Back in 1990, Honda and Acura introduced the NSX, an aluminum sports car that, at the time, was a bit of a disappointment: The styling was a bit bland and the performance, while competitive for its day, didn’t have that visceral feeling of a Ferrari or Porsche even though you could rev the engine all the way to an 8,700 RPM redline. The Japanese sports car didn’t have the scream of a Ferrari and it didn’t shout “look at me” like a Lambo. But there were some areas where the car really shined.
[Editor’s Note: Welcome back to dynamics engineer Huibert Mees’s column, where the former Tesla Model S/Ford GT suspension designer can write whatever the heck he wants. It’s been a little while since you last read his work, but that’s not on him — that’s on me! There’s a bit of an editing backlog, but I’m being honest when I say that editing Huibert’s nerdy engineering pieces is among the most joy I’ve had as a car journalist. You’ll enjoy this one, I know it. -DT].
One example where the NSX stood out was its ride, which is the main subject of this article; I’ll get to that in a second, but for now you’ll have to indulge me as I briefly talk about the shifter. Oh.…. My….. God! That shifter! Anyone who owns or has driven a first generation NSX will know exactly what I’m talking about. It is pure magic and adds significantly to the joy of driving the car.
The NSX’s Shifter Inspired The Ford To Run The Ford GT’s Shift Cables Through The Engine
Look at that thing. It’s gorgeous! The handle is a polished teardrop shape that fits in your hand perfectly, and the snick-snick feeling of changing gears has, in my humble opinion, never been equalled in any other car. When we designed the 2005 Ford GT, our goal was to make our shifter feel as nice as the NSX and I think we got pretty close. It took an inordinate amount of effort to get there, though. Good shift feel is all about keeping the number of bends in the cables to a minimum and keeping the radius of the bends as large as possible. Anything else adds friction and ruins the feel.
In the NSX, with its transverse engine mounting, it was relatively easy to route the shift cables from the shifter to the transmission with a minimum of bends, but in the GT, with that big engine sitting between the shifter and the transmission, it was much more difficult. Going around the engine with the cables would have meant many bends and lots of friction, so the decision was made that if we couldn’t go around the engine, we would go through it. And that’s what we did. The GT’s shift cables go through the valley of the engine underneath the intake manifold. It made for an almost straight shot from the shifter to the transmission and very good shift feel.
How The NSX Suspension’s Incredible ‘Pivot Assembly’ Gave It Such A Good Ride
But that’s not what I came here to talk about today. I want to talk about the NSX front suspension, because there is something unusual about this design that is the key to the excellent ride the car has. There is an extra piece in the front suspension that no other car before ever had and no one has used since. It connects the forward bushings of the upper and lower control arms to each other. In fact, they aren’t really bushings at all, they are ball joints, which is highly unusual in itself.
So why would Honda go through the effort of putting this extra piece in there? Well, it goes back to something called “caster wind-up.” To understand what that means we have to first talk about two of the primary parameters that go into a front suspension design: caster angle and caster trail.
The caster angle is the angle of the line between the upper and lower ball joint centers when viewed from the side of the vehicle. This line is called the Kingpin Axis, and like a hinge on a door, it is the hinge that the front suspension rotates around when you steer the car.
Caster angle is very important for steering stability and returnability. It’s the same as on a bicycle. The front fork of a bicycle is angled forward to give stability in steering. If the fork were perfectly vertical it would be very difficult to control and you could forget riding no-handed. It would be all over the place. The same thing is true in a car. Caster angle helps the car go in a straight line and be stable. It also helps the steering come back to center when you let go of the wheel.
Another important thing caster angle does is provide steering feel. The forces coming back from the tire act through the caster angle and the caster trail (see diagram similar to the one above below) to give the feedback forces we feel in the steering wheel (look at the image above, and you’ll see that the lateral forces between the ground and the tire are a distance from the steering axis; this means that cornering loads on the contact patch will try to create a torque about that axis that the driver will feel in the wheel; this is steering feedback or “feel,” and it’s highly dependent upon caster and caster trail). Of course, too much caster means these forces get too high and we would then need to increase steering boost to compensate which would hurt steering feel. It’s a delicate balancing act that the suspension and steering engineers have to play. We’re talking about small values though. In my experience, between 4 and 6 degrees of caster angle strikes a good balance between all the competing requirements the suspension designer needs to consider.
Caster trail is the distance along the ground between the center of the tire contact patch and the point where the kingpin axis intersects the ground. The reason it is important is that during cornering, the side force and the caster trail create a moment around the kingpin axis that works to return the steering back to straight ahead. This moment also helps create understeer which is important for safety should you lose control of the car in a turn. Understeer makes sure the car is pointing forward when you lose control so that if you hit something after running off the road, you hit it with the front of the car, not the side or rear. Again, we’re not talking about big numbers here. Between 25 and 35 mm is very typical in modern cars.
Both of these parameters are very important for the functioning of a front suspension and it is important that they change as little as possible while driving.
To fully understand how all this fits together, we need to also consider that a car suspension contains rubber bushings to make them quiet and comfortable. When a tire rolls over imperfections in the road, like potholes, expansion joints, broken pavement, etc., it needs to be able to move out of the way slightly or else the impacts would be very jarring and noisy to the passengers. The bushings allow the tire and the suspension to move not just up and down with the spring and damper, but also fore and aft slightly so that the impact and energy of the tire hitting the pothole or expansion joint is absorbed as much as possible. If you’ve ever driven a race car you will know what I mean. Race car suspensions have very little, if any, rubber in their bushings and the results is very crisp handling but also a very jarring and uncomfortable ride. It’s a good thing race tracks tend to be smooth!
