New Chain Ring

chasbwdc said:

I recently had my Giant OCR tuned and the shop recommended that I replace the chain and the rear cassette as the chain had stretched.

Makes sense from a shop perspective. Drivetrain parts do wear together, and the odds for good performance are better with all new than with a mixed set.

chasbwdc said:

... After this repair the chain now slips off of the middle chain ring to the granny ring when I am in the lowest rear gear.

That's a bit odd. Jumping sideways is quite a different thing than slipping on the same ring.

chasbwdc said:

.. The shop is now suggesting that while they don't see anything wrong with the middle chain ring that I replace this as well

Well, chain rings wear too, if you've run them with elongated chains. But it usually takes several cassettes and chains worth to get there.

chasbwdc said:

...He is also suggesting that I replace my bottom bracket with a shorter one to change the chain angle. Does this sound reasonable?

While not impossible that too would be a bit unusual. Giant should certainly be capable of delivering a bicycle that was correctly spec-ed from the beginning.

dabac said:

Makes sense from a shop perspective. Drivetrain parts do wear together, and the odds for good performance are better with all new than with a mixed set.

That's a bit odd. Jumping sideways is quite a different thing than slipping on the same ring.

Well, chain rings wear too, if you've run them with elongated chains. But it usually takes several cassettes and chains worth to get there.

While not impossible that too would be a bit unusual. Giant should certainly be capable of delivering a bicycle that was correctly spec-ed from the beginning.

Thanks to all of you. I am leaning towards replacing the middle chain ring and leaving the bottom bracket as is. The bike worked fine for almost three years and I am reluctant to change the geometry now.

The chain wont jump off a front ring left or right if it's worn it will slip. ie ride over what's left of the teeth. What moves the chain left or right? Derailleur adjustment. If the ring is worn you can see it from the shape of the teeth, cassettes wear out quicker than chainrings as they get moved about on more often.