Headset and Geometry question

Will changing headset from zero stack to one with external bottom cup changes the geometry or handling of the bike? Will it be significant?

I can see online the external cup height can range from 10-15mm

I have an old steel frame that I will be rebuilding it has a 44mm headtube with a zero stack headset.
HA is 73
SA is 74
Bb drop is 71

The frame comes with a a straight steerer 1 1/8 with a 365 a-c and 44mm offset fork. However, by the looks of it. The fork is not in a good condition and i cannot find any replacement 1 1/8 fork. So I am leaning towards getting a tapered fork with a similar specs.

Super quick sketch with your data and some other guesses for those not stated. This is with a 15mm higher effective A-C from the external headset. Units are Inches [Millimeters].

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How can increase the stack height change the geometry of the frame? Just changing the stack height, changes your hip angle and how close you are to the top tube (how tall you sit on the bike). Assume we are talking about the same fork, am I missing something? If the rake is changed as a result of replacing the fork, it’ll change the handling. Whether the combination will be significant depends on you and your fit. For me, increasing the stack height does not change my bike’s handling but my back was happier.

  • That is the key. He wants a NEW FORK and can’t find a straight 1-1/8" version. So he is looking to see what happens if he adds the External Cup headset needed to swap to a tapered fork (that is available).
  1. His current setup is a “zero stack” headset and 1-1/8" size. This means the fork pretty much matches the bottom of the head tube on the frame.

  2. He is proposing a change to a tapered fork, with a 1-1/2" lower size, which will require an “external cup” headset, not a “zero stack” equivalent. As such, the new lower cup on the headset will be 10-15mm of new space between a fork and head tube.

    • That acts like a spacer, and effectively adds to the effective axle to crown distance of the fork. This assumes he finds a fork with the same functional axle to crown and offset as mentioned above. That means the bike gets “choppered” a bit from the longer fork length. Hence the decrease in the two main angles by about 0.8* and increase in height.

He was mainly asking about change in handling:

  • I now realize that I gave an estimate of the changes, but didn’t comment on the handling aspect. To that end, this will “slow down” the handling from the slacker head tube angles and slightly longer wheelbase. May be minor and go unnoticed since the deltas aren’t huge, but some riders may be able to feel the difference (for good or bad results).

Does that make sense?

ETA: reference

  • He is going from a setup with the Zero Stack (middle), and potentially getting an External Cup (left), at least on the bottom side. That EC in the same frame head tube will be a new “spacer” between the fork and head tube, like I mentioned above.
  • I think most of the confusion here comes from the dual use of “stack”. He is talking about it mainly in terms of HEADSETS, while you are talking about it in the normal Reach & Stack sense of FRAME geometry.
  • This proposed change of an external cup headset will impact final Frame Reach & Stack since it inclines the front. I thought about including those values, but without a head tube length that matches his bike, my sketch may not represent actual his numbers.
  • Overall, this change will increase functional Stack (higher bars) and shorter functional Reach (closer to bars. All that assumes no other changes to the bike besides the taller external cup headset.

Thanks. I was only thinking of the top cap and missed the differences between the headset type.

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Alright. Just pulled the trigger and get the lower cup and fork and see what happens. Hopefully not a drastic change. If i remember the geometry is copied from an Specialized SL6 so bike is snappy.