Handlebar question
 

When we assembled my son's GMC Topkick we tightened up the faceplate that holds the handlebars on pretty tight. However yesterday the handlebars came loose so that they would not only rotate within the clamp, but slid a bit sideways. Apparently not enough torque on those bolts.

The steel handlebar has metal ridges running parallel to the tube, also steel are the two bolts and the whatever it is called part that goes between the handlebar and front vertical tube. The front faceplate is aluminum. The ridges in the handlebar wore down part of the faceplate to powder. We don't recall if there were originally grooves in the faceplate to match the ridges in the handlebars, but if there were, they are completely gone now, as the inside is smooth except for some scratches.

After cleaning out the dust we put the handlebars back on and applied a lot more torque by hand with the Allen wrench. Should these bolts be tightened with a torque wrench, and if so, how much torque would be appropriate? At the moment they are hand tightened to the point where they no longer rotate and it feels like the hex wrench is just beginning to bend (with the short end of the L in the bolt).

Thanks.

Have you considered replacing the bar and stem? some of the current components from better companies

have the torque specs engraved around the bolt in question.

If you have arrived at a fixed location, loc tite Red on the bars clamp will hold them better

cannot say what the torque spec is , size of bolt, [you can find general torque charts on the web] quality of the steel is un known and if threads are cut in aluminum.. (there is a different torque for dry and greased threads too)

the 'GMC' brand is just paint , someone else, some big box type supplier,
made those, in Asia.. no surprise there..

It is made by Kent bicycles. The specs say that these parts are used:

Stem: A-head TDS63K-8, EXT: 100mm, 15D
Handlebar: HL-MTB 153, W: 600mm, R: 30mm

but I have not been able to find any further information on that stem. It has only two bolts holding the face plate, and googling a bit it looks like better ones tend to have four bolts.

For my son's purposes a handlebar is a handlebar, but it would be nice if the stem clamp wouldn't let go of the handlebar while he was riding!

Is this being built from new? if so, sound defective, and could do with being returned, appears that the alloy in the face plate has been worn away by the harder steel from the handelbar. If you have tightened the bolt on the face plate of the stem to the max they will go by hand, you will most likely only shear them by using a regular torque (1/2 or 3/8 drive ) wrench unless you have a small one (1/4 drive).

For the number of bolts on the stem, this does not equate to quality, the trend now is for 4 bolt, especially for carbon bars, but go back 10 years, the most you got was 2 bolts, and with quill stems, it was normally just one bolt

4 bolt faceplates mean the compression load is shared by 4 threaded aluminum places rather than 2.

the aluminum is the weak spot.

have an Azonic DH stem with 6 bolts, and the thread is coarser
so the thread cut in the aluminum is stronger, too ..

A new stem with 4 bolts should not be expensive. Up to $20.
No need to replace the handlebar.
Take the bike to the shop with you to make sure you get a stem for the correct diameter handlebar.

Would say in this case, if replacing the stem, do the handlebar at the same time, not only will you save a lot of weight going from Steel to Alloy, but will eliminate the possibility of your current handlebar damaging the alloy on your new stem, as the current handlebar seems to have been working like a file.

If the parts had serrations (before they were worn off) that implies that the bite or interlock of the serrated faces were factored into the design, and you will not be able to tighten it enough now that they're worn smooth. You might be able to relate the the difference if you ever tried gripping with pliers whose jaws were worn.

You might try putting some coarse lapping compound (essentially grit mixed in grease) on the parts and clamping with as much force as you can short of stripping or snapping the bolts. (You might be able to beg some lapping compound from an auto mechanic, or engine rebuilder). The lapping compound increases the holding strength by biting into both parts creating a mechanical interloc between them (like spreading sand on ice for traction).

If you try this route be sure to position the bars exactly where you want them before tightening, and telling your son to tell you immediately if there's any slippage. This is critical because any relative movement causes the grit to make small grooves, and lose the ability to hold fast.

If the lapping compound doesn't work, your only other option is to replace both the handlebar and stem (the part from the bar to the fork).

