Does your car have body roll? I’m not talking a little bit, I’m talking A LOT of roll here.
Sure, some amount of body roll is a desirable characteristic for good handling. If the car was too flat, or too stiff, it would handle like a rollerskate and work the tires too hard, ultimately reducing max grip.
But there is such a thing as too much roll, especially when your car’s front suspension is of the McPherson-strut variety.
Note: I am beginning my rant assuming that you already know the basics of alignment settings, like camber and toe, what they are and what they do. If you want a refresher, check here.
Pictured left is a typical Mcpherson-strut suspension. To the right is a typical depiction of a Double-wishbone suspension.
The McStrut configuration is relatively simple. The strut, which is a coil-over shock type of suspension is mounted to the strut towers up top, and to the steering knuckle at the bottom. The knuckle is then mounted to the lower control arm and in BMW’s case, the tension strut. Instead of one large arm with a “Y” shape, that connects the front subframe to the knuckle, BMW prefers to use two indepedent arms to control the movement of the steering knuckle.
Image taken from 1Addicts, with my labels.
Note that there is no true upper control arm as seen on the Double-wishbone arrangement. One of the consequences of this is limited control over suspension movement.
What do I mean by this? Well, when the car is cornering, or it hits a bump, there is not just unidirectional movement. The forces exerted on the car’s tire are multiple. As the car corners, the tire flexes, the suspension compresses and rebounds and the car leans, moving the car’s weight along with it.
A double wishbone suspension, with links on either side of the steering knuckle, can better control wheel motion. This makes the steering more predictable, as the front wheels feel better located under load.
In addition to that is the big reason that McStrut car owners should be jealous: the dynamic camber curve.
With the double-wishbone suspension, the upper control arm is shorter than the lower control arm. That means that when the suspension compresses, and the shock goes through it’s suspension travel, the upper arm moves more than the lower arm, tilting the steering knuckle (again, where the wheel is mounted) inwards. That means that under compression, the double-wishbone car gains negative camber, which keeps the tire in full contact with the pavement as the tire loads up from cornering forces. When the wheel has negative camber, it can, in affect, push back against the weight of the vehicle as it leans on the tire.
A McPherson-strut, being very simple in design, and lacking an upper control arm to control the movement of the steering knuckle, cannot hope to replicate the performance of a Double-wishbone design.
On a McPherson-strut car, as the strut compresses, there is initial negative camber gain as the lower control arm moves down, however, as there is no additional arm pulling the knuckle “in” under compression, the camber curve eventually inverts, causing POSITIVE camber gain after a certain point. It does this because the lower arm can only go so far, and once it’s done with it’s usable stroke and the strut continues to see force exerted on it, it presses harder against the knuckle, ultimately causing it to push outwards.
If you push a McStrut-equipped car hard enough through it’s suspension travel, the wheels will actually experience net positive camber, reducing the tire’s contact patch with the pavement, reducing available grip.
Autocrossing my car with stock suspension.
As my car’s front suspension went through it’s available travel, it went beyond the limited negative camber gain that there was to receive, and the wheel’s camber curve changed, resulting in positive camber. However, note what the inside is doing. It’s doing very little is what it’s doing. The inside corner of the car has gone light, meaning that the tire is supporting very little load.
This is where a swaybar, or anti-roll bar, comes into play.
A swaybar literally links the left and right suspension components together with a sturdy bar, in an effort to reduce roll (hence the name). With a big enough sway bar, the load experienced by the outside wheel can be transferred to the inside wheel that would otherwise have gone light. Generally speaking, the bigger the bar is, the stiffer it is, and the better able it is to transfer load from wheel to wheel.
Reduced cornering loads to the outside wheel is most commonly seen as a reduction in body roll, or lean. With reduced lean, a McStrut car is less likely to enter the “bad” part of the dynamic camber curve. So a big sway bar helps keep a McStrut car’s limited negative camber in check, meaning that the front tires are better able to cope with roll, meaning more front wheel grip.
