JBL Big Block Poll

[Richard Hudgins]

Dart,

I would think that you could certainly twist a chassis too hard with torque. (If you could hook up the tires) We bent a couple of T300's monocoques this way. But they were so weak you probably could of bent them just by jacking them up wrong.

Ant,

Using the engine as a stressed member has been done for many years in racing chassis. You have to do it correctly in order to not introduce loads in the wrong places on the block.

Harmonics et al.

The basic high amplitude frequencies of the engine are fairly easy to calculate. (The engine harmonics are quite difficult due to the many factors involved, However, one is only concerned with the major amplitude events as they relate to harmonic periods of the engine and chassis.)

In a standard layout V8 engine, one cylinder fires every 90° of crankshaft rotation. This is a "fourth order" excitation, (Four power pulses per revolution) which at an 1000 RPM idle, produces 66 pulses per second (Hz), and at 5000 RPM, produces 333 pulses per second (Hz).

Therefore, If your chassis has a natural frequency of 66Hz, it will be in "harmony" with the engine frequency at 1000 rpm. If it had a natural frequency of 333Hz it would be in "harmony" at 5000 rpm. In other words, if the engine and the chassis had the same natural frequencies, the amplitude of the instantaneous torque peaks would excite the chassis to the greatest extent as you are dividing by 1.

The JBL chassis has a natural frequency of 22.3Khz. (This was found by sonic testing, FEMA studies of the design are really beyond my capabilities.) Therefore, at 1000 rpm you have a result (JBL chassis) of 337. This number reduces the transfer amplitude by 337. As the revs go to 5000, you now have a result of 66.9.

It appears (from SAE papers and other NVH studies) that anything below a 20-1 ratio becomes quite evident to the occupants of the vehicle and very annoying. Conversely, anything in the 700>5K range is also considered quite bad as this is the range of human hearing that is the most sensitive to amplitude. (The buzzing that you hear in some econoboxes is due to this ratio.)

Now, this subject is very complex and there are all manner of papers relative to even and odd frequency divisors, second and third order harmonics, and relative fatigue points, etc. etc. You will have to speak to someone smarter than me to get the true story.

Jeez guys, how asking me some easy stuff that I can understand?