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-   -   Xi Understeer Questions (http://forum.e46fanatics.com/showthread.php?t=938573)

Marshmallow 08-14-2012 02:58 AM

Xi Understeer Questions
 
I'm assuming that I am not the only person who has experienced this, but my 325xiT occasionally understeers on fast curves. It isn't very bad, although once I almost plowed directly into a curb. Luckily I was able to give it a bootful of power and make the turn thanks to the RWD biased system. However, I still don't like that I can't turn as fast as RWD. Can I do anything to help this? I'm thinking about adding a fender strut to help with the weight shift. Any ideas?

SamDoe1 08-14-2012 09:24 AM

Thick rear sway bar will help immensely. Unfortunately, the AWD system on this car was designed for idiots in the winter and not performance. Your choices are either sway bar, stiffer rear suspension, both, or sell it and buy a RWD car.

Kubica 08-14-2012 11:58 AM

Camber plates up front, and a thicker rear swaybar. Then tinker with tire pressures.

Marshmallow 08-14-2012 04:04 PM

I like the rear sway bar idea, but in what way could i tinker with the tire pressure? Is it something you have already tried and figured out good numbers?

Kubica 08-14-2012 05:08 PM

^yes but it won't help you with stock suspension, alignment, and tires. I'd need to drive it.

Marshmallow 08-14-2012 05:27 PM

ok well I have plans for all 3 of those things, so hopefully that will fix the issues. thanks for the help!

Kubica 08-14-2012 07:15 PM

Quote:

Originally Posted by hcollins (Post 14652704)
ok well I have plans for all 3 of those things, so hopefully that will fix the issues. thanks for the help!

Well what tires/pressures/suspension are you working with now?

Marshmallow 08-14-2012 07:35 PM

Tires- Michelin XSE 205/55 R16 (They are in need of replacement and I will be putting the new ones on ZHP alloys. Not sure on which type of tire just yet, but definitely all-season
Pressure- 38 psi front, 45 rear, just as my door jam says
Suspension- Completely standard, non ZSP. If I'm not mistaken, Xi models with sport pack didn't receive the sport suspension at all?

Kubica 08-14-2012 08:22 PM

Try 35f/38r.

ZHP wheels are very heavy. Lighter is better- they will slow you down compared to 16s. If you want handling, spend money on summer tires, not all seasons. Keep the 16s and get some cheap all seasons used on craigslist and run them in the winter.

There is no sport suspension on xi's, but I think eibach and koni make springs and shocks for wagons.

Marshmallow 08-14-2012 08:30 PM

Wow thats a lot of great information! I'm going to start with the tires since I have barely any tread. The suspension was very low on my list of future mods but now I am going to bump it up top. As for the tire pressure, the lower numbers make a lot of sense now and I'll try that immediately. In the meantime, I need a servicing so I'll ask about what BMW thinks of the suspension on it. I noticed it rides a little high on the driver's side (very weird, should be the opposite).

SamDoe1 08-15-2012 08:49 AM

Quote:

Originally Posted by hcollins (Post 14653127)
Wow thats a lot of great information! I'm going to start with the tires since I have barely any tread. The suspension was very low on my list of future mods but now I am going to bump it up top. As for the tire pressure, the lower numbers make a lot of sense now and I'll try that immediately. In the meantime, I need a servicing so I'll ask about what BMW thinks of the suspension on it. I noticed it rides a little high on the driver's side (very weird, should be the opposite).

Famous last words.

Kubica 08-15-2012 10:40 AM

Quote:

Originally Posted by SamDoe1 (Post 14654136)
Famous last words.

+1.

Find a good BMW Indy.

Marshmallow 08-15-2012 03:08 PM

I know what you're thinking, but BMW won't screw me over on repair costs. I work at the dealer and I'm just going to ask what they think about the suspension.

hassmaschine 08-15-2012 03:37 PM

Quote:

Originally Posted by hcollins (Post 14653001)
Tires- Michelin XSE 205/55 R16 (They are in need of replacement and I will be putting the new ones on ZHP alloys. Not sure on which type of tire just yet, but definitely all-season
Pressure- 38 psi front, 45 rear, just as my door jam says
Suspension- Completely standard, non ZSP. If I'm not mistaken, Xi models with sport pack didn't receive the sport suspension at all?

in many years of autoxing and tracking an E30 ix, which has the same torque split and similar weight distribution, I always had the best results running significantly higher pressures up front, rather than the other way around.

With hoosier slicks I'd start at 50 psi front, 38 rear (cold), and work my way down to 45-47 psi front, 35-38 rear (hot). With "normal" tires I found a similar bias worked but 5-psi or so less. in street trim I run a couple PSI more up front.

Marshmallow 08-15-2012 04:00 PM

50 psi seems like a lot but I can understand why you would want so much at the front. I'll try it a few different ways to see what works. Luckily, I have the fact that it's a wagon on my side. That adds more weight to the rear getting closer to the 50/50 weight distribution. I've read that Xi sedans are 52% front, 48% rear.

hassmaschine 08-15-2012 05:21 PM

they were hoosier race slicks, they take more pressure than street tires.

Kubica 08-16-2012 10:23 AM

Quote:

Originally Posted by hassmaschine (Post 14655212)
in many years of autoxing and tracking an E30 ix, which has the same torque split and similar weight distribution, I always had the best results running significantly higher pressures up front, rather than the other way around.

With hoosier slicks I'd start at 50 psi front, 38 rear (cold), and work my way down to 45-47 psi front, 35-38 rear (hot). With "normal" tires I found a similar bias worked but 5-psi or so less. in street trim I run a couple PSI more up front.

This is not at all relevant.

Kubica 08-16-2012 10:29 AM

To get the most grip from your tires, you need to maximize the contact patch. To do this, you chalk your tires along the outer rib of the tread down to the sidewall. You get the most grip when you roll the tire all the way to the rib of the tire without riding on the sidewall itself.

First you maximize grip at all four corners by determining the lowest pressure you can run without rolling on the sidewall. Then you figure out if the car is oversteering or understeering. If you are understeering, you add pressure to the rear to decrease the contact patch, until the car is neutral. If you have oversteer, you add pressure to the front.

Maximize grip at the front, then reduce rear grip to balance the handling.

Kubica 08-16-2012 10:33 AM

You also increase the contact patch of your tires by reducing bodyroll, which keeps more of the tire in contact with the road.

Springs, shocks, swaybar, camber plates all accomplish this.

hassmaschine 08-16-2012 11:40 PM

Quote:

Originally Posted by Kubica (Post 14656880)
This is not at all relevant.

What makes it not relevant? Other than having a vc to control slip instead of abs/dsc, the layout and balance are very similar. I had many, many runs on street and race tires with chalk and pyrometers to determine my pressures and having it higher in the rear never worked - you just can't get as much heat in the rear tires as the front.


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