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).
Identifying and mitigating car body vibrations and booming noises by analysing periodicity.
| Identifier | WestWitteringFiles\V\March1931-September1931\ Scan024 | |
| Date | 3rd March 1931 | |
| HS.{Lord Ernest Hives - Chair} DA{Bernard Day - Chassis Design}/EV.{Ivan Evernden - coachwork}) FROM R.{Sir Henry Royce} E.{Mr Elliott - Chief Engineer} BY.{R.W. Bailey - Chief Engineer} ) (At Le CanadelHenry Royce's French residence.) C. to SG.{Arthur F. Sidgreaves - MD} WOR.{Arthur Wormald - General Works Manager} PN.{Mr Northey} FP.{F. Potts} ORIGINAL RL/M7 3.31. x634 x7820 P. 2. BOOMS HS.{Lord Ernest Hives - Chair} writes that he has tried my suggestion of replacing the engine with an electro motor which is out of balance, so that we could find the periodicity of the engine vibrations. If I read and understand correctly he indicates that the body does not boom objectionably except at 2 periodicities equal to 2 per engine rev., and 1.5 times if the engine revs. indicated here are the same - 2500 - then it is 3700. & 5000 revs. on the motor - i.e. complete periods. You will remember that I pointed out that if the body responded all the way up increasing in vigour then probably the body could not be considered to have a definite period of any of its main parts which were not effectively damped, but if the body itself shewed a distinct booming period which came on at some particular speed then the body is faulty. (That it has 2 periods does not alter the argument.) You will see that the out of balance electro motor has no period but is all the while trying to find a period in something it is in contact with. So also our engine and transmission, which we know cannot be free from impulses to set up vibrations, is all the time (even if faultlessly designed and constructed) trying to find something undamped that will respond to them. These impulses may be 3 per rev. or 1 per rev. or some octave or harmonic of these numbers. Some of Mr. Grylls photos shew a kind of square diagram which might mean 2 per rev. at right angles, or something peculiar to every other rev., but might have something to do with HS{Lord Ernest Hives - Chair}' 2 complete periods per engine rev. Anyway it suggests that if the electro motor shews the body to have a distinct period which is not damped then I think the body is definitely wrong and would be wrong on any chassis having an engine with impulses. I am not quite sure that if very little damped even a rotating well balanced steam turbine would give a boom if run at the correct speed. It did occur to me that after finding the periodicity we could try to alter it if we knew the part by firmly attaching to the part some sheet lead which should make the period lower and less noticeable. Very little motion of high frequency is very audible and trying, and I am convinced that car body roofs should have very slow periodicity of natural vibration, and it goes without saying that effective damping is of the utmost importance. The stiff wooden bars across the roof should I think be made of laminated wood not too well fastened together like a road spring which is a well known dead beam. (1) | ||