I've discovered a problem on my new road bike. I'm new to road bikes and new to vintage bikes so it's a double wammy. I've been riding it for a week now with no rear mech cable. I decided today it's to hard to ride up hills in my top gear so I bought after searching and scrounging I found a local place I can get classic cables. Now that I'm able to shift the rear when I shift the front the chain falls between the sprockets. Having to stop to fix this every time I downshift made slow progress home. Any ideas? Is my chain just to narrow?