Front Forks

This page was written by Dawn Light, with the incredible help of the GSTwins Forum community, and is dedicated to them. Thank you all for the time and effort!

DISCLAIMER: Your safety is your responsibility. Check your local laws concerning vehicle modification. I am not a mechanic. I am not an engineer. I don't know anything about anything. Modify your vehicle only if you are sure that you know what you are doing. Have every modification you make, every part you build and every change you make checked thoroughly by a professional. Making modifications without the proper knowledge can, and probably will, cause the motorcycle to break apart while riding!

Please note that if all you want to do is replace stuff INSIDE the fork, we have our Upgrades.FrontSprings page.

Table of contents:

Why would I want to replace the front fork?

You may want to replace that stupid thing for several different reasons:

  • Sport performance: With a fork from a sport motorcycle you may have:
    • Two brake rotors providing more braking power.
    • Adjustability for compression, rebound and preload... so you can screw with a bunch of knobs.
    • A bigger wheel that means better tires.
    • A motorcycle that can do this (Video of a GS500 with Katana 600 front fork and Race Tech springs and emulators racing on a track).
  • Appearance: There are some FINE lookin' front ends out there!
  • A different feel: By changing the trail, offset, height, brakes and wheel you can determine the feel of the ride.

Note that the stock GS500 fork is an old style "damping-rod" suspension type fork. If you swap it out, you can give yourself a newfangled Cartridge Fork suspension. Orrr.. for the "cheap but almost as good" approach, you can install a cartridge emulator inside stock forks, as mentioned on our Upgrades.FrontSprings page.

Cost of a replacement fork

According to forum posting in 2011, "A complete GSX-R front end can be had for $500-$600 " There is also the hassle (and potential cost, if you pay someone else to do it) of removal, installation, and potential machining of parts to get things "Juuuust Right". Note that handlebars, controls, etc. may need adjusting of their mounting position, with potentially new length of cabling for brakes, etc.

Choosing a fork

You can fit almost any fork from any motorcycle on the GS500. From a chromed out custom style fork, to a track performance fork. Every front end will change the GS500 handling and feel.

Don't fit the GS500 with a fork that isn't strong enough for it.

Some forks may be easier fit than others. With any fork you choose, you won't need to do more than:

  • Make a custom steering stem
  • Make some spacers
  • Grind some weldings and use a press machine
  • Make custom brackets for the headlight and turn signals
  • Solve the gauges issue

Ways of replacing the fork

The following are various ways to fit the donor's fork to the chassis neck.

Custom steering stem

This allows us to determine the height at the front to some degree.

The sections in red are to be copied from the donor's stem' and the parts in black from the original.

Since the donor's lower clamp had a stem pressed into it, it may be slightly oversized and lost some interference. Measure and check it, and make the section of the stem that is pressed into it accordingly.

It's recommended to make a step above the section that is pressed in to the clamp, to prevent pressing it too far.

It will be a good idea to make a groove for a circlip below that section for safety.

We can lower the height at the front. We do this by making a length between the head and the outer thread.

I was suggested to use an alloy called EN19 for the stem. Its a medium strength High-tensile steel alloy with good corrosion resistance, as well as being easy to machine its a common grade and freely available from specialist suppliers.

Modifying the donor's stem

The modification is shortening the distance between the bearings to 131mm (Can be a little less), to match the original steering neck. Do this by moving down the shoulders that hold the upper bearing. Then you need to compensate for the shortening by making spacers above and below the nut that holds the upper bearing. dgyver did it.

Fitting the fork this way is possible if the donor's stem isn't shorter than the original stem and if it uses bearings with outer diameters of 55 (lower) and 47 (upper), and widths of 17 (lower) and 15 (upper). The ISO reference numbers for the bearings are Top: 32005 Bottom 32006. These are industry standard Taper-roller bearings and easily available from bearing supply companies Here is a list of possible matches.

Pressing the GS500 stem into the donor's lower triple clamp

If the hole for the stem on the donor's triple clamp is considerably larger than the GS500 stem, we can make a ring to compensate for the difference.

The GS stem's diameter was measured to be 30.15mm where its pressed into the triple clamp.

Make sure you press the stem top-to-bottom. In other words, while holding the lower triple clamp, you should apply pressure to the the part of the stem where the top triple clamp was originally. This will ensure that the circlip (which is used during original assembly of the lower triple clamp to set the length of the stem, this part may be rusty/invisible/ymmv on your bike) the lower clamp and the stem which you're interested in will survive the pressing process, more or less.

Same width forks swap

The GS500 forks are 37mm in diameter. Different forks of the same diameter can replace the originals.

An example of these are the forks from an Honda CBR600F1. This is a fairly easy modification.

Keeping the original wheel

You may want to keep the original wheel because it matches the rear wheel, because you like it, or because you spent all your money on a fork. Anyway, in order to keep the original GS500 wheel when replacing the fork, the following should be considered:

  • Side clearance: We want some safety distance between the tire and the forks. Make some measurements and compare. the original tire width is 110mm. That is the widest part of the wheel.
  • Fender clearance: We want some safety distance between the tire and the fender. This is how to measure it: If the donor's fork has it's wheel on it, we can measure the fender clearance, calculate the wheel's size and compare it with the original. If the donor's fork doesn't have it's wheel on it, we can measure the distance between where the axle should be (put something there), and the fender. That should be a little more than the original wheel height divided by two. If it's not you will have a problem with fender clearance. The diameter of a wheel = rim diameter converted to millimeters + 2(tire width x aspect ratio / 100). The original tire dimensions are 110/70/17. So the diameter of it=17x25.4+2(110x70/100)=585.8mm. Some forks are fenderless. If there is a problem with fender clearance we can remove the fender (Dirty engine, the wheel throws stuff at you and there may be a problem with the law), make a custom fender or find one that somehow fits.

Twin brake rotors

The GS500 wheel is ready for two brake rotors. Part number 10 comes off and is replaced by the left rotor.

Fitting it

Various ways to fit the GS500 front wheel on another fork:

  • Make a custom axle
  • Use the GS500 axle
  • Use the donor's axle

Here are some measurements that may help:

Other issues to address

Don't forget to consider these issues:

  • Gauges
  • Headlight
  • Turn signals

Links

These can be useful: