Stem - Clamp Strength
 

Hey guy,

I'm on my second Concept EX stem after the first one had a bolt snap in it after i tried tightening. The stem was dangerously loose and the bars had shifted 25-30% whilst riding

Got home after a ride today and my paranoia got the best of me. Chucked the wheel between the legs and gave the handlebars a twist. At first there was some flex, but under torque surely the stem started twisting on the headset.

Should stems do this? It was under considerable torque but it moved regardless. I don't remember ever having these issues with my avanti's stem.

I called the LBS and they advised the stem should be able to hold up (even under my considerable weight) and they mentioned they could install carbon compound, but im dubious of its success.

I'm wondering if its just crap stem design and investing in a stronger stem like a thompson/bontrager/rictchey in the search for a stronger clamp design, but is that just a pipe dream?

looking for peoples thoughts and opinions.

Regards

Which bolts? At the steerer or at the handlebar?

Steerer tube

Alloy steerer tube? I assume you are using a torque wrench? If you're not, you should be...

It was fitted by the lbs less than a week ago after the first one went

I am not familiar with that particular stem, but if it has a single clamping bolt I would find another type. A two bolt stem is better but still has a single point of failure; if one bolt comes loose or breaks the other will not hold reliably. The stems with a 4-bolt clamping means are better in this respect but the bolts must be tightened gradually so that the clamping force is evenly distributed. Carbon assembly compound or coarse valve grinding compound will indeed help hold the adjustment with less bolt torque, I think it would be worth a try.

Thanks.

It is a 2 bolt headset, 4 bolt bar stem, seems only to come OEM with focus bikes and can be found on ebay for about $25

eg: Bike Stem CEX Concept 110mm | eBay

Carbon assembly paste works great, even on aluminum to aluminum connections. I use it on my seatpost, on the seat rails, on the stem-steerer and stem-handlebars. I need less torque to make a secure clamp.

I don't think a different stem would make much difference. Your test of moving the stem on the steerer with a lot of sideways force on the bars is way more force than you'd have when riding. Much more typical is riding on the hoods, hitting a big bump, and having the bars rotate downward. And even that isn't likely to cause a crash.

If the stem-steerer is a little loose, there's a slight chance it could gradually move up a tiny bit, loosening the headset. The symptom is feeling the headset bearings knocking a little if you hold the front brake and rock the bike forward while standing over the top tube. So check that occasionally at the start of a ride.

That concept stem looks like a typical 4-bolt:
http://bikeforums.net/attachment.php...hmentid=439528

You shouldn't have to crank down on the bolts anywhere near enough to break them. Maybe the bolt was defective? For reasonable tightening, Torque Keys are preset to a specific torque, usually 5nm. Or see how tight the LBS sets the stem after the carbon paste is applied.

Another problem that just occurred to me: if the bar is slightly undersized you may not be able to clamp it tightly. Make sure that there is a gap between the stem and faceplate at each of the four bolts when the bolts are torqued. If any of the gaps close up you will be clamping the clamp against itself instead of against the bar. If this is the case a small shim, perhaps made from emery cloth for friction, may enable a secure fit.

Back in the day when 'sensible engineering" was common we had a separation of the stem/headset clamping duties. One fixture secured the stem into the steerer and another would trap the headset bearing adjustment. This was the threaded steerer/headset and the quill stem. With this system one could let the stem be only as tight as needed to allow for steering forces (which are very minor if the headset works halfway well). Then when one fell of bumped the bars hard and they got off center it was straight forward (Yes, intentional pun) to re align the bard with only your hands and legs (ft wheel between the legs).

Today with both functions combined the stem/steerer clamp needs to be tight enough to resist the bump and bearing getting loose forces which are greater then the steering ones. So when the stem gets shifted on the steerer it takes a wrench to loosen before straightening, or the possibility of damage exists.

As we march forward we go backwards too. Andy.

At either the handlebar clamp or the steerer clamp, one cannot set bolt torque by gauging clamp tightness. There needs to be a gap in the clamp at each bolt, and the bolts should be torqued to specification. A broken bolt usually indicates over-torque. BTW, which bolt broke -- at the steerer or the handlebar?

As for the stem spinning on the steerer under force, I advocate a tightness similar to what you describe -- provided bolt torque is correct. That is: the stem will not spin in use, but will yield to moderate impact.

So the Stem is fine, i have crushed the carbon steering tube.

those 2 bolts around it does not require a lot of force .. they hold the headset preload adjustment , and have enough friction so the handle bars turn the wheel.
in a crash if the stem slips there may be less damage to parts than if rigidly fixed.

You don't have a problem at all, and any attempt to "fix" it is misguided at best.

The stem clamp to the round steerer tube attaced to your fork. It has to be tight enough to transmit steering inputs without twisting. Steering forces are actually very low, and far (very far) less than the twist you subjected the stem to. You were able to cause flex and twist in the fork before it finally let go, so what is the issue.

FYI old timers and race mechanics tend to keep stems much looser than most people. We have a concept called "race tight" which means tight enough to do the job, yet loose enough to slip in a crash, hopefully mitigating some force an saving part from bending or braking.

i'm certain that if grabbed both ends of the handlebars on any of my bikes with threadless headsets, held the wheel between my legs, and twisted i could move the stem off center. as long as they don't move while riding i'm satisfied. occasionally, when i crash for instance, the stem will twist. i accept that.

back in business... Running an alu fork with steel steering tube.

http://i.imgur.com/ySnEJan.jpg

Where's the Beach?

6nm

the opposing pinch bolts on modern stems allow you to tighten both bolts in unison. ideally, this how you should clamp your stem down for numerous reasons most don't care about.

Henley Beach

in Adelaide?

Yep