikes are designed to have some squat (dw himself explained that most riders prefer some, and that none is bad).
    Will
    Squat is good. How much I would leave to personal preference. Remember that the very act of braking causes weight transfer forward which will unload the rear suspension. So you have two forces, squat wanting to compress and weight transfer wanting to extend. Which is greater and thus dominates is dependent on too many factors.

    Also remember that a bouncy not compliant rear suspension can happen because of a setup that is too stiff. Consider that when braking hard you are transferring a lot of weight off the rear, in effect making the rear end possibly over-sprung for the conditions while braking but okay when off the brakes.

    For me, at 215 and 6'4", on my Kona Coiler when I go into a parking lot and nail the brakes I always see the rear shock extend some. My high center of gravity and weight really dominates the squat.

  17. #17
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    Quote Originally Posted by NormanPCN
    Squat is good. How much I would leave to personal preference. Remember that the very act of braking causes weight transfer forward which will unload the rear suspension. So you have two forces, squat wanting to compress and weight transfer wanting to extend. Which is greater and thus dominates is dependent on too many factors.

    Also remember that a bouncy not compliant rear suspension can happen because of a setup that is too stiff. Consider that when braking hard you are transferring a lot of weight off the rear, in effect making the rear end possibly over-sprung for the conditions while braking but okay when off the brakes.

    For me, at 215 and 6'4", on my Kona Coiler when I go into a parking lot and nail the brakes I always see the rear shock extend some. My high center of gravity and weight really dominates the squat.
    well said.The change in geo and interaction with the suspension is small fries compared to the shift in weight. You may see the shock extend slightly because of the shift in IC, but it would shift way more with less squat or even a lawill design.

  18. #18
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    Sounds like some very good points guys Seems like a good way to avoid all of this is to just not use your brakes
    Honestly... ahh I give up

  19. #19
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    Quote Originally Posted by highdelll
    Sounds like some very good points guys Seems like a good way to avoid all of this is to just not use your brakes
    hah! the engineers designing these buggers know what they're doing. Like it or not, the rear brake is going to do something to the suspension, and they've turned a bad thing into a good thing with squat. Its the marketing guys who (forgive the pun) don't know squat. Squat is NOT a bad thing. Theres a certain thing as "too much" just like with everything else in life, but its a good thing. Think of it like heroin, awesome, but theres a limit!

  20. #20
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    william and a couple others are spot on with the jack (non techincal and incorrect term..should be anti-squat) and squat definitions.

    The simplist and first way to visualize brake interaction is this:
    The part the brake is attached to, will tend to rotate in the direction of the wheel when the brakes are appled.

    So a (simple) single pivot will tend to squat as the chainstay (what the brake is attached to) will tend to rotate in the direction of the wheel.

    as mentioned above, other systems cam verr greatly as the brake is not attached to a member that is attached directly to the main frame pivot. So the brkae interaction is/can be determined by angles with other linkage parts....and will change with suspension movement.

    the VAST majority of bicycle frames show some squat...at the least to counteract mass transfer and front suspension compression.

  21. #21
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    Here's the schematic picture of what happens to almost all "true 4-bar" linkages under braking. By "true 4-bar" I mean all designs where rear weel axle is mounted on "rear link" separated from main frame with 2 (or more?) additional links - chainstays and seatstays in case of HL and short links in case of all "short links 4-bars" (DW, Maestro etc).

    As you can see, regardless of link lengths, braking torque transfers to main frame and tries to swing it forward which results in suspesion decompression and brake jack. Don't be fooled by marketing, see for yourself. It's not magic, it's pure topology
    Personally I can feel rather severe brake jack on my Ellsworth Moment '09 despite all the hype.

    P.S. Yes, VPP does squat under braking because of counter-clockwise rotation of top link. It's an exception.

  22. #22
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    Great explanations (and diagram) guys! I think i can come pretty close to explaining it properly to someone else now
    Now, would i be in the ball-park saying brake jack/squat are inverse to pedal-bob? or is that a whole new bag o' worms. I imagine, in my little brain, that the forces on the axle are opposite. Is this correct? (i bet it's not that simple for some reason )
    Honestly... ahh I give up

  23. #23
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    Quote Originally Posted by highdelll
    Great explanations (and diagram) guys! I think i can come pretty close to explaining it properly to someone else now
    Now, would i be in the ball-park saying brake jack/squat are inverse to pedal-bob? or is that a whole new bag o' worms. I imagine, in my little brain, that the forces on the axle are opposite. Is this correct? (i bet it's not that simple for some reason )
    whole new can o worms with the pedal bob thing

  24. #24
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    I don't think any one has ever had issue with a bike squatting when applying the brakes. The issue with brake jack/squat has always seemed to be more of an issue with the inability of the suspension to react properly when the brakes are applied.

