You can measure caster with a camber gauge. I'm still using the cheap $39 bubble-type camber gauge that I bought 15 years ago - I think Racer Wholesale still sells them.
Caster = 1.4 x 'camber change' for +/- 20 deg. steering angle
So...crank the front wheels ('wheels', not 'steering wheel'
20 degrees to one side, then measure camber. Crank the wheels back straight, then 20 deg. to the other side (40 deg. total) and measure camber again. Then just do the math - multiply the total change in one wheel's camber by 1.4.
It helps to have some 'slip plates' under the tires to do this. I use two ~1/8" aluminum plates under each tire, about 12" x 12". Smear'em with grease, or thick oil. I use a cheap protractor at the edge of the plates, so I know when I've gotten to 20 deg. of wheel angle.
More caster, in theory, gives you better loading of the inside tire when turning. In practice, it makes you look like 'Popeye' if you don't have power steering, or the steering wheel isn't big enough. Start eating spinach.
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