1991 Acura NSX: Suspension Walkaround
At long last the 1991 Acura NSX suspension walkaround you've been clamoring for is finally here. Turns out there's lots of cool stuff to see under there, even if this car is old enough to drink and play blackjack.
Let's not waste any more time with preliminaries. The NSX is racked on our 2-post Rotary lift and fully exposed.
The front end of the NSX rides on double wishbones, coil-over shocks and ... what the hell is that?
"That" is a unique front structure called a compliance pivot, and it defines the forward pivots of the upper and lower control arms. On a very basic packaging level this structure simplifies and lightens the design of the all-aluminum unibody -- a production-car first when the NSX came out -- because as it allows these two suspension "hard points" to be positioned apart from the structure.
But that's not why Honda's engineers did this -- not really.
Honda press materials released in 1991 are hard to follow because they were translated from Japanese to English by someone who didn't seem to understand the nuance of the thing.
What they say is this device minimizes front bump steer. Bump steer is the toe change that naturally occurs as the suspension passively strokes from full rebound (droop) to full compression (bump) due to the differing lengths of the control arms, the mounting of the steering rack and the design of the knuckle -- all of the various hard point locations of a given design.
But if producing zero bump steer was all it was for, if that's all it actually did, the compliance pivot wouldn't be so thoroughly hated by a certain subset of NSX owners that take theirs to the track.
Despite the name, however, simple bump steer does not include all of the impact forces and displacements associated with striking an actual bump.
After staring at the thing I think this device has more to do with how the NSX's steering responds to actually hitting a bump when the tires are loaded, something more along the lines of deflection steer and compliance steer. The very name of the thing -- compliance pivot -- seems to bear this out.
Back to our picture: the two control arm forward pivots that attach to it are made up of ball joints, which are great for accurate suspension location but are not known for their ability to provide the "give" needed to absorb and dissipate the fore-aft component of pothole strikes and the like. Ball joints are rigid and unyielding. But the NSX responds quite well to such disturbances. It does not rattle one's teeth. This, I think, is what the compliance pivot is all about.
Say your front wheel hits something. The spring and shock take care of the vertical component, but that still leaves the fore-aft component.
In the NSX, that portion travels form through the wheel and tire to the outer ball joint (green) and knocks it backwards. The lower control arm (LCA) rotates around its main inner pivot (blue), which is fairly rigid in order to fix things steadily in position in the lateral direction when cornering.
As the arm pivots around this point, the forward pivot (yellow) of the LCA hammers outward in a clockwise direction around the blue point. The compliance pivot, to which the LCA's ball joint is attached, then rotates in a counter-clockwise direction around the pivot axis (orange) to soften this blow.
All street car suspensions make allowances for this in the form of some sort of bushing or other, and the compliance pivot does contain a rubber bushing element inside, but the NSX compliance pivot is unique in that everything I just described is, by design, happening to an equal extent at the upper control arm at the same time.
Because of this the caster angle and toe angle don't change, which should theoretically keep the steering unperturbed by such rough road pulses. The wheelbase, however, does contract for an instant.
I think the racing community hates the compliance pivot because, well, all racers and track day participants tend to gravitate toward harder bushings (or no bushings at all) because very wide and sticky competition rubber and continuous limit driving begins to induce such deflections in the normal course of events. Compliance is intended to provide instantaneous relief from bumps and impacts; things start to go all swimmy if such movements persist, say, for the length of a braking zone or through a corner.
Since the forward pivots of the NSX control arms are already ball joints, the aftermarket community has come up with a "non-compliance" device for the NSX, a simple clamp that bolts over the thing and prevents it from pivoting at all. Street ride no doubt suffers just as it would when any sort of conventional hard bushings are substituted for factory pieces.
Once more without arrows so we can see that beautiful lightweight lower control arm. Incidentally, the black cap on the forward pivot is there to deflect rock damage. Yes, it's that low.
Cool-looking lightweight goodies abound. The delicate web-like control arms and the compliance pivot components are made of forged aluminum. The front knuckle is made from cast aluminum.
