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Sportster Motorcycle Suspension, Frame, Forks, Handlebars, Fuel Tank, Oil Tank, Fenders Discuss Sportster Motorcycle Suspension, Frame, Forks, Handlebars, Fuel Tank, Oil Tank, Fenders problems, advice, and/or how tos.

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  #1  
Old 15th April 2012
johnwestphal johnwestphal is offline
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Default Fork Oil Weight/Spacer Length with Progressive Fork Springs?

I recently added Progressive fork springs to my '04 1200 C model. The bike already had the 15w SE fork oil and I used the PVC spacers that came with the springs.

I weigh 175 lbs and ride mostly unloaded/solo. The bike feels a bit stiff with too little sag.

I was wondering what others have done to fine tune their forks after adding these springs. Im thinking of trimming 1/2" from the spacers to lessen preload/increse sag and going back to stock fork oil for more compliant dampening.

Any thoughts/advice besides seven pages of suspension reading? lol
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  #2  
Old 15th April 2012
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I lost a spacer b4 I installed the springs
Cut new ones 3/16 longer and used 10 W Amsoil
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  #3  
Old 15th April 2012
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Have you taken sag measurements or are you just assuming you have too little do to ride harshness?

It is a lot easier to make a call without even looking at the bike, if you have as much data as possible.

Things like- I have X preload. I have x oil height- I have x total and x free sag. I have x oil weight- I have x spring rate.

Fork oil height plays a considerable role in terms of harshness, especially in the last half of the stroke.

I would be more inclined to use 10 wt, but there are so many other variables I would be inclined to look elsewhere first.

Generally about three quarters of an inch of preoad in the front will put you in the ball park if the spring rate is right.

Generally too much compression damping- think heavy oil- and too much oil will cause harshness.

It is almost impossible to have a too harsh front suspension as a result of having too high a spring rate. It is hard enough to get a spring with enough spring rate to fit inside the tubes on a bike with 39mm fork tubes. It is possible to wind it up a spring with too much preload and therefore making it act weird.
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Old 16th April 2012
johnwestphal johnwestphal is offline
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Default

Thanks for the replies guys, I sent Progressive Suspension's tech guys the same question too. I think Im gonna dump the 15w SE fork oil in favor of the OEM 10w and then cut the spacers to correct sag. Think I got used to the mushy OEM springs with 15K on them, need to figure out how much sag is the optimum for my weight/riding style.
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  #5  
Old 16th April 2012
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Just my experience:

I replaced my oil with the OEM oil from the dealer, mostly because that is all that I could get locally on a Saturday. Then I filled the forks to the level Progressive recommends in their instructions, I think they give a range, I just filled to the middle of that range. For the spacer, again, I just used what they recommend in the installation instructions.

I couldn't be happier with how the front suspension performs now, I'm afraid to make any changes because I don't think I can get it much better with just a spring upgrade. For reference, I am just about at 200lbs with a coat/boots and everything when I ride.
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Old 16th April 2012
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About an inch and a half rider sag works in most cases on the front. That is from a fully toped out position, not a static sag position.
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Old 17th April 2012
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Quote:
Any thoughts/advice besides seven pages of suspension reading? lol
WTF??????????

Start by measuring rider sag and oil level.

15w oil is mainly used by bikes with the short forks with damper tubes with larger oil bypass holes.

Replace with 10w, proper rider sag, stock oil level and then decide if you need more fine tuning.

Oh, yea, read the "7 Pages of Suspension" again.

Quote:
It is almost impossible to have a too harsh front suspension as a result of having too high a spring rate.
Absolutely NOT true. Too high a spring rate is only one of several things that can cause a harsh ride.
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