Front suspension

Discussion in 'Motorbike Technical Discussion' started by Joe something or other, Nov 8, 2006.

  1. So I've been fixing up this 93' Yamaha Xt 350, and I have heard alot
    about the problems with the front suspension but I have never heard
    anything about a solution for it. Is there any upgrades that I can
    buy? Or is there any home jobs that I could do?
    Joe something or other, Nov 8, 2006
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  2. Joe something or other

    John Johnson Guest

    Cheapest suspension "upgrade" on the planet is fresh oil. Lots of people
    neglect it, but it makes a _huge_ difference in handling. I'm not
    familiar with this particular bike, so can't really help much beyond
    this, but HTH.


    'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.
    John Johnson, Nov 8, 2006
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  3. Joe something or other

    B-12 Guest

    Check www. to see if they make a Gold Valve Emulator for
    the XT350. GVE's are a compression blowoff valve that overcomes the
    fixed orifice stroking speed limitations of the stock damper rods.

    Say what?

    If that sounds like technojargonese to you, what happens is that fork
    oil can only flow so fast through the holes in the damper rod, and then
    the fork locks up hydraulically and won't compress over sharp bumps.

    Riders who try to improve operation of cheap damper rod forks have two
    ways to go. The first is to use a good quality locally made fork oil
    that may or may not be *lighter* in weight than the stock oil.

    Gawd only knows what kind of oil the Japanese manufacturers use.

    Some writers have claimed that original equipment fork oil is made from
    fish oil. So a known quality locally manufactured fork oil of about 10
    weight is indicated for a fast-stroking competition fork. 10 weight to
    15 weight is good for small amplitude, high frequency "stutter bumps"
    such as are found on graded roads.

    But the rider finds that he has less control over larger bumps that he
    encounters at much lower frequencies, such as "whoop de doos". So he
    tries a heavier weight fork oil (like 30 weight) and discovers that
    riding his motorbike has become like trying to control a pneumatic
    jackhammer over stutter bumps.

    What the heck is going on?

    Since damper rod forks are *orifice limited*, oil has to flow four
    times as fast through the damper rod fixed diameter hole when the fork
    is stroking up and down only twice as fast, so experimenting with
    heavier weight fork oils only makes the hydraulic lock up problem

    The GVE is an adjustable spring loaded valve that sits on top of the
    damper rod and opens to relieve excess pressure over sharp bumps and
    then closes again when the pressure is relieved. Race Tech also
    specifies drilling one of the holes in the damper rod larger to allow
    more oil to flow through the rod...

    You could use 20 or 30 weight oil with GVE's installed and a bigger
    hole in the damper rod.

    The stock forks have an air valve on the fork caps. I remember back in
    the late 1960's, when my Yamaha 250cc Single Enduro was equipped with
    check valves on the fork caps, and the experts said that air would get
    trapped in the forks at sea level and then when we got to the high
    desert, we were supposed to "burp" the excess air pressure to get the
    fork to work well over the stutter bumps.

    The other claim was that the forks sucked air in as they stroked up and
    down and that air pressure built up, and we should burp the forks as we

    The truth of the matter was that we had damper rod forks, and damper
    rod forks can either be tuned for stutter bumps or for whoops, but not

    Then, in the late 1970's, street bikes began showing up with "air
    assist" forks. We were supposed to inflate them with as much as 7 psi
    air for a special hand pump. And the fork springs were a lot lighter
    spring rate, since they depended on air to help them.

    Problem was, the air assist would cause the fork seals to grip the fork
    tubes too tightly, and the forks had "stiction" which described the
    sticky friction of unresponsive forks.

    So the factories abandoned air assisted forks in favor of forks without
    air fittings and the forks had stiffer springs.

    And, my 1986 GSXR750 had an externally-adjustable compression blowoff
    valve with four settings. I can feel the difference in settings as I
    ride over frost heaves on mountain roads.

    The 1986 GSXR1100 had an electrically controlled compression blow off
    valve that would react in about 1/100th of a second.

    Then Yamaha and Honda began supplying cartridge forks with shim stacks
    inside the internal car-ytpe shock absorbed, and the many flexible
    shims in the cartridge did the compression blow offfunction.

    Most riders wouldn't want to buy a whole new fork to get the
    compression blowoff valves...

    Race Tech's Gold Valve Emulator is an infinitely adjustable aftermarket
    blowoff valve, but you do have to remove the fork spring to get at the
    adjuster bolt.

