F650GS Aftermarket Forks FAQ

edited by Kristian #562
Updated by Scott, ID, #1244
Please read the Disclaimer before attempting any work in this FAQ.
Last updated 21 July 2006, Winter #1935

For FAQs related to forks, shocks, maintenance and aftermarket options:


This FAQ is in addition to the main Aftermarket Forks FAQ. There are many more options for the fuel injected F650's, and where possible these have been included in the main FAQ.

RaceTech (RT) Emulator Install For F650GS/GSA

by Iceman #975


Since most GS's are relatively new bikes with lower mileage, I'm going to skip the fork seal replacement for this upgrade, to make the discussion simple. If you want to replace the fork seals, the procedure for changing them is the same as for a pre-GS F650. The Fork Oil Change FAQ (for Classic F) and the Fork Seal FAQ GS Dakar may also be of some use.

This FAQ is limited to the GS/GSA at present as on the Dakar, the fork tube caps are threaded. I'm also not positive the damping tubes have the same holes in them as the GS's. See below for more information on the fork tube component differences between the Dakar and the GS.

The Parts you receive from RaceTech

2# Emulators Part #FEGV S4101

I also ordered 2# Springs Part #FRSP S344380, which are rated at 0.8, however these have not yet been installed as I want to test the effect of each component separately.

Tools you'll need Working Supplies you'll need
  • 6mm Allen Socket
  • 8mm Allen Socket
  • #45 Torx Socket
  • Mechanics Pick, 90 degree
  • 3/8" torque wrench
  • A Drill and 29/64 or 12mm Drill Bit.
  • Measuring Cup for Fork Oil (600ml min.)
  • Tape measure
  • Small Fine Round File or Rotary Tool
  • 10mm Socket
  • Large Tubing Cutter (or Hacksaw, but see below)
  • 3/8" box wrench
  • A Clean Work Environment
  • An Old Towel (makes a good work mat for oil drippings)
  • 1200cc of 10wt fork oil (I used Bel-Ray)
  • RTV Black Silicone Sealer
  • Oil Drain Can
  • Blue Loctite
  • Contact Cleaner

RT-Removing the Forks from the Bike

  1. Start by lifting the bike off the ground to remove the forks. Remove the two bolts holding the front brake caliper to the fork leg, using the 8mm Allen. The caliper can be kept out of your way by tying it back with some wire. If your bike has ABS, you'll need to remove the speed sensor. Remove it with the 10mm socket, and pay attention to a small spacer/washer that may be between the sensor and the fork leg.

  2. On the bottom of the fork, there's an Allen bolt, which holds the damper tube to the bottom of the fork leg. Loosen it enough just to break it loose, not enough to let fork oil drip out. This bolt can be tight, so loosening it when the fork is out, or with a worn Allen socket can be difficult. Loosen the axle clamp with the #45 Torx Bit. Unscrew the axle with the 8mm Allen, and slide it out to release the wheel. Watch for spacers, there's one on each side, that can drop down when removing the wheel.

  3. Now's a good time to drain the forks of their fluid, OEM fluid is 7.5 weight. With a 10mm socket, loosen the drain plug. Have a drain can available, and while holding it close to the drain, remove the drain plug and allow the can to catch the fluid. If you set the can on the ground first, you're going to miss the can, Murphy's Law applies. After they are completely drained, re-install the drain plug and tighten to 4.4 ft lbs or 6 Nm.

  4. Remove Fork brace with 6mm Allen.

  5. Next, loosen the three fork clamping bolts on the upper (1# for each fork, marked yellow) and lower (2# for each fork) triple clamps with the 6mm Allen. The Allen socket you use should be fresh, a worn one can strip the Allen hole when loosening or tightening. When loosening the third bolt, keep a grip on the fork tube, as it will slide down and drop on it's own, once loosened.

RT-Disassembling the Forks

  1. If you decide to hold the fork tube in a vise, you'll need to use wooden or plastic protectors, to protect the finish of the fork tube. Any nick in the fork tube is bad, it can ruin the fork seal. Clamp it toward the top of the tube, and just snug enough to hold it still. Remove the black plastic dust cover, it pops out with your fingernail. With one thumb, push down on the fork cap against the fork spring pressure, to expose the inside retaining Circlip. While pushing down slightly, use the mechanics pick to get behind and pop out the Circlip. Be careful to have a hand on top, so it doesn't fly into orbit. Remember, Murphy's Law takes no day off.

