I had to address an issue last week on a client's bike that had to do with the front derailleur mount on his 2011 Felt F1. I did the frame-up build with Shimano Dura Ace Di2 in the early part of this past summer. This bike was not abused and I'd have to give a gold star to this client because of how well he takes care of his bikes. So, here's a short description of the issue and then my fix...
After inspection, it was obvious that the front derailler mount seperated from the frame. Two of the three rivets used to attach the mount to the carbon seattube were floating in the hole that they were innitially installed in and both of the holes were now larger in diameter than they originally were because of the damage caused by the rivets installed at the factory.
I initially thought that the failure was mainly caused by the different and greater forces introduced by the Di2 front derailleur in addition to possibly a poorly installed blind rivet.
After retreveing from inside the frame and inspecting the other two rivets I came to another conclusion.... I believe that this failure was because of the type of blind rivets used. Based on my inspection and research, the blind rivets that Felt used were either completely the wrong spec based on the materials being joined (composite and aluminum) or they didn't account for the stronger force that is applied to the area by the Di2 front derailleur. From what I can tell, the rivets that Felt used are sometimes called multigrip rivets because they can be used in applications where the hole is irregular or oversized. This type of blind rivet body expands to fill the hole. All three rivets did not expand against the internal surface of the carbon seattube. Because of this, one of the three rivets separated completely from the frame while one of the others was loose.
So here's what I did to repair and make it better that it was when it came out of the factory....
I special ordered a blind rivet that with the dimensions that would work with the existing components and the combined material thicknesses. This type of rivet is sometimes referred to as a press plate rivet.
I wanted to test the press plate rivet and compare it to the OE rivet. I did this with a section of carbon steer tube. The one on the left is the OE rivet and the one on the right is the upgraded replacement.
I was satisfied that this was a good solution. Besides the different rivets, I wanted to bond the mount to the seattube with epoxy just for good measure.
The repair was pretty straight forward. I started by applying some epoxy in and around the two damaged holes in the frame. After in cured, I redrilled/cleaned the two holes to the correct diameter. I then burnished the inside surface of the front derailleur mount in order to improve adhesion. I then applied epoxy to the frame, installed the three rivets, insured alignment and fit and then I let it cure.
As you can see the the last image, the rivet heads are obviously a larger diameter and not as pretty as the original rivet heads. In this situation, I choose structural integrity over cosmetic appeal. The end result was better than when it was new.
In closing, I believe that we are going to see more of this problem moving forward with more and more Di2 front derailleurs and the like being used, specifically on carbon frames that do not use a front derailleur clamp. Let me know what you think. Thanks for checking in. -John