Safety Alert Ford Sierra Compression strut problems

[mocad2016]

Cars affected: All Marlins with Ford based front suspension that have fitted solid compression struts. BMW engine cars with an anti-roll bar that has had the mid section of the anti-roll bar removed to clear the sump. May also affect cars fitted with polyurethane bushes.

A number of members, including me, have replaced the Ford Sierra anti-roll bar (ARB) with adjustable compression strut kits. One or two members have seen problems with the ARB mounting showing signs of pulling away from the chassis. It is the metal of the chassis that is sheering not the weld that is failing.

Our initial thoughts are that the solid fixture of the compression strut is transmitting more shock energy to the mounting point than the rubber bushed ARB. It may also be because without cross bracing the forces are absorbed by the chassis in a different way.

We would like all members to be aware of this potential problem and regularly inspect the mounting point for any possible problems.

Some members are looking at possible improvements in this area and we will let members know if a better design is developed.
Paul

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[mocad2016]

Re: Safety Alert Ford Sierra Compression strut problems

My own car Millie was converted less than a year ago. She had covered about 26,000 miles with no signs of cracking. She has done about 2,700 since conversion and has signs of cracking in the paint. I will be investigating the underlying metal shortly.

Meanwhile, does anyone have a spare ant-roll bar they don’t need?
Paul

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[A_Hogg]

Re: Safety Alert Ford Sierra Compression strut problems

That was how mine went [BMW powered with centre removed from roll bar]. Doug and Liz Billings son plated the chassis so that forces were distributed over greater area.
Probably better to reinstate the original anti roll bar if possible rather than adopt this modification.

[greyV8pete]

Re: Safety Alert Ford Sierra Compression strut problems

That was how mine went [BMW powered with centre removed from roll bar]. Doug and Liz Billings son plated the chassis so that forces were distributed over greater area.
Probably better to reinstate the original anti roll bar if possible rather than adopt this modification.

What D bushes were fitted with your cut apart ARB? Poly or O/E rubber?

[A_Hogg]

Re: Safety Alert Ford Sierra Compression strut problems

Interesting you should ask that question Peter. I had O/E at that fitting but Poly on the Compliance bushes {the ones that attach the anti roll bar to the track control arm]
Mileage may be a factor as mine had around 30,000 so not dissimilar . I previously had a Pinto engine SWB cabrio that showed no such signs of failure.
My view is that the removal of the centre section places too much stress on one side and fitting compression struts further distresses that area , especially as they are sometimes used to control the amount of caster and improve self centering.
It'll be interesting to see whether or not any others come to light.

[dave_clutterbuck]

Re: Safety Alert Ford Sierra Compression strut problems

My nearside ARB mounting suffered this problem early last summer, plate made and welded looked good will keep a close eye on both sides.
Mine is the standard ARB set up

[cabrioman]

Re: Safety Alert Ford Sierra Compression strut problems

This doesn’t look too great.

Its interesting that cars fitted with standard ARB setups, cut ARB and compression struts have all been affected.

It makes me wonder if this is just a weakness in the chassis design now the cars have been on the road for a number of years. Maybe simply not enough meat (metal thickness) in the chassis at that point to withstand the stresses. I think if I were to fabricate a repair plate I would want to try to come up with some sort of U shape plate so it could be extended and welded around the chassis sides in an effort to triangulate the welded mounting.

The pictures seem to show some signs of corrosion where the cracks have appeared but I suppose that could be water ingress into the crack before it got significant enough to be visible.

It will be interesting to get your feedback Paul on the damage to your chassis when you have taken a more detailed look at the areas showing cracking in the paint finish.

The attached image shows my chassis in the blasting booth when I had it taken back to bare metal. There were no signs of any problems at that time and the car had done 70K on a standard ARB setup, that said there is no way of knowing what was or is going on inside the chassis cavity in terms of corrosion.

The car is now running compression struts and the retro fitting of the ARB would not be an option anyway, as the car now has an M20 lump, which precludes use of the standard ARB because of the sump. The engine is as far back as it can go and still allow for maintenance, even after doing a cut and shut on the bulk head to take the M20 engine. You can just see the bulkhead cutout in the picture.

