VFR CARTRIDGE FORK SERVICE Part Four of Four ---------------- Version: 1.1, Thu Jan 23 12:51:12 EST 1997 Author: Robyn Landers, rblanders@math.uwaterloo.ca Copyright (c) the author, 1996 Distribution: May be copied as long as this header is retained. Improved versions will appear on my web page. --------------- This is the fourth and final installment in my report on VFR fork service. We left off with the forks cleaned and mostly re-assembled with just the fork cap bolt and related bits plus the fork spring waiting for reinstallation. Before doing that we need to install the fork oil and bleed the air out. 5.3 Fork Oil Filling -------------------- Measure into a clean container the specified quantity of fork oil: Oil Quantity Oil Level USA Model 1990-91: 383cc per leg 175mm 1992-93: 386cc 178mm 1994-97: 412cc 177mm Canada Model 1990-91: 394cc 187mm other years as per USA chart European Models -- sorry I have no info Compress the fork fully. Pour the oil in slowly. Avoid "glug-glug" sounds -- this is air getting trapped. To bleed the air out, follow this two-part procedure. 1. Extend the fork fully. Hold it vertically. Seal the end of the fork tube with your hand. Compress the fork slowly. You'll feel air pressure build under your hand. When the fork is compressed, release your hand slowly to let the air pressure equalize. (Don't do it suddenly or oil may squirt out.) Look inside and you'll see air bubbles coming up in the oil. Repeat this procedure about 10 times or so, until there are no more oil bubbles appearing. 2. Now bleed the cartridge. Compress the fork fully and reach in to grab the ends of the wire you previously wrapped around the locknut on the end of the damper rod. (The dedicated fork service person will instead have welded a nut onto a scrap steel rod so it can be threaded onto the damper rod to provide a convenient handle.) Slowly extend and compress the damper rod fully. More air bubbles will appear. Repeat until air bubbles are gone and resistance is felt throughout the full stroke of the damper rod. Take your time doing this and be patient to ensure that all the air is out. Any air trapped in the cartridge will defeat the damping performance of the cartridge. Now that the air is out, measure the oil level, i.e. the distance between the top of the fork tube and the surface of the oil with the spring not installed and the fork fully compressed. Ideally you want the number specified in the chart shown above. For me, my 386cc resulted in a level of about 190mm instead of 178mm. That's quite a discrepancy in volume. Let's see, 42mm fork with say 2mm thick walls, pi R squared, 12mm height, that works out to ... about 10.9 cc less. I know I didn't spill that much. :-) Plus the conventionally accepted rule of thumb for fork oil level is 6" (150 mm). What to do? My local trusty service advisor suspects that the quantity (volume) specification is probably a drain and refill amount, not a complete disassembly amount. He suggested starting with that amount, and then at least raising it to match the specified level. Furthermore, the spec of 178 mm seems rather large given the 150 mm rule of thumb. Based on the experience of other VFR riders who have come in complaining about soft suspension, he strongly recommended that I raise the oil level by about 10mm above spec, so that would be 168 mm. So that's what I did. The slightly larger volume of oil reduces the size of the air pocket, which won't make much difference in the early part of the fork's travel, but will firm up the action as the fork gets closer to bottoming. Why? Because air pressure increases progressively. Repeat oil filling and air bleeding for the other fork leg. Regardless of what volume of oil you use, it is very important that you have the same level of oil in each fork! Do not get carried away with raising the oil level to firm up fork action. Never fill past total fork travel, or you'll blow the seals with hydraulic lock. 5.4 Final Fork Reassembly ------------------------- Slide the clean fork spring over the damper rod and into the fork leg. The tightly coiled end goes down. Make sure the locknut is fully seated on the damper rod. Install the flat washer so it sits on the fork spring, and then the notched washer over it with the dish down. Note that the chamfered end of the locknut sits in the dish of the notched washer. (If the chamfered end of the locknut is facing up, you have it on upside down.) Thread the fork cap bolt onto the damper rod and snug it up against the locknut using two wrenches. Then back the preload adjuster out. Now extend the fork fully and thread the fork cap bolt back into the fork tube. Recall previous warnings about being careful not to cross-thread. You needn't get it fully tightened; just in as far as the O-ring is good enough for now. 6. Put the Forks Back on the Bike --------------------------------- Hey! You're almost done! Reinstall the forks on the bike. Refer to the notes you made prior to disassembly when positioning the fork in the clamp for proper height. The shop manual calls for 1.5" between the top of the upper triple clamp and the edge of the fork tube (not counting fork cap bolt) on 1994+ models. For 1990-93 models it says to position the fork tube groove even with the upper edge of the triple clamp. But there is no groove on my 1993! I measured it at the same 1.5" as it turns out. Tighten the lower triple clamp bolts first, and then the fork cap bolts, and finally the upper triple clamp bolts. There's enough room to get your torque wrench in to the lower triple clamp from the top, but not really enough from below. You needn't worry about alignment of the fork lowers because they can rotate on the fork tubes. Rotate the fork lowers as needed and then slide the axle into place to ensure they are straight. Position the oil line brackets but do not insert the bolts yet as the manual says. This is clearly a mistake -- the bolts have to thread into nuts which are permanently mounted inside the fender! Now put the front fender in place. You'll need to pinch it to prevent it from scraping on the fork legs. Line up the oil line bracket holes with the rear nuts in the fender, and line up the allen head bolts with the front nuts in the fender, then install the oil bracket bolts and the allen bolts. 7. Reinstall Remaining Components --------------------------------- From here on, it's straightforward reinstallation of front wheel, axle, fork pinch bolts, brake calipers, handlebars, switches, wiring restraints. As you slide the forks up into the triple clamp, make sure you have the brake/clutch hoses, switch wiring, and throttle/choke cables oriented properly! Congratulations, you're done! Now your forks should be in good-as-new condition (assuming the seals and bushings were okay to begin with). Enjoy. Refer to various other articles on how to adjust pre-load, measure and adjust sag, tune damping characteristics with fork oil viscosity, and so on.