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how does single pivot suspension cause brake jak?

yepyep Posts: 45
edited August 2009 in MTB general
hey i was just wondering how single pivot suspension causes brake jaking it would be really nice to find out. (please don't call me an idiot for asking it just i have never heard or read any thing where someone explanded it)

thank you


  • supersonicsupersonic Posts: 82,708 Lives Here
    The don't always. Depends on the pivot position, the weight transfer when you brake, the shock. Some extend, some compress.

    Some good info here: ... index.html
  • yepyep Posts: 45
    hey thanks i hope this will clear some things up :)
  • AnonymousAnonymous Posts: 79,667
    On a single pivot swingarm, it's not the actual suspension that "causes" brake jack as such, it's your deceleration. I've always founf the effect to be minimal when compared to fork dive.

    Anyway, the mechanics of it....
    On a single pivot, the rear wheel must trace an arc as it goes through the travel. Because of this arc, the wheel base changes, depending on where the rear wheel is at in it's travel.
    When braking, the rear wheel tries to extend to it's most rearward (longest wheelbase) position.
    If the suspension has to extend in order for the wheel to reach it's furthest point on the arc, then the bike is jacked up, and if the suspension must compress in order for the wheel to reach it's furthest arc position, the suspension squats.

    Does that make sense?
  • yepyep Posts: 45
    well yes but what if the single pivot suspension also has a link like in the yeti asx? here is a pic if it helps
  • supersonicsupersonic Posts: 82,708 Lives Here
    The linkage on a monopivot bike controls the rate of the suspension and leverage - these are secondary to the main effect here.

    Also have to factor in how we shift our weight when braking.
  • yepyep Posts: 45
    so it would make no difference to brake jaking?
  • AnonymousAnonymous Posts: 79,667
    I've nevere studied the movement on a Yeti linkage, but from seeing them static, it appears that the link actually does nothing, except help prevent twisting.
  • supersonicsupersonic Posts: 82,708 Lives Here
    yep wrote:
    so it would make no difference to brake jaking?

    If the exact same leverage and rate could be engineered without it, no.

    As Yeehaa says, another reason is to isolate the shock from side loading.
  • yepyep Posts: 45
    soz but is side loading twisting or flexing?
  • AnonymousAnonymous Posts: 79,667
    It means not applying force directly along the shaft of the shock.
  • AnonymousAnonymous Posts: 79,667
    according to the bloke on the link you posted, every manufacturers claim is a lie and all suspension works the same?

    is that the jist of it
  • supersonicsupersonic Posts: 82,708 Lives Here
    Well, not exactly - but it is a case of what works best for you. Many manufacturers claims (as I said in the Spesh thread) are incorrect.
  • AnonymousAnonymous Posts: 79,667
    according to him, everyones claims are wrong and he seems to have some real beef with elsworth for some reason
  • supersonicsupersonic Posts: 82,708 Lives Here
    Because Elsworth contradicts himself badly. He does concentrate on what he feels are incorrect claims. However:

    "Our advice is to ignore all suspension theories and other claims put forth by frame manufacturers and industry magazines, and base your buying decisions exclusively on experimentation. That is, make your decisions by test riding the bikes, even if it is just a parking lot test (you can get a lot from a parking lot test). Ignore all marketing!"

    Which is good advice for the novice!
  • AnonymousAnonymous Posts: 79,667
    sound like good advice all round
  • nicklousenicklouse Posts: 50,675 Lives Here
    Oh brake jack and the all the other things that the manufactures thing are great marketing topics.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • From what I can see the kona DOPE braking arm, (designed to elliminate brake jack) it works in principal, but only if your locking up the back brake at high speed, and in the video i saw they had removed the shock to enhance the action for the viewer so i wonder how effective it really is when the suspensions jacking against a 150psi + shock, well not noticable enough to give the rider any great benefit i would say - another marketing ploy perhaps ? These kits are like 200 quid or something for what basically amounts to a bar and bracket !!!

    My theory is this and I may be wrong (I dont have a massively funded R+D department like Kona, only my limited mechanical knowledge and common sense) if your going that fast on terrain bumpy enough where brake jack is going to cause you serious problems, and remember I only think this DOPE would work while grabbing big handfuls of brake, then I think your in big trouble and no metal bar is going to get you out of it !!

    However, if I had some spare cash and nothing else needed upgrading on my bike I would fit it, just so people would ask me what the metal bar was and I could say it was DOPE + it looks cool as well !! :lol:
  • supersonicsupersonic Posts: 82,708 Lives Here
    The DOPE basically gives four bar archicture to the system - and how that is done can make it feel different. DIfferent, not better or worse. Not to some.
  • AnonymousAnonymous Posts: 79,667
    The ironic thing about Kona's DOPE bar, is that of ALL the bikes I've ridden, I've never really thought brake jack was an issue - APART from a DOPE-equipped Stab Supreme.
    It really was so bad it was dangerous. :shock:
    In fact, I find Kona's suspension to be the perfect example of how to maximise brake jack!
  • AnonymousAnonymous Posts: 79,667
    Oh, just remembered as well - Text book perfect example of the effects of brake jack...

