Hi guys,

I just bought a cheap old mountain bike that has some slippage in the smallest back gears and wasn't changing into the smallest gear at all (I've managed to fix this by winding out the H limit screw on the derailleur). The slippage doesn't happen on the biggest gears at the back, but once changed into the 4th biggest one and below, the slippage happens on every gear whenever you put reasonable pressure on the pedals.

I've been googling the things that may cause slippage in gears and I have checked the following:

* Chain stretch - looks perfect
* Wheel alignment - looks perfect
* Worn cogs - they all look pretty good
* Crooked derailleur - this is what I think the problem is, but I'm not sure what it *should* look like, or exactly how to straighten it

I have uploaded some photos (h ttp://imgur.com/a/laoZU) to see if you kind folk could confirm for me that the derailleur isn't straight, and perhaps suggest a way for me to correct the issue.

If I'm off the mark and it could be something else, I'm open to suggestions!

Thanks in advance.

It seems like there are two things going on.

I haven't checked the pictures but, based on your having to back off the high gear limit, I suspect that the hanger is bent inward. An easy way to check is to shift to a gear where the derailleur cage is close to vertical. Then set a straight edge, ie. a broomstick, against the rear wheel as a reference, and see if the cage is parallel to it when you look from the back.

The other issue may be a new chain on a worn cassette. This tends to cause the skipping under load on smaller sprockets. It's possible that the prior owner replaced the chain, but not the cassette.

For the moment, I'd get the hanger alignment right, then adjust everything and hope for the best.

Hello and welcome to the forums!

I had such "slippage" symptoms happen to me for several years before finally figuring out what the actual problem was.
I guess you can say it intensified when it rained. In my case (and i suspect in your case as well) usually the largest chainring
on the CRANK - has significant wear to it - (the teeth are heavily worn down) and will not pull your chain properly anymore.

They will infact skip a beat so to speak..

You would be excused for not thinking the chain is able to skip a cogwheel of such considerable size and while the chain is committed
to such a big size wheelcog (50T in my case) So i naturally assumed it was the rear (smallest) cogs that was the problem because
the chain is in contact with cogs of such a small size. Well how wrong i was. Pretty distressing to look down and seeing it happen
and not have any idea where the problem lies. This was a real eyeopener to me as in all my years of cycling never thought this
could be possible and that cogs/chaindiscs did not wear, only the chain, lol. Maybe there is a significant difference between 26
& 28 inch bicycles. The 28" roadbikes always try for the lightest of alloys and i suspect these wear more easily than heavier
and more durable 26" parts where weight is not of the essence.


Try to find a picture of your chaindisc online and compare the teeth/tooth with the one you`re riding now. Maybe you`ll have
yourself an ephiphany. My two cents have been tossed.

sound like your derailleur hanger is bent.

I flip the bike upside down, then position the tension cage on the derailleur at a vertical position... then, look at how it aligns with the front sprockets. It should vertically align with them.

If, after you get the cage aligned, the derailleur limit screws will need set again.

it is also possible for the der. hanger to get TWISTED out of alignment to the bike's center line... shift the chain onto the smallest sprockets, then note how the cage aligns to the chain line... correct as needed, then once again check how the der. shifts, and adjust if needed.

NOW, once all that is ok, test ride the bike, and make sure to put it into the highest gear, at a medium speed... pedal as hard as you can... if it still skips, you may have worn sprockets, OR a bad freewheel or free hub ratchet!

And CHAINS don't " STRETCH" under normal conditions... they WEAR, at the pin to roller interface, and at the roller to tooth interface.... just sayin'...

and a bent/sticking chain link or two will also cause a "skip"... pedal the bike, and watch for the der. cage to twitch every time the tight link passes through the cage/rear sprockets.. this is more typical with rusty chains, or poorly installed chains.

[QUOTE=maddog34;19517462]I flip the bike upside down, /QUOTE] As you learn more about bike repair, you'll want to invest in a proper work stand. You'll be happy you did!

Look for broken teeth on the cassette, too. My BIL managed to do that to his own MTB.

[QUOTE=shelbyfv;19517777] i have two stands... for derailleur hanger tweaking, i like to have the bike on the floor, and upside down.

Thanks for all the responses!

So, I've made some progress in actually tracking down the problem.

