MacPherson Strut <BR>
<BR>Double Wishbone <BR>
<BR> <P>Nearly all cars in production today use either a MacPherson Strut type suspension or Double Wishbone type suspension for the front and rear of the car (the MacPherson Strut is known as a Chapman Strut when used on the rear of the car, but they are identical in every way but name). Both of these suspension types are independent suspensions (meaning that on whatever end of the car they are installed on, vertical movement of the suspension on the right side of the vehicle does not directly effect the suspension on the left side; basically the two sides of the car are not directly linked to each other so they can move independently of each other). Both of these systems also use a "coil-over-oil" design for the springs and shocks on the car, meaning that the shock absorbers are mounted inside the coils of the springs. <P>Now let's talk about what each of these are designed like, and then we'll go on to the advantages and disadvantages of each of these systems. <P>I'll start with the MacPherson Strut (Chapman Strut) design. This was invented by Earl S. MacPherson in the 1940's. The MacPherson Strut is basically the simplest of the front independent suspension designs. Like I mentioned above, this system uses a "coil-over-oil" design. The piston rod of the shock absorber is used to serve as a kingpin axis at the top of the strut. At the bottom, the spring and shock combo pivot on a ball joint on the single lower arm (which is often an A-arm). When turning, the entire strut column (basically the shock body) is twisted in order to turn the wheel, which would cause the spring to "wind-up" (the strut is twisting, but the spring wouldn't, causing the spring to be twisted, or wind-up). In order to solve this problem a thrust bearing is placed at the top of the spring to prevent spring wind-up during turns. Now in a MacPherson Strut type suspension design, the strut itself is the load bearing member, with the springs and shocks merely performing their respective duties instead of also holding up the car. There is a lot of confusion surrounding this, so I would like to address that quickly before moving on. <P>Many people believe that the MacPherson Strut design doesn't use shock absorbers at all, that it uses what they call "a strut" which they think is completely different from a shock absorber. So when they talk about MacPherson Strut type suspensions, they will tell you that MacPherson Strut type designs do not use shock absorbers, that they use struts. I have encountered many other misconceptions about this issue, but that one is by far the most common. Now there isn't anything that you can do to damage your car if you use the term strut instead of shock absorber, but I thought I'd better clear up the terminology problems here. These misconceptions are completely understandable, as there are so many different people in the business using almost as many different terms. <<hehehe, I told you I am long-winded>>. I'm going to try and explain the difference as best I can. A strut is the load-bearing member of the MacPherson/Chapman Strut suspension design. A shock absorber, or more correctly a damper, is the thing in your suspension that counteracts the movement of the springs in order to stop the car from constantly bouncing up and down over and over after you hit a bump in the road. Now here's where the confusion comes from: the strut and the shock absorber are integrated together in a strut type suspension. They are two different things, but they come in one package all integrated together. It is not a major problem, but since I'm writing this anyway, I thought I'd clear that up. So even though the strut and shock absorber are integrated into the same housing basically, they are two different things...the strut bears the weight of the car, effectively holding the car up, and the shock absorbers counteract the movement of the springs. So cars with a MacPherson Strut suspension have BOTH struts and shocks. Anyway, I'll shut up about this now. <P>Ok, back to the MacPherson Strut. So, we've got the wheel of the car attached to the bottom of the strut by a lower A-arm (in almost all cases today), and a near vertical strut. Steering is accomplished by the steering gear being connected either directly to the lower part of the strut, or to an arm from the front or back of the spindle. <P>Before I move on to Double Wishbone suspensions, a quick side note about the way the MacPherson Strut allows the twisting that takes place when the steering wheel is turned. The spring is seated in a special plate at the top of the strut assembly. When this plate, or the springs get worn, a loud "clunk" is usually heard at full lock (the steering wheel turned as far as it will go in one direction) as the spring frees itself from the proper place on the plate and then snaps back into place. The reason that I mention this is that this noise is often confused with a CV joint knock, so if you're hearing something that you think is a CV joint problem, you may want to have the springs and plate checked as well. <P>All right, now onto the Double Wishbone suspension type. Unfortunately it is very difficult to explain all of the parts of this type of suspension and how it works, so I'm just going to do a quick overview so you understand basically what is going on. With this type of suspension, the upright supporting the wheel is attached to the frame of the car with a pair of links in the shape of a wishbone. These links are connected to the frame by bearings called suspension pivots (which are either metal or rubber, or now-a-days polyurethane). These links (arms) are not always parallel, and are usually of unequal lengths. The Double Wishbone suspension is a type of Double A-arm suspension. Double A-arm suspension systems have been around for about 50 years now. Early versions had equal length arms, but this caused the wheels of the car to lean outboard in turns (increased positive camber). When unequal length arms were developed the positive camber change problem was solved, and in fact was changed to a huge advantage because it made the wheels develop increased negative camber during vertical suspension movement. Double Wishbone suspension also use the "coil-over-oil" design, much like the MacPherson Strut, which often leads to people calling the shock absorbers on their cars "struts" even when their cars don't use a strut type suspension. <P>I've included pictures of each suspension type to try and make what I'm saying a little clearer. Especially with the Double Wishbone suspension, the pics can probably give you a better idea of how the suspension works than I can with words. Of course, the best way to figure it all out is to get under a car with a Double Wishbone suspension type and look at it to try and figure out how everything works. Don't feel bad if it doesn't make sense though, there are plenty of mechanics out there that don't understand how a Double Wishbone suspension works, it's a very complicated system. Today, engineers use computers to help them design the systems because there are too many variables to deal with to do it by hand without taking an extremely long time.<P>Now, onto the advantages of each...