Fork fix hints and a few questions......

[oode]

OK so I finally got around to fixing the front seals on the B4, not as daunting a process as it could have been.  I did find a few pieces of cheap magic that I want to pass on.

1 - special tool for holding damper.  Conventional wisdom and myth says use an air wrench and if you spin it fast enough or cram a broom handle in there you are good to go.  I did not experience this, and I was also concernded about torquing down on assembly. But... I found a $4 tool that holds the damper perfectly. 
  -Go to basic hardware store and get 4   3/4"-10 nuts and a 2' section of 3/4" threaded rod. 
 - Put 2 nuts on each end with a 1/2" of rod protruding then wrap the whole bit in electrical tape to keep from taking chunks out of your forks. 
The 3/4" nut fits the damper perfectly and you can hold the other end with a 3/4 socket for adjustable wrench.

2 - Seal installer.  I got the advice to "just tap around the seal to seat it" and I was not comfortable with this and did not trust myself with a hammer an punch that close to my precious baby, so....  2' of 1-1/2" PVC, square off the ends, trim the strings then slit the tube end to end so it can expand to fit the fork tube.  Beat with the weapon of choice.  Seats the seal perfectly with little chance of damge to anything.

3 - GSXR750/600 rear shock (mine was '92 but '88 - '93 should be same) is a massive improvement, lots of adjustment and minimal if any rise in seat height.  well worth the $100 investment.  You get pre-load, compressions damp and rebound damp all in a nice unit that you can upgrade later if you choose to (spend a bunch of cash...)

Now for the questions.....

The PO left me fork oil which I didn't read before putting in and it is 15W and thusly a bit stiff considering my moderate weight and the progressive springs.  Is there a way to soften this up a bit without swapping out the fork oil????

If that isn't possible, can you swap out the fork oil without dissassembly????

Well, that is my contribution for the moment......

Getting chilly but still riding....

[pmackie]

The PO left me fork oil which I didn't read before putting in and it is 15W and thusly a bit stiff considering my moderate weight and the progressive springs.  Is there a way to soften this up a bit without swapping out the fork oil????

If it is really too high a viscosity, this would result in too much compression damping making the forks feel harsh over bumps, and too much rebound, which might cause the forks to "pack down" over a series of bumps. But given these are old style damper rod forks, with new springs, I would have thought that 15W viscosity would be about right.

If you really need to thin out the viscosity, you can take out the springs, let the forks drop and use a "turkey baster" or another suction device to remove a bunch of the existing oil. If you can remove 50% of the 15W and replace with an equivlent amount of 5W, you will end up with a 10W.

[gsxr400 racer]

take 30-60 cc's of fluid out of each leg to increase the air gap

[oode]

It definitely feels harsh on bumps, but I haven't noticed any packing.  Just feels like  a brick up front. 

So air gap will soften it up???   I assume I can just raise it up, and undo the fork caps (being careful not to launch them..).

Which raises another question I had, I put in a fair bit of preload on the spring on installation, is that also something I should trim down???  I think I left the spacer just a little above the top of the fork tube (1/4 - 1/2 "....)

[gsxr400 racer]

you really only want to preload them 10-25mm any more than that get new springs

[oode]

Is 25mm that relative to the top of the tube or the bottom of the (screwed in) top cap???

[gsxr400 racer]

from the bottom of the cap or whatever starts to press on the spring first

[pmackie]

Two things to consider...

1. Air gap/fork oil level. The level of the fork oil in the tubes controls the air gap above it, which operates as a partial air spring. Higher fork oil level (less air gap) increases the effective spring rate near the end of the stroke. Conversley, lower oil level decreases the effective spring rate near full bottoming. As Jay mentioned, if your forks feel harsh, you may have too high a fork oil level.

When I installed the new "Progressive Suspension" springs in my 600, they recommend much LESS oil than stock. This is because the spring is longer, with a much shorter, if any, preload spacer than stock. This effectively raises the fork oil level as the suspension compresses much more that stock, causing the forks to get harsh or even hydraulic lock, near the end of the stroke. Check the instructions that came with your new springs and ensure that you have the correct amount of fluid in the forks.

2. Preload - Again, on my 600, "Progressive Suspension" recommended that the springs be flush with the top of the tubes when installed, resulting in approx 1/2" of preload as you screw in the caps. I found this too little preload for my weight, luggage and riding style, and I have gradually added approx another 1" or so of preload to get the forks feeling right to me.

On the 400, with your own variables, you results will likely differ.

Once you ensure that your oil level is approx correct, you can do a couple of tests to help set preload. Put a "zip-tie" around one of your fork legs to serve as a stoke gauge. Lift ALL the weight off the front forks, and push the zip tie down, let the bike settle on it's own weight, then raise the bike again and measure the amount the forks compress with only the weight of the bike. This measurement is call "Unladen sag".

