Bob Murphy requested that I write an article on suspension systems for the Tii’s with a bit of general discussion and some of my experience with the cars. What follows is just that, although I will emphasize that I have not had an opportunity to try every combination of every component made by everyone. I will also emphasize that by now each of the cars, and certainly each of us reading this, is an individual with our individual tastes. Thus, what one owner finds to be the ideal compromise between ride and handling may be too soft for another owner and too hard for another. Furthermore, I believe most of the more “radical” alterations such as megawide tires requiring flared fenders or solid suspension bushings requiring flared kidneys as well as “competition” type systems are outside of the scope of the Tii Register. That caveat aside, the 02-series has two main impediments to spectacular handling. These are: 1) an unfavorable camber pattern and 2) insufficient roll stiffness.
These problems mean the chassis doesn’t keep the wheels upright and perpendicular to the road surface. Every tire, especially the wider, lower profile tire of today, works better and generates more cornering force when upright. The first problem cannot really be overcome without extensive alterations to the suspension mounting points on the chassis. The largest portion of the problem resides in the front MacPherson struts. These simple divices are too simple to allow enough bump camber change to “keep up” with body roll during cornering. Because of the design of the front struts, the outside front wheel camber becomes positive as the body jacks and rolls outward during cornering. To offset this, we use negat ive camber plates to make the static camber about 10 more negative (going from about 0.5 0 positive to 0.50 negative). Thus, as the body rolls in cornering the outer wheel carries less positive camber.
This article was originally written by Jeff Mulcahey in 1987. Some of the parts mentioned are either obsolete or rare in the modern days, but the logic and approach are sound methods even today in this excellent article.
These plates move the upper strut mount inboard and rearward. The former causes the static camber change and increases bump camber change slightly while the latter increases the caster angle slightly and improves the straight line self-centering of the steering. At the rear, the original camber pattern is fairly good. The original static camber can be preserved in a lowered car (see below) by flipping the rear subframe mounts and modifying the rear differential mount to “raise” the rear subframe in the body. Note that in ’74 and later cars there are different left and right mounts for the rear subframe.
The next area to work on is body roll in cornering. We seek to reduce body roll in order to reduce the positive camber change on the heavily loaded outside front tire . The three classic steps to reduce body roll are stiffer springs, stiffer shocks (that is, more bump or jounce damping) and stiffer anti-roll bars. The first two alterations also reduce brake dive under braking and squat under acceleration, making the car more stable in those transitions. They also affect the ride properties of the car. Stiffer anti-roll bars do not affect the ride in two wheel bump motions and thus do not alter brake dive or acceleration squat. Stiffer anti-roll bars usually mean larger diameter. Most aftermarket anti-rol l bars are low carbon cold bent steel and, finish excluded, are very similar. The optimal size of anti-roll bars for most cars are 19mm front and rear. Smaller diameters are not a substantial improvement from the stock 15/16mm diameters while larger bars (22mm) tend to compromise the “independence” of the two sides of the suspension. They also increase weight transfer to the outside tires which, in an extreme case, unloads the inner tire, causing it to lift and overload the outer (street) tire.
The use of polyurethane bushings and/or Heim joints allows one to get the most out of the stiffer anti-roll bars because all the initial deflection is transmitted to the bar whereas with rubber bushings the initial deflection simply compresses the rubber at a lower rate than the bar torsion rate. If you know what you are doing in terms of altering the oversteer and understeer characteristics with the adjustors, adjustable anti-roll bars will allow you to fine tune the suspension to your particular tastes and equipment specifications. However, I am underwhelmed by various adjustable bars which are adjusted by a sliding pinch clamp along the bar or by moving a Heim joint in a drilled tab welded to the bar. The proper way to make an adjustable bar is by flattening and cross-drilling the bar end and bars with this feature are rare. The next area to consider is the shock absorbers. These are probably the largest single determinant of the ride-handling balance you want to achieve. Koni shocks were popular in the early -02 tuning days by virtue of being adjustable until people realized you had to dismantle the struts to adjust them. Recently Koni has developed a new line of chassis kits with easily adjusted inserts. I have not sampled their current -02 kit. In the interim, Bilstein was acknowledged as THE shock to use. Complete dotted struts aside, these are available in “street” (023) and firmer “sport” (025) settings for the front with “street” (112), “heavy duty” (803) and “sport” (118) settings for the rear; the intermediate “heavy duty” setting appears to have superseded the “street” setting. At the front, I find the “streets” a good compromise between ride and handling for those who prefer the original ride bias toward the soft side. The “sports” are the components of choice for more vigorous driving.
