There's a result - free uprated springs

[Grimly]

Few months ago I bought a pair of 78 GS1000 single-disc forks because the stanchions on them were in perfect condition.
I took a bit of a punt on Suzuki part-bin engineering, reasoning the matching twin-disc legs would use the same stanchions, so shortly after bought a pair of twin-disc forks with rough stanchions but good lowers, with the aim of combining the two sets to give me a replacement pair for the '80 850. Same 37mm leg diameter, same discs, same etc, etc.
Hmm... only thing I didn't know at the time was the GS1000 legs are ~20mm shorter overall, so the trail and ride height might be slightly affected - but probably not by much and it's only going to give the same effect as dropping the yokes down the stanchions by a similar amount, which is a common mod to slightly sharpen up the steering anyway.

However, on stripping the single-disc set, I find they've been overhauled not too many miles ago and have what looks to be a set of Progressive springs in there. They're about 3" longer and definitely a heavier rate than the standard springs in the other set, and even those standard springs are much livelier and fresher than the dog-tired 250K miles set in mine.
It occurs to me the stronger springs will compensate for the shorter legs and likely restore the ride height to not far off where it would be on the 850.

Things look set to get interesting...

[GSX1000E]

Great news! Don't forget to add a bit of pre-load too.

[Grimly]

Great news! Don't forget to add a bit of pre-load too.

Well, the PO of these has gone to a deal of trouble to get the pre-load just right for him, with several washers and spacers above and below the main spacer. However, I'm a bit of a stranger to the air caps on these forks, as many (certainly both of mine) of the Euro bikes were sold with mechanical pre-loaders at the top of the legs and no air suspension.
I'm just wondering whether to keep that system or go for air. I've been very happy with air in the rear end and found it to be very controllable and predictable either with oil or rubber damping.
Only drawback is the fiddly air balancing, but I could easily enough make a bridge between the two air caps - indeed, I recall kits used to be sold for that, but it's been a long time since they were around and since they're only a couple of standard NPT inserts and pipes, there's no real hassle in making one sometime down the road. I even now have a spare set of caps and forks I can use to jig them up with, to minimise downtime on the bike.
This is getting better and better...

[ShadowFocus603]

Most folks recommend 10-15w fork oil and no air as far as I've seen in countless searches on the subject.

[Steve]

The heavier-gauge springs are encouraging, but be aware that many of the stock spring also had progressive windings.

The fact that there appears to be "great care" to set preload is also encouraging, maybe they really are Progressive springs.

Personally, I would ignore the air setup in the forks. Just a little too "fiddly" for normal use. Relying on strictly mechanical suspension will be more reliable in the long run. I do agree with the concept of having air-adjustable suspension at the rear, though. It is much easier to compensate for the additional loads of a passenger or gear.

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[RichDesmond]

Couple of things.

First, by "ride height" you really mean that the you think the sag will be less. Hopefully that's the case, but sag still needs to be the correct value. Total sag right around 35mm is good for general street use.
20mm of ride height in the front a pretty big change, typically 10-12 is the most you would go. A (very) rough rule is every 3mm of ride height is 1mm of trail, so you would be shortening trail by 6-7mm. May be just fine, but may a bit twitchy, so just be aware of that when you ride it the first few times.
Ditch the air assist if you can, it's never a good thing.
How much static preload do the forks have now? In other words, how much is the spring compressed when they're fully extended and the cap is on? Should be 10-20mm.

[Grimly]

Most folks recommend 10-15w fork oil and no air as far as I've seen in countless searches on the subject.

Noted, thanks.

The heavier-gauge springs are encouraging, but be aware that many of the stock spring also had progressive windings.

The fact that there appears to be "great care" to set preload is also encouraging, maybe they really are Progressive springs.

They might actually be factory because the pics of Progressive ones I see are agressively progressive at one end, and these aren't quite so much. Also, at the tighter-wound end, the end two coils are tapered slightly. I think they're factory or another make. Still a lot better than mine, though.

Couple of things.

First, by "ride height" you really mean that the you think the sag will be less. Hopefully that's the case, but sag still needs to be the correct value. Total sag right around 35mm is good for general street use.
20mm of ride height in the front a pretty big change, typically 10-12 is the most you would go. A (very) rough rule is every 3mm of ride height is 1mm of trail, so you would be shortening trail by 6-7mm. May be just fine, but may a bit twitchy, so just be aware of that when you ride it the first few times.
Ditch the air assist if you can, it's never a good thing.
How much static preload do the forks have now? In other words, how much is the spring compressed when they're fully extended and the cap is on? Should be 10-20mm.

From stripping them, the static sag was ~15mm, so that's something I'll keep in mind when putting them together.
My old rule of thumb for sag on the bike was 25mm difference between unloaded and loaded with rider - worked well for me for years, at least as a starting point.
Looks like I might ditch the air setup - was curious about it though, as it was the Donkey's Nuts when it came out, with every bike mag blathering on how it was infinitely adjustable, smooth riding, etc, etc. Still, the irony is the Euro mechanical cammed adjusters were probably better after all. Strange.

