sorry for bad alt text, I lack the terminology to describe this accurately
(i'm not a mechanic, i have no earthly idea if this is accurate. Don't sue me)
this post was submitted on 18 Mar 2025
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What's going on with the caster picture? Not sure what it is showing.
In addition to what others have said, it also affects camber in turns. Positive caster when turning gives the outside wheel negative camber and the inside wheel positive camber to keep a larger patch of contact when turning at high speed.
Most regular cars won't have an adjustment and are factory set, but you get into sports cars and after market stuff you can change it.
What it does is controll how hard the car/steering wheel want to point themselves back straight ahead(and a couple other things, but thats the main/most obvious one). All these drawings are way overdramatized to make it easy to see differences in everything that is being talked about. Just a few degrees in caster angle can make a huge difference in how hard it is to turn the steering wheel and how quickly the car will straighten itself if you just let go of the steering wheel.
Pretty much every vehicle you buy now will have a caster angle a bit on the positive side. That way the vehicle always tries to go straight and stable unless you want to turn the wheel. It makes it "more work" to turn, but thanks to power steering that's not really an issue.
A negative caster would be very unstable feeling and pretty much useless to have on a vehicle.
As far as telling from the picture what caster is, it's sort of a bad image for showing it. It would have been more intuitive to picture where the strut was mounted to the car body in relation to where the middle of the wheel is. A positive caster will have a strut angled like ( / )--------( l ) if the car was facing to the left.
it has to do with the angle between the point the tire pivots (steers) on and the point the wheel spins on. The steering point needs to be forward of the spinning point to help the car go straight down the road. Think of a shopping carts front wheels (they're called casters), and how they'll pivot around with the direction you're pushing the cart. That angle can be adjusted so the car tracks properly...
It's showing the 'knuckle' the wheel is mounted to leaning forward and backward.