Pasadena, CA? That's where I did my undergrad. :)

And in fact, during that undergrad time (before I got properly into bikes), I rode a real piece of garbage from Target (a Magna Glacier Point) whose handlebars also came loose despite tightening them down. Such is the way of cheap components. Properly made stems and bars don't do things like that.

My solution was to reinstall the bars as well as I could, drill a hole through the handlebar and the stem, tap the hole, and put a machine screw through both components.

That solution got me through undergrad. That bike injured me twice before I graduated, but both times it was from other failures than the stem/bar interface (if I remember the failures were the seatpost and the stem/steerer interface).

Sorry to be so grim, but I really get mad when I think of things that are so shoddily made yet so important for personal safety.

That's the place.

We have been looking at this hardware more closely and even with the face plate clamped down all the way one can make the handlebars rock slightly in the clamp. This is apparently because the splined central part of the handlebars, which has a slightly larger diameter than the surrounding tube, is slightly barrel shaped instead of cylindrical, but the clamp around it seems to be cylindrical. So when one pushes down on one side of the handlebars more than the other, there is enough torque to rotate the splined part within the clamp slightly. This tiny motion acts kind of like a rasp on the inside of the aluminum faceplate and apparently over time removes enough material so that the handlebars come loose.

I don't think that will work here since the handlebars and the clamp do not seem to be the same geometry. If the aluminum face plate was ever splined it must have been over a very small area, as careful inspection didn't show any sign of it, and it these would have had to have completely worn off.

The stem is marked "Kalin", I could not find any identifying marks on the handlebars. I think in this case the problem is actually more with the handlebars than the stem, since the stem does seem to be a simple cylindrical geometry, but not the handlebars. There may be a stem that will hold this particular handlebar design well, but the one on this bike is not it.

I am beginning to share your views about the safety of cheap generic bicycle parts...

Alright. Given that doing what I did is marginal anyway (doubly so on aluminum; my handlebar/stem were steel), your best bet is probably to change both the stem and bars. The stuff holding the saddle and the steering column are really safety-critical parts of the bike, so it's worth it. You'll need to pay some attention to dimensions and sizing conventions to get parts that are compatible with your steerer and with each other. Drop a line here if you get confused.

I contacted Kent Bike's customer support and they are sending a new handlebar and stem. Hopefully the first set had a manufacturing defect of some sort, and the replacements will fit together better.

Received the replacement parts. The handlebar was identical and the bike side of the stem was too, but the aluminum clamp for the front of the stem was a little different.

http://bikeforums.net/attachment.php...hmentid=188108

I think the original ones may have had some grooves, but they could not have been as deep as in the replacement - the original clamp was worn, but not that much. The thing is, although this clamp has some grooves they are not the same spacing as the shallow splines on the handlebars, which are much closer together. In other words, the grooves don't fit into the splines to lock the position of the handlebar, one can rotate the clamp pressed on the handlebar and not even feel much of a clicking as one set of ridges passes over the other.

Anyway, we tried bolting the replacement on and it could still be wobbled up and down slightly when maximally tightened. So we took the new parts to a local bike shop and the fellow there whipped out his calipers and said that the handlebar diameter was 25.4, but the clamp diameter was bigger, around 26.0. He had some stainless steel shims for a 25.4 to 26.0 fit (the shop sells a lot of Italian bikes) and we took them home and reassembled using those. That seems to have helped, I cannot feel the handlebars moving relative to the stem now, when pressure is applied vertically on one side or the other, and before I could.

Hi pasadena_commut,

Glad to hear your LBS was able to solve this with a simple shim. :)

Just to clear up something, I don't believe that handlebars and stems are supposed to have splines that engage in a lock-and-key fashion. It's been awhile since I've looked at a handlebar/stem interface. If I recall, the handlebar had some horizontal texture on it, but the inside of the stem was smooth. This was for a Nitto quill stem and Nitto drop handlebars, which is a rather different setup than yours but a high-end one that has never given me a lick of trouble.

(Someone correct me if I'm wrong.)