Torque- rear caliper onto carbon frame

chez_worldwide

Hi folks.

I have a Planet X pro carbon.

The rear brake- standard Shimano BRR451 (I'm about to upgrade to Ultegra to match the drivetrain etc) is very loose, and as I'm riding it rotates so that the LH pad actually touches the rim. I only notice this the other day after a particularly tough flat stretch! It can easily be centred by hand, but then road vibrations etc rotate it again 10 or so miles later.

I have a torque wrench, and I understand from talking with Planet X and general searches that I shouldn't go past 6nm on the bolt which attaches the caliper to the frame, but this still leaves it waytoo loose- much looser than the front brake, or the brakes on my alu commuter (which I can obviously tighten as much as I like without breaking into a sweat).

Any thoughts on what's wrong, or indeed whether I can tighten this further. It's hilly round here so I need confidence in my brakes!

Cheers for any advice.

sungod

is the actual bolt rotating, or are the calipers moving wrt to the bolt?

if it's the bolt, shimano says 8-10Nm...

http://si.shimano.com/php/download.php? ... 03-ENG.pdf

tbh if px says the frame can't take that it seems a bit odd considering the force the brakes could apply to the mounting point under braking load

the other thing i'd check is the cable housing, if it's too long then it's going to push on the caliper and push the lh pad onto the rim

mr_evil

Calipers typically come with thread locker pre-applied, which will significantly increase the torque needed to achieve the correct clamping force. I see that even Shimano have failed to specify the conditions that the given torque is for.

chez_worldwide

Thanks guys

I do get the impression that the bolt simply isn't tight enough, and as I tighten it it does get closer to where it should be, just not nearly close enough at 6nm. I also got the impression that the chap on the end of the phone at Planet X was giving me the stock "never tighten anything over 6nm" answer to cover their arses, rather than considering the specific issue I was trying to discuss.

Thanks for the Shimano tech pdf- I had seen this previously and did wonder whether it came with the caveat "refer to individual frame manufacturers tolerances..." etc.

As this is my first carbon frame and I can't afford to replace it, I'd rather not balls-it-up by being ham-fisted.

I've ordered a pair of Ultegra calipers, so maybe i'll see if the problem persists once these are fitted.

frisbee

It should be tight enough at 6nm, take the calliper off and make sure it tightens smoothly and doesn't close up before it'll clamp the brake stay..

norvernrob

Is the actual nut next to the frame rotating? I had the same issue with my old bike until I realised the caliper was rotating but not the nut, I removed the whole thing and tightened the nut to the caliper before refitting to the frame and it never moved again.

lapavoni10

is the bridge deep enough for the bolt? Should be, but worth checking, you may be torquing it onto the unthreaded part of the bolt but only just tightening on the frame.

chez_worldwide

Thanks for all the advice folks. Much appreciated.

I think you were right Sungod & NovernRob- looks like the nut needed tightening in relation to the caliper. Just after I'd finished the job, the postman came with my new Ultegra calipers... on they went! The old (now fixed) ones will trickle down to my trusty old cheap commuter, so thanks again.