Corvette braking test
Taking stop-testing to the ragged edge
Jim Moore - October 20, 2011 10:00 AM
There’s much more to inspecting bearings than just washing them off! 1. Inspect inner race for signs of spinning on the spindle. 2. Inspect ends of rollers that ride against the lip of the inner race. 3. Inspect roller surface that runs against “outer race”. 4. Inspect lip of inner race that roller pushes against. 5. Inspect surface of inner race that roller runs against. And of course, the outer race must be inspected in the roller surface area and for any signs of spinning in the hub.
Among the many amazing lubricants that Royal Purple manufactures is their Ultra Performance Grease. This stuff solved the bearing issues on a 200 mph bullet train ... it will work fine for us! A simple bearing packer makes the task a lot easier.
Here’s what you get when you open all the boxes.
The billet aluminum calipers are available clear anodized or with black or red powder coating. We knocked off 25 pounds with the new rotors and calipers.
You can see the three different sized pistons in the D8-6 front calipers.
Wilwood’s max run-out tolerance of .005 to .008-inch is larger than the .002 to .003-inch GM spec’d because of the more forgiving nature of their seals, but it’s best to get it as close to zero as you can. Sometimes you can correct it by rotating the rotor to a different position on the hub, but usually the issue is the hub, not the rotor. Thin brass shimstock (.005-inch) under one stud can help or you can do like GM did at the factory and true up the entire assembly. Mine all came in fine, but you’ll want to check to be sure.
Another Corvette racing advantage is that the pads are easily serviced without removing the calipers by just pulling the retainer pin and swapping them out. Wilwood uses a positive C-clip versus the OEM cotter pin.
Don’t forget to make sure the adjuster hole for the parking brake is aligned with the hole in the axle so you can adjust it.
You’ll want to check everything for clearance before driving. In my case, the non-stock extended trailing arm snubbers I use for drag racing were very close to the new Wilwood stainless brake line. Easy to fix by moving the snubber over to a different hole, but disastrous if I hadn’t checked!
If your rotors have never been replaced, you’re going to find them riveted to the front hubs and rear axles. The factory did it this way so they could machine the entire assembly for trueness. You’ll need to remove the rivets, but the good news is they are soft and can be drilled easily by hand. I also needed to drill the rear rotors to fit my ½-inch wheel studs. No problem! You don’t need to rivet the new rotors; the wheels will hold them in place just like nearly every other car on the road today.
You’ll be reassured every time you approach the car and see those calipers and rotors peeking through the spokes!
Now THIS is testing! This was on the superheated gooey asphalt — but the Wilwood’s STILL beat the stockers! A good stop usually slammed the headers into the pavement!
If you notice the paint stripes behind the tires you’ll see they are “wet”. You could smear the asphalt with your toes!
“Everything fit as intended right out of the box and there were no parts store runs looking for little odds and ends.”
I’ll admit it, if there was a 12-Step Program for Acceleration Addiction I would have never made it past…“Hi, my name is Jim and I’m addicted to speed”!
I’ve pretty much spent my years driving and racing stuff as if my hair was on fire. The solution to every problem has always been to just add more power.
For example: Rear end breakage? Solution: You don’t have enough power to spin the tires! More power! See how it works? Except that I’ve created some combinations that may not have been as balanced as they probably should have been. A stock ’57 Chevy and a ’72 Nova stuffed with insane big blocks and tiny stock drum brakes come to mind. Hauling these things down from way over triple digit speeds was a “two feet on the pedal” exercise in futility once brake fade entered the picture.
Then I discovered Corvettes and man … what an incredible package right out of the box. From ’63-’82 the same basic chassis arrangement provided style and soul as well as all the things you try to add to other cars like a huge engine setback, a suspension design that works, real gauges from the factory, easy engine swapping, and on and on it goes. But one of the best features was the four-wheel disc brakes they added in 1965. Cutting edge at the time, these anchors added a huge margin of safety so that as I chased speed, I didn’t have to worry about slowing down afterwards.
Corvettes through these years utilized a relatively expensive braking system comprised of 11¾-inch brake rotors along with four-piston fixed calipers (as opposed to floating) at all four corners. With two pistons on each side of the rotor applying even pressure to both pads, instead of one big piston pushing on the inner pad and essentially “pulling” the outer pad against the rotor (floating design, read cheaper) the advantages in performance and wear are obvious. The use of the large rotors with the four smaller pistons also improved “brake radius” which moves the “moment of inertia” further out from the rotating center point to provide stronger braking forces.
