Ugh, another PMM Thread

[PMB]

I installed the JBR 80a PMM. Starting up sounds AWESOME. Barely any vibes at all at idle, unless the A/C is on.

HOWEVER, I notice quite a bit of vibration at 1500-1700rpm. It's a pretty brutal range to have bad vibes. 60km/h in 5th gear is RIGHT in that range.

I have an '06 2.3L Hatch with automatic transmission. As the transmission automatically shifts down, and the RPM's fall through 2000-1700 RPM the vibes come in again.

I'm just curious whether this is in line with the experience of most people who have installed a stiff PMM. I have the Chrome Moly RMM on order. I'm not worried about vibes, they don't bother me, I'm just find it interesting that the vibes are in that range, instead of just at idle, as most people experience.

[cSPEED]

I have all 3 mounts.. installed PMM + TMM yesterday. That range is exactly the range where the vibes are most obvious. I imagine in an A/T car the vibes will be pretty steady because the car will always keep you in that low RPM range for fuel consumption. With the 88duro the vibes are going to be waaaay more obvious too. If you don't mind the vibes you'll love it though.

[PMB]

That's good to know. Other than through that 1700RPM range, it's as smooth as butter. You can really hear the engine though. I think at a stop, with the radio off, you can hear the valves ticking away merrily. Stomping on the gas, it almost sounds angry like a Vtec in the high range.

I'm hoping that the RMM will actually change how the engine vibrates, either to change the RPM range where it’s noticeable, or at least the type of vibrations being transmitted. There is an interior panel which rattles right now. I can’t figure out which one it is. Once I do though, I’m going to have to stuff it with foam rubber or something.

[cSPEED]

If you're hoping it will rattle less.. It won't. Expect at least 50% more vibes with the 88 duro. At this point you might as well get the tranny mount too and not leave the weak spot on that side as it will get worn quicker with the other 2 mounts being upgraded.

[PMB]

We'll see about the vibrations. Like I said, I expect the type of vibrations being transferred into the cabin should change. JBR doesn't make a transmission mount for the MZ3 the last time I checked, only the speed3, so I’m out of luck there.

Saying that the softer mount will wear out faster now that the other two mounts are stiffer is not correct in my opinion. If there is evidence to support your argument, I’ll gladly accept it though.

In reality, it works more like this (below you will find an oversimplified example of an idealized case, I didn’t want to start deriving second order differential equations and such, and start the discussion on over/under/critical damping in spring-mass-damper systems):

You have 3 supports, and their stiffness is let’s call it.... B = 30N.s/m of damping each. For a total of 90N.s/m. Each damper, or mount in this case, receives an equal portion of the vibration (assuming that is how the engineers designed it).

If you double the stiffness of two of the mounts, to each 2B (60N.s/m each) 2x2B + B = 150N.s/m
The distribution of the vibration energy does not actually go to 1/3 each spring anymore. The stiffer dampers absorb proportionally larger amounts of energy.
Math : 90/150 * 60 = 36N.s/m of energy to be dissipated now by each of the stiffer mounts, and only:
90N.s/m (of energy to be dissipated) – 2x36N.s/m (of energy absorbed by stiffer mounts) = 18N.s/m of energy remaining to be dissipated by mount that has not been replaced.

So in actuality, by doubling the stiffness of two mounts, you are reducing the amount of energy to be dissipated by the remaining stock mount by 40%.

It's a concept that took my a while to grasp in university, but it deals with deflection theories etc. Because the stiffer mounts deflect less, they reduce the amount of energy that needs to be dissipated by the softer mount which in it's previous role, actually deflected more.

[6strings]

^my head :(

[sarujo]

Jamie (JBR) is currently working on an MZ3 tranny mount but likely won't be out for a little while as he indicated other larger projects on the go right now. This was according to posts in M3F (USA) and Jamie himself via email.


JBR doesn't make a transmission mount for the MZ3 the last time I checked, only the speed3, so I’m out of luck there.