another tech talk,
Happy to post the following on different appropriate thread if someone shares which one.
Since the time we were on about rear mechs & cassettes etc., I wanted to figure out the maths of why does a 46/34 front works with an 11/40 rear and also compare with the new GRX.
We know that the Ultegra RD-R8000-GS & RD-RX-800-GS, that is meant for a 34 tooth max rear cassette can handle a 40 with the B screw adjustment. Presumably due to a larger body size than previous generations.
But why does Wolftooth say that with a 46/30 front with an 11/42, it requires an XT rear mech? = mech capacity.
Ultegra RD-R8000-GS = 39 Teeth
Ultegra RD-RX-800-GS = 39T
GRX RD-RX812 = 31T
Medium cage? Or is it longer?
GRX RD-RX810 = 40T
Long Cage :
XT RD-M80000-SGS = 47T
If we look again at the 46/34 front with an 11/40 and do the maths: (46-34) + (40-11) = 41
To go 2 teeth over the capacity of 39 is something like a chain link difference so I can see how the medium cage Ultegra can handle this. With only a 12T difference at the front this makes sense of why this setup works well.
Interestingly the new GRX capacity are not what I expected. They are meant as an either/or option of a 1x, or 2x with their new crank and a max 34T at the rear.
If we look at a 46/30 front with an 11/42 then (46-30) + (42-11) = 47
Et voila long cage XT mech to take up the chain slack from 16T at the front and 31 at the rear.
The GRX RD-RX810 has a rated capacity of 40T. Presumably one more T to take up the slack from the 17T difference in their 48/31 crank offering.
What if we wanted to run that new crank with an 11/40 rear? (48-31) + (40-11) = 46
That is XT territory. Will the GRX RD-RX810 work? Can a 40T capacity be stretched to 46? Or 48 if we wanted to run an 11/42 at the rear?
It depends on the length of that cage on the RX810. Maybe it is longer and more like the long cage XT than the mediums?
After some googling I have not found anything. It would be good to know before spending the time and money on another experiment.