I bought some rubber caster bushes from International Auto Parts a little while a go and finally got around to installing them. When I actually took the plastic off I realised all was not as it seemed. One of the bushes is rubber but the other is poly! I am actually quite happy with this arrangement as I have been meaning to try experimenting with a rubber/poly mixture to try and get good steering with minimal shock through the steering wheel.
My question is, which way do I put them in? Rubber closer to the wheel or poly?
Depends on what you're after. I'd image the car moving forward, the wheel will want to be dragged backwards, hence Poly in engine bay will hold wheel forward more than rubber so geometry wont wander as much I guess? The inside bush would mainly be to allow flex (in conjunction with the engine bay bush) up and down with suspension. Can you say the poly bush is hard than the rubber having been cast that way?
Since the poly is stiffer and all the brake reaction is taken by the bush in the wheel arch, that's where I'd put the poly.
Hi GTVeloce,
Send an email to the supplier there may be a specific way they are meant to fit, it may just be a mistake and they do not work together e.g. the poly bush may put more strain on the rubber one. I do not know for sure but this seems the easiest thing to try first. Good luck.
Andrew
If they don't get back to you I would agree with Scott. Under braking the upper part of the upright will want to move forward.
Therefore, I would put the harder material in compression. E.g the Poly push inside the guard and the rubber in the engine bay.
check your physics guys. harder compound should be in engine bay for compressive strength, softer in the wheel well to allow easier vertical movement of castor arm with suspension. remember, under both acceleration and braking, front wheels are wanting to stay put and car body wants to go forward. hence castor arm is inveriably always wanting to pull itself out...
Quote from: Cool Jesus on June 15, 2012, 10:44:24 PM
check your physics guys. harder compound should be in engine bay for compressive strength, softer in the wheel well to allow easier vertical movement of castor arm with suspension. remember, under both acceleration and braking, front wheels are wanting to stay put and car body wants to go forward. hence castor arm is inveriably always wanting to pull itself out...
Until reading this, I would also have agreed with Scott. Not that I care, I got 105 ball joints ;D
Anyways, nice work, Cool Jebus ;)
Hi Duk,
Did you get the adapters from the UK or did you open up the original hole?
Quote from: Tristan Atkins on June 16, 2012, 07:54:04 PM
Hi Duk,
Did you get the adapters from the UK or did you open up the original hole?
I made my own from 10mm steel flat bar.
Yeah Duk, I have the 105 balljoints as well, but the argument then is whether braking is trying to pull the joint apart(bad!) or trying to push the together (OK). I made my spacers from 10mm aluminium plate, and cut a piece of 16mm wide by 8mm thick steel with two M8 tapped holes to sit on the engine bay side for the bolts to screw through into.
Cool Jesus, Duk, here's a sketch of why the caster bar is in compression and why the caster bar bush in the wheel arch is taking the brake reaction.
Scott
Think of it like this: Try to stop a spinning bike wheel by grabbing the rim. Which way does your hand want to go? In the direction the wheel is spinning. Your brake caliper wants to do the same thing and hence with the caster arm mounted on the top of the upright it pushes forward.
Unless of course you are reversing..... ;D
Yeah, I'm still not convinced. What's happening to the tonne of inertia that still has forward motion? Actually just thinking about it the front suspension is after all attached to that moving mass, so yeah i can see the rotating mass wanting to twist, so to say, and push the arm forward. Mind you, these wheels aren't driven and the friction being created on the road would either negate the twist or once again want to pull the arm out??? Could really debate this one all night I guess.
GTVeloce, it looks like a matter of trial and error, unless we have a physics geek on the forum. Would be curious if there is any noticable difference.
Actually, what does the solid castor unit do for the suspension?
Makes no difference whether you're on the bike or standing beside it. The wheel still wants to pull your hand in the direction the wheel is spinning.
All you're really doing with a brake is trying to lock the motion of the wheel to the upright. You're not trying to spin it the other way.
Regardless of pushbikes and Newton, what's the right answer?
My intuition agrees with Cool Jesus - under braking the castor arm will be under compression, so the bush inside the engine bay will be taking the force. I looked at Scott's sketch, and I still think it agrees with me...
To my way of thinking, if the wheel is braking the car, the body inertia is trying to pull forward whilst the brake is (theoretically) pulling backwards, that would try to extend the arm and thereby compressing the bush in the engine bay. At least that is my two cents.
I just checked the IAP website and the front and rear bushes are different part numbers, are they marked or did they come in bags with the number on? if so have a close look at the diagram.
http://www.international-auto.com/file_center/files/suspension/alfetta_suspension_front.gif
Hope that helps.
Andrew
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 ;D ).
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 ;D (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'.
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.
Quote from: GTVeloce on June 19, 2012, 10:23:02 AM
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...
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.
Very little on this page has made things clearer to me. Please none of you take up a career in distance education :D
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?
Quote from: Evan Bottcher on June 19, 2012, 02:14:33 PM
Very little on this page has made things clearer to me. Please none of you take up a career in distance education :D
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.
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 ::)
Quote from: Evan Bottcher on June 19, 2012, 02:14:33 PM
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?
Quote from: Cool Jesus on June 19, 2012, 10:50:29 PM
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