[RichDesmond]

From stripping them, the static sag was ~15mm, so that's something I'll keep in mind when putting them together.
My old rule of thumb for sag on the bike was 25mm difference between unloaded and loaded with rider - worked well for me for years, at least as a starting point...

Ah, the sag ratio method. Good in certain situations. The 25mm "rider sag" rule is for the rear, way too much for the front.
The main assumption that method makes is that the ratio of rider weight to bike weight is in a certain range. A 300lb guy on a Ninja 250 is going to have very different numbers from a 160lb guy on a Goldwing.

On the air assist, the bike mags used to be all gaga over anti-dive forks too, don't see those much any more either.

[Grimly]

Stripping the remaining legs today, I measured the static sag - on the recently rebuilt set, it was 27mm; on the standard gnarly old set it was 20mm.
The progressive springs aren't really progressive at all, more twin-rate than anything. Half the coil length is fairly standard, the other half is more closely wound, but not progressively wound.
I don't know if these are factory or aftermarket.

[Bass Man]

Glad to see Progressive and progressive springs being discussed in the same thread! My stock 850 has its original progressive springs. So what's the beef against air assist in the front? So many of the after-market rear shocks advertise 'gas charged' - the gas being typically nitrogen, air is 85% nitrogen, what's wrong with doing same in front? Even if you remove the valves and cap them, the forks are still filled with air at a pressure relative to your location's altitude, and the barometer reading that day, and will compress just like it would if you pumped a bit more in.

I guess you could vent them...

[Rob S.]

So what's the beef against air assist in the front?

For one thing, you need a purpose-built pump and gauge. You can't use your tire pump or gauge - that would be like performing surgery on a fruit fly with large Vise Grips.

BTW I don't have a beef with the air forks (I bought the fork air pump).

[Steve]

Glad to see Progressive and progressive springs being discussed in the same thread! My stock 850 has its original progressive springs. So what's the beef against air assist in the front? So many of the after-market rear shocks advertise 'gas charged' - the gas being typically nitrogen, air is 85% nitrogen, what's wrong with doing same in front? Even if you remove the valves and cap them, the forks are still filled with air at a pressure relative to your location's altitude, and the barometer reading that day, and will compress just like it would if you pumped a bit more in.

There is a BIG difference between the setups for front and rear suspension. At the rear, air is trapped in a bladder and hasically has nowhere to go, so it stays there. However, on the forks, any trapped air can easily get by the fork seals. When the air leaks out, you will be relying on the mechanical springs, which were weak when they left the factory (relying on additional air for support) and have not gotten any stronger over the years.

Another strike against the stock springs (which might be progressively-wound) is simply the fact that they are progressively-wound. While it sounds good in theory, the trapped air is also a progressive-rate spring. The two progression rates will add up at an alarming rate, possibly giving you some interesting results. it is better to go with a straight-rate spring that is adequate to hold up the front of the bike, then use the air that is trapped inside the forks (normal pressure, nothing added) for some additional anti-dive support that will not bleed off, because there is nothing added.

The "nitrogen charging" for rear shocks is different yet again. That nitrogen (usually rather high pressure) just pushes against the shock fluid to keep it from foaming up, which would lose its damping qualities.

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[RichDesmond]

There is a BIG difference between the setups for front and rear suspension. At the rear, air is trapped in a bladder and hasically has nowhere to go, so it stays there. However, on the forks, any trapped air can easily get by the fork seals. When the air leaks out, you will be relying on the mechanical springs, which were weak when they left the factory (relying on additional air for support) and have not gotten any stronger over the years.

Another strike against the stock springs (which might be progressively-wound) is simply the fact that they are progressively-wound. While it sounds good in theory, the trapped air is also a progressive-rate spring. The two progression rates will add up at an alarming rate, possibly giving you some interesting results. it is better to go with a straight-rate spring that is adequate to hold up the front of the bike, then use the air that is trapped inside the forks (normal pressure, nothing added) for some additional anti-dive support that will not bleed off, because there is nothing added.

The "nitrogen charging" for rear shocks is different yet again. That nitrogen (usually rather high pressure) just pushes against the shock fluid to keep it from foaming up, which would lose its damping qualities.

.

Well put.

This is an important but subtle point, "it is better to go with a straight-rate spring that is adequate to hold up the front of the bike, then use the air that is trapped inside the forks (normal pressure, nothing added) for some additional anti-dive support that will not bleed off, because there is nothing added.".
The crux of it is that, because of the pro-dive nature of telescopic fork front ends, the overall spring rate (metal + air) does need to be slightly progressive. The trapped air volume at atmospheric pressure provides all that is needed though, and varying the oil amount gives a sufficient tuning range. Making the overall rate more progressive than that causes problems, and won't allow the suspension to make the best use of its available travel.

Another factor is that a too progressive overall rate, either from progressively wound springs or from added air pressure, makes matching damping to the spring rate harder.