spongy (disc) brake

Discussion in 'Motorbike Technical Discussion' started by Dave, Sep 4, 2007.

  1. Dave

    Dave Guest

    Took my XS650 out for a ride this weekend, first time in a month or so. The
    front brake was soft... by that I mean that I had to pull the lever maybe
    1-1/2" before I felt resistance and I had to squeeze damn hard to get much
    feel out of the brake. Bad, I thought. But after riding for a couple of
    minutes (less than 10) the brake felt completely normal with the catchpoint
    about 1/2" pull and brake action was strong. Fluid is at the normal level.

    Ideas?


    Dave S.
     
    Dave, Sep 4, 2007
    #1
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  2. Dave

    Dave Guest

    About a month.
    I've only had it since last fall, haven't changed the brake fluid yet.
    it doesn't appear to be
    I'll have a look but... I would think I'd see brake fluid leaking OUT if any
    air was leaking IN. I'll change and bleed the brake fluid, see what
    happens.

    Thx

    Dave S.
     
    Dave, Sep 4, 2007
    #2
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  3. I've had that, and been foxed as well, because there were no air leaks.
    I just put it down to a very small amount of pad creepback while the
    bike was parked up.
     
    The Older Gentleman, Sep 5, 2007
    #3
  4. Dave

    Dave Emerson Guest

    message
    You can also get strange effects if the breather hole above the reservoir
    diaphragm gets blocked, e.g. by dirt or alloy corrosion crud.

    If the bike was last used in hot weather and then taken out on a cold
    morning, you can have a partial vacuum above the fluid that works against
    the lever action and pulls fluid up the lines - it takes very little to
    create the effect described - and it clears itself when the trapped air
    warms up or the dirt gets shaken out; more likely on an XS650.

    It's also possible for the breather holes to be partially blocked such than
    it acts as a one-way valve rather than allowing free flow.

    Worth checking anyway.
     
    Dave Emerson, Sep 5, 2007
    #4
  5. Heh. Trouble is, I find myslef scanning adverts and bidding on Ebay
    again. I've had (counts on fingers) five XS650s now, and I feel the
    urge for another nice rutting Japanese vertical twin. Still the best
    360 degree big vertical twin engine ever
     
    chateau.murray, Sep 7, 2007
    #5
  6. Dave

    Dave Guest

    Why do you think this is such a great bike? It's got a crappy suspension,
    decent handling (better with fork brace), huge vibration from the 360-degree
    firing angle and a top speed of about 95mph stock. And it only puts out
    48HP. I've got a couple of bikes, have owned many more and can't really
    think of anything that makes the XS650 stand out as a stellar performer.
    Not a bad bike, not a great bike. But... to each their own.

    Dave S.
     
    Dave, Sep 7, 2007
    #6
  7. Dave

    Dave Emerson Guest

    Having had an XS2, I can confirm that the XS650 was a great improvement.
    Both had a strong engine but at least the XS650 handled (given an 1970's
    definition of "handling").

    The most disconcerting thing about the XS2 was the we the front disc would
    just make an 'orrible howling noise we first applied in wet weather, without
    slowing the bike at all, and would then lock the (EOM Japanese Dunlop)
    front-tyre solid for the next ten feet. My mate had an XS500 and that was
    just as bad.
     
    Dave Emerson, Sep 7, 2007
    #7
  8. Agreed. Easily rectified, though. And almost all Jap bikes had crap
    suspensiuon in the 1970s.

    The vibration is far, far less than on most Brit big twins. And every
    single component that needs to be rubber mounted, is. Seat, bars,
    footpegs, even the battery box.
    Rather faster than that. The custom version suffers from aerodynamic
    drag more, but the roadster version is capable of about 110.
    It's just the feel of the thing, I reckon, and that fabulous engine. The
    engine makes the bike. It's unbelievably strong, handsome, and makes the
    right noise.
     
    The Older Gentleman, Sep 7, 2007
    #8
  9. Dave

    OH- Guest

    Perfectly normal.

    Do a couple of emergency stops and the brakes will be even better.

