Should I or shouldn't I, to bush or not Options

[iankarr]

Hi Guys,

I've disassembled the rear of the bumblebee and the control arm pivots and bushings look to be in good shape.

My plan was to blast and powdercoat the control arms, which would require removing the pivots/bushings (due to the heat)...but I hear re-installing these can be a beeotch. And I generally prefer keeping as much OEM as possible.

So...do you think it's worth leaving the bushings and pivots alone and just painitng the arms? I'm aiming for this bumblebee to be a solid #2 car when done.

Thoughts? Thanks!

[Mark Henry]

How should the rear trailing arm hang?

Without shocks or the axles hooked up my trailing arms fairly quickly fall till they point close to straight down.
My front A-arms stayed relativity fixed with the old rubber bushings, but now I have McMarks full motion (bearings) bushings they also will hang straight down like the rears.

I'm debating whether to rebuild the trailing arms or not, but I'm thinking seriously about bracing the console as I have 250+hp.
If I let her rip up to 7K rpm I can feel the ass end getting a little squirrelly, mostly from the right rear.

[Superhawk996]

How should the rear trailing arm hang?

Without shocks or the axles hooked up my trailing arms fairly quickly fall till they point close to straight down.
My front A-arms stayed relativity fixed with the old rubber bushings, but now I have McMarks full motion (bearings) bushings they also will hang straight down like the rears.

I'm debating whether to rebuild the trailing arms or not, but I'm thinking seriously about bracing the console as I have 250+hp.
If I let her rip up to 7K rpm I can feel the ass end getting a little squirrelly, mostly from the right rear.

@Mark Henry

The bottom line it can hang whereever you want. (IMG:style_emoticons/default/shades.gif)

But here's the considerations why that is true in your case.

When you migrate to a low friction trailing arm mount like Bronze Poly, Delron, Rebel Racing etc, you esentially have no torsional rate from the bushing and it will hang free as you state. There is no torsional rate contribution to the suspension kinematics so it doesn't matter where you start from.

Why would you want a torsional rate from the trailing arm bushing?

The short answer is that it does two things:

1) As the angle of the torsion arm winds up or down you're effectively adding or subtracting spring rate in addition to what the coil spring provides. Removing this torsional rate means you need a larger spring. Well that doesn't seem like such a big deal. So you add a higher rate spring.

1a. Now that you have a higher rate spring you also need more damping from the damper to keep the wheel end under control. As an example 140 lb/in spring with a OEM damper is going to be underdamped and feel too floaty or bouncy.

2) The inherent nature of rubber is that it also adds some damping. It reduces the tendency of the training arm to oscillate after you hit a bump (i.e. it adds damping). It also adds NVH isolation and reduces transfered noise into the vehicle structure.

2a. Since most of us can't afford to tune our own dampers we take what the market offers and just settle for what we get. Some might use an adjustable damper which is fine but usually that is only adjusting rebound damping and only makes modest adjustements from one position to another. Sure there are better dampers out there (Ohlin, Penske, etc.) that are very adjustable both in compression, rebound, and even at high/medium/low damping rod speeds but that isn't what we're usually talking about for 914's for street use.

OK so now what? Where should my (Mark Henry's) trailing arm hang?

Since you don't have a torsional rate contribution to consider, the short answer is it doesn't matter to you due to the bushings you've chosen.

But what if others have have rubber OEM bushings?

The short answer is that the factory manual says that it shoud basically be horizontal to the outer control arm mount.



This is the "neutral postion" unloaded (no weigtht on it) position. It's been a while since I installed a set of trailing arms but I recall having to move the trailing arm slighlty with a jack to get the rear damper bottom bolt to line up. When you place the car on the ground the bushing is under some pre-load at curb height.

When you put the trailing arm into compression the bushing is adding torsional rate to the rate provided by the coil spring. As stated earlier the bushing also provides some damping in addtion to the actual damper.

So could it be positioned in other way? Yes.

It could be positioned higher or lower to change the wheel rate curve. Since I don't know anything about the actual rate curve of the OEM trailing arm bushing I can't say how much effect this would have as a percentage of the wheel rate. I know from other vehicles, the bushing contribution can have a sizeable effect depending whether the bushing is tightened in pre-loaded or not.

