Technical question

[Tomwoodbury]

One for the bike design experts on here if you don’t mind:

I’ve found a newish small company dealing exclusively in ti frames presumably built by Walty or similar. I had some questions regarding geometry and they advised that there had been a change in their published specs in that they had changed the fork to another with an axle to crown 10mm longer meaning the stack height had increased by the same amount.

My initial thought was that surely you would design a frame around a specific axle to crown height and changing it would indicate an issue with the original design in that it doesn’t handle as intended?

Is this a red flag in terms of bike design or am
I being overly cautious?

Thanks

[fatbikephil]

Cheap way of kicking the head tube out by a further degree and allowing them to say it's compatible with longer travel bouncy forks?
Will raise the BB by circa 4mm....

[Tomwoodbury]

It’s a road bike!

[Bearlegged]

It points to them keeping the head tube the same length. They may or may not have adjusted the rest of the geometry to compensate.

If a-c has gone up by 10mm, and stack height has done the same, that would suggest the BB has stayed the same height above the floor, and they've adjusted the rest of the geo (HTA etc.) to compensate.

If they've kept the geo of the frame the same, and simply lengthened the fork, you'd expect the BB to rise, and (as per Phil's comment) the stack would rise by more.like 6mm than 10mm.

What's actually changed, the manufacturer should be able to confirm. Whether or not it makes a difference is up to you.

[fatbikephil]

Have they switched from an external to an integrated headset? Would lose about 10mm off the stack height and so need a 10mm longer fork....

[Tomwoodbury]

No changes to the frame so ZS44/EC44 on both iterations

[jameso]

+/-10mm on a road fork with no frame adjustment isn't something I'd be happy with but it may work ok. I've tried it on one of my bikes and didn't like the result. I have got a road frame that I intended for either 370 or 375mm forks, everything was specced with that in mind to begin with though.

[Tomwoodbury]

Thanks all. Your answers have enabled me to make a conclusion- going to avoid it.

Tom

[stevenshand]

It seems odd. I'm not sure how James would have designed stuff when he was doing relatively high-volume production stuff but fork dimension is pretty much a starting point for me. If you don't know that, then you don't really know anything. If doing one-off design then it's not uncommon to have to change the fork part way through a design, maybe the customer has changed what they want to use or there's a supply issue. If that happens, then the drawing gets tweaked so that everything else (well, the important stuff) stays the same.

This just sounds like a mismatch between the frames that have probably already been built and the intended fork now being unavailable. It's not ideal and if I was in that position I'm not sure what I'd do.

I think it brings up an interesting point around designing bikes for general sale (not custom fit). You don't have a customer for these bikes and don't know their fit details so for some dimensions you're just picking a number. Ironically, the fit dimensions (reach, stack etc) are the ones that I'd be less worried about as you're having to work within a relatively large range to ensure the bike fits the most people but the important actual outcome of those dimensions (saddle setback, handlebar reach and handlebar stack) can actually be tweaked by components anyway.

The other dimensions (which can have a bigger impact on the ride quality*) I'd be less keen on being different from the spec. If I've come up with 68mm BB drop for example, I presumably ended up with that number deliberately and for good reason. If it ends up being 71mm because of a fork change, then I'd not be happy.

(* I'm not discounting the effect that a riders position on the bike can have on ride quality)

[Tomwoodbury]

Thanks for adding to this Steven. Your thoughts appreciated as always!

[jameso]

Agreed Steven. I'd want the fork dimensions before starting to draw the frame, or I'd work on the fork first if it's steel / made to spec rather than bought-in carbon.

[Tomwoodbury]

I love this forum! My own personal advice from people infinitely more knowledgeable than me.

Thanks again all.