Random Transmission Failures

I have a solid bottom mount and solid dog bone. I like them. No more engine rocking. When I gave gas, the car is moving, no delay no jet lag. But I will be happy to have spinning tires. Maybe I will with my rebuilt engine and DCOE's...:rolleyes:
 
If I read your description correctly, you devised a stiffer lower mount to reduce the shock load. Interesting, I might have thought a soft mount would soak up the shock more while a firmer one would transfer that load to the diff more. In other words a bit of the opposite effect. However I see where allowing the tires to spin would allow the load to essentially pass through the driveline altogether. But it seems the moment before they spin, when the shock first hits the system, is when things would break. Obviously your experience showed otherwise, curious to hear more details on how this worked to prevent failures.

By the way, I assume you still have a open diff? So we're talking about spinning one tire. Or do you have a limited slip diff?


Jeff, the soft mount is the problem. There is enough driveline movement to absorb enough of the shock that the tires get the full weight transfer before the driveline motion is checked. That full weight transfer, and the spreading out of the driveline shock via the mount's motion, means that the total load through the diff is much greater and spread out over a longer period.

If driveline movement is strictly limited then the entire torque load reaches the tires before the weight has a chance to transfer onto the drive wheels. Less weight transfer and a higher/shorter shock easily breaks the tires loose and the diff experiences a far lower shock load over a lower period of time. Having a very light wheel/tire combination helps here too.

This is exactly the same phenomena that causes an over sprung/shocked car to skate (slide) at initial turn-in. The tire experiences 100% lateral load before the car has a chance to roll and transfer weight onto the outside tire. So it breaks traction at far lower lateral load than if the car had a chance to transfer load through body roll. That roll and load transfer puts the additional vertical load on the tire that it needs to maintain grip. I used this exact principle to transmit torque to the rear tires before the chassis had a chance to transfer weight onto the rear tires.

The torque suppressor is a strut with rod ends attached to the transmission mounting plate of the lower mount and also the cross member. It sits basically parallel to the main shift rot. It permits the lower mount to absorb vibration in normal driving and also prevents the shifter from binding when shifting on high g or high load shift.
 
Thanks Steve.
Regarding the torque surpressor, is that just like another dogbone mounted down low by the lower mount?

Here is the 5th gear.
Interestingly, the matching gear shows zero damage.View attachment 31079View attachment 31080

That's as expected. High load fatigue failure. I am surprised it didn't take out more and damage the other gear. The driver must have reacted instantly.

Is that gear as dark as it appears in the photo? I do see some 5th gears that get dark from heat stress. I know these will ultimately fail so they get scrapped when I find them.
 
Very interesting explanation of the torque loadings Steve. I might look into building a torque suppressor like that, or maybe a polyurethane lower mount like this suggestion. From PaulD, post: 342253, member: 566. " For the transmission mount I made a mold and cast it in high density urethane. The face plate is not on for the picture. The alignment was perfect".
DSC_0194.jpg
 
Very interesting explanation of the torque loadings Steve. I might look into building a torque suppressor like that, or maybe a polyurethane lower mount like this suggestion. From PaulD, post: 342253, member: 566. " For the transmission mount I made a mold and cast it in high density urethane. The face plate is not on for the picture. The alignment was perfect".View attachment 31096

The eurathane lower mount would be ideal for a racing/track application. I went with the strut/rod-end combination for two reasons. The rule set the car was prepared under prohibited alternate motor mounts but does permit a single torque suppressor. And, the suppressor is virtually completely unnoticed in normal driving.

Conventional wisdom was to add the torque supressor to the top of the motor. Most everybody else installed one right beside the original dog bone. Initially I tried one there but it actually made the problem worse as now _all_ of the driveline reaction was transferred to the lower mount. If you look at a factory lower mount you see that the two voids are in front and behind the center pin. This allows more fore/aft movement and I was actually ripping the OE mounts apart after only a few launches. After a more in-depth analysis of the driveline reaction I realized I had the supressor in the wrong location. And since it only restricts fore/aft motion it transmits little if any vibration.
 
Is that gear as dark as it appears in the photo? I do see some 5th gears that get dark from heat stress. I know these will ultimately fail so they get scrapped when I find them.

It may just be the photo. Here is a different one and the new gear.

EEFEC027-31D0-44B5-91AA-966F93D71850.jpeg
E58D4C1E-C926-453B-8056-3D944B05DCCA.jpeg
 
It may just be the photo. Here is a different one and the new gear.

View attachment 31108 View attachment 31109

That looks normal for color.

Most likely simply a load induced failure from the increased torque. Or, it could be a metallurgy failure. There could have been a flaw in the part when it was formed.

Gear tooth failures are not particularly common but they do happen. And typically in the higher gears. I see virtually zero such failures in the lower gears. Thus one can assume its the torque loading applied and the finer cut of the teeth. The fine cut teeth are quiet and more efficient but not as strong as the bigger teeth you get with a more coarse cut.
 
The eurathane lower mount would be ideal for a racing/track application. I went with the strut/rod-end combination for two reasons. The rule set the car was prepared under prohibited alternate motor mounts but does permit a single torque suppressor. And, the suppressor is virtually completely unnoticed in normal driving.

