S1 Trailing Arm Bushes

[88S1]

Ok, I’m trying to understand how the trailing arm bushes should work before I weld my new support brackets on as I have the opportunity to tweak things.

I currently have some metalastic rubber bushes (like the originals) and also some poly ones (not powerflex ones etc, I bought Landrover ones to see if they worked as I knew I would need something to assist in lining up the arm brackets - they would work with some slight mods, but that’s for a different post). My intention is to fit the rubber ones to keep the car as original as possible.

My question is. Should the trailing arm pivot and swing on the bushes?, with the rubber bushes I don’t see how this can occur if the 9/16 bolt is tightened and the metal tube in the bush is therefore clamped to the brackets and the assembly is a press fit to the arm. Or should the bolt just be tightened so it leaves just enough for the assembly to swing. Adrian Venn posted a torque setting last year which says not.

So when clamped up, the rubber is resisting the torque of the arm itself, and therefore can only be tighten once the car is built and fully laiden otherwise if you did it during build there would excessive strain and torsion on it as the arms go to their resisting place. I have read previously that the poly ones do rotate around the tube when assembled, so this isn’t an issue.

What I’m a little confused about, if the poly ones rotate, then surely this affects the handling as there isn’t any resistance from the bushes as there is with the rubber ones and the spring/shock do all the work, which sounds sensible as that’s not the bushes job, but why isn’t the design of the rubber ones like this.

Appreciate any thoughts / previous experience on this matter. @Magpies, you fitted rubber, what did you do.

[magpies]

Hi 88

I measured the chassis height from the ground that I wanted - see wikki on this site, there is a small spread - DO NOT USE MY FIGURES as I have altered mine significantly. I looked at the suspension movement and decided apprx 1" squash was mid range, then set the bodyless chassis at this height and tightened the trailing arm bolts.

[88S1]

magpies said:

Hi 88

I measured the chassis height from the ground that I wanted - see wikki on this site, there is a small spread - DO NOT USE MY FIGURES as I have altered mine significantly. I looked at the suspension movement and decided apprx 1" squash was mid range, then set the bodyless chassis at this height and tightened the trailing arm bolts.

Thanks for the quick response. I’ll get the bushes cut and installed in the arms so I’m dealing with the correct distances between brackets (was going to wait till the arms were blasted, will just have to protect them), I’ll leave the bolts loose until the cars back together then can have a play after.

You were right about the shackle bushes being a few mm too small, I bought both versions, so will cut the longer ones down to length and press in from both sides (my originals were like this anyway), it’s important that they bottom out on each other to ensure the clamping works.

[GreenV8S]

88S1 said:

What I’m a little confused about, if the poly ones rotate, then surely this affects the handling as there isn’t any resistance from the bushes as there is with the rubber ones and the spring/shock do all the work,

The forces involved are not enough to affect the handling. The issue is that it tears the bush apart.

[magpies]

GreenV8S said:

88S1 said:

What I’m a little confused about, if the poly ones rotate, then surely this affects the handling as there isn’t any resistance from the bushes as there is with the rubber ones and the spring/shock do all the work,

The forces involved are not enough to affect the handling. The issue is that it tears the bush apart.

I would have thought the trailing arms should pivot with only minimum resistance, there should be NO sideways or angular motion. So the bush should be solid in the arm 'eye but the bolt should be lubricated allowing the arm to pivot with only slight resistance. If the pivot bolt seizes in the bush then the bush may well tear under the forces of suspension movement. Both the metallastic rubber bush and the modern poly bushes should allow this. That is my theory and others may disagree.

[GreenV8S]

magpies said:

If the pivot bolt seizes in the bush ...

I think the issue is that the conventional installation instructions result in the bush inner being clamped solid to the chassis mounts; even if the bush was a nice bearing fit on the 'pivot' bolt, it would not be free to pivot unless the bolt is left loose.

[88S1]

GreenV8S said:

I think the issue is that the conventional installation instructions result in the bush inner being clamped solid to the chassis mounts; even if the bush was a nice bearing fit on the 'pivot' bolt, it would not be free to pivot unless the bolt is left loose.

Yep that’s my reason for the question. If bolted up tight the resistance of the rubber in the bush would surely affect the effectiveness of the spring and damper a little at least as the arms are not free to rotate.

[GreenV8S]

88S1 said:

resistance of the rubber in the bush would surely affect the effectiveness of the spring and damper a little at least

Not enough to matter in practice. Remember you've got a quarter of a ton of car with quite a lot of leverage via the trailing arm. The torsional stiffness of the bush is insignificant compared to that.

