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Thread: Girvin fork

  1. #51
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    Quote Originally Posted by pvd
    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.
    Paris-Dakar isn't racing?

    So, what's the verdict on the USE S.U.B fork?

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

  2. #52
    pvd
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    Like I said, BMW motorcycles aren't currently known for being racebikes. Let's see what happens in 2010.

    http://en.wikipedia.org/wiki/2009_Da...ly#Motorcycles

  3. #53
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    Quote Originally Posted by pvd

    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.
    Could you explain and quantify the "huge performance loss in tracking", ideally with something other than superlatives and handwaving?

  4. #54
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    Yeah, and Subaru isn't known for making Rally cars. Just look at the 2010 WRC.

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

  5. #55
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    Quote Originally Posted by dr.welby
    Could you explain and quantify the "huge performance loss in tracking", ideally with something other than superlatives and handwaving?
    I'm sure you understand what suddenly adding 150 lbs of unsprung weight to the front end would do.

  6. #56
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    Quote Originally Posted by pvd
    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.
    Why? Over bumps very little of the force is actually carried by the actual spring in a conventional suspension anyway. Its carried by the shock shaft/fluid (damping forces are many times spring force for any reasonable sized bump velocity.) The tire sees the same force either way. It makes no difference to the wheel if the force is distributed X = Y1% spring + Z1% damping or X = Y2% spring + Z2 % damping + W% telelever. For the ultimate traction/tracking you need to keep the wheel on the ground w/ as constant a force as possible (which is what an infinitely light wheel/susp yeilding zero unsprung weight would do - you could run an air spring w/ zero damping.) Anti dive systems that decouple some of the forces and allow a more optimal spring/damper setup really can work.

  7. #57
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    Quote Originally Posted by pvd
    I'm sure you understand what suddenly adding 150 lbs of unsprung weight to the front end would do.
    No, I don't, because weight transfer is going to add 150 lbs of load to the front wheel whether you have anti-dive or not.

    Whatever you're trying to say, I just don't think it can be said in one sentence.

  8. #58
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    I think that if you guys don't understand the concept of unsprung weight vs.sprung weight and why we want to minimize unsprung weight, then we need to start a new threat on the basic concepts of suspension systems.

  9. #59
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    Quote Originally Posted by pvd
    I think that if you guys don't understand the concept of unsprung weight vs.sprung weight and why we want to minimize unsprung weight, then we need to start a new threat on the basic concepts of suspension systems.
    Look, I don't have the patience for this. You don't need to accuse others of being clueless noobs when you can't properly express your own ideas. We understand sprung and unsprung mass just fine, and that should be obvious if you look two or so posts up at what smdubovsky wrote.

  10. #60
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    Quote Originally Posted by pvd
    I'm sure you understand what suddenly adding 150 lbs of unsprung weight to the front end would do.
    Imagine braking hard coming into a downhill corner. You're antidive is in full effect. At the limit of your traction circle, a few feet before you start to release your front brake and turn in, you hit a small root.

    The problem here is that your "suspension" front end has been tightend up (significantly). The wheelrate has been increased by double, maybe even triple. The result instead of "soaking up" that root, your front fork now feels more like a rigid than a suspension fork. Oops. Result is front tire leaves ground, you plow off the trail and into a tree.

    And now your antidive fork is cracked, and you bought it used so you can't even try and pass it off as a JRA failure to the manufacturer.

  11. #61
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    RoyDean, its the exact opposite of what you say.

    A coil spring fork does not change rate vs travel. Its stays X N/m. W/ a typ fork, X needs to be pretty high so that you don't bottom the fork during braking. W/ anti dive like telelever, you can actually run a LOWER spring rate, like X/2 (which means you can run less damping too.) Not only do you have more travel left to absorb the bump but it does so w/ LESS change in force to the wheel. It tracks better due to the lower(more plush) damping and doesn't run out of travel as fast. "More plush" w/o bottoming is what all DH guys ever seem to want Go read how telelever works. Or just ride a modern bmw They actually first designed it w/ zero dive but riders didn't like it because it felt too weird. Production version have a little dive left in so it feels more like other bikes.

  12. #62
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    Yeah, anti-dive doesn't change the rate, it effectively adds preload. The rate stays the same.

  13. #63
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    Their have been many anti-dive systems tried in the past. Some with geometry, some with damping, some on the spring side.

  14. #64
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    Quote Originally Posted by smdubovsky
    RoyDean, its the exact opposite of what you say.

