question about caster

[peej410]

ive been browsing around a little bit and i have what could be a simple question...

how does caster differ from suspension rake?

 

caster defined as "Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side."

 

rake defined as the angle at which the suspension pickup points mount to the chassis. the front pickup point being slightly higher than the rear angling the suspension up.

 

how does increased caster effect high speed bump absorption?

 

how does increased rake effect high speed bump absorption?

 

i know in offroad vehicles a certain amount of rake is used in order to make the suspension a bit more compliant to bumps at high speed.

 

anyone have input on this stuff?

[Challenger]

Try thinking about rake in the form of a motorcycle. If you have more rake the suspension will absorb bumps better and Im guessing youd have the same results on a car. If you had no rake at all the front end would tend to probably just bounce if you hit a bump or put extra stress on the suspension.

[boodlefoof]

Caster is the "lean" of the spindle in side view. Adding caster (top ball joint further back towards the driver than the lower ball joint) will increase the tendency of the wheels to return to straight when in a turn. It will also cause the car to feel more stable at high speeds with less required steering wheel input. I can't see any way in which it would impact how the car handles a bump.

 

I've never heard the term "rake" as you've used it, but I can visualize what you are talking about. Rake will cause caster to change over suspension travel. If the upper control arm is not parallel to the ground, as it rises and falls, the upper ball joint will move forward and backward slightly (in side view). The same is true of the lower ball joint if the lower control arm is not parallel to the ground in side view.

 

If you place the front UCA mount higher than the rear (or the front LCA mount lower than the rear - or both) you're building anti-dive into the suspension. I'm not sure what effect it would have on bumps.

[260DET]

Yeh, the term 'rake' usually refers to the height of the body off the ground front to rear. A car has rake if the rear is higher off the ground than the front.

[johnc]

rake defined as the angle at which the suspension pickup points mount to the chassis. the front pickup point being slightly higher than the rear angling the suspension up.

 

The angle from level of the control arms is normally not called rake. If the angles are parallel between the upper and lower arms (viewed from the side) that's done to get the desired caster and is referred to as caster. If the angles are not parallel between the upper and lower arms then the suspension has some type of dive control (either anti or pro dive) and is generally referred to as anti-dive.

 

how does increased caster effect high speed bump absorption?

 

It adds a very small some amount of bind. In most cases its less then the binds induced by the suspension bushing so on a road car its not a concern.

 

how does increased rake effect high speed bump absorption?

 

Same as for caster unless there is anti-dive. Anti dive increases the bind in the suspension and its a tradeoff between anti-dive and compliance until spring rates get to a racing car levels. Then in most cases all anti-dive is removed from the suspension because the springs are stiff enough to take over that chore.