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    your avatar nails my response to what you just said red5.

    The second part anyway.

  26. #26
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    Quote Originally Posted by William42
    your avatar nails my response to what you just said red5.

    The second part anyway.
    When you grab the brakes on a suspension bike the suspension has a tendency to lock/stiffen. Watch the Kona video I posted above, it will make it much more clear.

  27. #27
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    Quote Originally Posted by RED5
    The issue with brake jack/squat has always seemed to be more of an issue with the inability of the suspension to react properly when the brakes are applied.
    Absolutely agree there and I would include the front suspension in that too. In general the bike handles like crap on the brakes verses off. I would include motorcycles also and that is smooth pavement. In the moto world they do not call it the "thrill uphill" for nothing.

  28. #28
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    Quote Originally Posted by NormanPCN
    Absolutely agree there and I would include the front suspension in that too. In general the bike handles like crap on the brakes verses off. I would include motorcycles also and that is smooth pavement. In the moto world they do not call it the "thrill uphill" for nothing.
    True, however it is a much larger issue with the rear suspension, at least the front is still able to track the terrain when being used instead of skipping over the bumps. Unless you run some sort of floating brake mechanism, like the Kona DOPE system or the Bullits floater.

  29. #29
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    Quote Originally Posted by RED5
    I don't think any one has ever had issue with a bike squatting when applying the brakes. The issue with brake jack/squat has always seemed to be more of an issue with the inability of the suspension to react properly when the brakes are applied.
    Quote Originally Posted by RED5
    When you grab the brakes on a suspension bike the suspension has a tendency to lock/stiffen. Watch the Kona video I posted above, it will make it much more clear.
    No no, I got it, you're just saying "When you do X, X doesn't happen." You're saying two completely contradictory things in the same sentence. I think you just don't really understand what brake squat is. Our explanations left something to be desired obviously.

    When a bike "squatts" it is exactly that "stiffening" feeling you're describing. The reason for this is because the brake causes the link, the arm, whatever its connected to to rotate (equal and opposite reaction). Essentially, the swing arm is taking the force of the wheel spinning onto itself. This causes it to rotate, which pushes into the suspension. All things equal, this would result in the back end of the bike sinking down.

    The reason you're getting confused here, is because at that same time the riders instant center pitches forward and so weight is removed from the rear wheel at the same time as the brake attempts to compress the wheel. This is especially compounded by the fact that usually when you brake, you're using the front brake too, which causes you to pitch forward even more. And so the shock extends despite "squatting" from the brake.

    The reason your fork or shock wont react as well under braking is because its deeper in the travel then it would otherwise be. This results in the feeling of the spring being stiffer then normal because you're deeper in the travel then you otherwise would be. The deeper you are in the travel, the less room there is for the shock to move and the stiffer it is, and so the result is your suspension doesn't track as well.

    Again, like I and davep said, the vast majority of bikes squat under braking. The reason your suspension doesn't track as well is because of brake squat. There are other benefits that you may not notice, but I'm sure if you rode something with truly no squat you would learn to appreciate (such as not feeling like you're getting pitched forward PLUS the springrate being off because your instant center is so much further forward).

  30. #30
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    Pulls up a chair (always enjoy these threads)


  31. #31
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    haha, well, I think this thread is pretty much over, theres not really too much to debate. I think red5 just didn't know what the terminology refers to, and now he does. Still good information for somebody who doesn't know what brake squat or brake jack is!

  32. #32
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    Go rigid......case closed.

  33. #33
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    Quote Originally Posted by William42
    haha, well, I think this thread is pretty much over, theres not really too much to debate. I think red5 just didn't know what the terminology refers to, and now he does. Still good information for somebody who doesn't know what brake squat or brake jack is!

    actually there are a few over simplifications and thus incorrect conclusions. (geo can have a significant effect...anti-squat as shown to pro-squat, depending)...

    then there is the true net effect on the chassis when things like mass transfer, traction force, etc are taken into account....

  34. #34
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    Quote Originally Posted by William42
    No no, I got it, you're just saying "When you do X, X doesn't happen." You're saying two completely contradictory things in the same sentence. I think you just don't really understand what brake squat is. Our explanations left something to be desired obviously.

