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Thread: Rear Suspension Pre-Load

  1. #31
    RaYzerman's Avatar
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    Re: Rear Suspension Pre-Load

    My theory is that if you stiffen up the rear end with preload it would be less prone to weave. I also think weave is more to do with aerodynamics of the fairings, saddlebags and if you have a top box.
    Ray

  2. #32
    John Heath
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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by RaYzerman View Post
    My theory is that if you stiffen up the rear end with preload it would be less prone to weave. I also think weave is more to do with aerodynamics of the fairings, saddlebags and if you have a top box.
    I can understand why you might think this - when the suspension unit is out of the bike, adding pre-load certainly seems to be making the suspension stiffer, as the distance between the fixing points is at their full extent and the spring compresses.

    When the unit is put in the bike, the suspension is already under some load due to the weight of the back end (and rider), and will typically be somewhere midway in the available range of 'travel'. Adding more pre-load simply increases the height of the rear end of the bike - it doesn't make the suspension stiffer, or compress the spring further.

  3. #33
    Be Thou My Vision dduelin's Avatar
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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by jfheath View Post
    Snipped...........Adding more pre-load simply increases the height of the rear end of the bike - it doesn't make the suspension stiffer, or compress the spring further.
    Adding preload does compress the spring further. That is how it raises the ride height.

    Just look at the top of the shock when you wind in preload. If the mechanism works properly the top spring seat moves downward as preload is wound in. (about 10 mm of travel if oil is full and all 36 clicks 18 full turns is used). Imagine for illustration it takes 100 lbs to compress the spring 10 mm. That would be a 10 lb/1mm spring rate. A free spring of this rate held vertically will compress 10 mm when a 100 lb weight is set on top of it. The spring is held between two seats on the shock which get closer together as weight (force) is added.

    Move the preloader down 10 mm adding 100 lbs "pre" - "load". The extra 10 mm of compression on the spring means it takes more force (100 lbs more to overcome the extra 10 mm added "pre to "load"ing it with rider) to initially compress the shock. Once the shock starts compressing it does not take any more force than before to compress it - 210 lbs of force compresses it 11 mm, 310 lbs of force compresses it 21 mm. Since the bike did not physically get any lighter then same weight of bike compressed the shock 10 mm less and the ride is higher by 10 mm.


    Dave

    Honda ST1300
    Honda NC700XD
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  4. #34
    John Heath
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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by dduelin View Post
    Adding preload does compress the spring further. That is how it raises the ride height.
    I had this discussion with SMSW, and I still don't buy it. Agreed, what happens when the spring is out of the bike is that adding pre-load compresses the spring. This means that it takes more load than before - before the spring will start to compress.

    When the suspension unit is on the bike, the spring is already partly compressed by the weight of the bike (it is on mine anyway). Adding more pre-load tries to compress the spring further, but there is no extra weight pushing down - so it pushes the rear of the bike up.

    And I checked this out. Measure the spring. Measure the height of the bike. Add preload. Then measure again. The spring is the same length as before, the bike is higher.

    However, if you are adjusting the preload when the bike is on the centre stand - I entirely agree with what you say. The suspension unit is already at its maximum travel, and all that adding preload can do is compress the spring - it cannot push the rear of the bike up any higher. As soon as the bike is on its wheels, the spring is compressed by exactly the same amount as it was before (it has the same load on it), but the bike is lifted higher.

    If you think this is wrong, can you let me know which of the diagrams in post #1 is incorrect ?

  5. #35
    Be Thou My Vision dduelin's Avatar
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    Re: Rear Suspension Pre-Load

    However we choose to understand or describe it the result is the same. Adding more preload compresses the spring. Because the spring is free to pass the additional force thru the swingarm to the tire to the ground the rear of the bike rises and the spring in this case measures the same length before and after but that does not always hold true with all shocks on all bikes.. If the spring rate of the shock is too soft the additional preload required to get rider sag correct tops out the shock before it fully extends when weight of rider(s) is removed - there will be no free sag. The distance between spring seats will be less than before additional preload. If this happens too much preload was required to get rider sag correct. This illustrates how the relationship of free sag to rider sag indicates if spring rate is in the ball park for individual riders.

  6. #36
    John Heath
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    Re: Rear Suspension Pre-Load

    Quote Originally Posted by dduelin View Post
    However we choose to understand or describe it the result is the same. Adding more preload compresses the spring. Because the spring is free to pass the additional force thru the swingarm to the tire to the ground the rear of the bike rises and the spring in this case measures the same length before and after but that does not always hold true with all shocks on all bikes.. If the spring rate of the shock is too soft the additional preload required to get rider sag correct tops out the shock before it fully extends when weight of rider(s) is removed - there will be no free sag. The distance between spring seats will be less than before additional preload. If this happens too much preload was required to get rider sag correct. This illustrates how the relationship of free sag to rider sag indicates if spring rate is in the ball park for individual riders.
    I agree entirely. My original document (post #1) made the assumption that fits with my ST1300, that the suspension isn't set to top out without a load or bottom out when being ridden over bumps. Of course if the suspension tops out - with the suspension fully extended, then applying additional preload will compress the spring.

    In writing Post#1 and drawing the diagrams, I was really addressing the notion that adding pre-load makes the suspension stiffer (first sentence). When I have read statements like this, I get the impression that someone has been riding along and the suspension is working fine - not hitting the top limit of the suspension travel and not hitting the bottom of the suspension travel. It is somewhere in between. The rider then gets off and increases the pre-load, and then continues riding. There is no change in weight. Again, the suspension is not hitting the top or the bottom of its travel. The impression that I get from the comments that I have seen is that the rider thinks that he has made the suspension stiffer, when in fact, all he has done is raise the height of the rear of the bike a tad. As a result, the bike handles a little differently because of the effect on the steering geometry.

    Thanks for bringing in the comment about some bikes will be adjusted so that they do 'top out' before load is added. Hopefully when the rider gets on, the suspension is sitting somewhere mid-travel, with room for a little adjustment in either direction. I didn't include this scenario in the diagrams because it never happens with my ST1300 ! However, in post#1, I did show how having no pre-load (pic 2) compresses the spring when the load is added, with the result that the ride height is too low; and how adding preload (pic 5) results in the correct ride height, with the spring compressed by exactly the same amount as in pic 2.

    Maybe I should add the scenario that you describe when the unloaded bike is topped out with the pre-load added ?

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