Shinkers

Hi,

So I finally got my fixed gear MTB built up, and during hard impacts (like on some bunny hops) and during some instances of braking with my pedals, I get a loud pop from the hub (142 Paul Word). At this time, I'm unsure whether it's the cog making the noise, or my hub moving in my dropouts. If I pull the wheel off, I cannot get the lock ring any tighter than it currently is. This is using Paul's lock ring, and a Park lock ring spanner.

Anyway, I'm contemplating just putting a freewheel on since fixed riding hasn't been the awesomeness that I was hoping it would be on the trail. I'm thinking whether it's the cog slipping, or my hub slipping, perhaps freewheeling will be a bit easier on the bike.

My question is, I have a dura ace cog on the hub, and would likely be using a White Industries freewheel. I've heard with cogs, that if you are swapping them, you should use the same brand to help preserve the threads on the hub. Would moving to a different branded freewheel be problematic?

Thanks.

Trakhak

Loud pop + fixed gear? I'd first check whether the chain is too tight, unless you're using a chain tensioner. If you're not using one, you probably should.

People have swapped sprockets and freewheels from different manufacturers on the same hub with no problems for decades. But installing a BSA-threaded fixed gear sprocket on a French-threaded hub---as I did when I was 13 and had just been given my first track bike---now that's a good way to ruin the threads. But even then, the mechanic at the local bike shop fixed the problem by wrapping the damaged hub threads with aluminum foil and screwing the sprocket back on.

IAmSam

I have no long distance diagnosis for the OP's problem...but you can not use a chain tensioner, if that is what you mean rather than a chain tug, on a fixed gear. If you are in fact suggesting a tug - those things are worthless...

Shinkers

So here's what I've got so far:

I have a regular threaded cog on one side, and a 6 bolt cog bolted to the rotor mount on the other side. I can get the pop on both cogs, which tells me it is likely not my chain tension, or the threads on the track cog.

I tend to think that it is the hub slipping in the drop out, but it hasn't been enough to notice the wheel moving off center, just a small drop in chain tension (which has been normal in my experience singlespeeding). I do have a chain tug on the bike.

The only time I can replicate the noise is with an occasional bunny hop in my yard, and even then it doesn't happen every time. My chainline is dead nuts, within .5 mm verified multiple ways with calipers.

With regards to swapping cogs, I don't remember where I'd heard that but I'm glad that I'm good to switch if I decide to.

Thanks.

Leukybear 

Freewheel thread pitch is the same as fixed gear cog thread pitch: 1.37x24 tpi. You're good to swap!
A good plan B if you did end up stripping the lockring threads.

79pmooney

How tight do you run the chain. You should be able to wiggle it up and down close to a 1/2" total. It should never go tight. (And you may have to pedal the chain through several chain revolutions to get the tightest and loosest combinations. Many cranksets do not sit on the BB centerline and chainrings might likewise be drilled inaccurately (especially true with road cranksets where a little wobble often improves front derailleur shifting).

A chain that is nominally tight may go very tight at one point in the chain's cycle. When it does, the load on the chain and the bearings in the hub's drive-side and likewise the bottom bracket's drive-side go sky high. Basically you are asking steel to stretch (the chain) or compress (the bearing balls). Nothing you can do with your legs will ever get remotely close to those loads. Doing that and hearing a noise that shouldn't be there? Am I surprised?

If you go to a velodrome, you will get to see a whole bunch of really nice bikes with high end equipment, all featuring noticeably loose chains. Yes, they are racing on a smooth surface and that helps. But I take from that the simple message that getting the right chain tension on the road is just harder to do because the window is smaller but, like those trackies, we don't want that chain too tight.

Ben

Shinkers

Well I tried deliberately running less chain tension than I normally would, and it still popped. However, I also tried tightening the axle to a lower torque and got the noise more frequently. But when I tighten the bolt to maximum recommended torque (16 NM) it still pops occasionally. When I had the chain set loose, I could see the wheel movement in the frame (it is slight), so I'm pretty sure it's my axle moving.

Now to figure out how to remedy that.

I did go ahead and order a freewheel online as a 'just in case'.

Thanks.

Shinkers

Update:

So I've now figured out that it is 100% the cog moving on the threads. If I stomp on the pedal backwards, it will make the noise, and then if I do the same thing forwards it will do it again. So the cog is moving a tiny bit on the hub (about the same amount as what you feel with a loose chain on a fixed bike if that makes sense).

I'm going to take everything apart and re tighten with anti seize (I used Phil grease before).

What are the chances that this has damaged the hub at all? It's had two rides with maybe 15 miles total, not to mention hopping around my yard trying to figure out what was making the noise.

Thanks.

hairnet

I once sheered a lockring right off the hub from this action, but I was trying to pedal brake from about 18mph, the cog threads were fine though. tighten everything up, ride around the block and double check.

Shinkers

Ha! I spoke too soon!

I pulled everything apart and used some anti seize, and puzzled over the fact that the cog was dead tight on the hub. It was a bear to get off. Once I retightened it, I felt very confident that the cog/lock ring interface was as tight as possible. Yet the bike still made the sound. I could now make the sound at wheel, by stomping on the pedal in the forward position, and then back. So something is moving back and then forth.

And then it hit me like a ton of bricks. My chainring is direct mount.

So off came my SRAM crank. I grabbed it in a vice and used a chain whip on the chainring to try and move it back and forth. Sure enough, it did. So I pillaged my parts stash for an old Race Face crank that I had laying around that is also direct mount, but their cinch interface seems like it would hold better. I grabbed that in a vice and couldn't get the ring to move. Regardless, I pulled the ring off and wrapped the crank mount with teflon tape to keep everything tight.

Anyway, I got everything on the bike and the sound is completely gone for now. I feel very confident that that was the issue.

I hate these wild goose chases, but god damn it's good to win one once in a while!

Thanks for all of the suggestions and help. I will be keeping the freewheel I bought for a rainy day so no harm done there.

seamuis

Shinkers

Yep, I get that. At the time though, I would have thought that some slop in the cog threads was more likely than my chainring moving on it's mount. Having been a singlespeeder for years, I feel like I'm pretty careful about my bike setup but I have very little experience with threaded hubs. I loathe direct mount cranks and chainrings for the very reason that I think they are noise-makers. I should have thought to check that first. But what can you do.

The whole fixed MTB thing is mostly an experiment. I have another SS bike that will still be my primary bike, but I thought it would be fun to try something different. We'll see how long it lasts.