The Concours has a soft fork that normally gives it "a plush ride." Kawasaki intended the Concours to be a GT in the European tradition like the BMW K100RT. Not a full blown sport bike yet not a bagger tour bike. Hence the soft suspension. There are fixes and upgrades. Read on.

FORK OIL

The ZG fork uses a Damper Rod. This rod contains holes that allows oil to flow from one chamber into the other. The flow of oil through these holes regulates the rate of compression and rebound of the fork. It keeps the fork from bottoming under most circumstances and prevents the chassis from pogoing. Kawasaki filled the forks with 10 weight fork oil, combined with dual rate springs. Their spring rate is somewhere between sponge and Jell-O. If your fork bottoms easily the oil has probably thinned out and been contaminated with water. As with any fork you can play with the fork oil weight to adjust the rebound damping. Use 10, 15, or 20 weight oil straight from the jug or mix weights to get something in between. After that you're stuck with one constant Compression and rebound rate. I suggest 15w. Pre-94 fork oil level is 355 mm (13.97 in.) with the springs removed and forks fully EXTENDED. Post-93 level is 171 mm (6.73 in.) with the springs removed and forks fully COMPRESSED.

NOTE: The method of measurement has changed in the manual for the 94-06 model years. It is now taken with the forks compressed. The measurement went from around 14 inches to about 6 3/4 inches, since now the forks are compressed instead of extended.

SPRINGS & EMULATORS

I'm a heavy guy and ride semi aggressively. I needed more spring than Kawasaki saw to provide. Most think the original springs are progressive but in actuality they are Dual Rate. One half of the spring has a 0.7 kg/mm while the other half is 1.7 kg/mm without any progression between them. The problem with Dual Rate springs is that the stiffer half doesn't come into play until the softer half is fully compressed and its coils are all touching.The sag was excessive and I was bottoming the fork during normal braking. Sag is the amount (in mm or inches) the springs compress while supporting the weight of the bike, riders, and cargo. To set the proper sag I increased the fork preload. Preload is a compression of the fork springs that pushes the bike up and supports its weight. This preloading was built into the fork at the factory. It can be fine tuned using the adjusters. The early (86-93) models use air pressure and the late models (94-06) use screw-in adjusters. Preload doesn't change the fork's spring rate or damping. The preloaders' job is to set the front and rear spring sag and hopefully prevent bottoming. Adjusting the preload only changes the ride height. For me, even with the adjustment set to maximum, there was still too much sag and the brake dive was still there. I needed a stiffer spring.

Stiffer fork springs are the best place to start handling improvements and reduce brake dive. Aftermarket springs in an early model (86-93) will also eliminate the need for air pressurizing but many still use it to fine tune the sag. Installing a stiffer spring in all model years slows the vertical speed at which the wheel travels when encountering bumps. It allows the fork to be more effective during low speed bumps and lets the fork oil flow through the damper rod holes slower. Aftermarket progressively wound springs are better than stock but still on the soft side, fine for cruising the backroads and commuting but I recommend a straight wound spring for "spirited" riding.

Progressive Suspensions only offers one spring for the Concours (all model years) with a varying rate between .80 and 1.25 kg/mm. The .80 end is still way too soft for a bike of the Concours' considerable weight. It will sag some from the Concours' weight and totally compress under braking. The stiffer 1.25 end may be stiffer but softer than the 1.7 half of the OEM springs and too short since it is basically only 1/3 of the spring (soft > medium > stiff). Their average spring rate is 1.0 but the initial .8 is too soft for a bike with Connie's heft.
http://progressivesuspension.com

RaceTech has a 2 part system of fork springs and valves. More on the valves later. Their springs have worked well for me. They sag less, and dramatically reduce brake dive. RaceTech springs are straight wound and come in 5 different spring rates though considering the Concours' weight the .95 and 1.0 ratios are the only real choices.
http://www.race-tech.com

Sonic's straight wound 1.0, 1.1, and 1.2 kg/mm springs are similar to the RaceTechs just stiffer. They provide a firmer, but not necessarily harsher, ride than the available progressively wound springs. Being straight wound their spring rates are constant though the travel of the fork. The fork rarely bottoms during braking so the tire follows the contour of the pavement without skipping and bouncing. The bike also quits wallowing in the turns. The improvement is more noticeable during spirited riding but commuting and touring is likewise improved. Anything that helps the bike respond to your steering and braking inputs better is worthwhile.
http://www.murpskits.com

Cartridge Emulators are the progressive companions to RaceTech's staight wound springs. They smoothen the ride over sharp, high speed, bumps and holes. High speed meaning quick fork travel and not the bike's road speed. I can see how they wouldn't be noticed on a smooth racetrack but on frost heaved, potholed, real world roads they are. They fit between the damper rod and the spring and override the damper rod's compression holes (larger additional holes are drilled to allow this). The key is a springloaded disc on the emulator that regulates the oil flow. The valve opens, according to the fork oil pressure. At low fork velocity the oil flows through the new damper rod holes reaches the emulator. There is a bleed hole that relieves the oil pressure. At faster fork velocities the pressure overwhelms the ability of the bleed hole to flow oil. The oil pressure builds and opens the emulator valve. A simple device but very effective. It's probably worth pointing out that emulators indirectly affect rebound damping. The new damper rod holes allow the suspension to recover faster. No mechanical rebound adjustment though. That is determined by selecting different weights of fork oil. RaceTech provides the valve setting according to the data you supply and they should be used only with straight wound springs like the RaceTech and the Sonic Springs. Using them with progressively wound springs makes for a softer, mushier fork.

Conclusion (and a little extra)

Determine how you use your bike and set up the suspension accordingly. Touring and daily commuting might be better served with Progressive Suspensions springs. Experiment with the oil viscosity. A 10w will probably suit the "easy rider" touring the country. A 15w might better handle the daily stop and go commute.

Use stiffer straight wound springs for spirited backroad sprints. Add the cartridge emulators (with straight rate springs only), a fork brace (just trust me), and good 15 weight fork oil and you've got a front suspension as modern as a Connie will ever have short of a transplant.

Add sticky tires and the Concours will handle more like the Ninja she came from. There are brake upgrades too, but that's another long story. Now, this is not an anti-Progressive Suspensions rant. They may work better for a tour rider who doesn't push it in the turns but just not for me. Feel free to disagree. I've done some of this stuff to my bike and the results are amazing.

One finishing touch. The late model forks ('94-'06) lost the oil drain plugs. To restore them use a center punch and mark the slider for drilling along the rear casting seam, at 2-7/8 inches from the bottom of the fork. This will put the hole as close to the bottom of the bottom chamber as you can. Use a #30 drill bit and a 8/32 inch tap for nice deep threads. Finish with a bottoming tap. After the hole is drilled and tapped file the surface flat for good sealing. Plug with an 8/32 x 1/4 inch pan head screw with Permatex number 2 (non-harding) on the threads.

Here are more Fork related links:
http://www.pbase.com/image/20443928 (T-Cro's Preload Knobs)
http://www.themcdonalds.net/richard/bike/concours/brace/conbrace.shtml (Fork Brace Installation)
http://tinyurl.com/s04a (Neoprene Fork Gaitors)
http://belray.com/consumer/products_dealerpiece.fsp?vehicle=superbike (Fork Oil)

Read the other fork related articles in the FAQ for more ideas and suggestions.