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Double Wishbone vs. MacPherson Strut II: Compared

Posted 12-07-2002 at 12:00 AM by MichaelDelaney

PREFACE

This is a continuation or Part 2. of the Double Wishbone vs. MacPherson Strut Suspensions Comparison Series written by my friend Bampf which first appeared in my Tech Archive Stickies when I was a Moderator on another Honda board. The Basics are posted in the Articles FAQ/Info section:

http://www.team-integra.net/sections...?ArticleID=525

Once again, my gratitude goes to Bampf for his efforts.


ADVANTAGES AND DISADVANTAGES

All right, I guess that I'll go into the advantages and disadvantages of each type now.



MACPHERSON STRUT


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 , 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.


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.

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).

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.


DOUBLE WISHBONE


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).

CONCLUSIONS

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).

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, because 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.


ADDENDUM



Note, in the second part of this I stated that:

Quote:

unsprung weight has a bigger effect on acceleration than weight
inside the car.
This isn't quite correct. It is the rotational mass that increases
acceleration, and in the suspension all of the rotational mass that you would be talking about for acceleration also happens to be unsprung weight.

However, the struts would have no effect on acceleration other
than the weight reduction. What I meant to say there is: reducing the
unsprung weight (like with MacPherson Struts) has a big effect on ride
comfort, and it greatly reduces the work the suspension has to do, which ends up in some parts lasting longer and staying in peak condition longer.

Also, I noticed that my use of the word "flexible" when I said:

Quote:

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.
I think it would be easier to understand what I meant if you replace the word "flexible" with "versatile".
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