02-15-2011, 07:13 PM | #1 |
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BMW saves on rear suspension.
Where are thouse times when the car needed to have a aluminium suspension arms for lighter unsprung mass.
Now steel is enought. Here more : http://www.auto.cz/test-bmw-x3-xdriv...-trakare-54204 |
02-15-2011, 08:18 PM | #2 | |
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02-16-2011, 01:32 PM | #5 |
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Sorry, No. The F25 chassis X3 uses high-strength stamped steel for the upper and lower control arms in the multi-link rear suspension and also uses a forged steel lower control arm in the front strut suspension. The front arm is plated and sure looks like aluminum, but it is not.
The rear upper/lower arms are thin stampings and are just painted black. Aluminum is lighter than a comparable amount of steel, but the currently available high-strength steels can often make a part stronger than an alloy component and still weigh less and take up much less room. |
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02-16-2011, 01:44 PM | #6 | |
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02-16-2011, 02:19 PM | #7 | |
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02-16-2011, 04:09 PM | #8 | |
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When I received my car from the local dealer immediately after it went through an “extensive 3 hour check” (CA’s words, not mine) the tire pressures were all exactly equal and were 2 psi (cold) lower than the lowest pressure recommended for any driving condition. Otherwise, the X3 is supposed to incorporate the late 3-Series “bump-steer” geometry, which as best as I can recall, is supposed to enhance stability if the rear end gets “loose”. With the much higher center of gravity and more body roll of the X3, you may be feeling the car’s suspension geometry trying to apply a little rear-wheel steer to compensate for the high roll. Another factor is the relative lack of steering feedback in the X3. I have “pushed” our new F25 in some fast but tight corners, and noticed a slight tendency to (have to) make a second steering line correction just before exiting a tight 90deg. turn as the body roll flattens out. The lack of steering feedback means thet you don't do this automatically, and have to think about it. I’ve only been driving with the VDC set to “Normal” and hope the "Sport" or "Sport+" settings reduce this. The other issue is the tires. I have the P7 All Season's tires which are certainly a compromise. On a previous car I owned (a 1999 Audi A4 Quattro Estate), going from the OEM All Seasons Dunlop’s to German Goodyear F1 DSG4’ s (set to optimal pressure by experimentation) made a dramatic difference. They almost totally eliminated initial understeer, reduced the steering angle at the limit, and made the car much easier to control in fast direction changes. Last edited by Lotus7; 02-16-2011 at 04:15 PM.. |
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02-16-2011, 04:22 PM | #9 |
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02-16-2011, 05:14 PM | #10 |
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Know what you mean. My other car is a Lotus 7 replica: No power steering, no power brakes, coil-over suspension F&R, no VDC, no radio. Of course it corners perfectly flat (the geometric roll centers are below the center of gravity of the chassis) and the handling is almost perfectly intuitive. A quarter turn of the steering wheel is all you need for most sharp corners. Of course, if you ride over anything thicker than a small coin, you feel it through the suspension.
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02-16-2011, 07:02 PM | #11 | |
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Do you have DHP? The variable steering ratio might also explain what you experienced. A review I read somewhere on the web noted almost exactly the same thing and blamed it on variable steering.
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02-16-2011, 07:51 PM | #13 | |
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02-16-2011, 08:26 PM | #14 | |
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There have been SAE papers published on the rear suspension and the BMW "Insights" page states: "Multi-link rear suspension keeps your BMW securely on track. Thanks to its intelligent lightweight construction, you can enjoy a high standard of agility and freedom from adverse forces or interference even on winding roads. The lightweight steel construction of the multi-link rear suspension delivers remarkable weight savings and ensures the characteristic BMW near-perfect weight balance. The result is even greater agility and sporting performance. Despite its low weight, the axle offers superb rigidity. Wheels are individually suspended and each is equipped with a number of guiding links. The precise design of the axle beam delivers excellent chassis support while enabling a system of dual elastic bearings for the rear-wheel drive and outstanding wheel guidance, all of which maximises passenger comfort." Yep, whenever I consider buying a new car, "...freedom from adverse forces" is right at the top of my priorities list. The above [BMW statement] clearly contains advertising copywriter drivel, but the basic idea comes through. Although BMW does not specify the exact steel alloy they are using it’s probably ASTM A/1008A/M or A/1011/A/M, which are sheet metals suitable for “steering and suspension parts, door-intrusion beams and chassis members”. This material has a yield strength of up to 80,000 psi as compared to 36,000psi for conventional automotive steels. This allows thinner parts to be stronger, so provides a very real unsprung weight reduction. Production costs of these “high-tech” alloy steels is about 24% more than plain old A36 sheet metal. Last edited by Lotus7; 02-17-2011 at 12:56 AM.. |
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02-17-2011, 12:37 AM | #15 |
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Did you notice any difference (apart from the steering) between VDC in Normal and non-VDC? E.g. did it iron out the bumps in the road better?
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