01-05-2012, 02:13 PM | #23 |
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My '11 setup was staggered 309's with summer tires - so 245 front/275 rear. I don't think you would gain much, if anything, by going to 275 all round except more understeer. And since the thing already understeers like a willful truffle pig looking for lunch, I would bother.
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01-05-2012, 04:52 PM | #24 | |
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01-05-2012, 09:35 PM | #26 | |
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01-05-2012, 09:39 PM | #27 |
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01-05-2012, 09:49 PM | #28 |
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It depends on what you drive. With rear biased torque in "Sport" I'm happy having more rubber at the back than the front. I can key the front in with less rubber: what I need is grip at the back to balance the car under power. So thanks but no thanks to more rubber up front.
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01-05-2012, 10:32 PM | #29 |
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Almost all BMWs, especially the SAVs, typically have notably more negative camber in the rear than other automobile makes (ultra high performance models aside). As we all know, rear negative camber is intended to create a flatter contact-patch between the outside rear tire and the road surface, reducing oversteer as the lateral forces create more positive camber. This is especially important on RWD cars, obviously, as they are prone to oversteer when power is applied too early coming out of a corner (yada yada yada blah blah blah).
Now under normal driving conditions, the X3 has a 40/60 front/rear torque bias. This is done to facilitate more spirited, RWD-like driving characteristics (but let's not get carried away...this is not a RWD car). Similar to a RWD car, then, the X3 should require more rear grip to reduce "excessive" () oversteer. But because of the increased negative camber at the rear wheels, the straight-line contact patch is effectively less than than it would be if the camber angle was at 0˚ (see numerous X5 4.8is threads regarding significantly uneven rear tire wear). The car, then, has less straight-line, high speed stability. In order to counter this loss of stability, the rear wheels are fitted with wider tires to increase the width of the contact patch when quick, high speed maneuvers that won't change the camber angle nearly enough need to be made (i.e. avoiding a large pothole on the expressway at 80 mph). This combination of a rear-oriented torque bias, negative rear camber, and wider rear tires gives the car a more balanced dynamic of fun, maneuverability and control, and safety. While the staggered setup increases understeer at the limit, the car is actually more agile at road-safe speeds. There is a line, though, and if you intend to track your "people-mover-with-an-identity-crisis" (a frugal attitude towards high performance SUVs and SAVs), the squared setup does lower rear grip ever so slightly enough to ever so slightly increase agility at high cornering speeds. Whether to actually use a staggered setup on track with, say an M3, well that's up to the technique and driving style of the driver himself. Bren, while you're right about wider front tires decreasing understeer, this mostly applies to cars whose aerodynamic and geometric limits exist beyond the limitations of the engines by which they are propelled. BMW and insurance companies would rather argue the safety of an SUV more prone to understeer than oversteer; loss of front grip is easily remedied by slowing down, while loss of rear grip takes more skill to correct. An SUV is heavy; it takes a lot to undermine its curb weight. But when weight can't keep it all together, good luck fighting the straight-line momentum that suddenly went sideways. Last edited by xDrive35i; 01-05-2012 at 10:39 PM.. |
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01-13-2012, 08:46 PM | #30 | |
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huh? having 275s all around will only help handling...it will reduce understeer without increasing oversteer and you'd still have the 275 out back so no less grip for the main power wheels
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01-14-2012, 12:13 PM | #32 | |||
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A car not only needs lateral traction to turn but it also needs yaw moment. The yaw moment is generated by the relative difference in front to rear lateral traction and left to right longitudinal traction. If you traction is biased too much to the front and inside (during braking) or outside (during accelerating) then you will have too much much yaw moment which leads to what people describe as "oversteer". If you traction is biased too much to the rear and outside (during braking) or inside (during accelerating) then you have too little yaw moment which leads to what people describe as "understeering". Stability control manipulates the left to right bias using the brakes to try and correct under or over yaw moment. So it is possible to have too much traction, relative to the traction available at the other end of the car. Having too much traction at one end will degrade the performance of the car for many reasons... to much rotation mass, too much rolling resistance, ect. I have the 245 front / 275 back setup and I am convinced it is not necessary. I cannot spin my tires, even with traction control off. That means that my car is no where near being "loose"... I think 245's all around would probably give the driver a better experience... and is why BMW switched to them for the 2012 year. If bigger tires produce more grip (as suggested in this very thread), then why don't we have extremely large tires? Compromise and the law of diminishing return. Tires are load sensitive, meaning the relationship between lateral grip and vertical load is not linear but regressive (double your vertical load and your lateral grip increases by a smaller amount). The target tire size is going to be in the more linear region of the vertical versus lateral grip relationship. The down side to larger tires is more rotating mass, more self aligning torque (resistance to being steered), and more difficulty packaging the tire in the wheel well (especially in the front). Balance is required. Although all tires are different it is generally true that camber creates a lateral force in the direction the tire is tilted. I have seen the rule of thumb that each 1 degree of camber is roughly equivalent to .1 degree of steering but I don't know if that is only applicable to racing tires. Camber above 5 degrees tends to reduce grip as the tire starts to carry load through the side wall (although this is extremely dependent on tire construction, profile, and stiffness). Downsides to camber... uneven tire wear is the only one I can think... however, this is a big concern for passenger vehicles. I disagree that the goal of static camber is to have the tire "flat" in a corner. Maximum grip would be generated with the inclination angle of all tires into the turn. Balancing tire size, tire stiffness, camber, toe, caster (front), roll stiffness, spring rates, damping rates, wheel base, track width, tire pressure, anti-drive / anti-squat, and many other things are all used to generate the handling properties of a car. It is foolish to justify a decision because "racecars" do it. Race cars have different functions, setups, speeds, and rules that they are governed by. Because F1 used grooved tires does that mean we should all groove our tires for maximum traction? Nonsense. |
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01-15-2012, 01:42 PM | #33 |
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YIKES.
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01-15-2012, 07:29 PM | #35 | |
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Regarding the race cars, I couldn't agree more. |
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01-15-2012, 11:31 PM | #37 |
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hahaha trust me you dont need to explain anything to me.. just the fact that noone ever said anything about larger tires up front than out back lol
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01-15-2012, 11:33 PM | #38 | |
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