So for me if i had to, it would mean plating it, but as I say if I were to think of modding it I would want to extend the ARB bracket round the edge of the chassis leg to spread the loads.

John

[A_Hogg]

Re: Safety Alert Ford Sierra Compression strut problems

Peter Morris's Cabrio, now in other hands also had the M20 engine. I know that when he re -engined it to take the BMW lump he modified the bulkhead to get the engine as far back as possible. He also managed to install the full ARB from I think a P100 Sierra .

[milliemarlin]

Re: Safety Alert Ford Sierra Compression strut problems

I have had a chance to take of the compression strut and clean off the paint.

This is the o/s mounting
OS block 01.jpg

Here in close up
OS block detail.jpg

Near side close up
Split 01.jpg

Both sides show the weld is pulling away in the corner and beginning to sheer down the side.

surface distortion.jpg

The rule is parallel to the long side of the mounting block in this picture. It shows that the chassis is not flat in this area. I can’t be sure it was flat to start with but it certainly looks like the metal has been pulled out of shape.

Position.jpg
This picture shows the positioning of the block looking up from the ground. It is well supported by the vertical face of the chassis. That will dissipate the fore to aft forces easily. The red line shows the inside vertical face of the chassis. It seems like the block is rotating around that axis. With the ARB the two mountings will avoid that motion. The corner of the block is close to the middle of the chassis section, the weakest point for up/down forces.

Inside distortion.jpg

Inside distortion NS.jpg
View of the inside of the chassis towards the blocks. The internal web shows signs of being bent up. I did note when first assembling the car that these webs were distorted. At the time I assumed it was distorted by heat from the welding. So I would not attribute too much significance to this distortion.

Bent brackets.jpg
The mounting brackets are distorted. The o/s adjustment nut has been rubbing against the bracket sides. This is probably caused because the bracket has bent.

I had wondered if the compression struts may have been hitting the mounting bolts when the suspension is forced up but there is no sign of them touching.

ARB.jpg
This picture allows a comparison between the ARB and the compression strut. I have positioned them approximately 100mm apart but in similar alignment.

The compression strut is much more rigid than the rubber mounted ARB. I think this would make the difference between the blow from a hammer and a rubber mallet. I think the peak force would be spread over a longer time as the rubber distorted.

Looking at the forces, assuming that on average the forces on the wheel will be from the front, that would be downwards from the top of the picture in the direction of the blue arrow. The strut will pass all of the force onto the bracket at the angle of the red line.

The ARB is located by bushes on both sides. I can’t see that a bump on the n/s wheel will transmit much force to the o/s mounting.

I’m not confident in this supposition but it looks as though the ARB will convey forces parallel to the car’s front to back axis; that it is vertically in the picture in the direction of the blue arrow. By virtue of its “S” shape side to side forces will be partly absorbed by the ARB. Not easy to explain this but I’m suggesting front to rear forces are absorbed on one side by the ARB but side to side forces are shared by both sides. The most important thing is that the ARB will not let the mounting twist.

With the strut the front to rear forces is absorbed by the chassis in the same way but all side to side forces are absorbed on one side. This will cause a twisting motion on the mounting plate.

I can’t tell how much unseen damage has been done to my chassis so I will have to weld on a reinforcement plate. That should spread the load across the width of the chassis member. I think I will try to weld a fillet on the inside of the mounting plate. The brake pipes and wiring are in that area which might complicate matters.

My ARB was bent. Which is why I fitted the compression struts. So I really do need an anti-roll bar if anyone has one to sell.
Paul

[cabrioman]

Re: Safety Alert Ford Sierra Compression strut problems

Peter Morris's Cabrio, now in other hands also had the M20 engine. I know that when he re -engined it to take the BMW lump he modified the bulkhead to get the engine as far back as possible. He also managed to install the full ARB from I think a P100 Sierra .

