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rjp996

Rear Shock - better with the wife on it....

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rjp996

So, ive always found the rear shock on my NC750x a bit harsh - I changed to it to a Wilbers after 5k and have the high speed and low speed compression dialed up to have only a small amount - it improved things a bit but still not fantastic.

 

Anyhow, past few days I have been out with my wife on the back and with her extra weight (.....), I find the bike much more compliant and less hard over bumps..

 

So the question is.... do you think this could be because the spring rate is to high (it was for a 90kg person..). I think the Wilbers 641 is no a progressive spring so assuming that its not related to the pre load ? (as on a progressive spring that may make a diff).

 

Or have i messed up elsewhere.

 

other alternative is to make my wife ride on the back of my bike when I got to work, hang around and come back with me in the evening - but I don't think she will buy that solution ;-)

 

 

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rjp996

Ohhh, doing a search it looks like the Wilbers 641 may well be a variable rated spring, and if my understanding is correct, then the additional load on the spring with my wife on the bike would have put be into a different section of the spring and its spring rate - if thats the case, adding preload to a variable rate spring would achieve the same ?

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electric_monk

It is said that the first sign of madness is talking to yourself and that the second sign is answering yourself.

This thread gives us a great view into your mental state then....:drool:

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embee

The preload doesn't alter the point at which the spring operates, it alters the ride height of the bike.

If the spring needs to provide a force of, say, 400kg to support the bike and rider (bearing in mind the linkage is nearly 3:1ratio), then it will compress to the height at which that is achieved. The preload (assuming it is less than this required load) simply alters the starting point, not the point it ends up working at when loaded.

 

For example, say the spring is 20kg/mm stiffness/rate. You apply 5mm of preload and that means 100kg load in the spring, the shock is fully extended (wheel at lowest point with bike on stand). When you then put the bike on its wheels and sit on it, needing a total of 400kg to support it, it will compress another 15mm to provide the additional 300kg. The spring will then end up with a total compression of 5+15= 20mm at this load. If the linkage is 3:1, the wheel will have moved 15mx3=45mm compared to where it was with the bike on the stand.

 

If you start off with 10mm preload=200kg force in the spring, when you then put it on its wheels and sit on it, the spring will compress a further 10mm to provide the additional 200kg to make a total of 400kg. The spring will be compressed a total of 20mm exactly as before, but the  bike will only have sagged 10x3=30mm compared to the 45mm with the lower preload.

 

...if that makes sense?

 

Having measured my 700 Integra spring I concluded it was too stiff but had too little preload so the static sag was too much before it supported the weight but was then too hard. I fitted a lower stiffness spring with more preload (on a Nitron shock) to get the sag where I wanted it and the right sort of stiffness of ride. Of course if you are going to carry a pillion it makes sense to have a variable rate spring which works in a lower rate region when solo and compresses to a higher rate condition when two-up. A remote preload adjuster is also a huge advantage for switching between one/two people. I imagine Honda fit a compromise spring, stiffer than you want for solo use but possibly not really stiff enough for pillion use.

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rjp996

embed - agree with you on a linear spring, however it looks like the willers 641 is a variable rate spring, so the more the spring is compressed the firmer the spring rate - so adding preload to a variable rate spring would increase the spring rate (firmness), however if the spring is of the wrong rate, then the sag will be out and the bike not sitting level.

Having a think, I'm thinking that reducing the preload will mean that the softer initial part of the spring will be utilised (so long as the spring is not underrated for my weight).

 

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trisaki

Sounds like you need to back off the preload 

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