Im aiming to do a frame design with:
Reach: 408
Stack: 548
HT length: 160
What am I doing wrong since I cant keep the HT length with the reach/stack settings?
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If you locking in stack and and HT length from the get go then you’d have to massage the BB drop/height and fork A-C to get a geometry that works. I’d say your constraining yourself too much.
You’ll be better off starting with your BB height, handle bar position (reach/stack) and fork length as driving dimensions, among others. Based of these inputs HT length will be a calculated number. Hope that helps!
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It’s a good input, thanks!
For this case it’s for a customer, and I’d like to meet these design parameters for them. Do you know if it’s possible to ‘lock’ them in in BikeCad? Then as you say, it’s the rest of parameters that needs to be managed.
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Hmm I don’t know of anyway to lock that specific combo. Probably because it’s a bad way to design a frame. I also can’t imagine your customer would be that fixated on an exact heat tube length.
If you really want to go down that route you can display these dimensions on the model and then just adjust things as necessary.
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There is some sentimental value for an old frame with those specs. We are trying to build that again, but I need to plot it in to get my values right.
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You can set your reach and stack, display your head tube length dimension, and tweak you fork a/c until you get the right head tube length.
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Head tube length is a resultant dimension. Not a driver. Bar position is fixed then work back from there.
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I think those that responded already have given great answers. I’ll just add that although BB drop and fork length are likely the best variables to play with if you want to achieve a certain head tube length after having set stack and reach, you could also play with fork offset (rake), lower headset stack height and head angle. Changing any of these dimensions will also influence the head tube length.
When you speak of your desire to control the front end by stack and reach AND head tube length, it would theoretically be possible to add such an option, but it would come at the cost of losing control of some other critical dimension such as fork length. For a summary of the six ways you can currently control the front end of the bike, see bikecad.ca/front_end_6_ways.
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That’s true, but it then got me thinking. What do you guys do when a lot of HTs then actually come in standard sizes?
Fair point but I don’t buy them because exchange rate and shipping mske the hideously expensive. If I did use them Id size fown one step and either make a nice custom spacer or extend the bottom of a custom stem.
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I just cut off pieces of 44mm stock as needed.
On the stack/reach question, you have to remember that stack affects reach, so they’re not truly independent variables. As others have suggested, you should figure out where you want your bars and then let the stack and head tube length be whatever they need to be.
-Walt
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