Hi there,
I've had this issue before with trying to get a stack/reach, front centre/head tube and wheelbase to all add up on a Ribble.
This time it's a Ribble Endurance SL Disc sport size medium (and large) https://www.ribblecycles.co.uk/ribble-endurance-sl-disc-sport/?colour=red
The only way I can get the stack and reach to make sense with the rest of the bike is to reduce the fork length AC to 353. This didn't feel right so I called their helpline and someone went and measured it for me at 380.
I'm trying to help a client decide between a medium and a large. This could be a deciding factor.
They've also confirmed that it's an integrated headset and there are 3x10mm spacers available. The guy thought the handlebar reach on a 38cm bar would be 90mm, but I'm not convinced by that either.
I didn't upload the file. The first one is using the stack and reach, the second using the FC and headtube. If the stack and reach are correct, the large works well. If it's the front centre, he's going to be slammed
FYI, on geometry geeks they list the seatpost as c-c whereas on bike insights and ribble's website, it looks like c-t
Normally, I like this kind of challenge. I feel like more often then not, I can massage numbers to make all the dimensions in a geometry spec sheet work out in BikeCAD. When a head angle is listed as 73° that might actually be rounded up from 72.8°. Sometimes, it is unclear which method is being used for measuring effective top tube length, but if one works with the numbers I'll go with that. It is frustrating when fork length isn't given, but sometimes being able to freely adjust that is what makes all the other numbers fall into place.
Having said that, this Ribble Endurance SL seems to have some irreconcilable issues that cannot be fixed simply by playing with tenths of millimeters. In the large frame, once we assign a stack of 562mm, a reach of 390mm, a head angle of 73°, a BB drop of 68mm and a fork offset of 45mm, the front centre measurement has to be 592mm. The spec sheet lists the front centre as 598mm but this is impossible given the aforementioned combination of dimensions. There are no other dimensions that have any impact on front centre beyond the ones I've listed.
Looking at their frame dimension diagram, some of the dimensions are attached to unexpected references, I'm not sure if this is a mistake or the explanation for why the dimensions aren't adding up.
For example, the head tube length dimension seems to include part of the integrated headset. Meanwhile, the effective top tube and reach dimensions appear to be ignoring that part of the headset. Adding to the inconsistency, the stack dimension seems to be going to somewhere above even the integrated headset. Is it going to the top of the spacers? I assume it was just a mistake that it was drawn to the reach dimension and not to some physical part of the frame, but at the end of the day, I don't know which if any of these dimensions to trust.