Engine cradle theory

[yeeter]

Im curious, can anyone explain to me the purpose/function of the Quietline engine cradle modification from a theoretical standpoint?

Its my understanding (and I could be wrong), that the stock mounts go through two rails which get bolted directly to the engine block.

Adding a cross brace is to keep the two braces moving together. But if these rails are bolted to the engine, Im not seeing how there is motion between them to begin with (engine block is a rigid body)

Then some are using solid mounts with good results. Would seem to make the cross brace even more important if there was allowable movement, but again doesnt seem like it works that way.

I see a lot of posts here on how to do it, and testimonies that it help.

But Im not understanding why...

[cubs-n-bxrs]

I think one of the bigger problems is that the bolt holes in the oil pan can and do get stripped out. Tying the two frame rails together eliminates that problem because you are bolting the engine to a solid fixture then. I believe the oil pans on the quite lines were made out of aluminum which doesn't take the wear and tear as well.

[R Bedell]

The rails are "isolated" from the frame due to the rubber mounts, thus, reducing vibration. The vibration is absorbed (or reduced) by the mounts.

[ol'George]

Also, some ppl like to attach the iso rail/cradle assy to the frame with the rubber iso's first, then set the engine on it and tighten the 4 bolts into the engine.
The mod. allows the bolts to line up quite well with less chance of a cocked bolt cross threading.

[CADplans]

Originally Posted by yeeter

Adding a cross brace is to keep the two braces moving together. But if these rails are bolted to the engine,

Im not seeing how there is motion between them to begin with (engine block is a rigid body)

But Im not understanding why...

Everything has a natural frequency, think of something like a tuning fork.

The cradle design from IH inadvertently was designed so that its natural frequency was VERY close to the frequency of the engine it is supporting.

The "mod" changes the frequency of the cradle, stiffening it with a brace raises the natural frequency,,,,

The problem is resolved unless you "hotrod" the engine up to the problem area and recreate the issue.

Anyone got a 6,500 RPM Kohler - K engine so we can find out if there is an issue??

[dvogtvpe]

We had a customer running a pro stock 50.5 in a QL with the rubber mounts still in it. not really an issue. had the cross brace added.

[red56turbo]

I think the cross brace keeps things from twisting by tying the two engine rails together. The combination of the engine torquing/twisting, worn out isomounts, and loose engine/frame bolts would destroy the threads in the oil pan, allow excessive movement, and take out the driveline.