MOT failure - front suspension play - please help

[jimiller]

Hi All,

My 1969 saloon just failed its MOT today, and I was wondering if I could get advice. It failed because “nearside front suspension has excessive play in a pin or bush of an upper wishbone inner bearing.” Essentially, when you yanked on the top of the wheel, the whole suspension (pin, damper arm, etc.) moved more than the opposite side. The tester speculated it was the top trunnion bush.

The curious thing is trying to sort out the cause. The bushes seem pretty tight, and there is no play between the wheel and backplate, so the bearings are fine. It really just looks like suspension travel. When my friends and I worked on the car, we did notice two things. First, both sides of the front suspension were missing their rebound bump stops. Second, in the absence of the stops, the nearside suspension hangs lower than the offside when the car is raised. This is to the point where the nearside damper arm contacts the bottom of the hole in the inner wing through which it passes, and the offside arm doesn’t. When the nearside suspension is lifted so that it is in the same position as the offside suspension when the offside suspension is fully extended, the play in the nearside suspension disappears.

This raises a few questions. In the absence of the rebound stops, what in the suspension is acting as the rebound stop? Why would one side hang lower than the other (maybe a worn suspension part or steering joint)? Are the rebound stops necessary, and will fitting them get me past my MOT? Is it worth (ie. easy enough) replacing the trunnion bushes for good measure?

In general, the steering on the vehicle feels a bit more vague than usual, but I wonder if this is just a placebo effect.

Any advice you could offer would be well appreciated. Thanks,

Jonathan

[eastona]

In the absence of the rebound stops, what in the suspension is acting as the rebound stop?

Nothing. Except maybe the first bit of bodywork the wheel contacts Seriously, not sure if there's enough suspension travel to do that, but it's worth fitting bump stops.

Are the rebound stops necessary, and will fitting them get me past my MOT?

A good idea to fit them, but it'll not likely get you past the MoT as they've failed you on worn components, not missing bump stops.

You need to raise the car [SUPPORT IT PROPERLY!] and really study the trunnions and rest of the suspension while someone raises and lowers the wheel with a bar underneath it like the MoT chap does. You'll likely find the top trunnion needs replacing.

Not likely to be a wheel bearing with that MoT statement.

They can be tricky to see (I can't work the wheel and see what's moving on my own! I don't have enough hands and eyes!) and it's often worth getting the MoT inspector to point at the worn component and show you how it is moving, that way diagnosis is easier.

Andrew

[rayofleamington]

This is to the point where the nearside damper arm contacts the bottom of the hole in the inner wing through which it passes, and the offside arm doesn’t.

normally both will make contact to the lower bump stop plate. If that's missing, the limit is probably the damper travel.


Unfortunately the description from the MOT guy is useless. If the top trunnion is failed it should be obvious to him and he should be able to describe the problem. If there is excessive play, you need to know what it is before you fix it!

[Peetee]

Would the angle or relative position of the steering arms restrict movement?
And what about the rear mounting for the torsion bars? If one has been reset so it is on the upper hole and the other (on the next spline) on the lower hole? Or maybe one torsion bar is tired?

[aupickup]

just hope it is not the thread on the kingpin, as they are difficult to get and i have just sold my last ones on here

could be a trunnion

[MarkyB]

What do you mean by "rebound bump stops"?
There should be cone shaped bump stops above the suspension leg on the inner wing and some small rubber blocks on metal things ( note technical term ) that the arms of the shock absorbers rest on when the car is jacked up or if you are rally driving and go over a jump.
The only thing that I can think of that might give the symptoms you describe is the "set" that the drivers side torsion bar takes on due to always being the loaded side.
This might have been adjusted out a few times over the years and might explain what you are seeing.
Was a previous owner very fond of pies by any chance?

I'd try and speak to the bloke who did the MOT and ask him was it the metal part of the top suspension that failed or the rubber part?

The outside of the rubber bush can look OK but still be shot inside, especially with modern rubber.

[aupickup]

the ride height which is of course the torsion is is not an mot failure

[rayofleamington]

Would the angle or relative position of the steering arms restrict movement?

The steering arms wont limit the suspension travel.
As for torsion bar pre-load, if it is so badly out of adjustment that it doesn't meet the bottom stop, then it will be sitting up to the top bump stop.

[jimiller]

Thanks for all the advice and suggestions. The MOT tester did show me what he was talking about. With the vehicle raised, supported at its frame, and with the suspension hanging down, when you grabbed the top of the tyre and rocked it back and forth, the whole pin/trunnion/damper arm rocked back and forth with it. The motion seemed like suspension travel, and to me it looked like the nearside was worse than the offside because the kinematics of the suspension were different (caused by the nearside wheel/suspension hanging lower). The MOT guy told me to replace the rubber in the trunnion, but I’m not sure I believe that’s the cause, since he was fairly vague about it, the rubber looked solid, and I’m not sure whether a worn bush would cause the problem we’re seeing..

