Vibration/tires/Road Force/smooth ride!!!!!!! (archive)

The complete automotive resource for buyers, sellers, and owners like you.

I have to share my recent experience. I wanted to rid my car of vibration. And at the same time I wanted to get a smoother ride. I reach my objective. Here is the story.

I was one of the many seeing small but distinct vibrations from the front end of my E32 (91 750iL). I have had the wheel balanced, did a 4-wheel thrust alignment, replaced control arms bushings, and replaced all four brake rotors and pads. All seemed to make minor improvements. But I could still feel a vibration at lower speeds. Between 15-50 MPH. And it made me crazy to have such a great car that vibrated like a piece of ____!

I found the solution! I had new tires installed. And had the tires/wheels balanced and OE matched. What is OE matched? Most tires are not perfectly round. And some rims might not be perfectly round either. This is more likely with older rims (E32 rims might fit this category) that might have hit a curb or pothole in their faithful careers. By identifying and positioning these tire and wheel anomalies appropriately, the potential wheel/tire vibration can be eliminated or at least minimized. This of course requires special equipment that can identify these roundness/�Road Force�/balance variations. I am told balancing does almost NOTHING to address Road Force variations.

I had my work done at a local Discount Tire shop. They have a machine that can measure for the Road Force variations and even tells the machine operator how to rotate the tire to the appropriate location of the wheel to minimize the variance in load force. Not all tire shops have this technology. So make some phone calls first. And remember these tire guys probably work on a commission basis and list price is only something to put in the book for reference. I purchased my tires at a considerable discount (~ 25%) from list and they did the OE matching for free!

After mounting the tires on the rims, I watched the operator measure the �run out� in my rims. 2 of the four stock alloy BMW rims had small variations. But they were within specified tolerances. But, these two were not perfectly round as they were when the car was new. Then he performed the �Load Force� variation test on each tire/wheel combination. The worst tire/wheel combination generated 18 pounds of road force variation prior to making any adjustment (moving the tire to a different location on the rim). After testing and moving/rotating tires on rims to the machines suggested location; the maximum road force variation was 3 pounds on the worst tire/wheel combination. One was still not perfect. (it was placed on pass rear location) But three were able to reach zero road force variance. But I believe this addressing of the Road Force variation makes a big difference. We could have just balanced the tire/wheel combinations and not even looked for Load Force variations. All four tire/wheel combinations were in fact balanced prior to performing the initial Load Force variation tests.

Balancing is done in combination with the road force/OE matching. So it took 2-10 minutes per wheel/tire combination as opposed to just balancing which take less than 1 minute. This extra time is spent testing, shifting, balancing, re-testing, re-shifting, and re-balancing until you reach an acceptable point. I would be willing to guess their acceptable point would be less than my or possibly your acceptable point. So being in the shop and watching the process might deliver better results.

Also as important is the tires I selected. I am not going to be racing or performing in an autocross or rally anytime soon. I drive my 750iL to and from work in highway traffic and to dinner and a movie with my family. Tire loading and lateral g-force capabilities are not as important to me as wet traction capabilities, long tread wear (UTQG Rating: 620) and mostly a smooth and comfortable ride.

I replaced the Michelin Z rated P225R60-15 with a Michelin T rated P215R65-15 touring tire. I know taller and narrower. It is only slightly different in size. And the difference is only noticeable if you were to measure it. You can�t see the difference with the naked eye.

But man does it ever deliver a smoother ride! I installed a Michelin tire called the X-ONE. http://www.michelin-us.com/us/eng/tire/catalog/xone.htm I also have these tires on my wife�s Tahoe and it makes huge difference in ride comfort on the Tahoe too. We don�t race the Tahoe either in case some of you were wondering. If you would like to improve the ride qualities and willing to trade some lateral g-force capabilities, then I would strongly suggest you look at these Michelin tires for your 7 series!

Questions, comments, or problems, please visit the Roadfly help desk.