Ever since I put in my progressive springs, I've been wondering about how one goes about tuning the forks.  Oh, that was a horrible pun    sorry!

Okay, so the questions are:

1. Changing spacer height affects front end sag and spring rate?  Shorter spacer may increase sag and make ride softer?

2. Fork oil weight.  Should affect damping rate, but how would I feel this effect while riding?  Lighter weight = less damping, right?

3. Temperature vs. Fork oil.  Was reading an article on the SV650 cup and this was mentioned.  How does temperature affect the suspension?

4. Fork oil level.  How does changing the level affect the suspension?

I think I might want to make a couple of adjustments in the future to soften the front forks up a little bit... you want 1/2 to 1 inch of sag?

Quote1. Changing spacer height affects front end sag and spring rate? Shorter spacer may increase sag and make ride softer?

Recent discussion.  The general rule is that spacer preload is for setting sag (perhaps 1 1/2 to 2  inches for the fork).  A few other symptoms may occur if the spacer is shortened.  First, the fork will have an increased tendency  to pogo over series of bumps since there will be more travel available above the ride height before the suspension tops out.  Second, decreasing preload may soften some of the suspension movement since the higher the forks rebound the softer the next suspension movement will be.  Third, lowering fork sag shifts weight distribution more to the front which will increase the burden on the suspension.  

Quote2. Fork oil weight. Should affect damping rate, but how would I feel this effect while riding? Lighter weight = less damping, right?

Right.  Lighter damping means less suspension control under extreme cornering and braking.  Less compression damping means softer ride.  

Quote4. Fork oil level. How does changing the level affect the suspension?

You must have the minimum amount which the manual specifies to keep the damping assembly submersed in fluid.  Adding more fluid beyond that amount will help the fork resist bottoming since the air pocket reduction will increase air spring rate exponentially.  Too much fluid will reduce suspension travel.

To sum up, to ensure that the Progressive suspension is no firmer than it needs to be, have maximum sag (above) and 10 weight fork oil in the specified quantity.

One thing I had never read clearly, was whether the fork oil level should be measured with the front wheel on the ground, or with a block under the engine so there is minimum compression of the fork.  Or maybe it doesn't matter, so long as you measure it the same way before and after.

I've got some time tomorrow, but the bike shops are all closed Sundays.  I've seen some synthetic "15w shock/fork oil" at the auto parts places.  Is that the same thing?  I think the "synthetic shock/fork oil" is pure silicone (like DOT5 brake fluid, it will never absorb water).

The fork oil level is measured with the springs out and the forks compressed all the way down. The level should be 99mm (3 7/8") from the top edge of the fork tube.

The stuff from the auto parts place sounds like the right thing. I think they're catching on and carrying more motorcycle related items.
Kragens carries a Valvoline oil for ATVs. I've found it to be very good for non synthetic engine oil.

You are about right on the spacers affecting sag. They don't affect spring rate though. You CAN soften the springs a certain range by taking preload out, just like you can stiffen it up a bit, but to much either way will make the fork do wierd things. Assume you have between 1" and 1 & 1/2" of sag to play with. If you have over 1 1/2" of sag and the bike still feels stiff and bouncy, then you need lighter springs.

You should measure the fork oil level with the springs out and the tubes completely collapsed, I donno if an assembled GS front end will allow this, I always R&R my forks off.

As long as it says its a specific weight "SHOCK/FORK" oil then it should be ok. The viscosity (thickness) varies some from brand to brand, but you shouldn't notice much for street riding. Same goes with heat. Oils become thinner the hotter they get, even on single viscosity oils. But you shouldn't be working your forks hard enough for that to be a problem.

Old timers used to use motor oil or even ATF. But DO NOT put that crap in your GS. ATF will attack fork seals and motor oil will get gunky with aluminum particles and you'll never get it out.

With the primative damping rod fork on a GS you will never be able to get it "perfect" for every speed/road/load condition. The best you can hope for is to get it at least comfortable and competent over the roads you most often travel. For the street, if you want a fairly soft ride, I would just go with 10wt. 15wt if you do any kind of hard riding on GOOD roads. Beyond that a set of cartridge emulators would be a good idea.

Quote from: Bob BroussardThe fork oil level is measured with the springs out and the forks compressed all the way down. The level should be 99mm (3 7/8") from the top edge of the fork tube.

I was thinking I'd just measure the oil level before removing the old springs, and again after adding the new springs.  But I'll try the 99mm check before I put the springs back, and see how close I would've gotten.

Also, my Haynes says to note the alignment of the fork tubes before removal.  Would it help alignment of the forks to connect them to each other with the axle and fork brake before reinstallation?

No, I'm willing to bet that the forks will bind up if you try to push them back up as a unit.

Progressive reccomends less oil than stock because their springs are way beefier.  5.5 inches from the top is what they recommend for the GS.  Plus if you are going to add a beefier spacer like sch 40 pvc..you may want to go 5.6 or 5.7.

The amount of air trapped in the forks acts as a second spring..there was a really good article about that, but i forgot if it was linked from an earlier thread or if i just found it online.  Basically though, less air=stiffer spring constant for the air.  Now, theoretically...(and thinking theoretically has gotten me in trouble on this board before...so watch out)  Fork oil should be more or less incompressible, like most fluids.  The forks are air tight, so if you had NO air, they would not compress at all.  The less air you have in there, the quicker it becomes impossible to compress anymore...uggg..headache now..

Quote from: DiderichProgressive reccomends less oil than stock because their springs are way beefier.  5.5 inches from the top is what they recommend for the GS.  Plus if you are going to add a beefier spacer like sch 40 pvc..you may want to go 5.6 or 5.7.

The amount of air trapped in the forks acts as a second spring..there was a really good article about that, but i forgot if it was linked from an earlier thread or if i just found it online.  Basically though, less air=stiffer spring constant for the air.  Now, theoretically...(and thinking theoretically has gotten me in trouble on this board before...so watch out)  Fork oil should be more or less incompressible, like most fluids.  The forks are air tight, so if you had NO air, they would not compress at all.  The less air you have in there, the quicker it becomes impossible to compress anymore...uggg..headache now..

That might be what I need to change.  I filled it up to the 140 mm suggested height.  A little less oil and maybe shave off 1/4" from my pvc spacer... Long Island roads are mostly cratered and buckled all year round, so I think the usual swap is a bit too stiff for the road conditions.  Just a bit of fine tuning... progressive springs are way better than those mushy-soft stock springs!