Chain slipping over chainring
 

When riding my vintage bike with a Campagnolo crank and Origin 8 chainrings, I have noticed from time to time when I drop the chain from the big to small chainring, usually just before a climb, the chain will not engage the small chainrings and skip over the top of the chainring. The skipping will continue until I stop the cranks and coast momentarily and the chain will drop into the teeth. The chain I am using is an 8 speed SRAM chain. This happens maybe once every tow or three rides, so it's not a big deal, I'm just wondering if it's the nature of the beast or can something be tweaked to stop the skipping.

There are two causes, either of which manifest this way.

1- chainring separation is slightly too much allowing the chain to ride the crests of the inner chainring, while leaning on the face of the larger ring. If you narrow the gap slightly it'll bump the chain over, so it drops onto the sprocket.

2- Chairing phasing. If you have matched chainrings, the inner ring is phased so that as the teeth are lined up with the gaps as chain comes off the outer at the shift gate. You can see this if you slowly turn the crank through a shift and see the alignment. When the rings aren't phased correctly the chain lands with the roller hitting the crests, and if you're turning the crank fast enough it'll skate along before the chain drops in. It's analogous to shifting a car transmission before synchromesh.

BTW- if your rings are mismatched, you can improve the phasing, by rotating the inner to the position with best engagement as the chain comes off the shift gate. If you have 5 pin cranks, and the chainring isn't a 40 or 45t the phase will change slightly in each position.

As I said this is a vintage (Nuovo Gran Sport with GS friction shifters) crank with Origin 8 chainrings 52/42 144BCD, that have no ramps or pins. I have another bike with a Vintage Super Record crankset (with OEM SR chainrings). I will check and see if the rings have the proper spacing. Thanks

Edit: well I checked and both have 8mm spacing (CtoC); I also saw that the big CR is not an Origin 8 but an the Ofmega 51t (my memory is not what it used to be as I installed the Ofmega several years ago and have the Origin 8 52t CR waiting in the wings). I will see if turning the big ring will help but I'm thinking it maybe user error. It only occurs when I am spinning in the big ring and shift to to small just before a rise, still spinning the cranks at a good clip (if I slow down the pedaling the skipping does not occur). The issue is once it starts skipping I have to slow my pedaling way down to almost nothing before it will engage; as I change this habit the skipping does not occur so I have to be HUA when it happens now..

OK, the spacing is probably OK, but check it anyway. The remedy is to time your shifts, usually with either foot coming to 12 o'clock, then experiment with the 5 possible positions of the inner ring, so the chain hits the sprocket phased (as well as possible) when you shift at the right time. If you wish, you can help the process by carefully filing one or two teeth lower to create a shift gate. Be careful to bevel them correctly otherwise it'll be a cause of dropped chains. (look at the timing and cut of the gates on any modern ring for a guide).

Long before there were gates, ramps or pins, I used to do this when setting up team race bikes, and it helped make for smother shifting.

You need to change your name. ;)

Maybe his name is correct. The bike might have 10 speeds, but he only rides at one speed regardless.

Uh, wouldn't that obviate the need for the post?

BTW, we should not overlook the simplest explanation - the front derailleur should be checked for alignment with the chainwheels, height over the chainwheels and inner adjustment.

Or maybe he moves across the earth at only one speed.

I think you missed my meaning. I meant that because of engine limitations, he could only ride at one speed, ie. 12mph. OTOH, if he's normally an SS rider, that might be why he's not too familiar with FDs, and multiple chainrings.

...I guess I should have added a wink so you knew I was half jesting!