How much less fork oil with heavier springs ?

Discussion in '1st & 2nd Generation 1983-1989' started by straycat, Nov 21, 2018.

  1. straycat

    straycat New Member

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    Another question. Ive rebuilt one set of forks using the stock springs etc. The second set of forks will be rebuilt with the heavier springs I took out of the track/race bike.

    since there are more coils in the track/race springs and the spring is longer, I assume I need less fork oil.

    The Factory Shop Manual calls for 430/450 CC of oil in the forks (Right/left), there is also supposed to be 180mm of air space with the spring OUT (not installed).

    So how do I decide what the correct amount of fork oil is with the longer heavier springs ?

    id rather not have to install a stock spring and take a measurement with the spring "In" and then drain the fork again and get the same measurement with the heavier spring, but I cant think of any other way.
     
    Last edited: Nov 21, 2018
  2. NorcalBoy

    NorcalBoy Member

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    You are asking a multi faceted question, but I will do my best to simply answer the height question with a little extra info thrown in:

    The oil height, in the most basic terms, controls the bottoming of the fork, typically under heavy braking. The amount of oil you pour into the tube determines the volume of air that is contained in the fork tube (above the oil, I'll just refer to it as the air "gap" for simplicity) when the spring is in and it is fully assembled. As you might guess, or not, as the fork compresses, that volume of air also compresses, as it compresses the air space is reduced, putting this air under pressure. This is where we start talking in tongues.....as the air is compressed, it acts on the cross sectional area of the fork to create a force that works in unison with the spring, thus assisting the spring as the fork compresses...I hope I'm doing OK, so far, lol. The next bit is a physics lesson, and I won't bore you with that, but it has to do with the absolute gas law, etc, etc.. In simple terms, as the volume of air is halved, the pressure contained increases by double...I'm skipping a little here, as the air gap is compressed it is aiding the spring when it comes close to the end of it's travel, i.e., bottoming. This is an exponential force when considered in conjunction with the spring, which by itself , is a linear mechanical force. So, if you take oil out, the fork will bottom easier (due to more air "gap" and the distance the fork has to travel to compress it) no matter what the spring rate is. If you put more oil in, it will make the fork more resistant to bottoming (due to a smaller air "gap", the distance the fork has to travel to compress it is less).

    So basically, if the manufacturer gives you a height, use it. If you find yourself bottoming easily under braking, increase the height of oil in the tube in small 5mm, increments. (This is all based upon the fact that you have set the sag, correctly i.e. the spring you have chosen is the correct rate for your weight). You can also control bottoming by, depending upon how fussy you are about your setup, changing the preload, but remember, changes in preload also changes the ride height, which changes the entire geometry. So, the oil height is a quick and easy way to adjust the bottoming effect of your forks without changing your geometry. If you weren't having any problems with bottoming out the forks with the stock height, start with that and then add, or remove, as necessary to get the feel you are looking for in your forks resistance to bottoming. TA DA! I hope I didn't put you to sleep.
     
    Last edited: Nov 21, 2018
  3. straycat

    straycat New Member

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    Thanks for the info.

    Ive never ridden the bike as yet. Im building one from 3 bikes and one bike (race bike) had heavier springs. I rebuilt 2 sets of forks from the 3 I had, one set is factory spec (Springs, oil volume etc), one set has the upgraded heavier springs. So, to have the same volume of air as the factory fork setup, the heavier and longer springs forks would have to reduce the oil by 20-30cc. (or some such amount), right ?
     
  4. Terry Smith

    Terry Smith Member

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    Good job on the explanation NCB.

    You're asking a tricky question Stray, and I think starting with the stock 180mm compressed/springs out and then adding or subtracting depending on feel would be a simple approach (albeit involving occasional disassembly).

    Or...
    you could make an estimate of the total spring volume based on the mass and a typical density for steel, Mr Google says 7.860 g/mL for spring steel. If you do the same with some stock springs you can calculate the difference in their total volumes. If you were then to estimate the proportion of spring that is immersed in oil you could calculate the difference in volume occupied by the heavier spring and subtract that from your oil volume.
     
  5. Terry Smith

    Terry Smith Member

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    I had a post-posting thought, "heavier" springs may well equate to straight rate springs instead of the progressive springs that Honda's usually come with. If so, it is reasonably common practice to run a much higher oil level/smaller air gap to provide more progression and final bottoming resistance. For reference, Racetech specify 130mm air gap with straight rate springs in a VF1000R.
     
  6. NorcalBoy

    NorcalBoy Member

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    Typically the springs aren't longer, it's just that the wire used for the spring is heavier, thus making it harder to compress. If they are longer who knows where they came from. That is why I also must suggest that you NEVER use progressive rate springs, you will chase the settings until your eyes bleed, as the spring rate isn't linear, which means it's constantly changing as it compresses, thus the air volume isn't changing at a linear rate either, and the setting keeps bouncing around. Basically, I would not even bother fucking around with the longer springs, as you don't know where they came from, nor do you have any baseline setting guidance. You also have no idea what the rate is, i.e. KG of force required to compress the spring per mm. Because you don't know the rate, you could go through a large quantity of unwanted effort to find out the springs are too heavy, or too light, to hit your sag numbers correctly, then it's all wasted effort. Step 1 is ALWAYS to set the sag, if you can't get in the correct range, your fooked anyway. I always use the KISS principle when dealing in unknowns, as in in completely eliminating them as easily as possible
     
  7. NorcalBoy

    NorcalBoy Member

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    Thank you. You typically have a lot more patience for this type of thing than I have at this point in life. Normally, the first thing I suggest is buying Andy Trevitt's or RaceTech's suspension book and educating yourself first. I am a firm believer in the "you can fish for the person, which helps them one time, or you can teach the person to fish and he can take care of himself for the rest of his life". But it is the holidays and I was feeling a little more generous than usual, rofl.
     
    Last edited: Nov 21, 2018
  8. Terry Smith

    Terry Smith Member

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  9. NorcalBoy

    NorcalBoy Member

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    You are a better man than I, Mr. Smith!
     
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  10. straycat

    straycat New Member

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