Caster bushes

[Darryl]

I'm amazed this is still going on. Don't mess with Newton or practical experiments. Aggies bike example is a straight forward "trial and error" (otherwise known as experiment) that I'm sure most of us can remember doing at some stage.

Under braking, I'm sure we can agree that the wheel is having force applied by the road in a rearwards direction, at the point where the tyre contacts the road. The body of the car has mass with momentum applying force in a forwards direction via whatever connection from the car body to the wheel. The wheel has attachment to the car via things attached to the steering upright (the brake caliper and the axle). Lets keep any possible complicating factors out of this (it doesn't really matter) by assuming the wheel is locked. You can stop trying to think about spinning things.... Its just force and distance (and force x distance = torque).

Now we can ignore/cut away anything (in physics models we can just assume things don't bend...) to get just the simple collection of moment arms and forces to figure out.

So we have a near vertical rigid rod/line that connects the contact point with the road through the lower control arm / upright attachment point and on up to the upper control arm/caster rod attachment point.  That line and the points at which it has forces applied are all that matters to Newton.

That line is on Scott's diagram.

So for a gravel rash free alternative to the bike experiment:

Get a pencil (representing that line), push down on it with one index finger hard enough that it will be hard to move on the surface its on (pad). This finger is the upper balljoint.  then use a finger of other hand somewhere on the side of the pencil (lower balljoint)  to try to move it across the page.

Which way are you pushing/pulling with the finger at top of the pencil to counteract this (or which way does the pencil fall over if you don't?). There is no way it will ever want to move the other way under braking, no matter what you do (unless you put the lower control arm underground, or remove it   ).

[Cool Jesus]

Just to add more fuel into the discussion. Was at the shop working on my fetta and thought to have a look at the castor anchorage. The photo shows 30+ years of wear from standard bushings. From what I can see, inside the engine bay it is realtively even in wear, prhaps leaning a little towards wear for upward flex of the suspension. Inside the wheel well shows obvious wear of the castor arm flexing upwards with the suspension. Both side are virtually identical.

I can also see now the spinning torque forces acting on the castor arm. I'm no Izzy Newton, but I still stand by my first theory on bushing placement (this would be my own personal preference), both theories are equally correct in their logic.

Came across my own question too, solid castor arms hold wheel geometry better, at the cost of ride softness can anybody confirm this? Tossing up whether or not to use this on the rebuild and haven't came across any reviews on the 'upgrade'.

[GTVeloce]

When I went back and looked at the plastic packages, it did mention which was front and which was rear. According to IAP, the poly is the front bush while the rubber is the rear (wheel well) which is the way I have them installed at the moment. However, I intend to do a bit of testing and then swap them around soon to see what effect it has.

[Darryl]

When I went back and looked at the plastic packages, it did mention which was front and which was rear. According to IAP, the poly is the front bush while the rubber is the rear (wheel well) which is the way I have them installed at the moment. However, I intend to do a bit of testing and then swap them around soon to see what effect it has.

Interesting. I wonder if there is more to this in terms of component wear/life characteristics rather than the peak load questions we have been focused on? Or maybe transfer of vibration (the rubber will absorb higher frequency vibration better than the poly I expect)? These real world "complications" aren't really covered by simplified gross static force modeling with a pencil...

FWIW the poly bushings I have on the 75 are of a "ball and socket" design and obviously have only one way they can sensibly be assembled - its not just a simple "make the stock bush shapes in poly" approach. They seem to work (produce consistent geometry under braking) better than worn out stock bushings and don't feel obviously harsh or noisy but that is all I'm prepared to say without a complete front-end overhaul and a change one component at a time test...

[aggie57]

The apple falls from the tree, it hits you on the head.  Or does it really?  Maybe it falls beside you as the earth is spinning.

[Evan Bottcher]

Very little on this page has made things clearer to me.  Please none of you take up a career in distance education 

So now my thinking has reversed, and under braking the castor rod is in tension?  Forget the bushes for a sec - which is it - tension or compression?

[aggie57]

Very little on this page has made things clearer to me.  Please none of you take up a career in distance education  

So now my thinking has reversed, and under braking the castor rod is in tension?  Forget the bushes for a sec - which is it - tension or compression?

Compression.  The upright is trying to rotate in the same direction as the wheel.

[Cool Jesus]

No...Tension...

Actually, both tension and compression.

The caster arm (also called a radius arm/rod or a torque arm) is a suspension member intended to control wheel motion in the longitudinal (fore-aft) plane and not wanting to state the obvious, but it's responsible for maintaining caster of the front wheels.

Caster rods typically are mounted ahead of the wheel. In that position they resist dive under braking forces (compression) and wheel hop under acceleration (tension). When the car stops abruptly, this can cause brake dive. In contrast, when a car accelerates suddenly, it can cause the wheels to move up and down enough to come off the ground for a few seconds. To prevent brake dive and wheel hopping, the caster rod is mounted in front of the wheel.

When I chimed in, GTVeloce was after best fitment. It was my opinion that being an Alfa you rarely have your foot on the brake, so best to keep the wheels on the ground and in the straight ahead orientation.

Evan, consider yourself schooled

[aggie57]

So now my thinking has reversed, and under braking the castor rod is in tension?  Forget the bushes for a sec - which is it - tension or compression?

Caster rods typically are mounted ahead of the wheel. In that position they resist dive under braking forces (compression)

http://www.carrollsmith.com/books/tune2win.html