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Thread: Girvin fork

  1. #26
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    Look fork

    Used without permision from the original poster.

    Spring is fender time

    There are numerous threads out there regarding this fork. Cool stuff.
    Zip ties? Not on my bike!

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  2. #27
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    Heavy, expensive, complicated. Motos have motors.


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  3. #28
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    If it (the Hossack) can be done in a way that is durable, structural, and close to the weight of a telescoping fork their are some huge benifits. Constant wheelbase, constant trial, or constant head angle to say the least. There is also the fact that the wheel path of the front wheel can be adjusted to follow the ground with a minimum of drag against the forward momentum of the COG.

    It's all bound to the 'if'. People have been trying for over 120 years. BMW seems to have successfully brought it to street motorcycles. That means that bicycles may be able to apply a similar concept.

    Most developers have pretty much given up on the idea of a leading or trailing link fork designed as a separate unit from the rest of the chassis. Building the front end into the chassis is still being persued and shows some promise. One day...one day...

  4. #29
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    Quote Originally Posted by pvd
    or constant head angle to say the least.
    On a full suspension bike, I don't think this can ever be possible.

  5. #30
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    Here's another linkage fork. It was something Honda was experimenting with some time ago.
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    May the air be filled with tires!

  6. #31
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    Walt, I've sent my buddy the link to this thread to see if he'd be interested in donating it.

  7. #32
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    Well, I have nothing constructive to add, but I find it a very interesting thread. I have often wondered while riding my SS if a better constructed version of an AMP/Girvin type fork would not be a viable fork in a lesser travel environment like 100mm and under. If...if it can be done and end up lighter and stiffer than a telescopic fork. Or why bother? I have little complaints on brake dive with a well tuned fork.

    You go Walt.

    Oh, and the other bike shown...the link one...wow. I remember that. I am not sure I could ride something that looked so unlike a bicycle no matter how well it worked.
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  8. #33
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    Quote Originally Posted by pvd

    Nothing can elliminate brake dive since the physics dictate that more weight goes to the front during braking. Any system that minimizes dive is basically shutting the fork off at a time when it's needed the most to maintain traction. Motorcycles gave up on this concept about 25 years ago.
    Huh? You feed braking force into the fork/linkages to balance out some or all of the weight transfer. Nothing is getting "shut off". Same thing with anti-squat.

    I think motorcycles gave up on it because it was needlessly complicated or unmarketably expensive. Definitely used in cars though.

  9. #34
    pvd
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    Quote Originally Posted by dr.welby
    Huh? You feed braking force into the fork/linkages to balance out some or all of the weight transfer. Nothing is getting "shut off". Same thing with anti-squat.
    NO! The front end dives because more weight is being placed on it due to the deceleration of the bike under the COG. The shift HAS to happen. If you are doing anything that prevents the front end from compressing in this condition, then you are effectively shutting the system down.

    Squat is exactly the same thing, but we use anti-squat geometry to control it. We need to get the power to the ground for maximum drive but we still need the bike to stay composed. Anti-squat slightly reduces our traction to keep the bike composed. Too much anti-squat and the rear end will completely loose traction. Very bad.

    When we are braking very hard it is paramount that we keep maximum grip for control. Any anti dive mechanism will reduce grip and cause us to loose traction and control at the worst of times.

  10. #35
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    Quote Originally Posted by pvd
    NO! The front end dives because more weight is being placed on it due to the deceleration of the bike under the COG. The shift HAS to happen. If you are doing anything that prevents the front end from compressing in this condition, then you are effectively shutting the system down.
    BUT YES! The shift has to happen, but the fork does not need to compress. It's simple physics. Just add up your forces (positive in the direction of compression):

    + Rider weight
    + weight transfer (dive)
    + bump force
    - anti-dive
    ---------------
    = total force on suspension


    If you have 100% anti-dive, the dive and anti-dive forces cancel out. So the total force on the suspension is the rider weight plus the bump force, just like it is when you're not braking.

  11. #36
    Relax. I'm a pro.
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    I'll continue riding my hard tail and not worrying about all this stuff.

  12. #37
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    Quote Originally Posted by dr.welby
    Definitely used in cars though.
    Please explain how it's used on cars. Typical road cars?
    May the air be filled with tires!