Many modern cars have a suspension that is tightened down or "neutralized" at curb ride height or even a moderately loaded ride height (say full tank of gas + driver). This is done by leaving the control arm fasteners loose, drawing the body of the car downward to a given ride height by ratchet straps or other means (it's automated in the plant via fixturing) and then tightening the fasteners. When this isn't done properly by the dealerships when a repair is made, the difference is very noticeable and results in a harsh, abrupt ride quality that even average customers will complain about. If the suspension was tightened in full rebuound, the "extra" wind up of various bushings can be excessive at curb height and even worse when fully loaded and leads to premature bushing degradation since the bushing is being cycled though an angular range of motion beyond what it was designed for.

[914_teener]

How should the rear trailing arm hang?

Without shocks or the axles hooked up my trailing arms fairly quickly fall till they point close to straight down.
My front A-arms stayed relativity fixed with the old rubber bushings, but now I have McMarks full motion (bearings) bushings they also will hang straight down like the rears.

I'm debating whether to rebuild the trailing arms or not, but I'm thinking seriously about bracing the console as I have 250+hp.
If I let her rip up to 7K rpm I can feel the ass end getting a little squirrelly, mostly from the right rear.

@Mark Henry

The bottom line it can hang whereever you want. (IMG:style_emoticons/default/shades.gif)

But here's the considerations why that is true in your case.

When you migrate to a low friction trailing arm mount like Bronze Poly, Delron, Rebel Racing etc, you esentially have no torsional rate from the busing and it will hang free as you state. There is no torsional rate contribution to the suspension kinematics so it doesn't matter where you start from.

Why would you want a torsional rate from the trailing arm bushing?

The short answer is that it does two things:

1) As the angle of the torsion arm winds up or down you're effectively adding or subtracting spring rate in addition to what the coil spring provides. Removing this torsional rate means you need a larger spring. Well that doesn't seem like such a big deal. So you add a higher rate spring.

1a. Now that you have a higher rate spring you also need more damping from the damper to keep the wheel end under control.

2) The inherent nature of rubber is that it also adds some damping. It reduces the tendency of the training arm to oscillate after you hit a bump (i.e. it adds damping). It also adds NVH isolation and reduces transfered noise into the vehicle structure.

2a. Since most of us can't afford to tune our own dampers we take what the market offers and just settle for what we get. Some might use an adjustable damper which is fine but usually that is only adjusting rebound damping and only makes modest adjustements from one position to another. Sure there are better dampers out there (Ohlin, Penske, etc.) that are very adjustable both in compression, rebound, and even at high/medium/low damping rod speeds but that isn't what we're usually talking about for 914's for street use.

OK so now what? Where should my (Mark Henry's) trailing arm hang?

Since you don't have a torsional rate contribution to consider, the short answer is it doesn't matter to you due to the bushings you've chosen.

But what if others have have rubber OEM bushings?

The short answer is that the factory manual says that it shoud basically be horizontal to the outer control arm mount.



This is the "neutral postion" unloaded (no weigtht on it) position. It's been a while since I installed a set of trailing arms but I recall having to move the trailing arm slighlty with a jack to get the rear damper bottom bolt to line up. When you place the car on the ground the bushing is under some pre-load at curb height.

When you put the trailing arm into compression the bushing is adding torsional rate to the rate provided by the coil spring. As stated earlier the bushing also provides some damping in addtion to the actual damper.

So could it be positioned in other way? Yes.

It could be positioned higher or lower to change the wheel rate curve. Since I don't know anything about the actual rate curve of the OEM trailing arm bushing I can't say how much effect this would have as a percentage of the wheel rate. I know from other vehicles, the bushing contribution can have a sizeable effect depending whether the bushing is tightened in pre-loaded or not.

Many modern cars have a suspension that is tightened down or "neutralized" at curb ride height or even a moderately loaded ride height (say full tank of gas + driver). This is done by leaving the control arm fasteners loose, drawing the body of the car downward to a given ride height by ratchet straps or other means (it's automated in the plant via fixturing) and then tightening the fasteners. When this isn't done properly by the dealerships when a repair is made, the difference is very noticeable and results in a harsh, abrupt ride quality that even average customers will complain about. If the suspension was tightened in full rebuound, the "extra" wind up of various bushings can be excessive at curb height and even worse when fully loaded and leads to premature bushing degradation since the bushing is being cycled though an angular range of motion beyond what it was designed for.

This is a great thread and comments.

I did this replacement many years ago on my car and the bushing replacement is definately a compromise to the stock arm. I ended up customizing the Billy's with progressive valving and upping the spring rate slightly IIRC.