Safety Alert Ford Sierra Compression strut problems

[ggrum]

Shortly after this issue came to my attention I brought it to Marlin Sportscars attention. Following that I had a constructive discussion with Terry Matthews of Marlin and she has sent me the following letter to publish here and in the magazine at a later date, bearing in mind that not all club members regularly use the forum.


"18th January 2019



Dear MOC, Marlin owners & Marlin Enthusiasts


It has been brought to our attention that a very small amount of Marlin Cabrio and Hunters built using a BMW engine, have over time, developed stress cracks around the antiroll bar to chassis mounting plates.

Marlin Sports Cars wish to stress that this fault is confined to the Ford based cars where the central section of the anti-roll bar was removed to accommodate the BMW engine. We have also been told that some owners have exasperated this problem by fitting a solid tie bar with no rubber bushes.

We currently have a BMW based Marlin Hunter in our workshop and we are working on a solution that we will be more than happy to pass on to any interested owners. In the meantime we would urge anyone who has this design of suspension and has not yet inspected it, to do so.

We would also like to add that this issue does not affect any Marlin Sportster models regardless of which donor has been used as they have an entirely different front suspension design.


Yours Sincerely
Terry Matthews
Director of Marlin Sports Cars

[marlin munroe]

The mileage on Simons Gregory's car was indeed low; however the chassis was an early model and had been the subject of two uncompleted earlier attempts to create a Marlin from it, therefore had been lying around in damp garages for decades before Simon acquired it and stripped it back to basics.

From the pictures, it does look as if the original factory welds have been the subject of some corrosion, and Simon had cleaned up the chassis and coated it with two coats of Isoflex liquid rubber at the time of his construction. The original powder coating was failing badly at that stage so irrespective of how well he was able to prep the chassis , without grinding those welds back to shiny metal and possibly re-welding them, any corrosion within the micro ridges and troughs of the weld was already well established and free to develop

The photos seem to show that at least 50% of the problem is the failure of the weld through corrosion. ( under heavy duress of though clearly)

[marlin munroe]

The mileage on Simons Gregory's car was indeed low; however the chassis was an early model and had been the subject of two uncompleted earlier attempts to create a Marlin from it, therefore had been lying around in damp garages for decades before Simon acquired it and stripped it back to basics.

From the pictures, it does look as if the original factory welds have been the subject of some corrosion, and Simon had cleaned up the chassis and coated it with two coats of Isoflex liquid rubber at the time of his construction. The original powder coating was failing badly at that stage so irrespective of how well he was able to prep the chassis , without grinding those welds back to shiny metal and possibly re-welding them, any corrosion within the micro ridges and troughs of the weld was already well established and free to develop

The photos seem to show that at least 50% of the problem is the failure of the weld through corrosion. ( under heavy duress of though clearly)

Quick correction!

I have just checked the receipts. The chassis and kit for this Cabrio were purchased in late 1997, new from Marlin
So 11 years and two previous owners before the build was completed by Simon in 2008; then approximately 8000 miles and ten more years before the weld failure was detected

[milliemarlin]

<snip>
The photos seem to show that at least 50% of the problem is the failure of the weld through corrosion. ( under heavy duress of though clearly)

I think it unwise to believe that corrosion is a significant factor in this type of failure. The welds are a focus of the stresses involved. The transition from the thick mounting plate to the thinner chassis is bound to focus the stress. Any corrosion seen is most probably due to the exposure of unprotected metal. There was no sign of corrosion on the unstressed side of my mounting plates.

My own car had covered more than 10,000 miles with the shock absorbing anti roll bar mountings. With the rigid compression struts signs of failure were apparent after a few thousand miles.

I strongly recommend that compression struts are not fitted to these cars and that the mounting plates of all cars are regularly inspected.
Paul

[ggrum]

I agree with Paul in that the failure is probably caused by stress concentration as a result of the difference in thickness of the mounting plate and chassis. I suspect the thinner chassis section is flexing slightly under load. However, there will often be a higher level of corrosion in the weld area caused by changes to the crystalline structure in the heat affected zone (HAZ). Correct choice of filler, quality of weld and subsequent heat treatment can all affect corrosion in the weld area.
Geoff

[Patrick]

Outside observer, not seen any of these in person so I may be talking nonsense:

Is there a gusset or any triangulation above the part that seems to be floating free air (circled in red) - other than a thin part of the chassis?

If there is not then potentially a great deal of stress on the part that is fatiguing and pulling out from the chassis with the red line being the pivot point.

Untitled 2.jpg

[cabrioman]

I think you hit the nail on the head, there is no support structure of any kind behind the inner hole on your picture.

I felt when I had my Cabrio the lack of support on the inner mounting was the problem, as it potentially allowed the arb mounting plate under load to rotate along the inner edge of the chassis leg. It certainly wouldn't bend under load as it is far to thick to allow that.

I think any fix to a failed mount should include support in that area and ideally have a revised mount that wrapped around the chassis leg to introduce some triangulation.

[marlin munroe]

I agree re. the stress and the moments of force; not only on the overhanging corner but right back to the upright inner surface of the chassis which is further in than the outside edge of the flange; as pointed out in the photos of the second example on the first page of this stream.

However just from the photos and the extent of the corrosion on my own Marlin , it would seem that in my case, corrosion was an exacerbating factor that caused premature separation of the weld, rather than a clean stress crack of a good clean weld.

So perhaps a block welded on the inner surface of the chassis to the extent of the edge of the flange would prevent that " see-saw" flex around that pivotal point.

However even with the flex potential engineered down to the minimum, repetitive impact stress fractures may still eventually cause the weld to fail




Line indicates where the inner upright surface of the chassis starts, area below the red line is an overhanging flange. ( picture "borrowed " from first page)

[milliemarlin]

I agree that an upright web on the inner surface of the chassis would be a good enhancement. To fix my car I concentrated on thickening up the chassis and spreading the load around the mounting block. But most importantly reinstating the rubber mounting bushes.

For a fuller description see:
https://www.marlinownersclub.com/for...roll-bar/page8

Paul

[dogoncrazy]

As this problem to the best of our knowledge is confined either to those Cabrio's fitted with the BMW engine and consequently a 'chopped' anti roll bar , or those vehicles that have been modified from the original design and have a compression strut set-up, I would suggest that it's more likely a result of entirely different stresses being imposed on that area.
There are a number of Cabrio's out there with the standard set up and greater mileages that have not had the problem.
Unless you could cut a section from the mounting plate and incorporate the chassis rail and welds, and then put it under the microscope you cannot categorically say that the failure is due to corrosion or whether the corrosion is a result of failure.
Marlin got it wrong in respect of chopping the anti roll bar to fit the M20B25 engine and a number of owners by installing what they thought was an improvement .
I was one of those members who had the BMW powerplant in my previous Cabrio