  13. #38
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    Ah, the Ribi Quadralateral. It was said to work great and was promoted by Roger Decoster(MotoX Legend). It was too complicated and heavy to produce. Same reason the Boyseen Link never saw the light of day. Even though Edvind Boyseen still uses it on his bike.

    I think motorcycles gave up on it because it was needlessly complicated or unmarketably expensive.
    All anti-dive is internal on MX forks. Before they developed the technology they ran this.....



    Why not try leading link instead of teloscopic?? Would be much easier to machine the parts for.

  14. #39
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    Quote Originally Posted by dr.welby
    So the total force on the suspension is the rider weight plus the bump force, just like it is when you're not braking.
    Wrong.

    The "total force" on the suspension never changes, regardless of what type of fork you have. (well, ok, it may change very slightly due to length of the fork changing, thus affecting the realtime location of the center of gravity).

    A simple free body diagram shows that the reaction force at the front wheel of a bike is same under all conditions (assuming the wheelbase and center of gravity location remains fixed).

    Peter is right. Anti dive increases the wheelrate of the front wheel during activation.

  15. #40
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    Quote Originally Posted by RoyDean

    A simple free body diagram shows that the reaction force at the front wheel of a bike is same under all conditions (assuming the wheelbase and center of gravity location remains fixed).
    Well, could you draw it then and post it? I'm not following what you're saying.

  16. #41
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    I rented a bike with a Girvin fork, and that baby felt PLUSH.
    I guess the bushings would always wear and make the whole fork wiggle, but good bearings and beefy construction might solve that. Besides no stiction, I think the axle path helps with absorbing bumps.

  17. #42
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    Quote Originally Posted by dr.welby
    Well, could you draw it then and post it? I'm not following what you're saying.
    See attached image (sorry for the poor quality, I had to rush as I'm late to meet my inlaws for dinner, somebody check the math).

    Basically, the final formula

    R[SIZE="1"]B[/SIZE] = [F(H) + W(O)] / WB

    means that the vertical reaction at the front wheel (R[SIZE="1"]B[/SIZE]) (the primary force seen by the fork, but not total as I didn't bother doing both the x and y directions and resolving them into a single force based on the head tube angle) is based solely on the braking deceleration force (simplified as "F"), the height of the center of gravity (H), the weight of the bike+rider (W), the distance of the center of gravity from the rear wheel (O), and the wheelbase of the bike (WB).

    Like I said, this is simplified. Doesn't account for slight changes in the dimensions during suspension movement (but they would be minimal), and doesn't resolve x direction forces into the fork reaction force.

    But the simple response is that the geometry of the CG versus the two wheel positions is the only thing that affects the force on the front wheel/fork. Slight anti-dive variations thrown in WILL affect the absolute numbers that result, but they absolutely will not "eliminate" the weight transfer phenomena.


  18. #43
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    Quote Originally Posted by RoyDean
    See attached image (sorry for the poor quality, I had to rush as I'm late to meet my inlaws for dinner, somebody check the math).
    Hey, wow, thanks for taking the effort! Giving it a cursory glance it all looks correct. I've always seen it in a much simpler form, where you just calculate the weight transfer as F*h/l where F is braking force, h is height of C.G., l is wheelbase.

    So I fully agree with your free body diagram of the rider/bike system. In my above summing of forces, I have your two terms - the weight of the rider/bike (though techically it's the fraction of the weight on the front wheel), and the weight transfer (f*h/l).

    However, we're interested in what's going on in the fork, and if you have anti-dive then you have that force F channeling into the fork through torque F*r, and then through some linkages or pistons into the fork. Which is why I add the anti-dive force into my sum of forces on the fork.

    You mentioned something about that force increasing the wheel rate, which is where this discussion probably needs to go from here.

    For me, and maybe this is just a semantic pothole, wheel rate indicates some sort of spring constant that relates a force to a displacement. My position is that the wheel rate does not change - the spring rate of the fork is the same, but the anti-dive essentially "preloads" the fork to match some or all of the weight transfer.

