Variable rate fork springs

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alan1949
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Variable rate fork springs

#1 Post by alan1949 » Wed Jul 30, 2014 8:21 pm

I have 2 sets of 1988 Canadian RZ350 variable damper forks fitted with variable rate springs that are nominally 503mm long. Should the closer wound coils be at the top or the bottom? I know the spring rate is the same in whichever orientation but the depth of the oil in the lower legs will be significantly different. Also should there be any packers on top of the springs for preload as both sets are built differently.

silverstrom
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Re: Variable rate fork springs

#2 Post by silverstrom » Thu Jul 31, 2014 1:24 am

Progressive says it doesn't matter which way they are installed. Up or down will give the same performance. Don't worry about the oil level with springs installed a certain way. It will make no difference when riding as the total volume is unchanged.

Using a heavier oil will certainly help. 15 wt works well. I also use preload adjusters and recommend them.

I use a 1 inch spacer in my 90 forks. That is a PVC spacer and a washer at both ends for a total length of 1 inch. Without the spacer you have no preload with the progressive springs made for 83-85 bikes and used in 86-90 forks.

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RuZty
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Re: Variable rate fork springs

#3 Post by RuZty » Thu Jul 31, 2014 6:42 am

alan1949 wrote:.... I know the spring rate is the same in whichever orientation but the depth of the oil in the lower legs will be significantly different.
Good point, and the center of gravity will be lower with the tighter coils at the bottom. (splitting hairs)

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nokturnal
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Re: Variable rate fork springs

#4 Post by nokturnal » Sat Aug 02, 2014 11:21 am

What do you mean both sets are built differently? Could you have a set of 85s (oddball year)
:smt017

hondaror
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Re: Variable rate fork springs

#5 Post by hondaror » Sat Aug 02, 2014 12:17 pm

What colour are the legs?
Rory
2 1984 RZ350s
1985 RZ350
9 1986-1990 RZ350s
2000 ST2
2005 749
2005 749 with 999 engine

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T.RexRacing
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Re: Variable rate fork springs

#6 Post by T.RexRacing » Sat Aug 02, 2014 12:28 pm

No it's not the center of gravity it's the displacement of oil. The closer the coils are to each other the more oil they'll displace having an effect on trapped air volume. That changes the air spring action in the last third of the stroke affecting bottoming resistance. When tuning suspension with oil height I use 5MM steps and you can definitely feel that small an amount even in 43/45MM big tube USD forks. It will have an even larger change in 35MM noodles like the RZ. So even though the spring rate is the same up or down for consistency in tuning always insert the progressive springs in the same direction.
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RuZty
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Re: Variable rate fork springs

#7 Post by RuZty » Sat Aug 02, 2014 2:13 pm

I was referring to the overall CoG resulting from heavy end down (and I know the difference is minute)

alan1949
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Re: Variable rate fork springs

#8 Post by alan1949 » Sat Aug 02, 2014 9:44 pm

I infact have 3 sets of forks. One is as parts list for year 1988 with shorter springs and spacers. The other 2 sets are from 1986 and 1988 Canadian f2 bikes with the same aftermarket variable rate springs in each. One set had close coils to top and a spacer between spring and top cap. The other had springs the other way up and no spacers. I' m rebuilding both at the moment.

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