Bmx sprocket out of round?
 

I have been fixing up an old 2008 Haro.

I replaced the sprocket with a 25T Theory Verify.

The chain tightens & loosens once each rotation of the cranks, as if the sprocket were slightly egg-shaped.

Is it possible that these could be manufactured out of round? Or is it more likely to be another problem causing these symptoms? What is most likely? What are other possible causes?

I have a theory, but don't want to bias possible answers at this time.

Thanks!

Once per revolution of the pedals indicates chain ring unless the sprocket & chain ring have the same tooth count.

It appears you are talking about the chain ring.
Loosen the chain slightly. There's bound to be tiny manufacturing tolerances. If the chain is too snug, they become VERY apparent.

Does the chainwheel run true laterally, or is some side to side wobble? This would happen with a misaligned/bent spindle/BB.

How is the mounting point of the chainwheel to the crankarm? Could there something that causing lack of concentricity at this point?

Just guessing, since it's been a while since the BMX days.

The sprocket is almost always round. However, it's likely not perfectly centered.

Try slightly loosening the mounting bolts, turn the crank to find the place where the chain is tightest. Now use a hammer to GENTLY tap the chainring at 3 o' clock to slacken the chain. Test and repeat until you get it as good as you can, then tighten the bolts.

Note that you will never get it perfect, so adjust chain for minimum, but not zero, slack at the tightest position.

Sheldon Brown has this routine nicely written up on his website. I believe it is in the fix gear section.

My observations about this have led me to the fact that all cranksets are not equals when it comes to roundness. That the better makes and models do better and also that cranksets for single and fix gears tend to be better than the same level of "road" or derailleur bike intended cranksets. That eccentricity doesn't matter on the road. There was even a time when some believed that eccentricity improved shifting so why spend money in careful tool setup and QC?

The cranksets intended for fix gear, and especially cranksets intended for the velodrome are much better with the best being near perfect. My Sugino 75 crankset at 20k road miles is so close I rarely feel a need to improve my first guess at chain slack after spinning the cranks and it is mounted on a medium priced (but velodrome worthy) square taper BB.

most BMX cranks and sprockets are "out of round" when assembled.

If you have other bikes, or friends with bikes, see if you can "test" theirs just to get an idea of "what's out there".

Yes. I guess I did mean the chain ring (front).

Could this be caused by a bent bottom bracket axle/shaft? That is what I think may be the problem. It is an old style cup & cone American bb, fwiw.

Pictures? Just so everybody knows what your definition of "old style" is-

Bent spindle usually doesn't cause the chain to tighten and go slack to any great extent, but you can get an indication of a bent spindle OR warped chainring by turning the cranks with the chain off and sighting from the back of front of the bike, along the chainline, checking to see if the chainring wobbles back and forth at the teeth.

It's likely the middle hole either isn't concentric with the circle described by the bottom diameter of the teeth or the middle hole has too high a tolerance. I typically just reduce chain tension 'til there's no binding.

In BMX parlance, American typically refers to Ashtabula (or one-piece)

Here's a pic of the chainring.

https://external-content.duckduckgo....8ca&ipo=images

I don't do BMX and with all the BB standards that keep popping up, one person's "Old Style" is somebody else's nth generation. I didn't want to presume- "adapters for 19mm and 22mm spindles" aren't in my old school memory.


"The Theory Verify Sprocket is simple and effective, yet wallet-friendly. Constructed of 6061 aluminum sprocket, it has a 15/16" bore and includes adapters for 19mm and 22mm spindles....
Begs the question if the proper (if needed) adapter was used as Schweinhund mentioned.

Ah yes, could be the issue, cuz man even if the bike has a 22mm spindle, that setup has the potential to be VERY non-concentric without the adapter, if 19mm spindle, holy cow!

So, Midawi , pull the cranks out of the frame, loosen the lockring and jiggle the sprocket. Is there a lot of play? You'll need an adapter in there.

To the OP.

Why are you arguing against the answers you're being offered?

Occam's razor implies that the simplest answer is best.

An eccentric chainring - round but turning off center - explains exactly what you described in the OP. Moreover, it's extremely common, not some out of left field construct.

Like they teach medical students, "if you hear hoofbeats, think horses, not zebras".

Neither of the adapters fit over the spindle at all.

So that isn't the problem.

Thanks!

This is when you are pedaling forward isn't it? If you are turning the cranks backwards and seeing this, then it's just the freewheel or freehub dragging some as it turns.

You should be able to make a mark on the chain stay with a crayon or piece of tape and use it to see if the ring egg shaped while you turn it or that something is bent or not concentric with that crankset.

They do make egg shaped rings on purpose. But probably not your ring.

The quick and easy fix is something that came up regularly, within the Little 500 race bikes. When the front ring was put on the crank arms it was not perfectly centered and produce the tight chain / loose chain issue. The fix is to put the chain in the tight position, and hold the crank arm steady. Then put a cloth on the chain midway between the front and rear sprockets, and push the chain on the bottom section against the top section. That procedure normally centers the front chain ring on the crank and fixes the problem. HTH, Smiles, MH

i'll try again...

