Mysterious flat at Quassy Half

kmepham

I raced Quassy Half on Sunday.  My rear wheel was a PT-equipped Mavic Open Pro, disc cover, Vittoria Open Corsa EVO CX 320 TPI Clincher (2nd-ever ride) with Vredestein latex tube (brand-new).  Pumped it up to 110psi at 6:20 am..  During the bike leg, I didn't notice anything amiss, power output was fine , but when I look at the results afterward I didn't do as well on the bike as I'd expected - I actually lost places from  the swim  to the bike, which never happens, and this is a very hilly course where my low weight (3.6w/kg) should have helped.  Strange.

Stranger still, at 2:30pm, when I'm putting my bike in the car, I discover the rear tire is nearly flat.  Had I been riding on a slowly leaking tire, which had slowed me down but so gradually that I didn't notice it?  The next day I checked the tube for leaks (none found), then put it back on the bike, re-inflated it, put weight on the bike, and a full day later it's still full.  How did it go flat on race day?  I wonder if, in my hurry to get everything set up before they closed transition 10 minutes ahead of schedule, I failed to screw the Presta valve all the way closed, and as the wheel bounced around on the road it jiggled the valve open and shut and let out a bit of air each time?  I can't come up with another explanation.  What do you think might have caused this flat?

Chris G

That does not really happen unless you hit something really hard and then you can usually feel it. Sometimes with the zipp extenders it can, as the presta valve is always open with them. I never screw mine close and that never really happens though. Strange. You may need to ride it as latex can be different, almost self sealing. Try riding it. Might be the kinda thing where it slowly leaks with weight on it but not just sitting there.

kmepham

Yes, maybe I didn't weight it enough for my test. Now I've put 160lbs of weight plates on the saddle, will check back in 4 hours.

kmepham

After 5 hours with 160lbs on the saddle, no loss of tire pressure (and on race day I was only on the bike for 3:15). So the air loss is still a mystery. Next I'll try riding on it, with the valve open.

mancona

Was there a big change in temp or weather conditions during the race day?

I have noticed if the temp changes (usually only for a drop) or if there is a pressure change due to weather fronts moving through latex tubes will loose a good bit of air quickly. However it is usually both tires not just one.

As Chris mentioned I did have just one tire low after my race on Sunday but it was due to a very hard impact with a pothole that send both my bottles airborne and knocked me right out of my aerobars so it was VERY noticable.

I've also had bad experiences with the zipp style valve extenders that go over the open valve core. For tht reason I run challenge tubes with removable cores now and move the core to outside the rim so I can tighten and loosen it by hand. In theory you should be able to leave them open and not have issues but I just like being able to close them.

Coach Patrick

How was your actual split and pacing on the bike? We can see if you were putting out 250 W for 5' early and later in the day and, if conditions were similar, can compare the output MPH. I am guessing it wasn't an issue as you would have noticed that when racing (or at least I would have!)

HayesSanborn

Any chance you just miss read the tire pressure in the morning? I rarely close my valve's, that really wouldn't impact the tire pressure unless you hit something major.

A slow leak would have been dublicated. My WAG, you just under inflated on race morning.

 

kmepham

@Hayes, since I haven't been able to duplicate the leak, and you've all reassured me that an open valve wouldn't cause the problem, a pumping mistake on my part seems seems the best remaining explanation. But it would have to be that I missed the rear wheel entirely (not underinflation since I grip the tire with my thumb to hold the 90-degree adapter in place), which would be especially spacy on my part since I know I turned the bike around to get at the rear wheel because I knocked the helmet off the bike next to mine in the process, and then I remember wondering what PSI to use on the tire and not having time to look. It's hard to imagine that after all that I would have skipped the actual pumping, but I was in a hurry since they'd just announced they were clearing transition 10 minutes earlier than scheduled so who knows.

I guess the lesson I'm going to take from this will be to quickly grip-check the tires as the last thing I do before clearing transition, and then again in T1 and anytime I dismount the bike for a pee break - at least until I've rebuilt my confidence that this isn't going to recur.

mancona

Another possibility is that you didn't have the valve open enough when pumping and the pump hose filled up to whatever PSI the gauge showed but none of the air actually went into the tube and was just trapped in the hose. I know someone who did this a few years ago ;-)

kmepham

@Matt, that certainly could have happened if I was pumping a regular wheel, but with the disc cover I'm pushing down on the tire with my thumb to hold the 90-degree pump adapter on the valve, so I wouldn't have missed how underinflated it was. If it was operator error, it must have been me failing to pump the tire at all. At least that should be an easy mistake to prevent in future LOL.