Cotic Rocket?

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  • Cotic Rocket?
  • Mbnut
    Member

    Just to prove I don’t understand…. does disc size make a difference then?? Should I be looking to run a smaller disc to counteract brake jack….

    Premier Icon ahwiles
    Subscriber

    no.

    (because..)

    the position of a brake caliper has the same affect on the torque generated as the shape of a spanner has on the torque it applies.

    it might be bent, it might be straight, it might be thick, it might be thin, it might be hollow, it might be solid, but the torque generated around the point of rotation is equal to the (input force) x (the perpendicular distance).

    input force = the deceleration force on the contact patch of the tyre.

    the perpendicular distance = the vertical height of the pivot above the ground.

    everything inbetween the contact patch, and the pivot, is just a funny looking bent-spanner.

    caliper postion has no effect.

    i’ve come to appreciate brake-squatty single-pivot bikes, a big handful of rear brake and the back end sits down a touch = they’re less nose divey than true brake-neutral bikes*.

    (*which i also like – you can brake through really heavy braking bumps and simply not feel them, everything is good, but for different reasons: i might suggest that slow steep and techy trails might suit a single pivot, and rumble-strip alpine Dh tracks might suit a parallelogram design, i might suggest this, but that would start another massive argument 🙂 )

    Premier Icon AlexSimon
    Subscriber

    Just to prove I don’t understand…. does disc size make a difference then?? Should I be looking to run a smaller disc to counteract brake jack….

    Well I don’t understand either, but in my head (again) it would just be the same as braking less. i.e. you’re reducing the leverage, and the power, but the angles would be the same.

    druidh
    Member

    Hang on… a brake caliper set behind the axle will have the effect of “pushing” the rear tyre onto the ground, increasing the amount of braking which can be carried out without skidding? One in front of the axle will have the opposite effect?

    Premier Icon ahwiles
    Subscriber

    no.

    caliper postion has no effect.

    druidh
    Member

    It will if the wheel is trying to rotate around the caliper?

    Premier Icon ahwiles
    Subscriber

    it’s trying, but it can’t move, it’s a solid structure.

    Premier Icon AlexSimon
    Subscriber

    Hang on… a brake caliper set behind the axle will have the effect of “pushing” the rear tyre onto the ground, increasing the amount of braking which can be carried out without skidding? One in front of the axle will have the opposite effect?

    Well that’s the question we’re asking.
    ahwiles is saying ‘no’ because the only force of significance is the force of the tyre.
    So the relationship between that and the suspension design is what creates squat/jack.

    But Cy is saying that moving the calliper made a big difference on the Hemlock, so I was just trying to get my head around it.

    When you put the bike in a stand and spin the wheel and brake, you can see what goes on, but that is totally ignoring the tyre on ground force. How large each of these forces is, is one for the physics/maths guys.

    Premier Icon ahwiles
    Subscriber

    But Cy is saying that moving the calliper made a big difference on the Hemlock…

    i wonder how he quantified that…

    the hemlock is one of the ‘more’ brake-neutral designs out there (especially with the long rocker fitted: it’s basically a parallelogram), but this means that heavy braking causes a lot of nose-dive, because there’s very little compressive force applied to the rear system.

    you can counteract the nose-dive of a hemlock by leaning back a bit, it’s not difficult, and quickly becomes second nature.

    I suspect that Cy ‘fixed’ the nose-dive of his hemlock by unconciously leaning back a bit, then concluding that his modified Seatstay was a success.

    just a guess of course…

    (and happy to be persuaded that i’m completely wrong about all of the waffle above with some lovely force vector diagrams – ooh, i do like a nice diagram)

    All I ask for is an explanation…

    Did you? Or did you say “I don’t buy it” which suggests you don’t believe what someone is saying, whether or not you don’t get it. Maybe you should take your cynic hat off and put on a confuzzled one instead, thus eliciting a more positive response. 😉

    ahwiles, you’ve missed off half the force vectors! If you want to explain why something doesn’t happen, ignoring the parts that don’t fit into your incorrect hypothesis isn’t the way to go about it. If the calliper is mounted to something that moves relative to the wheel (as with any calliper mounted to a suspension arm but not with any calliper mounted to a telescopic fork lower) then the forces will be non-internal and reactions will be exibited.

    cynic-al
    Member

    I said “ask” not “asked” and you said you CBA explaining 😀

    Pretty please I can haz a vector diagram?

    Premier Icon AlexSimon
    Subscriber

    ok – mentioning the difference between the fork and the swingarm, might have made a lightbulb come on (although it’s only dim).


    no – it’s gone.
    🙂

    mildred
    Member

    I’m thoroughly baffled by this, though strangely interested (its making me feel a bit unclean though).

    Ahwiles can you explain this from the Commencal Supreme DH V2 Tech’ book?

    Disc brake mount adjustment
    Your Supreme DH comes with an adjustment that allows you to modify the handling balance of your bike while under braking.
    During this phase, the rear braking forces act to compress the rear suspension, resulting in a loss of traction, and therefore its
    efficiency. This is sometimes referred to as “brake jack”.
    Your disc brake hanger has three positions; position number 3 being the one that least interferes with the rear suspension during
    the braking phase. Positions number 2 and 1 are intermediate positions. Don’t hesitate to go out and try each position to determine
    which one works best for you.

    (source: http://www.commencal.com/news/supreme_dh_uk.pdf)

    Unless I’ve totally misunderstood, isn’t this a similar thing to what Cy is suggesting, that caliper position can have an effect? Its certainly the 1st time I’ve seen this type of adjustment on a frame.

    Yes. When you think about it more it also suggests that the larger the disc, the more it matters. I imagine it causes some fun on high performance cars with semi-trailing arm rear suspension – one of the reasons old 911s throw you backwards off the road if you brake when cornering?

Viewing 14 posts - 46 through 59 (of 59 total)

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