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Just completed a 65 mile sportive on my basic Trek 29er with knobblies. Still got around in 3hours 40minutes +/- Quite shocked as I honestly told everyone that I'd just sit off the back, but ended up doing alright. The big big big downside is you can't get aero, in a headwind you might as well be peddling a shed.
Only did this on the MTB as there was a lady in my office doing the same ride and it was going to be her longest hardest ride, and I thought I'd look a tit taking it seriously when the were mums and dads escorting toddlers on the short routes
The mums and dads liked my saddle cowbell, the roadies hated it!
Which in a roundabout way brings me to the question 'what road tyres for a 29er'.