I've been searching, but so far no luck, so maybe you smart peeps can help me out.

I hear (and feel) a mild clunk or pop when I start pedaling hard. It only does it once and then it doesn't happen again until I brake with the rear brake (but never when I brake with the front).

I thought it might be the rear disc brake itself, but I have since inspected and re-tightened everything and it did not change a thing.

It feels like it could be coming from the BB/crank area or the rear hub, but I don't have any experience with taking those apart...so I don't know what I would be looking for.

I'd take it to the LBS, but I'm worried that they will have it more than a day (and I can't afford that since we are down to one car - which the wife takes - so I have to commute to work on my bike on the weekdays).

Any ideas would be appreciated.

I have a Felt V55 with about 1000 miles on it now, if that helps.

Not much to go on, but you might check for evidence that your rear axle is moving in the dropouts. My hypothesis is that the chain is puling the drive side forward under power and the brake is pulling it back. Try checking the clearance to the chain stays before and after the accelerate/brake phases.

I agree that's not much to go on. Just for fun, grab the top of your rear tire and try to wiggle it from side-to-side. If it moves, you've got loose hub bearings and that needs to be fixed pronto.

I had that once, chainstay was cracked.

I checked this and it's definitely not an issue (thank goodness)

I'll take a closer look at the chainstay when it gets lighter out...

If it turns out to be a cracked chain stay you're going to wish that it was just a loose hub bearing.

Broken rear axle, now held together by the quick-release? In the old days, taking the wheel out to see would have been the worst possible thing you could do if you needed it ride the bike. Release the QR and the loose ball bearings would spill all over the floor and putting them back was impossible. Maybe you can reconstruct a sealed bearing hub's axle after pulling the wheel but if I really needed the bike, I wouldn't gamble on it.

If I were you, I'd pick up another rear wheel and only then, pull out your current wheel and check the axle.

To follow DiabloScott's thought, maybe ride the bike, do that hard start and feel the clunk, glide to a stop, put the bike in a solid strand (or tie the front wheel to a tree) and try pulling the rear wheel back and see if you can get that second clunk. Look for any cracks between teh BB shell and the rear dropout, especially on the right chainstay. Special attention the the chainstay to BB joint and chainstay to dropout joint. If nothing there, I'd then look along the seatstays.

Ben

My '91 Eros does a similar thing. It's some combination of worn freehub pawls and grease migration from the wheel bearing. Since the latest Ultegra hubs are largely the same construction, you might check if your freehub feels gummy -- does the chain droop at all, or try to push the crank around while coasting?

Not saying that's the cause, but it's easy to check.

If the chainstay is cracked, it ought to show itself with a little force applied from the side - just hold the bike up with the brakes on and sort of step on the chainstay or BB laterally and vertically (like we used to do for evaluating how flexy a frame was) and listen for crunchy noises. OP's bike is aluminum so it should be obvious.

And yeah, near the dropout and the BB are the most suspect places, unless there's damage from a kickstand or something.

<blind guess> loose headset.. are there prizes?

Well, there's bragging rights but, after the problem is determined, the thread disappears and nobody cares anymore.

I was thinking that perhaps there might be a a sleeve or inserted part similar to a steel frame lug or dropout where one piece is sliding deeper into the other with chain tension, then being pulled back out with the brake. The flex test wouldn't catch that. (I wouldn't expect the failures the stress test would readily catch to behave with the "clunk" the OP describes. My chainstay failures didn't.)

Ben

Thanks for all the feedback fellas!

I briefly checked during lunch and I don't see any obvious cracks in the chainstay or BB areas, so I'll have to apply some stress like DiabloScott & 79pmooney said and check again just to be sure. I'll start taking some of it apart to see if it's a sleeve, hub, or axle. I wish I had a spare rear wheel so that I could simply swap that on to see if it eliminates things (which would help me isolate the issue). oh well. lol

If I don't see anything obvious, I'll likely just drop it off at the LBS tomorrow and then let you all know what it was. Either way, more to follow...

So it's definitely the rear hub according to my LBS.

I just dropped it off and they quickly found that it was something loose in the hub. I think he said that whatever it was, it should be finger tight but he couldn't tighten it with his fingers. He said they will completely take apart the hub, clean it up, and see if it fixes the issue. If not, they'll have to order part(s).

Update: Turns out the bearings were worn/pitted. It was allowing the axle to shift under force (both braking and stopping). If I had waiting much longer to address it, it might have put enough stress on the axle to the point where it could snap it. Yikes!!

New bearings are on order.

I was open to getting a new hub or new wheels instead, but I have Quick Release design and apparently they don't carry that anymore since everyone has gone Thru-Axle now. My bike is only 2 years old and it's already out of date!! lol