Understeer...BMW's achiels heel

[kdw215]

First Post:

I've read a lot on here and on e90 about understeer... having come from a long line of BMW's I can say this is normal, what is odd is the concensous on here that the answer is a 'big rear bar'

E30, E36, E46 all have the same 'issue' as the current e92, e82, and all respond to the same solution. (being all rear wheel drive, front wheel drive responds differently)

The issue is 'roll induced understeer' the front slips first as it has less grip compared to the front. (put a bar which is too big on the front - and you can make it oversteer. Why? because the front has MORE grip)

Basically it's comes down to the roll ratio and the springs. If you look at the KW2 club sport you will see the answer. A quick calculation reveals a front to rear spring ratio of approx 63.5%

M3 KW Clubsport 63.5%
Front: 508 lb/in
Rear: 800 lb/in

KW Clubsport (non M3) 70%
Front: 400 lb/in
Rear: 570 lb/in

(this is most likely higher due to the difference in sway bars between the two)

Bilstein PSS10 62%
Front: 385 lb/in
Rear: 615 lb/in

Incidently this ratio is pretty close to all the Group A e30, e36 race cars which are between 59% and 61%.

The e30 crowd have known this for years, so it's well proven as an approach.

I would 'solve' the e82 understeer issue with a similar F to R spring ratio.

Stock the 135i is approx 34% Given 120f and 350r (not accounting for the progressive nature of the springs)

I notice the BMW Performance springs are estimated to be 190f 505r
(thats a 37% ratio, so not much different but a slight nod in the right direction) so we still have the same built in 'safety undeersteer'

make those front springs around 300-320lbs. And i would bet it would be a dream... (and note no sway bar changes) so rear traction should be unaffected which it can be by installing too big rear bar.

Really sway bars should be used to fine balance an already correctly set-up spring and damper combination, not the other way round.

Obviously this is just scraping the surface of chassis dynamics, and is a point of view based on expirence with BMW's in particular. and IMHO

[WAY]

I have KW Clubsport on my car and a square tyre setup, and it still understeers at the limit but no where near stock. Beyond the suspension setup, what you really need is a widebody front which is what I am doing.

[sparoz]

So, I guess - forget the sway bar and tamper the springs?

[My135]

I am not a suspension expert but based on my personal experience by installing a lareger front sway bar did make the car front end much tighter in terms of minimizing the body roll and floaty feeling over high way joint; and turned in sharply around corner. This is how I feel and I don't know whether this setup will introduce more or reduce under steer. But my sense told me this is a much improve setup over stock. I am so happy with the current setup and I am thinking about stop putting more money into the suspension department. This is my 2 cents.

[Victory Road]

Originally Posted by kdw215

First Post:
blah blah blah blah blah

Damn good first post. I'm not a suspension expert, but definitely a suspension nerd and I could not agree with you more. What most people don't understand is when using a larger sway bar your actually decreasing overall traction at that end.

The great thing about suspension is there is no "right way" to do things...but in my opinion sway bars should only be used to fine tune balance (ie not so much a bigger bar, but an adjustable bar) while springs should be used to control body roll and overall vehicle dynamics.

[deamon32]

Originally Posted by sparoz

So, I guess - forget the sway bar and tamper the springs?

Thats been my plan all along, the swaybar is an easy and quick upgrade but it really is just a bandaid fix. If you are serious about upgrading your suspension go with the coilovers first to improve basic grip, responsiveness and to reduce understeer. The added dampening should also improve weight transfer and reduce corner entry and exit understeer.

Once the coilovers have been dialed in you can tweak the suspension with swaybars to remove any additional body roll and to increase grip during slow to medium speed cornering.

[kdw215]

hi.

Yes a bigger FRONT bar will help as it gives more grip to the front = reduces roll understeer.
Fine for the road, but on the track not so good - seen a few end links snap doing this.

I put H&R coilovers on my e30 M3, it already had H&R bars - what a pig it was, completly mismatched, way to much bar for the springs, went back to stock bars and it was great.

There is a very good suspension calculator somewhere on the web, if you can be bothered to measure and weigh everything it works pretty good, see if I can find it. It comes down to sprung and unsprung weight and the relative frequencies.

But the secret is always the dampers in the end...


found this - not the one I was looking for.

http://buildafastercar.com/tech/Sway...ate-Calculator

[My135]

You may be right about which setup provides the best improvement, but from my perspective, I am not ready to spend $1200-$3000 on the suspension. So for me, the quick fix from front sway bar is the best cost effective alternative ($215).

[techS54]

Just ran Willow Springs Horse Thief Mile yesterday in my bone stock 135 (I used to have an EVO 8) . The car was great but I did not like how much body roll and understeer I was experiencing. I was driving home thinking springs and adjustable sway bars. I like the logic of adding more front end grip via springs versus decreasing rear grip in order to reduce understeer so my question is, could you recommend a set of springs with that magic 59-61% ratio? Although I know they are the answer, I cannot justify a coilover setup right now ($$$$$$+lease) but I was thinking springs and adjustable swaybars would be a good setup. Your thoughts fellas?

[spam052]

Originally Posted by techS54

Just ran Willow Springs Horse Thief Mile yesterday in my bone stock 135 (I used to have an EVO 8) . The car was great but I did not like how much body roll and understeer I was experiencing. I was driving home thinking springs and adjustable sway bars. I like the logic of adding more front end grip via springs versus decreasing rear grip in order to reduce understeer so my question is, could you recommend a set of springs with that magic 59-61% ratio? Although I know they are the answer, I cannot justify a coilover setup right now ($$$$$$+lease) but I was thinking springs and adjustable swaybars would be a good setup. Your thoughts fellas?

