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      11-12-2018, 07:33 PM   #10
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ashiohsiao's Avatar

Drives: 2012 F20 118i
Join Date: Oct 2018
Location: Taiwan

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Originally Posted by ovekvam View Post
I have driven quite a few BMWs the last few decades, and none of them were balanced towards oversteer from the factory. They all have some understeer unless provoked. The older modes with semi trailing arms could very easily be thrown sideways with a throttle lift or slight braking during corner entry, particularly the E21, but also E36 Compact. After the introduction of the Z axle, the willingness to oversteer has been reduced.

My two underpowered F20s have been very difficult to oversteer. I have to make a very aggressive corner entry during braking to make it happen. Fortunately it is also rather unwilling to end up in terminal understeer. Just a very slight understeer. With a square tyre setup, it is more balanced, but still understeers a bit. If I go too fast into a corner, the car ends up with a four wheel drift.

Based on my experience with chassis modification on my own BMWs, I would do the following to make the car handle better:

- Narrower rear tyres (same size in all corners)
- More negative camber up front (M4 front control arms?)
- 15 mm rear swaybar (mine also has 13 from the factory)
Yup~ the "unless provoked" was what I meant about being slightly tail-happy. My previous couple of cars were all FWD, and so the more neutral turning and the tendency to be slightly tail-happy feels comparatively stronger for me.

I definitely agree with your three directions of reducing understeer, and I had done similar experiments in the past, including the bump stop trimming, and what I've found is that the most prominent change actually came from trimming the front bump stops, and the first time I tested a Touge time attack here in Taiwan after trimming the front bump stop, (with everything else the same), I actually shaved 76 seconds off my past record, and I simply could not get any understeer! (Not the F20)

On my F20, trimming the front stops does the same magic for me. There are these two big turns on my daily commute route, and before trimming, I could take the first turn at about 70km/hr and already I could feel the car was starting to have a bit of understeer, the same for the second turn at about 88km/hr. After trimming the front bump stops, so far I have test the first turn at about 82km/hr, no tyre squeak and no understeer, the second turn at 100km/hr, again no tire squeak and no understeer.

However, I do need to point out that trimming the front bump stops would also give you slightly more sway at low speed or slightly more initial roll when you begin a turn-in. Therefore, the ideal way may be changing the stiffness of the front and rear bump stops. The trimming method is just easier and more cost-effective.

I then told a few friends about this, and they also tried it on their cars, and they also were shocked at how much a different it made

Last edited by ashiohsiao; 11-12-2018 at 07:52 PM.. Reason: Add more information