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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: jllphan on April 19, 2016, 02:22:02 PM
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I'm still freshening up this K75 and decided to change out the fork oil. Removing the dust caps, i noticed an "S" stamped on the plug which according to the popular ibmwr link, tells me i have sport shocks. A bit surprising given this isn't an S model, I guess sport shocks were on option for the C bike?
At any rate, I drained (and I wish i would have measured what I drained out to have some type of reference, but failed to do so) and refilled to 280cc per side but it's not right. At speed, it's fine albeit a bit soft. But if i come up to a stop light and let off the front break, the fork "pops up" just a bit as if it has air in it or something. I didn't feel the spring binding, so i figured I'd just add another 10cc per side and see what happened -- same result, no change.
Also of note, i used a 5w oil.
Now for my question -- exactly how far can one go beyond the recommended amount? I'm debating if i should add another 10-15cc per side or just drain out what i have and refill with a heavier weight oil? I'm reluctant to drain a refill with something else for two reasons:
1. I wouldn't think this new "poping up" issue would be alleviated simply by a heavier weight, seems more like an issue w/ lack of oil.
2. The dumb dumb that had the bike before me stripped the drain plug at the bottom of each fork, and while i managed to get it back together and am leak free for the first 100mi, I'd like to limit the number of times i take this apart and put back together.
Thanks in advance for your thoughts fellow brickers.
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I dunno if it helps but I used 10w Bel Ray fork oil and liked how it felt afterwards
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I refreshed my forks recently, too. S-forks as you have, in a 75RT model. Filled them with 280cc of 10w wich gave it a nice firm ride opposed to the mushy feeling it had before that. Did you have your wheel out to do the refresh? Are you sure it is back in correctly as i found that it is easy to bend them inwards on install causing them to stick. Maybe that's causing it to pop? Anyhow, 5w sounds to light in my opinion, maybe take the forks out completely and drain them by holding them up side down to save the damaged threads?
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To add to Dennis's observations, what seems to be happening with you and your bike is that the fork is compressing as you brake then rebounding when you stop—not popping up, exactly. You've got the right amount of fluid for a Sport fork. Don't overfill. Try 7.5 weight next. It's a trial process for everybody to find the most effective oil.
Another cause of the diving/rebounding effect is a worn rear shock. You should determine its condition, too.
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A rider's weight and the amount of gear the bike is packing also figure into the action.
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Thanks mucho for the advice, gents. I should have been a bit more specific w/ the issue. The "popping up" is not normal/correct, or at least, it's not simply the spring rebounding. It's a single pop up action. Not a fluid rebound of the spring. One single, very notchy pop. Not as in the sound, but the motion. It jerks up, not spring up. Also, this is not for the entire motion and it doesn't do it all the time. Best case i can give you is as follows:
Come to a stop on a level road surface, bike dives a bit and then comes to rest (suspension is fully rebounded). Then i turn the front wheel slightly and pop -- a notchy last inch or two of travel upwards. It's like the spring is binding, but it never does it when in motion (I'm hypersensitive to anything i feel through the handlebars). It's almost like there's not enough fluid in, but at this point, I'm positive there's at a minimum 285cc per side, probably closer to 290-295ish.
If you honestly feel a heavier weight oil would alleviate this, I'll give it whirl, i simply thought a heavier weight would create a situation where it made it more difficult to compress the shock and not much more. Thanks again.
Oh and no, i did not take the wheel off to do it -- just followed all the helpful hints on this site. Seals appear good or else I would have just pulled it all apart and rebuilt. Bike only has 15k and seems really well looked after so I don't necessarily suspect anything is worn/broken -- I just like to make things perfect.
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One other thing here. Just went out, got it off the center stand and with the front brake applied, I can't get it to do the pop/binding thing at the top. So it's only after riding for a while and then coming to a stop. Just trying to give you experts as much info as possible in hopes of helping me out!
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this place is so whack... the more i read the more apparent it becomes...
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Considering your characterization of the previous owner in your first post, jllphan, you'd best get to disassembling both tubes to determine if the springs, spacers and washers are all present and in the correct order. You may as well repair the drain bolt threads, too, while you're doing all this. They are essential and will be problematic if leaking. If it were my bike, I would not be riding it until the fork condition and components were sorted and in good shape.
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take a video of that moefoe popping and post it right here...
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Reading so many of your posts, that scares me a bit Johnny. But your contribution to coaching me up is appreciated, your tutorial on replacing the fork oil was well placed and followed.
Disassembly and analysis seems like the next step forward.
Like I said, it seems to take the humps and bumps just fine and never feels erratic, unstable or twitchy; it's simply the odd motion at rest after riding. Was hoping this was something some of you may have experienced and determined it to be attributed to either not enough fork oil or something that develops from using a lower weight (5w) despite the fact that BMW does recommend a number of 5w oils. Cheers.