    The brakes are sort of a self adjusting mechanism but in order to
    stay adjusted they need to get used.

    OTOH, if they get "soft" more quickly or get worse each time, you
    have had very good advice in other answers.
     
    OH-, Sep 7, 2007
    #9
  10. Dave

    Dave Guest

    message
    I guess it's hardly fair to judge it by today's standards. And folks are
    still opting for Penske or Ohlins shocks as one of the first mods on a new
    bike anyway.
    I don't see why they didn't go with an 90-degree angle by offseting the
    cranks... would've added a few pennies for the modified ignition but once
    designed the cost in parts would likely have been less than $10. I'd do the
    83-degree mod but the cost of a new cam and ignition is prohibitive.
    Like I said, when I got this bike I thought I'd throw a set of cheap tires
    on it, fix the obviously broken things and unload it. And yet, this is my
    2nd year on it. It does sound good. The problem with this bike is that I
    KNOW it could perform a lot better, but it would cost me more to mod it than
    to toss it and get a better performer. Shocks, fork brace, good tires,
    maybe different bars, double discs up front, drilled rotors, new pipes,
    rejetted carbs. How much are we up to? I saw one in the paper the other
    day, a street tracker, for $3,000.00 which had come down from $4,000.00 and
    you know the guy's got hundreds of hours into it. Anyway, I've got clutch
    parts on order so it looks like I may be in it for the long haul with this
    one, one more year anyways. I've got collector plates on it so insurance
    runs me $72 for 6 months... hard not to keep it.

    Dave S.
     
    Dave, Sep 7, 2007
    #10
  11. Dave

    Ron Gibson Guest

    If this is a circa 1979 Yamaha 650 Special (There is also a non-special
    model) it's best quality is handling. It used to be quite good in racing
    back then.

    But in the next few years the technology really took off and the 650
    twin got left in the dust.

    I've got a 83 XS400RK that is nearly 90 pounds lighter, same HP but with
    DOHC and a redline of 10,000. Of course it would run circles around the
    79 bike but the 650 was very dependable and a lot of fun on curvy roads.

    Next bike is gonna be a Kawasaki though. I want to add a newer model and
    the new Yamaha's require you to keep a chiropractor on speed dial to keep
    your back in shape.
     
    Ron Gibson, Sep 15, 2007
    #11
  12. Absolute nonsense.
    Dirt track, yes. Think Kenny Roberts. Hardly applicable to the street,
    and the bike doesn't even register a blip in road racing.
    Yes.
     
    The Older Gentleman, Sep 15, 2007
    #12
  13. Dave

    Ron Gibson Guest

    Oh, gee. That came form a book I read and comments from the dealership.
    And your claim to superior knowledge is....???

    Having said that what *is* that models best quality?

    <chirp, chirp, chirp>

    I thought so.

    Thanx for sharing.
     
    Ron Gibson, Sep 15, 2007
    #13
  14. Having owned five XS650s (three roadsters[1] and two Customs). Not from
    "a book I read".

    And this:

    http://www.classicbike.co.uk/pdf/676/293869.pdf

    The XS650 only handled (reasonably) well after Percyt Tait sorted it.
    The Custom threw huge wide rubber-mounted bars into the mix, along with
    odd weight distribution and a silly 16" rear wheel. Which buggered the
    handling up again.

    Lovely bike, nice to ride, but the handling is *never* the best point of
    any XS650, and especially not the Custom
    The engine, without a scintilla of doubt
    Are you feeling silly now?

    [1] One of which had a 750 kit in it, Fun, it was.
     
