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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Tracking, Autocrossing, Dragstrip, Driving Techniques > 328i e90 zsp stock understeer



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      11-09-2010, 11:27 PM   #1
accel
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328i e90 zsp stock understeer

Hi,

I have several years of autocross experience with my previous car - Honda Prelude, and I bought 328i with some future occasional autocrossing in mind.

I did search this forum and it sounds like common agreement here is - stock ZSP setup understeers.

I didn't have a chance to autocross my 328i yet, however, I did some moderate spirited driving last weekend when it was raining. This allowed to exceed adhesion threshold at some safe moderate speeds and my conclusion is - this setup might naturally under or even neutral steer in mid corner, but it over-steers under power. At least I was consistently able to induce corner exit over-steer in two consecutive corner exits.

Can someone provide some more details on how stock e90 behaves at the limit? Just "e90 under-steers" tells me nothing. I'd like to know how it behaves at corner entry, corner exit, under power, under braking, etc.
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      11-10-2010, 01:17 PM   #2
The HACK
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This is a loaded question. It's going to earn me another "you just typed up a 1,000 word dissertation without answering the OP's question" ire from J Tyler.

Your assessment on the 328i chassis is spot on. For low traction conditions. On wet or limited traction conditions, the DIFFERENCE between available grip up front and in the rear are a lot closer, therefore the "staggered" tire setup OR BMW's factory suspension and spring settings doesn't necessarily promote understeer, but the rear drive nature of the car will allow for power-on oversteer. It's the nature of the beast, even when I had my underpowered E30 318is, that car will still oversteer when you get on the throttle way too early and aggressively in wet.

The same said chassis will also exhibit some corner entry understeer tendencies because there's not a lot of negative camber up front in dry conditions. Although, to be honest? The quickest way to cure that is to either brake harder, brake earlier, or slow down the hands. But like ALL BMW chassis, it exhibits very neutral mid-corner behavior and typically will "understeer" on corner exit in the dry rather than power oversteer...Unless you have a little bit more power than what the 328i carry.

HAVING SAID ALL THAT. Like almost all BMW chassis the E9X platform is surprisingly neutral, and most of the corner exit understeer tendencies can easily be cured by altering your line, or you can regain some of that lost traction up front by temporarily opening up the steering. I'm sure, having come from a FWD Honda Prelude, whatever you experience in the E9X in terms of "understeer" will pale in comparison to how much a FWD car will "understeer" into and exiting a turn. But whenever ANYONE on any form mention "understeer" without mentioning the condition in which the understeer occurred, WHERE the understeer occurred, and what they're doing with the throttle or brake at the time, have little to no knowledge of what vehicle dynamic on any car is and is just regurgitating what he/she heard or read on the Intarweb. And the myth that the E9X platform understeers from the factory? Greatly EXAGGERATED.
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      11-10-2010, 01:22 PM   #3
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With the zsp suspension, and included tire stagger, the car does understeer on dry pavement. I am assuming that you are running the factory tire sizes of 225/255 F/R, correct?

Yes, you can induce oversteer on exit under power, but thats a trait of having a RWD, not necessarily the chassis balance. Entry and mid corner balance will still favor understeer. Also, on dry pavement, it will be harder to induce oversteer.

But a simple tire/wheel change the balance. Put a square setup on (like 235s on all 4 corners), and the car is much more nuetral with the zsp suspension. I have zsp on my 325, but run a square setup, and the balance is very, very nice. I do have to be a bit more careful in the rain, because it will oversteer a lot easier at mid corner, but the DSC tends to keep it in check. In the dry, the car is very neutral and controllable though. Assuming you drive correctly, of course.
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      11-10-2010, 02:05 PM   #4
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I found the stock set up VERY neutral and I am always a little suprised when I see these posts.

Chances are if you experience understeer- you are probably going too fast into the corner. With a near perfect balance, the e90 is just a very neautral car. I have altered my car a bit and with the new alignment, there is more front end grip than I would ever need on the street. Haven't autocrossed it with the new set up yet.

I came from a Subaru WRX that was very front heavy. That car had loads of understeer. Maybe thats why my e90 is so pleasing.
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      11-10-2010, 05:17 PM   #5
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Thank you, guys for replies.
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      11-10-2010, 05:56 PM   #6
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Quote:
Originally Posted by mike-y View Post
With the zsp suspension, and included tire stagger, the car does understeer on dry pavement. I am assuming that you are running the factory tire sizes of 225/255 F/R, correct?
I'm really enjoying:
255/225 F/R
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      11-10-2010, 08:17 PM   #7
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Interesting thread. My E90 is the DD and doesn't see much track time (nor did I purchase it with the intent to track it), but the post above by Hack makes a lot of sense to me. Excellent post, and very well-explained.
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      11-10-2010, 11:17 PM   #8
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Quote:
Originally Posted by mike-y View Post

Yes, you can induce oversteer on exit under power, but thats a trait of having a RWD, not necessarily the chassis balance.
But isn't that is exactly what is desired with RWD?

Here's what I did with my FWD car with some mods - tune the car to have natural neutral-steer chassis balance. I could then control the car by varying amount of power applied to the front wheels - releasing the gas would shift weight towards front wheels giving them more traction and unloading rear - to induce over-steer. Applying more gas would do the opposite (plus some front wheel slip) for under-steer.

I would imagine that close to neutral RWD chassis balance would allow to over-steer the car by applying more gas in the turn. Even 328i that corners close enough to the limit of traction should have more than enough power to control level of traction at the rear wheels in conjunction with centrifugal forces.

And on corner exit, where centrifugal forces are already negligible, staggered setup should let apply full gas with minimal risk of undesired over-steer - as I imagine you'd like to accelerate as fast as possible going out of the corner.

P.S. Natural over-steer in conjunction with RWD might be too much I remember once I tried MR2T... I was only able to get to the finish with my 3-rd attempt That was LOTS of fun though

Last edited by accel; 11-10-2010 at 11:29 PM.
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      11-11-2010, 12:55 PM   #9
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Quote:
Originally Posted by accel View Post
But isn't that is exactly what is desired with RWD?
Not when you need to mash the gas pedal to force the car into oversteer because it's already understeering too much. Though that can be fun, it's not ideal

An understeering car is still going to be slower around the corner, even if you can light up the tires and induce oversteer under at the exit. Like you said, with your FWD, you set the balance up neutral, and then can use the throttle to balance. But you want it to be EASY to balance, as in small throttle inputs. if you need big changes in throttle input, you are more likely to upset the car (provided you have enough HP to do so).

And that's what will happen if you have significant understeer and need to mash the gas to put it into oversteer.
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      11-11-2010, 02:08 PM   #10
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Quote:
Originally Posted by mike-y View Post
Not when you need to mash the gas pedal to force the car into oversteer because it's already understeering too much. Though that can be fun, it's not ideal
In general, over to neutral steer is desired in corner entry and mid corner. At corner exit some neutral to under-steer is better, so that you can floor the gas pedal for acceleration with minimal risk of spin.

Having said that - it should be easy to break traction in rear wheels while in the corner - simply because they are under centrifugal forces influence as well. So it sounds like you could use gas pedal to control car's rotation at that stage.

But once you are at corner exit - centrifugal forces are much less than in the middle - so it should let you apply some gas for better acceleration.

This is my understanding... and it sounds like ZSP staggered setup is suited pretty good for this scenario.

But I won't know for sure until I try
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