At the rear start by forgetting the “sports” unless you only drive on perfectly smooth roads. Small irregularities can actually cause the rear wheels to unload due to the excessive rebound damping of these components and unloaded tires do not generate lateral acceleration. For the average enthusiast with conventional tires, I have used “sport” fronts combined with “heavy duty” rears with good results. One aftermarket specialty firm recommends “sport” fronts and “street” rears. Th is results in the damping at the front and rear of the car not really feeling matched as the rear floats over disturbances the front handles with clarity, resulting in some pitching moments about the front axle . For the owner less interested in an absolute go-fast state of trim, “streets” all around work well. Good quality replacement-type, that is softer riding components, are the Boge Turbo and TS shocks.
The final area for alteration is the chassis springs. Most cars can tolerate a spring which is about 20-25% stiffer than stock and about I” shorter installed. Lowering the car more than this really lowers the roll center and limits the size of tire you can use within the confines of the stock fenders. As with anti-roll bars, there are a variety of suppliers offering these components at everything from very reasonable to exorbitant prices. When they are available, I use the Miller and Norburn Touring springs because they work well, are durable and are very reasonably priced. For the most part, aftermarket springs are less than or equal to OEM springs in price so if you have a car with tired springs, an aftermarket performance spring set can also be an economical alternative. More economical, assuming the springs have not sagged, is to cut half a coil off each end of each spring. This will lower the car 0.75-1.25 inch and increase the spring rate 15-20%.
One drawback to lowering a Tii is that the roll center sinks faster than the car and thus the roll moment actually increases as the car is lowered.
A combination I have enjoyed for somewhat sedate street use is 4 street Bilsteins with OEM springs cut in this fashion and 19mm anti-roll bars front and rear. While on the topic of lowering a car, don’t forget that these cars had 0.5 or 0.75 inch aluminum spacers between the front strut bearing mounts and the body. These can be removed for some .inst ant lowering. Also, the upper rear spring rubber “dampers” are available in three thicknesses varying in height by 9mm. The availability of the shorter dampers is questionable but you can always cut a taller damper to make it a shorter damper. While you are tweaking around with the ride height, I like to set the cars up with about a I” nose down rake as measured at each end of the rocker panel. This helps the straight line stability and aerodynamics. More or less rake causes an unfavorable camber setting/pattern at the rear or front wheels, respectively. If you’re going to all the trouble to tune the ride height, you should also make sure the car doesn’t sit higher on one side than the other as each inch in side to side height disparity translates into about a degree of side to side camber disparity. Lastly, it is interesting to note that all the 2002′s used the same springs. The front springs are all the same except for the later cars (’74+) and those with factory air conditioning kits. At the front, these cars used the factory “heavy duty” springs which are 0.4 inch taller and slightly softer than regular. The later cars also used the “heavy duty” springs at the rear which are similarly taller and softer than standard. Interestingly, the 2002 Turbo used the same front springs as the other cars with taller (but not softer) rear springs to accommodate the increased weight of their 70 liter fuel tank.
One drawback to lowering a Tii is that the roll center sinks faster than the car and thus the roll moment actually increases as the car is lowered. This means that, all things (spring rates, shock valving and anti-roll bars) being equal, a lowered car actually rolls more than an un-lowered car. Of course all these things are not equal and this doesn’t happen. An aftermarket spacer is available to insert between the ball joint and lateral arm to raise the front roll center. This does indeed raise the front roll center and reduce the roll angle but also alters the relation of the lateral link and the steering arm and introduces some bump steer. A spacer fabricated to insert between the strut and steering knuckle lowers the roll center without altering bump steer. The correct thickness of spacer is equal to the ride height reduction, assuming such a spacer will still allow the ball joint and lower lateral arm to clear the inside of your wheels.
