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Recently I've installed a front derailleur and while doing it used nearly all hex screwdrivers from my multitool. For example, on the left hand of the handlebars I have a brake, a shifter and a fork lockout. To unscrew them I have to use 3 different screwdrivers (the one on the grip is the same as on the lockout). Needless to say that to put a cable on the frame I have to use different ones.

handlebar view

What is the reason for that (except for the multitool manufacturers conspiracy)?

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  • If manufacturers prioritized standardized fasteners there are opportunities to move to fewer sizes. Priorities are set by market factors. The fact that some don't standardize means that it's not a priority - meaning the market factors don't drive them to fewer sizes. – David D Feb 6 '20 at 20:21
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Generally, bigger sizes of screw heads allow higher torque to be applied to them. This is also often expressed in lengths of wrenches used for them in workshops: typically a 3 mm L-shaped hex key is shorter than a 4 mm one, which in turn is shorter than 5 and 6 mm ones etc. Multitools are exceptions because of size constraints of tool bodies; it is typical to see a 8 mm hex head on a multitool with inadequately short leverage.

An unnecessarily big screw would also mean more "dead" space is needed in the parts such screw connect. These parts are collars of brake levers, derailleur shifters, grips etc. "hugging" the handlebar's tube. Collars interfere with each other, preventing you from finding the most comfortable position of e.g. brake lever. By the way, this is why top-level brake/shifter combinations basically use a single collar and are connected together (Ispec, MatchMaker etc.)

Bigger torque is needed to counteract bigger force × leverage applied to a specific component. For handlebar components, forces are produced by user's hands/palms and have the same order of magnitude regardless of which component is being "touched". The applied force being fixed, it is the leverage (in centimeters) that should decide achievable range of user-supplied torque and thus required screw-supplied counter-torque.

  • Lock-on handlebar grips tend to have the smallest screws (1.5 - 2.5 mm) because grips sit very tight to the handlebar. Leverage applied to them is defined by their thickness and is an order of 1 cm.
  • Shifters typically have relatively large paddles sitting far away from the handlebar's axle, let's say 3 cm away. A 4 mm screw would seem to be adequate to hold them securely.
  • Brake levers have dimensions and leverage comparable to shifters'. Thus, screws of the same size should suffice for them. However, brakes are more critical to rider's safety, and them rotating around the handlebar is even less desirable. As an additional precaution, they are installed with higher torque and thus bigger screws (5 mm).
  • Fork lockout control is smaller than both shifters and brake levers, but bigger than grips, so I would say it may have a screw somewhere in-between (2.5 - 3 mm).
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    But why can't all items on the handlebars (ok, except for grips) use the same size - the biggest one? Or nobody cares? – k102 Feb 6 '20 at 9:22
  • As I've answered above, bigger screws means wider collars. Wider collars mean that brake levers cannot be moved closer to grips to achieve the most ergonomic position; or shifters cannot be moved farther away from the grips for those who prefer them be that way etc. etc. But I guess yes, nobody cares enough at the price point of such components (again, higher end components provide more integration and adjustability with fewer collars) to go after any sort of standardization. Given that there are even more bolts and bolt sizes on the rest of the bike, I am just glad I don't have to use Torx. – Grigory Rechistov Feb 6 '20 at 9:35
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    @k102 Sometimes it is just random. The bolts for the saddle clamp and stem are often 4 mm, sometimes 5 mm. The bottle cage bolt can be 4mm, can be 3mm. It is just what they choose to put there, but you are free to choose your own bolt with the same threading but a different head. You can put there a torx instead, if you wish. Just use the right torque setting on your torque wrench. – Vladimir F Feb 6 '20 at 13:37
  • @VladimirF I have one bike with all "auxiliary" bolts (bottle cage, rack, fenders etc) to be low-profile torx. I tend to replace them with regular hex-headed bolts whenever I actually install anything to their mounting points. I've heard about people doing the other way around, going with torx everywhere it is possible on their bikes. – Grigory Rechistov Feb 6 '20 at 14:18
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    Different head shapes also have different standard hex sockets for the same thread (countersunk<button-head<cap head, though bikes sometimes use non-standard button-heads). This is why bottle holder screws (for example) can have different sockets on them. Brake levers need to be more securely mounted to the bars (stronger clamp/bigger screw) than lights, bells, or even shifters. If every bracket on the bars was big enough to take an M6 cap-head you'd run out of room for accessories – Chris H Feb 6 '20 at 15:52

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