Through our 26 years of racing BMW’s, and acquiring 11 National Championships along the way, we have learned a lot about suspension and how to maximize performance from it. Because there is a tremendous amount of misinformation out there, we are offering this document to help you make a more informed decision on what performance suspension to put on your car. Here are some facts to consider:
#1 - The overriding purpose of a performance suspension is to maximize the tire contact with the pavement. A tire has a set of parameters which need to be satisfied in order to perform at its maximum. These include; pressure, camber angle, operating temperature, spring rate, damper characteristics and load capacity. Some of these factors can be dealt with through intelligent choices of tire brand, size and compound. However, the things which are much harder to control are the suspension design and the ability of the dampers to keep the tire in contact the pavement the maximum amount of time. BMW has an excellent suspension design on their cars, so the parts we can look to improve are the damper and spring package, suspension bushings, and alignment settings, all engineered to optimize performance.
#2 – The suspension and sub-frame bushings are part of, and play an important roll in the performance of a suspension. OE suspensions are designed to work in concert with the bushings on the car. Raising the spring/shock rate too much will reduce handling and feel, unless the bushings are upgraded as well. There are several levels of bushing flexibility available depending on the objective. Harder rubber bushings like BMW M brand, are best for street where ride quality is important. Urethane and delrin bushings are desired for vary aggressive street, or in racing where spherical ball joint bushings are not legal. Ball joint bushings offer the most control, but can be harsh riding and will make noise as they wear.
#3 – A tire likes to have load applied to it in a progressive manner – load the tire too quickly, and it will just give up, (slide). Too much spring and/or too high a shock rate will materially reduce the performance. So will a shock that ramps up the damping too quickly. High spring rates can improve the lap time on fresh tires, but as the session/race progresses, to stiff a spring will prematurely wear out the tire from too high peak loads. If your finding the handling deteriorates quickly, try a lower spring rate to see how it improves tire grip later in a session. We see this even in F1. The teams which have figured out to get the tires to work longer usually win. There is a constant battle between chassis stiffness and tire traction at every level of motorsport. Finding the balance is the key to success.
#4 – The higher the traction potential of a tire, the more spring and damper load it can take. R-compound tires can take on a firmer suspension than street tires, and race tires can take on a firmer suspension than R compound tires. However, as stated above, if you have too firm a suspension on your car, it will actually reduce the performance potential, because the tire can’t deal with the loads and just gives up.
#5 – Damper construction: Whether twin tube or mono-tube, high pressure gas, low pressure gas or no gas at all, single adjustable or multi adjustable, with remote reservoir or not – Valve technology, durability and low flex of the parts are what matter. For example, the dampers which come on BMW’s are twin tube, and have excellent durability characteristics. “Twin tube shocks” are a design in which a second tube inside the main housing contains the valves which control the damping. “Monotube shocks” have the valves contact directly to the inside wall of the damper housing. There is a constant debate as to which is better, and in reality both are capable of excellent results, so the proper choice is the ones which accomplish the desired goals in the most durable way.
#1 – More damping and higher springs rates offer more performance:
#2 –If it costs more, then it must be better: Not necessarily so. Although it is true it costs more to build a sophisticated suspension, there is no guarantee it will offer the optimum performance for the car you are driving.
#4 – High pressure gas is better than low pressure gas: Nitrogen is used in high pressure gas dampers to control foaming of the fluid. However, if you are using a shock fluid which doesn’t foam, then gas pressure is not required. Also, high gas pressure introduces the need to control the additional damper movement for the gas as it compresses, while a low pressure damper has all of its damping in the fluid. And last, high pressure dampers ride more firmly on low deflections like road seams because it take more force to displace them. The advantage of high pressure dampers is the gas pressure can be used to make small changes in effective spring rates, and they can be mounted upside down.
#5 – A lower ride height is better – Over lowering a suspension will take it beyond it optimum working range, and will actually reduce the traction, feel and performance. It is a common practice to increase spring rate in overly lowered suspensions as a ‘band aid’ way to increase performance and feel by limiting suspension movement. Once a suspension has dropped below its working range, the suspension ‘falls away’, which in reduces the effective spring rate. A suspension works its best when as it is loaded, the suspension arms go through the path where they areparallel to the ground.
#6 – Remote reservoir dampers are better than self contained units: The need for remote reservoirs comes from when then damper housing can’t carry enough fluid throughout its travel. This happens when the valves are larger and require more fluid for a given displacement. Using larger valves are desirable to have more control for a shorter amount of travel. This is great for suspensions with very limited movement like formula cars and/or cars with ground effects. (Because down force is way more valuable than spring rate, cars with high down force constantly battle between mechanical grip and the need for a stiff chassis to keep the aero package at it optimum level). However, small, high quality precision parts can do an excellent job of controlling damping inside the main housing.
#7 – You need a higher spring rate on a race car than a street car: Optimum spring rate for a given traction level never changes. The only reason for higher spring rates on a race car is because the chosen tires have a higher level of traction. However, on a race car, more damping rate is desirable to better control the speed at which the body rolls, and this also offers better feedback to the driver. Great race damping is usually to firm for most street cars – the cure is to have adjustable dampers which have their adjustment range accommodate the best of both worlds.
#8 – A go-cart feel is faster. Body roll is not an undesirable characteristic. When the body rolls, it softens the transition of load to the tire and can dramaticly improve its long turm traction and durability, as well as offer the ability to trail brake more, and get on the power earlier. – However, many drivers like the go-cart feel for the responsiveness, and they are willing to trade long term maximum traction for it.
#9 – Higher spring rates make a car ride more harshly: Actually shock settings have more impact on ride quality than springs. In fact, reducing the actual weight of a spring improves ride quality as well. So a coil over conversion to a lighter springs allow an increase in spring rate while maintaining good ride quality.
Final Note: It would be truly helpful if all of the participants on the forums and chat rooms had to a résumé attached to their comments, because there is a tremendous amount of misinformation out there. However well intentioned these people may be, if the knowledge and experiences aren’t there, the suggestions offered and products endorsed are based on their personal experience, which many times does not represent the best advice.
For example: If a new to BMW enthusiast purchases brand x suspension because his friend or someone on line recommended it, he may feel the change to the car and appreciate that there is a performance improvement, but, because he has no other BMW experiences to compare it to, how can he/she know their new suspension is the best option?