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Thread: Girvin fork

  1. #1
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    Girvin fork

    Who was looking for a Girvin fork? One of my riding buddies was joking about having one "for sale" and really does. I'm sure you've got it by now for whatever project it is, but I know it was someone on here that needed one. If you still do, I have lines on a couple it sounds like.

    Later,
    Jay

  2. #2
    pvd
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    One of the worst forks ever before the DUC.

    I used to call those forks 'StapleGuns'. Many of the itterations had no provisions at all for rebound damping. Litterally dangerous at any speed. Other than that, they had no tortional stiffness to speak of.

  3. #3
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    Quote Originally Posted by pvd
    One of the worst forks ever before the DUC.
    Nice! Good to see your sense of humor is turned on today, Peter.

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    Walt was looking for one

  5. #5
    pvd
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    I gotta admit, the Shiver SC was amazingly bad as well. I dealt with that for too long before giving up.

  6. #6
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    The linkage fork? We used to ride bikes with those forks at full speed into curbs. They were kind of fun for that.

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    I'm feeling PVD on this one. Girvin.. everything Girvin is shite. Unreal stomach turning, projectile vomit bad. Look at the dude.... bad bad bad. The only site I could find a pic of him is:

    suwonalpha.co.kr

    And google told me I would die if I opened the link.

    Bad.

    -Schmitty-

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    That was me.

    Yes, it's a stupid idea, but I am thinking of building a "modern" version of the Girvin/Noleen/K2/whatever linkage fork. I wanted a garbage one in hand to mess around with.

    Reason? I need a stupid winter project, I hate brake dive, and I think the fork could be improved with bearings instead of bushings and a 20mm axle. Not to mention an RP23 instead of gnarly elastomers.

    I'll probably end up with something even crappier than the original. But hopefully it'll be fun. So Jay, if your pal wants to *give* me a fork, great. I'm not going to pay money for one, though, considering that I really only want it to help solidify my thoughts on how to go about building my own. 1 1/8" would be preferable, in the case that I actually wanted to ride around on it at any point, but it's not necessary.

    -Walt
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    Should be a good exercise in machining!

    Replacing the many bushings with bearings will drive weight through the roof.. both from the bearings themselves and the needed thickness on the linkages to accomodate the bearings.. it will get very wide as well for the same reasons.

    Try to use needle bearings it seems...

    I think I just saw one in the bone pile in a shop here.. I'll let you know.


    -Schmitty-

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    Yup!

    I'm aware that the weight will be, um, heavy. That's not particularly a concern for me, though - I regularly ride a 40 pound DH bike on XC rides, and I have no interest in selling this thing to anyone.

    I'm not a great machinist and not good with computer-aided drawing/design, so I thought this would be a fun project for me to work on those skills. That's the main idea, really.

    My initial thought was to recess the bearings (well, 4 sets of them, anyway) in the fork legs to keep things from getting too crazy wide, but I'm really only at the random idea stage right now. Hence my interest in an old fork to play with.

    -Walt


    Quote Originally Posted by Schmitty
    Should be a good exercise in machining!

    Replacing the many bushings with bearings will drive weight through the roof.. both from the bearings themselves and the needed thickness on the linkages to accomodate the bearings.. it will get very wide as well for the same reasons.

    Try to use needle bearings it seems...

    I think I just saw one in the bone pile in a shop here.. I'll let you know.


    -Schmitty-
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  11. #11
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    I think there is absolutely no reason why a linkage fork couldn't be viable with modern technology (Just ignoring that German A fork for a second). In fact, think of the three main components - legs and two swing links - perfect for composite construction.

    Walt, this is a cracker of a project and one I've been daydreaming about for years.

    I say just ignore the Girvin and do one from scratch out of steel. Use a Turner Igus bushing kit as your pivots, grab a Fox RP23 and go for it!
    No longer member of the bike industry nor society, so don't hassle me.