    An interesting aside I have probably mentioned before: I was talking to a guy who was experimenting with anti-dive bike forks, and one of his first iterations had a lot of anti-dive - around 100%. Initially, he really liked it because he could run the fork much softer than he normally would, until one day he was riding a trail that had a technical switchback that required a trials- style front wheel endo move. He rolls up onto the front wheel, his forward momentum stops, the anti-dive no longer activated, the soft fork relaxes and drops down through its travel, shifting his point of balance and he goes over the bars. I remember seeing something similar playing around with those Lawill Leader forks - you could slowly roll up into an endo and watch the anti-dive reduce as you slowed down and stopped.

  19. #44
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    What shock is going to be appropriate for the front?

    Seems sub-optimal to use something valved and sealed for much higher loads...

    (I still like the idea)
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  20. #45
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    Quote Originally Posted by dr.welby
    You mentioned something about that force increasing the wheel rate, which is where this discussion probably needs to go from here.

    For me, and maybe this is just a semantic pothole, wheel rate indicates some sort of spring constant that relates a force to a displacement. My position is that the wheel rate does not change - the spring rate of the fork is the same, but the anti-dive essentially "preloads" the fork to match some or all of the weight transfer.
    Right, gotcha.

    Ok, so "wheel rate" simply means the force vs. displacement characteristic of the wheel. In a typical bicycle fork, this will pretty much equal the spring rate of the fork, plus any misc forces involved (seals, damper effects, etc.). However, in linkage style suspensions, the rate of wheel movement does not always equal the rate of spring movement, so in those cases the wheel rate doesn't equal the spring rate. Additionally, any amount of anti-dive or anti-squat will also change the wheel rate (in a fashion that seems way more difficult to calculate, although it's definitely doable).

  21. #46
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    As RoyDean states, the force @ the front wheel does not change. If you neglect the small CG movement, both anti-dive and non anti-dive forks have the EXACT same normal force.

    W/ mechanical anti-dive the shock damping is not affected (Note: this is NOT how 'platform' damping works). Only a counteracting force is added INTERNAL to the FBD. This is how the para/telelever systems work. The dynamic spring rate doesn't change so the forks bump response is unaffected. Since the fork doesn't have to resist dive forces by using the springs, lower rate springs and damping can be used to soak up bumps better. For a big travel bike that needs high rate springs/damping to prevent bottom out there may be no benefit.

    As mentioned before cars can implement anti-dive/squat. On both A-arm and Mcpherson strut suspensions can angle the front of the A-arm(s) downwards. AFAIK, its not done very often on production cars. Anti-squat in the rear can be done similarly. That's very often used on semi-trailing arm designs.

  22. #47
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    Quote Originally Posted by smdubovsky
    As RoyDean states, the force @ the front wheel does not change. If you neglect the small CG movement, both anti-dive and non anti-dive forks have the EXACT same normal force.
    Yes, exactly. Anti-dive does not eliminate weight transfer. It counteracts it within the suspension.

  23. #48
    pvd
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    Quote Originally Posted by smdubovsky
    This is how the para/telelever systems work.
    Telelever has no anti-dive properties. It does provide additional support to the telescoping fork legs during braking. This is to fight the flex that occurs during braking. The flex on traditional telescoping forks during braking is pretty outragous.

    Paralever is a rear system designed to reduce an effect known as 'shaft drive climb'. A standard shaft driven bike will jack as the gears of the shaft climb over each other. Essentially, paralever would be a pro-squat system.

  24. #49
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    Quote Originally Posted by pvd
    Telelever has no anti-dive properties.
    Yes, it does. Don't you own this book?


  25. #50
    pvd
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    Sorry. I'm going to have to be corrected. Thanks. But I stand by my argument against anti dive systems. I've never bought into that argument that the BMW system is done for anti-dive. I belive that it is for struture first and formost. As typically fully loaded tourers, Structure is paramount.

    Under heavy braking, nearly 100% of the system weight is on the front wheel. If any less than 100% of the initial unsprung weight is not carried by the spring during braking, then there is a huge performance loss in the tracking of the front wheel. Essentially, a massive load is applied to the wheel and not the spring during braking. That blows. Since we all know BMW is not know as any kind of racing motorcycle except ancient history and 2010 MotoGP, we can accept the design as not optimal.

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