The video shows someone installing a gps bike tracker, however there is not much room in the top of the head tube, so he hammers down what's already in there in order to make room for the tracker.

In this video, (starting at 1 minute 30 seconds)

I wanted to make a custom gps tracker for my bike as I'm into electronics, but I have no idea what this 'hammering' does. What is this part he's altering? And is it safe to do so?

  • It is used to put tension on the headset bearings. Read up on how to install a fork and tension the bearings.
    – paparazzo
    Dec 4, 2015 at 22:04
  • You may be better off with starting out with using the seat tube. There a bit of fabrication in the headset implementation.
    – paparazzo
    Dec 4, 2015 at 22:44
  • A simple trick to hummer it with no problems: Check that your stem is well tightened. Then even if you release the upper bolt, the bearings will stay at the tension. So you can remove the bolt, check it's length and hummer the star nut bottom, but be sure to check that the bolt have enough length to get tighten back. insert your GPS and put back the bolt. Tighten it just for the case that you will lose your stem some day. Don't be afraid of bearings: you can't harm them while stem is tightened.
    – Alexander
    Dec 6, 2015 at 13:17

3 Answers 3


What he hammered down is the star nut. It's a gription (yes I made that word up) device that serves as an anchor point in the steer tube to allow the top cap to properly compress the headset during a headset adjustment. Old or damaged star nuts are often driven all the way through the steer tube to remove them (that's why your steer tube is open at the bottom). They are generally only hammered in a short ways because being longer serves no purpose. In this case, so long as that GPS unit is as stiff as a steel bolt and allows for proper compression during an adjustment, it should be fine.

  • 1
    Good ol' gription devices.
    – zxq9
    Dec 5, 2015 at 4:19
  • I was going to say something concerning "uni-directional", but the fasteners used on carbon steer tubes are not. However, they are still gription devices. Dec 5, 2015 at 4:37

The hammering is to make room for the unit. The purpose of the bolt is to compress the bearing on the headset.

I would not use the unit for compression.
I suggest:

  1. Remove unit
  2. Loosen pinch bolts
  3. Use the bolt (shown in video) to put proper compression on headset bearings
  4. Tighten pinch bolts
  5. Re-install GPS unit

Mainly I suggest you start with a seat tube based unit.

Headset is a lot of fabrication. In a headset during test you could just let the unit rattle around (pack it with some something).

  • 3
    The bolt that runs through the unit is attached in tension, not compression. The cap and headset bearings will be under compression, not tension. That said, the amount of torque being applied to the bolt is minuscule — probably not even 1Nm. Thus the tension on the bolt and compression on the cap are extremely minor. Once the headset is properly compressed, it is locked in by tightening the pinch bolts, so the unit won't experience forces during riding. The extra steps you suggest seem unnecessary to me. Dec 5, 2015 at 0:12
  • @StephenTouset Tension / compression - feel free to edit the answer. I think people will get what it means. So the steps seem unnecessary to to you. Me I will use a bolt to put tension / compression / or what ever you want to call it rather than (chance) stressing the unit every time.
    – paparazzo
    Dec 5, 2015 at 0:47
  • This method would work quite well AND avoid damaging the unit or producing a dodgy headset adjust. However, I would use an actual star nut tool (and not the long bolt) to set the start nut deeper to start. Star nuts are not always easy to hammer in, and there would be a high chance of bending the bolt. Dec 5, 2015 at 4:35

The part he hammers down is called the "star nut". It's purpose is the attachment point between the fork and the headset. Having it moved that low, and something inserted between it and the top cap seems like a bad idea.

  • 4
    Doesn't seem like a bad idea to me. The purpose of the star nut is to "grip" the inside of the tube as it's pulled upward by a screw. Where it does so inside the head tube seems largely immaterial, as long as you have a screw long enough to reach it. Torques on this bolt are minimal. That said, you will want/need a star nut tool to ensure the star nut is kept straight. Winging it like this guy does it seems like a poor choice. Dec 4, 2015 at 21:03
  • 2
    @Stephen Touset So to clarify, it doesn't really matter how far down the star nut is, just as long as it is actually connected? How pivotal is this piece? Will all hell break lose if it becomes unattached? Dec 4, 2015 at 21:20
  • 2
    Look at what a star nut is. It's meant to jam up against the inside of the steer tube (I mistakenly said head tube earlier) as a screw attempts to pull the center upward past its "wings". There's nothing to detach. It's literally hammered into the tube in the first place. The important part is tightening the bolt to properly tension your headset when you reinstall the cap, such that you find the sweet spot between the headset binding and there being too much play in the fork. Dec 4, 2015 at 22:20
  • 1
    The star nut is only used to properly compress the headset during adjustment. After that (and the stem is tightened to the steer tube) you can actually remove the star nut, bolt and top cap without any ill effects (other than having to reinstall them all if you need to do a headset adjust). The star nut bolt and top cap are not mechanically necessary parts after the headset is properly compressed. Dec 5, 2015 at 4:32

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