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I would like to know why the tires pressure is often different between front and rear.

I thought it depended only on the weight distribution but that seems wrong. Indeed, I have a BMW 420d xDrive (engine in front and 4 wheels driving) and the pressure must be higher at the rear. I guess the weight is heavier at the front. So, why the pressure must be higher at the rear in this case ?

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  • Welcome to Motor Vehicle Maintenance & Repair! Jan 2 '20 at 18:59
  • I once read from somewhere that putting the higher tire pressure in the rear decreases oversteering and increases understeering. Btw, it's what I read.
    – Jolie
    Feb 5 '20 at 8:54
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Actually on many BMWs (the 4-series included) the weight distribution is 50:50 between the front an rear axles. (I think it might be heavier at the rear in the case of the cabriolet)

While weight is one reason that manufacturers will indicate a different tire pressure for a given axle (as you can see when they usually suggest a higher-than-normal pressure for the rears when the car is fully loaded with passengers and luggage for example) it's not the only reason. The suspension geometry will be significantly different from front to rear - given the fronts need to do the steering and there is also the torque split to consider. xDrive is permanent all-wheel drive and varies torque split according to detected grip levels but by default it's 40:60 (rear-biased). All this means that the demands on the tires varies significantly between the front-rear axles and BMW will have had to take this into account when they determined the recommended pressures in order to give the desired handling characteristics while optimizing tire life.

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