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Thread: Diavel suspension is too firm

  1. #61
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    Quote Originally Posted by Mike.Gayner View Post
    Of course preload compresses the spring - that's the entire point of it. By what mechanism do you propose preload raises and lowers the swing arm?



    This is precisely what altering preload does. You compress the spring without changing the length of the shock - it therefore takes more weight to make the bike sag by the same amount. So for the same given weight, more preload means a higher ride height.
    Bit late back into the fray and have to say sorry but this is all arse about face.

    Thing is, it's so easy to prove it to yourself.
    Go out to your bike with a tape measure and with the bike resting on its wheels (side stand OK but not centre stand) : -
    1. measure the spring length
    2. measure the distance from the back axle to some point on the frame
    3. ensure that the shock has available movement in both directions i.e. that its not bottomed out or topped out.
    4. increase the preload
    5. take measurements 1 and 2 again with proviso 3.
    You will find that the shock has extended, the ride height has increased but the spring is the same length as at the start. It must be because you haven't changed the load on it.
    All you have done is push the top of the spring down relative to the frame, but the bottom is restrained only by the swing arm which is free to move away.

    Of course, if you take it so far that the shock tops out, no more movement is available and so you will start to compress the spring, but that's out in ridiculous territory with sumo pillion passengers.

    I have been trying to explain this for over a decade now, so I must be a truly hopeless tutor.

    Maybe I should put a week's pay on it and retire on the winnings.
    I may not be as good as I once was, but I'm as good once as I always was.

  2. #62
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    My understanding is that preload affects the sag and hence the stiffness of the spring because you are adjusting how much the spring is compressed when the shock is fully extended. I think it should usually not adjust the unloaded ride height, only the sag (and hence the loaded ride height). To make the shock the softest it can be, the preload should be adjusted right out so the spring is fully extended. This will however reduce the effective travel of the shock because more of it is now taken up by the bike/rider sag. On the other hand if you have the preload wound right in so the spring is more compressed and sag is reduced, then you may run the risk of spring bind which effectively reduces the travel of the shock. I'm not sure if this is possible with most motorcycle shocks or not.

    Maybe there is some confusion between preload and ride height adjustment.

    Ride height adjustment is not available on all bikes but you can see an example of how it is done here http://www.sportrider.com/suspension-setup-guide#page-7 . Ride height adjustment should not directly affect how the shock behaves but it will affect the geometry of the bike and hence how it behaves over bumps etc.

  3. #63
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    Quote Originally Posted by Bass View Post
    Bit late back into the fray but have to say sorry but this is all arse about face.

    Thing is, it's so easy to prove it to yourself.
    Go out to your bike with a tape measure and : -
    1. measure the spring length
    2. measure the distance from the back axle to some point on the frame
    3. ensure that the shock has available movement in both directions i.e. that its not bottomed out or topped out.
    4. increase the preload
    5. take measurements 1 and 2 again with proviso 3.
    You will find that the shock has extended, the ride height has increased but the spring is the same length as at the start. It must be because you haven't changed the load on it.
    All you have done is push the top of the spring down relative to the frame, but the bottom is restrained only by the swing arm which is free to move away.

    Of course, if you take it so far that the shock tops out, no more movement is available and so you will start to compress the spring, but that's out in ridiculous territory with sumo pillion passengers.

    I have been trying to explain this for over a decade now, so I must be a truly hopeless tutor.

    Maybe I should put a week's pay on it and retire on the winnings.

    In this case aren't you talking about measuring the bike sag? Ride height would be the distance with the shock topped out? Maybe I have my terms mixed up.

  4. #64
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    Quote Originally Posted by Crasherfromwayback View Post
    I have a rear shock out of a 1998 CR250 here for a rebuild. If I wind the preload collars right off, I can compress the shock myself (a wee bit only) by just pushing down on it. If I was to wind the collars right up, I couldn't budge it no matter how hard I might try. So it's softer one way, and harder the other. So for people to say preload makes no diffence to how the shock/bike behaves, and is no softer or stiffer is simply not correct.
    Mate, that's a limiting situation. You are taking your measurements with the shock fully extended and so the forces are being restrained by the internal mechanism.

    Look, do what Ken Macintosh does.
    Go and get your bathroom scales and put them on your drill press table.
    Put one end of the shock on the scales and the other in the chuck mount. Use the drill press to force the shock off its end stop slightly.
    Use the drill again to force the shock through a further increment (e.g. 1 cm) and use the scale to measure the force INCREASE required to achieve that increment.
    Do it several times with several preload settings
    Do it at several places in the stroke
    You will find that the incremental force required is completely independent of the preload setting.
    That is what Hooke's law is all about.

