TRZ Transmission Mount Concern........(long winded)

[Novej]

First let me begin by saying I love, love, love this mount.....the feedback you get as a driver with it installed is simply amazing.....My car has an AWR rear mount, and PG engine Damper kit also installed. I've already decided I think Im going to change the engine damper out for the AWR passenger mount and now Im thinking I may have to replace the TRZ mount as well...

You see I track my car (1/4 mile) as often as I can....As we all know the wheel hop this thing has stock is rediculous....and now that Im running the hypertech tune...GOOOOOOD LORD....So needless to say Im very happy I have the motor strapped down nice and tight inside the engine bay.......OR DO I?????......

You see while installing both the TRZ and engine damper I discovered to my extreme dislike that neither replaced the vertical bolts which supported the engine to the mount.....and what really upset me was theres not even a nut that you could install to keep the tranny and engine from litterally sliding down these bolts should the threads in the mounts or on the bolts themselves gave out....in essence we still have the same problem with what could cause the dropped motors......~sigh~

Now I understand tensil strength and the metallurgy behind why the bolt shouldn't break especially if installed correctly...(mine was installed with red locktight also).....however that doesn't inspire confidence when looking at the tripod the single mount screws down into on the tranny itself or the actual engine portion the two bolts for the damper screw down too.....they just look weak...(hince the failure of these and the bolts in the past)

I've seen PT performance has a transmission mount that in essence takes the vertical bolt and distributes the weight over it by turning it horizontal, the way Mazda "should" have done it, I'm seriously contemplating buying it..........Basically what I need to know is am I tripping or is this a legitimate cause for concern especially considering all the vibrations these three bolts are now being suscepted to overall......??? Appreciate any help/opinions.....thanks guys

Oh to my surprise my stock tranny mount was still torqued to specs when I removed it after who knows how many passes down the track

 

[cudaman]

If you are going to be at the track a lot, an ounce of prevention is worth a pound of cure. Broken bolts and sagging engines do not a nice sound make.

 

[Novej]

^^^^^Soooo um...............what's that mean exactly??? LOL

 

[JamesCH]

Here's a little nuanced insight:

I did some quick hand-calcs, I assumed the transmission bolt was an M12 of 10.9 grade and found that it's maximum tensile load is just under 22,000lbs. That's a lot of force. I would not be worried about that bolt failing in tension.

As for the 'PT performance transmission mount' that "takes the vertical bolt and distributes the weight over it by turning it horizontal, the way Mazda "should" have done it"... well... that puts the bolt in 'shear' stress, as compared to tensile stress when it's vertical. Steel's shear strength is about 55% (plus or minus a little bit) of its tensile strength, also the bolt is likely cold-formed in a way that preferentially improves its tensile strength. The overall strength of the mount is probably the same, or worse, with a horizontal bolt of the same size.

My opinion is that bolts typically fail because their torque was out of spec, which causes all sorts of bad fatigue, shear, and bending stresses that the bolts were never intended to sustain.

That is a long-winded way of saying "make sure everything is torqued to spec and you'll be fine"

 

[Novej]

Thank you very much for your replys and your tests there James...thats really reassuring....The PT mount doesn't use the stock bolt in that horizontal position.....in fact it doesn't look like they use a bolt there at all.....(now that I look at that peice doesn't look to strong either) LOL

http://www.pt-performance.com/showImage.php?pID=191&imageNumber=1

I totally agree about the tension required to break the bolt, and wrong torque being the culprit for its failure. However, what about the peice that it screws into on the tranny and the actual threads on/in both......??? I understand that you have to pretty much strip them (again wrong torquing) in order to do this, but.......could the excess vibrations lead to premature failure in either case??? Again thank you for your reply :two thumbs up:

 

[JamesCH]

Shearing, or stripping, the threads in the engine block would require some serious torquing. Internal threads spread loads out over a fairly large area and would take a lot of tensile force to break. A 12mm bolt torqued to 75lb-ft is supplying about 9,600lbs of clamping force, torquing the bolt to 100lb-ft will increase that force to 12,800lbs. That's not a whole lot more torque, but it's about equivalent to hanging an extra MS3 from that bolt. My point is that normal loading forces should be far less than the forces put in the fastener by torquing it.

Well, you have answered your own question: maintaining adequate torque, and therefore tension, in the fastener is critical in preventing vibration and fatigue related failure.

Vibrations are forces that push, then pull, then push again.
Bolts fail from fatigue when they are loaded, relaxed, loaded, relaxed, etc.

Lets say the vibrations have +1000lbs of pulling force and -1000lbs of pushing force, and your bolt is torqued to provide +2000lbs of clamping force and has a 4000lb proof-strength. This means that as the connection experience vibration the bolt will cycle between (+2000lbs-1000lbs)=+1000lbs and (+2000lbs+1000lbs)=+3000lbs of tension. Now, because it never fully relaxes and never exceeds its 40000lb proof strength the threads and the bolt fatigue (loose strength) very very slowly, if at all. If the clamping force (from torquing it) is too low, the connection will fatigue quickly causing premature failure. If the clamping force is too high the yield-strength of the bolt may be exceeded with vibrations causing immediate failure.

 

[Novej]

(bowdown) Very well put sir. Yeah I totally didn't take into account the actual clamping forces we're contending with here........So in your opinion.....was there anything wrong with the stock mounts (other than allowing way to much drive train movement), or was mazda just torquing those bolts wrong? And the last question that comes to mind is this.....It sounds like you really did your math with calculating the forces needed to break these bolts......with that being said....is there any way that you could measure the actual forces being applied to them when were launching the car and shifting (especially that 1-2 shift) just to see how close the bolt is being subjected to these limits...because even with the mount despersing alot of that energy through the bolt to the chassi......the bolt is still ultimately receiving the brunt of the initial force correct???

 

[Saskatchewan17]

I mean, I didn't read too in depth to everyone's responses... but, the way I see it is that the reason the stock mounts fail and are pretty bad is because of their design and bushing material. I feel as if the stock bolts are more than strong enough to hold the forces of extreme engine movement. But the stock rubber bushings are not up to the task. That is why in pretty much every engine mount failure situation I have seen, the bushing has torn or ripped and the bolt has stayed in place. So with the TRZ mounts you get more of a solid bushing and less rubber mush for the engine to squish around.

 

[JamesCH]

I believe it's more or less how Saskatchewan17 put it: the stock engine mounts allow too much movement.

There are ways to potentially measure how much force is going through those connections, like strain-gauges and load cells, but they would be far more trouble then they're worth.

Keep your torques in check, inspect your stock motor mounts regularly for tearing, or replace them outright with stronger AEM parts. Then (drive2)

 

[Novej]

Thanks guys. I guess I'll buy the AWR passenger side mount and call it a day......hmmm I see they're designing a tranny mount too....maybe I'll just go AWR all the way around hahahaha.....