Now this may not sound like a big deal, but doing the math exposes the advantages. Disc brakes need a master cylinder and caliper combination that creates approximately 900 to 1,200 psi of pressure at the caliper to work effectively. With a given master cylinder, it’s going to take a caliper piston area of “X” to generate that amount of pressure. By using four smaller pistons with the same total area as one large one, we can generate the same braking force, but now something magical happens. If the centerline of the smaller pistons is placed at a longer distance (however slight) from the center of the rotor by using a larger rotor and/or a redesigned caliper, the amount of torque applied to the spinning rotor is increased.
It’s just like using a longer wrench to apply more torque to a bolt with the same amount of input. For example, if we manage to move the piston centerline one inch further out on a rotor this size, we will see an increase of 20 percent in stopping torque with the same 1,200 pounds of force applied. This concept also allowed GM to use disc brakes without power assist on many ’Vettes, since the huge rotors (for the time) were able to stop with less pressure than a smaller rotor would require. All of this is why we see giant diameter rotors on real high performance factory cars. As engineers like to say … “It’s not really magic … it’s engineering!”
Now we understand that GM went overkill on the Corvette brake system from the start, which explains why I’ve had no real mishaps while stopping my 3,750 pound (race weight) Corvette from 140-plus mph passes at the dragstrip or in normal street driving. It’s just a good basic design. But as good as it is, there are still a few quirks. This is where Wilwood Engineering has stepped in and created their bolt-on brake kits for ’65-’82 Corvettes. Knowing that the original setup worked pretty well, they went about improving things by using the engineering knowledge and racing experience they’ve garnered over the past 34 years to increase braking efficiency, while still fitting the package within our stock 15-inch wheels. Their efforts produced the D8-6 and D8-4 calipers.
Building a Better Brake
First, they designed the new calipers using their in-house FEA design and 3D modeling technology and then manufactured them from forged billet aluminum bodies to save weight and dissipate heat more efficiently. Stainless steel pistons equipped with Wilwood’s proven square ring Hi-Performance seals eliminate corrosion and leaks. Anyone owning an older Corvette has dealt with this problem. The original cast iron housings and lip seals work well as long as corrosion from moisture doesn’t set in, but lip seals will begin to seep if left in storage indefinitely. And if fluid can get out, air and moisture can get in.
With the weight and service concerns addressed, the next step was to increase braking power. This is where the D8-6 caliper fits in. The “D8” signifies the type of pad design and the “6” denotes that there are six pistons in each front caliper as opposed to the four that are in the D8-4 and OEM calipers. Both four- and six-piston calipers are available for the front, and the rears remain as four-piston versions. The reason for adding more piston area to the front is to provide that increase in braking power, since the majority of braking is naturally handled by the front brakes. When you slam the pedal, weight transfer applies more downforce to the front tires and with enough traction, we can apply more braking torque without locking the wheels and thereby, stop quicker.
You may have noticed the three different sized pistons in the D8-6 calipers, which is a common design trait among Wilwood racing calipers. They are 1.88-inch, 1.38-inch and 1.25-inch, respectively. The reason is to correct a phenomenon that occurs under hard braking when the leading edge of the pad creates incandescent material and debris that becomes trapped between the pad and the rotor. This debris tends to lift the trailing edge of the pad away from the rotor. By putting a larger piston on the trailing edge of the pad, more pressure is applied in that area to compensate for the debris and keep it flat against the rotor and reduce uneven pad wear.
The next part of the equation is the available Wilwood ProMatrix slotted and drilled rotors that were primarily designed to provide unsurpassed bolt-on high temperature durability. Interestingly, Wilwood says that the newer brake pad formulas of today have made the need for slotting and drilling rotors unnecessary in most cases. In the old asbestos pad days, there were some gassing issues during racing that were helped with these mods, and today it doesn’t hurt for the street as it will help sweep debris from the rotor, but it’s really more for aesthetic reasons than anything. Wilwood uses a black E-Coating for corrosion protection on the rotors that wears away in the area where the pad applies, leaving the dark coating in place in the slots and drillings to really make them stand out.
