Upside Down Forks on a C10

The C10 is a great motorcycle, but most owners will agree that two areas that need improvement on the stock bike are the front suspension and the front brakes.  The front forks are undersprung from the factory, resulting in a wallowy feeling over bumps and excessive dive under braking.  The damper rod forks tend to give a harsh ride over small bumps.  The flexibility of the front forks lends a vague quality to the steering, especially for those who like the sport side of sport touring.  The front brakes, while adequate, don’t inspire the confidence required for a bike of the size and speed capability of the C10.

Fortunately, all of the issues mentioned can be addressed in one way or another.  Installing stiffer springs will eliminate the wallow and excessive dive.  Cartridge emulators will reduce damping harshness, and a fork brace will help with the flex of the forks.  Brake caliper adapters will allow the installation of 4 or 6 piston calipers and/ or larger brake rotors, greatly improving brake performance.   But even with these improvements available, some owners won’t be content without a complete upgrade of the front end to one from a more modern motorcycle.  This option results in even greater improvements in the areas mentioned, and offers the ability to mount a 17” front wheel.  One such option involves swapping the Connie front end with one from a mid-90’s Kawasaki ZX-9.  The ZX-9 front end includes more modern upside down cartridge forks, 320 mm brake rotors with 4 piston calipers, and a 17” wheel with an axle and bearings that are substantially larger than the C10’s.  This article will discuss the steps necessary to adapt the ZX-9 front end to the C10.

First, some general notes about the project.  The conversion requires modifying the triple clamps from the ZX-9 to fit the C10, and requires a machinist and some light fabrication work.  When done properly, the result is as safe and reliable as the original front end, but if done improperly, the result could be unsafe, so it is critical that any machine work and assembly be performed by a qualified person.  The conversion results in the front end being lowered approximately 1” from stock.  The descriptions in the article do not include any details on operations that are described in the service manuals for either the ZX or ZG, so it is important to have these on hand.

As a minimum, the items required to do the conversion are as follows:

-  Forks from a ZX-9, model year 1994 thru 1997 model (B1 thru B4).
-  Upper and lower triple clamps from a 1991 thru 1996 ZX-7, or the combination of a lower triple clamp from a 1994 thru 1997 ZX-9 and the upper triple clamp from a 1991 thru 2003 ZX-7.
-  Wheel, axle, speedo drive, and 320 mm rotors from a donor compatible with the ZX-9.*
-  Calipers compatible with the ZX-9.  For the 1994 thru 1996 zx-9 these would be 4 piston Tokico calipers, and for the 1997 they would be 6 piston Tokicos*
-  Some way of mounting handlebars to the ZX-7 top triple clamp, since the ZX-9 used clip-on handlebars that will not clear the Concours tank.  Dirt bike handlebar risers for 7/8” tubular bars are a good choice, and allow the use of any tubular handlebar you choose.
-  A fender from a ZX-9 or ZX-7, or an adapted fender.  The ZX-9 fender is a direct fit, and the ZX-7 fender requires some adaptation to fit.  It is important to use a fender that covers the front of the lower chromed portion of the forks, since these will be exposed to even more debris than conventional forks.
-  A new lower steering bearing and seal for a C10.
-  A donor lower triple clamp from a Concours.  This can be the part from your current front end or one that you obtain for the mod.  Having an extra one just for the mod allows you to minimize the down time for the conversion.

* See the table at the end of the article that gives compatible donors

Figure 1 is a picture of the items required.

Figure 1 – Required Parts

Making the ZX forks fit the Concours requires modifying the ZX lower triple clamp to work on the Connie.  This involves removing the stem from the Concours lower triple clamp and installing it into the ZX triple clamp.  The easiest way to remove the C10 stem is to heat the clamp portion while pressing down on the top of the stem.  The stem can also be removed by cutting and grinding the clamp portion away from the stem.  The important thing is to remove the stem without doing any damage, including the circlip at the base of the stem, as it will be reused in the modification.  Figure 2 shows the ZX triple clamp on the left and the C10 stem on the right. 

Figure 2 – ZX Lower Clamp and C10 Stem

Since the Concours stem is smaller than the ZX stem, the most straightforward way of installing the C10 stem is to leave the ZX stem in the ZX lower triple clamp and use it as a bushing for the C10 stem.  The inside of the ZX stem must be machined to accept the Concours stem,  the ZX stem must be cut down, and the top of the ZX triple clamp and stem must be machined to accept the Concours stem bearing and seal.  Figure 3 shows the necessary finished dimensions for the ZX clamp prior to installing the C10 stem, along with a photo of the modified clamp.  Again, it is extremely important that a skilled machinist perform this work to the dimensions shown.

