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Thread: Air in forks?

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    Atomic City Boxer 154048's Avatar
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    Air in forks?

    Greetings,
    Just changed my fork oil (1980 100RT). All went well, but do I need to pump the forks a bit with the top caps out to remove any air? I had to do this on an ancient dirt bike I had years ago is why I ask...

    Also, I put in the listed 235 cc's of 7.5 wt fork oil. I know that guys adjust their ride by adding or removing volume, but I am not sure what I am looking for. Style of riding is laid back...non aggressive. Never have a pillion, and even when touring I am not heavily laden....235 sound good?

    Thank You...
    Steve in Santa Fe
    1980 R100RT
    2005 DR 650

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    Not positive, but I don't think you have to pump the forks, don't even think you can as springs are in forks so you can't work them up and down without taking off center stand. If you pump them with the top filler bolts off, you might get sprayed!

    I don't think you change fork action by changing oil quantity. Rather, you change the weight of the oil - let's say go to 10w oil if your 7.5 isn't stout enough.

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    Registered User lmo1131's Avatar
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    do I need to pump the forks a bit with the top caps out to remove any air?
    No.

    I don't think you change fork action by changing oil quantity. Rather, you change the weight of the oil - let's say go to 10w oil if your 7.5 isn't stout enough.
    +1

    7.5 ought to do the job for you, riding as you describe. Is the front suspension "pumping"; if so go heavier, as jimmylee suggests. You might also want to take a look at your rear shock performance.
    "It is what you discover, after you know it all, that counts." _ John Wooden

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    1973 R75/5 - original owner

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    Administrator 20774's Avatar
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    Quote Originally Posted by jimmylee View Post
    I don't think you change fork action by changing oil quantity. Rather, you change the weight of the oil - let's say go to 10w oil if your 7.5 isn't stout enough.
    I actually thought you could...don't know by how much. As the OP suggested, reducing the air volume above the oil will change the spring compression as the air is just something else that needs to be compressed along with the spring.

    I would do 7.5w and 235cc per the manuals...then go from there.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Quote Originally Posted by 20774 View Post
    I actually thought you could...don't know by how much. As the OP suggested, reducing the air volume above the oil will change the spring compression as the air is just something else that needs to be compressed along with the spring.

    I would do 7.5w and 235cc per the manuals...then go from there.
    You could be, and are probably correct on this.

    On other shocks, forks, the volume of oil (unless way overfilled, or underfilled) had nothing to do with the hydraulic oil action of the shock.

    I haven't seen these forks apart or studied how they were designed, but usually, shocks are designed to transfer oil from one chamber to another through a sealing disk. On the disk (or disks) there are a series of holes that allow oil to flow through at a predetermined flow rate. Some even had a "reed" type backing that allowed flow one direction more dampened, and back the other direction pretty free flow.

    It was the weight or thickness of the oil that changed the flows both during fork compression (not much obstruction of flow - spring did the work) then highly dampened flow on the fork extension back to full length - which keeps the forks from just being "springy" (boing - boing) as the forks go back and forth. this is why shocks/forks use fork oil as opposed to other types of oils, because this back and forth through little holes is what causes the oils to go "foamy." Foamy oil will "cavitate" and trap air causing the which clearly affects the dampening.

    This trapped air is probably what the OP has heard of, pumping forks to get rid of air pockets. On these forks, I think this naturally happens as the forks work - as long as one uses a good quality fork oil.

    The air above the oil level pretty much didn't do anything, and some were actually vented to allow the air chamber to "breathe."

    At least I have over the years never heard of changing oil volume to affect fork dampening. It has always been changing the oil viscosity. I usually start with 5w BelRay oil and then up the viscosity. I settled on 7.5, often mixing 5w & 10w where 7.5w was unavailable.

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    Administrator 20774's Avatar
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    Where does the fork tube breathe in and out of? The fork gaitors breathe, but the tube itself is sealed...not sure how much air can escape past the fork seals. You pour fluid into the hole at the top...it comes up the tube some amount of distance. Now you have fluid sitting on the bottom with air above it...no place for the air to go. You compress the forks...fork oil has to move through the holes in the bottom of the fork tube out into the slider area...on rebound, the fork oil moves back into the tube. I don't see where the air can ever get to the outside.

    I'm sure I don't have a good grasp of the working internals, but that's how I think of it. Correct me if that's not right.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Atomic City Boxer 154048's Avatar
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    I remember when as a kid I had a Yamaha 100 'Trailmaster' One of my knucklehead buddies thought my front forks were too soft (turns out it had a broken spring-they were external on these things). So he filled my forks up to the top with fluid and sent me on my rocky and jarring way. Sheesh.

    Thanks again for all the great replies! I have been riding older BMW's for 10 years now and have benefited so many times from the accumulated wisdom. And it is done in a civil teaching tone...Much appreciated!!!
    Steve in Santa Fe
    1980 R100RT
    2005 DR 650

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    Quote Originally Posted by 20774 View Post
    Where does the fork tube breathe in and out of? The fork gaitors breathe, but the tube itself is sealed...not sure how much air can escape past the fork seals. You pour fluid into the hole at the top...it comes up the tube some amount of distance. Now you have fluid sitting on the bottom with air above it...no place for the air to go. You compress the forks...fork oil has to move through the holes in the bottom of the fork tube out into the slider area...on rebound, the fork oil moves back into the tube. I don't see where the air can ever get to the outside.

    I'm sure I don't have a good grasp of the working internals, but that's how I think of it. Correct me if that's not right.
    I was speaking generically when I said that some shocks actually do have a breather to allow some air to escape.

    You are correct in that these forks do no have a breather.

