Auto-shifting under load

[Paul]

I have 3x8 set up on one of my bikes. In lowest gear, under load, the chain drops from 1st to second sprocket. It stays in 2nd fine 
It runs perfectly up on the stand. However, there’s a little bit of play in the rear mech at the pivot. Is it just worn out or is there something I can do about it?

[robgul]

Two thoughts:

If you have sideways play on the rear derailleur linkages/pivots that might be the problem with it "flexing sideways" under load

Can you tweak the cable adjustment a tiny amount to take the chain just a gnat's further onto the 1st sprocket (and check the limit screw isn't restricting the chain position on the sprocket)

[Paul]

After my last ride I tweaked it as you describe but I haven't tried it in anger since.

In its original set up, when it would stay in 1st gear, the mech would sometimes ping on the spokes, which is probably not a good thing and which is why I adjusted it. If I can't find a sweet spot (and god knows I need that first gear!) I'm wondering whether there might be enough room on the hub to put a small spacer on before the first sprocket, without compromising the lock ring, or causing the chain (when it's on the smallest sprocket) to foul the stays.

[LittleWheelsandBig]

Hanger alignment?

[Paul]

Ooh, that’s a good question. I don’t think so, but I’ll investigate. Is there a quick and easy method? Something about screwing something into the hanger and checking for parallel-ness with an axle?

[LittleWheelsandBig]

A rear wheel with a Shimano (10x1mm thread) hub is the same thread as a gear hanger.

[chris]

Is the sprocket flexing? Years ago I had a fancy titanium XTR 8 speed cassette where the larger sprockets were mounted on a spider. Whenever I pedalled hard while in the lowest gear the chain would jump to the next sprocket. I changed the cassette for a cheap and cheerful steel one and the problem went away.

[Paul]

A rear wheel with a Shimano (10x1mm thread) hub is the same thread as a gear hanger.

I think I have a shimano axle - is that what I need?

[Paul]

Is the sprocket flexing? Years ago I had a fancy titanium XTR 8 speed cassette where the larger sprockets were mounted on a spider. Whenever I pedalled hard while in the lowest gear the chain would jump to the next sprocket. I changed the cassette for a cheap and cheerful steel one and the problem went away.

I’ll add this to the list if other efforts don’t succeed.

[LittleWheelsandBig]

It is more accurate if you use a complete wheel, rather than just an axle.

[rogerzilla]

Other things to check:

Is the low gear stop adjusted a bit too tight, so bottom gear isn't fully clicked in at the shifter?
Is the shifter badly worn?
In the unlikely event that you're using a friction shifter, frame flex can tug the cable enough to move the lever*.  This shouldn't happen with indexed gears unless the shifter is really worn out.

You can check hanger alignment with a long steel rule and calipers, but you should remove the BB, wheel, and mech first.  This method also assumes the BB shell is in straight.  You should check for inward/outward bend and twist.  I have an old Cinder Cone MTB frame that had a visibly bent hanger, but after 20 minutes' careful tweaking it shifts impeccably.


*my old Thorn would always change to the middle chainring if I tried to sprint in the big ring

[Tim Hall]

Here's a YouTube link showing how to do the hanger straightening thing with another wheel

https://youtu.be/TnwreRrorIA?si=Pvff-gcV8idyeKiq

[Paul]

Here's a YouTube link showing how to do the hanger straightening thing with another wheel

https://youtu.be/TnwreRrorIA?si=Pvff-gcV8idyeKiq

Brilliant! I get that now.