    Of course it will make a difference to how the bike behaves but the stiffness remains the same as your measurements will have just proved.

    P.S. If I'm wrong, I want to know about it immediately cos it means something new and so interesting. Also, I believe that some springs don't follow Hookes law well at the very start of their deflection.
    I may not be as good as I once was, but I'm as good once as I always was.

  5. #65
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    Quote Originally Posted by mangere View Post
    In this case aren't you talking about measuring the bike sag? Ride height would be the distance with the shock topped out? Maybe I have my terms mixed up.
    They are pretty much the same measurement for all practical purposes, except sag increases as ride height decreases and vice versa. Any two points which allow you to measure one, will also allow you to measure the other
    I may not be as good as I once was, but I'm as good once as I always was.

  6. #66
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    Quote Originally Posted by mangere View Post
    My understanding is that preload affects the sag and hence the stiffness of the spring because you are adjusting how much the spring is compressed when the shock is fully extended. I think it should usually not adjust the unloaded ride height, only the sag (and hence the loaded ride height). To make the shock the softest it can be, the preload should be adjusted right out so the spring is fully extended. This will however reduce the effective travel of the shock because more of it is now taken up by the bike/rider sag. On the other hand if you have the preload wound right in so the spring is more compressed and sag is reduced, then you may run the risk of spring bind which effectively reduces the travel of the shock. I'm not sure if this is possible with most motorcycle shocks or not.

    .
    The first thing to realise is that you don't want the shock either topping out or bottoming out when you are riding. Your suspension just goes away if you allow that to happen. Anywhere in between those 2 points, the only way to change the equilibrium length of the spring is to add weight or remove weight.
    To change the length of the spring you have to change the force on it and so if the shock is not at an end stop, the spring will be the length it needs to be to carry the load on it.
    If you don't change the load, you don't change the spring length.

    The only way that you can change the spring stiffness is to fit another one

    As for ride height, yes, changing the length of the shock shaft is the true and pure adjustment that alters no other geometries. However, extending or retracting the shock shaft equilibrium position (as preload alterations do), has the same effect albeit with penalties to available shock movement.
    I may not be as good as I once was, but I'm as good once as I always was.

  7. #67
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    Bass, maybe pictures would help or a web page address.

    Sometimes diagrams help.

    Some geometries are not that straight forward.

    READ AND UDESTAND

  8. #68
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    Quote Originally Posted by eldog View Post
    Bass, maybe pictures would help or a web page address.

    Sometimes diagrams help.

    Some geometries are not that straight forward.
    The tricky part is to know how far to go into the discussion. I think that my knowledge is better than average but it still has gaping holes in it.
    I once sat down and tried to describe the whole thing mathematically and the theory is actually not too bad if you are comfortable with a bit of calculus. The problem to getting a good picture was getting a measurement of the effect of shock seal friction and stuff like that.
    Getting a gut feel for the whole picture is hard because it's all about change and rates of change.

    Anyway, as for diagrams etc, the very best diagram I have for the effect of preload is my bike. It's all electrically adjustable and so I can demonstrate everything I have been saying to anyone interested in a few seconds and in glorious living colour. Just press the button and watch it all happen.
    I may not be as good as I once was, but I'm as good once as I always was.

  9. #69
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    I had a GPZ 900 22 years ago... fk i'm getting old

    Anyway the original post does relate to comfort yes, it's ok on the motorway and street dragging like it's originally designed for i guess... my cock up on that one. It's not ok on our NZ back roads. I contacted Robert Taylor and he's getting an Ohlins in for me so i'm looking forward to getting that fitted in a few weeks and he reckons it will be way better for our roads. So on the balance of things I think that is the best way to go to get the best outcome.

    I have been reading the posts you guys making about preload etc... I'm no expert (i do have some chicken strip left and did do daily power wheelies on my Monster but not clutch wheelies so i'm average, not expert) but my input is... Yes it raises the rear height, any bike I have changed this on (including dirt bikes) absolutely raises the height. I wouldn't say it makes it firmer necessarily but it makes the suspension work in a different part so the net effect is usually a "slightly" firmer feel through most of the preload adjustment... with the exception of the bottom (say 10%) of the adjustment makes the bikes feel soft and wallowy.

    Of course the geometry changes with more preload, lift up the rear and the front tips in much quicker and the bikes become less planted... drop the rear too much and the front rides high or wide into the corner and mid corner along with a whole lot of other variables. So with raising the rear up high then yes you should also raise the front preload to get the right geometry and turn in.

