It is grossly slack when you actually put the saddle up to pedalling position and it is 70 degrees. The 73.7 is the effective seat tube angle when the saddle rails are in line with the top of the headtube. The actual angle is very slack likely in the mid 60 deg range so when you raise the saddle the esta is slacker as you go up.
For an XL the esta is basically when the saddle is dropped almost all the way down. Enough with the geometry lesson though - when you don't get it I guess you don't get it.
WRONG! the stated ESTA is the point at which the angle created from the middle of the BB dissects the midline of the actual seattube angle at the point parallel to the middle of the centre of the top of the headtube… the seat rails have no affect on the measurement of this angle… But when you don't get it, I guess you don't get it!
Also, it's trigonometry not just geometry: some basic math can give you an estimate providing a couple assumptions. Main assumption is that the diagram provided by Norco is a good approximation of an XL frame (big if because who knows what size it's based on)
I'll do some rudamentary math below for a 37" inseam that jbv quoted… feel free to use my math and figure out your value based on your own seat height measurement (remember again, assumptions are being made)
ESTA is at the stack height… 620mm
ST is 510mm
37" inseam = 939.8mm = ~940mm (I'll make a few other approximations)
crankarm length 175mm
estimated extended length at seat provided shoe + pedal is 15mm thick (this value is likely higher for most) = 940 - 175 - 15 = 750mm
root of (TTH-Reach)^2 + S^2 = root(182^2 + 620^2) = ~646mm at ESTA
750 - 646 = 104mm of post extending past ESTA height.
This is where assumption of diagram being accurate to a XL frame sizing come in:
ESTA appears to be inline with edge of ST, ST is probably 34.9mm thick (assumption #2)… lets assume over the ESTA height minus ST height that the post moves rearward half the distance of the ST thickness (Aft) = 17.45mm over a length 136mm (ESTA height minus ST)
So extended height minus ST = 750 - 520 = 230 = Visible post (VP)
VP divided by actual length of post at ESTA multiplied by Aft measurement = distance behind ESTA line at the top of the seat in the middle of the actual line created by the seattube
230/136 x 17.45mm = 29.5mm
So if you had a 37" inseam… the seat would sit 3cm behind the ESTA line… Depending on your saddle position, you should easily be able to make up 1.5-2+cm forward on the rails.
I can't see how this could put you at a ST in the mid 60 degrees… unless you have a mid 40" inseam…
Even if the Aft measurement was another 5mm longer it would put you at ~38mm behind the ESTA line. 10mm would be 46.4mm
Again, you can always decrease these values by moving the saddle forward on the seatrails.
I'm just wondering if anyone has had a chance to ride a 2017 29er Sight yet? All the speculating at number crunching won't serve any good until someone is ACTUALLY RIDING THE BIKE…