    If Race Tech doesn't make a Gold Valve Emulator specifically for the
    XT-350, ask them what diameter GVE would fit. They probably only make
    about half a dozen diameter GVE's.
    If Race Tech doesn't make Gold valve Emulators for the XT-350, you
    could make your own in a junior college machine shop. It should only
    take you about 3 months to learn to make your first set...
    B-12, Nov 8, 2006
  4. Cheapest and easiest solution, as John Johnson says, is to change the
    damping oil. Next easiest thing is to shove in a couple of decent
    aftermarket springs. Google for them.

    Beyond that, you're getting into the realms of complicated rebuilds and
    engineering - not exactly a home job.
    The Older Gentleman, Nov 8, 2006
  5. Joe something or other

    oldgeezer Guest

    B-12 schreef:

    <Snip the lot, sorry B-12>

    An absolutely devastately correct story.

    I did change my front fork oil from SAE 10 to SEA 30. It made
    like I used to ride on an punctured front tire into a tire
    that was correctly inflated.
    It totally changed the behaviour of my bike.

    For your info;
    I have two identical bikes (as matter of fact, they were on
    the Honda assembly line one after the other --frame
    numbers differ one last digit, as do licence plate numbers--) .
    And I rode MG87NN with grade 10, and MG88ZN with
    grade 30 in the front forks..

    You wouldn't believe wat difference that makes. It feels
    as if I have two totally different bikes.

    When I bought the (newer) MG88ZN, I swapped springs from my MG87ZN
    because I thought : 'something is weird, this new bike must have
    been in an accident, or springs are deteriorated', because it gave me
    the deflated front tire feeling, like I was driving an HD in the
    But it only was the grade of the front fork oil that made a totally
    different feeling. --Hard to describe though---

    But today, temperature dropped below 6 centigrade, and
    I stutterered all the way home (SAE 30) on my (newer) MG88ZN.
    This got me puzzled as to "I drive all seasons, What is it that I
    really want?" Dunno yet.

    oldgeezer, Nov 9, 2006
  6. Joe something or other

    B-12 Guest

    How about a motorcycle that originally came equipped with a
    fully-adjustable cartridge fork?

    Or, how about swapping a fully-adjustable cartridge fork into *your*
    triple clamps? A swap like that is very common in California, the
    SV-650 racers will stick a GSXR front fork on their motorbike.

    Or, how about checking to see if they make an emulator
    that will fit your present damper rod fork?

    When you install the emulator and racetech springs, you will throw away
    the preload spacer and use the longer springs. And you can fiddle
    around with different weight oils.

    The major hassle with the emulators is that you have to remove the fork
    caps to adjust the compression blow off point.

    The externally-adjustable compression relief valves on my GSXR come
    from an era when the endurance racing guys wanted a motorbike that
    wouldn't nose dive under heavy braking on the same corner in a long
    race. The street riders still wanted suspension compliance, so the
    compression blow off valve was the solution.

    Canyon riders don't do a lot of heavy braking, or they would complain
    about nose dive too.

    Canyon riders want a fork that can handle frost heaves in the spring,
    so the cartridge fork with the adjustable shim stack works just fine
    for them.
    B-12, Nov 9, 2006
  7. Joe something or other

    Ian Singer Guest

    30 weight and heaters for the shocks?

    Ian Singer


    See my homepage at
    hosted on
    All genealogy is stored in TMG from
    Charts and searching using TNG from
    I am near Toronto Canada, can I tell where you are from your reply?
    Ian Singer, Nov 9, 2006
  8. Joe something or other

    oldgeezer Guest

    T Shadow schreef:
    Right, I didn't think of 'single weight'.

    I googled for thumpertalk and front suspension.
    Probably the best solution will be to use Dextron ATF
    and experiment with the oil level in the forks.

    oldgeezer, Nov 10, 2006
  9. Joe something or other

    oldgeezer Guest

    Ian Singer schreef:
    Or 90 weight and making the shocks part of the cooling system. ;-)

    oldgeezer, Nov 10, 2006
  10. Joe something or other

    Ian Singer Guest

    Is that OK on the seals or will it destroy them?