    As for the real details about how it all works... i'll stick to my day job that i'm better at.
    hard work pays off later... laziness pays off now

  10. #70
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    This is precisely what altering preload does. You compress the spring without changing the length of the shock - it therefore takes more weight to make the bike sag by the same amount. So for the same given weight, more preload means a higher ride height.[/QUOTE]

    This is a concept on one of the effects of altering preload I can understand. Well put that man!

  11. #71
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    Quote Originally Posted by Bass View Post
    The tricky part is to know how far to go into the discussion. I think that my knowledge is better than average but it still has gaping holes in it.
    I once sat down and tried to describe the whole thing mathematically and the theory is actually not too bad if you are comfortable with a bit of calculus. The problem to getting a good picture was getting a measurement of the effect of shock seal friction and stuff like that.
    Getting a gut feel for the whole picture is hard because it's all about change and rates of change.

    Anyway, as for diagrams etc, the very best diagram I have for the effect of preload is my bike. It's all electrically adjustable and so I can demonstrate everything I have been saying to anyone interested in a few seconds and in glorious living colour. Just press the button and watch it all happen.
    If i have time. I will have a demo.

    READ AND UDESTAND

  12. #72
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    Oh good grief! Anyway its a myth that you need to somehow ride a bike hard as to gain advantage from decent suspension. Bikes are built down to cost and sometimes crazy setup. As an example RT has stated that Ohlins valving spec for various Ducatis is well different from how they are made on Ducatis supplied with Ohlins std.

    Maybe they want you to have your teeth rattled out on the test ride to make you think its a sports bike like no other.

    Spend the money and enjoy.

    I'm not going to enter the preload debate other than to say;
    Draw an x/y graph.
    Draw a diagonal line from zero
    Draw the same line but a bit further up the x axis.

    Congratulations you've just added preload without changing the spring rate.


    But preloading is a bad idea. You still end up buying the same amount of drinks when you are out, just get far messier.
    Don't you look at my accountant.
    He's the only one I've got.

  13. #73
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    Quote Originally Posted by Bass View Post
    Mate, that's a limiting situation. You are taking your measurements with the shock fully extended and so the forces are being restrained by the internal mechanism.

    .
    Mate, I hear ya. But how will that shock behave, from the very top of it's travel right through to the bottom, if I take the spring off it completely? The very same feel all the way through the stroke right? How will it behave if I wind that much preload on it, that it coil binds?

  14. #74
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    Quote Originally Posted by Crasherfromwayback View Post
    Mate, I hear ya. But how will that shock behave, from the very top of it's travel right through to the bottom, if I take the spring off it completely? The very same feel all the way through the stroke right? How will it behave if I wind that much preload on it, that it coil binds?
    Good points and everything I have said assumes the shock is good throughout its' stroke and that its not operating against any physical stops.

    As a bit of an aside, but still on topic since the OP's comments were about a harsh ride, has anyone out there had a go at taking some specs to a spring manufacturer and getting a spring made? If so what problems arose?

    I'm far from being a professional in the field but have picked up a couple of rules of thumb that worked OK for me on sport/tourers and adventure bikes. As I indicated a couple of posts ago, you can measure the spring rate with a drill press and a set of scales. However, I struggle to tell from riding the bike whether I stuffed up the damping settings or whether the wrong spring is in there. Can anyone point me straight?
    This is one of the major holes in my experience. So I tend to use the difference between static sag and rider sag (in % of total stroke terms) as a first order indicator. Often, I've considered changing the spring but they're not always available and so I've wondered about getting one made.

    I've also noticed that a new spring relaxes a bit during the first few days of its working life and so I guess I'd have to allow for that when specifying the length?
    I may not be as good as I once was, but I'm as good once as I always was.

  15. #75
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    Quote Originally Posted by MD View Post
    I've met so many riders who obsess with suspension tuning yet ride like a big blouse, (snip) Never pushed their suspension anywhere near it's limitations.
    It me.

    The improvement in comfort is worth it. Ducati are commonly too harsh for our roads and the stock hardware offers insufficient adjustment. The Ohlins equipped Ducati models are set up for the track and are very harsh. To get any level of comfort they need rebuliding to a more suitable spec.

    An Ohlins improved the Hornet amazingly. My Ducati came with a spring to suit a midget, the Ohlins as supplied was far too harsh at Ducati specs, softened it was brilliant. The Triumph is a much nicer ride with the Ohlins fitted.

    It's about more than the just the inherent sport riding limitations.
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