+1

[WhiteOne]

BUMP!

[WhiteOne]

Swapping the front springs for stiffer ones (120 to 210), to go with the stock rears (350) would give us about a 60% ratio. Any idea where we might find springs close to that. This sounds like a very good setup for the street. Racers might want to go stiffer on both ends.

[Mark M]

Believe Ground Control is now marketing a complete coil over package for the e82.

I have their kit on my e36 and it is excellent for tuning. You can specify from any of their available Eibach springs, so you can customize the spring rates to your choosing, or just run their off the shelf version. Additionally this kit comes with the Koni adjustable dampers for even greater tuning. Further, the GC Eibach springs are linear rated which is much more favorable and predictable for track use when compared to progressive rate springs.

[Evolution Racewerks]

Great post. Front grip is key to reduce/eliminating understeer on this car.

Our front bar is a monster. We've snapped 1 front sway bar end link so far.

Our rear sway bar is unchanged.

The secret is always in the tires

Originally Posted by kdw215

hi.

Yes a bigger FRONT bar will help as it gives more grip to the front = reduces roll understeer.
Fine for the road, but on the track not so good - seen a few end links snap doing this.

I put H&R coilovers on my e30 M3, it already had H&R bars - what a pig it was, completly mismatched, way to much bar for the springs, went back to stock bars and it was great.

There is a very good suspension calculator somewhere on the web, if you can be bothered to measure and weigh everything it works pretty good, see if I can find it. It comes down to sprung and unsprung weight and the relative frequencies.

But the secret is always the dampers in the end...


found this - not the one I was looking for.

http://buildafastercar.com/tech/Sway...ate-Calculator

[WhiteOne]

Running M3 springs (160) on the front would improve ratio to about 45%.
Not 60%, but an improvement.
Would the M3 springs lower the ride height?

[cpt97m3]

Originally Posted by WhiteOne

Running M3 springs (160) on the front would improve ratio to about 45%.
Not 60%, but an improvement.
Would the M3 springs lower the ride height?

If they fit, which they probably don't, they would raise the car since the M3 is a bunch heavier.

[HP Autosport]

Originally Posted by kdw215

First Post:

I've read a lot on here and on e90 about understeer... having come from a long line of BMW's I can say this is normal, what is odd is the concensous on here that the answer is a 'big rear bar'

E30, E36, E46 all have the same 'issue' as the current e92, e82, and all respond to the same solution. (being all rear wheel drive, front wheel drive responds differently)

The issue is 'roll induced understeer' the front slips first as it has less grip compared to the front. (put a bar which is too big on the front - and you can make it oversteer. Why? because the front has MORE grip)

Basically it's comes down to the roll ratio and the springs. If you look at the KW2 club sport you will see the answer. A quick calculation reveals a front to rear spring ratio of approx 63.5%

M3 KW Clubsport 63.5%
Front: 508 lb/in
Rear: 800 lb/in

KW Clubsport (non M3) 70%
Front: 400 lb/in
Rear: 570 lb/in

(this is most likely higher due to the difference in sway bars between the two)

Bilstein PSS10 62%
Front: 385 lb/in
Rear: 615 lb/in

Incidently this ratio is pretty close to all the Group A e30, e36 race cars which are between 59% and 61%.

The e30 crowd have known this for years, so it's well proven as an approach.

I would 'solve' the e82 understeer issue with a similar F to R spring ratio.

Stock the 135i is approx 34% Given 120f and 350r (not accounting for the progressive nature of the springs)

I notice the BMW Performance springs are estimated to be 190f 505r
(thats a 37% ratio, so not much different but a slight nod in the right direction) so we still have the same built in 'safety undeersteer'

make those front springs around 300-320lbs. And i would bet it would be a dream... (and note no sway bar changes) so rear traction should be unaffected which it can be by installing too big rear bar.

Really sway bars should be used to fine balance an already correctly set-up spring and damper combination, not the other way round.

Obviously this is just scraping the surface of chassis dynamics, and is a point of view based on expirence with BMW's in particular. and IMHO

You don't need super wide front tires to reduce understeer. The key is to get the proper spring rates front and rear.

On E90post you will find lots of threads on suspension set up. Lots of good reading there! If you still have questions after doing some reading...PM or email me.

Harold

[HP Autosport]

Originally Posted by kdw215

hi.

Yes a bigger FRONT bar will help as it gives more grip to the front = reduces roll understeer.
Fine for the road, but on the track not so good - seen a few end links snap doing this.

I put H&R coilovers on my e30 M3, it already had H&R bars - what a pig it was, completly mismatched, way to much bar for the springs, went back to stock bars and it was great.

There is a very good suspension calculator somewhere on the web, if you can be bothered to measure and weigh everything it works pretty good, see if I can find it. It comes down to sprung and unsprung weight and the relative frequencies.

But the secret is always the dampers in the end...


found this - not the one I was looking for.

http://buildafastercar.com/tech/Sway...ate-Calculator

Bigger is not always better!

A big bar may be okay with a stock or mild suspension system, but with an aggressive coilovers, you will find smaller bars will yield more traction.