Raising the rear subframe prevents excessive rear wheel negative camber on a lowered car and also raises the rear roll center. However, when I lower a car according to the formula mentioned above, I do not find it necessary to alter the rear subframe mounting. On a lowered car with modifications to raise the roll centers, and thus the roll axis, the roll moment is reduced and even in the absence of stiffer springs, shocks and anti-roll bars, body roll in cornering is reduced. The first, if not obvious, place to begin a discussion of Tii suspension systems really should be the wheels and tires. These are, of course, the medium through which your car interacts with the pavement. They are also the first link in the suspension system and have profound influences on the ride and handling of your car. They also, in part, dictate the character of the suspension system you might choose to install in your car. Also rea lize, that the selection of wheels, and therefore wheel and tire sizes, often involves esthetic considerations which I will leave to you.
The oldest, simplest and most conservative choice in the tire area is something in the vein of the 80-series 165 HR 13 radials which were standard on the cars. Let’s assume that if your car is riding on tires this size, you are either a: an enthusiast who values original specifications and therefore is also willing to alter springs, shock absorbers or anti-roll bars or b: just beginning to upgrade the suspension and therefore will also want a more modern “low profile” tire. If you insist on running a 165×13 tire but want to uprate your suspension, the following comments on 70-and 60-series tires also hold for 80-series tires.
These are more contemporary size selections and common fitments to Tii’s are 185/70×13 and 205/60×13. Both sizes will fit on a 6″ wheel without interference l[ you select wheels of the proper offset. Recently, 195/60×14 tires on 318i/325e wheels have turned up on ’02s. These appear to fit, that is clear the front strut and steering knuckle, if the rest of the car and suspension is straight and correct. Another tire size is 185/65×14 but, frankly, this is a retrograde step in load rating and not much of a handling improvement over the 13 inch tires. All these sizes do not really require any special attention to the suspension as they still have fairly generous sidewalls which a: do a fairly good job of absorbing initial shock impacts without transmitting them to the rest of the suspension and car and b: will tolerate some additional camber change during cornering. Therefore the general formula of stiffer springs, shocks and anti-roll bars with negative camber plates mentioned earlier will work very well. If the truth were known, a car with a 205/60 HR 13 tire on a 6″ wheel with this formula is probably the best OVERALL combination of ride and handling. Indeed, for several years I ran 205/60 HR 13s on 6×13 wheels (13mm offset although BMW AG also say 20mm will fit) with the sport front/HD rear Bilsteins, Miller and Norburn Touring springs, negative camber plates and 19mm anti-roll bars and found it to be an outstanding combination.
This is the leading edge of “high-tech” tire design embodying the now familiar “plus two” concept of mounting a very wide low profile tire on a wheel larger in diameter to maintain the original rolling diameter. The typical fitment is a 195/50×15 tire on a 6″ wheel although some tires will fit when mounted on 7″ wheels (20mm offset for either width). This is the combination which requires some deviation from the above formula. These tires have a very short sidewall which translates into very little shock dissipation by the tire sidewalls, especially on a 7″ wheel. Thus a suspension which yields that nice overall balance with a 205/60×13 or 195/60×14 tire is very likely to produce a very harsh ride with a 195/50×15 tire. To get around this, some aftermarket suppliers produce variable or progressive rate springs for the newer series cars. Such springs have a varied wire diameter or coil winding to produce a non-linear spring rate and feature a soft initial segment allowing additional compliance to compensate for that lost from the tire sidewall. Unfortunately, such springs are, to the best of my knowledge, not readily available for the 2002 Tii. Thus, we will not discuss this possibility further and must stick with the original suggestion of stiffer springs with a rate of around 135-145 pounds per inch in the front. However, we can trade a bit of roll stiffness from the firm shocks, particularly at the front where it is pronounced on a two wheel bump, to the anti-roll bars, where it is not as perceptible in two wheel bump motions. For my own car with this tire selection, I retained the Bilstein intermediate (“heavy duty”) rear shocks but have replaced the “sport” front shocks with the conventional “street” setting Bilstein shocks.
To compensate for the loss of initial roll stiffness from this change, I increased the front anti-roll bar diameter from 19mm to 22mm. These changes had the dual advantages of making the ride more pleasant and the car a touch less prone to trailing throttle oversteer without increasing understeer.