Unfortunately, all this rubber causes a problem during braking. When we apply the brakes, we create a braking force which tries to push the suspension rearward in the same way a pothole does. However, braking also generates a moment that tries to rotate the suspension in the same direction the tire is rolling. The rotation of the suspension caused by this moment reduces, or even eliminates, the caster angle and the caster trail. This is called the caster wind-up. The caster angle and caster trail are changed as a result of the “wind-up” in the suspension caused by braking forces. This means that if we brake during a corner, the understeer and steering returning forces created by the caster angle and caster trail are reduced or even eliminated by the caster wind-up.
For the most part, more rubber will make the car’s ride quieter and more comfortable, up to a point. Too much rubber can lead to other problems, like shake, which refers to the vibrations in the car just after the pothole or expansion joint impact. You can feel this very clearly in some older, softly sprung Lincolns or Cadillacs. Hit a pothole in one of those cars and the shaking seems to last forever. Too much rubber can also make the steering feel numb and uncommunicative and make the handling dull and uninteresting.
How Honda’s Design Prevents ‘Wind Up’
Let’s look at how rubber in the bushings causes caster wind-up. When a suspension encounters an impact, like a pothole, the force of the impact tries to push it rearward. (In the image below, the NSX is moving towards the bottom left; the rear of the car is located towards the top right)
In an ordinary double wishbone design, the impact force causes the upper and lower ball joint forces to be in the same rearward direction which tries to rotate both control arms. This rotation is resisted by outward reaction forces at the forward bushings of both arms. The rubber in these bushings deflects as a result of these forces, which allows each arm to rotate. This rotation allows the upper and lower ball joints to move rearward and allows the tire to move rearward, thereby absorbing some of the impact.
During braking, on the other hand, the upper and lower ball joint forces are in opposite directions because there is the addition of a moment created by the brakes trying to slow the car down. The car doesn’t want to slow down because of Newton’s first law of motion which states that an object in motion (or at rest) wants to stay in motion (or at rest). The motion, or momentum, of the car wants to keep the wheels rotating even though the brakes are trying to slow them down. This creates a moment that pushes on the suspension.
Since the upper and lower ball joint forces are now in opposite directions, the reaction forces at the bushing are also in opposite directions and any deflection in these bushings will cause the control arms to rotate in opposite directions. This rotation causes the upper ball joint to move forward and the lower ball joint to move rearward. Remember that the kingpin axis, caster angle, and caster trail are defined by where these ball joints are so if they move then these parameters change as well. If the upper ball joint moves forward and the lower ball joint moves reward then the caster angle will be reduced and the caster trail will also be reduced. The more rubber is in the bushings, the more motion of the ball joints can occur, leading to more change in caster angle and caster trial.
Compare what you see here with the corresponding image above and you can see how the motion of the upper and lower ball joints have reduced the caster angle and caster trail.
So, suspension engineers are left with a dilemma: Do we add lots of rubber to make the car comfortable and live with the caster wind-up, or do we keep the rubber to a minimum, sacrifice ride but keep the handling and steering nice and crisp? The answer, of course, depends on the type of car you have. It might be fine to have dull handling and steering in a Town Car but it wouldn’t be fine in a Corvette. The same goes for ride; a Town Car needs to have a soft and quiet ride but a Corvette can get away with a much stiffer and noisier ride.
But what if there were a way of getting a soft ride without sacrificing handling and steering? Let’s see what Honda did to try to overcome this dilemma. Let’s look at pothole impacts and braking again and see how the NSX front suspension handles these situations.
First, let’s look at the extra vertical link (Honda calls it a “Pivot Assembly”) Honda put into the NSX suspension and see how it’s mounted.
The vertical link, located towards the front of the vehicle (on the left) is mounted on two rubber bushings that form a vertical pivot axis which allows the link to rotate around this axis. It is also connected to the upper and lower control arms by two ball joints.
The rubber bushings allow the link to rotate around the pivot axis when the upper and lower control arms push on it and return it to center when the forces coming from the control arms go away.
Now let’s look at what happens during a pothole impact.
As we saw before, the pothole impact tries to push the suspension rearward which tries to rotate the control arms in the direction shown. This creates an outward force at the front control arm bushings, which in the case of the NSX are ball joints connected to the vertical link. Since both ball joints are pushing outward on the vertical link, it will rotate around its pivot axis and allow both ball joints to deflect outward which allows the control arms to rotate and move the tire rearward. This is exactly what we want for a good ride and impact absorption. Sounds perfect so far.
Now let’s look at what happens during braking.
As in the ordinary wishbone suspension, we have the braking force but we also have the braking moment, which causes the upper and lower ball joint forces to be in opposite directions. We also have the same opposite reaction forces at the control arm forward bushings, but in this case instead of having rubber bushings connected to the body, we have ball joints connected to the vertical link. The ball joints won’t deflect very much on their own and since the upper and lower reaction forces are pushing on the vertical link in opposite directions, it won’t rotate around its pivot axis and therefore the upper and lower control arms are not allowed to rotate like they did in the ordinary design. Since the control arms can’t rotate, the upper and lower ball joints can’t move which stops the change in caster angle and caster trail from happening and ensures the handling, steering feel and returnability are not changed when braking in a corner. Pure. Genius.
So why did Honda choose not to have this feature in the new NSX, and why has no one copied this idea? The NSX proved the concept worked and worked very well, but the extra part looks stupidly expensive and even in $100,000+ cars, component cost is still important. The other problem is that the upper and lower control arm forward bushings must be almost directly above each other which limits what you can do with the overall suspension design. Still, it seems to me this concept could have been adapted by other high end sportscars to reduce the compromise between ride and handling.
I still think this is a brilliant design and it blew me away from the moment I first saw it all those years ago.