  12. #12
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    I had the coil version on my 857. I quite liked it, actually. Stiff and responsive (for the time, perhaps it wouldn't be great by today's standards). The older elastomer versions were pretty bad, though. I'd be curious to see a modernized 29er version of this.

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    Yeah...

    I'm not planning to *use* the fork for anything. I just want one to play with (and see how they did what they did) because I've never really looked at one closely. From everything I've heard, they sucked pretty badly in terms of damping and lateral/torsional stiffness. I think some bearings and a thru-axle might solve the latter two issues, and an rp23 (or whatever modern shock) would certainly help with the first.

    I'm guessing that with nice composite/alloy construction, you could make a ~4# fork that worked pretty darn well. I could be wrong, and I'm sure that what *I* build will be at least a pound heavier than that and possibly quite crappy. But as I said, I like goofy projects. I'm glad *someone* doesn't think I'm an idiot...

    Does Turner sell the bushing kit you're referring to? I was sort of looking at doing bearings, but I'm not married to anything at this point.

    -Walt

    Quote Originally Posted by Thylacine
    I think there is absolutely no reason why a linkage fork couldn't be viable with modern technology (Just ignoring that German A fork for a second). In fact, think of the three main components - legs and two swing links - perfect for composite construction.

    Walt, this is a cracker of a project and one I've been daydreaming about for years.

    I say just ignore the Girvin and do one from scratch out of steel. Use a Turner Igus bushing kit as your pivots, grab a Fox RP23 and go for it!
    Waltworks Custom Bicycles
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    Just some background...

    Just as explanation - I've never ridden a mountain bike linkage fork, but I come (sort of) from a motorcycle background (mostly enduros). I used to borrow a friend's 1100gs (a BMW street/touring bike which had a linkage type front suspension setup) and just be *amazed* how much better the bike handled in any kind of braking situation. I never got around to trying out a linkage setup on a mountain bike (and in fact I'm pretty much a rigid bike dude now) but I always wondered about it. So there it is.

    -Walt
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    Hey Walt, give me a history lesson here: wasn't the Girvin designed by the brother or some relation to Kevin Hines (the national enduro champ years ago) or was that Pro-Flex? I have a lengthy enduro past as well.

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    Yeah, you can buy bushing kits straight from Turner. Nice, easy, lightweight solution.

    This would be a great little fun project. You could make the linkages from formed laser-cut sheet, turn the bushing seats from tube.....legs would be simple.

    Awesome.
    No longer member of the bike industry nor society, so don't hassle me.

  17. #17
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    Check the WW forum.

    Quote Originally Posted by jay_ntwr
    Who was looking for a Girvin fork? One of my riding buddies was joking about having one "for sale" and really does. I'm sure you've got it by now for whatever project it is, but I know it was someone on here that needed one. If you still do, I have lines on a couple it sounds like.

    Later,
    Jay

    IIRC, there was some guy who slapped a Fox Float in a Look linkage fork (a clone of this fork made of carbon, I think) and he said it was actually pretty good. Not sure how much I believe it. Those fork legs look like they would allow a ton of twist flex to me.

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    I have a parafork and they are stiff and responsive and handle fine at low speeds. I had a bad crash and I don't know what happened, I just lost it plowing into sand (nothing I haven't done before), I blame the fork. I haven't used it since, I just don't trust it.

  19. #19
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    I loved my old Amp linkage fork, never had any issues with it, I have been eyeing up the German linkage fork although the fact the linkage points forward not back like the Amp does look a little odd.

    Go for it mate I think it will be a fantastic project, Linkage forks can be great no stiction, light weight and the ability to play with the axle path

  20. #20
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    Quote Originally Posted by moto367
    Hey Walt, give me a history lesson here: wasn't the Girvin designed by the brother or some relation to Kevin Hines (the national enduro champ years ago) or was that Pro-Flex? I have a lengthy enduro past as well.
    Yes, when Pro-Flex was in Rhode Island, Kevins brother was part of the crew.

