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Discussion Starter · #1 ·
Finally got everything I need for my fork upgrade so I was going to assemble today.
'74 CB/CL450 damper rod forks on a my '70 CL450 using Racetech springs and 20w fork oil. How do I measure for the spacers?
Also how much oil do I put in - there's no definitive amount it seems that I can find so how crucial is it..

Cheers

DW
 

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Do you have CL or CB shocks? The CB K5, CL K5 & K6 take 155 to 165 cc per side, the CB K6 & K7 take 135 to 145 cc per side. I got to ask, why are you using 20 WT? They came from the factory with ATF that is about 7 WT. Why are you putting spacers in, preload? These are not modern cartridge forks. Trying to tune the spring compression to the rider weight is a little over the top. These bikes are fun to ride, but they are a 40 YO bike and ride like one.
 

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Put it together and measure how much spring sack(fork compression) there is with you sitting in the seat. 30% suspension travel is a good starting point.
 

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I would start with 10W oil, ride it for awhile and maybe you'll want to go to 15W. 20W is going to be way stiff and a very harsh suspension
 

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I would start with 10W oil, ride it for awhile and maybe you'll want to go to 15W. 20W is going to be way stiff and a very harsh suspension
I put 20w in everything and have not really noticed any undue harshness or stiffness. Isn't the spring rate that controls stiffness? I thought the oil viscosity dictated rebound control. i.e., thicker oil means less oscillation (bounce) upon rebound.
 

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Not all forks are designed the same. 20W may be fine for many, but older bikes mainly just used ATF fluid, which is not as heavy. There were heavier oils back then too, but these forks are just not designed for them, and do not work as well as they could that way.
 

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Most forks control the fluid in both directions. Simple ones use the same orifice for both, while others will change the damping rate for compression and rebound. I haven't seen any that don't have some damping in compression, so a stiffer oil will increase the shock transmitted to the frame. Of course, this all depends on which specific brand and variant within that brand, because the weight system allows for thin 20W to have the same viscosity as a thick 7W, due to overlap in the ratings. Brand A's premium 10W may be thicker than their plain label 20W, and still fall withing the ratings.
 

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Discussion Starter · #10 ·
OK so some more info...
This was based on the recommendation from Matt the vintage specialist at Racetech. My bike is a 1970 CL450 that had the wack ass old 60's suspension setup so I am swapping the forks to a 1974 CB/CL450 set which had the better plunger style internals. The new springs are linear (from Racetech) NOT progressive which originally came in the forks, hence the need for spacers. The 20W oil is what he said he's been recommending to his clients and it seems to work well with these - he mentioned the same thing that since it's the new specific fork oil it's weight is different that the old ATF style. Plus that's something I can always drain and replace with 15W after test rides...I was more confused with how to figure out the damn spacer length.
 

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Discussion Starter · #12 · (Edited)
View attachment IP FRSP S Street Fork Springs.pdf

Yes thanks all I've done my research including the Racetech spacer instructions but I can't wrap my head around it so can someone break the spacer thing down?
My springs are .80 and I've now got 15W oil as I agree the 20W may be too much to start with. Oil isn't my biggest concern as it's fairly easy to drain and try another weight but I'd rather not have to remove the forks again to rework preload or anything else so I'd like to get it right the first time. Racetech sheet states standard preload to start with for street bikes is 20mm. Obviously the new spring is shorter than the original so I'm thinking I cut the PVC pipe to make the new spring length match the old one and then add on another 20mm for preload? Sorry for my confusion but I've never done this and there's so many variations on what to do I just need help from someone who's done this...
Frustrating...just want to keep getting the bike together...


Thnx
 

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I'd start by verifying the travel limit of the forks, then using the RaceTech article as a guide, determine the recommended sag, and measure the actual sag without spacers. Just put the forks together and sit on the bike to measure it. Then cut spacers equal to the measured amount of sag, after subtracting the recommended sag, more or less, then re-test. You can always trim some from the spacers if you cut them long, until the sag is where you like it. So, if the sag without spacers measures 3", and you want 2.25", make spacers 0.75" long.
 

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"thanks all I've done my research including the Racetech spacer instructions but I can't wrap my head around it so can someone break the spacer thing down?"

I went through this last year with new RaceTech springs. It helped me to think of the spacer as setting your ride height (measured by sag). Using no spacer would allow maximum sag and give you the lowest ride height while leaving little compression travel in the fork tubes. Too much spacer and you're raising your ride height by compressing and pushing the spring forcing the tube out to its maximum length. You want to ride your bike so that the forks are somewhere in the middle of their travel. That is, there is room to compress when hitting a bump and room to extend when rebounding. This is measured by sag. That is, how much does the suspension sag when when loaded with the weight of the bike, rider and gear.

I sat on my bike with gear and let the suspension settle. I then had a friend put zip ties around the tubes just above the seals. We carefully lifted the front wheel off the ground without moving my zip tie markers and let the front wheel fully extend the fork travel. Measuring from the top of the lowers to the zip ties gave me the sag. I cut spacers out of pvc pipe and after a couple of tries I had the final sag set where the manual suggested.
 
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