When I put it in the largest gear at the back, the largest cog on the cassette, the jockey wheel and the idler wheel all line up perfectly. The same for the 2nd and 3rd largest gear. BUT, when I put it into the 4th largest I can see a visible difference in the alignment - the jockey wheel is slightly outside and the idler is outside ever further. In the smallest gear they're both way out (not sure how I didn't notice this before). So it looks like the derailleur is bent outwards.

So, my new questions are:

1) how can I tell if it's the derailleur that's bent or if it's the hanger that's bent?
2) how do I go about fixing either of the above?

Thanks again for all your responses, very helpful lot you are!

"So, my new questions are:

1) how can I tell if it's the derailleur that's bent or if it's the hanger that's bent?
2) how do I go about fixing either of the above?"


is your hanger part of the FRAME, or part of the DERAILLEUR?

sounds like it's TWISTED... FLIP the bike onto it's seat and bars... then, look at the jockey/idler cage from above, with the gears in the smallest sprockets, front and rear... apply an adjustable wrench to the hanger if it's part of the derailleur, and twist it back into position, plus a bit, since metal has a MEMORY, and will want to return to it's former state some... IF the hanger is part of the FRAME, remove the derailleur(this prevents damaging the der.), and twist the hanger in the manner described above... Remount the Der., and take another look at the alignment of the der. cage from above... is it now aligned? if so... go ride it after resetting the limit screws, checking/setting the cable(if needed), and testing the shifting... if not aligned, repeat as needed.

Tip: if your bike has INDEXED shifting, turn the high gear limit screw in a turn or so before setting the cable tension, then turn the screw back to it's original position after setting the cable...... this pre-loads the cable a bit, and takes up any slop from sharp bends in the cable routing. ;-) i do this "pre-loading" freehand... works just fine, and saves having to use up adjustment at any adjusting ferrules in the cable run... been doing the pre-loading for 50 years now... i've owned hundreds of bicycles, and have never taken any of them to a shop for repair. Now, i am the "shop" in my town.

Then you should get one of these DAG-2.2 Derailleur Hanger Alignment Gauge | Park Tool Would save you having to turn the bike upside down and give more accurate results. Works for me....

Normally, deraillers are pantograph mechanisms. So if the guide and idler wheels are in a parallel plane to one cog, they should be parallel to all other cogs, unless somehow the cogs aren't parallel.

If the hanger bracket is bent, the guide/idler cage will be out of parallel to ALL the cogs, not just the small one.

I'd like to see a picture, but could it be that the pins and bushings, the ones that fix the derailler pantograph mechanism, are worn? Could you have a worn out derailler?

Alternatly, could your cassette have been assembled poorly, with the small cog or cog spacer not seated correctly so that the small cog is not parallel to the others.

I guess I'd put the bike in low gear (big rear cog) and spin the wheel backwards (so the chain and crank move). Do you see wobble in the small cog? Then spin the crank backwards (so that the freewheel clicks). If you still get no wobble, I'd investigate how "loose" the derailler pantograph is. A loose setup would be unusual, but if the cage is parallel with all other cogs but not the small one, and the small cog is parallel with all other cogs, then the derailler is not maintaining the proper constant angle it needs to.

How old is the derailller? What brand? If it's "Golden Happy Butterfly" brand from over the Western ocean, and the rest of the bike works great, I'd be tempted to try a new inexpensive derailller.
Alternately,

[QUOTE=shelbyfv;19517777]Thats' the one I got: https://www.amazon.ca/RAD-Cycle-Products-Bicycle-Adjustable/dp/B0065PHDZE/ref=cm_cr_arp_d_product_top?ie=UTF8. Not expensive and does the job.

Thanks again for the input everyone.

More developments - after taking a closer look at the rear cogs, it's possible they ARE worn out as winding the pedals backwards and watching for a wobble revealed that some of the teeth seem to be worn a lot more on one side than the other, could this cause slippage? I will post a picture of the cassette once I manage to get the back wheel off (the nuts look to be rusted on) so you guys can take a look and give your expert opinions.

Also, it turns out I was using an unreliable method to see if the cog, idler and guide were lined up. The idler is in fact too far inwards on the big cogs and not perfectly lined up. I attempted to bend the hanger a bit to get things lined up but wasn't really successful.

Given that the large cogs seem to work fine with the idler not quite lining up, I think my inability to gauge whether or not the cogs were worn led me down the garden path a bit.

Success!

Turns out it was the rear cogs - I bought a $20 replacement set and I'm back in action.

http ://imgur.com/a/xWB4E

Thanks again for your help everyone.