    PK

  21. #21
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    All the discussion about using Turner bushings, these as mentioned are from IGUS.

    If my memory is correct, the Girvins used IGUS bushings also.

    I owned and rode and 853 ProFlex with the Girvin fork back in 93. By todays standards it was very flexible. Compared to the forks of the era like the Manitou 4 and RS Mag 21, I don't recall it being much stiffer nor softer in torsional movements.

    I too have been deciding on building a new fork. Since the majority of our riding now is all tandems, the fork situation and performance is pretty slim pickins.

    Currently our Cannondale MTB tandem has a Moto Freeride. Yes stiff but maintenance intensive with no new parts available. Plus these things are prone to cracking triple clamps, and tough to get enough spring rate for the tandem.

    So I've been contemplating building a leading link style fork similar to that used in sidecar motocross racing. The tandem can not float or lift the front wheel, so every thing passing beneath the tire is basically an uncontrolled impact.

    My thoughts for a leading link is more based on terrain following ability first, rigidity can be designed in as weight is not a primary concern, but somewhat important.

    Rather than the more common two damper design, I planned to use the typical swingarm and vertical leg structure, but run a second set of verticals into a link driving a single damper. This second link would be similar to the Lawill leader type design. The swing link would dial in the leverage ratio and I'd hopefully stuff any quality 6.5" rear damper in there. This second link I'm hoping would compliment the side to side rigidity, this may allow building a slightly lighter swingarm.

    At the moment, and until I draw it on the computer coupled to a drawn frame, my greatest concern is toe clearance for the swingarms rear loop.

    Crazy or not, any thoughts.

    PK

  22. #22
    pvd
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    Quote Originally Posted by Walt
    I used to borrow a friend's 1100gs (a BMW street/touring bike which had a linkage type front suspension setup) and just be *amazed* how much better the bike handled in any kind of braking situation.
    That's Telelever. It's still basically a telescoping fork, but the spring and damping is not inside the tubes. The link to the extermal shock allows the fork to get a maximum amount of support during braking. It's a very good idea. It has honest advantages for tuning and in structure. I've seen somebody doing it on bicycles. I'll need to find that.

    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.




  23. #23
    pvd
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    http://en.wikipedia.org/wiki/Motorcycle_fork

    Actually, that bicycle was a Duolever configuration. I think that this would be a much better direction for you to go.



    Whyte PRST-1



    Whyte PRST-4

    You may want to give them a call. http://www.whytebikes.com

  24. #24
    pvd
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    Here's the page of the system designer, Hossack.

    http://www.hossack-design.co.uk

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    Quote Originally Posted by pvd
    . I've seen somebody doing it on bicycles. I'll need to find that.
    This it?

    -Schmitty-

  26. #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!

    Want:
    650B rims or wheel set. 80's vintage 32 or 36 x 135mm

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    Heavy, expensive, complicated. Motos have motors.


    -Schmitty-

  28. #28
    pvd
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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...

  29. #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.

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

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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.

  32. #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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  33. #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.

  34. #34
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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.

  35. #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.

  36. #36
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    I'll continue riding my hard tail and not worrying about all this stuff.

  37. #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!

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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.

  39. #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.

  40. #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.

  41. #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.

  42. #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

    RB = [F(H) + W(O)] / WB

    means that the vertical reaction at the front wheel (RB) (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.


  43. #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.

  44. #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)

  45. #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).

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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.

  47. #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 <em>eliminate</em> weight transfer. It <em>counteracts</em> it within the suspension.

  48. #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.

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

    <iframe frameborder="0" scrolling="no" style="border:0px" src="http://books.google.com/books?id=84hF-qoR5I8C&lpg=PT317&ots=FYA0vFMaKo&dq=foale%20telele ver%20anti%20dive&pg=PT317&output=embed" width=500 height=500></iframe>

  50. #50
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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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