pyf wrote:
Does anybody need to tension the chain with a rear derailleur ? Because that's what horizontal dropouts are for ;-) !
Seriously, there is no reason for horizontal dropouts on a TT bike, it adds weight and complicate something that does not have to be complicated. With a well designed seat tube the tire has to be almost a centimeter away from the seat tube before aero is ever so slightly compromized so if you design the frame with vertical dropout so that a 23mm tire rolls at let's say 5mm from the back of seat tube, then a 25mm will be at around 3mm and a 21mm tire will be at 7mm and aero will be ok in all cases. The time when people thought the rear tire has to rub (or almost rub) the paint of the seat tube is over (so I hope). 99% TT bikes users will use tires from 21mm to 25mm so I think vertical dropouts are best for TT bike application, again IMHO.
If you are willing to compromise, you are right, vertical dropouts are acceptable. 3mm tire clearance however is not ISO/EN compliant so these bikes could not be sold in Europe with 25mm tires. Even 23mm with only 5mm of clearance is a no-go in Australia.
If you aren't willing to compromise aero performance and you want to remain CPSC, EN, ISO, JIS, etc, etc, etc compliant then horizontal dropouts solve these issues.
-SD