    The Older Gentleman, Sep 15, 2007
    #14
  15. Dave

    Ron Gibson Guest

    Nonsense. You made a bad tire selection. See...

    http://650wiki.org/index.php/1.08._Building_a_Better_Twin_%
    28The_Minton_Mods%29#STAGE_I_CHASSIS

    or...

    http://tinyurl.com/ytxa4z

    TIRES: Roadrace-quality handling will do nothing if you dont have tires
    to go with it. We tried several sets of tires before setting on our
    preferred set: a sport-compound Metzeler on the front and a Pirelli MT28
    on the rear. The Metzeler is a round-profile tire and complements the
    steering geometry of the Yamaha twin perfectly. The sport compound
    version of this tire allows the rider to take advantage of the excellent
    brakes that result from our modifications. The Pirelli is also a round
    tire that retains the same feel at any lean angle. Its traction is above
    reproach and it lasts. But there are other tire combinations that perform
    well on the XS, depending upon how you use your twin. There is no better
    touring setup than the Conti twins. These tires will give the best ride,
    last at least as long as anything else, and will give more traction than
    the stock chassis can handle. One rear tire (16-inch) gave one staffers
    XS650 11,000 miles before needed replacement. If you must ride in the
    wet, we strongly recommend Dunlop K181s. These tires are magical in the
    rain. They probably wont last as long as the Contis, but they will make
    you last longer in the rain. I have used ACP balancing fluid for four
    years and have never had a balance problem during that time. Tires wear
    evenly, and even at 130 mph I have a smooth ride.
    If you mean reliability then yes. Performance, especially sucked. I also
    owned one and 4 more Yamaha's from that era plus or minus a few
    years including a 81 750 Seca and still the 650 was the best handling.
    That's for the 1979 model. I can't speak for any other model that was
    made from 1970-1984.

    Engine performance can be easily improved and pretty cheaply by just
    putting pipes on, dropping the jet needles and using aftermarket air
    filter element.
    Nope. Not at all.

    How about you - Feeling like a condensing arse with tunnel vision
     
    Ron Gibson, Sep 15, 2007
    #15
  16. I really, really hate to throw this back in your face, but the bike they
    used in that write up was not the Custom. It was the ordinary roadster.
    Which proves my point - it was the roadster version which handled
    reasonably well, and not the Custom. I've actually got a photocopy of
    the original article (I bought th mag at the time) and the picture
    shows....

    ....well, it isn't a Custom.
    Nope again. I've pointed you at an article in the most authoritative
    classic bike mag in the world. I've pointed out that the Custom was not
    a good handler, and to back up your argument, you've chosen to point to
    a piece that wasn't written about the Custom.

    Your lack of knowledge is showing, I'm afraid.
     
    The Older Gentleman, Sep 15, 2007
    #16
  17. Dave

    Ron Gibson Guest

    Oh I'll lose a lot of sleep over that. Perhaps I'll return my Mechanical
    Engineering degree to my university.

    And I'll have to travel back in time and tell that bike not to handle so
    good.

    Opinions are like assholes, and sometimes so are people. They both can
    stink up a conversation.
     
    Ron Gibson, Sep 15, 2007
    #17
  18. Dave

    Ron Gibson Guest

    Try reading the thread. I qualified my remarks and remained civil until
    some self anointed expert told me what I observed empirically didn't
    happen. That is the height of stupidity. Appears to me you got a personal
    problem with engineers.
    Look dumb ass, I can't help it if you didn't want to make the
    sacrifice to learn something. If not for Mechanical Engineers you'd be
    pushing a donkey down down the road at 2 MPH. Do you even know what
    mechanical engineers do, what their expertise is in? I doubt it or your
    post wouldn't be so ignorant.

    So In summary FOAD. I'm not impressed with your lack of manners or foul
    mouth. Maybe it's the source of your problems.

    Grow up and get some manners cretin.
     
    Ron Gibson, Sep 16, 2007
    #18
  19. It's no substitute for ignorance, I'm afraid. And trying to use a link
    to a different bike to back up your argument merely reinforces your
    appearance as a know-nothing.
    Or ditch the rose-tinted specs.
    Well, that was your approach, sweetie.
     
    The Older Gentleman, Sep 16, 2007
    #19
  20. Not self-anointed, I'm afraid. Did you look at the link I posted?
    It didn't. And you tried backing it up with a link to a different bike.
    In your case, yes. Especially as the link detailed everything they had
    to do to make a stock roadster (not even a Custom) handle well.
    Appears to me you assume too much.
     
    The Older Gentleman, Sep 16, 2007
    #20
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