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Thread: Rear Suspension Pre-Load

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    John Heath jfheath's Avatar
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    Rear Suspension Pre-Load

    Occasionally I come across a comment - (rarely on this forum however) - which is related to adding pre-load to rear suspension - and it is sometimes described as 'making the suspension stiffer', or 'compressing the spring'.

    My own opinion is that both of these assertions give the wrong impression, and that adding pre-load simply raises the rear of the bike. This is to compensate for the fact that an additional load has already compressed the spring and reduced the ride height. The pre-load adjustment is required to prevent the suspension from 'bottoming out' and restoring a reasonable ride height.

    The diagrams are my interpretation of what happens to the suspension.
    Comments / criticisms welcome - but if you disagree, please explain. I want to understand this.




    Fig 1 - Weight of bike and rider. No preload. Spring partly compressed, ride height is OK and damper piston is clear of the top and bottom.

    Fig 2 - Extra weight is added. Spring compresses. No preload. Suspension is in danger of bottoming out on the damper piston. Ride height needs to be increased.

    Fig 3 - On centre stand, Swing arm drops. No weight on suspension. Suspension is at maximum extension.

    Fig 4 - Preload is added (red ring). With the suspension at full extension, the lower and upper mounting points cannot be moved further apart. So adding pre-load compresses the spring.

    Fig 5 - With pre-load added, bike is removed from centre stand. The load is same as in Fig 2 so the spring compression is the same as Fig 2. (Hook's Law - Same Weight = Same Compression). Ride height is now OK. Damper piston is clear of top and bottom (as in Fig 1).

    Result - ride height is increased, spring compression is the same as before the pre-load was added. (See Fig 2).
    --------------------------------------------------------------------

    If the bike is not put on the centre stand when pre-load is added, what happens is slightly different, but the result is the same:




    Fig 1 - Bike and rider, no preload, ride height is OK (as before)

    Fig 2 - Extra weight added. Spring compresses. Ride height is now too low.

    Fig 5 - Preload is added while the weight is still on the spring. There is no extra weight to compress the spring further and the rear of the bike is now moveable. So the rear rises.

    Note that the amount of spring compression is the same as in Fig 2. Note that the ride height has been restored to the same 'OK' point as Fig 1 before the load was added.

    -----------------------------------------------------------------------------------------------------------------
    Following from the comments from SMSW and dduelin later on in this thread:

    On some bikes without weight of the rider or pillion, the suspension is at the maximum extent of its travel. (Pic 3)

    When pre-load is added the suspension unit cannot extend any further, so the spring compresses. (Pic 4)

    The compressed spring is able to support some of the rider's weight without compressing further. The rest of the riders weight causes the spring to compress a little and the entire suspension unit to lower the bike slightly. (Pic 5)

    In fact, the total spring compression is identical to what it would be if the weight of the rider was placed directly onto the spring - but in this case, due to the pre-loaded spring, the amount of movement from the suspension unit being 'topped out' is much smaller. When the rider gets off the bike, the suspension is at the very top of its travel. This doesn't happen with the ST1300. When the bike is on its wheels, the weight is supported by the spring which is already beginning to compress.

    Whichever way you look at it, providing that the unit doesn't top out or bottom out when riding, if the rider gets off and adds a little more pre-load, all that happens is that the rear of the bike will be a little higher when the rider remounts.



    -----------------------------------------------------------------


    Over to you ..... !
    Last edited by jfheath; 08-14-2017 at 01:59 PM.

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  3. #2

    Re: Rear Suspension Pre-Load


    As always, great diagrams John.

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    Re: Rear Suspension Pre-Load

    Great info

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    Dave.David's Avatar
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    Re: Rear Suspension Pre-Load

    Ok, first inquiry. If preload is adjusted buy the black knob on the left side under the seat of bike, what are we adjusting by the set screw at the bottom of the shock? Is there a diagram for that?

    Sent from my SAMSUNG-SM-G935A using Tapatalk

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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by Dave.David View Post
    Ok, first inquiry. If preload is adjusted buy the black knob on the left side under the seat of bike, what are we adjusting by the set screw at the bottom of the shock? Is there a diagram for that?

    Sent from my SAMSUNG-SM-G935A using Tapatalk
    The screw at the bottom of the shock is to adjust the speed of the shock/spring rebound after absorbing bumps. It is adjustable so with different loads, the tire will always remain in contact with the pavement after absorbing hits.

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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by Dave.David View Post
    what are we adjusting by the set screw at the bottom of the shock? Is there a diagram for that?
    Rebound damping.
    1 drop in reverse, so is that a minus 1? 205/50-17 Goodyear TripleTred, 130/70-18 Bridgestone BT-45 (rear)
    While you're alive, shine: never let your mood decline. We've a brief span of life to spend: Time necessitates an end.

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    Site Supporter ToddC's Avatar
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    Re: Rear Suspension Pre-Load

    But does our preload adjuster on the ST1300 compress the spring...? Or just add fluid to the bottom space in the shock...??

    ToddC
    I Just wana Ride man, I just wana Ride!

