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When to replace front forks?


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I have a 1987 xl600r and I think the front forks are bad. Everytime the front wheel comes off the ground it feels as if the front end kind of pops out of place. I think they need to be rebuilt. Does anyone know what I would need to replace and how hard of a job is it? Thanks for your help!

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Pops? Sounds like the top clamp stem nut could be loose,or the oil level is low for some reason.(leaked or didn't get the right amount last seal replacement)

A seal replacement and the two bushings with the correct amount of oil should get it back to new again.

But a USD setup would be nice..

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There are bushings that actually do the sliding in the forks. They do wear out and that gives extra play in the system. They are also discontinued which sucks big time. The steering head bearings are another source of play. They often rust really bad before you notice. I've seen some so bad that the steering stem could clunk around.

The fact that the bushings are discontinued makes the USD conversion more appealing.

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Most likely it is something you done when you reassembled it.

Did you just slide the fork out of the tree to put the boots on or did you take the forks apart for some reason?

If you took the forks apart and didn't put the top-out spring in the correct place or left it out you are heading towards disaster.

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Or even easier than USDs, just put 85-90 XR600 forks on. No wheel or bearing changes at all. They will bolt on with no mods unless you have the occasional bike that had loose ball bearings in the stem instead on tapered rollers. You would need an XR front brake line and have to fab a speedo mount. At least they have something to mount your speedo up to. USDs are cool but you have to do some or all of the following, machine/buy custom stems and spacers, mill top triple clamp, replace with special bearings, make axle/spacers, change wheel bearings, replace entire wheel, replace stock speedo with electronic. The newer the USDs the more expensive or complicated the swap. I've swapped 6 sets and ended up going back to 43mm cartridge legs on all of them. All were 89-94 USDs. My avatar pic still shows the old USD setup on one of them:worthy:. Some have done it cheap and easy (me for one) and others have had to do all the mods at great trouble and expense. Used USDs WILL need the fork seals replaced before you install them too. They always wear the left leg seal out faster than the right because of brake torque. I have 02 CR250 forks sitting here that I may install some day but I'm not in any hurry to do it. These late forks do work bitchin though. Big problem for an XL600 is the rear shock is going to be too short for any conversion and will need to be replaced with a longer XR shock. You would have to find space to mount the remote reservoir which the XL doesn't have. The XR forks can be dropped in the clamps almost 2" which would keep them close to matching the rear end height. On the "popping" note, check the oil in them. If they have gone dry they can make some real weird noises and actually change the balance of the bike in a wheelie when they clunk out so fast. I believe XL forks don't have any topping springs in them just oil passages to stop them hydraulically. If you need a stock set of triples and stem I have a set I would gladly part with for a sixpack or so. I'll probably get flamed for blaspheming the holy USDs:worthy: but I think they're not necessary for the greater majoriy of riders IMHO.

Edited by valvesrule
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Thanks for all your help. I think I will try to add oil to the forks first. Where do I put the oil in at and what type of oil should I put in?

The caps at the top of each leg. The manual says use ATF but it may be too light on XL forks. I personally would use 10-15 weight fork oil. ATF and fork oils both have seal conditioners added that may help seal leakage that forks that old are almost certain to have. Don't worry about how many ounces as much as oil height. XL forks are pretty softly sprung and a bit heavier oil should help overcome that. I don't recall exactly but there's either a 10mm bolt or phillips head screw at the bottom of each leg. Pull them first and drain whatever comes out. Sometimes the threads come out with the screw so be forwarned you may have to rethread them to the next size. I like to hang the frontend from the rafters in the garage so you can let the bike down after pulling the caps and springs. Loosen the top triple clamp pinch bolts before you try to pull the caps out. The springs will try to pop out and kill you so be careful. When both caps are off pull the springs, washers and spacers out noting the order. Refill the legs to no higher than 6" from the top with them fully bottomed out. Replace all the springs etc. and see if the clunk is gone. Another option if you're streeting your XL, I put a complete 85 Yamaha Maxim frontend on my XL frame. It only needs the lower stem bearing machined or replaced with the Allballs bearing to work. 19" mag wheel and double disc brake setup for really cheap. The rear Yammie wheel can be made to work too.Good Luck with it.

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I believe XL forks don't have any topping springs in them just oil passages to stop them hydraulically.

Part 13.

3214_FRONT%20FORK.gif

All conventional, dampening rod forks I've been into, including bicycles have a top out spring. Hydraulic oil does not compress and if that was used to stop the forks from topping out, it would be just like steel on steel. The spring allows them to top out more softy.Trying to tune dampening rod forks is full of compromises because they use the same fixed orfice to control compression and rebound dampening. Thick oil used to resist bottoming also means the rebound will be too slow to allow the wheel to stay in contact will a rough surface.

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Part 13 . You win the rebound spring debate. Jus' an old man takin' 'bout walkin' uphill to school in the snow both ways here. Every Honda fork fiche I looked at shows you to be correct with a topping spring stock. Guess I've just worked on too many old 70s or modified rod forks. Virtually all Terry Kit and Progressive Suspension damper rods eliminated the topping spring if one was used. I inadvertently assumed that TSs had been dropped on more recent forks. This was standard practice for White Bros and other old world suspension gurus to do their magic. Toss the T.S.s, gain travel, raise preload & final spring rate, and improve damping control all with a few holes repositioned, removed, resized or added. We used to do this in the early 70s when forks were less complicated. I have a set of PSs for XR350 in my hand as I type. I also know that hardly anyone mods rod forks anymore so you may never of seen this. There's a roughly 1/4" hole about 2" from the piston that expedites oil transfer from the top of the leg restriction to below. In the area between the actual #19 and #13. As the fork extends past that hole 3/4" another appx. .050" hole takes over to slow movement further. After passing that hole rebound is controlled by damper rod to restriction clearance as movement is down to very low speeds already. The last 5/16" has a sub piston that nearly stops all motion completely until piston clearance evacuates the remainder of the oil. If you've ever worked on large industrial hydraulic cylinders you have seen the cushion piston to be very common. A 10" 1500psi tractor cylinder needs far more end of stroke control than a spindly 1 1/2" motorcycle fork. All forks are just hydraulic cylinders designed to bleed down at a controlled rate rather than seal completely. Any tendency to hydraulic lock is countered by minimal spring preload and the very short length of the subpiston. When using very light oils as is common nowdays you may feel or hear a light thump but it is in no way damaging or even distracting. If you use very high preloads it could become a problem. Works just like a progressive pressure relief valve. Hydraulic topping holes work just like the bottoming cones (part 15). Some even relieved one side or reduced the diameter of the rod for the first 3-4" to further lighten damping those few first inches of travel. If you ever have a damper rod with holes near the top of the rod it will likely not have a topping spring hiding below. I know the Race Tech Emulators for an 88/90 XR600 fork do something similar but I don't know if the TS is there or removed. The emulators are nothing more than an adjustable blowoff valve added to the hydraulic system of the fork. They only work under high pressure spike loads. I would post a pic of the rods I have here to illustate but I don't have a photo bucket account setup. It's a controlled leak that gradually decreases the orifice until hydraulic lock is nearly total. Since they are not o ringed or v ringed as in a true hydraulic cylinder they work just fine with no clanking or harsh metal to metal contact. I'd venture to say the very set of XL forks being discussed could have the same mod done and get a better fork. Problem is guessing size and position of the holes is a matter of experience or trial and error. Could take a lot of drilling and plugging to find the perfect combo. And heavier oil doesn't mean 50w. With ATF being under 10w and original oil being at least that 15 to 20 is more than enough even with the inherent "looseness" of old forks.

Edited by valvesrule
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