jsnmglbrt

My son's bicycle--a mid-size 21-speed Diamondback--has developed a problem I need help figuring out:

When he is pushing on the pedal, the crank will intermittently (but frequently), "slip". It will jump from, say, the three-o'clock position down to the six-o'clock position, with a jolt that can cause him to slip off.

I don't think the chainring is slipping in the chain. I think that would be easy to tell--it would make a particular noise or I would simply be able to see it happen (as I jog along next to him). I don't think the cranks are moving independently of the chainring. So my only conclusion is the hub is slipping independently of the rear wheel(?) or maybe the chain is slipping on the cassette? I am mystified.
Between the quarantine and Corona Recession, the bike shop is not really an option. I am capable of the repair, if I can figure out what the source of the problem and it's solution is.

I would be grateful for any guidance. My socially-isolated 10-year-old would be even more grateful.

Iride01 

Is he perhaps trying to shift when this happens? If too much pedal force when shifting that can happen.

Crankycrank

Unless you've installed a new chain recently with an old freewheel/cassette which can cause the chain to slip the most suspicious culprit would be the freewheel/freehub. Here is a video explaining how to lube a freewheel without taking everything apart. What can usually happen is that there are little pawls inside the part that get stuck sporadically when the lube dries out causing it to slip as you describe. Try this, or check some of the other videos and let us know if it worked. You may need some WD-40 or other solvent to free things up if this doesn't work and then apply some real lube afterwards. https://www.google.com/search?q=how+...hrome&ie=UTF-8

jsnmglbrt

He is not trying to shift. I was able to reproduce the problem riding myself and I wasn't shifting either. I have experienced that kind of slipping while shifting. This is different.

joeruge

Could be that the ratchet mechanism in the free-hub is dis-engaging under load. Listen to the sound of the wheel rotating while the chain is still. It should sound pretty regular, maybe with some rhythmic pulsation.

If this is the problem, it could need some service to clean out old, dried up grease. Since you probably don't have the tools to disassemble the hub, you should do a little research to learn what would be the best short term remediation, maybe some spray-in lubricant. W-D 40 will help loosen things up, but it will also get rid of any useful lubricant in there.

jsnmglbrt

The problem is worse now, but easier to see. I watched the video and cleaned the hub. Now, I can simply hold the back tire still with my hand, push on the crank, and feel it slip forward about an inch. Then I have to pedal it back a few notches and I can do it again. So I assume the problem is with the ratcheting system in the hub.

Iride01 

If you are certain the chain is not slipping over the rear cogs or front chain wheels which might indicate wear. Or that front or back is not trying to shift itself from something being misaligned, then the free hub will probably be your next suspect, I'd think.

You said 21 speed. 3 front and 7 back?? Probably a freewheel instead of free hub. If so I'd just go wild with a can of WD40. But that may wash the lube off the axle bearings. The axle bearing should be repacked with grease. But the bearings in the freewheel or free hub really don't need it, IMO, because they don't do any work unless you are coasting. And hopefully you aren't coasting down 10 miles of hill at a time.

Freewheels are an easy and relatively inexpensive item to replace if you have the tools and skills. Different models require different tools. Some can be home-made. But if there are any open bike shops then they may be less expensive than you think. Some may have mobile services that are still allowed to function even if the shops have to be closed to customers.

Miele Man

Can you swap in a spare/different wheel and see if that corrects the problem?

Cheers

GamblerGORD53

Just get a new cluster. IMO. Shouldn't cost much.

dedhed

Throw a new freewheel on.
https://www.cambriabike.com/products...SABEgL7JvD_BwE

https://www.rei.com/product/117164/p...E&gclsrc=aw.ds

Between the freewheel and tool should be under $20

mostaza

Can you tell us how the cranks are attached to the axle? Is it three part? Splined (as opposed to square)?
Could it be that the bolts securing the crank to the axle are loose and the crank arm is slipping on the splined axle?

smashndash

Are we assuming this uses a 7 speed freewheel? If so, that’s pretty lucky. A modern freehub system would be a lot more expensive to replace. I second this diagnosis. Also, that first link seems to be a cassette, not a freewheel. https://www.amazon.com/SHIMANO-Tourn...reation&sr=1-6
^ that’s a freewheel. I believe “HG” refers to the Shimano freehub system. Lastly, wouldn’t OP need a chain whip as well?

BTW OP: The difference between a freewheel and freehub is that a freewheel combines the ratcheting mechanism with the cogs and threads onto the hub. A freehub mechanism is part of the hub, and needs a cassette (a bunch of a cogs) to be slid onto it, and then locked into place with a lockring.

I think this is impossible on a typical 2 piece crank that has the spindle bonded to the drive side arm. Also, if something on a 3 piece were slipping, wouldn’t it cause the crank arms to go out of phase? They’d be at a 120 degree angle or whatever instead of 180.
Also, if this bike uses a 7 speed freewheel, it almost certainly uses square taper. Slipping is unlikely without catastrophic failure.

dedhed

Good catch, I grabbed the wrong link.
https://www.cambriabike.com/products...peed-freewheel

They don't show part numbers but typical Shimano part number nomenclature is CS-XXX for a cassette and MF-XXX for freewheels


Given the level of bike I'd be surprised it's anything other than a freewheel.
No, a chainwhip is not needed to remove a freewheel, it is on a cassette. Even that has work arounds if need be.
Another option is to have a shop remove the freewheel. I've paid (usually $5) to have them remove obscure ones I didn't want to purchase the tool for one time use.