Condensed Answer: Thumb shifters are designed for handlebars that have a 22.2mm diameter grip area. Since modern drop bars are thicker, thumb shifters cannot be installed on them by default.
Nonetheless, it’s possible to modify the shifters to make them compatible with drop bars.
How To Install Thumb Shifters On Drop Bars
- Find Thumb Shifters That Fit By Default
Some thumb shifters have a wide enough clamp for a set of drop bars.
One example of such shifters would be Shimano A050. The model has a 25.4mm clamp and is therefore compatible with a modern drop bar that has a diameter of 23.8mm.
The downside of those shifters is that they’re limited to 7-speeds and some people may find them a bit difficult to use. Also, they don’t have the shape of classic thumb shifters. Nonetheless, most of the work is still done by the thumb.
Dia-Compe Wing Shifters
Dia-Compe produces a thumb shifter installed right behind the hoods. The clamp can accommodate handlebars ranging between 22.2mm and 24mm. This is a premium product designed for 11-speeds.
Suntour Command Shifters
Another option would be the original Suntour Command Shifters which go behind the brake hoods too. Finding them, however, will be quite difficult as they’re no longer in production. Thus, the only available models are sold online for a very high price.
- File The Clamps
One of the ways to make a set of thumb shifters compatible with drop bars is to remove 1.6mm of material from the inside of the shifters’ clamps.
This procedure can be done manually with a file, but it’s quicker and easier to use a rotary tool or a drill with a sanding attachment.
By modifying the shifter, you will more than likely lose the warranty even though the clamp and the shifting mechanism are not dependent on each other.
Another downside is that the clamp will get weaker. If the clamp of the shifter is already thin, it’s wiser to avoid this hack because the clamp may break either on its own or during impact.
- Vintage Drop Bars
Vintage drop bars made of steel are thinner than modern aluminum bars and can accept a thumb shifter designed for a 22.2mm grip area. The downside of vintage drop bars is that they’re heavier and prone to rust. Also, their shape is fairly aggressive and some people may find them uncomfortable to use.
That said, this is a solid option because decent units can be found for cheap online.
- Switch To Bullhorn Bars
Another solution would be to switch to bullhorn handlebars with a 22.2mm grip area.
- Paul Thumbies
Paul Thumbies are clamps that turn non-thumb shifters into thumb shifters.
For example, if you have Shimano bar-end shifters, you can get a Paul Thumbie clamp for Shimano shifters and install the shifters on a set of drop bars.
Note: Paul Thumbies come in two sizes 22.2mm and 31.8mm. If you have modern drop bars, you can get the 31.8mm version and install a shifter near the stem. You could also use a shim and mount the thumbie close to the grip area.
- SRAM Shifter Adaptor
Paul components make a special adapter allowing you to mount any under-bar SRAM trigger shifter to a 31.8mm drop-bar too.
FAQ: What about installing shifters on a handlebar extender?
Installing shifters on a handlebar extender is not ideal for the following reasons:
- The diameter of the extender is about 20mm. Thus, you will need a shim to secure the shifter. (This is a minor inconvenience.)
- You will have to move your arms away from the handlebars to shift. The process is uncomfortable and hurts the bike’s balance.
- Cluttered handlebars are not aesthetically pleasing. People who care about the lines of their bikes might find the final result unsightly.
That said, this approach is technically a solution, albeit not a pretty one.
The Advantages Of Installing Thumb Shifters On Drop Bars
- Mixing Parts
Most thumb shifters are created for MTBs and hybrids. By switching to thumb shifters, you will also gain the opportunity to use MTB mech that isn’t compatible with road components.
However, keep in mind that some thumb shifters are limited to 6-7 speeds. Those cannot be coupled with a modern MTB drivetrain.
- Avoiding Downtube Shifters and Bar-end Shifters
If you have a retro road bike, you might be looking for a way to get away from its downtube or bar-end shifters. Installing a set of thumb shifters on the flat part of the drops is one way of switching to a different system.
Modern brake-shifters are among the most expensive components on a road bike. Meanwhile, thumb shifters are fairly cheap as long as you don’t go for exotic models.
The Disadvantages Of Installing Thumb Shifters On Drop Bars
As already explained, the thickness of modern drop bars makes them incompatible with thumb shifters unless you use alternative models or modify a set.
People who simply want to install their shifters and ride away will probably find this issue annoying.
Of course, there are thumb shifters that fit on drop bars by default but apart from Shimano A050, they’re all expensive and sometimes outprice entry-level brake-shifters which will be more comfortable to use anyway.
- Shifting Only From The Tops
Thumb shifters are designed for flat bars and the like. The only exceptions are the wing/butterfly models mentioned above.
Thus, most people who do this conversion install the shifters on the tops. While this position is fairly comfortable, it still requires you to move your hands away from the brake levers/hoods to shift. This maneuver slows you down and hurts your stability.