E21 Shimmy

---------------- Date: 06/03 23:56 Received: 06/04 07:18 From: Brian C.W. Jarvis, woodside@atcon.com To: E21Autos, E21Autos@ix.netcom.com

Back in the 70's my father was partial to BMWs and owned four at one time or another - a couple of 2002's, a Bavaria, and a 1977 320i. He turned over all of his old Roundel's to me shortly after I bought my 1981 320i last summer. Going through the old issues has been an extremely educational experience. A lot of the topics discussed on the E21 Digest today were written up in these magazines. I thought it might be interesting to post an article or two from the old Roundels to the Digest every now and then. Here's one:

320i VIBRATION RESOLUTION by Michel Potheau (Tech Ed.)

For several years a lot of 320i owners have had a vibration in their cars between 48-62 indicated MPH. This problem has been diagnosed by many repair shops and dealers as being either in the tires or wheels used on the car. Several well known brands of radial tires were falsely accused of being at fault.

The actual cause of the vibration is inherent in the design and weight distribution of the US version of the car. Strangely enough, it does not manifest itself in every single car. Manufacturing tolerances for cars are laxer than those for missiles and hence some deviation from a norm occurs.

Every single BMW 320I lists to the left. This can be observed on level ground with no occupants and correctly inflated tires. A measurement was made from the lip of the left fender going straight to the ground, through the center of the BMW emblem on the center cap. This measurement was made with the wheels cut straight ahead.

The left side of each 320I in the sample group was lower in every instance. Generally, the left side of the car was 24 1/2 inches. The right side of the car generally was measured at 25 inches. Now, this is used as the "norm" for all cars. Deviation from this occurred as a function of equipment, i.e. sunroof, A/C, automatic transmission etc. Also mileage on the car and the brand of shock absorbers also influence the deviation.

The vibration the cars experience is due to a wind buffeting which occurs at a specific speed. Lacking a wind tunnel to determine if the buffeting is due to "crouch" or "lift" it can only be speculated upon.

The amount of vibration varies in direct proportion to the difference in the height of the left front of the car. There is also a difference in the left rear but it is a result of the vagaries in the left front. In the test group a difference of 1/2 inch resulted in no vibration. Again, all measurements are made without a driver. Depending upon the mass of the driver the numbers change by differing amounts. Also, a given driver changed the ride heights of different cars by different amounts. Based upon this, it can be surmised that different spring rates have a different effect under static load. Also, different cars have individually different balance points. When the left to right vibration difference is between 7/16 and 5/8 no vibration is noticed. As soon as the difference is greater than 5/8 inch, vibration occurs. Differences up to 1-1/16 inch were observed.

Crude as it seems, the solution is to install a pair of spacers within the coil spring on the left side in both the first and last coil. This brings the left side of the car up by about 3/8 inch and results in a car which is level and then can have the headlights properly adjusted. The cure works on one type of vibration only. It is possible that there are others in the same car. The type of spacer used was rubber face, the metal ones are not satisfactory. A 2002 sway bar bushing was found to be the most satisfactory one since it could be wedged very firmly when the car was jacked up and not fall out. Due to the size of the springs generally a spacer intended for pick-up trucks is in order.

October, 1979 (Vol. X, No. 10) ----------------- End Forwarded Message -----------------

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