  2. This will release the fork cap up above the fork tube with the pre-load spacer under it. Remove the fork cap, the pre-load spacer (looks like a piece of exhaust tubing), the washer and then the spring. Remove the fork from the vise and compress it to bottom it. While it's laying flat on a work surface, remove the Allen bolt from the bottom of the fork leg, and notice a copper washer that should be on the base of this Allen bolt. While keeping the fork compressed, slowly tilt the fork upside down till the damping tube slides out, and set the fork assembly aside.

  3. Carefully remove the damper tube piston ring, and set it aside. Trial fit the emulator to the damper tube top cup. If the fit seems loose, there are snap rings in the kit that fit around the base of the emulator. This expands it's diameter, for a closer fit, and I used them on mine.

RT-Modifying the Fork Damper Rod

  1. You'll notice the two 12mm holes at the bottom of the damper tube. Measure from the bottom of the tube 2 5/8" or 66mm, and on the back side of the tube (opposite side where the two factory holes meet), drill a 12mm or 29/64" hole, starting with smaller sizes first. Chamfer the inside and outside of the new hole to remove the rough edges from the drilling. Clean the tube from shavings, and reinstall the piston ring.

    Note: For more detailed information on calculating the size of hole to drill. The flow through an orifice is commonly expressed as:

    Flow in GPM = 19.636*C*(D)2*(h)1/2
    C is the discharge coefficient which is equal to 0.61 for a sharp edged orifice,
    D is the diameter of the orifice in inches, and h s the differential head across the orifice in feet.

    All that really tells you is that the flow is proportional to the square of the diameter of the orifice and the square root of the apparent head. In this case the larger orifice will have twice the area of the two smaller orifices combined. Flow is proportional to the square root of the apparent head and the coriolis force would act to reduce the apparent head at the orifice." Scott C, posted by Flash#412.

    If you are curious enough to want to know the 'spring rate' of your springs you can learn how to calculate it by reading the Spring Rate FAQ.

  2. Clean the hole at the bottom of the fork leg, and the Allen bolt that holds the damper tube in place. I use a dab of RTV black silicone at the base of the Allen bolt. When dry, it helps seal the bottom of the leg. You can re-use the copper washer on the Allen bolt, or replace it if the forks have previously been apart. While holding the compressed fork assembly upright, start to re-thread the Allen bolt up into the damper tube, and make snug.

RT-Re-installing the Emulators & Springs

  1. The emulators need to be adjusted before their installation. Loosen the 3/8" locknut on the bottom side, and unscrew the Allen adjuster until the adjuster spring is neutral. Turn the adjuster to two turns in, for an initial setting.

  2. Re-mount the fork into the vise, and leave the fork compressed. Pour 550cc of fork oil into the fork tube, and pump the fork assembly to get the air pockets out of the fork. When you can't see bubbles looking down into the tube, you're done pumping. Drop the emulators into the fork tube, and let them settle down through the oil. With the fork compressed, measure from the top of the fork tube to the top of the oil level. RT calls for a fluid height of 120mm from the top of the fork tube, so adjust this fluid height accordingly. You may wish to consider changing the weight of your fork Oil to 10 wt.

  3. Extend the fork leg, and add your choice of spring, RT recommends their springs for a higher tension. The washer goes back in, then the preload tube, then set the fork cap on top.

  4. If you want a factory pre-load, you need to cut off 12mm (the height of the emulator) from the pre-load tubes. Hacksaws rarely cut straight lines, so if you have access to a tubing cutter, use it to make your cut. Many owners may not have a large enough tubing cutter available. If you must use a hacksaw, fit a stainless hose-clamp around the pre-load tube and tighten it down at the line to be cut. The hose-clamp will act as a guide, then use a flat file to clean off the sharp edges.

  5. RT calls for a pre-load of 20mm, which means you're going to compress the spring 20mm as a head start. Measure the distance from the top of the fork cap, to the top of the fork tube. Since the top of the fork cap sits recessed into the fork tube by 10mm, cut the pre-load tube so the top of the fork cap top will sit 10mm higher than the top of the fork tube. Now, when assembled, the pre-load is 20mm. Push the fork cap down against the spring, and re-install the retaining Circlip.

