I have an early 70's Peugeot U0-8 which is now a fixed gear, and a very noticeable problem that has been bothering lately has to do with speeding up and slowing down. Whenever I try to accelerate suddenly its as if there is a grinding/rumbling feeling in the bottom bracket. For the record, I replaced my cottered cranks with a newer crankset that attaches via the square tapered spindle. I also overhauled the bottom bracket. I also had to acquire another fixed cup, as mine was shot. I had to replace my chain as well, and I'm using an 1/8" chain. I don't recall having this problem before I replaced everything, so it must be something I've caused. I know its not the chain tension either, because the tension is near perfect. Sorry for my lack of technical names, I'm "new" to bikes. Anyone have ideas?

is it a loose ball bottom bracket, and what kind of tensioner are you talking about?

Loose ball? I meant the chain isn't too tight or too lose.

Can your cranks spin smoothly when a chain is not attached to it?

loose ball means the bearings are just individual ball bearings, not in a cartridge.

A chain tensioner for a conversion? Out of curiosity and ownership of a peugeot aswell, OP what chain tensioner are you using on your peugeot?

i'm not seeing where the op says he's using a chain tensioner.

Yes, I just took my chain off and tested it, it spins very smoothly. However, I noticed that there was way too much play, as if the floating cup wasn't screwed in far enough. So I screwed it in farther, screwed the lock ring on, reattached the crank arm, and gave it a test ride. The grinding/rumbling problem seemed to have become less noticeable, however, after about 15 minutes of riding, there was way too much play again, and I had to screw the floating cup in farther. This also isn't the first time this has happened, I've had to do this every time I've gone out on a ride. The floating cup seems to somehow loosen itself, even though the lock ring is on TIGHT.

They are in cartridges I believe. If memory serves correctly, I remember one of them is Campagnolo, and the other didn't have any sort of brand name on it.

Oh right...you have French bottom bracket...it becomes loose every once in awhile.

Maybe try to grease the cup and torque it down as tight as possible without cracking the cup.

No, I used to have that problem with the fixed cup, but I tightened it down extremely hard and it hasn't moved a bit in the past month.

are you using a proper french threaded fixed cup?

yr adjustable cup is coming loose, right? if it's campagnolo, it'll be english threaded- which means it's not exactly right but it works. the bottom bracket does need to be adjusted correctly though to hold an adjustment and maintain any strength.

if you get it properly adjusted and it still comes loose, i'd use teflon tape.

I don't know if the fixed cup is french, I just know that it threads on easily. The floating cup is the same one from when I bought the bike. The thing is, I'm using random parts, and they're all used. The cranks are two different brands, the chainring I have didn't come with the crankset, and the spindle is just some random one that fits. I saved $200 by fixing up my bike myself with used parts, rather than taking it a shop and having the parts ordered. I don't understand how the floating cup is coming loose, because the lock ring is still on super tight every time I undo it and readjust the floating cup. How would I use teflon tape?

Test to see if both cups loosen/tighten in the same direction.

Oh I'm positive they're french threads. That's why out of a bin of about 50 fixed cups only one would fit. And why the fixed cup unthreads itself sometimes.

This might be stupid, is your chainline straight?
Because you wear your crank arms everytime you install/remove them. This could mean that it might get drawn in deeper into the spindle each time, thus affecting the chainline.

Yeah I checked the chainline, its pretty damn straight. However, the chain ring IS slightly bent, I see it wobble a bit when I look down at it while riding. It almost makes a "zipping" noise when I put pressure and accelerate fast.

There, I think you just solved your problem. I had a wobbly chainring before and everytime I get off my saddle to climb a hill or sprint, my chain starts to make annoying clanking sounds. That is probably why your ride comes noisy.

Still doesn't explain the unthreading floating cup. What's really frustrating is the chainring attaches to the arm with three bolts instead of 5, so i have to fuss with the position because otherwise one part of the chain is saggy while another part is almost too tight for the pedals to turn. I should probably just buy new parts...

Holy crap, aren't those cranks the cottered cranks that came with the bike?

I use used parts too, but I don't have French parts to worry about. All you really need is a set a of cranks and maybe a set of bottom bracket cups that are French threaded, or have one of the cups replaced.

They WERE the original cottered cranks, and I had to replace them because the chainring was coming unattached from the crank arm. To get the cotter pin out I had to actually drill straight through it. I now have a more modern set up.

Just get the Velo Orange bottom bracket. It seems that the BB is the problem.

My UO8 conversion does not make any sounds

He edited it out....

Nope the reason I edited was because I had written "I have a early 70's Peugeot U0-8"

Velo Orange? What exactly is this and where can I purchase it?