<P>The MacPherson Strut is the simpler of the two suspension designs, and as such, there are fewer things that can go wrong with a strut type suspension. Also, the MacPherson Strut takes up a little less room horizontally, which allows for more room for the front drive axle to pass through the front hub, and it allows for more passenger compartment space. MacPherson Struts are also relatively inexpensive compared to any of the other independent suspension types. Another big advantage to a MacPherson Strut design is reduced unsprung weight, which not only reduces the total weight of the car, but unsprung weight has a bigger effect on acceleration than weight inside the car. For example, if you reduced unsprung weight by 20 lbs, you would have to remove nearly 10 times as much weight from the passenger compartment (I'm not sure about the "10 times" part, I haven't heard any exact number, but it is fairly correct, and I put it in here only to impress upon you how important unsprung weight can be). Also, this reduction of unsprung weight increases the ride comfort exhibited by the car. With no upper arm (and sometimes no lower arm) engineers are able to directly control the vibration of the car with no need for the vibration to travel through initial members. This results in better ride comfort than most other systems have. <P>There are some disadvantages to this system as well. First, it's a very tall assembly, making this system impractical on race cars, and means that you won't be able to lower a car with MacPherson Struts as much as other systems. MacPherson Struts also have a problem with the amount of room available for wider wheels, without increasing the scrub radius. The scrub radius is the distance from the ball joint line to the centerline of that wheel. Basically you want to minimize the scrub radius because any bump or cornering force that is applied to the tire can exert a twisting force on the steering that is proportional to the length of the scrub radius. So if you were able to get the scrub radius to be zero, the car wouldn't really need power steering (the twisting forces due to the length of the scrub radius would be gone, so the tire would easily rotate about the steering axis). This means that getting wider wheels will increase the scrub radius, and you will need to use more effort to steer the car. Another disadvantage of MacPherson Strut suspensions is that there is very little camber change with vertical suspension movement. This means that the tires on the outside of the turn are going to have positive camber as the body rolls. This means that the contact patch of the outside tires is reduced as the body rolls during a turn. Since the outside tires are the ones that are providing the most cornering force, you want them to have as large a contact patch as possible. <P>Despite the disadvantages of the MacPherson Strut suspension design, many cars use this suspension (probably mainly due to the ride quality, and because it is so inexpensive). <P>Now, there aren't very many disadvantages to a Double Wishbone suspension. One of the biggest is the cost, because these systems are so complicated and difficult to design and because of all the different parts involved. However, many companies believe that the handling gains are worth the extra expense. Another disadvantage would be that with so many different parts, there is more that can go wrong or break, and again because of the complexity of the system, repairs are usually more expensive. Also, again due to the complexity, modifying the suspension of the car properly is a little more difficult because it is extremely difficult to predict all of the effects changing one variable would have. <P>There are quite a few advantages to the Double Wishbone suspension design. First of all, because of the length of the upper and lower arms, vertical suspension movement results in an increase in negative camber. This means that the tires on the outside of a turn stay in better contact with the road, because the negative camber gain that occurs as the body roll helps make sure that the contact patch of the tire is as large as possible. Also, this allows the car to keep a larger contact patch (the exact length of the upper and lower arms determine how much camber gain there is) during all conditions (except for the tires on the inside of a turn, but since they don't provide as much cornering force as the outside tires, this trade-off still ends up with an overall gain in handling performance). Because the camber changes when there is vertical suspension movement, it is possible to have the proper negative camber during a turn without having that same amount of camber when the car is going in a straight line, whereas, with other systems you would have to dial in a certain amount of negative camber that would always be there even when the car is going straight which would lead to increased tire wear. Also, the rigidity of the system prevents deflections during hard cornering, which keeps the steering and wheel alignment constant, even under a lot of stress. Also, because the length of the arms can be specifically designed for the car, and because those arms can be mounted at various angles to the ground, the engineers can use computers to design the suspension for certain amounts of camber gain, a certain amount dive resistance during braking, and can design the suspension for just about any roll center height and swing-arm length. With so many options open to the engineer, and with the computer programs available today, chassis engineers can tune the suspension to perform as well as possible in all conditions (though for production cars they end up leaning towards safety and ride comfort instead of the best handling possible). <P>Please don't come away from this believing that MacPherson Struts are the worst things on the planet, they aren't, they just aren't as flexible as a Double Wishbone suspension is. Also, don't allow yourself to believe that just because you have a Double Wishbone suspension your car automatically handles better than cars with MacPherson Struts. Plenty of the best handling production cars have used MacPherson Struts. All that having MacPherson Struts means is that you will have to spend a little more time planning your suspension upgrades if you want the best possible handling from your car (and you will end up sacrificing a few more things than you would with a Double Wishbone suspension, like tires - you can dial in the amount of negative camber you want in a MacPherson Strut, but you will end up using tires faster). Because of the simplicity of the MacPherson Strut design, it is sometimes easier to modify the suspension to fit your exact needs than it is with a Double Wishbone suspension. Also, with MacPherson Struts you will need to be more aware of your driving techniques (which to me is actually an asset for someone just starting to race, and even for some experienced drivers, but that all depends on your point of view). <P>For the average driver (even the slightly more performance oriented drivers) will not notice very much of a difference between a MacPherson Strut versus a Double Wishbone, because they probably aren't using more than 60% of either system's capabilities anyway. When modifying, the Double Wishbone suspension's advantages can become it's disadvantages, 'cause the complexity of the system makes it more difficult to predict what making one change will end up doing to your handling characteristics. But, on the other hand, if you know what you are doing, a Double Wishbone suspension can be more perfectly tuned to match the particular driver.<p>[ December 05, 2001: Message edited by: ProtegeSTS ]