A sway bar also has the benefit of acting like additional spring rate as the car leans. What this means is that a car with moderate spring rates, and a good sway bar, can have comparable roll and lean characteristics to a car with very stiff springs, but no sway bar. Having a softer actual spring rate means that the car is more composed over bumpy and rough surfaces, meaning that the car would have more grip and available traction versus the more stiffly sprung car. However, by linking the left and right sides of the car together with a firmer bar, the suspension in effect becomes less independent. On a car with no sway bar, an impact to the right wheel would be absorbed only by the right strut, leaving the left strut fully compliant. With a stiff sway bar, the load of the impact is dispersed between the two struts more evenly, so having only one wheel hit that bump would affect the chassis almost as if both wheels hit it at once. This not really an issue for road racing or autocross, but if you’re offroading or rallying, sway bars are usually the first suspension component to be removed entirely.
All of this does come with an asterisk though. Making the front end of your car too stiff will exacerbate understeer issues, and reduce ultimate traction. Your suspension should only be as stiff as you have tire grip to match. If the car is too stiff at either end, and the tires aren’t up to snuff, the car will “skate” over bumps and lose grip. Don’t just throw parts at the car without considering how it will affect the car’s dynamics. If your spring rates are already sky high, but you want better wheel control, consider a softer spring with a bigger bar. Conversely, if you feel like you’re compromising grip on rougher pavement, consider a softer bar with a heavier duty spring rate.
For E82/E90 folks’ reference, this car has a race weight of 3400lbs, and uses 255 section width street tires. I am using a 26mm sway bar with 400lb front springs and 700lb rears. With this combination, I would not recommend any additional front spring rate or bar without a serious uptick in front tire grip.
So, less body roll, better dynamic handling behavior and tire control, and sharper front end response. All of these are things that will greatly aide the handling of your BMW.
This all sounds great, and fortunately, installation is easy.
1. Jack the car up and put it on stands.
2. Remove endlinks.
These are literally the links between the struts and the ends of the sway bar. Use your trusty 17mm socket and 17mm wrench and get ‘er done. Tighten spec is ~40ft./lb. so it’s not hard work.
3. Undo chassis mounting.
Again, pretty straight forward, the sway bar is secured to the front subframe via two U-bolt style collars with rubber bushings. Remove them, and the bar should be free to drop down and out. I want to say that they are 14 or 15 mil.
4. Gawk at the new toy.
Bigger is better. ;) Oh yeah, and red is the fastest color.
Pretty much the opposite of removal, with one caveat.
When installing swaybar endlinks, they need to be tighten with the suspension under load (i.e. sitting on the ground, with the wheels not hanging in the air). This is done to remove abnormal preload on the bar, and ensuring that the bar responds as intended when the car is leaning. If the preload isn’t set up right, the sway bar and endlinks can clunk over bumps as the bar is putting uneven load on the links side-to-side.
So, how do you tighten the endlinks if the car is on the ground? How do you even get under the car? Here’s my top tip for that one: use wood blocks like a plinth.
With everything bolted up, drop the car down, and you’re done.
Congrats! The car now rolls less, has a better dynamic camber curve, better steering response and is generally more fun with limited, if any, down sides.
In conjunction with the T.C. Kline coilovers the car feels excellent. Enough compliance for a street car, but stiff enough when it needs to be. Hopefully you’ve learned something about setting up your car for superior handling. My rants and raves here apply to just about any car with a McStrut front end, from rental to race car, so use my insights as you see fit. My full BMW suspension setup diatribe is coming soon, so stay tuned for that, as it covers my philosophy in how I set my car up, and how similar cars can be setup.
Jake Stumph is a freelance writer and notable Southern California track day bro. When people ask him to help with their cars, his go-to reply is that “I’m not an engineer, but I’ll take a look anyway.” If you find his posts informative, amusing, [insert adjective here] then you can follow him on Facebook, where he posts track-side commentary, pictures and additional rants and raves.