Figure 3 – Lower Clamp Machining

Once the triple clamp is ready, the C10 stem is pressed into the ZX triple clamp, making sure that the retaining ring on the C10 stem bottoms out in the counterbore in the ZX clamp.  Figure 4 shows the stem installed. 

Figure 4 – Stem Installed

The steering stops on the lower triple clamp must be modified slightly to clear the Concours steering stop (more on the steering stop later).  The corners of the stop tabs need to be trimmed approximately 1/8”, otherwise the clamp will only be able to turn a few degrees either way.  The lower bearing and seal can then be installed.  Figure 5 shows the modifications to the stop tabs and the bearing and seal installed.

Figure 5 – Finished Lower Clamp

The ZX-7 upper triple clamp must also be modified to work with the C10.  To place the upper clamp at the same location as the stock C10 clamp, the underside of the triple clamp must be machined at the stem boss to reduce its thickness to .78”.  A bushing with an O.D. of 25 mm and an I.D. of 18mm must be made and installed in the ZX top triple clamp.  The bushing should be made as a press fit and/ or epoxied in place.  The inside diameter of the bushing needs to be a sliding fit over the C10 stem.  The bushing must be flush with the upper and lower surface of the triple clamp.  Figure 6 shows the ZX7 triple clamp before and after modification.  Once again, it is important that a skilled machinist perform this work.

Figure 6 – Top Clamp Machine Work

The ZX-7 top triple clamp must then be modified to accept handle bar mounts and to allow the Concours ignition switch to be installed.  There are any number of options for handlebar mounts, including a complete upper triple clamp for the ZX-7 from LSL that has integral handlebar mounts.  Dirt bike handlebar risers work just as well for a fraction of the cost, and require drilling holes in the triple clamp for mounting the risers.  To mount the ignition switch, the underside of the switch loop needs to be trimmed, and the inside of the hoop needs to be enlarged.  This can be done using a mill, a die grinder, and/ or a combination of belt and drum sanders.  Figure 7 shows the various stages of the modification and the finished product.

Figure 7 – Upper Clamp Modifications

To allow the ZX lower triple clamp to clear the lower part of the Concours fairing frame, there are two options.  One option is to trim some material off the ZX lower triple clamp in the area of the pinch bolt bosses.  This is basically a chamfer on the outer upper corners of the bosses.  The exact amount will be clear with the first attempt to mount the triple clamp in the frame.  The other way is to expand the fairing frame slightly in the area of the triple clamps using some sort of spreader setup like a port-a-power or a short screw jack.  Figure 8 shows a makeshift screw jack being used to expand the frame.  A block of wood was jammed in the opposite side to prevent racking the frame.  Very little spreading of the frame is necessary.

Figure 8 – Spreading The Fairing Frame

With the ZX triple clamps modified, the only modifications remaining are to the Concours.  The steering stop tabs on the ZX triple clamp are not compatible with the Concours steering stop, so a new steering stop must be made for the steering head of the Concours.  Figure 9 shows what the new stop looks like. 

Figure 9 – Steering Stop

The fabrication of this piece requires some trial and error to get the maximum turn angle without the triple clamps contacting the Concours fairing frame.  It is best to make the stop larger than needed and grind away material as needed.  Figure 10 shows the stop bolted in place, but it can also be welded.  The key-activated steering lock will not engage the tab on the C10 frame, so to make the lock functional, a tab must be bolted or welded to the existing lock tab or the top leg of the C10 fairing frame.

Figure 10 – Steering Stop In Place

The remainder of the work is a matter of installing the lower stem race in the C10 frame, installing the triple clamps in the frame, installing the fork tubes, installing the front wheel and brakes, and mounting the handlebars.  Any procedure you are not familiar with can be found in the ZX/ ZG service manuals.  Assuming you were successful in getting all the right parts, everything should bolt together with no issues.

Just a note about fork setup.  Depending on your weight, the stock ZX-9 springs may be too light, since the Concours is a heavier bike than the ZX.  Heavier springs are available for the ZX from various spring manufacturers to bring the sag adjustments within the recommended range, if required.  Also, the used ZX-9 parts are more than 15 years old, so it is a good idea to replace the seals, as a minimum.  The ZX-9 forks are adjustable for compression and rebound damping, and the adjustments are noticeable, so with some trial and error, it is possible to find settings that work for your riding style and road surfaces.  The ZX service manual is crucial to understanding the disassembly and assembly of the forks, and how damping adjustments are made.

The modification results in reduced trail and rake angle, which in turn reduces steering effort in turns and transitions.  Even so, the front end has a much more solid feel than stock, due to the greatly increased rigidity of the fork tubes and front axle.

Compatible donors:

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