What makes some frame's headtubes longer than others? I have a 55cm soma rush with a 130mm ht length, and the 52cm EAI bareknuckle has a 130mm headtube as well... what determines headtube length?

This is kindof a dumb question, but thanks to anyone that can help me answer this!

Old joke,

"your legs are too short"

Whadaya mean too short? They reach the ground, don't they?!!"

Down tubes meet the head just above the fork crown to maximize rigidity at the lower headset. Top tubes vary in height to establish frame size, seat tube length or handlebar height, or some of mix of these.

The head tube needs to be whatever it takes to reach from one to another.

Often manufacturers just measure their frame sizes from different points. Could be that EAI measures 52cm from the center of BB to the center of the top tube, while Soma measures 55cm from the center of BB to the top of the seat tube.

Sometimes the fork is longer or shorter, to accomodate different size tires, and the headtube varies inversely.
Sometimes the top tube has an upward slant making the head tube longer for the same seat tube length.
Sometimes the headtube just extends upward a bit over the top tube.
Sometimes the bottom bracket is higher off the ground, this will also make the head tube longer for the same seat tube length.
"internal" or inset headsets require a longer head tube for the same effective length.

Since the handlebar height is less adjustable than the seat height, the headtube length (or really, the vertical distance from top of headtube to bottom bracket) is often a better indication of the frame's 'height' than the seat tube size.

so wait, the shorter the top tube in proportion to the seattube = taller headtube?

No, it's about height not length. At some level there's some degree of proportion between handlebar height and saddle height. Taller frames usually call for a higher range of handlebar adjustment. Since you want decent cross bracing, you move the top tube height up rather than use extra long stems, and that means you need a taller headtube.

It might help you if you drew yourself a sketch of 2 frames of different heights.

Head tube length is pretty much determined by what the other specs of the bike need to be. It's not like they design the bike and say, "we want this one to have a 130mm head tube." Head tube length is a mostly useless measurement, in that it doesn't tell you any exact information about the rest of the frame.

Generally, as the frame size goes up the head tube gets longer. This is most easily seen comparing two bikes with level top tubes (see pictures.) Things get more confusing with sloping top tubes, because the head tube needs to be longer to meet up with the higher top tube.

Sloping top tubes generally slope down, ie they don't affect head tube length but make for shorter seat tubes.

Some companies do longer head tube variants of some of their framesets for less aggressive riders; a top tube on such a frame could be said to slope up, I guess...

But generally, sloping top tube = compact frame, meaning it slopes down to the seat tube.

BTW, I heartily approve of compact frames... shorten three tubes and lengthen one, which gives you a lighter, stiffer frame with a more compliant ride... pure win.

It's one of the many choices that frame designers can specify.
If its a custom frame , you have a say in that...
buy it pre existing you do not.

Head tube length is a very important dimension, since it really does tell you the true vertical size of the frame. What's actually more relevant is the vertical distance from the center of the BB to the top of the head tube. A number of companies are now listing this dimension om their geometry charts and call it the stack or stack height. This dimension eliminate errors in fitting that might occur from differences in the BB drop. A few brands, like Serotta use an 8cm BB drop on some of their frames and that allows the head tube to be 1cm shorter than a frame with a 7cm drop and still produce the same saddle to handlebar drop (with the same stem setup).

You also have to consider the headset stack height. Frames that use conventional press fit threadless headsets will have a headset stack height of 25-35mm, while a frame with an integrated headset can be as little as 8mm. The frame using the conventional headset can have a shorter head tube length, but still produce the same saddle to bar drop.

Fork length also affects the head tube length. Road bikes use forks with lengths ranging from 360-375mm, while cross bikes use a length around 395mm, so the head tube would be at least 20mm shorter and still have the same stack height as a road frame (if the BB drops are the same).

https://www.cervelo.com/en_us/bikes/2011/R3/geometry/

What makes some frame's headtubes longer than others? --> frame characteristics is what dictates how big or long the headtube will be. Somebody mentioned fork also. Now a days with compact frames you can have frames with the headtube in any length the manufacturer wants for example some treks or specialized have a headtube so long that "at least for me" makes no sense to even put road handlebars to them, better just go and put flat bars.

All depends of the frame you are taking about also, racing track and racing road frames are so different that you could have a difference up to 2 cms in the headtube, being the track one the one with the longest head tube. The crown fork is barely touching the wheels, since it does not use brakes it is useless to even have room for them. The other factor that the BB is higher in the track bike, adding that maybe to a .5 to 2 cms shorter frame the same frame size for a track bike could add 2 cms more in the headtube.

Now if we talk about chap ass bikes as the old varsities or colegiate treks that are still moving around, or cheap asian stuff i would say that there is no rule, those bikes geometry is really weird so sometimes u have a 53 with a super short head tube and a 55 with a heaadtube so big that looks like a 60cm frame.

But basically that;s it, there are more details but why go onto them.