Do the whole proceedure again, but this time sit on the bike in your normal riding position, feet on the pegs. This is called "Laden sag".

Also, do a ride around your normal circuit, and see how much the forks move. As well as hiting some good bumps.

At this point you  have a few measurements that will help you. As a guideline/starting point you should have approx:

Unladen Sag - 10-15% of total fork travel. So about 1/2 to 3/4"
Laden Sag - 25 - 35% of total fork travel. So about 1-1/4 to 1-3/4"
Normal riding with NO serious bumps - about 2/3 travel or about 3 to 3-1/2"
Bumps should allow the fork to stroke fully, so about 4-1/4 to 4-1/2"

You want the forks to stoke completey on bumps. If they don't, you can lower the oil level. If they bottom too much, you can raise the oil level. More preload will decrease sag. Too much sag will cause the forks to use up too much of their travel too soon, dive excessively on braking and usually cause the bike to run wide on corner exits, as the bike dives too much on the way in, then rebounds too far on the way out.

Just some suggestions to try...

Also, check out this thread:
http://forums.banditalley.net/index.php?topic=10853.msg87057#msg87057

[Vidrazor]

I ran across this over at Kieth Code's Superbike School, it may be of help:
http://www.superbikeschool.com/multi-media/sag.php

[pondingie]

One vendors fork oil weight system is quite often not the same as another's.  Just grabing a bottle of 10 weight off the shelf could put you in a place you don't want to be.  The service manual for my B12.5 calls for Suzuki SS08.  My dealer tells me that is 8 weight (makes sense).  Well I drained that crap and let me tell you that's some thick shit.  I put in Ams 5 weight and found improvement.  Disclaimer; I use Iranian (iridium)spark plugs and Amsoil - I am not a terrorist nor an Amsoil salesman.

[pmackie]

One vendors fork oil weight system is quite often not the same as another's.

This can be kinda true, since the SAE viscosity rating system was origionally designed for rating engine and gear oils into general categories. for eg:
SAE Grade - Cst @100 C
     0W          -3.8 
     5W         3.8 - 4.1 
    10W         4.1 - 5.6 
    15W          -5.6 

 Using SSU or ISO (Cst) viscosities are much more accurate. However, anything rated as a 10W will be close...I have NO IDEA what someone means when they say they have a 8W? Maybe the light end of a 10W or the heavy end of a 5W?

This chart gives a pretty good list of data that someone put together:

http://www.pvdwiki.com/index.php?title=Suspension_Fluid

Anyway...for those who want more info on suspension tuning, I ran across this guys two web sites, and like his overall approach and attention to detail. I haven't read over all his thoughts, but he seems to have done his research and explains things pretty well. Have a read over:

http://www.pvdwiki.com/index.php?title=Main_Page

http://peterverdonedesigns.com/index2.htm

[97af]

This can be kinda true, since the SAE viscosity rating system was origionally designed for rating engine and gear oils into general categories. for eg:
SAE Grade - Cst @100 C
     0W          -3.8 
     5W         3.8 - 4.1 
    10W         4.1 - 5.6 
    15W          -5.6 

 Using SSU or ISO (Cst) viscosities are much more accurate. However, anything rated as a 10W will be close...I have NO IDEA what someone means when they say they have a 8W? Maybe the light end of a 10W or the heavy end of a 5W?

This chart gives a pretty good list of data that someone put together:

http://www.pvdwiki.com/index.php?title=Suspension_Fluid

Anyway...for those who want more info on suspension tuning, I ran across this guys two web sites, and like his overall approach and attention to detail. I haven't read over all his thoughts, but he seems to have done his research and explains things pretty well. Have a read over:

http://www.pvdwiki.com/index.php?title=Main_Page

http://peterverdonedesigns.com/index2.htm

Too cool....Somebody else who dives into oil #'s as much as me. I bet we could get the perverbial dreaded "oil discussion" thread going to the point where the mods would lock it down!

Anybody got a surplus of Mobil 1 15-50 Red Cap they want to part with?

Running a little low....

[gsxr400 racer]

I like tranny fluid in my forks LOL

[97af]

I like tranny fluid in my forks LOL

Jay,dont laugh.....Thats all I used in my forks as a kid..late 70's early 80's motocross manuals called for it.

[pmackie]

Thats all I used in my forks as a kid

I still use DEXRON III ATF as fork fluid in the damper rod forks of the small bikes (XR-80R/TTR-125L/KDX 175). Works just fine in these old tech forks, easy to get (I seem to always have it on the shelf), and the cost is almost nothing.

Most newer forks need something else however, AND not all ATF's are the same. Lots of spec changes now on ATF with lots of changes to Dexron, Mercon and Plus specs.