My legs hurt
Join Date: Apr 2008
Location: Farther behind you than I'd like to be
Bikes: Vaya, Brompton, '73 Schwinn Super Sport, Cresswell Fold-it, '81 Trek 610
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If you want to fit bullhorns, there are a couple of issues you need to deal with.
I'm assuming Downtubes use a mountain bike stem. The clamp size on these are 25.4 mm. Most bullhorn bars are 26mm. You might be able to put a larger bar in the smaller stem if the stem has a removable face-plate, if you really tighten the bolts. Personally, I'd prefer to use the right size stem.
The other issue is the shifter / brake levers aren't likely to fit on the bullhorns. So, you are likely going to need bar-end shifters, and brake levers that will fit on the bullhorns and work with V-Brakes.
All in all, it's fairly involved conversion, but certainly doable. I did this on my Xootr Swift, and love it. You might want to see if you can live with bar-ends mounted to the existing bars. Much easier and less expensive.