The asymmetry in the suspension may be a torsion bar thing, though the previous owner was a short lady, but I also wonder whether there’s something worn there. Judging from the posts, there are 3 likely possibilities: top trunnion, kingpin, or trunnion bush. I’m fairly inexperienced with this, so I was wondering if you could advise in how I could diagnose which problem it is. How do I identify whether each of those three parts are worn out? For the bush, it’s just rock the wheel and see if the rubber is compressing, but what about the other parts? Also, could it be something worn lower down in the suspension causing the whole pin to lean over as a result?

Thanks again for the help!

Jonathan

[kennatt]

probably the only thing to do would be to strip the top trunnion down,once all the load is off it you will be able to see exactly where the problem is.there should be no discerable movement in the top trunnion joint.and the bush should be solid with no sign of deterioration to the rubber.If the threads on the king pin are sharply pointed then that is done for ,they should be flat at the tips.

[cormorant]

If you support the car securely on an axle stand, remove the wheel and just jack up a little under the lower arm then you can release some tension from the torsion bar and may stand a chance of seeing movement in the top trunnion (or none if it's ok). If the rubber bushes are perished this should be fairly obvious on a visual inspection.
If the trunnion is badly worn the kingpin is bound to be too and replacing both is the only answer

[eastona]

where are you Jonathon? Or can you video the movement?

Is it between the front upright (leg) and the trunnion

or

is it between the trunnion and the damper?

or

is it movement in the damper itself (other than suspension movement)?

Andrew

[MarkyB]

Er...

The MOT guy told me to replace the rubber in the trunnion

Just do what he says, while it is apart you can see how the top trunnion threads are.
If there is play but the threads look OK you may easily get away with replacing the trunnion but NOT the leg.

They were designed this way, all bearings have a soft side and a hard side and the soft side is easy to replace.
That's why big end shells are coated with lead and can be replaced easily.

As a rule of thumb I would say that one suspension leg will wear out 3 bottom trunnions.
Obviously there are lot of variables like the conditions the car is used in and how often they get greased but I think it's about right.

Use plastic bushes as the quality of modern rubber seems to be very poor.

[jimiller]

I’m based in Cambridge. The play looks to me to be between the trunnion and damper, but it’s more that the whole leg is rocking back and forth and that’s where you see the most travel. Everything seemed tight... I’ll go for a closer look, and try to capture a video of it, as well as take the top trunnion apart and check the threads on the leg. And, I guess according to MarkyB I should scope out the bottom trunnion as well if the top trunnion seems fine? Andrew, am I right that:

Play between the front upright and the trunnion is possibly a king pin or trunnion issue?
Play between the trunnion and damper is a bush?
Movement in the damper itself is a worn damper?

Thanks again for the help. Some parts are already on order (failed on front brake cylinders as well) and the 10 working day clock is ticking

Jonathan

[bmcecosse]

If it's not the upper bushes (and it likely IS , except you keep insisting it's not) - then the next possibility is that the damper has ben damaged by running with the restraint of the bump and rebound stops - and that is what is moving about. Also check if the dampers are securly bolted to the crossmember! Fit bump and rebouns stops asap - they are essential. Top bushes are best in polyurethane - waste of time fitting rubber bushes there!

[eastona]

BMC and MarkyB are right (as usual ), change the trunnion and bushes, check the damper bolts and for play while the suspension is stripped, check the thread on the upright also while it's stripped.

You've got some work to do!

Andrew

[jimiller]

Fitted the bushes and rebound stops, pin threads looked fine (whew), tightened up the damper bolts.... and passed the MOT. This is the second year in a row you guys have saved my rear with MOT stuff. If you happen to find yourself in the Cambridge area, stop by for a pint

Jon

[rayofleamington]

Well done to you too!
I'm glad it turned out to be just a bush - as mentioned before, if the MOT guy knows his stuff, he should be able to explain where he found the play.
I'm hoping there was (and is) a tab washer with a D shaped hole that was (and is again) bent over the nut to prevent it working loose - you don't mention it so I thought I'd ask.

Your guy sounds good, as he was helpful enough to get it up in the air and show you the issue - I've had some helpful ones and some really unhelpful ones (your only opportunity is to vote with your feet next time if you're unlucky enough to get a difficult MOT tester)

[bmcecosse]

Well done - another year of Minor motoring !

[MarkyB]

MMMMMMMMbeer