19 replies to this topic

[josh.goddard]

Hi

 Quick question i have solid mounted my front subframe. I have got my engine out if i solid mount the engine will i have problems with it tearing the shell to pieces when the engine tries to move? and will i also have a problem if i put solid teardrop mounts on with it breaking the front panel?

Thanks Josh

[Carlos W]

If you've solid mounted the toe board, you need to put the solid tear drops and tower mounts on it too!

Mixing and matching will cause problems!

People weld strengthening plates into the toe board to ensure it doesn't crack!

Sorry I don't know about solid engine mounts!

[Cooperman]

You MUST solid mount all front sub-frame mounting points or none at all.

The engine cannot and must not be solid mounted unless it is a race car, and even then it's not really advised. Why would you want a solid mounted engine? The vibrations through the shell would be simply horrendous.

[Vipernoir]

I see this everytime solid mounts get mentioned - WHY musn't you mix and match ?

I've been running solid tower mounts but rubber floor and front mounts on several cars for years with no problems.

[tiger99]

It will eventually cause cracking and ultimately fracture somewhere in the shell. In your case, the subframe is trying to rock fore and aft on the solid tower mounts, so what will probably fail is the bulkhead cross member, and that is the most dangerous failure mode. The cracking will probably start on the underside, where it is hidden from view. The crossmember is not designed to take the kind of load that your setup is imposing on it. In other circumstances of mixed mounts it is often the inner wings that crack from the bottom edge upwards.

 

Anything that is rigidly mounted at some points, flexibly at others, will have similar problems, as one or both of the connected parts will try to flex at the rigid connections when subjected to dynamic loads.

[Cooperman]

It will eventually cause cracking and ultimately fracture somewhere in the shell. In your case, the subframe is trying to rock fore and aft on the solid tower mounts, so what will probably fail is the bulkhead cross member, and that is the most dangerous failure mode. The cracking will probably start on the underside, where it is hidden from view. The crossmember is not designed to take the kind of load that your setup is imposing on it. In other circumstances of mixed mounts it is often the inner wings that crack from the bottom edge upwards.

 

Anything that is rigidly mounted at some points, flexibly at others, will have similar problems, as one or both of the connected parts will try to flex at the rigid connections when subjected to dynamic loads.

Well explained and absolutely correct, as usual.

[Spitz]

This is what can happen if you have solid bottom mounts and poly or rubber tops

http://i5.photobucke...er/SDC10017.jpg

[Artstu]

This is what can happen if you have solid bottom mounts and poly or rubber tops

http://i5.photobucke...er/SDC10017.jpg

 

Where's that?

 

[samsfern]

Looks like the floor mount.

[Spitz]

lol........yes sorry, one bolt of the solid floor mount

[Cooperman]

Unless all fixing points are solid, the full dynamic loads will go through whichever mounts are solid. I know that's stating the obvious, but it what has to happen if you think about it. Then, as 'tiger' so rightly says, there is the movement between sub-frame & shell which causes fatigue failure as shown in 'Spitz's' photo.

[tiger99]

Yes, and considering, for example, braking, the torque tries to rotate the subframe in the same direction as the wheels, i.e. front down, so download on the teardrops, and back up, so upload on the toeboard. The two rigid points at the tower mounts are in a straight line, and can't resist the torque, so the subframe will rock, because the rubbers will deflect. So all the torque and drag load is being reacted on the width (fore and aft) of the subframe to bulkhead crossmember spacer, which is about 1.5 inches if I remember correctly. The crossmember is sheet metal.....

 

But it gets worse, much worse, if suspension loads due to bump and cornering are added in.

 

All rubber mounts, and the entire subframe moves about (which is why we don't like rubber mounts here!), and if the stiffness of the rubbers is roughly similar, the loads will be spread reasonably well over all 6 mounts.

 

All solid, as Issigonis originally designed it, and the shell and subframe mutually stiffen each other, which is very good.

 

The problem with fitting solid mounts and not reinforcing the toeboard is because the original design had one bolt at each side vertically through the floor, and the other at an angle through the toeboard, so the subframe end was clamped to the corner of floor and bulkhead, which is stiff. The newer, rubber mounted design has shorter subframe rear extensions and the solid mount just substitutes steel for the rubber, putting the load into the flat metal of the toeboard. Because the front end is not all that stiff, due to the tower cut outs in the inner wings and the wheel arches in the outer wings, the front mounts in themselves will not stop the subframe rocking fore and aft, so the toeboard mounts, in the middle of a flat sheet, will make it deflect, and eventually crack, which is why a reinforcing plate is added around each mount.

 

So, all solid with toeboard reinforcing is best (or an early solid mounted car), and all rubber is second best.

 

We don't mix rubber and poly because not only are the poly teardrop and toeboard mounts currently on the market complete structural abominations, which will stretch, but they have very different stiffness than the rubber mounts, so the stresses will spread very unequally, and something will suffer.

 

And don't get me going on flip fronts, with the teardrop mounts no longer carrying loads, and the bracing bars, usually ineptly designed, carrying subframe fore and aft torque loading only a very short distance above the tower mount seatings, so there is an enormous bending load induced..... Not that the obsessive front end chopper-offers will understand any of this or believe what I say, of course.

 

Issigonis introduced the subframes after many thousands of miles of thrashing the "orange box" prototypes around Oxfordshire, because the previous structure tended to crack, and many billions of miles of Mini use on the road has proved that he got it right, because fatigue cracking around the mounts was completely unknown, except in very severe conditions such as rallying on very poor surfaces, which break any car eventually. The wimpy rubber mounts were designed by some relatively junior engineer, and in my opinion had insufficient testing, as I have seen toeboard mounts break after about 10k or road use. Sometimes, the original is best, and that is one of these occasions. A proper solid mount kit, with reinforced toeboard, is basically as good as the original.

[Cooperman]

Well explained, tiger.

 

What you have said about flip fronts is also absolutely correct. They are about the least safe thing you can do to a Mini bodyshell, apart from trying to make a standard saloon shell into a convertible.

 

The rubber mountings were introduced in an attempt to make the NHV (noise, harshness, vibration) less for a new primary target market of middle aged ladies rather than young drivers. So, if you want to drive like an old lady, then the rubber mountings will suit you best. otherwise, for optimum steering response all-solid with reinforced front bulkhead is the way to go.

[rally515]

Very good right up tiger and you,ve addressed a few extra points of detail there

 

Cliff

[Jacko]

After reading the solid mount posts on this forum I’ve decided to go for it, solid tower and bottom mounts, for heel board strengthening I was thinking about making a plate to sandwich the heel board in and spread the load, save welding, that’s what I’m thinking