BC

How much are the shocks?

danglerb

Right, they are for the abrupt torque changes, engaging the clutch, shifting the automatic, the twitch from a throttle blip, that sort of thing.

Shocks are like a low pass filter, slow events go right through, but very fast events hit a brick wall (shock acts "solid"). Except for some kind of rubber "mounting" I would expect a shock to pass the force of engine vibration right through to the chassis.

Jim bailey - 928 International

According to Porsche the forces involved are about 3 Gs ... page 844 of Porsche Excellence was Expected... " To assure that the least transmission of noise and vibration from this power train to the body, it was essential to position the two pairs of mounts at the nodal points of the complete assembly, those point which remain at rest when the whole system resonates......... small hydraulic shock absorber was added ,in a sea-legged position. These shocks helped the engine transaxle mass serve as a damper for the movements of the body. ..."Freeway" hop subsequent analysis showed that wheelbase length was a crucial factor in the excitation of this unpleasant ride motionand that the hydraulic dampers served to suppress it in the 928" ..... And so it goes the more I think I know the less I know I know ... Porsche designed the engine suspension using the mass of the driveline to damper movements of the BODY ! Improving on Porsche engineering .....Yea right !

heinrich

Yes Jim, long before Porsche made the 928 or 924, there were issues with American roads where the freeway sections are of such lengths that a car with 50/50 weight distribution (like the Corvette) was having major freeway hop issues. It was decided (by porsche) that the length of the wheelbase had to be reconsidered because of this. And this was a major deciding factor for them. The 924 was I believe the first recipient of porsche's attention in this regard. From http://www.pavement.com/Concrete_Pav...ign/index.asp:

"....... In the late 1950's, manufacturers of automobiles started receiving complaints of a peculiar phenomenon called "Freeway Hop" that was unique to concrete pavements on the California freeway systems. A study found that, with the suspension systems of certain models of large cars (a 1959 Buick was representative), a harmonic wave of objectionable vertical motion was induced by the interaction of small but repetitive road irregularities and tire-wheel imperfections; the cars were resonant to the repetitive 15-ft. joint spacings. Engineers suggested an irregular join-spacing sequence.

As a result, randomized joints became the standard practice for major roadways in California. Sequences of 13, 19, 18, 12 ft. were used through the 1960's and 1970's. In the same time period, a dozen western states adopted a similar practice for their undoweled pavement designs, and four states extended the use to doweled pavements. Skewed joints accompanied the use of randomized joints. Experience in several states showed that the 18- and 19-ft. panels were too long - they sometimes developed intermediate cracks - so the spacings were changed. For example, California has used 12-, 15-, 13-, 14-ft. sequences in the last ten years or so.

A 1987 FHWA survey reported that about 15 states employ the randomized joint feature. These include seven states that use it in undoweled pavements, three states that use it in plain, doweled pavements, and five states that use it in both undoweled and doweled pavements. Joint spacings and sequences vary from state to state; most have shortened their longest panels......"

heinrich

Btw the length of the torque tube was also decided on this factor.

SharkSkin

Nope -- solid only for the early cars, unless you want to swap crossmembers like Brendan did.

SharkSkin

~$50 ea., upper bracket around $10 ea. used IIRC. I was missing one bracket.

SharkSkin

Then you don't seem to understand shock absorbers very well. The resistance will increase with speed, but they never "act solid". Engine vibration, the short, quick, back & forth movement, is exactly the sort of oscillation that shocks are supposed to damp. If a vibration is at a resonant frequency, the oscillations will increase until something breaks. Maybe the following links will help you understand:

Brick
Damping

BC

I guess you can drive holes at the right places on the old version K-Member, but it need to be done exactly, and there may be a sledge hammer involved.

danglerb

What is "acting" solid if it isn't a great increase in resistance?

Shocks only behave like true dampers at frequencies low enough for some movement to occur. The energy absorbed is the force times the travel.

Sorry if I failed to explain it well enough the first time.

SharkSkin

Isn't there a slightly different shape to the crossmember too? I guess that's what you mean by sledgehammer. Oh, and you need the brackets that attach between the block and the mount, if you don't happen to have them already from wherever you found the x-member.

SharkSkin

Getting back to your original statement, my own experience is that the car is smoother with the shocks than without. This would seem to contradict the notion that the shocks transmit more vibration.

The fluid mounts emulate the damping that was originally provided by the shocks. Using suspension as the analogy, these features are the shock absorbers. The rubber in the mounts(early or late) is anaogous to the spring in the suspension. The rubber can wobble and bounce back and forth if there is nothing to damp the motion, just like a car with toasted shocks will bounce and oscillate.

danglerb

Looking at the cut up MM it isn't clear to me that the hydraulic has any special damping quality at all, just that it might be a bit more wiggly than a solid, more compliant. When force is applied down on the top of a hydraulic MM the sides are going to flex more than a solid, providing more isolation, but I don't see a damping mechanism.

Porsche I am sure designed the motor shocks to match the vibration they wanted to control. My point was that motor shocks might be more suited to a MM that allowed more movement instead of the solid.

Foo, misspelled Hydraulic all day.

BC

Yep. Its a bit flatter where the mount base would be. Hence the sledgehammer to deform the 2 gauge steel or whatever its thickness.

SharkSkin

Dang Lerb, maybe Rog will jump in and clarify, but if you look at this post it appears that the damping effect is from the restriction between the upper & lower chambers.