Viewing 40 posts - 1 through 40 (of 61 total)
  • "brake jack" what's the accepted meaning Cos now I'm confused.
  • Premier Icon kaiser
    Free Member

    I always used to think that brake jack was the opposite of brake squat
    ie brake jack being the rear suspension extending causing a stiffening of the back end and brake squat was the suspension compressing .HOWEVER,,,am I wrong ? I ask this as tonight i was reading the Orange 5 thread and someone who seemed to be confident in what they were saying indicated that brake jack involved the suspension compressing.
    can someone who really knows clear this up for me .. googling seems to show I’m not alone and opinion is divided.
    thanks
    Bill

    Premier Icon Herman Shake
    Free Member

    I know brake jack is when the momentum of the wheel locking transfers into the swingarm contracting (shock compressing). Which also would make the frame “squat”. The Kona DOPE system is there to eliminate jack (I don’t know if they still use it?)

    Maybe one is the rear locking and the rider’s weight shifting toward the bars=rear opens up. The other is the momentum of the wheel rising toward the front triangle=rear closes in.

    Unless brake squat is fork related, aka dive?

    Or not 😆

    Premier Icon TandemJeremy
    Free Member

    brake jack is the suspension extending – the opposite of squat either can happen depending on the geometry.
    do I care? not one jot. I ride a hardtali

    Premier Icon coffeeking
    Free Member

    Brake jack is lifting the rear end, i.e. extending. Squat is indeed compression. FWIW jack is the exception I think, rather than the rule – normally frames tend towards squat under braking (much akin to pulling the handbrake on a hot hatch).

    Premier Icon Herman Shake
    Free Member

    Ah, jack as in lift. Squat as in err…squatting.

    Premier Icon jam-bo
    Full Member

    Brake jack is something shit riders complain about in an effort to justify buying another expensive frame that they ride just as slow on.

    Premier Icon crotchrocket
    Free Member

    Brake jack is something magazines and manufactures use toconvince riders that they NEED to ‘upgrade’ to newer ‘better’ frame in order that they don’t die in an horrific brake related incident which could have been avoided by the careful application of money and consideration of ‘kittens’.

    Oh and don’t omit to have the ‘correct’ amount of travel for your chosen riding style. If you use an XC bike on an AM trail you WILL DIE!!!!!!!!

    Premier Icon PeterPoddy
    Free Member

    Brake jack is something shit riders complain about in an effort to justify buying another expensive frame that they ride just as slow on.

    Nope. I found selling a £2500 boutique bike with a £1350 frame and replacing it with a £999 full bike from a mainstream brand cured the problem.

    🙂

    Premier Icon thepodge
    Free Member

    The 5 also suffers from a loss of active suspension under braking. all single pivot bikes do

    Premier Icon bazzer
    Free Member

    The 5 also suffers from a loss of active suspension under braking. all single pivot bikes do

    Why ?

    Premier Icon scaredypants
    Full Member

    The 5 also suffers from a loss of active suspension under braking. all single pivot bikes do
    Why ?

    in resisting the rotational force of the wheel, the brake ends up transmitting force to the frame where it’s bolted on. Single pivots don’t isolate that area of the frame from the rest of the back end and so it affects the movement of the suspension as a unit
    Why no,I’m not an engineer – how did you know ? 😳

    I have a long travel SP bike. Sometimes the back end seems almost locked out under braking. It also kicks back a bit on rough clims. Hasn’t changed my life though

    Premier Icon Shackleton
    Full Member

    Less freedom to pivot around the rear axle as the brake is stopping it rotating. This then stops the suspension arm moving as much (rather than actually stiffening up the suspension….). Just brake less on bumpy bits!

    Jack vs. squat. (I ride a 2005 Heckler.) In real world riding I’ve never experienced “squat” other than when I was actually trying to make the bike do it by putting my weight so far over the back end the front wheel was almost lifting. The only time I’ve observed it in isolation was when the bike was in a stand without a shock, I spun the back wheel up to high speed and applied the rear brake and the swing arm moved up / tried to compress the shock that wasn’t there. In the real world the act of braking and momentum puts my weight forward counteracting this squat along with fork compression and whatever clever gubbins Fox put in their DHX5 shocks.

