Yup, most important on a e9x is putting a thicker REAR bar to reduce the inherent understeer designed in.
Most combos give a slightly thicker front bar with a MUCH thicker rear bar to shift the bias rear.
If you just upgrade the front you will make it more numb and understeery.
I had the Hartge bars, which were +1mm front and +8mm rear, note the rear almost doubled in size from 13 to 21 mm.
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