progressive front springs with air ride

Discussion in 'Custom Mods' started by TURBO200R4, Oct 24, 2019.

  1. TURBO200R4

    TURBO200R4 stand up straight and grab the world by the a$$

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    i installed the progressive front springs and while i was installing them i drilled and tapped the top cap for a Schrader valve. all went well i then put 20 lbs of air in it to bring the forks to the top. when i set on the bike it compresses just a little. the forks has held the air for about a month w/o probs.
    i have put it through some good tests i did drag the floor boards (pucker time) a couple of times (running the maxs on rear and the avon 140 on front. i think to complete the set up i will have to go back to the 150 on the front.
    pros
    good to 120 mph. seems to take bumps good. excellent in corners(more clearance) more travel when braking.

    cons
    on a drop off of pavement (about 4-5 inches) the fork banged to max travel:(

    i wanted to go to RAA west to try it out but i never made it. i though it might give me a small advantage to maybe help me to keep up with the some of the guys.
     
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  2. Tripps

    Tripps Retired superhero

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    I'm not following, I don't think, wouldn't they bottom out even worse without air pressure?
     
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  3. TURBO200R4

    TURBO200R4 stand up straight and grab the world by the a$$

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    before my shocks rode in the middle area so when i went off the pavement the shocks would take time to extend and therefore no bang now they r real close to the top so they close that 1/2 inch gap real quick,
    i have went off at the same place at slower speed and it was not the hard hit like the first time.
     
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  4. Claviger

    Claviger Aspiring Student

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    Very interesting. Must think on this.
     
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  5. TURBO200R4

    TURBO200R4 stand up straight and grab the world by the a$$

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    i wanted to go to raa west and let some of the faster riders give a try to get opinions but i never made it.:(
     
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  6. barbagris

    barbagris Mad Scientist

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    I have been riding over 20 years with air assist on my Guzzi forks. Here's the thing. Most seals do not cope well - I have twin double lip seals on the Guzzi which is nowhere near the weight of an R3. This increases seal "stiction" unless you can keep the seals very well lubricated and CLEAN.

    When you drop off the kerb the build in pressure is DRAMATIC. The air gap reduces to almost zero - that additional 20Psi could well result in too much pressure.

    One alternative could be a smallish plenum between the two forklegs.

    Once they start to purge air - ime pretty much all the air rushes out. And the result is vacuum once the forks then attempt to rebound. The effect is catastrophic failure of the system. New seals time. Also as the air pressurises the hydraulics the valves can cease to function correctly - we need Mr @Speedy to relate the technical part.
     
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  7. Speedy

    Speedy Supercharged

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    So the bang is the forks topping out. Likely that with the combined rate of spring and air your rebound travel is under dampened. Not much you can do to change that with out new fork cartridges. You may find better results by backing off the air pressure some. This will reduce spring rate and slow front fork extension speed. Unfortunately while the R3 needs more fork spring rate, it needs good dampening even more. Heavier weight oil helps but it is not quite enough.

    Forks need set in just like rears. Too little and when accelerating the forks top out. When this happens, front tire loading changes abruptly as now the weight transfer to the back tire is lifting the wheel and tire from the surface (no more sprung weight left so un-sprung weight transfers rearward.) This works okay drag racing but is not so nice when rolling into heavy throttle mid-apex.

    With stiffer springs and more dampening, you can increase fork set in which improves both handling and ride quality.

    As Barbagris mentioned an accumulator between the two forks will insure that the forks are pressure balanced, and it reduces the rate of progression in pressure increase due to the volume reduction from the forks compressing. This would allow you to adjust set in height without as much increase in spring rate. (20 psi to 40 psi versus 20 psi to 100psi, for example, depending on fork air and accumulator volumes.)

    As to the function of dampers under pressure, increased air pressure reduces the volume of air entrained in the oil. So, in twin tube low pressure gas designs, the shock will perform even better than normal, as these shocks keep oil and air separated (shock OEMs use vacuum de-gassing to reduce air in the oil when they assemble shocks, but some always remains.) In emulsion shocks, the oil and air are combined so the valving is designed to function managing a fluid that is somewhat like shaving cream. When this type of shock is put under pressure the air bubbles in the oil are reduced in size so the effective viscosity can change dramatically (the whipped cream can turn back into a liquid.) Thus emulsion shocks use a compressible fluid versus the uncompressible oil in twin tube or high pressure monotube shocks. Therefore, they are extremely sensitive to static pressure changes and will become very stiff if the fluid condenses back into a liquid.

    Most forks and old twin tube shocks with no initial internal pressure are intended to function with uncompressible liquid oil, but when used extensively the oil and air begin to combine (are whipped into a lather) so the dampening goes to mush. This phenomena is responsible for fade much more than the minor viscosity change due to the oil getting hot.

    If you think I am wrong about the cause of fade, keep in mind the extreme pressure drops created with a high speed high pressure liquid flowing through a tiny orifice. It tries to expand. If the fluid is a liquid with no entrained air, it will remain a liquid but there will be voids in which there is no liquid. These do not grow in size and quickly collapse (same phenomenon as cavitation.) However, if there is entrained air (microscopic bubbles not seen by the naked eye) those bubbles explode in size turning the oil into a froth. These bubbles collapse over time too, but if they are generated too quickly and the oil viscosity slows their ascent to the surface sufficiently all the oil in the system foams. Imagine blowing air through a soda straw into a glass of soapy water. What happens?
     
  8. Nat67

    Nat67 Rockgoblin

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    wow ! I think I love you ? :rolleyes:
     
  9. barbagris

    barbagris Mad Scientist

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    Only Think? - Oh you slut!. I love reading @Speedy posts.

    It is not just the content, but the marvellous use of language. So much is conveyed with such efficiency of verse.

    The oil emulsion thing never occurred to me as the Guzzi has sealed independent dampers (which are all poor - OEM or aftermarket) - it's fork oil is just for lube and keeping air in.
     
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  10. Nat67

    Nat67 Rockgoblin

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    I know . That was the only way I could convey my admiration for that post ! There are a number of guys here that I would never even think to question their knowledge in their particular fields , when they are kind enough to share it , The likes of Neville lush , De cosse , pops , Ishrub , Paul Bryant , That grumpy polish engineer / tool maker , claviger and even yourself to name just a few . But @Speedy ? **** , that's poetry ! so succinctly put and clearly understandable on a subject that most people nod their heads to but are generally none the wiser ! whilst reading that post , my world stood still . Amazing ! :rolleyes:
     
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