Weight Weenies

I need advice from a tech wizard. I wanted a bit stiffer big chainring than the one that came with my SRAM S950 compact crankset. Thinking that I only needed to specify 50T 110BCD I bought a Red big ring with that spec and was surprised to find that the spacing between small (34) and big ring is wider with the new Red big ring than it was between small ring and the original big ring. I didn't measure it but it had to be at least 3+ mm wider because I couldn't even come close to shifting onto the big ring with the front derailleur limit screw setting that worked perfectly with the original rings.

What I expected was that the gap between rings would be the same given that both the new and old big ring are 50T 110 BCD, and that a difference as big as I found would require a fairly large widening adjustment of the front derailleur travel which I would expect to affect the angle of chain crossover and probably create noise and wear problems.

Has anybody ever tried installing a Red big ring on a S900/950 compact crankset and found a similar difference in spacing between the two rings? Advice will be greatly appreciated.

thanks, Jeff

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That does not sound right... Are you sure the ring is installed correctly? You should take accurate measurements to confirm this "at least 3+mm variance"

I feel like you did something during installation to disrupt your setup.. How did you change your rings? Take the crankset off or leave it on bike? Move the derailleur at all? Etc...


Got some pics you could post of your setup?

I'm kicking myself for not taking actual measurements of the gap between rings when I had the new Red chainring on the crankset. I thought of this only after I discovered I couldn't shift onto the new Red ring, removed it and put the old, original chainring back on the crankset. After reading your reply I did go measure the offset of the teeth of both chainrings from the back surface and found that the Red chainring appears to have a 1mm wider offset than the original ring. So much for my 3+mm estimate. ( I say "appears" because I didn't have a set of calipers and just eyeballed each on end with a metric rule)

But to answer a few questions: I installed the new Red ring onto the spider without removing the crankset from the bike. Simply lifted off the chain, removed the five bolts and swapped out the old ring, replacing it with the new ring while holding the small ring in place.

I did not make any front derailleur adjustments for the new ring; didn't think I would need to. And when I swapped the old, original big ring back onto the crankset and tightened the bolts I found that the front derailleur was again able to make perfect shifts just like before.

Anyway, now that it seems the gap is only 1mm wider with the new Red ring, I'm wondering if I shouldn't just put it back on, take confirming measurements of the gap between rings, and if it's really just 1mm, adjust the limit screw on the front derailleur to accommodate the wider offset and see what happens.