Sport Compact Car - October '98
Project Sentra SE-R
By Dave Coleman
[Put into HTML format by Mike Mager]
Part two: We shave 21 feet off our braking distance with an NX2000 brake upgrade.
It has been quite some time now since our last update on Project Sentra SE-R. Project 200SX SE-R has been going well, and we have been holding back on our "classic" SE-R to avoid being redundant with the Sentra's younger brother. But now, as the 200SX is busy breaking in its new engine, it's time to take a look at the classic SE-R's progress.
A used car with over 85,000 miles on it, the SE-R has naturally had a few maintenance issues to deal with. Since the engine really should be in perfect shape before we start making modifications, we have been concentrating our underhood efforts on getting the car into perfect tune. While we were working through the engine maintenance issues (which we will describe in more detail in the next installment), we put our modification efforts into the braking system.
The Sentra SE-R came with a fairly substantial set of brakes from the factory. Considering its 2600-pound weight, the 9.84-inch vented front and 9.21-inch solid rear discs provide what feels like healthy stopping power. Pedal feel is excellent; there is virtually no slop in the pedal, and it's firm and easy to modulate. Imagine our surprise, then, when our 60 to 0 braking tests revealed a stopping distance of 169 feet.
That stopping distance isn't quite as bad as it seems initially. Stopping distances are usually measured by applying full braking at some higher speed (usually 65 to 70 mph), and measuring only the portion from 60 mph to zero. Our stopping distances, on the other hand, were measured by applying the brakes at as close to 60 mph as possible. The advantage of stopping from a higher speed is that by the time the car is down to 60 mph, the transitional braking period, when the brakes are not fully applied and the suspension is still settling in to its nose-down braking stance, has already been completed. Our tests were performed with a Vericom VC2000, which can do the test either way, but the Vericom's interface is better suited to applying the brakes at 60 mph. Braking from a higher speed would have required saving the runs and downloading them to a PC for a bit of number crunching. As a result, our braking distances were a little longer. Even taking the test methods into account, the stopping distance seemed to be a little on the long side.
The most obvious problem with the stock braking system was that the rear brakes locked far before the fronts. Since this car doesn't have ABS, this necessitated backing off on the brakes until the rear brakes unlocked. Properly balanced, the front brakes should do the majority of the work, but with the rear brakes locking early, a large portion of the front brakes' potential is wasted.
The B13 Sentra (B13 was the chassis number for the '91 to '94 model) had a sport coupe brother, the NX2000. While the NX2000 was virtually identical mechanically to the Sentra SE-R, it had larger front brakes. Those larger brakes will bolt directly onto the SE-R with a minimal amount of work.
Just saying that brakes are "bigger" or "better" doesn't mean anything, brake performance is determined by a variety of factors. Lets take a look at what factors can affect breake performance, and how the NX2000 brakes compare.
Rotor size is the most commonly bragged about since it is the most easily seen and quantified. Rotor diameter determines how much leverage the calipers have. Brake torque (or any torque for that matter) is determined by the force applied by the calipers times the distance from the center of the rotor at which it is applied. If two calipers can deliver the same friction force, the calipers dragging on larger diameter rotors will deliver more stopping torque. A larger rotor will also have more mass to absorb heat from a sudden stop, and more surface area to dissipate that heat into the air.
The NX2000 rotors are slightly larger in diameter (10.12 inches vs. 9.84 inches), are significantly thicker (1.04 inches vs. 0.67 inches) than the SE-R's. The result is a significant increase in heat-absorbing mass (13 pounds vs. 9 pounds) and increased surface area on the internal vents.
The calipers themselves, of course, are the other side of the equation. Given the same caliper layout (a single-piston sliding caliper in this case), a caliper with a larger piston will apply more clamping force on the rotor for a given brake fluid pressure. The reason is simple. Pressure is measured in psi, or pounds per square inch. The force exerted by the piston is the line pressure times the area of the caliper piston. If the line pressure is 500 psi, a piston with a surface area of one square inch will exert 500 pounds of force, while one with a surface area of two square inches will exert 1000 pounds of force. The down side to this is that it takes more brake fluid to fill the area behind the larger piston, so the brake pedal will have to move more to pump that fluid.