    So I do experience “jack” and a reduction in suspension responsiveness when braking hard but have never had “squat” whilst riding “normally”.

    At the risk of sounding like a willy waver this hasn’t stopped me, or my other single pivoted riding friends, riding for the last umpteen years, nor stopped us beating VPP, Horst, DW link, etc. bikes up and down the hill. At the end of the day a crap rider on a posh bike will still be crap. A decent rider on a simple bike will still be a good rider. There is only so much the bike can do for you……….stop worrying about it and buy the bike that suits your riding style (and, sadly, budget) rather than the latest set of acronyms and get out and ride.

    Premier Icon Herman Shake
    Free Member

    Jack sh*t or diddly squat?

    Premier Icon Shackleton
    Full Member

    Jack sh*t or diddly squat

    A neat real world summation.

    Premier Icon cookeaa
    Full Member

    The 5 also suffers from a loss of active suspension under braking. all single pivot bikes do

    Actually pretty much everything sees some reduction in bump performance under braking, but in terms of bouncers single pivots are no better or worse in my experience, simple tester roll over some roots on a HT and then do the same whilst dragging your brakes, you’ll get a far rougher ride under braking, this is what alot of people percieve as “Jack”..

    The theory is that it’s all about how the loads from braking – via the caliper are imparted back into the bike’s frame, normally via whatever suspension component the caliper happens to be mounted on. The claims for Horst links and VPP/DW links that there is no effect from braking on the suspension is sheer coblers, there is always some energy that has to be dissipated somehow…

    Brake arms as used on Konas and Oranges of old were once accepted as the only possible “Cure” before it was realised the only actual cure is learning how to use your brakes properly…

    To be honest the people who notice “Brake Jack/Squat/lock” are either sensitive, tuned in riders, or proper Mary’s who ride every corner with their back wheel locked… the rest of us just pull on the brakes at the right point (Before the corner) and let the suspension do its thing…

    Premier Icon kaiser
    Free Member

    .stop worrying about it and buy the bike that suits your riding style (and, sadly, budget) rather than the latest set of acronyms and get out and ride.

    who’s worrying ? I simply have an interest in technology and like to understand things rather than just guess or pretend I know like many seem to do . I’ve often seen folks on here making statements after advice is requested which is total cobblers!

    also Mr S

    Less freedom to pivot around the rear axle as the brake is stopping it rotating.

    ….surely apart from treks ABP system no linkage pivots around the axle ..You sure you know what you’re talking about ?

    Premier Icon Iain Gillam
    Free Member

    Brake squat isn’t always be a bad thing, if you have a sharp bump that kicks the back wheel up then dragging the back brake on the face of it will keep the back wheel down. If you don’t want the suspension performance to be impaired by the brakes then lay off them on the rough stuff.

    Iain

    Premier Icon jojoA1
    Free Member

    Does brake jack cause the “kickback” feeling I get back from the pedals sometimes or is that something different?

    Premier Icon molgrips
    Free Member

    I think that some bikes squat, which counteracts the fact that your weight pitches forwards as you brake. A single pivot doesn’t squat like this so perhaps seems worse?

    However I think that more likely things like the axle path of a single pivot and the way you have to set the damping up compared to a multi pivot (ie more rebound etc) contribute to a lack of plushness when unweighted (ie braking). Some/many people think this is brake jack. IT’S NOT.

    Does brake jack cause the “kickback” feeling I get back from the pedals sometimes or is that something different?

    No, braking is nothing to do with pedals. That kick you feel is caused by the fact that on some suspension designs (like a 5) the distance between the cassette and the chainring varies as the suspension moves. This is what people mean when they talk about chain growth. As the suspension extends on a 5 the chain shrinks, and this slack is taken up by the freewheel. Then when it compresses again it tugs on your legs via the cranks.

    Actually, thinking about it, this is probably the cause of the roughness people feel when braking..?

    Premier Icon Shackleton
    Full Member

    who’s worrying ? I simply have an interest in technology and like to understand things rather than just guess or pretend I know like many seem to do . I’ve often seen folks on here making statements after advice is requested which is total cobblers!

