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spring preload calculator

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I have to install a stiffer shock spring for a heavier rider and want to set the spring preload before installing the shock in the bike, because it's a real pain to adjust the preload on a 09 KX450F.

so I did a draft to calculate the preload I probably need.

just a draft with assumed values, I will see if I am to far off🤣

preload.jpg

1. rider wt 75kg, stock 5.5kg/mm spring, 13.4mm spring preload to reach 100mm rider sag

2. rider now 90kg, results in 113mm rider sag

3. readjusted preload of 18.1mm to reach 100mm sag

4. new 5.7kg/mm spring, 16.2mm preload to reach 100mm sag

so I should decrease the preload approx. 2mm to the preload-value of the stock spring.

I will see 👍

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just leave the top spring but loose, and rotate the spring?

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just leave the top spring but loose, and rotate the spring?

👍🤣 yes, sometimes I can`t see the forest for the trees. but it's both a pain, turning the spring while smashing the chain roller with my hands, and losen and tighten the lock nut trying to find a space for the stem ...

nevertheless I hope to save some turns, and leave the lock nut lose for correction...

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i set them at 10mm and work from that.

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i set them at 10mm and work from that.

ok, this is what I will do in the future. 👍

I think it's not possible to set it 100% exactly after calculate it.

- the spring is not even (bottom and top), so it's at least 1mm you can vary the free spring length value

- the end of the coils are not binding at a unloaded spring, so it starts with a softer spring rate until the coil ends are binding. depending on the gap this will happen sooner or later

at the moment I have about 5 ideas each day what to do, to test, to prove, to develop... sorry for that, I hope soon I will concentrate more on riding than on bother you with my tech threads🤣

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i set them at 10mm and work from that.

+1

that's my default preload as well

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ok, this is what I will do in the future. 👍

I think it's not possible to set it 100% exactly after calculate it.

- the spring is not even (bottom and top), so it's at least 1mm you can vary the free spring length value

- the end of the coils are not binding at a unloaded spring, so it starts with a softer spring rate until the coil ends are binding. depending on the gap this will happen sooner or later

at the moment I have about 5 ideas each day what to do, to test, to prove, to develop... sorry for that, I hope soon I will concentrate more on riding than on bother you with my tech threads 🙂

dont worry that will change, you will have 5 ideas a day , and no time to think about them, it happens🤣

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I made a spread sheet a while back...

Biggest "assumptions" are linkage ratio etc...

But it's nearly impossible to get it right from a spread sheet!

I agree with mog...set at 10mm and go from there...if the body is clean, and top is loose, usually turning the shock spring is easy enough.

That new kawi 450 is a pain in general with the shock (getting it out, getting it in, bladder cap...etc)

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Obviously you have to factor in the length of the swingarm and the matter of the linkage if you want to get this as perfect as possible, which can be done or made simpler with a spreadsheet.

Leverage. 👍

Basic math in a lot of ways.

If you want less math and better results, try the Tony Foale software.

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How are you coming up with your numbers for force wheel and force shock?

as I wrote, thats just a draft, so I assumed some values:

- 50kg bike wt at rear wheel

- Factor of 62% for the rider wt to be effective at the rear wheel

- 2.8/1 wheel to shock ratio. this is the initial ratio, I didn't mind the change while entering the progression curve.

but IMO it's not possible to calculate it 100% (so you need not to readjust it after re-install) because of the inaccurateness of the free spring length and coil end gaps...

Edited by kawamaha

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-2.8/1 wheel to shock ratio. this is the initial ratio, I didn't mind the change while entering the progression curve.

but IMO it's not possible to calculate it 100%

Do you have the entire leverage ratio measured? That would be helpful.

And as the old saying goes, if you can put a man on the moon, you can probably calculate and predict the preload change accurately. It's just a matter of how much time you want to spend on it.

Here's a thought. If they did space travel like we do seat of the pants suspensiion testing, I'd hate to be the guy sitting in the capsule. I could just hear the engineers in mission control, "Well, we missed that one by about 100,000 miles. Maybe need to add another 5,000 lbs of thrust".

Edited by kevinstillwell

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Do you have the entire leverage ratio measured? That would be helpful.

https://thumpertalk.com/forum/showthread.php?t=832118&highlight=

And as the old saying goes, if you can put a man on the moon, you can probably calculate and predict the preload change accurately. It's just a matter of how much time you want to spend on it.

I know- but prefer the faster way with maltreated hands 👍

I think its really close if you just change the preload, then the result in sag change is no problem.

the problem is if you change the spring. the new spring probably differs in the items I mentioned...

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Ya, one problem is the fact that shock springs aren't linear. So you never actually know the spring rate at the point of sag, which could be considerably different than the overall rate of the spring.

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