Fork Spring Preload

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Oct 17, 2000
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#1
I recently swapped out the stock fork springs for Fredettes #21 springs and rebuilt the forks. What a difference in the whoops. The bike just plows through the heavy stuff. My question is: The manual stated to finger tighten the bolt that goes on the damper rods and then install the cap but I never set any preload anywhere. Did I install the springs with maximum preload? The fork works real well on the heavy stuff or going fast but a couple of times at slow speed on square edged bumps the front end felt like the tire was flat and bottomed against the rim. I looked down twice to see if I had a flat. I used 525 ml of 5wt or whatever the service manual stated. Am I running too much preload? Thanks.
 

Sage

dirtbike riding roadracer
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#2
The big spacer is your pre load adjuster, longer = more pre load, shorter = less pre load, (in a nutshell). the nut under the cap need to be more than finger tight, that something I'd go do right now! I uaually use the stock spacer and that has always worked good for me and other forks I've done. as far as the oil, set it by level not volume, fully compressed spring out set it to 100mm.

PS. look at this, not related but many should find it good, tell a friend - "Tire's"
 
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Mac

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#3
Dan,
Fredette usually includes a piece of pvc tubing so you can custom cut your own preload spacers if you want. I used the stock metal spacer. Sage is right, dont just dump in 620cc you really should measure the oil height properly so you can fine tune this height later if needed. Adding more oil helps prevent bottoming. I like to run 90 - 95mm of Mobil 1 atf which is around 7wt I think.

Sage,
Good advise but $63.45 for a Dunlop 756 110/100/18 rear tire?? I found that same tire HERE for $50.95. Do you guarantee yours for life or what? :think
 
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#4
I got the springs second hand and don't have the spacer. If i take off the caps and compress the forks there seems to be and inch or two where the fork boots bind and I can't get a true "fully compressed" measurement. Spring out? How about the damper rod? Damper rod in or fully out? Thanks.
 

canyncarvr

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#6
I got the springs second hand and don't have the spacer.
Then you used the spacer that was already there? The steel tube?

Set preload by:
1. Assemble fork with everything except the spacer you took out. Include the washers.

2. Measure the 'space'..between the two washers. Say it's 80mm.

3. Add to that measurement the amount of preload you wish. In this case, for a 10mm preload, you would cut a spacer to 90mm.

You can use 1" PVC for spacers. Use schedule 40 PVC. You can use the existing steel tubes, too. It's harder to fashion to the required length.

Yes, your boots will not allow full compression. Take them off.

Spring is out to check oil level. Pump the piston rod a few times to make sure you have all the bubbles out of the system. When checking the level, let the rod sit at the bottom of its travel.

Proper oil level is set by putting in MORE than you need, then extracting the excess out (use a big syringe with rubber tubing cut to the level you want).

The purpose of preload is to make sure everything stays tight. Putting in a bunch of it just wastes available spring movement. OEM preload is way too much imo. 5mm will do. A local tuner uses only 2-3mm.

Your springs will soften up on the little stuff alot if you set your preload to something reasonable.

Doesn't the 'finger tight' part comes from pre-assy to measure for preload? Certainly on final assy, it's not finger tight. Check the torque specs in the front of your manual. If you don't find it, ask.