Panhard Rod Bushes

[steve_a]

OK,

I need to consult the collective on Panhard Rod Bushes (PRB or problem as I am thinking of them at the moment!).

I fitted new metalastic, all makes to every area of the discovery D1 300TDi 2" lift back in Dec. I had a terrible power steering leak, but got let down on a replacement box before a decent trip out.

The ATF ate the top bush over the course of the 2000km and we did a bit of bush repair to keep it drivable.

New box fitted and new bush back in in March. 3 Weeks ago got the death wobbles again and on checking the bush was shot again, about an inch of movement on the chassis end, nothing noticable on axle end.

Now, lets get the various usual culprits out of the way:

o It was on the ground when tightened

o I was lazy and didn't replace the slightly worn chassis bolt at the time (naughty I know) and tightened it firmly.

I assumed it was one of the likely causes, eyed holes etc.

o The chassis bracket doesn't seem to have any particular eyeing of the holes

o No visible motion of the bolt (before bush change or afterwards)

o The bolt, while damaged wasn't actually that bad and was a snug fit in the bush

So, I am now starting to consider that the cause of the wear is the chassis mount has been bent in (by us tightening the carp out of while outback) and is gripping the central bush tube. My theory is that this then causes the tube to be held stationary, while the arm moves, effectively tearing the tube away from the rubber, it would fit with how clean the tube had been removed and the condition of the rubber.

However, searches show people suggesting that the inner tube *should* be gripped in the bracket. So that is big question/argument one. Should the chassis be a very light interference fit to the tube, minimizing the motion, but allowing free rotation or should it be gripped tight?

Next, I fitted two new bolts this time. Because I was being anal I note that the shank is a great length when you have the bush on the workbench, but when you add in a washer or two, the chassis brackets, it is too short. The bracket rests on the threaded section, the shank (grip length?) would need to be around 15-20mm longer to have it resting on the bracket and minimizing motion.

So, that's the next one, are the bolts the right ones? Even if they are, should they be replaced by ones with a longer shank. This is not too difficult on the top one, but the axle one would be tricky unless the overall length stays the same.

The only other factor I can think of is that the lift has altered the caster, this has caused the bushes to be loaded with 'twisty' forces, if you can envision what I mean. Though I haven't really noticed this being an issue on any of the other rovers I've had.

And, just to make up for reading a long post about a single bush... Here's a link to a few photos from Outback NSW https://www.facebook.com/media/set/?set=a.10200618218924089.198895.1418294738&type=1&l=070ce8b782

[Diablo]

Have you checked/redone the swivel preload recently? My death wobble was caused by this, and it ate about 3 sets of PRBs before I cured it.

[steve_a]

I can't be 100% sure, but I replaced all of the TREs back around 12 months ago. I would have checked the pre-load at that point I am sure. But haven't checked since.

I also checked and tightened all the wheel bearings around 3 months ago.

There is no steering issues *until* the bush is buggered, in fact it felt awesome with all round new bushes and a brand new (not recon!) steering box, Best rover drive I've had.

The steering damper was replaced in Feb, we changed that outback since we had to give the old girl every chance on the 6hr drive back..

Gut instinct is that it isn't the pre-load, but maybe I'll get around to checking again.

[Diablo]

I had the exact same symptoms.

you may be correct in that it isn't pre-load, but it's a relatively simple check.

[Retroanaconda]

I'm pretty sure the central tube of the bush should be clamped to the bracket, and therefore to the chassis of the vehicle, by the bolt. And the outer tube is a tight fit in the rod eye.

The idea being that all movement is done through the rubber medium.

[steve_a]

OK, tightly clamped - why? Why would you do or want this?

If the bush is free to rotate then the axle drops without binding or damping. That's fine, we have the dampers to do the damping.

What purpose would clamping the central tube do? It would resist motion, it has to, since the inner tube is bonded to the rubber and the rubber is bonded to the outer tube. The outer tube is a very tight (like 5 - 15ton) interference fit. It is not rotating.

I can't imagine anything *but* the rubber parting from one of the tubes unless the rubber is so soft as to not too. In which case it is too soft to do it's primary function of just absorbing the vibration and keeping the axle central front to back.

Logic suggests that you want the tube to be a close fit to the bracket and NOT clamped so tight as to really catch on it. In fact, if you look at the chassis bracket it is about 4mm on each side. That's not really something you expect to be able to bend under the kind of torque value that's mentioned (88NM). The bracket on the axle is thinner, only 2mm steel, but is supported on 3 sides.

Compare that to the radius arms at 197NM which definitely shouldn't rotate.

Also note that it says NOT to apply pressure to the inner rubber during fitting.. And also note that the WSM doesn't say that it shoudl be clamped.

[Nigelw]

What is the torque setting for the P/H rod bolts? I haven't my manual close but that will tell you whether or not they are clamping or lose fitting, but I cannot believe they would be loose fitting due to the stresses that are in play and if they were then they would surely elongate the mounting holes quite quickly under movement!

Although this is a subject I have been thinking on recently after swapping out my bushes from metalastic to poly bushes, now the poly bushes will offer no resistance to the inner tube being tight as they will roll inside the bush, you could see for yourself by fitting only a set of P/H rod poly bushes as a cheap sacrificial real world test and see how you get on?? That's kind of what I am doing with my own truck, read up loads on them and couldn't get a clear answer.

Interestingly I believe the tightening of the bolts onto the inner tubes is to help set and maintain suspension resistance through the bushes as well as the springs and dampers, yes the springs and dampers do set the height but the metalastic bushings will help to maintain it, shot bushes are something I often associate with sagging springs as there is more load on the bushes and they fail much sooner.

Sorry just thinking outside the box a little

[Retroanaconda]

If it's not clamped then the inner sleeve will rotate with suspension movement, which will result in wear against the insides of the bracket - resulting in a loose fit after a while if not straight away.

I, similarly to dieseldog69 above, always thought the bushes contributed to the resistance of the suspension movement and its damping.

[Aragorn]

The crush tube MUST be clamped tight. The twisting of the rubber is where all the movement comes from, and this is why they must also be tightened with the car on the ground. IE when the cars at rest, you want the rubber to also be at rest. If theres any movement at all between the middle tube and the chassis bracket, it'll just chew up the mounting point in no time.

If you tighten them with the car jacked up, the rubber ends up twisted with the car sitting on the ground, and twisting even further as the suspension compresses, which causes the bush to fail sooner.

Maybe try some genuine bushes if the "OEM" ones arent lasting?

[steve_a]

I offer up a rather neat you tube vid..

It shows the potential range of motion (though probably way more than a rod would see due to the radius arms) which is exactly what I was imagining and I thought might be the cause of the bushes.

I agree that there will be wear on the bracket, though I'd prefer to have a couple of washers on the inside to take the pain and be replaced.

In fact I wondered if this was a comprimise of having washers inside to allow easier rotation, less binding and yet still hold tight.

I think with the 4mm metal that the bolt, under normal torque (88NM as I out above btw), should be tight on the bracket, yet not deforming it. This may also be a case for having washers front and rear to increase the surface contact (though surface area has no affect on fritctional force... ).

I'm right off polybushes for panhard rods, I just didn't see good life out of them on the 90 back in the UK, back there it was the only bush I had the metalistic ones on.

No offence to anyone here, but I haven't seen a source yet which looks authoritive that the centre tube should be clamped.

Being overly picky on words for a moment, it is not a crush tube, you have those on the diffs, these are meant to not be crushed.

If it was a specification and vital, then I would expect that Rave would say so.