Integrated vs External Cup Lower HS in 44mm HT with Different Length Forks

Mostly a lurker here, but I've learned a lot by reading the postings of the hive mind. I've been thinking about a hypothetical custom frame and have a question regarding frame geometry that I'm sure you folks will be able to answer. Specifically, I want to know if and how a frame's geometry (ST and HT angles, bb height) can remain fixed with either of two forks that have differing axle-to-crown measurements.

The hypothetical bike has a 44mm head tube. I want to use fork A, but I also want the option to use forks B or C at some point in the future because they have cage/light/fender mounts that fork A does not.

Fork A: 383mm A-C. 47mm offset. 1 1/8" straight steerer. Integrated 45° crown race. Ritchey WCS Carbon Gravel fork (Outback v1 fork).

Fork B: 393mm A-C. 50mm offset. 1 1/8" straight steerer. Integrated 45° crown race. Ritchey WCS Carbon Adventure fork (Outback v2 fork).

Fork C: 395mm A-C. 55mm offset. 1 1/2"-1 1/8" tapered steerer. Allygn M.U.D. fork.

Could I keep the frame's geometry effectively* fixed if I used an external cup lower headset with fork A, but used an integrated/zero stack lower headset with fork B? Is the solution for using fork B or C as simple as getting an integrated/zero stack headset with the correct bearing?




* I realize a mm or two difference in fork offset, A-C, or lower headset cup stack height are probably inevitable, but I want to minimize the difference.

I realize now it wouldn't be possible to use fork C with a zero stack lower.

Could the frame be designed for fork A, but leave enough HT below the DT/HT intersection so that if and when I decided to use fork C I could cut ~10mm off the bottom and face it?

Definetly do not make that 10mm cut off stuff, too many reasons not to

If it's impossible for you to decide which fork you want the frame to work with, then I would suggest to design the frame for the "in betweens" point so it would only vary +5mm or -5mm from that ideal spot, depending on which fork you use. Depending which type of bike is, such variations are not such big deal (it's not the same to do it on a pure road racer, a bikepacking gravel or a trekking bike). You can also compensate certain things by tire set-ups (diamters due to widths)

But my best suggestion would be to give it a deeper thought and decide which fork would be best for such bike and just make things 100% right. Considering today's wide offer it's pretty hard not to find one that suits pretty much any specific need you might think of.

good luck

My question to the OP is this, if you’re building a custom frame, why not build two custom forks, with the options that you want for each, to fit the frame correctly?

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Originally Posted by Rody

My question to the OP is this, if you’re building a custom frame, why not build two custom forks, with the options that you want for each, to fit the frame correctly?

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I want to use a carbon fork. I'm not a builder; sorry if that was the impression and if this sub-forum is meant just for builders. I'm currently riding a custom steel fame that was built with a great steel fork with the mounts I wanted an offset to produce ~35mm trail. I wanted to try a carbon fork and neutral trail with the same frame and it's now wearing the Ritchey WCS Gravel which retained the original HT and ST angles, but changed trail to ~57mm. Also dropped about 2# off the front end which is noticeable to say the least. I am able to switch between the two forks, but have been thinking lately it would be nice to just put the steel fork back on permanently and leave that bike fendered, 650b'd, and dynamo-lit while having another frame built that only wears a carbon fork. The Ritchey Gravel fork is great and I'm very happy with it. I would just like to avoid a future frame being forever married to a fork that has somewhat unique specs (middle ground A-C, 1/8" straight steerer) and is already out of production (as far as I can tell). Seems like I could achieve this by designing the bike around the 383mm A-C fork with an external cup lower and using the 393-395mm A-C fork with a zero stack headset. But the latter will only work with a fork that has a 1 1/8" straight steerer if using a 44mm head tube, limiting my options.

My question is as much one about current headsets and head tube options and their ability to work with various forks, I guess. Since I posed this question I started looking into various tapered HT options (more than I thought there were). I think my issue essentially comes down to the question, is there a head tube standard in which you can use an integrated/zero stack headset with a 1.5" tapered steerer and also be able to use an external cup headset with a 1 1/8" straight steerer?

I certainly respect the "shit or get off the pot" rationale to just make a decision and go with it. But, in the last several years so many products and standards are here today and gone tomorrow. I appreciate anyone's time spent humoring me here.

To answer your question, no, not that I have seen to this point in time.

Something for you to think on is this; customers who try to create a do it all recipe for a bike typically find that they have a bike that is a compromise, doing nothing well.

I would encourage you to focus on a design that meets your performance and riding style, and make an effort to perfect it as closely as possible.

Best wishes,

Rody

Quote:

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Originally Posted by Rody

To answer your question, no, not that I have seen to this point in time.

Something for you to think on is this; customers who try to create a do it all recipe for a bike typically find that they have a bike that is a compromise, doing nothing well.

I would encourage you to focus on a design that meets your performance and riding style, and make an effort to perfect it as closely as possible.

Best wishes,

Rody

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I'm just throwing my my $0.02. Rody is spot on here.

I've never seen that Allygn fork before, but it looks like a fine way of connecting your stem to your front wheel while also serving as a nice conduit for your dynamo light. The geometry differences created by swapping out those forks can almost always be mitigated by an appropriate weight shift at the appropriate time. One of the most beautiful attributes of humans is that we're adaptable. I'd bet that if you have your bike built with that fork in mind, you'll adapt to the way you ride with it underneath you and you'll enjoy it as much as you've enjoyed any other bike.

My sincere thanks for the information and thoughtful responses here.

I agree with all you've written, Rody and Sean. Thanks again.