    A coil spring fork does not change rate vs travel.
    A: Please explain how it is "different than I say". Nowhere do I say anything about specific rates. All I state is that REGARDLESS of what your spring rate is, anti-dive increases your WHEEL rate. Spring rate and wheel rate are two totally different things.

    B. Nowhere do I say a coil spring's rate changes with travel. Reread my response. And even if I did, it's not necessarily wrong. There are progressive rate coil springs in use on all types of suspensions. Tighter wound portions of the coil offer less rate at the lower travel, and after they bind the rate goes up.

  15. #65
    pvd
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    Quote Originally Posted by RoyDean
    Nowhere do I say a coil spring's rate changes with travel.
    There is no such thing as a non-progressive spring. They all go progressive even if some are linear for a short period.

    http://www.pvdwiki.com/index.php?tit...re_progressive

  16. #66
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    Quote Originally Posted by pvd
    There is no such thing as a non-progressive spring. They all go progressive even if some are linear for a short period.
    Sure there are. Your own plot shows that if you design a spring application properly, you can stay entirely within the linear range. You even state it in your wiki.

    Not that I'm disagreeing with your overall statement, but in general, as an engineer who HAS designed suspensions (although for four wheeled vehicles, not bicycles), it is generally accepted that you can have linear rate springs or progressive rate springs.

    Saying there is no such thing as a linear rate spring is akin to saying steel or TI don't have infinite fatigue lives.... Sure, it's true, but for most practical purposes, its safe to assume the opposite.

  17. #67
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    Most folks have so little knowlage of what's going on with springs that they belive that Hook really wrote a Law. The schools really do a disservice on this subject. Yes, a properly designed system is going to use a spring that uses the linear portion of the springs range, but if a designer has no idea that a coil spring goes progressive past a point, they can easily pass this point and often do. You and I know that most folks have no idea that this happens. You and I both know how lame most engineers really are and how little they really know.

  18. #68
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    Nevermind. I'm tired of this. Feel free to delete my entry, Walt.
    Last edited by AndrewTO; 09-14-2009 at 07:02 PM.
    I ..... need ..... DIRT!!!!!

    ... and cookies.

  19. #69
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    Stay on target...

    Guys, if you feel the need to post something unrelated to the topic, whether it's pointless chest-pounding or personal attacks, DON'T.

    There's no problem with heated discussion - it's even ok if you call someone's idea(s) idiotic, as long as you're making some kind of on-topic argument about WHY. If you're not, please don't waste all our time.

    This thread has been excellent so far and I know I've learned a lot by following some of these arguments. When we debate honestly, we all win in the end.

    Thanks, and good night!

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  20. #70
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    Quote Originally Posted by RoyDean
    A: Please explain how it is "different than I say". Nowhere do I say anything about specific rates. Spring rate and wheel rate are two totally different things
    Sorry. You did say wheel rate not spring rate. (And by rate, I assume we're both talking about the first order differential of force/displacement.)

    I understand how in mcpherson strut or A-arm how wheel and spring/shock rate are different. On a typ telescopic fork w/ springs in the fork how is the factor not 1? (unless you're talking about the HA inclination factor & using defining displacement vertical vs displacement along the travel axis - is only a ~5% difference in bikes. 200++% is possible in cars depending on the spring/shock inclination and location on the susp arms. Heck and that even changes vs travel.)

    I understand that w/ telelever locating the spring on the linkage its identical to a mcpherson strut and has a wheel-spring factor. You are correct in that since we are talking about non-conventional designs we should be using wheel rate and not throwing around the terms interchangeably.

    Quote Originally Posted by RoyDean
    All I state is that REGARDLESS of what your spring rate is, anti-dive increases your WHEEL rate.
    Semantics may be an issue? If the rate is defined as (what I think is the more conventional) the wheels df/dx vs an external displacement then, no, telelever does not affect wheel rate. (some other designs might) If defined as the sprung masses resistance to movement then the answer would be different, but that would be a pretty non-common/unique definition.

    And by 'not affect' I mean mostly negligible amts (compared to other larger changes involved). As shown in welbys side view of a bmw, the telelever arm is not exactly perpendicular to the fork (but is close) so its force contribution will vary slightly based on travel. That variance along the arc is small though. The bottom line is that the wheel rate on, say, an 1100gs since walt brought it up is less than a comparable bike w/ conventional forks.