Yep I know Peter managed to do that but to be honest when I modified mine I just couldn’t get the clearance I wanted at the back of the engine when using the ARB. The M20 has a water outlet on the rear of the head which Peter initially blanked off, but it gave him cooling problems and he had to reinstate it. Maybe that was why he thought originally the clearance was ok.

I actually cut out a section of the bulkhead and then rewelded in a modified profile, which moved it back further than Peter had managed by panel beating his bulkhead, but I still didn’t think it produced enough clearance for maintenance at the back of the engine.

I think Simon Greggory had a similar experience and reached the same conclusion when he fitted the M20 into his outstanding build.

We both ended up using compression struts.

In the end if the problems highlighted in the original post are being seen with cars running standard ARB, modified ARB and compression struts, then getting rid of the compression struts won’t solve the potential problem.

[cabrioman]

Re: Safety Alert Ford Sierra Compression strut problems

Interesting findings Paul.

I tend to feel the ARB mount is a bit flawed in its design and could have done with being more substantial, especially the surface area of the mounting plate.

I will have to stick with struts, but I think I may look to fabricate a mount bracket that goes round the edge of the chassis. My thoughts are to add an additional mounting bolt to pass through the chassis side to side to remove any posibility of twisting, which looks to have possibly been the cause of the weld failure.

At least by modyfying the bracket rather then the chassis this will allow me to trial a solution, before I resort to a welding torch on my nice new shiny chassis🙂

[danny_nelson]

Re: Safety Alert Ford Sierra Compression strut problems

To my knowledge Dave clutterbucks car is the only one reported with a failure with a standard ARB. With due respect to Dave I don't think his car can be compared to any of the above. Daves car was subject to years of neglect left in a roofless garage with a tree on top, and in a sad state. The car was then bought and done up before Dave bought it. Dave will tell you that the tops of the rear turrets (shock absorber mounting) Rotted through as well, and had to be welded. So I think this one can be put down to neglect. The first pictures Paul has posted are of the ex Simon Gregory car!!

[milliemarlin]

Re: Safety Alert Ford Sierra Compression strut problems

It might not be practical to fit the ARB to beamers but could a cross brace be fitted. I have in mind something like this which is intended to fit between the tops of rally car suspension legs to provide bracing. Perhaps mounted under the compression strut bracket or even part of it.

I blagged these pictures from carrepairmart.com and bmrsuspension.com hope they don’t mind.

Suspension strut.jpg BMR brace.jpg

[cabrioman]

Re: Safety Alert Ford Sierra Compression strut problems

To my knowledge Dave clutterbucks car is the only one reported with a failure with a standard ARB. With due respect to Dave I don't think his car can be compared to any of the above. Daves car was subject to years of neglect left in a roofless garage with a tree on top, and in a sad state. The car was then bought and done up before Dave bought it. Dave will tell you that the tops of the rear turrets (shock absorber mounting) Rotted through as well, and had to be welded. So I think this one can be put down to neglect. The first pictures Paul has posted are of the ex Simon Gregory car!!

Quite concerning Danny to hear those pics are of Simons car, it can’t have done hardly any mileage I wouldn’t have thought, as it wasn’t road worthy when Simon took it on.

A cross brace could help matters Paul but I can’t help feeling a mount that bolted round the chassis leg like a large U shape would be the best option as done like that I can’t see how it could possibly rotate/flex to cause the damage seen, the welds on the strut mounting lugs would have to be well done though to make sure it didn’t just transfer the failure to the bracket itself. The joys of kit cars eh.

It a good job the owner of Simons car spotted it before it let go as it could have been quite a catostrophic failure with both sides showing the same damage.

[A_Hogg]

Re: Safety Alert Ford Sierra Compression strut problems

I thought that the intention of any ARB was to impose a motion on the TCA . The 'D' mounts offer fore and aft location but allow the ARB to essentially pivot within them. So as one wheel/suspension arm moves upwards that energy is imparted to the other side thereby stiffening the suspension . A compression strut, as I see it, only offers fore and aft location and nothing else.