I need to replace the middle (42T) ring on my 105 triple. I would like to replace it with a 39T so I can have exactly the same gearing as my other bike except for the 30T on one for when I get to the hills. I know that the middle and large rings on Shimano cranks have special tooth designs, ramps, pins, etc., to work with specific other rings. In other words, there are two different 52T rings, one designed for use with a 42T and one for 39T. The 42T middle ring also has ramps, etc. As far as I can tell the 39T Shimano ring is intended for use on the double crankset and does not have ramps/pins since you will never shift up to it from a smaller ring. Does this mean it won't work at all or just that the shifting up from the 30 might be a little slower? Ditto for shifting from the 39 up to the 52 if I don't replace the 52 right away with one intended for use with the 39? Using 105 STI 9-speed shifters and front der.