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It could be the steering head bearing starting to fail. With the wheel off the ground, slowly move the bar from lock to lock to find out if the action is smooth or notchy. Could by wiring or cables binding on rotation. Check that while you're checking the bearing action.
Doesn't matter how few miles are on it if it was owned by someone who was careless when maintaining it or riding it.
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sounds like binding. With the bike on the centerstand.loosen the fork tube clamps on the bottom triple clamp, loosen the 4 bolts on the fork brace and the axle clamps then bounce the bike to compress the forks a couple times. Tighten everything back up and that might un whack your whack.
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i believe it to be swingarm bearings whack or rear shock whack...
j o
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. . . bounce the bike . . .
. . . swingarm bearings whack or rear shock whack...
Solutions involving bouncing, flogging, and throttle-whacking are the most fun though.
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sounds like binding. With the bike on the centerstand.loosen the fork tube clamps on the bottom triple clamp, loosen the 4 bolts on the fork brace and the axle clamps then bounce the bike to compress the forks a couple times. Tighten everything back up and that might un whack your whack.
+1. This seems like really good advice.
I also have the sport forks on mine. I just learned (from this site) about cleaning the forks with ATF. I got huge, disgusting blobs of black goo out of my forks. Maybe there's some goo in the valves that ATF would clean out. My forks probably need a rebuild anyway.
FWIW - I used 5W, and it seems OK, but I think the springs are stronger than stock. Also, keep in mind that the "W"s are not necessarily the same between different brands of oil, e.g. two 5W oils may act differently. So if you're changing the "W", stay with the same brand to make the comparison meaningful.
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Another vote here for stiction. Combined with old springs it isn't surprising that there is not enough spring force to extend the last little bit of fork travel after a brake dive(btw, brake drive is pretty common on bricks as far as I can tell).
Short of new springs and fork seals, all you can do is clean the internals welll and retighten all the front suspension bolts and screws in the correct sequence to minimize misalignment and stiction.
Can;t tell you what weight fork oil to use. I use 10 in my 75 bricks and 7.5 in my 100. Doesn;t seem to matter. What is important is to have the right amount of oil. Too much and you can blow the seals, too little and you can get topping out.
Back in my misspent younger days, our Hondas that we used for enduros ran just fine with ATF front and back. Any reason not to use ATF in the bricks? It was cheap, and kept it;s viscosity and damping characteristics at high temperatures over rough terrain.
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I hope I'm not hijacking this thread but how come debris accumulates in the forks? What's wearing off and getting suspended in the oil?
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I'm guessing moisture mixing with the oil. I had my forks apart a couple years ago and one side was clean as a whistle, the other was full of black goo.
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The seals are made of black rubber. When they wear, little bits get suspended in the oil. Along with water, a little dirt can get past the seals - especially if they are a bit worn. If forks are pitted, big bits of broken bug bodies can hide inside the pits on the downstroke - to be washed off into the oil before the upstroke. Mix it all together - black goo.
Moral of the story - change yer forkin oil.
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Love this site, it's amazing the response given so few members -- thanks all for the tips/advice.
I have the bike stripped down in the shop (changing out the cooling fan motor and replacing a front brake line) and before I decide to pull the forks, i had one other question ---- that I'm a tad sheepish to ask, i just can't find any info anywhere, they aren't even pictured on the several parts fiche i've looked at....
Are the spring retainers in the pic below to be tightened down? If not is there a reference mark for positioning? I did not spin them in or out when changing the fork oil, but find it odd that they are moveable, not tight and not loose. Wondering if the previous owner had them set right -- obviously not if they are to be tightened.
(http://www.motobrick.com/index.php?action=dlattach;topic=7145.0;attach=11633;image)
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The spring retainer is secured by a clip, you need to push down the spring retainer to get acces to this clip.
It's normal that the spring retainer can be turned.
I don't see any "S" on your retainer, which means you have a standard fork and use 330cc in each leg.
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Got it. Not my fork, grabbed the image off the site. My retainer looks like the one in the photo but the top of the fork is indeed stamped with an 'S'. So to be clear, this retainer has no tightness or position setting as long as it's secured by the clip?
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Nope... Push down, take out clip, let back up carefully... Why carefully? There's a spring pushing up down there....
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Why carefully? There's a spring pushing up down there....
That's why I clamp my stanchions in soft vise jaws. I need to concentrate on controlling that thing.
. . . they aren't even pictured on the several parts fiche i've looked at
That's because you were looking at drawings of the tubes. Look at drawings of the stanchions instead. They're in MaxBMW fiche drawings for certain.