    Sorry. Intended as a general comment rather than specifically directed at you. Bad phrasing on my part.

    surely apart from treks ABP system no linkage pivots around the axle ..You sure you know what you’re talking about ?

    Yep. Single pivot suspension systems rotate about the suspension arm pivot AND the drop out. (I just checked my bike to make sure!)

    Brakes off – free to rotate and wheel rolls nicely over stuff. Result – suspension absorbs most impacts smoothly.

    Brakes on – wheel stops/slows in it’s rotating reducing the ability of the suspension to pivot around the dropout, and instead pivots about the contact patch of the tyre on the ground. Wheel also can’t rotate smoothly(ish) over stuff and “grabs” at it. Result – wheel skips, all impacts become sharp spikes resulting in the feeling of less plush suspension (and theoretical induction of squat, although as I said before, I have never noticed it in the field).

    So under braking you remove the ability of the wheel to roll over things and move the wheel based pivot from the dropout to the contact patch increasing the force on the suspension system for any given trail obstacle. The more you lock the wheel the worse this will get.

    Premier Icon ridethelakes
    Free Member

    An easy way to illustrate what happens is to put the bike on a workstand, take the shock out, then spin the wheels fast and brake. On a single pivot the swing arm will jump up – this is squat. On a horst link or abp it won’t move.

    Some people like the squat effect as it slackens the geometry going downhill.

    Premier Icon molgrips
    Free Member

    On a single pivot the swing arm will jump up – this is squat. On a horst link or abp it won’t move

    Have you actually done this? I have on a 5 and I think it’s angular momentum which is doing it not break induced suspension activation.

    Premier Icon ridethelakes
    Free Member

    Have you actually done this? I have on a 5 and I think it’s angular momentum which is doing it not break induced suspension activation.

    Yes I have (thats how exciting my life is…).

    Premier Icon molgrips
    Free Member

    A 5 does not squat under braking though.

    Premier Icon bazzer
    Free Member

    Less freedom to pivot around the rear axle as the brake is stopping it rotating. This then stops the suspension arm moving as much (rather than actually stiffening up the suspension….). Just brake less on bumpy bits

    But if I put the rear brake on I can compress the suspension and it moves freely in the car park 🙂

    I do know why braking is a problem I am asking the question to illustrate that most people just spout what they are told and don’t really understand it 🙂

    I have moved from a Patriot (single pivot) to a Hustler (4 bar)

    I must admit I don’t notice the performance under braking being any better, but that might be because I rode the Patriot for 5 years so got used to not rear braking over rough stuff. What I do notice though is the lack of pedal feedback on the Hustler when climbing. Going over a root or something, I could feel pedal force increase/decrease.

    Patriot in general felt much more active though, Hustler seems to steer better though

    Premier Icon ridethelakes
    Free Member

    molgrips – Member

    A 5 does not squat under braking though.

    How so? You proved it yourself?

    Call it angular momentum or whatever you like but the end result is the same.

    In reality the suspension still works and single pivot bikes are a lot of fun but theres no doubting that the suspension does compress under braking.

    Premier Icon molgrips
    Free Member

    The impulse exerted by the transfer of angular momentum in that situation is naff all compared to all the other forces in play. Plus when you are braking steadily down a hill say you’re not even slowing down, so there’d be no transfer of angular momentum.

    How so? You proved it yourself?

    Every time I brake and my hands go down and my bum goes up!

    theres no doubting that the suspension does compress under braking

    It’s not really possible in the real world because the caliper and dropout are attached to the same bit of metal. All the braking force would go into flexing the swingarm about a point between the caliper and the dropout.

    Premier Icon RoterStern
    Free Member

    [video]http://www.youtube.com/watch?v=BRf7KlTjPTc[/video]

    This shows it pretty well.

    Premier Icon bazzer
    Free Member

    Moly

    Just because the front goes down does not mean the rear suspension has not compressed. It just means your forks have been compressed due to the weight transfer. Your rear shock could still be on the bump stop. This is the effect people are talking about.

    In the real world the shock will probably not be on the bump stop so the rear will be still active to some degree.

    Premier Icon molgrips
    Free Member

    The rear definitely extends, a lot.