The 1991 Acura NSX steers the front tires from behind the axle centerline, perhaps in an effort to centralize the mass and leave more space for the spare tire under the ultra-low hood.
Peer into the shadows and you'll see the steering rack is quite slender because this early NSX has un-assisted manual steering. This is a variable-ratio steering rack, ranging from 18.2-to-1 to 20.8-to-1. This is done by crowding the rack gear teeth slightly closer together in the middle for less twitchiness around center at high speed in a straight line. Just off center the teeth start to spread out, which quickens the ratio when cornering.
But this only applies to the 5-speed manual, like ours. If you've got a 1991 NSX with the 4-speed automatic gearbox you'll have electric power steering and a fixed 18.6-to-1 steering gear ratio.
A fairly standard-looking front stabilizer bar connects to the lower control arm just inboard of the shock absorber. For the record it measures 0.7 inches in diameter.
This small deflector bounces cooling air towards the brakes.
Fifteen-inch front wheels (and late 80's thinking) don't exactly result in exotic-looking Brembo-esque front brakes, but these sliding calipers are nevertheless finned and fairly beefy. Twin pistons allow the clamping force to be spread evenly across a longer, skinnier brake pad that maximizes the available space. Numerologists will love the ventilated rotors because they measure 11.1 inches in diameter and are 1.1 inches thick.
Another coil-over shock and pair of wishbones suspends the aft end of the 1991 Acura NSX.
Forged spider webs of aluminum make up the upper (white) and lower (yellow) a-shaped control arms. The toe-link (green) is built with a turnbuckle in the middle, a far easier and less fussy way of adjusting toe-in -- not to mention stable -- than and eccentric cam. Sweeping in from the rear is a 0.6-inch rear stabilizer bar (black) made of spring steel, of course.
Aluminum sandwich: rear subframe (yellow) bolts to rear subframe-subframe (green) which bolts to aluminum chassis (orange.)
Meanwhile, the stabilizer bar's end link and the lower end of the coil-over shock connect to the cast aluminum knuckle using a shared bolted joint (white,) which means the influence of all of these components happens at a 1-to-1 motion ratio.
Another aluminum brace ties the lower reaches of the rear suspension subframe together for a little extra stiffness.
Neat and tidy -- even if it is dusted with 21 years of road grime.
Single-piston sliding rear calipers grab onto 11.1-inch ventilated rear rotors that are 0.83 inches thick. A cable-operated mechanism pushes on the same piston when the e-brake is pulled.
These center caps are perhaps the biggest example of committee-think on the Acura NSX project.
"I think the forged wheels should have exposed lug nuts for a clean look and easy access," says the first.
"Well I think the NSX should have a smooth center cap that hides the lug nuts altogether and makes the center of the wheel look really big and visually heavy -- I mean, strong," replies the second.
"OK, let's compromise: We'll make a center cap, but we'll put holes in it so you can still see the lug nuts."
"That's good, but we need more. Let's say we make the holes small enough that the center cap will still need to be removed to fit a socket onto the nuts."
Both together: "With a little indent in the cap so the tip of a screwdriver can gouge the wheel and dent the cap as you pry them apart? That's it! We're a genius!!"
At first it seems that 80's- and 90's-think is alive and well when it comes to the small tires and wheels, and you'd be partially correct. But so is the engineering imperative to achieve light weight and use no more than necessary to get the job done. This is, after all, a car that weighs just 3,010 pounds.
And so our NSX's 15-by-6.5-inch forged aluminum front wheels wear 205/50R15 rubber, and the mounted assemblies weigh just 36.4 pounds apiece. The weight distribution of the NSX puts 58-percent of the mass on the rear axle, where we find 16-by-8.0-inch wheels and 225/50R16 tires that weigh 41.4 pounds when married together.
These tires are directional, so they'll be no tire rotations. This, in part, explains why early NSX owners complained of terrible tire life on the order of 10,000 or 12,000 miles, in some cases.
Dan Edmunds, Director of Vehicle Testing.