    I have my forks apart right now, and I went out and tried to "understand" how they worked. As I see it, inside the fork tube, there is the lower rod (that connects to the outer tube at the very bottom.) that has a "piston" with piston-like rings on it, and also a springed ring on it as well. As the lower fork tube moves up, the oil above the piston (when the fork is fully extended) moves through the piston (rings keep oil from leaking around the piston) and fill the chamber created below the piston as the fork moves upward and compressing the spring. Thus, there is no volume change in the area above the piston as the oil above is drawn below.
    The exact opposite happens as the force of the spring overcomes the upward force of the road (bump or otherwise) and the oil then returns back to the top chamber from the lower chamber as the piston holes permit it to flow (therefore, the fork action's speed is controlled by the holes in the piston and springed obstruction over those holes and by the oil viscosity. The thinner the oil, the faster the exchange, and the thicker the oil, the slower the exchange.

    if I understand these fork's design, I don't think that the air compresses at all as there is no volume change in the total of the two chambers. The fork spring compresses at the spring rate (could be multiple rates as designed).

    I would have to measure out the oil quantity specified and visually somehow see just how full it fills the fork, and see just how much air space is available. It does appear to me that over-filing (but not filling up completely!) would not matter that much. Of course, underfilling would be problematic on two counts: not enough lubrication, and may not be enough oil to fill the chambers and would then force the forks to "suck air."

    It appears to me that oil viscosity is the only thing you can do to change the fork flow rate.


    Please don't think that I think I am the final word on this as I still would have to study the whole fork action more closely - but this is what it looks like the way they work to me based upon very little study.

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    Administrator 20774's Avatar
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    It seems that the air can act as a secondary spring. This is a quote from Tom Cutter on the Airhead list some time back:

    The whole purpose of careful oil level adjustment is to control the exact volume of the trapped air in the fork, which is the secondary spring acting on the suspension.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    As with almost everything airhead, Snowbum has a couple articles about front forks that might help:

    http://bmwmotorcycletech.info/frontforks.htm has some general info about construction and how they work, and

    http://bmwmotorcycletech.info/front-...ls-amounts.htm is about how much oil & how to measure.

    Probably more than you ever wanted to know, but, hey, that's Snowbum .
    1983 R100RS (Sold)
    2004 R1150RT
    BMW MOA 181289
    ABC 13558

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    Quote Originally Posted by 20774 View Post
    It seems that the air can act as a secondary spring. This is a quote from Tom Cutter on the Airhead list some time back:
    I definitely could be wrong here, but I have my doubts about this "secondary" spring concept.

    I took my one fork which is right now, disassembled on my bench, and looked at it, and I just can't see it. It still appears to me that the whole process is a transfer of oil from one chamber to another without compressing any air.

    Could be wrong though.

    I would have thought that if this were a common idea - i.e. that one could change fork rates and/or spring pressure by increasing/decreasing fork oil I would have heard of it somewhere. In 40+ years, this is the first I have ever heard of it. Hmmmmmm?

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    Registered User melville's Avatar
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    Quote Originally Posted by jimmylee View Post
    I definitely could be wrong here, but I have my doubts about this "secondary" spring concept.

    I took my one fork which is right now, disassembled on my bench, and looked at it, and I just can't see it. It still appears to me that the whole process is a transfer of oil from one chamber to another without compressing any air.

    Could be wrong though.

    I would have thought that if this were a common idea - i.e. that one could change fork rates and/or spring pressure by increasing/decreasing fork oil I would have heard of it somewhere. In 40+ years, this is the first I have ever heard of it. Hmmmmmm?
    There are forks that use only air for a spring. A few on MCs, but fairly common with early mountain bike suspension forks. Changing the air pressure would change the spring rate, and changing the oil level would change the progressivity of the spring rate. Higher oil levels (less air) would result in a fork that had a strongly progressive spring rate.

    Just for fun, take the top sealing caps off your fork and go for a short ride. Nail the front brake a couple times. Then put the caps back on and do the same ride. You will feel a difference, and that difference is the air acting as a spring with the caps in place.

  13. #13
    #4869 DennisDarrow's Avatar
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    There is a thriving industry and set of tech standards that have to do with "air ride suspensions". Not only are air/gas suspension components commonly used in the motorcyle industry but in heavy transportation of goods and services........God bless...........Dennis

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    Quote Originally Posted by melville View Post
    Just for fun, take the top sealing caps off your fork and go for a short ride. Nail the front brake a couple times. Then put the caps back on and do the same ride. You will feel a difference, and that difference is the air acting as a spring with the caps in place.
    have you actually done this? Seen it done?
    I ask, because I suspect that the only thing one would "feel" is a fork fluid shower upon the first compression action.
    Ride Safe, Ride Lots

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    Quote Originally Posted by DennisDarrow View Post
    There is a thriving industry and set of tech standards that have to do with "air ride suspensions". Not only are air/gas suspension components commonly used in the motorcyle industry but in heavy transportation of goods and services........God bless...........Dennis
    has nothing to do with this conversation! I have been in the very plant that makes the Good Year Air springs, and built the machines to make them!

    Dennis, you, again, have a penchant for stating something completely unrelated. Dennis, show me ONE conversation or article put out by BMW or anyone familiar with THESE forks/shocks that recommends using the VOLUME of oil to control the fork action or to purposely increase spring pressure? Just one.

    If it were that simple to use the air in the fork to use as a helper to the spring, one could simply put a tire valve on top of each fork leg (THESE forks) and pump them up! Would you go for that?

    The point of setting an UPPER limit for the amount of oil (whatever that is) is to avoid the air pressure that would ensue if overfilled. That is why some manufacturers of shocks (not these, obviously) put a "breather" to allow air to flow in and out so that pressure could not be built up, as my very first statement said.

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