My experiece was just over 4 weeks ago in a velodrome. Coming down the banking i went to go sit in behind another persons wheel but instead crossed his wheel by about 4 inches. I was aware what was happening so used my upper body strength to push of his rear wheel to hold myself up. Hearing my wheel rub his he accelerated away.
The result was me hitting the deck all tensed up snapping my collar bone first then my head hitting the ground smashing the left side of my helmet into pieces. I then proceeded to roll and slide with my whole body weight(90kgs) on the left side of my head at one stage(lucky i got a rugby neck).
If i hadn't been wearing my helmet i would probably have a cracked skull and missing all the skin on the left side of my head.
I've always worn a helmet and considered that if it wasnt law here in Australia i wouldn't wear one. That has now changed.
Im now in the process of getting a new helmet through crash replacement policy.
Just my two cents
