From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical analysis of the 'booming' vibration phenomenon in cars, discussing its source, mechanics, and proposed tests.
| Identifier | WestWitteringFiles\V\March1931-September1931\ Scan080 | |
| Date | 25th March 1931 | |
| TO HS.{Lord Ernest Hives - Chair} FROM EV.{Ivan Evernden - coachwork} EV{Ivan Evernden - coachwork}1/25.3.31. copy to ORIGINAL BOOMING CARS. P2. x634. x7830. We have so far not assisted in your work on booming, but having received your report on the tests which have been carried out, we venture to send you our comments. (I) QUANTITATIVE ANALYSIS. So far evidence is very conclusive as to the source of the "driving vibration". Towing a car without push-rods indicated this to be the varying kinetic energy of the pistons. It would appear therefore that the driving vibration should have a frequency of three times engine speed, and that it is always present except when neutralised by the gas pressures. We would like to make a test measuring the amplitude and frequency of this vibration over a working range of engine speeds by means of your recording tape instrument. Mr.Gryll's tests with R's Sonometer ( the out-of-balance starter motor) appeared to shew that the frequency of the boom in the body to be 1 1/2 and also 2 times engine speed at 60.MPH. This is a most peculiar phenomenon as it is unknown physically that a vibration can set up one of a lower frequency than its own. It is for this reason we would like careful tests to be made to measure the frequencies of the driving vibration and of the boom vibration on several cars. (2) THE MECHANICS OF BOOMING. We divide the phenomenon into three parts. Driver, Transmission and Driven. Driving Vibration This is the engine in most instances but can at times be the road wheels. The Transmitting Medium. As far as the engine is concerned tests go to shew that the transmission is via the engine to the frame for certain. ( Diamond Mounting is a cure) Were it not for the isolated mounting of the subframe one would expect the vibrations to travel direct to the body. We imagine that this will be a route when a body is mounted direct onto the chassis, But tests also go to prove that this is not the only route. The Steering and the dash is another. Now the only way in which we can make the steering and dash transmit torsional oscillations of the engine is illustrated in Fig. I. (see over) Without torsionally free mounting of the engine the frame is twisted each half cycle in opposite directions. The column is thrust in and out of the dashboard the latter being held square with the scuttle face by the hard rubber washers of the scuttle isolation bolts. The column is clamped to the dash and therefore the pot of the dash has to diaphram. In this way we suggest that the column and dash between them form a very good amplifier working on the principal used in diaphram amplifiers for wireless telephony. | ||