Accuracy of Manufacturers Stack and Reach Charts

[fogman]

How accurate are the manufacturers Stack and Reach charts?

I have two Pinarellos, both size 51.5.
My 2009 FP3 has a stack of 53.1 and a head tube of 130.
My 2014 Dogma 65.1 has a stack of 52.7 and a head tube of 125.
Yet the Dogma requires 0.5 cm LESS spacers than the FP3 to achieve the same saddle to handlebar drop. All other factors are the same (saddle, stem size and angle, crank length, etc.). Is there some other factor I am missing as this does not make sense to me. I did have a professional BG Fit and this did not make sense to the fitter as well.

Has anyone else encountered these types of discrepancies when using stack and reach to set up a new bike?

[fa63]

It could be that the bottom bracket drop between the two frames is different.

[maxxevv]

Check the stack height of the headset and BB drop differences as suggested. Different headsets can have significantly different stacks.

Most geometry charts have measurements that exclude headset stacks. Haven't seen the two for Storck or the Pina, but I suspect the differences might lie in one having it inside the headset measurements and the other not?

[wingguy]

It could be that the bottom bracket drop between the two frames is different.

And why would that change a stack measurement? Stack is measured from the BB, wherever the BB is. That's the whole point.

To the OP, where did you find the stack and reach of the FP3? I didn't think Pina published it then. Could be a dodgy conversion, could be ( as mentioned) headset heights.

[Calnago]

Suppose you have two bikes with identical forks and offset and headtube angle and headtube length. But one has a BB 7mm lower than the other (68 vs 75mm bb drop for example). The bike with the 75mm drop will have a stack height 7mm greater than the other one. So simply comparing stack heights without considering BB drop could throw things off in your analysis as @fa63 suggests.

[wingguy]

I'm sorry calnago but you've literally just explained why, in that situation, stack will already be correct when head tube height isn't.

The question is about the stack measurement in the manufacturers chart. Stack is measured from the BB. BB drop could be centimetres different between bikes and the published stack measurements will still be directly transferable with no correction needed.

[Calnago]

Lol... @Winguy: you are absolutely correct. Don't know what I was thinking. Good catch.

[fa63]

@wingguy - you are right, thanks for the correction.

[doubleD]

it is also worth checking the fork length from axle to crown.
Many forks have been opened up in the crown area for tire clearance, increasing the axle to crown and stack.

[wingguy]

Again, unless the fork has been changed on the bike aftermarket, the fork length is incorporated into the stack measurement.

The stack is the stack. There is nothing else in the geometry chart that you need to allow for to find out what the stack 'really' is, because it is what it says it is.

[cyclenutnz]

Did your fitter have an XY tool? Judging Bar x,y by relating to saddle position is introducing a source of error.

Pinarello are fairly new to the stack/reach game so 2009 bike figures could be a little off.

[basilic]

Measure it yourself? Place bike on level surface, vertical, rear wheel set against a wall.
Measure distance from wall to top of head tube, from wall to center of BB, subtract, get reach.
Measure height from floor to top of headtube, from floor to center of BB, subtract, get stack.

PS: obvious precautions: avoid crooked floors/walls, have same size tires on front/back

[Briscoelab]

I'm guessing it's a combination of difference in the headsets on the bike and the mfg numbers also being slightly off.

Either way, you've measured your saddle to bar drop and have it dialed, so who cares?

Also consider that measuring and setting saddle height, even by a good fitter, isn't always exact. 5mm of difference is really easy to get in there. If you are using the exact same saddle, they can measure to the center of the seat rails on one bike and easily translate that to another bike (if the other bike has a matching saddle). Measurements to the top of the saddle, even when using a level aren't easy to make and transfer consistently, if you don't have a measurement jig, like the pro teams use.

[wheelsONfire]

Fogman, did you measure the actual fork angle yourself?
I have two bikes in here, they are both told to have 73.5 degree angle at head tube and seat tube.
Just by looking at them, you'll see they are not at same angle.
One bike seems to be, on does not.