You could always measure it , if you have the differant ones . Axle location ( distance ) frame glued to a box ( Beer Crate ) as long as the frame / datum is not varied , axle posn will relate directly .
Somewhat of a fuss is made of it all , the tyres themselves have varying requirements ( per Type ) Trigonics liked ample ( 4 in trail plus ) others required less . 3.00 KR76 on front got light steering.
Trail figures published on the real ( early ) clamps would be with the 3.00 ribbed tyre , fitting 3.25 or 3.50 would increase trail , and raise front , so marginally increae rake , unless you fitted a bigger
tyre to the rear , raiseing it , which , if more , would decrease them . :lol: :lol: Then theres tyre profile , for ' roll steer ' , compounds , and . . . .
String and chalk , and a plumb bob ., a big square ( builders ) tape measure , and flat ground .
A ' big square thing ' that catches consistantly under the frame , so can measure below to axle is fine , or backed against a wall , indoors . :lol:
( or the other way round , Nose to wall , chock under frame for consistancy of hight , measure to footrests or sim , a half in more means its a half in more .)
COMPARATIVE positions . The steering axis inclination ( rake ) is fixed . ( the steering head ) . so AXLE position fwd = more trail , aft = less .
The angle / inclination of the fork legs is a seperate other issue .