Stem top bolt wouldn't screw in after transport

Question

I transported my bike via plane in a canyon shuttle 2. It was well secured and safe. To pack it up I had to unscrew the top stem bolt and unhinge the stem along with the handlebars.

Once I attempted to reassemble the bike it turned out that that stem bolt is now too short. I have found that the inside of the tube onto which the stem is mounted contains a 6mm allen key thread. I unscrewed the thread as far as I could which is about 4 or 5 mm and it went upwards until it stopped due to some block. After that I was able to mount the stem and the screw was able to catch the thread. I think I got about 4Nm or maybe 3 until I couldn't screw it in any further.

The bicycle seems fine. However I am concerned that I did something which is going to put me in danger. As tomorrow is the only day I got left I really want to go for a long ride.

Is what I did correct? Is it safe to ride like that?

Answer 1

It sounds like your steerer plug dropped down slightly during transport. This would indicate it wasn't torqued sufficiently. This could be an issue of maintenance or it could have come from the factory like this if it's quite new (no hate on Canyon, mistakes happens with all brands and preassembly).

I asked the brand and model because there are different designs for expander plugs. An expander plug plays the same role as a starnut, but you can't use a starnut in a carbon steerer as it would compromise the steerer. The expander grips the sides of the steerer by expanding uniformly and putting pressure over a larger area than the point connection of a startnut.

The general procedure would be to have the spacers and stem on the steerer (stem not tightened), then the plug is set at the correct height (depends on particular plug/expander but needs to be high enough for the top cap bolt to thread in). The expander is then expanded to the correct torque spec (probably in the range of 5nm, though it's important to look at the specs for your particular type). You want the spacers and stem in place so there is no chance of expanding the steerer. The correct height is important so that it counters the compression of your stem.

Next you would pretension the top cap to remove headset slop. Finally tightening down the stem bolts to the correct torque for a carbon steerer. If available I use friction paste between clamp-on parts and carbon parts (e.g. stem/bar, stem steerer). There is less chance of movement with the lower torque you are allowed to apply to a carbon part (as opposed to alloy).

Since you are travelling I'd recommend popping into a shop that services road bikes. It should be quick and cheap to adjust the expander and torque stuff down to spec (and they'll likely slap some carbon paste on there too).

Unfortunately I wasn't able to find the particular model of expander Canyon uses with a quick google search. There may be info in the manual for you model bike.

Answer 2

When you say "top stem bolt" what I am assuming you mean is the bolt used to preload the headtube bearings while aligning and tightening the stem to the steerer tube. That top stem bolt is not what clamps the stem to the steerer tube, the bolts that clamp the stem around the steerer tube are what attaches the stem to the steerer tube of the front fork (the handlebars attach at the other end of the stem). Once the stem is clamped to the steerer tube, you actually can remove the top stem bolt and cap - it has done its job (no one does this, but in theory you can remove it).

The steerer tube plug inside the steerer tube is what the "top stem bolt" screws into. In a carbon steerer tube this plug is tightened to wedge into the inside of the steerer tube and serves two purposes.

• The plug provides a connection to the fork steerer tube that the "top stem bolt" can pull against to preload the headtube bearings. Think of it like squishing the layers of a sandwich together to remove slop/play in the headtube bearings. This is done before aligning and tightening the 2 (?) stem clamp bolts to spec (what actually holds the stem to the steerer tube). You DO NOT want this top stem bolt too tight. It is not for clamping the stem to the steerer tube. If it is too tight, then the bearings would be overly loaded and you will reduce the life of your bearings, and the steering would not freely rotate either.

• The plug provides a little back support on the inside of the carbon steerer tube counter the force of the stem being clamped to the steerer tube. So location in the steerer tube is important for it to serve this function. Fortunately the common length of the top stem bolt makes this positioning pretty self-serving. The bolt is not so long that it would allow the plug to be much below the stem, and if the bolt is too short, it would not be functional.

Because you mentioned that the top stem bolt would barely reach the steerer tube plug (what has the threads to receive the bolt), indicates that the plug may have moved lower in the steerer tube. The 6mm hex on the steerer plug is what tightens (wedges actually) the plug into the tube. The 6mm hex needs to be tight enough to hold the plug from twisting in the tube and from moving up and down in the tube. Do NOT over tighten this plug, it just needs to not move/twist and do its job.

• If it is too low in the tube then you may not be able to engage enough, if any, threads of the top stem bolt AND the interior support that the plug supplies against the stem clamping bolts would not be high enough to do so.

• If the plug is too high, the stem top cap can bottom out on/make contact with the plug before it fully preloads the headset bearings.

Because this is a potential safety issue, if you are not 100 percent sure of what you are doing on this, seek a local bike shop for some hands-on advice.

Also, when transporting your bicycle, you do not need to remove the top stem bolt at all. You just need to loosen the two stem clamping bolts enough to allow the stem to rotate around the steerer tube. I would slightly tighten the stem clamping bolts a little once the new positioning for transport is made, just so they do not back out further due to vibration in transport and get lost in the packing material.

Answer 3

The top bolt is there for preloading the headset before you tighten the stem, it doesn’t provide the clamping force on the steerer.

Provided the two side clamp bolts on the stem are up to spec (normally 4NM) you will be totally safe.

You can test that the pre-load on the headset is sufficient by applying the front brake hard and rocking back and forth on the wheel. There should be no clunking feeling or sound. At the same time the steering action should be smooth and not gritty or notchy. If it was, that indicates there’s too much pre-load on the top nut.

When setting up your stem you don’t need to tighten the side bolts all the way until you get the pre-load correct. Just enough to lock the stem in place. It’s not an exact setting. It can be a matter of a 1/4 turn on the top bolt between right and wrong. You’ll get a feel for it after a few times.