Talk Morgan Forums Morgan Sports Cars Morgan Plus Four, Supersport & Plus Six (CX & CXV Generation Vehicles) Undertray vibrations

I have been working on our Plus Six over the past four years slowly locating and correcting various types of rattles and squeaks in the car. Over this summer I finally found the source of a subtle yet annoying rattle from the lower body that you could only hear at low speeds.. The source was found to be the mid car undertray that covers the area from mid-engine to the rear of the gearbox. I have always found this tray to be firmy secured by its bolts to the body ( there are quite a few securing it in position ). However, the tray is made from a fairly thin gauge of aluminium.. in 2 or 3 areas its securing bolts are some 8-12 centimetres away from the trays edge, this allows the tray alloy between the bolt and the trays edge to vibrate against the cars main alloy body beams. I also found that on our Plus Six there is an armoured wiring cable that is routed under the gearbox, this just sits on the tray, the armouring of the cable was sufficient to have partly worn away the undertray due to movement.

The cure to the the vibration noise was to apply some 5 cm wide strips of thin self adhesive neoprene on the inner tray surface between the securing bolts and the end edge of the tray in the areas where the securing bolts were over 5 cm from the trays edge, this compresses when the bolts are tightened and stops any vibration. I also applied the same neoprene in a 15cm square patch on the area that the armoured cable sat on the tray.. This coming winter I will investigate to see if the cable under the gearbox has been incorrectly located, as it seems strange to place a cable in this position, it would have made more sense to thread it over the top of the gearbox.

As a side note to anyone that has bottomed out their CX car.. On our Plus Six the lower edge of the exhaust is just about at the same height as the mid tray.. I have found that mostly its the exhaust that makes first contact, but it is very possible to also hit the undertray and being of a thin gauge it can bend/bow towards its rear.. This is not that obvious to see until you remove the tray and check its level on a floor. Ours did not show signs of any huge impact, but it was bowed at its rear, this also helped exagerate the clearance at the tray securing bolt locations and the trays edges.. It is fairly easy to straighten the tray using wooden blocks on a smooth garage surface, also same procedure to make sure the tray securing lips were parallel to their fixing locations.

I wonder if there's room to put some pieces of sound-deadening material like Dynamat on the inner/upper face of the under tray? You don't need much to damp out vibrations.

Good idea to eradicate the problem but a little noisy Tim

I'm very pleased to see that neoprene has a future on the CX range.

On a Plus Six I viewed a few weeks ago the mid exhaust silencer had been removed as "it was grounding out fairly frequently". The owner had removed it and just added a straight pipe in its place. It made NO difference to the sound at all i.e., not noisy or no drone.

So it might be worth, removing, if it a common occurance?

Do you have a picture Dave? Thank you in advance. 😎

Afternoon Clemens, I did not take any pictures when I applied the neoprene strips, but I am going to take the undertray down this winter, so will take some photos then, It does not take much to stop the vibration as its just in a few areas on the tray.

Interesting ref removal of the centre section of the exhaust not making any sound difference, that mid exhaust silencer is a Helmholz resonator box to dampen specific frequencies. Guess only the factory knows which and they must have added it for a reason..

Thank you Dave 👍