Front brake post mount bolt query

rolf_f

Dear all

Please help! I have one of these:


One of these:


One of these (180mm):



And one of these



Last time I put these things together, after not a great amount of riding there was a clattering noise and I discovered the front caliper knocking around no longer bolted to the fork; one of the bolt lugs had broken off from the body of the caliper.

After managing to source a replacement caliper, I now have it fitted and bled and need to be sure it doesn't happen again. In the first instance, either the caliper body was faulty or I my fixing of it to the fork was faulty - so I want to eliminate that. So can someone please tell me what bolts, washers and whatever I should be using to make sure I can avoid what ended up as being a year without the bike!

Thanks in advance.......

nicklouse

You did use longer bolts than the ones that came with the caliper?

Also what actually went wrong first time?

jimothy78

I reckon the only way one of the lugs is going to have snapped off the calliper itself is if the other bolt had worked loose and fallen out first. Then the calliper could have been pulled away from the fork with enough force to snap the lug off.

Like nick says, when using a bolt-straight-through adapter like the one shown, you should be using longer bolts than standard, to allow for the distances they have to pass through on the adapter itself. If you look on techdocs.shimano.com for the calliper you have, you'll find the appropriate diagrams showing all the different fitting permutations and the length of bolt required in each position for your particular combination.

02gf74

^^^ also some adapters do not have angled faces - notice the shimano in the photo above does - this means that the calliper does not quite sit square on the adapter so as the bolts are done up, there will be some bending in the calliper lugs.

rolf_f

jimothy78 wrote:

I reckon the only way one of the lugs is going to have snapped off the calliper itself is if the other bolt had worked loose and fallen out first. Then the calliper could have been pulled away from the fork with enough force to snap the lug off.

Like nick says, when using a bolt-straight-through adapter like the one shown, you should be using longer bolts than standard, to allow for the distances they have to pass through on the adapter itself. If you look on techdocs.shimano.com for the calliper you have, you'll find the appropriate diagrams showing all the different fitting permutations and the length of bolt required in each position for your particular combination.

The bolts were pretty long.

I think that may have happened was that the lugs weren't perfectly aligned - and therefore the bolts were stressing the lugs. Once one fractured, because the lugs weren't aligned it would have meant that the other bolt was automatically loose. That one then came out and the caliper swung freely. The fractured lug remained attached to the bike. But it could have happened just as you say. But that said, I'd only just built and checked the bike so I know that everything was done up tightly before the ride. I tend to be fairly paranoid/OCD about that sort of thing!

02GF74 wrote:

^^^ also some adapters do not have angled faces - notice the shimano in the photo above does - this means that the calliper does not quite sit square on the adapter so as the bolts are done up, there will be some bending in the calliper lugs.

Both faces are at the same angle so the lugs shouldn't be under any stress. But if this is the wrong adaptor then I need to know. But it is a 180mm Shimano post adaptor so I can't see it being wrong.

nicklouse wrote:

You did use longer bolts than the ones that came with the caliper?

I think I used some bolts I already had. I'm pretty sure that the bolts that came with the caliper were too short for the post mount adaptor. It's been a while though so I'm not completely sure. They certainly had plenty of thread inside the post though. ie if it was user error I'd accept the chance it was down to me failing to have done the bolt tightly up enough but not that the bolt was too short to start with.