Caster bushes

GTVeloce · June 12, 2012, 10:03:44 AM

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?

Cool Jesus · June 12, 2012, 10:34:11 PM

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?

scott.venables · June 13, 2012, 03:38:32 PM

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.

Andrew Bose · June 13, 2012, 05:34:27 PM

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

Tristan Atkins · June 15, 2012, 10:54:21 AM

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.

Cool Jesus · 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...

Duk · June 16, 2012, 06:41:14 PM

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

Anyways, nice work, Cool Jebus

Tristan Atkins · 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?

Duk · June 16, 2012, 08:28:28 PM

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.

scott.venables · June 17, 2012, 01:04:58 PM

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

aggie57 · June 17, 2012, 01:37:37 PM

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.....

Cool Jesus · June 17, 2012, 08:24:10 PM

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?

aggie57 · June 18, 2012, 06:42:05 AM

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.

Evan Bottcher · June 18, 2012, 08:21:45 AM

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...

Andrew Bose · June 18, 2012, 06:39:38 PM

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