Help a guy who's bad at math.

[DrMoab]

K so I have my axle out and I've rotated the knuckles 4*. I need to adjust the upper control arms out to get my castor back to the proper angle. (My pinion needed to be rotated up 4*)

In order to not have to keep unbolting the upper arms I thought I would try a little simple geometry (give me a break, it's been 22 years since I used any in school)

It's 9.25 inches from the lower arm mount to the upper. So using my kids protractor I drew a line 9.25 inches long and then another line 4* off of that. I measured the difference at the top of the two lines and it came out 5/8 in difference. If I dial out my upper arms 5/8 of an inch that should get me in the ball park....right?

It's amazing what you lose when you don't use the skills they teach you in grade school.

 

[DrMoab]

Or....do I need to cut that in half because I'm rotating around the center of the 9.25?

 

[rockclimber]

you aren't rotating on the center of 9.25. you are rotating it around the lower mount so 9.25 is the radius of the circle, not the diameter.... not the center of the distance between the lower and upper.

this math is pretty good... but I would still put an angle finder on it at ride height and check it out. chances are that not everything on the axle is perfect.

also, remember to set the caster with one upper floating and then adjust it until the bolt can go through by hand to avoid pre-loading the bushings.


ahh the joys of double adjustable UCAs

 

[DrMoab]

Yeah I figured it wouldn't be perfect. Just looking for a starting point and hoping ill get lucky.

 

[sdspearo]

Splitting hairs, but the actual number is closer to .645"
x=sin(4deg) x 9.25=.645

Didn't quite follow that last statement about rotating about the center of 9.25, but if you adjust the UCA's by the above amount, the axle should rotate 4 deg from where it was originally. Verify with an angle finder after adjusting.

 

[Vanimal]

they have handy online triangle calculators, or you can look up the formulas like Art posted. I did the math today when i adjusted my caster, and it worked quite well. i didnt need to adjust them after the first try.

 

[DrMoab]

Adjusting mine the 5/8" I hit it spot on first time too. 5* castor, 1* pinion.