Conventional wisdom was to add the torque supressor to the top of the motor. Most everybody else installed one right beside the original dog bone. Initially I tried one there but it actually made the problem worse as now _all_ of the driveline reaction was transferred to the lower mount. If you look at a factory lower mount you see that the two voids are in front and behind the center pin. This allows more fore/aft movement and I was actually ripping the OE mounts apart after only a few launches. After a more in-depth analysis of the driveline reaction I realized I had the supressor in the wrong location. And since it only restricts fore/aft motion it transmits little if any vibration.
Steve, when I got my car it had been used mostly for track events by the PO, and it had the dog-bone modified so that one end of it was solid tube and the other end retained the bushing. I had always wondered why, but maybe it had a similar effect in stiffening up the movement? Just to keep my thinking straight, upon launch is the engine pushing on the dogbone? or pulling?
 
Steve, when I got my car it had been used mostly for track events by the PO, and it had the dog-bone modified so that one end of it was solid tube and the other end retained the bushing. I had always wondered why, but maybe it had a similar effect in stiffening up the movement? Just to keep my thinking straight, upon launch is the engine pushing on the dogbone? or pulling?

The engine torque pushes against the dog bone. Simple physics, the engine tries to rotate the opposite direction of the rear tires. So all of the drive torque is pushing against the dog bone.

I have seen people put a stiff bushing in one end of the dog bone to stiffen it up some but not so much as to transmit a lot of vibration into the chassis. Seems to work OK for that purpose but I haven't tried it myself.
 
This is by far one of the best threads on xweb, or in fact any forum I've ever been on.

Thank you Steve. I finally got my flywheel so now I can install that gearbox you built for me a couple three years ago. :)
 
built a torque rod for the lower engine mount, and fitted urethane bushes to the dog bone
It will be interesting to hear what you think about the results.

I'm also wondering if adding a torque strut increases vibration/noise transfer from the drivetrain through the chassis. The urethane bushes may add a little more vibration, but the torque rod is a more solid connection. However it only acts in one direction so there may not be as much as it seems.

What did you use for the dog bone bushes?
 
Very interesting but I don't understand how you did it so please add some more detailed pictures. I certainly need this mod.

From the looks of things, he added onto the existing engine mount (a U shaped area) to serve as a mount for the rod end and then welded a tab across the opening of the jacking point on the cross member to bolt the other end rod end to (this is either tapped or has access from the other side to put a nut on the bolt.

He also added a reinforcing tab across the L of the cross member to reinforce it. I couldn’t say if it was part of this activity or something from the past. There is also a larger landing zone for the jack which has apparently served as a skid plate in its past...

Nice looking solution. The engine can still rise and fall but cannot move forwards or back. As it isn’t a rigid in all directions connection it shouldn’t add much in terms of harshness or vibration.
 
The dog bone bushes are Nolathane 46007A, duro 75, for an Aussie Ford Falcon, but there are others around. I had to machine it to the right size, od 46.5mm, id 18mm, length 40mm.
thumbnail_101_0174.jpg

Below is a pic of the torque rod on the bench
thumbnail_101_0182.jpg

Yes, a nut welded on the back of the 1st plate welded at the rear of the engine support. The engine mount I used was one that the 19mm shaft had broken off the back plate, so I made a 25mm dia collar to be a press fit on the 19mm stub, welded to back plate, and also welded from behind.
thumbnail_101_0181.jpg

Also had to cut about 20mm off the transmission lug for clearance.
95351077_759085111294457_8972190095843000320_o.jpg
 
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The engine mount I used was one that the 19mm shaft had broken off the back plate
I have a couple of used engine mounts that I cannot separate from the backing plate. The center shaft is so seized onto its collar that they will not budge even after excessive force, heat, penetrant, etc. The exact opposite as Greg's experience of them coming apart.

I wanted to separate them in order to reuse the backing plate as part of a mod, similar to what Greg did. These are the ones with the extension arm for a muffler support (example below). That arm could be repurposed as one of the anchor points the same way as the extra metal that Greg welded onto his backing plate. But the rubber mount portions are bad, so I planned to replace that.
Something like this, cut at red line and add mounting hole (yellow):
engmt.jpg
 
Below is a pic of the torque rod on the bench
View attachment 32102
Thanks for showing us your work, you didn't wait around. Nicely done. I just spent some quality time replacing the rubber mount, so I know the area. I'm wondering if this will also prevent the rubber mount from failing. Vendors say they don't have 1500 mounts, only 1300 meant for less weight. Is there any give in this set up, or solid mount?
I was still trying to figure out how it things all pushed and pulled, so in this case the engine will pull on this rod, correct? From Steve's description, I had pictured the rod to be next to the shift rod, on the other side of the engine/trans.
Thanks
 
I was surprised to see on the mwb site that the 1300 mount can't handle the weight of a 5 speed, I suspect it's more to do with the rubber that individual suppliers are using. The rubber in this one doen't feel any softer than my 1500 one, I did fill the voids with small wedges of neoprene bonded in, to stiffen it a bit. Time will tell if it can take the loads. The torque rod will have negligible affect on downward load carrying, mainly pulling on the rod during acceleration.
 
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