Hi

My bike has an annoying habit of changing to a smaller rear cog as I am riding up hills. This only happened since I had the rear cogs replaced due to being totally worn. My old LBS replaced it with a Sun setup which was fine at first but then started to slip, I moved house, took it to my new LBS, they said the cassette was moving and replaced it with a modern looking (ugly) Shimano number which was fine at first but is doing the same now - any suggestions?

(60s lightweight - Campagnolo Nuovo Record mech elsewhere ).


Thanks
Mark

Some freewheels do wobble due to poor tolerances in the machining operations.
As it shows the same problem with both units, The chain is suspect. Even if it is a new chain is it of the correct type. If no stiff links, is it old, stretched out?
Small chance the jockey wheels are binding.
An even smaller chance that the shift lever needs lubrication, take it apart, verify it is assembled the correct way, lubricate it with grease and put it back together.
Campagnolo downtube shifters of that era are really reliable if lubed and assembled correctly.
If you have braze on shifters, install the ringed screw prior with oil and make sure it threads in completely w/o too much effort.
You want to insure that the resistance you feel upon assembly is from the belvel washer spring and not from the threads.

was chain replaced?

also have you had the derailler hanger alighnment checked? I do a modicum of work on bikes for a charity and in many cases poor shifting is related to the hanger being bent. The park tool for this is https://www.parktool.com/product/dera...nt-gauge-dag-2

make sure the links are compatible with the new fw. some fw spacing can be narrower then others and the chain may also have different width's.

+3 on a new chain
Using a worn chain on a new FW/cassette results in a quickly worn out FW/cassette.

The only time this sort of thing has happened to me, the friction on the shifter lever for the rear derailleur wasn't set tight enough.

My friend Drew can Fix your problem.

This is where I am leaning ^^^^^ And +1 for oil on the "ring"-screw threads.


The "friction sandwich" in each shifter can vary quite a lot, and I often find washers missing or parts installed backward or in the wrong order (which can even lead to the thumb screw issue brought up above by repachage).
Even if the friction washers are installed wrong on just one side of the lever, slippage can result.

I often use only plain motor oil in my friction shifters, and am careful what kind of grease I use when I use grease.
Some shifters tend to slip for a while, right after servicing, until the surface layer lubrication seemingly gets contaminated by wear material.

One time when I had tis problem it was caused by the the shifter cable not being installed properly under the bottom bracket, so it did not slide properly and it would self shift. Check both the routing and the lubrication.

+1 on shifter tension issues. My brother's 82 super sport was basically a single speed for a while due to this problem... but much worse. Also, old cable might contribute.

I've had this problem as well. The reason that it becomes apparent going up hills is that there is additional frame flex. Because the cable moves less freely, the RD moves instead with the flexing.

Lots of great feedback - thanks very much - lot's to try now.

It did have a new chain at the same time they replaced the freewheel/cassette

This might help - when I tighten the shifter up so it's quite stiff to operate it stops the problem (but you can feel it trying to slip gear).

My bikes ghost shift on hills too. I just assumed it was the likes of Rudi Altig urging me on. "Courage Paul, courage!"

Is that going up or down, or both?

I slow down going uphill, but not downhill. Could that be due to a work chain?

Morksmith's follow up indicates that it is the shifter that slips first, then of course the derailer follows the resulting cable movement toward a smaller cog.

So Matariki, your comment is still valid.
I think that when the cable can't slide easily within the rear-most cable-housing loop, that frame flexing tugs more forcefully on the shifter, enough to cause unwanted movement. And yes, even the cable guide down at the bb can be a culprit here by preventing the cable wire from "going with the flow" of the downtube's flexing.

But a shifter should never have to be set so stiff and tight as to prevent easy shifting(!!!).
I don't think that a bike can be ridden well in the hills like that, and I always keep my shifters oiled up, and with tension settings as light as I can get away with.
And, except for the possible scenario where a 225lb "weightlifter" is perhaps riding a 753-tubed featherweight steel frameset, I think that everybody's shifters should ideally be set to a similar, very "light" action.

So, modern chain, together with silky-smooth cabling (really inexpensive on a downtube-shifted bike), and a very light shifter tension setting should, taken together, allow most riders to shift fast and effortlessly when needed.

I would take it back to the bike shop that did work on it in the first place, if you cant figure it out on your own. All the things mentioned could be it. I know you should not replace a worn cog without replacing the chain.

All I can say is I cant wait for the snow to go away so I can get on my FIXED GEAR lol

There's also the possibility of frame flex if you are mashing up hills.
Try spinning 1 lower gear and see if you still have the problem.

... shifting to a higher gear on a hill is how you beat the rest to the top...

Check the Campy NR RD pulleys for cracks. They tend to do this with age.
Check the BB and chainstay areas for cracks, since the bike is older.

Lube the friction shifter, cable housings, inspect the shift cable or replace it with a newer die-drawn one.

There must a lot of flex, if it is BB flex, to ghost shift a 5 or 6 speed freewheel.