[88S1]

GreenV8S said:

88S1 said:

resistance of the rubber in the bush would surely affect the effectiveness of the spring and damper a little at least

Not enough to matter in practice. Remember you've got a quarter of a ton of car with quite a lot of leverage via the trailing arm. The torsional stiffness of the bush is insignificant compared to that.

Ok, will tighten when cars back together.

[Kitchski]

You tighten the bolts up and the inner crush tube is clamped tight by the chassis. It is not supposed to rotate, and if it did, it'd slowly eat its way through the mounting bracket, causing the bolt to loosen further. You'd also have play in the suspension as the bolt & bush are not an interferance fit.
It's one of the drawbacks of the rubber bush design, for any car not just an S; The arm wants to go one way, and the chassis wants to go the other, and the rubber is bonded in the middle. Eventually the rubber perishes, and the rotational twisting motion causes it to fail.

The poly bushes are different, because aside from a few oddity designs, the inner crush tube and the bush are seperate. You lube the tube, insert it into the bush (honestly, I'm not trying to go all 50 shades here) and install as before, only this time the tube is free to rotate inside the bush, like a bearing would. You get no play, you get free movement (with the damper removed you can rotate the arm on its axis to your heart's content, even when fully torqued up). By fitting a standard rubber bush and deliberately not torquing it up, you're trying to achieve the same result, only it's not. Honestly, there are no advantages that I can see to using rubber bushes these days, certainly on an S, anyway.

You're right in pointing out that the bolt can't be fully-torqued until the car is sitting at its final ride height, yes. With regards to the tension of the bush alone contributing to the handling characteristics, they would play such a small role that I'd barely be perceivable. The best handling S' I've ever driven were all on poly bushes, and none of those had that additional tension. I would say what with the weight of the car, the spring rates....all factors considered, it's irrelevant. If anything, having no tension there improves the way the car drives, because it leaves the springs to do the springing alone, as they're meant to.

As some know, my main four-wheeled passion is old Citroens with hydropneumatic suspension, and all the original, pre-PSA stuff (which generally rode the best) usually had bearings along with bushes; Bearings to do the motion of the arm, and bushes to deal with NVH and take the strain off the bearings & subframes etc.

[phillpot]

Time for my two penn'oth . . . . .

I struggle to see how, with the weight and leverage involved, the big bolt can clamp the crush tube tight enough to stop it turning?

And, supposing it can, is there enough rubber in these bushes to give the suspension range from full droop (car jacked up) to full compression?

[Kitchski]

phillpot said:

Time for my two penn'oth . . . . .

I struggle to see how, with the weight and leverage involved, the big bolt can clamp the crush tube tight enough to stop it turning?

If it didn't clamp it tight enough, you'd be able to prove that theory by removing the rear damper and then moving the arm up and down; If the whole tube is rotating, the arm won't 'spring' back into place, it'll just stay wherever you move it to. But, having removed christ knows how many of them, I can tell you that doesn't happen.

phillpot said:

And, supposing it can, is there enough rubber in these bushes to give the suspension range from full droop (car jacked up) to full compression?

Seemingly so! Even if you remove the damper completely, the arm will pretty much stay in position, and the amount of travel the bushes have to deal with probably isn't as much as you'd think. I'd be amazed if it was more than 10 degrees.

[Adrian@]

Does anyone have access to the Reliant CC forums or know of the specs sizes etc. of the Reliant bush/bolt sizes (on the original design of the trailing arm) and if it had any ongoing changes through the production run. I am presuming the design pre-dates Reliant production usage of 84, by perhaps a year or so (TVR's usage is 86 on).
https://www.ebay.co.uk/itm/254646422038
I am thinking design specs would relate to weight and power being put through the arm.
A@

Edited by Adrian@ on Tuesday 29th December 13:58

[phillpot]

That's a new one to me, never heard mention of them being a Reliant part before!

Every day is a school day

[Adrian@]

Keep up at the back Mike! A@

[Kitchski]

Jesus, I know they were similar, but I didn't realise they were that similar!

[Adrian@]

I have to presume that the moving FROM the Reliant design of the shocker pick-up is TVR's first failing (it takes them 3 redesigns to put it back into the original position). BUT, having never seen a Reliant part in the flesh is it the same size as TVR bushes and overall size That is why I was asking the question! A@

[88S1]

usually had bearings along with bushes; Bearings to do the motion of the arm, and bushes to deal with NVH and take the strain off the bearings & subframes etc.
[/quote]

Thanks for clearing it up Richard.