# Thread: AC Height Effect on HT Angle

1. ## AC Height Effect on HT Angle

Anybody recall the ratio between AC height and HT angle? If memory serves (getting less likely every day) I thought it was basically 1" of increase or decrease in AC height translates to about 1 degree increase or decrease of HT angle. Is this right? Thnx

2. Originally Posted by cutthroat
Anybody recall the ratio between AC height and HT angle? If memory serves (getting less likely every day) I thought it was basically 1" of increase or decrease in AC height translates to about 1 degree increase or decrease of HT angle. Is this right? Thnx
That is the ratio. It is a rough equation, of course (so plus or minus a degree ).

Also, more AC also affects all your other geo and angles too. SA & BB height being key.

P

3. Originally Posted by cutthroat
Anybody recall the ratio between AC height and HT angle? If memory serves (getting less likely every day) I thought it was basically 1" of increase or decrease in AC height translates to about 1 degree increase or decrease of HT angle. Is this right? Thnx
That's a very close rule of thumb.

4. Ok - thanks for the quick replies. Guess my memory isn't totally shot yet. Any idea on the effect on the BB height?

5. Dusting off my trigonometry, it looks like a 1" change in A-C will produce a 1.27 deg HA change on a bike with a 45" wheelbase, 1.3 deg HA change on a 44" WB bike, and 1.33 deg HA change on a 43" WB bike. Someone check me. I suck at math.

6. tscheezy, my actual measurements agree with your math.

7. Originally Posted by tscheezy
Someone check me. I suck at math.
Sorry - they made me repeat Algebra II and wouldn't even let me in Trig. If you increase the AC length, don't you also increase the WB? So then what does that do to the HA measurements????
Too confusing to worry about - the point of the inquiry was just to see what effect the travel adjustment on my fork has on the overall geometry of the Spot. Assuming your #s are correct Tscheezy, it looks like every 15mm of adjustment equals about a .75 degree change - so running the fork at 115mm results in a 69.75 degree front end and conversely, opening it up to 145mm gives you a 68.25 HA. The change in ride feel confirms the change in HA - very Burner-like on the low side and more RFX/6 Pack like on the long end. Best of both worlds - I think I'll keep it.

8. ## Geek on

For a 44" WB bike raising the rear 1" would steepen the HA 1.3 -- (1/44) * (180/PI)

For the front you've got to factor in that the fork isn't vertical so you get less change per inch of increased fork A-C. That fudge factor would be 0.92 inch/inch for a 67 degree HA -- sin(67)

So for the front you'd end up with 1.3 * 0.92 or 1.2 degrees/inch

9. Originally Posted by .Danno.
For a 44" WB bike raising the rear 1" would steepen the HA 1.3 -- (1/44) * (180/PI)

For the front you've got to factor in that the fork isn't vertical so you get less change per inch of increased fork A-C. That fudge factor would be 0.92 inch/inch for a 67 degree HA -- sin(67)

So for the front you'd end up with 1.3 * 0.92 or 1.2 degrees/inch
Danno, is your (1/44) * (180/PI) the general formula? wat does PI stand for?? and also the units of measurement used

10. Pi = 3.14... etc. Units are inches and degrees (angle).

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