    An interesting observation is that telelever is identical to a mcpherson strut. The direction of travel is only 90deg different which isn't even a consideration since vertical displacement/force is vertical no matter which way you rotate it (susp analysis is not affected by direction of travel). On telelever, the spring is not coaxially mounted on the sliding strut (coilover) but thats not required, just convienient. Porsches torsion bars work that way - applying spring force though the arm vs the strut body. Im sure someone else has mounted the spring on the arm too. Struts car very effectively resist (sideways) 'dive' based on roll center/cg heights. In fact while analyzing roll centers, roll-stiffness, cg height, etc its EXACTLY analogous to two wheel vehicles analysis. Direction of travel is only different.

  21. #71
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    I had a friend that ran the Girvin forks exclusively for quite a while. I think they have a great concept, especially if you build a frame specifically for the fork. The fork had a tendency to jack the front end of the bike (or at least the cockpit) up due to the linkages.

    I have to wonder if a person tried to make a modern day "Girvin" fork with more travel if the linkages would be too long causing the fork's "J" curve axle path to be too pronounced. I am sure someone with far more knowledge than I could compensate for this...

  22. #72
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    *boing!*



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

  23. #73
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    so much complexity. So much cost. Is it really worth it?

  24. #74
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    Quote Originally Posted by smdubovsky



    Semantics may be an issue?
    Yeah. If wheel rate is the change in load divided by a change in wheel position, I don't see why this would change with anti-dive, at least for a simplified model of anti-dive. Consider:

    Your bike is at rest with 100 lbs on the front wheel. 200 lbs on the front wheel makes the front wheel move 1 inch. The wheel rate would 100 lbs/in.

    Now lets say you're braking,and anti-dive is trying to extend the fork with 50 lbs of force. You can do whatever math you want, let's say we just add that force in (even though anti-dive doesn't increase the wheel load). You'd have 150 lbs on the front, then 250 lbs, 1 inch of travel, and the wheel rate is still 100 lbs/in. The anti-dive is a constant preload, and therefore doesn't affect wheel rate.

    This of course may be too much of an oversimplification. I keep thinking there's some sort of free-lunchism going on, but I keep figuring out ways to justify that there isn't. Except for one:

    Let's say we are getting anti-dive strictly from wheel path. In other words, the wheel moves slightly forwards as it travels and when you brake the forces want to extend the suspension to keep it from diving. If you're braking, the anti-dive is trying to extend, and when you hit a bump the wheel has to move forwards relative to your center of gravity. Which means it has to speed up to move forwards, or you the rider has to slow down relative to it. I can see a few possible ramifications: if you're braking at your limit, could forcing the wheel to go a little bit faster cause a break in traction? Could making the front wheel go faster or slowing down the rider constitute, from the standpoint of energetics, and increase in unsprung "mass"?

    And then similarly, let's say we have a perfect axle path, but we are floating the caliper and using a linkage or piston to load the suspension for ant-dive. Hitting a bump would make the caliper rotate backwards and increase the apparent speed of the disk, which would do similar things as above, I believe.

    The above are loose mental models I've been thinking about. In it's simplest, most "perfect", on-paper form, it seems like anti-dive should be a winner. However, the real world doesn't seem to bare that out. Possible reasons include:

    * Economics - telescoping forks are cheaper to manufacture and offer "good-enough" performance.

    * Preference - Because of telescoping forks' dominance, riders are used to riding pro-dive suspensions in the front, and handling, both mental training in the rider as well as the characteristics of the bike, has been optimized around that.

    * Halfway-thereism - anti-dive may be a good idea, but all implementations so far haven't been optimized correctly - for example maybe front suspension are run too soft because they can be, but that makes handling suffer in other situations.

    * Free-lunchism - anti-dive looks good on paper, only if you idealize the model to get rid of certain small problems, like I mention above.

    Questions, comments, napkin-sketches welcome.

  25. #75
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    Welby,
    I think *Economics and *Preference are two (very) big factors.

    Telelever really works (go ride one). Its not easily adapted to a bike frame though. A DT has no real 'width' to be able to use a simple & light A-arm to resist the side forces the lower joint must take. Thats no big deal on a motorcycle though where the frame is 12+" to clear the engine/radiator. So you'd be stuck w/ a narrower link and stresses and play in the bearings are magnified. Though, I guess it wouldn't be any worse than the rear pivots on a FS bike. It also puts an odd load into the middle of the DT

    I don't know that small fore/aft movements would affect braking much. The knobs/sidewalls/spoke windup/fork deflection probably dwarf it for a long enough arm. For short travel linkages maybe it needs to be factored in.

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