XR4Ti Archived Tech Articles

    Rear Bushing Effect on Camber - A short writeup on the effect of worn out suspension bushings on rear suspension alignment and camber.

Post Date:    Wed, 28 Mar 2001
Subject:      Re: Rear Camber & Urethane Bushings
List:         xr4-hardcore@yahoogroups.com

Brad and others,

Brad wrote:

> What was the effect on rear camber when replacing 
> the stock (tired) rubber bushings with urethane?  I 
> am assuming it would have returned the wheels to
> a more upright position (less negative camber)

I thought this over quite a bit as it's something I've never considered 
before.  Let me try to work through this issue here.

Let's look at it like this.  First, don't be concerned with the beam 
bushings, only focus on the semi-trailing arm mounting bushings.  These 
are pivots, fulcrums.  The suspension rotates around this point.  If you 
ignore the spring, there should essentially be no leverage on this point.

Now let's change our thinking up a little bit.  Instead of a spring on 
the trailing arm, let's put something solid in there.  Now the fulcrum 
isn't at the bushings, it's where the spring belongs.  When a load is put 
on the wheel, what is going to happen?

I suspect most of the compliance will be at the inner bushing, the one 
closest to the differential.  This will increase negative camber.  How 
much are we talking though?  I suspect the change is extremely small.  The 
spring perch is closer to the wheel than the bushings.  I'm guessing the 
total deflection at the bushing is less than 0.5".  I haven't measured, but 
I'm going to guess that the movement at the wheel end will be about 1/3 that 
of the movement at the bushing end.  So we are looking at a deflection of 
about 0.167" at full loading.

However, remember that we have something solid in place of the spring.  Put 
the spring back in and the bushings go back to work as a pivot rather than 
the only source of compliance.

This is where things get out of my hands.  I'm not schooled enough yet to 
understand what's really going on, but I know there is a bunch of forces 
moving around in odd directions.  Remember that the trailing arm moves in an 
arc.  Since this is the case, we are bending the spring to some degree as 
well as compressing in.  This bending of the spring is going to put a load 
on the suspension that we haven't even considered.

Here's what I think all this boils down to.  The compliance of the bushing 
is not significant relative to static suspension alignment.  Naturally 
compliance is a big deal when it comes to maintaining alignment under 
changing loads (racing conditions).

> I'm wondering if anyone has actually taken before 
> and after measurements. 

I haven't heard of this being done.  But if someone knew what their 
alignment was beforehand and just installed the bushings as the only change, 
then did checked the alignment.  However, I'm thinking the change will be 
so small that other variables, such as installing the spring in a slightly 
different orientation, would have more of an effect than the bushing change 
or even a negating effect.

> Also wondering which one of the bushings failing is 
> most responsible for the increased negative camber 

If any bushing is going to have an effect, I think it's going to be the 
inboard bushing.  A couple reasons for this.  One, it's more perpenicular to 
the camber change than the outboard bushing.  Two, see the paragraphs above.

However, I don't think that it's the bushings affecting our camber.  I'm 
thinking 99% of camber change comes from a change in the loaded height of 
the spring (a reduction in ride height).  I don't know if Chris Senior is on 
this list or not, but it's been his theory that it's the bending of the 
crossmember that changes our rear camber.

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Published by Chris Anglin.