GS forks on a GT750

[tz375]

To summarize,
Using what you have a is a good place to start. AFAIK those 1100 forks have the same design features like teflon bushes that Allan likes on the 650 forks.

They are stiffer than GT or GS650 forks.

They are all ready fitted to the bike.

650 forks are a slight upgrade over GT forks but it's a small change relative to modern suspension.

I would be very surprised if the damping characteristics on any of those forks was close to what was needed. We know the GT was miles out.

The likelihood of the 1100 having so much compression damping that it packs down is fairly low but easily fixed.

All three sets of forks GT, GS650 and 1100 will benefit from improved damping and the best way to set up low and high speed compression damping is with cartridge emulators from race tech, YSS or others. When emulators were first tested by bike magazines, they were converting bikes with teflon bushes and larger diameter damper rods than a GT and found significant improvements in handling and street manners. There's a lot of scope for upgrade here.

Damper rods are better than no damping
Emulators are better than damper rods
Cartridges are better than both

Cartridge conversions are available but the entry price point is more than most of us would want to pay.

Rear suspension (the whole system) is usually a bigger issue than forks. Get good bushes in the swingarm and good shocks to significantly update handling.

Teflon bushes do help to reduce friction - both sliding friction and limiting friction. They do nothing to reduce seal stiction.

I think you will find that despite what was written in 1975 when there were no other options, GT forks have enough travel. The issue back in the day was sagging springs which absorbed most of the travel. More travel does not translate to better handling - having the correct springs and matching the damping does. 4-5" is enough travel if the forks are set up correctly. The latest GSXR by comparison has 4.&" travel at the front and 5.1 at the rear. More allows the bike to pitch too much and that upsets handling.

You can cut spacers with a hackswaw and trim them with a file. That's how we do race bikes and their suspension takes more of a pounding than street bikes - less potholes but we use all the travel most of the time.

Start with stock oil viscosity recommendations but best to use FORK oil in a specific grade and stay with the same manufacturer. Start with 20Wt and move up or down as required to fine tune damping. Start with 10Wt with emulators.

Increasing oil levels stiffen up the front end slightly (partial air springing).

I agree 100% with Allan's comments about air assist. Anything over 10% tends to blow seals but Suzuki at that time did use high pressures but no one does that any more. I'd start with zero PSI and add up to 10 if necessary with a small fork air pump. The volume of air is very small and it's hard to set air pressures accurately. Try to get the forks working without air is a better approach IMHO.

Suspension technology has come so far in the last 40 years that it's arguably better to fit a set of reasonably modern forks say 2001 GSXR/R6 conventional forks fit than messing with old forks unless we want them to look stock.

[jabcb]

End of season update on how the fork mods worked out.

The Progressive Suspension springs for the GS1100GK forks are part # 11-1129. These also fit the VS800.
Progressive Suspension also sells a lowering kit for the VS800: part # 10-1556.

So I got the lowering kit and installed then to lower the forks 1". Also slid the forks down the triple clamps to offset the lowering. Am currently using no air pressure.

Worked out quite nicely. Much better ride than my stock GT750 & no bottoming so far.