mx5 diff problems
 

evening all. Was having a think about the know issues with the diff arm snapping, and obviously I want a solution like the rest of you. So I have an idea and i'd like your thoughts on the subject.

in the mx5, the diff, gearbox and engine are connected in one solid structure, tight and stiff. where these connect to the subframes, they are on rubber bushes to allow the vibration and road shock to disperse.

On the roadster, you have no subframe. I think Stot may almost have the solution with his solid mounted bracket. the theory being that the solid mounting will prevent the diff from twisting. however, I think that by having the supporting bolts running straight down to a new plate at the bottom of the diff cage, any rotational force is transferred laterally down the bolt, preventing any shearing force to the support bolts. I would still use rubber bushes to the diff arms, so any small twisting of the diff is absorbed by the bushes, as in the original car.

What are your thoughts?:confused:

IMHO it needs to be all rubber mounted, or all hard mounted.
If it's half and half the hard mount is going to take all the stress and is likely to fatigue fairly quickly.

I've gone for a hard mounting solution now, similar to Stots.

If I were starting from scratch though I'd probably use polybushes in the arms, and bolt the nose to the chassis via bolt isolation bushes.
That way it'd be nice and solid, but still have a little isolation to reduce vibration and potential fatigue.

Whatever you do, just don't use landrover mounts in shear :D

I saw that thread - oops!

normally, id agree with a solid mounting all round, but I wonder if any flex in the chassis is going to damage the diff with no absorption. but would rubber mounting give too much flex? Afterall, in the mazda, the nose is solid mounted, but the arms are bushed onto the subframe.

I'm at the point in the build ready for the stripdown to paint, and I don't want to damage the coating on the chassis by changing my mind!

Yes, but the nose is solid mounted to the gearbox, which is then rubber mounted to the chassis, so effectively it is still rubber mounted all round.

Have you had a look at the locostusa site? The MX5 (or Miata as they like to call it) is a very common donor over there, and there's numerous threads/builds/ideas on mounting the diff.

Yes, but they are connected in the mazda via the ladder beam, to keep the diff from twisting. the rubber connections are where the diff/box/engine structure join the subframes, so they are isolated from the main loads on the body. Remember the gearbox mount on the mazda is not rubber, its stiff. I was thinking by using the transmission tunnel as the ladder beam (maybe with a few additional diagonals) and solid mount the drivetrain to the tunnel, it MAY act as the original ladder beam in the mazda. rubber mounting as the original car will give a little flexability.

for instance, if you hit a bump on the road with the rear left wheel, the rear left of the car would twist slightly. if the diff was solid mounted instread of bushes then this would twist the diff arm with the framework, stressing the diff arm.

On another note, can I ask why you revised the handbrake plate at the back of the tunnel a few times? im setting mine up now, and I cant see any problems. it cant be that easy can it?

I changed it because I didn't like the angle of the cables at that bracket.
With the bracket following the seat backs the cable outers point upwards into the tunnel, but the inners have to go down to the lever.
Think it took me a few attempts to get it right without the cables rubbing on the chassis, or the plate bending too much.
Probably would have been fine the way it was originally :rolleyes:

This is my take on the problem. In the original application all is well unless silly hp is applied. If you look at the way the diff is mounted as standard it relies on the cross bar top mounting and what is effectively a tether for the nose of the diff. As mounted in the haynes as soon as you side step the clutch the diff tries to buck instead of pulling on the tether as in the original and its this that is breaking the beam. ( I think). The nearest thing I can think of that would allow the diff to be rubber mounted and effectively tether the diff nose is by using something like this http://www.torquesolution.com/v/vspf...S-MC-R56-2.jpg and not restrain the diff nose. This is the third failure of the mx5 beam and in a minute someone will get hurt by a driveshaft stabbing them in the back.

Bob

Keith Tanner of Flyin Miata seems to have found no problems with his set up, which was a welded bracket.

Eventually someone will hit on a solution. I think the key is to prevent that drivers side arm from twisting, as others have said it's design to fail in the event of a rear impact I believe ( correct me if I'm wrong). The arm if you look at it is a H-beam which is a strong section but if it's twisted it will fail from that notch they have cut.

Only other option is to reinforce the arm.

Ugly as it is, if/when mine fails I'll be interested to see how.



These are volvo engine mounts one top and one bottom with big end bolts going through the centre to 10mm thick steel sections one on the left and one to the right. If the nose lifts enough it will then stop after 10mm (I'm guessing), which I think the rear diff mounts can accommodate, and fingers crossed that's not enough to cause a failure. Problem is will it then try to pull the rear of the diff downwards around the new centre of rotation as torque is continued to be applied and fail anyway. I think this is the scenario for failures in high powered MX5's

I think Skov is right though if you are solid mounting anywhere then it has to be all round.

Useful link...

http://foxed.ca/rx7manual/2003mazdar...101025T09.html

1 comment on that - the bolts are in shear. eventually, it will snap them if there not tough enough.

As for solid mounts all round, you need to allow SOME movement due to chassis twist. No matter how braced it is, the chassis will twist in corners and over bumps. the mild steel diff cage will twist, but the cast diff wont. To be honest, I think a stiff mounted diff is more likely to fail than a bushed diff. Think how much damage a car would get with no suspension at all?:confused: