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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, you just take a deep breath and say Fuck it.

  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, you just take a deep breath and say Fuck it.

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

  4. #4
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  5. #5
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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, you just take a deep breath and say Fuck it.

  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, you just take a deep breath and say Fuck it.

  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, you just take a deep breath and say Fuck it.

  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, you just take a deep breath and say Fuck it.

  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, you just take a deep breath and say Fuck it.

  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, you just take a deep breath and say Fuck it.

  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.

  25. #25
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    your avatar nails my response to what you just said red5.

    The second part anyway.

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