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Crap dodgy tyre valves leaking air at speed!

Discussion in 'General Motorcycling Discussion' started by [FLUX], Jan 22, 2006.

  1. So, went and did the Superbike School thing at Phillip Island today.

    Now the thing with the school is that you're not really there as like a track day to go flat out. The purpose of the day is to ride a5 or 6 tenths to practise the cornering drills, to get a good grounding in the techniques that they're trying to teach you, and then you can take that away, practice, and improve, both for the road and at any future track days.

    The first 2 sessions went well, and in the 3rd session (of 5) we're starting to pick the pace up a bit, so I take liberty with the throttle down the main straight as you do, and peel into turn one, and then the bike's steering feels a little sluggish, and I have to push on the inside bar hard to hold a line. Wondering if I'm just imagining it I continue around, back down the main straight again, and bike feels horrible through turn 1, and terrible through Southern Loop.

    Pull in, check the pressures, and the front tyre has like 5PSI in it!!

    Check over the front tyre, and there's a slightly suspicious thing in the tyre, so I pay an overpriced visit to the track tyre guy and get new tyres, since the front appeared to be punctured, and the rear was about 85% worn anyway. Always better to get new tyres in a matching pair.

    Anyway, fit a new set of Dunlop 208RR's on. Head out for the next session, first 6 laps doing well when practising the drill and I start picking up the pace again to practise the drill at higher speeds, and again! Bike's handling goes all sluggish. Pull in straight to the tyre guy, and we can barely even touch the front tyre it's that hot!

    Wait a few minutes, check the pressure, 0 PSI!!

    At this point my mind is boggling at the thought that I was peeling into turn 1 at ~180kph (at a guess) with 0 PSI in the front tyre...

    Tyre guy inspects the tyre with a fine tooth comb, can't find anything, and suspect that it's a cracked rim. I kept saying though that the tyre wasn't flat when I started, that it was fine for 5-6 laps, yet only started going flat when I went fast (i.e. >250kph down the straight).

    I asked about the valve springs. Could the valve springs not be holding the valve shut at speed? He says it couldn't be that. I tell him to humor me and offer to pay him to replace the valves. He refuses the money but replaced the valves anyway (in both tyres) to prove his point. Matter of tyre master pride here I guess.

    I take a look at the front valve and notice that even the slightest amount of pressure on the spring, which was very weak, would cause it to lose its air seal. I say it must be the problem. Tyre guy doesn't believe me, saying he's never heard of such a thing.

    Anyway, head out for the 5th and final session, which was a total hoot might I add. Able to use all the gears and basically set a track day pace, but this time with the added confidence of all the drills at hand.

    I'll digress here. Had a friend time me, and managed a 1m56s lap, and managed to do so without even touching a knee down anywhere, and using WAY less lean than I was ever using when I did my last track day time of 1m59s. This just shows the value of the training. At the last track day I always wondered how others were going faster than me through corners when using less lean, and now I finally understand just why that is, and how it works! I won't attempt to offer the advice/tips here. It really requires the tuition and hands-on experience to get the most out of it.

    Anyway, the tyres stayed to their true pressure the whole way through the final session.

    It WAS the valves. At >250kph, the centrifugal force was pulling the valve spring enough to lose the air seal and deflate the front tyre. Tyre guy was surprised and amazed, and put that into his bag of possible things to check in future whenever people complain of deflating tyres.

    Anyway, had a fantastic day! Can't wait to do a track day soon to put the skills to practise, push it even harder to the point where the knee will start touching down again, and see what I can do outside of a 55C track temp with near gale force winds pushing the bike wide at every point...
  2. Sound like u had gr8 fun!

    How much is it for the day?
  3. yeah sounds like a great day ..... but i think the tyre guru needs a wake up call , something as cheap as valves in my mind should be changed everytime you change tyres , it was standard practice with car tyres
  4. Man, I had a great day. What surprised me the most about the final session was that I was setting that time, and honestly felt like I wasn't even trying. The skill drills really manage to get you to explore the correct way to do things on your own terms. In the morning session I was dragging the knee through about 6 corners on the track, but by the last session I was going even faster than I've ever gone before, felt completely relaxed like I could do that sort of thing all day (55C track temps and severe dehydration notwithstanding), and probably only threatened to touch the knee down once (knee down purely being an indicator of lean angle).

    The day costs $399 normally, but today it cost $419, purely because it was a "Keith Code" special day, meaning that we had Keith Code there himself giving the training for the whole day, and I guess he charges a little more for his time. ;)
  5. How much will it be next time if u just riding?
  6. Ive seen the same thing mentioned somewhere before, Im supprised tyre guru didnt think it was possible.

    Isnt this one of the reasons for using good valve caps? They provide a second seal so even with complete valve failure nothing will leak.
    The valve stems should also be replaced every couple of tyres.
  7. do the dice type valve caps qualify as good quality ?? :LOL:
  8. A regular ride day is $189, if you bring your own lunch. That'll give you 6 x 20 minute sessions, or around 50-70 laps (~250-350kms) depending on how fast you are and how many laps you can fit within 20 minutes.
  9. Yes, when I bought the bike second hand, it had some "glam" model blue anodised valve caps on it to match the bike color, but the valve caps had zero air-seal potential.

    Tyre guy gave me a pair of new valve caps with a proper secondary seal, which I think I like better anyway (style wise) 'cos they're a polished metal type which matches the nickel plated rims (ah - the curse of style eh?), so I was actually happier with the replacements. They were lighter too.

    So definitely ensure that if you're out on a track day:

    1) Ensure that you have good quality new(ish) tyre valves installed
    2) Ensure that you're using valve caps that offer an additional secondary seal

    My mind just boggles at the danger of the situation. I told my instructor on the day about it, and he said he had a flat front tyre once which peeled itself off the front rim heading into turn 1 at about 190kph, totally killing the bike in the resultant crash and putting him out of action for a while.

    I was very lucky, it could've been so much worse.