Framebuilders - Actual head tube angle / changing fork atc

Hi, not sure if this would be the correct forum for this...

I am trying to figure out what my actual head tube angle is on my bike.

http://volumebikes.com/70026/wp-content/uploads/cutter-V6-53-drawing.png

You can see this frame has a 74 degree HTA with a 366 fork length. I am currently running a 420 fork. I have searched around, and read the rule of thumb is every 10mm of fork atc will alter the head tube angle by .5 degrees. So based off that, I determined a 71.3 HTA.

But not sure about that. I actually threw on a 375 atc fork with 38mm of rake this weekend expecting a dramatic difference, and it really wasn't much.

Tried some trig on it to attempt to get the angle, but didn't have much luck. The only part I know for sure would be the atc + head tube length. Which isn't enough.

Thanks for any help.

Focusing on only one dimension of steering geometry is not going to give the complete picture. You do give passing notice to another dimension, one of the forks' rake.

It is relitively easy to mock up the various forks, knowing ATC and rakes with a constant initial head angle. Run the math and see the changes of head angles. But don't expect radical real life changes to handling. There're more factors involved then steering end stuff. BTW, don't trust the published specs. Measurements are the best data. Andy.

Changing the fork makes the frame pivot around the rear axle. So the angle change is approx. arctan(fork length difference / wheelbase). 3 degrees sounds about right.

Comparing trail is what I am doing. Based on a 71.3 hta with 32 rake and 28c tires gives me a trail of 81. Putting on the other fork with an estimated 73.5 hta with 38 rake would give me a trail of 62. I was expecting much more of a change. Which is why I am thinking my estimated hta is completely wrong.

I was hoping someone could tell me how the math is done. There has to be some kind of formula. I found the 10mm of atc = .5 hta on the bike geo wikipedia page. Also found 1 inch of fork atc = 1 degree of hta in a mountain biking forum discussion. Which of course completely conflict.

Thanks.

Didn't notice your reply. Thanks tuz. Need to get an angle finder and just measure I think.

Interesting how some manufacturers are reporting hta. Especially on freestyle type bikes.

best bet is just to draw the triangles out and do the math from first principles. Or download and run rattlecad (http://sourceforge.net/projects/rattlecad/) with the appropriate inputs