Carbon handlebars - indentations from stem clamp

[sennder]

Hello,

I recently built up a Trek Domane SLR. I had purchased Bontrager XXX VR-C carbon handlebars factory overstock for it. After just a handful of rides, the handlebars started creaking. Since I had not used carbon assembly paste during assembly, I decided to apply that now. Upon removing the handlebars, I noticed that the handlebars had a dent where the stem clamp ends. The stem is a Bontrager Pro Blendr stem with the split face.

The top part had a similar but much shallower indent.

I had used a torque wrench at just under 5Nm. Stem says 5.2Nm max. I tried to as even as possible when tightening the bolts.

Trek told me to take it to a dealer, where the mechanic told me that it's normal and that the indentation is only in the reinforcement, not in the carbon. Is it safe to ride or should I pursue this further?

Thank you

--sennder

[progetto]

I would say toast

[IrrelevantD]

I'm with Progetto on this one. I've personally never seen bars, carbon or alloy, that have done that without taking some structural damage. from what I can find, the torque specs on the bars are 5nm at the stem. If they have a warranty on them, I'd reach back out to Trek or go to another shop.

[TobinHatesYou]

What kind of torque wrench did you use? Also use carbon fiber grip paste on any carbon interfaces other than the steerer/stem.

[sennder]

Thanks for the replies. I have opened a ticket with Trek and sent them these pictures along with the shop mechanic's response.

I torqued the bolts to around 4.5Nm with a "Venzo" torque wrench. I had checked the torque wrench against a calibrated one so I'm sure that I didn't over torque the bolts.

i had actually removed the bars just to apply fiber grip when I noticed the indents. I had decided not to use it when I was building up the bike on the advice of a Trek technician.

[alcatraz]

I'm going out on a limb and say that this stem is unfit for lightweight carbon bars. The clamping area is unsufficiently large to spread out the clamping pressure. It is possible that these bars had a manufacturing weakness around the clamping point but I would definitely not put a replacement bar in the same stem. I'd also make sure I undertorque them even if I'm happy with the stem design. Also lots of carbon paste.

How big a guy are you? Do you have a fairly exteme position where you have a fair amount of weight on your bars? What would happen if you try to float your hands 1cm over the hoods? Unstable?

I think that a combination of weight/fit/clamp design/light bars causes this. I had to bin a handlebar I really liked because it delaminated. I since then changed to bontrager xxx integrated bars/stem copies and have never had a problem.

/a

[sennder]

Thanks for your response. I'm fairly heavy at 80kg/176lbs. Pretty standard position though. Not any excessive weight On the bars.

I agree with your comment on the clamping of the stem. Not a fan. But Trek is spec'ing these with all their carbon bars now so I guessed it was fine. Once I get a replacement for the bar, I'll look into replacing the stem. I was actually considering the xxx integrated bar/stem but it was just too pricey.

--sennder

[alcatraz]

If only there were a way to use like a colonoscopy camera with a hex bit to install a compression plug inside the clamp area, it could save a few handlebars from the bin. Kickstarter?

/a

[TobinHatesYou]

I'm going out on a limb and say that this stem is unfit for lightweight carbon bars. The clamping area is unsufficiently large to spread out the clamping pressure. It is possible that these bars had a manufacturing weakness around the clamping point but I would definitely not put a replacement bar in the same stem. I'd also make sure I undertorque them even if I'm happy with the stem design. Also lots of carbon paste.

How big a guy are you? Do you have a fairly exteme position where you have a fair amount of weight on your bars? What would happen if you try to float your hands 1cm over the hoods? Unstable?

I think that a combination of weight/fit/clamp design/light bars causes this. I had to bin a handlebar I really liked because it delaminated. I since then changed to bontrager xxx integrated bars/stem copies and have never had a problem.

/a

It's a Bontrager alloy stem with Bontrager carbon bars. That combo is exceedingly common. However I have never been a fan of that 2x band-clamp style. Something like a Ritchey C220 would be my preferred alloy stem barring weight weenie choices.

[Lewn777]

I just don't like carbon bars, it just seems they are in a danger area. Especially with so much counterfeiting and then being clamped with fairly high torque on the stem that they seem to be in a place that a few bangs or a slightly out of whack torque wrench it's delamination city. I think if you're gonna go carbon just get the whole thing integrated bar stem combo.

[Wookski]

The bars are f&$ked. Carbon bars are fine with assembly paste torqued correctly (I.e tight enough to prevent slipping). Anywhere near 5nm is ridiculous.

[wheelsONfire]

This is typical for 3T bars also.

[c60rider]

I have two pairs of deda carbon bars, Superleggera RS and some Superzero. There's nothing on those other than superficial marks where part of the bar has been exposed to the elements and the rest hidden by the clamps. I torqued them up just shy of the recommended (I think it was 5nm) so these look really poor to have crushed in the way they have. Certainly it looks pretty structural and I wouldn't want to use them again other than on an indoor bike. Just don't take the chance for the relatively small cost of bars.

[commendatore]

The bars are f&$ked. Carbon bars are fine with assembly paste torqued correctly (I.e tight enough to prevent slipping). Anywhere near 5nm is ridiculous.

Do you have something to back that up or is your comment that “5 nm is ridiculous” baseless?

[TobinHatesYou]

I torque my Ritchey and ENVE bars to 5n-m and never have an issue. Most carbon bars are heavily reinforced in the 31.8mm section.