jim p

I have a bike with a 40/52 chain rings up front. The derailer is a suntour BL. This is friction downtube shifter that I am using to operate the derailer. My problem is that if I move the shifter to maximum travel and hold the shifter as I pedal the chain will shift past the large chain ring and drop onto the crank. If I move the shifter to the max travel position and then release the shifter and then pedal slowly it will usually shift fine but I always have to be aware that it could drop the chain onto the crank arm. The derailer has two adjusting screws. One adjust the travel when shifting to the small ring but the other screw does not seem to adjust the limit when shifting to the large ring. I have moved the adjusting screw which I think should adjust the travel for the larger ring but either it does not have enough adjustment or it is not actually ment to adjusts the travel.

What am I missing?

Do I need one of those plastic rings that goes on the large chain riing to prevent dropping the chain?

Al1943

I'm not familliar with your particular shifters, others may have better suggestions.
When adjusting the high limit screw the chain must be on the large chainring and smallest cog.
Likewise when adjusting the low limit screw the chain must be on the small chainring and largest cog.
Most friction shifters have an adjustment for the amount of friction on the shifter, you should not need to hold the shifter in place while shifting. See if you can increase the friction a bit.

Al

due ruote 

I've never heard of a FD that didn't have adjusting screws for both travel limits, and since yours has two screws, one of them must be for the outward limit. I'd take another look at it. I assume that if you had changed your BB spindle length, you would have mentioned it?

jim p

I'll take another look. I am thinking that maybe I need a longer adjusting screw. I have the screw all the way in and I think that it needs to go in even further. Thanks for the suggestions. If I can't get it adjusted I will either live with it or pull a derailer off one of my junk bikes.

jim p

Just to let you know what I found. The adjusting screw worked fine. I was just a little timid to turn it as hard as needed. A little extra torque and the derailer moved like it was supposed to and I was able to adjust it so that it shifted from the small ring to the large ring with about one pedal revolution. This worked fine but it still wanted to throw the chain off the large ring onto the crank arm. My solution was to bend the front outside plate of the derailer about 1/8". This holds the chain and keeps it from falling off the large ring while I am shifting. I have now put about 500 miles on the last adjustment with no problems so I am now calling it fixed.

Thanks for the help.