Most bolts on bikes are using hex sockets. However, there is at least one exception: the 6 bolts of brake discs, which usually have Torx sockets.

What is the reason for this? Has the Torx socket a significant importance in this area in particular?

Related question: I'm considering replacing these bolts with titanium version (saves 6 grams per wheel, woohoo!), and I can buy them with either hex or Torx sockets (hex would be a bit more convenient as I have hex keys readily available everywhere, on the other hand, the Torx socket is technically better). Can I just pick the socket I prefer, or would you strongly advise for Torx?

  • Refs this related question I just found out: Is there anything special about rotor bolts? May 9 '20 at 3:33
  • There may be warranty and liability issues because the brake was designed like that and it is a safety relevant item.
    – Carel
    May 9 '20 at 7:25
  • That related question was me. I haven't had any issues with my stainless socket hex head bolts (though I have had other brake trouble). I've also never needed to remove or tighten a rotor on the road and I have plenty of Torx tools at home
    – Chris H
    May 9 '20 at 13:53
  • Because everyone already has Allen keys. May 9 '20 at 19:23

The magic of lobes allows Torx to handle a given torque value with a shorter bolt head than the alternatives, and some frame/fork designs need the clearance in this area. That's the only reason for it.

Some secondary bolt retention designs won't work with just any bolts, ie Shimano.

  • Could we say that, basically, the Torx socket is here to help preventing the key from ripping off because of the small button heads? May 10 '20 at 1:38
  • 2
    Yes. Or in other words, the M5 Allen option for a similarly low-profile bolt head would be a button head screw with a 3mm Allen fitting. That would be borderline for the ~6Nm torque needed. May 10 '20 at 1:46

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