Spare tire for IM???

i don't carry spare clincher tire but if i did i'd attach it to the back of my seatpost.

I am thinking about putting one of my old tires in my special needs bags just in case, but I seriously hope I do not need it. I do not plan on stopping for my bike special needs bag.

No

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Nope only carry two tubes and two CO2's.  I put extra tubes and CO2 in my special need just in case it is one of those days.

I had a friend break a spoke at IMLP in 2007 and bike support actually swapped her entire wheel out and gave her a loaner for the rest of the day.  She had to wait 25 minutes on the course for them to get to her, but her problem was fixed.  After the race she had to give them back their's and got her busted one back.  My guess is if you shred a tire that badly that bike support will probably do something similar for you, but I'm not 100% sure. 

I ride tubular and last year at L.P.I carried 2 on the bike and put 2 in my special needs bag and I will do the same this year. A bit much, but better to be safe than sorry. Last year I got 3 flats.

I carried two tubes as well.  Had my normal gear bag and just threw a tube in my jersey pocket, since there was nothing else in there.  No weight issues plus a little additional insurance

I broke a spoke in my last race with 8 miles to go.  I twisted it in the spoke next to it, opened up the front brakes, and kept riding.  It was rubbing a bit, which probably slowed me down a little, but i figured the faster i went, the less friction it would cause.