"Clunk" from disc brake on emergency stop

[confusedcyclist]

Sorry for the unusual description of this problem, I am not quite sure how else to describe what's happening, I can't reproduce the problem on a bicycle stand, it's only when on the bicycle, and travelling at speed with a sudden stop.

On the commute to work this morning a car pulled out of a junction as I was descending with speed. I had to do an emergency stop, I stopped fine, but there was an unnerving "clunk" as I felt something shift in the front of the bike just as the bicycle came to a stop. The wheel is seated correctly in the drops, and the pads are aligned correctly. I'm perplexed!

I have removed the disc brake caliper and refitted, all bolts correctly torqued.

I have experienced this a few times now when stopping suddenly, but it doesn't occur during less aggressive braking. Thoughts? Should I expect this from the discs? I'm running Avid BB7 MTB brakes.

 

[Threevok]

I suspect it's not the brake but it may be your headset.

Stand over the bike - pull the front brake with one hand and rock the bike back and fore. With your other hand - check to see if there is any play at the base of the stem.

 

[confusedcyclist]

Will report back.

 

[Red17]

Are you on telescopic forks as you mention mtb brakes ? Could be the forks bottoming out if its only happening under heavy braking

 

[Jody]

Are you on telescopic forks as you mention mtb brakes ? Could be the forks bottoming out if its only happening under heavy braking

More likely that the bushes in the forks are worn than them bottoming out under braking. You can get some nasty clunks from heavy stops with worn forks.

 

[Threevok]

More likely that the bushes in the forks are worn than them bottoming out under braking. You can get some nasty clunks from heavy stops with worn forks.

Indeed - that was going to be my second suggestion - should the headset (and tightness thereof) not be the culprit - in which @confusedcyclist repeat the same test but look for play in the stanchions of the fork

 

[gareth01244]

I have had a similar issue after building up a disc bike around a second hand frame. I went out on a test run after picking it up from my bike shop who built it and found that apon hard braking i would hear a clunk as i released the brakes. With it being a used frame i was very wary of riding the bike in case the frame had a crack in it somewhere so i went back to the shop who looked it over and they diagnosed it as the headset. Been perfect ever since.

 

[bikingdad90]

Are the pads moving when your brake hard? Broken retaining/spring pin?

I had it on a pair of calipers once.

 

[confusedcyclist]

Thanks all for replies.

It's a new build albeit with a secondhand frame. The forks are solid, this is a touring build that has v brake levers so I needed to use the MTB avid calipers. Very surprised to find play in the headset on my way home from work. Is it likely I need to replace the bearings?

 

[TheDoctor]

More likely they just need tightening. Search YouTube for adjust headset. Its very straightforward.

 

[lpretro1]

Try nipping it up first - if it becomes rough when you take the play out then the bearings will need attention

 

[Threevok]

@confusedcyclist - any news on how this turned out ?

 

[confusedcyclist]

Sorry, thought I had posted an update. Yes, it turns out it was the headset. I had to loosen the stem first, but tightening the top bolt did the trick. It caught me off guard because my LBS fitted the headset and stem for me. It was otherwise 100% DIY build. I had assumed I didn't need to fiddle with the headset as they never mentioned it would need further tweaking, but now it's plainly obvious they couldn't have checked it was on tight enough without the handlebars being fitted at the time.

 

[Threevok]

Result !

Glad you got it sorted