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Low sprocket min GRX-812

Old 07-24-22, 10:59 PM
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gk3
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Low sprocket min GRX-812

I have a Jamis Regnegade S2 1x11 with a GRX-812 RD. Specs are:

GRX 812 specs:
Top sprocket: Max. 11T/Min. 11T
Low sprocket: Max. 42T/Min. 40T
Total capacity: 31T

It currently has a 11T-40T cassette. I'm looking at a set of build wheels for the road with a 11T-30T cassette. Would this be pushing the Low Sprocket Min too much? Would it just result in poor shifting or would it actually be dangerous? I'm guess I don't really get the minimum spec since there is probably a sprocket close to 30T somewhere in current cassette and everything works fine.

Thanks in advance,
George
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Old 07-24-22, 11:32 PM
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The low sprocket rating of 40-42 teeth is because of the angle the derailleur is designed to travel & be a moreorless consistent distance from the relevant cog.

Too close & the derailleur will shift by simply crashing the chain into the next cog or dragging it away.

Too far & the angle necessary to derail the chain won't be enough. Shifting will be vague & inconsistent.

There is little safety risk in installing a cassette too small, but at best it'll shift like garbage. Even if you get half the cassette to "sort of work ok" the B-screw adjustment will only be appropriate for one side of the cassette or the other. The other half stands a good chance of not working at all.

The key to smooth, reliable shifting is the derailleur "floating" a consistent & proper distance iaway the cogs. So that the chain is guided over until it self selects the cog you desire. This is the function of the B-screw adjustment. The angle of the parallelogram, spring tension, cog stack, & other factors are specially selected to this end.

Last edited by base2; 07-24-22 at 11:42 PM.
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