The NX2000 calipers have a larger piston (1.94 inches in diameter, vs. 1.81 inches, for about 15 percent more surface area.) for increased clamping force and reduced pedal effort compared to the SE-R calipers.
We used a set of Autospecialty remanufactured calipers for this swap. One of our main reasons for using the Autospecialty calipers was the fact that they plate their calipers with yellow-colored zinc plating instead of leaving them bare cast iron like many remanufacturers do. While leaving the cast iron caliper body bare doesn't hurt performance, the calipers look awful when they rust--something they start to do almost immediately. When we picked up the calipers, we were surprised to see that they were not plated--or even remanufactured. It seems that Autospecialty just started remanufacturing NX2000 calipers, and since you can't remanufacture calipers if you don't have any cores, they only had brand-new calipers in stock (though they were being sold and priced as remanufactured). Lucky us.
Cross drilling, slotting, etc.
Cross drilling or slotting rotors is a common technique for racing or high-performance street rotors, and there is a fair amount of debate over which is better. Done properly, cross drilling increases the surface area of the rotor, improving its ability to cool. Cross drilling also reduces the mass of the rotor, reducing the amount of heat the rotor can absorb. How effective cross drilling is depends on which is more important, the ability to quickly cool between stops, or the ability to absorb heat from a single stop.
Though cross drilling increases the overall surface area, it reduces swept area, or the surface area that the pads actually contact. Depending on the types of pads, this can sometimes be offset by the extra "grab" the pads get as they encounter the edge of a hole. Cross drilling also lets gasses escape from between the pad and rotor.
Cross drilling also lets gasses escape from between the pad and rotor. Under certain conditions of heavy braking, some of the resins in the pad can vaporize and get between the pad and rotor, making the pad float on a cushion of air, and reducing braking force to virtually nothing. This is called green fade, since it usually happens on new or "green" pads. Unfortunately, since pads used on the street can stay green for months if the car is not driven hard, green fade can strike the first time the brakes are used severely.
Cross drilling on a vented rotor allows these gasses to escape into the vent. Cross drilling on a non-vented rotor allows the gasses to escape into the hole where it is trapped until the hole is carried past the edge of the pad. Though green fade can still happen with cross-drilled rotors, it is far less severe. Slotting will also help reduce green fade, though how effectively it does so depends on how many slots there are and how they are designed.
Besides the reduction in mass, the other drawback to cross-drilled rotors is increased susceptibility to cracking. Any inconsistency in the surface of a highly stressed object will increase both the stress in that area and the possibility of a fracture developing there. A hole certainly counts as an inconsistency in the surface, as does a slot. In general, a hole causes a larger stress riser than a slot, but both must be designed and machined with care. Chamfering, or better yet radiusing the edges of a hole can greatly reduce the possibility of cracking, and for that reason, every reputable manufacturer of cross drilled rotors chamfers or radiuses their holes.
Take a look at rotors on a race car and you can find holes, slots, neither, or even both. The quantity and placement of the holes or slots also varies. The difference can be based on class rules, track conditions, or even driving style, but the variety just proves that neither solution is always better.
Since our brake upgrade would significantly increase rotor mass, we felt cross drilling would be the better solution in this case. The mass reduction from cross drilling would be more than offset by the larger overall size of the rotor (the 13-pound rotor weight mentioned easier was actually for a cross drilled rotor). We used Power Stop cross drilled rotors on both the front and the rear. Since Power Stop is actually a division of Autospecialty, one of the world's largest brake parts distributors, their experience with the failure modes of stock brake parts under extreme use should give them the experience to design cross-drilled rotors that are safe from the common types of abuse.
Brake pads are critical, of course, since they actually apply the braking force. The coefficient of friction of the pads determines how much force is applied for a given pressure. Besides the simple, stickier-is-better factor, brake pads have to be able to behave consistently over a large range of temperatures, and last a reasonably long time. If they're for a street car, they also have to be able to work well when wet, work quietly, and produce as little dust as possible.