    When a bike "squatts" it is exactly that "stiffening" feeling you're describing. The reason for this is because the brake causes the link, the arm, whatever its connected to to rotate (equal and opposite reaction). Essentially, the swing arm is taking the force of the wheel spinning onto itself. This causes it to rotate, which pushes into the suspension. All things equal, this would result in the back end of the bike sinking down.

    The reason you're getting confused here, is because at that same time the riders instant center pitches forward and so weight is removed from the rear wheel at the same time as the brake attempts to compress the wheel. This is especially compounded by the fact that usually when you brake, you're using the front brake too, which causes you to pitch forward even more. And so the shock extends despite "squatting" from the brake.

    The reason your fork or shock wont react as well under braking is because its deeper in the travel then it would otherwise be. This results in the feeling of the spring being stiffer then normal because you're deeper in the travel then you otherwise would be. The deeper you are in the travel, the less room there is for the shock to move and the stiffer it is, and so the result is your suspension doesn't track as well.

    Again, like I and davep said, the vast majority of bikes squat under braking. The reason your suspension doesn't track as well is because of brake squat. There are other benefits that you may not notice, but I'm sure if you rode something with truly no squat you would learn to appreciate (such as not feeling like you're getting pitched forward PLUS the springrate being off because your instant center is so much further forward).
    Trust me I'm not confused. My point was that the "squat" isn't the issue, it's the stiffening action of the suspension. Let me put this another way.Say your bike squats everytime you grab the rear brake and the suspension didn't stiffen, I doubt anyone would still have issues. My point was it's not the squatting thats the issue, but the suspension stiffening up. Believe me I have quite a bit of knowledge regarding brake jack/squat. Which is exactly why my 07 Coiler has a floater on it.

  35. #35
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    Quote Originally Posted by davep
    actually there are a few over simplifications and thus incorrect conclusions. (geo can have a significant effect...anti-squat as shown to pro-squat, depending)...

    then there is the true net effect on the chassis when things like mass transfer, traction force, etc are taken into account....
    Sure, if I were to design a bike just based on the above, it'd ride like poop. But then again, as a simplification for getting a concept off its to far off the mark either. There are lots of variables, but all things equal, thats basically what brake squat is.

    Quote Originally Posted by RED5
    Trust me I'm not confused. My point was that the "squat" isn't the issue, it's the stiffening action of the suspension. Let me put this another way.Say your bike squats everytime you grab the rear brake and the suspension didn't stiffen, I doubt anyone would still have issues. My point was it's not the squatting thats the issue, but the suspension stiffening up. Believe me I have quite a bit of knowledge regarding brake jack/squat. Which is exactly why my 07 Coiler has a floater on it.
    You clearly do not know what squat is. I'm not trying to be mean, it took me awhile to figure it out, but the "stiffening" action that your describing IS squat. It is the function of the brake inducing force into the shock that is not transmitted from bumps in the ground causing the shock to stiffen up and lose its ability to react to and absorb bumps.

    I think you may be thinking along the lines that when you grab the brake, it makes the swingarm more resistant to movement. That is not the case at all. Grabbing the brake does nothing to the swing arm other then try and push it up, which in turn compresses the shock. The stiffening feeling is brake squat. I don't know how else to say that. Keep thinking about it, it'll come.

  36. #36
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    Quote Originally Posted by William42
    ... Still good information for somebody who doesn't know what brake squat or brake jack is!
    hehe... it is good stuff...i feel a bit smarter
    Honestly... ahh I give up

  37. #37
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    so would I be correct in calling a bs on trek's ABP in reducing brake jack, as it should actually have less brake squat than their previos designs. Thats not to say it's braking performance isn't better than before, just that it is wrong to say it has no brake jack, instead it has more anti brake squat.

  38. #38
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    it has no brake jack. it has more anti-squat. Its quite similar to FSR really

  39. #39
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    Quote Originally Posted by William42
    No no, I got it, you're just saying "When you do X, X doesn't happen." You're saying two completely contradictory things in the same sentence. I think you just don't really understand what brake squat is. Our explanations left something to be desired obviously.

    When a bike "squatts" it is exactly that "stiffening" feeling you're describing. The reason for this is because the brake causes the link, the arm, whatever its connected to to rotate (equal and opposite reaction). Essentially, the swing arm is taking the force of the wheel spinning onto itself. This causes it to rotate, which pushes into the suspension. All things equal, this would result in the back end of the bike sinking down.

