School me on the affect of minor geometry tweaks

[robertbb]

Hey PL,

I'm comparing two frames right now, and the important numbers (reach, ETT, STA and HTA) are pretty darn close. Stack's a bit higher on one, and it also has a longer head tube... this should fit me better.

I could make either work, so in terms of handling I'd like to understand what a few minor differences "on paper" should translate to.



Specifically, given both have an identical wheelbase, will the slightly shorter (2mm) chainstays of the bike in left column translate to a front wheel that is slightly further forward of the BB and imply a fork with a higher rake (reducing toe overlap)?

Does BB drop come into play here?

[Heisenberg]

Originally Posted by robertbb

Hey PL,

I'm comparing two frames right now, and the important numbers (reach, ETT, STA and HTA) are pretty darn close. Stack's a bit higher on one, and it also has a longer head tube... this should fit me better.

I could make either work, so in terms of handling I'd like to understand what a few minor differences "on paper" should translate to.



Specifically, given both have an identical wheelbase, will the slightly shorter (2mm) chainstays of the bike in left column translate to a front wheel that is slightly further forward of the BB and imply a fork with a higher rake (reducing toe overlap)?

Does BB drop come into play here?

bb drop doesn’t matter re wheelbase.

guessing 43 vs 45mm rake fork?

the abstract of all of the numbers you’ve presented is otherwise meaningless. i can tell you the taller bike will probably have a quicker steering feel, but that’s a 60% guess at best.

[Dave]

The F-C is really needed. Why are they both missing? To compare reach accurately, the reach on the frame with the shorter stack should be reduced by 3mm, assuming that a 10mm spacer will be added.

Other than the stack, the differences are too small to make much difference.

[Mark McM]

Based on those numbers, the fit will be very close between these two frames. The biggest difference I see is the head angle. In order to keep nearly the same front center (as is the case here with similar chainstay lengths and wheelbases), it would require that the frame with the shallower head angle have less fork offset. Both changes increase trail, so this bike would be more stable/less agile than the one with the steeper head angle.

[dddd]

Stack and Reach dimensions were intended to simplify frame fit measurements such that it would no longer be required to calculate what effect on reach that the seat tube angle was having re the fore-aft positioning of the top tube relative to the bottom bracket.

But Stack and Reach has it's own problem, in that comparing the Reach of two frames is only accurate/meaningful when measured at the same Stack height!

In this case, the bike with the taller stack has a documented Reach of 386mm, but if you want to compare that to the shorter-Stack frame then you should first add about 3mm to the taller frame's Reach measurement.
And note that this will also of course affect the front-center.

So the taller frame might also end up needing a slightly-shorter stem length, which would further quicken the steering beyond what the taller front end is going to cause.
Well, if the bars do end up higher then the stem length will not end up needing to be shorter, further complicating the equation as again the whole fit equation has changed.

Stack and reach makes things simple, right?

[Mark McM]

Originally Posted by dddd

Stack and Reach dimensions were intended to simplify frame fit measurements such that it would no longer be required to calculate what effect on reach that the seat tube angle was having re the fore-aft positioning of the top tube relative to the bottom bracket.

But Stack and Reach has it's own problem, in that comparing the Reach of two frames is only accurate/meaningful when measured at the same Stack height!

That's true. But for bikes with sloping top tubes, what's the alternative? You can't judge how 'tall' a frame is by the seat tube length anymore, so you need some vertical dimension closer to front of the frame (which is now the Stack dimension). And there's always been the problem of judging the 'reach' of a frame, even on frames with horizontal top tubes: Top tube length alone is not enough, because changes in seat tube angle and length affect where the back of the top tube is.

So it is true that you have to do Reach dimension compensation when comparing frames with different Stacks, but this is much easier than the calculations you have perform without Stack and Reach.

Originally Posted by dddd

In this case, the bike with the taller stack has a documented Reach of 386mm, but if you want to compare that to the shorter-Stack frame then you should first add about 3mm to the taller frame's Reach measurement.

That's not really much of a difference in Reach, when you consider that 3mm is smaller than the smallest increments in stem length (which are typically only available in 10mm or sometimes 5mm increments).

[colker]

Originally Posted by Mark McM

That's true. But for bikes with sloping top tubes, what's the alternative? You can't judge how 'tall' a frame is by the seat tube length anymore, so you need some vertical dimension closer to front of the frame (which is now the Stack dimension). And there's always been the problem of judging the 'reach' of a frame, even on frames with horizontal top tubes: Top tube length alone is not enough, because changes in seat tube angle and length affect where the back of the top tube is.

So it is true that you have to do Reach dimension compensation when comparing frames with different Stacks, but this is much easier than the calculations you have perform without Stack and Reach.



That's not really much of a difference in Reach, when you consider that 3mm is smaller than the smallest increments in stem length (which are typically only available in 10mm or sometimes 5mm increments).

I judge how tall a bike is by the length of the headtube. The seat tube measure gives me an idea of stack on traditional threaded fork road frames. The head tube length is the key along w/ top tube. If it´s cross or gravel than fork is taller and i am lost.

[Mark McM]

Originally Posted by colker

I judge how tall a bike is by the length of the headtube. The seat tube measure gives me an idea of stack on traditional threaded fork road frames. The head tube length is the key along w/ top tube. If it´s cross or gravel than fork is taller and i am lost.

Head tube length is not enough, because there are other variables that affect the 'height' of a frame. In addition to the fork length that you mention, bottom bracket drop also affects frame 'height'. That's why Stack was introduced - it is a complete and direct measure of frame 'height' which eliminates the no need to calculate from multiple variables.

[unterhausen]

effective seat tube length is the same as stack, except measured along the angle of the seat tube. I guess I could get used to stack and reach, maybe I should try.

[HenryA]

Yes bottom bracket drop will come into play. I’d want the lower drop.

[Dave]

Originally Posted by unterhausen

effective seat tube length is the same as stack, except measured along the angle of the seat tube. I guess I could get used to stack and reach, maybe I should try.

Stack is a vertical measurement, not one made along the centerline of the seat tube, so it is not the same.

[Mark McM]

Another important fit dimension is saddle setback. This is generally measured to the nose of the saddle. Do we measure from the nose to the seat tube? No. If you did, you'd also have to take into account the seat tube angle and saddle height. Instead, we do the smarter thing and reference it to a fixed point by measuring from the vertical line through the BB. This removes the variables of seat tube length and angle.

Stack and Reach just extend this concept by referencing the top of the head tube to the same fixed point.

[ColonelJLloyd]

I keep thinking of all the frame builders on this forum who will check this thread.

[lhuerta]

Originally Posted by ColonelJLloyd

I keep thinking of all the frame builders on this forum who will check this thread.

... no need. DDDD’s explanation above accurately sums it up....case closed.

[unterhausen]

Frame dimensions don't include components at all. I happen to feel like the position of the top of the effective seat tube is a good point to know. Other than mountain bikes with curved and laid back seat tubes, this gives a good idea of how hard it will be to put the seat in the place where the rider wants it. As far as those mountain bikes, I have no idea how to easily represent the information in a usable way.

My frame fixture is centered around the bottom bracket centerline, so I could modify the tapes on it to read stack and reach. Still need another measurement for the seat tube. But stack, reach and seat tube angle suffice. It can hold curved seat tubes, but measuring it would be a different story. Have to think about that if the occasion ever presents itself