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I'd like to install a bullhorn handlebar on my existing hybrid bike and would prefer to continue using my flatbar Shimano STI R42O gear/brake lever system rather than a bar end brake/shifter solution. I have a 2/8 gear configuration and caliper brakes.

I've been researching it for a couple of hours now and it seems as though this may not be possible. Why? Is it to do with the space required for mounting the brake/shifter? Or is it simply down to the market not providing compatible bullhorn bar and shifter ring diameters?

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You need to determine the diameter of your bullhorn bars where the shifters will mount.

Then find out the range of diameters that the shifters can fit around from the manufacturers specifications.

If they don't fit safely then you need to find components that do match, either chunkier bars or shifters with smaller brackets. Without more information in the question there's no way to answer whether they are compatible.

However the 'width' of bullhorn bars is much less than typical flat bars so the lack of space along the handlebars may inhibit your hands/levers/shifting.

There simply might not be enough space to fit the levers, which is why this isn't done more often

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If you have flat bar shifters, most bullhorns have a different diameter than flat bar shifters use. But you can find bullhorns which have the same grip diameter as flat bars and clamp the shifters on (though these are comparatively rare). Then you have to make sure the ergonomics work for you.

My suggestion if you want a few more hand positions is to buy some bar ends. Ergon makes some fantastic ones, albeit a bit pricey.

Edit: I saw STI shifters, which normally is just used for the drop bar brifters, so the remainder of the answer may be useful for that.

The bullhorn diameter has to match (and most bullhorn bars satisfy this, though you can also get bullhorns for mountain diameter). Worst case, you can always cut a drop bar to get a bullhorn.

The bigger problem though is that you have to shift -- the STI shift motion is pretty hard to pull off on anything other than a standard drop bar. The cable routing is a bit weird, and its not necessarily the case that you'll be able to fully use the brake lever when using STI shifters depending on how its installed (actually thats probably the biggest problem -- you don't need shifting but you do need good braking). Also, the ergonomics of a STI hood were designed for a drop bar

People have done this though and some people like it. Bar ends and aero brake levers are neater to route and likely easier to shift/brake with though.

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I can't find that model of shifter, but presuming it's sometihng like this Shimano st-8s20, I'd say your problems would be:

  • The shifter parts would not line up or be easily accessible from the position of your hands on the bullhorns. That might not actually be too bad if you change gear infrequently and they were not positioned so that using them would be dangerous.
  • The cables would extend outwards from the end of the bullhorn handlebars. So you'd have two large almost spring like attachments extending from each end of the bars and arching round to where they join onto your frame: front brake; downtube/toptube etc. Which would have what I would imagine to be a detrimental effect on handling, especially considering that it could change depending on what gear you're in, or whether you're braking or not.

Point two could be mitigated by routing the cables back to the stem and cable tieing them there before they go onto their end destinations.

But, if you've decided that bullhorns are the way you want to go, and your budget would stretch to bar end brake/shifters, then you could always buy the bars and try out your existing STIs to see whether they are actaully impractical before purchasing the bar end solution.

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