Chain "slips" on highest gear

paulwn

Hi, I'd be very grateful for any advice on the following problem as I've tried quite a few ideas to solve it without any success and its driving me nuts ......

I recently purchased a second-hand MTB which has all new mech installed as follows :

Shifters : Shimano Altus ST-EF51 8-speed
Rear Derailleur : Shimano Tourney RD-TY300 **
Cassette : Shimano CS-HG 31 8-speed 11-34
Chain : SRAM PC850 8-Speed

** I did spot that the rear derailleur is marked "6/7 speed" but the general opinion on various forms etc is that it should be fine on an 8-speed like this.

All seems to work absolutely fine, except that the chain "slips" very frequently whenever it is in the highest gear - regardless of which chain ring is used.
All other gears work absolutely fine and the problem only occurs when the bike is being ridden, not on the work stand.

Having asked a few other experienced riders and read a few posts on various forums etc, here's what I've already tried to do to fix this :

1/ Checked that the rear derailleur is properly set up according to all the usual instructions (I've set up and adjusted several derailleurs previously on
other bikes with no problems at all)
2/ Adjusted the derailleur cable tension so that the gears only barely change (in both directions, up & down)
3/ Backed out the "high" limit screw completely and detached the derailleur cable to ensure there is enough movement on the derailleur - which there is.
4/ Added a washer between the derailleur hanger and the derailleur itself
5/ Removed a link or two from the chain to change its tension
6/ Adjusted the tension screw on the derailleur to its extremes and in the middle
7/ Checked the cassette for signs of damage to the highest gear and to ensure that there is nothing that the chain could foul on
8/ Ensured that all the mech is undamaged, clean and free running - its all brand new so no real problem there

In each case, I’ve tested to see what the effect is (if anything) and then returned that setting to ‘normal’ before trying another change. None of the above come anywhere close to fixing the problem : some adjustments make it slightly worse, some slightly better but the chain still slips regularly (every pedal rev or two) on the highest gear (only).

All ideas and suggestions gratefully received,
Thanks, Paul

Bojangle

I would guess that the chain was renewed without changing the cassette, get a new chain and cassette and the problem will go away 100% Guaranteed !

JGTR

Hav you checked your chain line?

I had this problem and I adjusted H screw to move the mech nearer the wheel even though I had set it up using Parktools previously.

paulwn

Thanks for the suggestions guys....but both the chain and cassette were replaced at the same time and I've also already tried adjusting the H limit screw as suggested but without success - so I'm afraid those ideas don't solve my problem.
Paul

LimitedGarry

Did you check the chain for any stiff links?
Really can't think of much else, and all of it you probably tried already.
If it's only a problem with that one sprocket and everything else is flawless, I personally would see no alternative but say that the derailleur is indeed incompatible with an 8 speed drivetrain.
If you have the option, might be worth trying out a different chain entirely (not just a different piece of the same make).

Normally would suggest cabling, but you tried it with disconnected cable.

JGTR

Can you get the chain to go past the big ring with your hand when the screw is fully adjusted and the cable disconnected? If not then maybe wrong derailleur. Mine slipped in the big ring when I hadn't adjusted the limit screw enough so if the derailleur hasn't enough movement could cause same issue?

bobbydigital

I'm not really sure what slips means in this context, but could it be that the freehub is the issue?

My current MTB works beautifully when in low gears, I switch to the highest gear on the rear and I cannot put any power through the pedals as the drive slips. If I pedal backwards the chain often comes off due to the freehub not spinning as it should, might be worth a shot. I had it on a previous MTB and went through the whole process of indexing, shortening the chain and making sure everything was straight, finally put a new hub on the rear which solved it.

LimitedGarry

Highest gear means the smallest sprocket.

And what he means, at least what I assume he means, is that the chain is catching onto the larger sprocket next to it and then falling back, or simply sliding over a cog, causing a momentary slip.
This commonly happens for about 100km of riding after you put a new chain on a used cassette. On completely new gear, however, it's definitely a problem with the setup.

paulwn

Thanks for all the additional suggestions, really appreciate it. I have tried a different brand of chain but it made no difference sadly. Both chains are brand new, no stiff links etc. Its hard to tell exactly what the slip is due to as it only happens when riding, but presumably the chain is riding up onto the adjacent sprocket. I'm starting to feel that there must be something wrong with the derailleur itself, but it beats me what that could be !
Paul

JBA

You have a 7-speed rear mech so I suspect it just doesn't have enough travel to properly cover the highest gear. It sounds like it is almost there, but any load causes the chain to slip.

Fit an 8-speed mech and your problem should go away.

larkim

Have you tried riding it with the cable disconnected? Presuming my logic is right and then the bike should be in the gear which is causing you issues, at the very least you'd be able to rule out any issues with cable tension etc at that point. Just saying this as I've often found that what works fine on workstand doesn't work quite as nicely when the bike is actually being ridden.

It's stating the obvious that its not a very good mech either, so I suppose if you could isolate it to being an issue with the mech then it's a relatively cheap fix to get an Altus or Acera mech to replace it.

paulwn

Thanks guys. I have indeed tried riding with the cable disconnected and unfortunately it doesn't help. General opinion seems to be that the problem is with the derailleur itself so I think I'll invest in a better 8-speed one and try that. Will report back once I have done that.
Thanks again, Paul

JohnSync

Hi Paul, did you ever get this fixed? I'm having the same problem just now, it's driving me mad.

steve_sordy

In addition to the suggestions above:

Check that the bolt that holds the mech to the hanger is not loose.
Or the bolt that holds the mech hanger to the frame - same symptom.
Check the the mech arm is not bent. (You will need a tool for this).

JohnSync

Thanks for the tips Steve.