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Are they neccesary or are they in the nice to have category?
I'm assuming you mean tri-specific cycling shoes rather than regular run of the mill road cycling shoes. Normally if the prefix tri- is attached to cycling it seems to be an excuse to jack the price up by a few quid. That said I see wiggle are now doing some bargain bucket ones (http://www.wiggle.co.uk/dhb-t10-triathlon-cycling-shoe/). At the end of the day you want a shoe that is quick to get into (single velcro fastener and perhaps a heel loop) which you can find in regular road shoes.
If you're meaning clip in shoes with special pedals over regular shoes and pedals then definately go for it (although you are guaranteed to fall over at some point when on the bike and you forget to unclip).
I've just taken delivery of the dhb shoes with the carbon soles, so I'll let you know! I was in the market for a new pair anyway and they seem very good VFM, if not the flashiest looking shoes you can get.
I notice that tri-specific shoes are designed not just for easy get-on-and-offability but also for the natty trick of having them attached to your bike for quick transition; they'll have a loop at the back for pulling on more easily, but also for attaching an elastic band for this clever little trick:
(Not that I've tried it yet. Probably another excuse for a comedy f*ck-up...) Also note how the velcro tab fastens the opposite way to a normal shoe so that it doesn't tangle with the bike when unfastened.
If you're serious about triathlon and every second counts - then get the tri shoes.
If you're not at the pointy end - normal cycling shoes are fine. I only race long distance tri - so I use normal road shoes. Its not worth me getting a pair of lightweight race shoes for just 6 hours use per year.
Thanks for the responses, I meant tri specific cycling shoes, my road bike shoes can be a faff to fasten with the ratchet system they have
Also found a bargain on the shoes, so plumped for some subtle BRIGHT SHINY SILVER shoes to anounce my arrival! Me gear tart....if the cap fits