Recently I was able to acquire a pair of the Bilstein/Alpina front struts. These are color coded green and, by the ride feel similar to the “street” settings which carrying Bilstein part numbers similiar to the “sport” front inserts. These particular struts also have 2° negative camber built in at the axle spindle so the negative camber plates are no longer necessary. To go along with these struts I had fabricated a set of 19mm spacers to fit between the struts and steering knuckles; these spacers are a close match to the 20mm reduction in ride height induced by the lower spring perches on the struts. As the struts are intermediate in roll stiffness between the “sports” and the “streets” and the spacers reduce the roll moment, I have elected to use an. intermediate setting front anti-roll bar. The front bar I am using is the 20mm bar from the 2002 Turbo. This bar, and the appropriate bushings, are available from BMW NA stock and list for about $55. The bar is bent so that it will clear the air conditioning compressor IF you have an OEM-type drive to the compressor which comes off the crankshaft behin9 the injection pump belt and IF the compressor is not mounted too far outboard. In combination with the “heavy duty” rear shocks, a 19mm rear anti-roll bar, no alterations to the rear subframe, and a 40% limited slip differential (see below), I find the ride good for the tire size and the handling very sure, stable and quick. The rear of the car also stays where it should unless it is provoked to come around with additional power or a severe trailing throttle maneuver.
An old “trick” to improve the turn-in response of the car is to place a large fender washer with a I” diameter hole in it between the rubber bushing (not between the original washer and the metal sleeve in the bushing) and the retaining washer at the front end mounting of the lower trailing link in the front suspension. This reduces the rearward compliance of the bushing which must be compressed on the outer wheel as the car turns. The reduced compliance means the car does set and turn into the corner faster. However, upon encountering a bump the wheel must move up (due to the bump) and rearward (relative to the body, due to the forward motion of the car relative to the bump). The loss of rearward bump compliance with these washers is not noticeable with conventional tires but is noticeable with 50-series tires and stiff shocks. Because of the deleterious effects of these items on the ride, I removed the extra compression washers I had added previously to the front suspension when using the sport inserts but have replaced them now that the softer struts are in place. A similar tweak was used on the Turbo in that these bushings were of a harder material . The Turbo bushings are no longer available except with the entire Turbo front subframe. An alternative suggested by BMW AG is to replace the foremost front flat washer on the bushing with a concave washer like that found just behind the bushing. One bit of advice is to plan your overall suspension package, including the wheels and tires, before you start any modifications or make any purchases. This logic of this is obvious in that you want to purchase any given component only price . It would be expensive if your tastes in wheels change or unfortunate if your wheel width and offset are incompatible with the ride height of the springs you purchase later. A corollary is to not get too “radical ” in your selections of anyone component lest you find that choice compromising your subsequent options .. First, “radical” components tend to extract a price in day-to-day practicality in exchange for the few days each year you go out on the race or autocross track. Secondly, a radical selection in one area may compromise a later, previously unforeseen modification. As a personal example, when I purchased my 1511 wheels and had the offset altered for my car, the Bilstein front struts were no longer available. Nevertheless, I opted for the 611 width.
A year and a half later I tracked down a fresh set of struts and installed them. I am convinced that the 711 wheels, if altered to fit inside the Tii fenders, would not clear the struts (or vice versa), whereas everything fits marvelously with the 6″ wheels. The final comment about suspensions doesn’t really concern the suspension but does affect the handling. A limited slip differential is a very useful piece of hardware when it comes time to press hard in the corners . These components reduce the tendency to trailing throttle oversteer and make the car feel more stable in power on-power off-power on transitions. One manifestation of high lateral acceleration is a tendency to lift the inside rear wheel, either from excessive body roll on a stock suspension or due to stiffer shock (rebound) damping and anti-roll bars in an upgraded system. Once this happens, the inner wheel spins and you lose acceleration out of a corner. The limited slip will also reduce but not eliminate this effect. At low speeds, they can introduce a slight understeer but I would much rather have a touch of low speed understeer than high speed tail twitchiness. Limited slip differential or differential inserts are available in 25, 40 and 75 percent locking effects. I use 40% unit, finding it to be a good compromise between the unobtrusiveness (to the point of wondering if its there) effect of the 25% unit and the low speed tire scrub and heavy understeer in tight corners of the 75% unit. The “real” Tii 3.45 final drive ratio makes a very nice combination with a close ratio five speed and offers a slight reduction in engine revolutions when cruising while retaining excellent gear spacing, but that’s a topic for another day. Another topic I hope we’ll discuss at a later time is upgrading the brakes, including several strategies for vented front rotors, rear drum upgrades and front to rear brake proportioning. Last update: 2007-08-14 19:43 Author: tiiregister