    The reason you're getting confused here, is because at that same time the riders instant center pitches forward and so weight is removed from the rear wheel at the same time as the brake attempts to compress the wheel. This is especially compounded by the fact that usually when you brake, you're using the front brake too, which causes you to pitch forward even more. And so the shock extends despite "squatting" from the brake.

    The reason your fork or shock wont react as well under braking is because its deeper in the travel then it would otherwise be. This results in the feeling of the spring being stiffer then normal because you're deeper in the travel then you otherwise would be. The deeper you are in the travel, the less room there is for the shock to move and the stiffer it is, and so the result is your suspension doesn't track as well.

    Again, like I and davep said, the vast majority of bikes squat under braking. The reason your suspension doesn't track as well is because of brake squat. There are other benefits that you may not notice, but I'm sure if you rode something with truly no squat you would learn to appreciate (such as not feeling like you're getting pitched forward PLUS the springrate being off because your instant center is so much further forward).
    You've got it wrong; Red5 has it right. And you shouldn't accuse someone of misunderstanding the terminology and concepts when you've just misused the term "instant center" twice. You meant center of gravity or mass.

    You misunderstand what causes the brake squat because you're leaving out half of the action/reaction relationship. The friction at the brake pads creates a torque trying to rotate the wheel backwards. The reaction is that the forward angular momentum of the wheel tries to rotate the swingarm forwards. The two cancel each other out.

    That means you can consider the swingarm and wheel (or floating link and wheel on a four bar) as though they were tied together into a single rigid unit. What's then left to consider is the backward acting force at the ground and its relationship to the pivot point for the wheel/swingarm unit. And you have to consider the relation between the rider/bike center of mass and the force at the ground. The first tries to compress the suspension; the second tries to extend it.

    The first moment--the brake squat-- is greater the higher and more rearward the pivot point is. On single pivot bikes with a fairly high pivot there will be about enough squat to just cancel out the extension caused by the forward weight shift. You won't find many bikes that will actually squat from rear braking. A bike like the OP's will probably stay right at the sag point from rear only braking.

    But that bike braking at the sag point will not handle bumps nearly as well as if it were coasting. And it's not any deeper in its travel than normal. The reason the suspension stiffens is explained on the Trek website referenced by gotboostedvr6 up above: http://trekmountain.typepad.com/king...explained.html The explanation is kind of buried in their promotion of their "active brake pivot" so I will summarize.

    The farther back the pivot point, the more the brake/swingarm or brake/floating link unit rotates during compression. As that unit rotates, the contact patch has to move forward around the tire. The ground force, pushing back against the contact patch is trying to keep the patch from moving forward. The two tendencies are fighting each other. The result is suspension stiffening. The more brake squat, the more stiffening.

    To conclude: a bike with a lot of brake squat will stay right about at the sag point from rear only braking (which is a good thing) but the suspension will stiffen and behave badly (obviously a bad thing). A bike with a far forward instant center, as on a nearly parallel four bar linkage with the caliper on the floating link, will extend a lot from rear only braking (a bad thing) but will not noticeably stiffen and will track the ground better (a good thing).
    "Don't criticize what you can't understand."

  40. #40
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    Quote Originally Posted by 006_007
    Pulls up a chair (always enjoy these threads)

    hey... gimme some of that popcorn
    Honestly... ahh I give up

  41. #41
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    Quote Originally Posted by Flystagg
    so would I be correct in calling a bs on trek's ABP in reducing brake jack, as it should actually have less brake squat than their previos designs. Thats not to say it's braking performance isn't better than before, just that it is wrong to say it has no brake jack, instead it has more anti brake squat.
    As should be clear from reading through this thread, different people mean different things by "brake jack". I think the term should just be dropped.

    The Trek bike with ABP moves the caliper from the chain stay to the seat stay. Now the braking pivot is the instant center of the linkage. That's way out in front.

    The bike will have little brake squat. The rear will rise up from weight shift under rear only braking. The suspension will not stiffen and will track well.
    "Don't criticize what you can't understand."

  42. #42
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    Its taking me a bit to get my head around where we disagree Steve, so I'm going to start from the basics and you can tell me where I'm going wrong. I don't claim to be an expert, and if somebody can show me how I'm wrong then I'll believe them, but your explanation isn't really doing it for me. And yes, you're right, I should have said COG, lets skip IC for now though.

    Brake squat is the tendency of the suspension to sink in under braking.