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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by ToddC View Post
    But does our preload adjuster on the ST1300 compress the spring...? Or just add fluid to the bottom space in the shock...??

    ToddC
    As I understand it, and others who have taken the shock apart will chime in to correct me if I am wrong, the preload adjuster (as we screw the knob in) pushes oil into a hydraulic cylinder that sits atop the spring - much like the red collar in John's sketch. The difference is that instead of a collar with detents or threads on the shock body, we have a hydraulic cylinder. As the cylinder expands, the top of it presses up on the shock body and the bottom of it pushes down on the spring. Does it compress the spring or push up on the shock body to raise the rear of the bike? Aye, dat's da rub. John and I have discussed this at length, and I think we are talking semantics - heads I win, tails you lose. What is happening is we are adding 'preload' to the spring. If the bike is unloaded and we add 100 lbs of preload, it will take 100 lbs of weight on the seat to return it to its unloaded condition (assuming the shock was not either topped or bottomed out).

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    John Heath
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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by ToddC View Post
    But does our preload adjuster on the ST1300 compress the spring...? Or just add fluid to the bottom space in the shock...??
    I have represented the preload adjuster as a red ring on top of the spring. Some suspension units use a threaded collar, the ST1300 uses a hydraulic system to achieve the same result. The spring is already supporting the weight of the bike and the weight of the rider. To push it down further requires more weight, or something fixed to push against. When on its wheels, the bottom end of the suspension is fixed, (it cannot be pushed down further, the road is in the way !), but the top mounting point can move up and down. The pre-load moves that.

    But I put this here obtain other views on this. I believe it is correct, but I'm trying to confirm that as I have never had the ST1300 suspension unit apart.


    Quote Originally Posted by Dave.David View Post
    Ok, first inquiry. If preload is adjusted buy the black knob on the left side under the seat of bike, what are we adjusting by the set screw at the bottom of the shock?
    That screw accessed through the hole in the right hand footrest bracket is for adjusting the damping. What I have not shown on the diagram is the complex valve arrangement in the piston which allows the oil in the cylinder to pass through the piston rapidly in one direction (to respond to the wheel hitting a bump) but more slowly in the other direction (to allow the bike to return to its normal ride height). The screw adjusts the rate that this happens. The suggestion is that if you have a heavy load, the damping needs to be harder to stop the bike from boing-boing-ing.

    But there are people on these forums that know much more about this than me. I'm trying to get responses. I put the pictures there to say - this is what I think. Let me know if I am wrong and where.
    Last edited by jfheath; 06-08-2017 at 02:12 PM.

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    John Heath
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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by SMSW View Post
    .... the preload adjuster (as we screw the knob in) pushes oil into a hydraulic cylinder that sits atop the spring - much like the red collar in John's sketch. The difference is that instead of a collar with detents or threads on the shock body, we have a hydraulic cylinder. As the cylinder expands, the top of it presses up on the shock body and the bottom of it pushes down on the spring. Does it compress the spring or push up on the shock body to raise the rear of the bike? Aye, dat's da rub.
    That paints an excellent picture of the hydraulic pre-load. At one end pushing down on the spring. At the other end pushing up on the body of the suspension - which is firmly connected to the upper mounting point. The weight on the upper mounting point has not increased. The spring is already compressed and is supporting that weight already, and the upper mounting point is well able to move up.

    So does the act of raising the upper mounting point compress the spring further? I really suspect not because there is no extra weight on it.
    I need a physicist, or I am going to have to go to the garage with a tape measure and find out.

    [edit]I had to know - so I've been doing some measuring. Amongst other things, I discovered that I am riding the bike with almost full pre-load dialled in - We live on a bumpy road, and if when we set off fully loaded it hits bottom, I wind it up a bit more. I've obviously never bothered to wind it down again.

    So I measured the spring from its base to the top of the top coil. I measured the height of the rear of the bike to the base of the joint between the rear cowl and the side panel. No panniers or side panels. Some weight in the top box. Bike upright on its wheels, me measuring and balancing the bike - very nervously !

    Result:
    No pre-load - spring length 16.5 cm. Rear height 75.5 cm
    Full pre-load - spring length 16.5 cm. Rear Height 78cm.

    So in this experiment, adding pre-load does not affect the spring compression, Ride height is increased by 2.5 cm

    I had to turn the pre-load 10.5 clicks (5.25 turns) before any resistance was felt - so perhaps I am losing some available ride height after 30k miles.


    Quote Originally Posted by SMSW View Post
    What is happening is we are adding 'preload' to the spring
    And this is the confusion - you see I don't think that we do anything to the spring. I think that we just add height to the suspension unit and the spring stays the same, supporting the same weight as it did before the pre-load was added. You add the weight. The spring compresses. You add what is confusingly called 'pre-load' and it raises the rear. The spring still has the same compression as it is supporting the same weight, but now the ride height is correct.
    But you and I have been through this - I'm just trying to let others in on the discussion points !
    Last edited by jfheath; 06-09-2017 at 05:56 AM.

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