    Take a french car, mostly any french car, but Peugeots in particular, and point it down a hill.
    Stop it with the foot brake, then apply the handbrake (REALLY hard, they never really work on peugeots :lol: ) now let go of the foot brake, and notice how the back end squats down an incredible amount.
    THAT, is the same as a bike that would squat under braking.

    Thing is, it makes no difference to the car, they still handle great, and the vast majority of deceleration is done with the front brakes, just like it is on a bike.
    so, as long as you're not riding a Kona, brake jack can be ignored.
  • I think, (have you seen the video demonstration ? It was on konaworld but I cant see it anymore ) what there saying in the demo is that the arm absorbs the rotational momentum of the wheel whilst braking and transfers it to the frame, all very well and good except -

    The bike was in the air at a standstill, so no rider weight, no forward momentum and no friction on the tyre.

    So my point is this, when you brake as your riding, your weight tries to carry on forward as well as the bikes, the grab of the rear tyre tries to pull the bike back, now obviously if the back tyre is trying to brake on rough terrain and the back wheel is skipping then the tyre may momentarilly lose traction but by then the wheel has lost most of its rotational force so it's unlikely to be jacking up with more force than the rear shock is pushing it back.

    I'm not saying it doesn't work at all, just perhaps the effect isn't as Kona would have you believe ?? :roll:
  • supersonicsupersonic Posts: 82,708 Lives Here
    Exactly one of the points I made out earlier - the big wibbly wobbly thing sat on the bike; us ;-)

    We react in different ways, so suspensions works differently for all of us. Sure, some of the concepts are sound, but testing is the only way.

    Some pro riders have their brakes set up to provide squat or extension under braking - CG was one.

    The 4 bar defines an IC for the caliper that is not the centre of rotation, hence the braking can be tuned independantly of the pedalling effects.
  • I cant slate them too far as my dream bike is a coilair magiclink with dope - dont ask me why im a freak and the more moving parts a bike has to play with and fettle and clean in the shed then the happier I would be :lol:
  • AnonymousAnonymous Posts: 79,667
    Well that kind of highlights DOPE as a bullsh*t marketing ploy. The Brake jack effect from rotating wheel mass is immesurably small compared to the wheelbase extension effect - unless your suspension is INCREDIBLY badly designed.
  • AnonymousAnonymous Posts: 79,667
    I cant slate them too far as my dream bike is a coilair magiclink with dope - dont ask me why im a freak and the more moving parts a bike has to play with and fettle and clean in the shed then the happier I would be :lol:
    I honest to god wouldn't bother. It REALLY annoys me that they get a suspension genius in to design some specific part to their spec, rather than asking him to come up with the be-all and end-all in suspension.

    It's like asking a lotus engineer to make a tractor handle better, but he's not allowed to change anything pivotal.
  • supersonicsupersonic Posts: 82,708 Lives Here
    I think DW is on to a good thing - the links that can be tuned to purpose to give more anti squat or less pedal feedback depending on usage. Basically the more anti squat you have with a design, the more pedal kickback you will get as the chain length alters. The DW concept was to alter the VPP through the travel so you got the anti squat in the areas you pedal, and not where you don't need it and therefore less kickback. To many this is a great trade off.

    A DH biker may not pedal as much - so might like a more active suss right from the start. DWs bikes do follow this concept. Combined with tuned leverage rates they can give many possibilties.

    Of course the above still applies - try them out.

    I also like the GT Idrive is it also combines good pedalling performance with minimal pedal kickback.
  • I know mate I just think they look the mutts nuts and would like to ride one just to see how they feel !

    I've ridden loads of full sussers including a marin rift zone quad link, a carrera LRS, a Gt i-drive, a trek fuel, a giant yukon and a few others if i cared to remember, and I can honestly say without a shadow of a doubt, as an all rounder I love my kona dawg the best, I cant explain why apart from it feels like a grown up bmx with suspension and I love it - its such a simple design, its never bothered me having a single pivot frame and I certainly dont suffer from BJS (brake jack syndrome :lol: )
  • Saying that I have never had a proper go on a big travel downhill bike - which is a different ball park..........
  • I wonder what an I-drive with magiclink and DOPE would be like :lol:
  • supersonicsupersonic Posts: 82,708 Lives Here
    edited August 2009
    In some respects the Idrive is a magiclink ;-)
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