Re-installing the Forks on the Bike

  1. Slide the fork leg back into the triple clamps, and tighten them to 18 ft lbs, or 25 Nm. Refit the front wheel, making sure your spacers are in place, and tighten the axle to 58 ft lbs. or 80 Nm. Do not tighten the axle clamp yet. Install the caliper, and tighten the mounting bolts to 37 ft lbs or 50 Nm (I use Blue Loctite here, and on all critical fasteners). The ABS Sensor has a tightening Torque of 4.4 ft lbs or 6 Nm.

  2. Install fork brace and tighten bolts to ____ ft lbs or ____ Nm.

  3. Take the bike off the stand, and compress the forks a few times to set the axle to center.

  4. Now tighten the axle clamp to 17 ft lbs. or 23Nm. Tighten the damper tube bolt on the bottom of the fork leg to 14 ft lbs or 20 Nm. Pump the front brake lever until the pads meet the rotor, sometimes the first pump feels like there's no brake.

  5. Now you're ready for a test ride, then a re-check of your fasteners.

RT-Tuning it in

1) To adjust your forks, note the following:

RT-NOTES: Alternative requirements due to F650 DAKAR to GS Differences

  1. If you have a Dakar, there are a couple of differences. First, the fork tube caps are threaded in, like the Classic F, not held in with circlips. Loosen the upper fork clamps first, so you can remove the caps. If you don't loosen the upper clamp, you'll end up stripping the hex off the cap, since the clamp squeezes on the threads. When tightening the cap during re-assembly, tighten the lower triple clamps first, then tighten the fork caps, and finally tighten the upper fork clamp last.

  2. Another difference may, or may not, be a difference in the Dakar damper tube holes. Since I've never seen a Dakar fork apart, I'm assuming the damper tubes are probably longer, but have the same holes. If they do, drill the same 12mm hole in the same place in the tube. If they're smaller, you'll need to a drill different sized hole(s). Basically, if there's a difference, post the difference, and I'll reply with an answer.

  3. Since the Dakar has a longer travel in the fork, It may require more fluid for an initial fill, and perhaps a larger distance from the top of the fork tube, for the fluid height. Since the stroke is longer, it may require a larger air volume in the top of the fork. I'd guess 140mm from the top, or the factory setting. The higher the fork oil level, the more resistance to bottoming (inner fork, air compression).

  4. I had to replace my fork tubes on my 7/00 2001 F650GSA. The part # from BMW is 31 42 2 345 376. This is the same part # used for the pre GS F650's. The fork tubes are identical in dimensions, except the fork caps are different. The pre GS uses fine threaded (1.0mm) screw in caps, the GS's use non threaded plugs held in with internal snap rings, and the GS Dakar's use coarse (1.5mm) threaded caps. So if you order tubes for a GS, you're going to get threaded tubes for a pre GS. BMWNA doesn't have a clue about this, it has taken me two weeks to sort this out. Until they supply the correct (Dakar) tubes, the current replacement requires pre GS fine threaded caps, or the Honda F2 adjustable caps.

Do the RT Emulators fit the Dual Spark models?

After searching through the Race-Tech site I was surprised to find no listing for the for the 04-06 GS. All the FAQ's also seem to only address the older models. Does anyone know if the FEGV S4101 emulator's will fit into the 06 GS (non Dakar) forks or are springs and oil my only choice? ... It appears after looking at the parts fiche on the A&S BMW website that the 99-05 GS and the 05-up Dakar models use the same damper rods. The 04-down Dakar model has a different part number for the damper rods. cadhopper #2041

Touratech Spring Install

I just received the TT springs for my GS. Next step is to get fork oil and decide how much to use. I've read the spent-shun knowledge and experiences in the FAQ which were very helpful. The reason for the upgrade is excessive dive and sag. First I should say that I am a big boy - scaring the h*ll out of 300 lbs., just over 6 feet tall with a 32 inch inseam - seat height is not a problem for me. My fork sag is about 2.1" (~53mm). The bike has about 10k mi. and is mostly road ridden. The FAQ says it should be set to about 1" (~25mm). Touratech recommends 575ml of SAE 15. The FAQ says 600ml of SAE 7.5 or 10. I think the 25ml difference is because the TT springs are larger. I guess I'll start with 575ml of SAE15 and see how the bike handles and see how much sag I have. If the sag is more than an inch, should I add a little more fluid or put a longer preload spacer in? The rear seems to work fine with the re-bound and the pre-load both set at max. Any wisdom is appreciated. Kevin #1092 Black 02 GSA