    Brake squat can occur at the same time as the shock extends. Heres where my explanations get tricky and hard to understand I think: The bike will "squat down" even as the shaft extends, because the shaft is extending due to shift in riders weight (forward). There are two parts to the mix here (and I'm going to assume you're not pedaling, because I have no idea how to explain what happens at that point). There is the brake/wheel/contactpatch whatever pushing the swingarm to compress the shock, and there is the forward shift in riders weight as they slow down (which unweights the shock, causing the spring to want to decompress and extend).

    I think we still agree at this point?

    If the sag point compresses, the the riders shift in weight is less then the brake induced squat.

    If the sag point decompresses, the riders shift in weight is more then the brake induced squat.

    The reason the bikes rear wheel stops working so well is because its suddenly oversprung when you apply the brakes. Squat attempts to keep the bike steady in its travel and keep it balanced without excessive pitch forward, but supposing your weight is 70-30 or 60-40 rear wheel-front wheel, when you brake it suddenly changes to 50-50 or 40-60 R/F. You have less weight over the rear wheel, and so whatever spring (or air pressure, whatever) you were running that was the correct springrate for you at a regular sag point and regular balance is suddenly too stiff. The suspension cannot react to bumps as efficiently.

    Hypothetically, your springrate should change throughout the normal movements of the bike. For regular riding, it should be (hypothetically) a 400 lb spring. Then, under braking, it should be a 350 or 375 lb spring. Then, as you stop braking and start pedaling, You should be back to a 400 or 425 pound spring.

  43. #43
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    Quote Originally Posted by William42
    Its taking me a bit to get my head around where we disagree Steve, so I'm going to start from the basics and you can tell me where I'm going wrong. I don't claim to be an expert, and if somebody can show me how I'm wrong then I'll believe them, but your explanation isn't really doing it for me. And yes, you're right, I should have said COG, lets skip IC for now though.

    Brake squat is the tendency of the suspension to sink in under braking.

    Brake squat can occur at the same time as the shock extends. Heres where my explanations get tricky and hard to understand I think: The bike will "squat down" even as the shaft extends, because the shaft is extending due to shift in riders weight (forward). There are two parts to the mix here (and I'm going to assume you're not pedaling, because I have no idea how to explain what happens at that point). There is the brake/wheel/contactpatch whatever pushing the swingarm to compress the shock, and there is the forward shift in riders weight as they slow down (which unweights the shock, causing the spring to want to decompress and extend).

    I think we still agree at this point?

    If the sag point compresses, the the riders shift in weight is less then the brake induced squat.

    If the sag point decompresses, the riders shift in weight is more then the brake induced squat.

    The reason the bikes rear wheel stops working so well is because its suddenly oversprung when you apply the brakes. Squat attempts to keep the bike steady in its travel and keep it balanced without excessive pitch forward, but supposing your weight is 70-30 or 60-40 rear wheel-front wheel, when you brake it suddenly changes to 50-50 or 40-60 R/F. You have less weight over the rear wheel, and so whatever spring (or air pressure, whatever) you were running that was the correct springrate for you at a regular sag point and regular balance is suddenly too stiff. The suspension cannot react to bumps as efficiently.

    Hypothetically, your springrate should change throughout the normal movements of the bike. For regular riding, it should be (hypothetically) a 400 lb spring. Then, under braking, it should be a 350 or 375 lb spring. Then, as you stop braking and start pedaling, You should be back to a 400 or 425 pound spring.
    I don't think I disagree with any of this. You're saying that when braking the load shifts to the front so that the rear is oversprung and the front is undersprung. That's always true, no matter what kind of rear suspension you have.

    What I was talking about is something additional to this. The more the caliper carrying link rotates during a given amount of suspension travel, the more the wheel gets a forward rotational force. The braking force at the ground is in opposition to this. It's also true that the more the caliper carrying link rotates during a given amount of travel, the more brake squat you have and the less the rear will tend to rise up from the forward weight shift. So more brake squat means more interference with suspension performance.
    "Don't criticize what you can't understand."

  44. #44
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    Quote Originally Posted by Steve from JH
    I don't think I disagree with any of this. You're saying that when braking the load shifts to the front so that the rear is oversprung and the front is undersprung. That's always true, no matter what kind of rear suspension you have.
    Ok at some point I must have implied something I didn't mean to, that was all I'm trying to get at. Brake squat is the tendency of the shock to compress under braking (barring shifts in rider weight). Brack jack is the tendency of brakes to decompress under braking (barring shifts in rider weight)

    Loss of traction is due to change in COG (shifts in rider weight) and the resulting change of correct spring weight. Brake squat is one tool used to keep the bike even and stable in its suspension despite the loss in traction because of the shift in COG.

    I think that (in complication with everything else I've said) sums it up pretty well. While I see what your saying about the change in caliper position in relation to the rotor, the amount of leverage enacted on the shock because of speed, bumps, and shifts in COG under braking effect the suspension to a way greater degree.

    Correct me if I'm wrong here though, you're one of the people I got this all from so I'll take your word for it if you show me how it works!

  45. #45
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    Quote Originally Posted by William42
    You clearly do not know what squat is. I'm not trying to be mean, it took me awhile to figure it out, but the "stiffening" action that your describing IS squat. It is the function of the brake inducing force into the shock that is not transmitted from bumps in the ground causing the shock to stiffen up and lose its ability to react to and absorb bumps.

    I think you may be thinking along the lines that when you grab the brake, it makes the swingarm more resistant to movement. That is not the case at all. Grabbing the brake does nothing to the swing arm other then try and push it up, which in turn compresses the shock. The stiffening feeling is brake squat. I don't know how else to say that. Keep thinking about it, it'll come.
    Actually, I have a very good understanding of brake squat/jack. But whatever I give up!! You do not seem to be grasping what I'm getting at and I haven't the patience to keep trying to explain myself, because frankly it's not really that important to me.

  46. #46
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    the simple explanation is..... Momentum wants to be conserved... Rear wheel is moving at a certain speed and when the brakes are applied, wheel slows down but momentum must be conserved. That motion will transfer into lifting the rear wheel up as the wheel cannot spin as fast. When rear is lifted up, suspension is compressed to do so and there, you have less travel and because of most suspension rates, they run stiffer in that range.

  47. #47
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    Quote Originally Posted by mrpercussive
    the simple explanation is..... Momentum wants to be conserved... Rear wheel is moving at a certain speed and when the brakes are applied, wheel slows down but momentum must be conserved. That motion will transfer into lifting the rear wheel up as the wheel cannot spin as fast. When rear is lifted up, suspension is compressed to do so and there, you have less travel and because of most suspension rates, they run stiffer in that range.
    kind of? whats moving wants to keep moving, so theres definitely a transfer of energy, but the reason it'll push into the suspension isn't because it can't spin any more and moves up, its because wheel is transmitting its spinning force into spinning the swingarm rather then itself. If thats what you're getting at the yes.

  48. #48
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    Quote Originally Posted by William42
    kind of? whats moving wants to keep moving, so theres definitely a transfer of energy, but the reason it'll push into the suspension isn't because it can't spin any more and moves up, its because wheel is transmitting its spinning force into spinning the swingarm rather then itself. If thats what you're getting at the yes.
    yea William... you hit it right on the money...

  49. #49
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    still kind of

    Quote Originally Posted by mrpercussive
    yea William... you hit it right on the money...
    this
    "wheel is transmitting its spinning force into spinning the swingarm rather then itself"
    should read
    "wheel is transmitting its spinning force into spinning the axle carrier rather then itself"
    the axle carrier maybe the swing arm or the seat stays
    and it may not be obvious at first glance which it is.
    "Be not afraid of going slowly but only of standing still." - Chinese Proverb

  50. #50
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    Quote Originally Posted by mrpercussive
    the simple explanation is..... Momentum wants to be conserved... Rear wheel is moving at a certain speed and when the brakes are applied, wheel slows down but momentum must be conserved. That motion will transfer into lifting the rear wheel up as the wheel cannot spin as fast. When rear is lifted up, suspension is compressed to do so and there, you have less travel and because of most suspension rates, they run stiffer in that range.
    Virtually no bikes actually compress in the rear from rear braking. So that can't explain the stiffening.

    The capture of angular momentum of the wheel, which is what you're describing, is a very minor effect because the wheel doesn't weigh much.

    The compressive torque from rear braking is almost entirely caused by the resistance to slowing of the entire bike and rider, which weigh much more.

    This resistance produces a backward acting force along the ground line against the contact patch. Since that force acts beneath the pivot point for the wheel and wheel carrying link, it causes a forward torque, clockwise viewed from the drive side, on that link.

    The longer the distance from the axle to the pivot point (the IC in the case of a four bar linkage), the less the compressive force is.

    The compressive force is at its most not much more than the extending force caused by the forward weight shift from deceleration.

    If you add any significant degree of front braking, all of this discussion becomes unimportant and it doesn't matter much what kind of rear suspension you have.
    "Don't criticize what you can't understand."

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