High performance brake pads come in a variety of compounds with a variety of properties, and selection is very often a trial and error affair. One important point with brake pads, though, is that racing pads are very often unsuitable for the street. Most racing pads are designed to work under conditions of extreme heat, and often don't work well at all when cold or wet. On a street car, your first stop with cold pads could be your most important one, so pads that work well when cold are essential.
Since pad choice is such a trial and error affair, we decided to try a few. Our initial tests were run with Axxis Metal Master semi-metallic pads. After we get some miles on these pads and have more experience with them, we have a set of Axxis Deluxe Plus organic pads to try. The Deluxe Plus Pads will have to wait until our next installment, however.
With the basics of braking out of the way, lets take a look our particular case: The NX2000 brakes simply bolt on to the SE-R, but getting better braking is not as easy as simply bolting on the big brakes. There are many problems that can be encountered when doing this swap. Lets take a look at what you should and should not do when making this upgrade.
Rotor backing plate
The NX2000 rotor is actually close enough in size to the SE-R rotor to fit within the stock backing plate. The larger caliper, however, will not fit without interfering with the plate, so it must be removed. Removing the backing plate exposes the rotor to dirt, water, and all the debris it was designed to block, but it also improves cooling by allowing more airflow to the back of the rotor. Realistically, the benefits of removing the backing plate outweigh the drawbacks.
The backing plate, of course, is not designed to be removable. The plate is pressed onto the hub carrier, and must be pressed (or hammered) off. After it is free from the carrier, it can't be slipped off without removing the hub. If you don't want to bother removing the hub, you can simply cut the backing plate at its narrowest point and bend it to get it off.
Mismatched Rotor and Caliper
It is, of course, impossible to put the smaller SE-R caliper over the larger NX2000 rotor, but it is possible to go the other way. The larger NX2000 calipers will fit over the SE-R rotors and may not be obviously mismatched at first. With brand new pads, the NX2000 caliper, which is designed for a 1.04-inch thick rotor, will barely be able to clamp onto the 0.67-inch rotor. Since the pistons will be almost fully extended, there will be a significant pool of fluid behind them, and the slight compressibility of that fluid will contribute to a mushy pedal. A mushy pedal is relatively benign, though, compared to the consequences of continued use. As the pads wear, the piston moves farther and farther out of the cylinder and since it is already out 0.37 inches too far, it will soon push out of the cylinder altogether. When this happens the fluid will gush out and the brakes will fail instantly and with no warning. This is generally considered a very bad thing.
The solution, of course, is not to use the NX2000 calipers with the SE-R rotors. Supposedly, some SE-Rs--namely those built in the Zama assembly plant in Japan as opposed to those made in the plant in Smyrna, Tennessee--came with these larger brakes from the factory, so some auto parts stores can get them mixed up. To be safe, check the only easily measurable dimension--the thickness of the rotor--before installation.
The larger brakes almost fit inside the stock SE-R wheels, but almost isn't good enough. You will, at the very least, have to upgrade to NX2000 wheels to use the NX2000 brakes. We had no problem with clearance with our 16 x 7-inch Black Racing Phantoms.
While the larger piston of the NX2000 caliper increases clamping force on the rotor, it takes more fluid to fill the cylinder. This means the commendable pedal feel of the stock SE-R brakes can be lost. When we first installed the brakes, the pedal was extremely soft, almost as if the system still had air in it, but further bleeding proved that this was not the case. While the car could stop reasonably well, the master cylinder would reach the end of its travel before the brakes would lock. Clearly this was not an improvement.
We had intended to use Goodridge braided steel brake lines with the upgrade, but since they were delayed, we had installed the new brakes with the stock lines. When the Goodridge lines arrived, we installed them and, surprisingly, they solved the mushy pedal problem! Stock brake lines can swell slightly, requiring more fluid to fill, and making the pedal mushy. With a healthy brake system, braided lines will make the pedal much firmer and easier to modulate. In our case, the improvement was so large that they made the difference between the brakes working and not working. The reduction in pedal mush was enough to prevent the master cylinder from bottoming out.
