Loads on Diff Mountings

[greyV8pete]

As this post is dropping down my original Cabrio thread I have re posted it, slightly edited for clarity, as a new topic in the general car chat forum as I think it is an important issue.

......... While lying under my Hunter I have been pondering the size of the four bolts that attach the rear diff carrier to the car. On mine they are only 8mm. Looking at the Sierra / Granada workshop manual it says 15/18lb/ft torque for these so I guess 8mm is original. Strange as the diff carriers have holes that will accept 10mm bolts. What size bolts does everyone else have?

I then started thinking what loads are imposed on this mount and have come to the following conclusions:

On a Sierra / Granada the diff is mounted on the bent beam subframe which has a three point location on the car. The front two locations are the big donut things at each front end of the beam. The third mounting is the rear diff carrier, which is effectively the rear subframe carrier!

On the production car therefore the top of the rear carrier rubber is under compression with the weight of the car resting on it. Under acceleration the rear of the diff will want to drop as a reaction against the drive shaft torque, so theoretically the bottom of the rear carrier rubber will be in compression. But as downwards force is applied to the rear mount the body is also tending to drop with it, so tending to subtract from the torque forces due to acceleration.

Now consider the situation with a kit car mounting of the same diff, like in the Marlin Cabrio / Hunter. With our cars there is no subframe beam so the weight of the car is all taken on the rear springs / rear wishbones / and chassis mountings. This means that the diff is effectively “hanging” on the two outrigger bushes plus the rear diff mount and the diff mount is being loaded totally downwards, by at least half the weight of the diff. Under acceleration the load due to torque reaction forces is then added to the load due to the weight of the diff. This may also explain why the rubber in the standard Ford mount doesn’t last long, unless the lower void is rubber filled. Does this all make sense?

Either way I have decided to open up the chassis holes on my car and will fit 10mm bolts. I have also made up two straps to fit in the chassis recess in the boot to reinforce this area. This has been prompted by the observation that after my fabricated bracket failure one of the bolts (albeit probably the only remaining one holding prior to final failure) has deformed the chassis area around the hole down a bit.

I will be interested to hear your thoughts on this. Peter.

[cabrioman]

Re: Loads on Diff Mountings

I have what might be quite an unusual Hunter in that it has done a high mileage, 75 k since it was built. It spent all that life as a 2ltr ford with an auto transmission and the standard Ford diff mount. When I stripped the car to rebuild it, there were no signs of issues with the diff chassis mount, which considering the mileage would seem to indicate the basic design is fine. Other higher power/torque applications may be of more concern but that's difficult to quantify.

The ford diff bracket has its problems in allowing diff movement but the design is fundamentally weld free, which might be a factor in the discussion.

John

[greyV8pete]

Re: Loads on Diff Mountings

........... the standard Ford diff mount. When I stripped the car to rebuild it, there were no signs of issues with the diff chassis mount, which considering the mileage would seem to indicate the basic design is fine. ............
The ford diff bracket has its problems in allowing diff movement but the design is fundamentally weld free, which might be a factor in the discussion.

John

Thanks for the feedback John. Perhaps a way forward would be to use the inherently strong outer U-shape bracket of a Ford unit and then weld something inside that to carry a Polybush which bolts on to the diff boss. I will however still feel happier when I use 10mm bolts for securing the rear mount to the chassis / body. Peter.

[greyV8pete]

Re: Loads on Diff Mountings

It seems that I am not alone in my concerns for the loading on the rear Sierra diff mounts when used without the Sierra sub-frame. I have just trawled through the Cabrio Technical Tips and found the following (extract):

CABRIO CHRONICLE Peter Hare Aug 93
……………… . Differential mounting · I am grateful to Bob Coppins for identifying this problem. We had both experienced definite mechanical knocks when changing up through the lower gears. I feared a worn differential or drive shaft coupling but Bob identified it as excessive movement of the differential on the sudden load reversals which occur when accelerating briskly through the lower gears. It appears that the difference between the Sierra and the Cabrio in the method of mounting the differential changes the loading on the rear differential mounting ……….

I am now looking again at my new billet machined rear mounting and wondering if even that will be strong enough? Although it is an uprated design for competition it is still intended for the original Sierra sub frame! It is made from HE15 grade “aluminium” but I wonder what the max tensile load is for this material is?

The U-shape that will have the downward forces applied is 9 x 30mm cross section on both sides, so we are looking at 18 x 30mm (540mm²) for any stress calculations.

I believe that the relevant forces are [diff rear weight + torque reaction due to acceleration] x 2 for suddenly applied load.


I am currently considering an additional M/Steel “strap” to go under the mount to provide added security. There would be a thin rubber sandwich between the two components to prevent chafing and to avoid possible electrolytic corrosion issues. Peter.