    Ian Singer


    See my homepage at
    hosted on
    All genealogy is stored in TMG from
    Charts and searching using TNG from
    I am near Toronto Canada, can I tell where you are from your reply?
    Ian Singer, Nov 10, 2006
  11. Joe something or other

    John Johnson Guest

    I wonder what the effect would be if you used a multi-weight oil like
    10w-30 or 10w -40 ????????????????????
    George C[/QUOTE]

    A dude on the VFRlist did this. It's not a comprehensive evaluation, but
    there is it:


    'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.
    John Johnson, Nov 10, 2006
  12. Joe something or other

    Wudsracer Guest

    Actually, the viscosity never goes up. It just doesn't thin as much at
    higher temps like a single weight oil would.
    In other words; "The multi-weight oil is never thicker at higher temps
    than it is at low temps,."

    Wudsracer/Jim Cook
    Smackover Racing
    '06 Gas Gas DE300
    '82 Husqvarna XC250
    Team LAGNAF
    Wudsracer, Nov 10, 2006
  13. Joe something or other

    B-12 Guest

    No, that wouldn't be the "best" solution, it would be a *cheap*
    solution because of the ready availability of ATF.

    And, it begs the question about what is supposedly "better" about ATF,
    compared to the unknown quality original fork oil or a locally
    distributed aftermarket fork oil that comes out of a 55 gallon drum,
    has some pretty colored dye added, is decanted into a plastic bottle
    with a snazzy logo, and is marked up accordingly.

    I had a local trickster try to sell me a 55-gallon drum of soybean oil
    for $5000 USD several years ago, but that's a whole other story, beyond
    the fact that he was trying to
    recruit people into his pyramid scheme.

    Back in the 1970's, one of the motorcycle magazines did a series of
    fork stroking tests on a suspension dynamometer and they published nice
    load cell readout graphs showing how the damping forces compared from
    fork to fork.

    The readership really ate that shit up...

    Of course the simple damper rod forks mainly damped in one direction
    (rebound) and the other direction was either a flat line or showed
    annoying spikes as the fork was unable to
    compress rapidly enough while it was in orifice limitation.

    And, for consistency, they used a certain hydraulic oil, referred to by
    a mysterious number. :-/

    Interested racers called in, wanting to know where they could get that
    magickal oil, and the magazine confessed that it was a special high
    quality laboratory oil that was used only because it was consistent
    from batch to batch.

    Ordinary people couldn't easily get that oil, so they couldn't
    duplicate the magazine's results. But, the article was typical of a lot
    of the things that the amateur engineers at motorcycle magazines do
    when they are pretending to do research. :-(

    It was about that time I first started hearing about racers who used
    ATF. The claim for ATF was that it didn't foam up, and that is
    important because a motorcycle fork is essentially an air over oil
    strut, similar to what an aircraft uses, except that motorcycles use a
    dependable steel spring, instead of compressed air that leaks away.

    So, high speed stroking would cause air bubbles to entrain in fork oil
    and the fork wouldn't damp consistently as the bubbly mixture passed
    through the fixed orifices.

    Look at the design of REAR shock absorbers of the early 1970's. Rear
    shock absorbers were being pressurized with nitrogen gas, either
    directly over the oil, or the gas was kept out of solution with the oil
    by putting it into a plastic bag, or a remote reservoir, behind a

    We don't see remote reservoirs on front forks very often. Why? Maybe
    it's because front forks don't stroke very fast under normal highway
    use. And they don't stroke very far, either.

    In fact, the most comfortable-riding fork is one where the laden ride
    height is only about 1/3rd to 1/2 of full compression.

    And, when a rider is going down the highway, using very little
    suspension travel at all, the
    air-over-hydraulic fork or the gas-over-hydraulic fork just isn't doing
    much damping at all.

    How can a hydraulic damping system work, if it depends upon motion to
    produce a damping force?

    Also, if street forks aren't moving much, and they have a large enough
    quantity of oil in them, how much air can get entrained in whatever
    fluid is in the fork, ATF or Super Slick Stroker oil?

    The ATF solution was originally for *dirtbikes*, where the front end
    strokes up and down rapidly and uses all the travel half a dozen times
    in 100 meters.

    Finally, the ATF solution also becomes problematic in that the
    viscosity of ATF can vary wildly from one brand to another. Nobody
    seems to be able to tell me exactly what the viscosity of the red stuff

    About all I can say for sure about ATF is that any experiment you do
    with ATF that allows you to decide on sticking with that particular set
    up would indicate that you always use the exact same brand of ATF.

    But that would also be true of any set up arrived at through the use of
    Super Slick Stroker oil, or whatever the dustributor calls his
    B-12, Nov 10, 2006
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