    You are talking to Mr Obsessive bike fettler here. Yes, I do look down at my shock when I brake, to see what it does. I also have noticed that sag increases slightly when going up even slight hills, and decreases when going down.

    Just because the front goes down does not mean the rear suspension has not compressed

    No but as I said, my bum goes up. This means the rear has extended. I’ve ridden and thought about this bike a lot, and I can confirm that it does not squat under braking. And even if it did the effect would be completely swamped by 85kg of me going forwards. On hard braking I throw my weight back to keep it level. Or if it’s a singletrack corner my weight goes back, then sideways… then forwrds if there’s a step up.. it’s all over the place. So the tiny force exerted by the sudden stopping of wheel in a bike stand has absolutely zero effect.

    Like I say, lack of plusness under braking has other causes in this instance.

    This shows it pretty well.

    No it doesn’t. For a start that’s squat not jack, and as above that effect is something different.

    Premier Icon Shackleton
    Full Member

    But if I put the rear brake on I can compress the suspension and it moves freely in the car park

    This is either trolling or just ill-thought through.

    I do know why braking is a problem

    Based on the evidence you have presented in this thread I don’t think you fully understand it.

    I am asking the question to illustrate that most people just spout what they are told and don’t really understand it

    As far as I can determine you fall into this category yourself. I’ve never come across auto-trolling before!

    Premier Icon tommmm
    Free Member

    The video is surely just a transfer of angular momentum from the wheel to the frame.

    Premier Icon tommmm
    Free Member

    I think Moly’s generally right, his weight is far greater than that of the wheel, and is much more likely to affect the performance. Though the statement about a flexing force between caliper and dropout seems unlikely. The swingarm will be designed to be strong enough to transfer this force away from the area, and will lead to squat, minimal though the effect may be.

    Premier Icon flow
    Free Member

    I had a Trek EX 9 2010 with ABP, it suffered from brake jack just as much as the Five I just bought, its all bullsh*it.

    Premier Icon molgrips
    Free Member

    Though the statement about a flexing force between caliper and dropout seems unlikely. The swingarm will be designed to be strong enough to transfer this force away from the area

    Not really – the swingarm will flex minutely because it’s very stiff. But increased stiffness doesn’t transfer the force away, it just resists it.

    It’d be like standing half way along a seesaw (between the seat and the middle) and pulling up as hard as you can on the seat handle. No matter how hard you pull the seesaw is never going to tip!

    I had a Trek EX 9 2010 with ABP, it suffered from brake jack just as much as the Five I just bought, its all bullsh*it.

    No, you just don’t know what brake jack is. Rough suspension under braking is unavoidable.

    Premier Icon ahwiles
    Free Member

    wheel rotation invokes swingarm rotation when the brake is applied.

    (the ground skidding past the tyre is an input force, which acts around the pivot, the swingarm rotates).

    your seesaw model is not valid.

    sorry.

    let’s say you’re slowing at 0.1g, and weigh aroudn 100Kg, the force at the tyre/ground contact patch will be 100N.

    100N acting at a distance equal to the height of the pivot above the ground (0.35m)= 35Nm.

    and that’s 35Nm of compressing torque – not insignificant (based on 0.1g)

    Premier Icon molgrips
    Free Member

    the ground skidding past the tyre is an input force, which acts around the pivot, the swingarm rotates

    well since that’s inevitable if you want to slow down, it’s hardly the fault of the suspension design.

    Brake jack is the action of the actual brakes on the suspension, not the ground.

    Premier Icon flow
    Free Member

    Ahh yeah silly me, been riding/cycle mechanic for 16 years and I don’t know what brake jack is, sorry oh wise one 🙄

    I know there isn’t much difference between the Trek and the Five when it comes to brake jack, I was there when I rode em.

    Premier Icon flow
    Free Member

    I don’t know why people see it as a bad thing anyway. You can use it to your advantage once you are used to it as it weights the front wheel giving it more traction.

    Premier Icon retro83
    Free Member

    flow – Member

    I don’t know why people see it as a bad thing anyway. You can use it to your advantage once you are used to it as it weights the front wheel giving it more traction.

    It also steepens the head angle and reduces traction on the rear wheel

Viewing 40 posts - 1 through 40 (of 61 total)

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