There are two kinds of mush in a brake pedal, we'll call them slop and squish for lack of better terms. Slop is the pedal movement before the brakes actually start working. This can be caused by warped rotors pushing the pads away from the rotors, mis-adjusted drum brakes (if you have drum brakes) or a mis-adjusted brake pedal. Squish is a pedal that moves a lot as you increase braking force. A squishy pedal can be caused by air in the brake lines (air compresses very easily), flexible brake lines, contaminated fluid (where either the contaminants are compressible, or their boiling point is lower, causing bubbles to be introduced into the system as the brakes get hot), or a poor relationship between master cylinder bore and caliper bore.
In our case, the Goodridge lines largely cured the squishy pedal problem, but there was still a fair amount of slop before the brakes actually started to work. Pedal adjustment is the most frequently neglected part of the braking system, but it is the easiest place to cure pedal slop. With the stock brakes, the pedal had already been adjusted to minimize slop, but with the new bore relationship between the master cylinder and the calipers, the pedal had to be re-adjusted. The brake pedal presses on the master cylinder with a simple rod. Adjustment is simple: Stand on your head under the dash, loosen the lock nut and turn the rod with a pair of pliers. It is quite easy to tighten the rod too far and make the brakes drag, so be careful. You should test the brakes away from traffic, small children and pets. Once you think you have the adjustment correct, take your tools with you and try a few repeated stops. If the adjustment is just a hair too tight, the heat of repeated use will cause the brakes to start dragging.
After driving with the new pads for about a week to bed them in, we tested the 60 to 0 stopping distance again. It took several passes to learn how to modulate the new brakes. While the stock brakes were severely rear-biased, the new brakes are, not surprisingly, front biased. After a few stops we encountered some green fade, but surprisingly, even as the brakes faded and more pedal travel was required to stop the car, the stopping distances continued to drop from run to run. Finally, after about ten runs, our times stabilized. Our final three-run average was 153 feet, a full 17 feet shorter than with the stock brakes!
In normal driving, the pedal is firm and immediate, and takes much less effort than the old brakes. The metal master pads are quiet and unobtrusive, something that you can't take for granted with high-performance pads. With our gunmetal gray wheels it's a little hard to tell, but brake dust seems to be minimal as well.
Not being part of the braking system, the importance of tires is often forgotten. Sticky tires can drastically reduce stopping distances though, so our braking tests would be incomplete without trying some different tires. Our baseline tests were done with Nitto NT505 tires in a 205/45-16 size. The shorter distances recorded with the new brakes were also on those same tires. In our next test, we switched to a set of Nitto NT555s in the same 205/45-16 size. The NT555s immediately surprised us with their seemingly contradictory merits.
The NT555's claim to fame is its contact patch. By utilizing a tread pattern based on the NT555R drag radial, they are able to make a contact patch with a large amount of rubber-to-road contact. While the concentration on contact patch would suggest an ultra-gummy pseudo race tire, the NT555 has an extremely high treadwear rating of 300. (Treadwear ratings are based on a scale where 100 is a standardized test tire. A rating of 300 indicates that the NT555 lasts three times as long as that standard test tire.) The NT505 had a treadwear rating of 180, which is far more common in performance tires.
Contradictory ratings aside, on the road the NT555 performs surprisingly well. They grip quite well, are progressive in breakaway, and are quieter at extreme slip angles than the NT505s were. Most surprising, though, is the sound and ride quality they afford. The tires seem downright supple, absorbing tar strips, botts dots and other pavement irregularities like an 80-series whitewall. This forgiving nature could take a lot of credit for their improved behavior. A tire that is too stiff will skip across the surface of anything but a glassy smooth road. A forgiving tire can maintain contact, providing stronger, more consistent grip.
Our braking tests bore out the NT555s superior grip. After another test session our three-run average 60 to 0 stopping distance was down to 148 feet, 4-feet shorter as a result of the tires alone. It's also important to note that in this second test session there was no hint of the fade we noticed in the beginning portion of that first session. It seems that after the first four or five stops of our first test session, the pads were finally bedded in properly.
As promised, our next installment will cover some of the common old-age problems that SE-Rs seem to develop, as well as some engine performance at long last. We have some interesting new treats lined up from JUN USA; it's a rare chance to test newly available parts for a car that isn't in production any more. The increased grip of our new tires also means we'll have to do a little more suspension work to take full advantage of their added grip. Stay tuned.