Hammering on the classic.
 

The Bike:
1985ish Benotto/Columbus SL tubing
Campy SR deraileurs, 6 speed regina freewheel
Friction shifters.

Problem: When I put the hammer down the bike consistently down shifts, sometimes putting the chain between the cogs which makes for a very eye opening moment. What is going on here?
I have checked all the mechanicals. Dérailleurs have no play in them and shift perfectly. The levers are nice and tight. The freewheel has no play. Am I running into frame flex? Any other idea's on things I can check?
Assuming it is frame flex I guess there is not really much I can do or is there?

tighten the shifters?

did it.....They are not moving at all unless I move them.

Check the chain.....very carefully!.....look to see if one of the pin ends are broken off or see if a link is bent...

Is the cable routed under the bottom bracket shell?

Yes, it runs through a small guide under the shell.

Sounds like frame flex induced ghost shifting to me. I would liberally lube up the area where the cable goes through the guide. Sometimes that helps a bit.

Did someone slip you a 9-speed chain when you weren't looking? If your chain can actually land between cogs, then that chain is too narrow (which would also cause it to jump off when stressed, because it can't get fully engaged on the cog teeth). Barring the remote possibility that you somehow got you cogs too widely spaced, that's about the only possible explanation for having a chain land between cogs.

good call. I will go out and make sure it is clean and lubed up. Thanks

It won't stay between gears but it slips instantaneously just for a brief moment. Just long enough to make you back off so you don't rack the boys on the top tube. I am sure it is a 6 speed chain......at least I think I am sure :)

I had the same issue on the Gazelle last time I put the hammer down (7sp Sachs freewheel, SRAM 8sp chain, Super Record derailleurs), I'm going to have to give the under the BB area some lubing tonight.

Here is a pic. Maybe someone will see something I don't.

http://i30.photobucket.com/albums/c3...to/Picture.jpg

It looks like a mighty nice drivetrain to me! Let us know if greasing the cable guide helps much. I had the same problem on a bike. The gunk and lack of lube in the guide made it act kind of like a one-way clutch with the cable. The frame would flex and pull the cable and it would kind of stick for a second. I nearly had a crotch on the top tube incident a few times. Cleaning and lubing the guide helped a great deal, but it doesn't make frame flex go away of course.

Looking at the weather it may be a while. I was going to throw her on the trainer to see but I am guessing that by the the trainer holding the bike by the rear axle that will help to eliminate the flex........yes?

Just change the cable and lube it up...Maybe change the cable housing. Changing cables is a worthy low cost upgrade to not have to worry about anything for thousands of miles.

Is the derailller hanger aligned? Looks straight to me, but you can't eyeball it. The shop will have a tool that checks it.

Have you checked to make sure the derailleur hanger is straight?

It eyeballs straight. Shifts perfectly until I am really stomping on it hard. Wouldn't a bent hanger cause poor shifting all the time?

In my experience, a bent DR hanger causes poor ****ing in some gears, not all. So you can get it to sit perfectly on the top gears, but it won't shift correctly all the way across the range.

[edit] Apparently, if you misspell "shifting" the censorbot will get you!

Stiff link in the chain?

Do you have a big enough "loop" in cable going from the chain stay stop to the RD? A big enough loop helps to keep the chain where you want it on the freewheel when you are running friction DT shifters.
Could you post a picture of this area?

I would create a bigger cable loop than the one shown here

NOTTA....

I cannot use my 6 speed Regina freewheel on my Guerciotti with 8 speed cassette and 8 speed chain because as was mentioned the chain is too thin and will 'fit' (more like slide over) between the cogs. If you are only not quite sure what kind of chain you have I'd investigate that again. Perhaps measuring with calipers will suffice but I don't know what the specs would be.