I would like to say that this happened while I was bending my bike up a steep climb, but in fact it happened while the bike was sitting on the bike stand and I was out of town. On the other hand, maybe it was better that way.
I took the bike off the stand to go for a ride and the aerobars started flopping around. It looks like metal fatigue; scary.
I could never get this stem to hold my aerobars securely; the clamp would bottom out on the body of the stem before it was completely tight.
Upon removal from the bike, I discovered another crack, that had not yet noticeably affected performance.
Are your bars oversized? If you think they are are you sure? Reason I ask is that you shouldn’t have been anywhere close to closing the gap. And the cracks you see would easily be caused by trying to tighten an oversize stem onto a 26.0 bar. However I can’t imagine that it would even deform that much. I wonder if there ought to have been a shim to go between the stem and carbon bars?
Definately send that back to Thompson, as either way that doesn’t look like their normal quality assuming that the stem has been used with the right sized bars.
If he tightened the stem and the gaps were closed then the HB had to be undersized, not over sized, although trying to tighten a stem on an oversized or undersized bar could do it. I suppose its possible that he tried both wrong sized HB and steerer, but most likely the OP is ham fisted and wanted everything really reallly tight, breaking both ends.
Yep, attempted to fit a 26 bar in a 31.2 stem was my thought too - possibly not clear frm my post. Not sure from the post / picture, but I assume that the stem clamp has been removed to get the bar out and then retensioned for the photo. But if what you are saying is right about the clamp actually being full closed then you can see from the offset at the break how much tension/compression there was in the stem.
I can’t understand at all how the steerer end of the stem also got a crack uunless something went very very wrong at manufacturing when the stem was made (very unlikely). Even the most hamfisted mechanic would normally be expected to snap the bolt / round the head of the bolt or do some serious nasty to the steerer before killing the stem, even on ultralights.
General points. With the bar part of the stem, then take the cover off the front, put the bar in and then lightly screw the plate back on and set the gaps to be even top and bottom then tighten both bolts gradulaly (ie 2 turns top, 2 bottom, or go diagonally if it’s a 4 bolt). The idea is at the end, the gap will be the same top and bottom with even torques.
Simmilar with the steerer, bit at the top bolt, bit of the bottom, back to the top, etc etc.
If you have any doubt, then use a torque wrench, but 5Nm is not a lot, a fairly easy twist of the wrist on the longer arm of a standard L shaped allen key. If you’re anywhere near maximum force with a 4-6mm wrench on a bike you’re likely to be over torqueing.
seen this just the other day,Thompson is really pushing the weight on there products,i had a seatpost break and measured the thickness at the break point,it was .032 thousands sorry that thin its not going to last
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Just an idea: If the bar wasn’t completely tight inside the clamp, then any loads you applied might not have been distributed equally across the faceplate. Instead of loading up both of the bands of metal that comprise the plate, you might have just been loading up one at a time, and in a different way than it was designed for.
Here’s the deal. There are some areas on the bike where you should never cut weight, especially since weight is relatively unimportant: tire tubes, and stems. There’s VERY VERY VERY little to be gained from swapping a stock stem for an ultra-light one. I’d say the same thing for bars, but there are some benefits to going with a a slightly flexy bar (namely, comfort). Quit being a weight weeny!
Given that you have a failure in two places on a single stem (and assuming that there was no crash involved), I would first ask if the stem had been heat-treated properly.
Yeah, there was another problem if you could not get it to tighten on your bar. I know some manufacturers put out 31.7mm bars for a while, though not sure if that little diff would stop a Thomson clamp from biting. What kind of aero bar?
I was out riding my recently built TT bike today and my Thomson X2 stem just let go of the aerobars. The bar just dropped, without warning, straight down. Luckily I was able to keep the bike upright and skidded to a stop. Limped home and checked the clamp, which was torqued to recommended spec, and sure enough, a small crack in the same location as the OP’s stem. This stem was brand new, and had never been used. I am just thankful it happened while training , and not during a race.
I don’t own a torque wrench, although I will get one, now. But as I stated in my original post, the stem would never hold the bar tightly enough to prevent rotating downwards if one put pressure on the extensions. I never tightened it anywhere near as tight as I could have, but the clamp had bottomed out; maybe it was stretching.
I don’t know about you, but I don’t want to put my life on the line for a plus or minus 1Nm difference in torque on any critical component on my bike. Torque tolerances should not be so close as to result in this kind of failure.