"Is it possible that these could be manufactured out of round?"

YES!
is your crank a one piece, "Ashtabula" type?
like this one?
https://www.ebay.com/itm/25609923033...029e9154cf723a

if so, it is, almost certainly, Manufactured Out of Round...

almost ALL of them are manufactured out of round.
and they also BEND when the bikes get jumped and /or Crash hard.
Not that BMX bikes ever get Abused then Sold.... :rolleyes:

you may want to try Re-Centering the Chain ring a few times, testing it each time.. is there any slop to allow a minor re-centering?
the "tight spot" will happen when the Ring is out towards the Front of the bike.. tap it back when it's at the "tight spot".. Place a chunk of wood over the teeth with CHAIN IN PLACE, then whack the wood with a hammer... see if it moved.

btw... the rear gear/freewheels and hubs are also out of round...they will cause extra-tight and loose spots that don't happen in exact rhythm with the crank turnings... been there a million times, and probably a few more this weekend,... sigh.

Loosened the chain... but
 

For now I loosened the chain a bit. It works, but I'm not happy about it as a long term solution.

I'm 90% sure the spindle is bent (or manufactured poorly?), since there was some visible & audible grinding of the chainring happening once per revolution. However, I havent taken the bb apart & the spindle out to confirm this.

I don't know if I want to replace it (the spindle or the bottom bracket) since the frame is very heavy at 7 pounds anyway. (Lipstick on a pig!?!?)

If I replace the frame with a newer frame it is sort of a can of worms:
I will probably need 1) a new fork, right? 2) new cranks, right? (Please confirm!)


I'll probably wait a bit until I can find a lighter frame & fork at a good price on Marketplace.

I'm in NJ. Are there any shops in my area that have good prices & a decent selection on 2nd hand gear?

I'm having fun riding this thing as is, but I'm sure it will be even better 4 pounds lighter! I ordered Demolition Momentum tires too which are about a pound lighter per tire, so 2 pounds. Then potentially another 2 pounds off when I get a better frame.

Thanks for any advice or input!
https://cimg7.ibsrv.net/gimg/bikefor...9ba2d08b94.jpg
Chain as is.
https://cimg1.ibsrv.net/gimg/bikefor...992e709c0d.jpg
Not sure if you can see much difference, but this is at the loosest spot in the crank revolution.
https://cimg3.ibsrv.net/gimg/bikefor...4b27516eb6.jpg
And at the tightest spot in the crank revolution.

There was some side to side wobble after looking at it again.

In fact the grinding I was hearing sounded aluminum, and sure enough the chain was grinding the side of chainring at the point of the revolution where the chain was tightest, which would be consistent with a bent spindle as well.

I loosened the chain, and thus I was able to get rid of all grinding of metal at least, but it is not an ideal (or permanent) solution.

Just not sure if I want to invest more in this frame (which is super heavy anyway) , or if I should just find newer 2nd hand stuff in decent shape on FB Marketplace.

Makes sense.

Only one problem in this case. There are no mounting bolts on this chainring. It fits directly onto the spindle (which, as I explain elsewhere in the thread, I am 90% certain is bent.)

Thanks for your reply. The advice will certainly be useful down the road on a future project!

If I understand the terminology correctly (and I'm learning on the fly here), the chainring is not spline drive, but the cranks are.

There was no adapter necessary. The chainring fit snug directly onto the spindle. The adapters didn't fit at all onto the spindle.

So I think that means the spindle is 15/16ths, right?

This.

If you have some sentimental attachment to the bike, you might have to spend some money and start replacing stuff. Otherwise, you can accept that it will never really run as it should and ride it anyway, or sell it to someone who doesn't care.

Nope. There was no play at all when I installed it.

The chainring came with two adapter rings, but neither fit over the spindle at all and the chainring fit snug onto the spindle directly.

Thanks!

If I was arguing against answers offered (and I'm not sure I was) it would only be because it didn't fit with what I observed with my own two eyes.

Part of the confusion is terminology. I am learning the bmx terms on the fly here, so it may take some time to sort that out so we are all talking about the same thing.

Also, a chainring mounted directly to the spindle can't be adjusted by "loosening mounting bolts" as was suggested. I may have initially ignored some replies if they were not actionable to fix the problem.

Finally, it is just a matter of my free time to spend on this forum. Between work & family, sometimes I am in a rush to test out the answers that jive most with my initial suspicions.

And, as it turns out, I got some good advice that confirms the bent (or non-concenttic) spindle theory.

But I appreciate all the responses, even ones that turn out not to be consistent with my specific issue.

There is no substitute for hands on experience when diagnosing a problem, so I understand that every possible explanation for my problem can't all be correct.

However, I am happy to learn about all the other possibilities that I haven't yet considered (whether they are all right or wrong is irrelevant).