Beyond the Brakes
We need to talk about tires, which are arguably the most important part of your braking system. It doesn’t matter how aggressive the pads are or how big the rotor is when trying to increase braking performance if the tires can’t handle that force without locking up. Keep that in mind as you think through your own system, and remember that moment of inertia deal works the other way too. The larger diameter wheels and rotors are harder to accelerate. Use only what is needed and bigger is not always better.
So, this brings us to our testing and installation today. As mentioned, my ’Vette gets used pretty hard at the track, and if you recall the cross country road trip we chronicled in the August 2011 issue of Auto Enthusiast, you know that it gets a lot of street use too. My brakes consisted of one of those stainless steel sleeved kits that I installed many years ago. While it’s worked well and I’ve had many years of practice herding this thing down from speed, there’s been a few times where the pucker factor increased ten-fold on some short little dragstrips. Stopping a lightweight drag car is one thing, stopping a heavy passenger car is another. Taking weight out of the car is always a good thing, and initially I was drooling over the non-vented lightweight rotors to really shave the rotating pounds.
But a clearer head prevailed as I acknowledged that this is foremost a street car and it has to have dead reliable brakes under all conditions. Drag racing brakes get used hard with a nice long cool down period, which isn’t always the case on the street, so the heavier vented rotors went back on the list. New rotors are not required with the Wilwood calipers, but you should verify the condition of your current ones.
Where the Rubber Meets the Road
Before we did anything, I did some testing to see just what I was really starting with, since I had never actually measured their performance. Now this isn’t going to be dead serious laboratory 60-0 testing where a “gun” shoots a pavement marker when the brakes are applied to get perfect accuracy. Truthfully, 60-0 testing doesn’t work brakes tremendously hard anyway, but it’s the most practical test to accomplish in the real world.
In my case we searched out a nice back road to place some pylons and I found that with a little practice I was able to hit the brake application mark very well. The only problem with this method in the friendly Texas countryside is that everyone wants to stop and see what’s going on and to offer help! Slows things down a little! I should also mention that my car is definitely not set up for maximum performance braking tests. My street tires are plain ol’ BFG’s on the rear and tire store specials on the front – not sticky at all. I also have a big-and-little tire combo for that nice cool rake and combine that with drag race springs and shocks and it is what it is. At the end of the initial testing I have to admit I was quite surprised at how well the originals actually performed.
•151 ft., 2 in. - Nice quick stop to get an idea of how hard I could push it on that surface.
•123 ft., 7 in. - Worked it much harder. No brake lockup or drama.
•118 ft., 3 in. - Back to back. Now I knew I could work it harder.
•109 ft., 5 in. - Hit it hard and kept it right on the edge of lockup. Never could repeat it.
•135 ft., 1 in. - Real hard and locked up front wheels early and rears near the end.
•111 ft., 4 in. - Very similar to test four.
•113 ft., 2 in. - Another good one!
Digging through some old road tests of the day, I found that a good mid-year ’Vette could stop in the 135-foot range, so we were in the game, considering that even my hard radials are surely better than what they had to deal with back then. I’ve also noted that Z06 ’Vettes of a few years ago typically seemed to hit the 105 to 110-foot mark with a lot less weight and bigger brakes/tires. The latest carbon fiber brakes on the baddest of the bad new ’Vettes will hit 95 feet, so you can see the type of equipment necessary to beat what we have already.
The Nuts and Bolts of it
The procedures for the brake pad/caliper/rotor replacement are really straightforward (follow your shop manual) and you can easily do this at home. Since my brakes and wheel bearings had not been apart for many years, I decided it was a good time to tear down the rear trailing arms to inspect and re-pack the bearings as well as the fronts. If there is any doubt at all, now is the time to do it. Here’s the short version.
Once the car is in the air and safely supported with jackstands, remove the wheels. You can do this one end at a time or all at once. Using a line wrench, disconnect the hard metal brake lines where the rubber hoses connect since you will be installing the new braided stainless steel lines included in the kit. On the front there is a small clip that attaches the rubber line to the frame bracket that you can pry off with a screwdriver while leaving the hose attached to the caliper. Next, just remove the two caliper attaching bolts that hold it to the caliper bracket and slip it right off with the brake pads intact. It’s the same idea on the rear except the metal lines will remain with the calipers as you remove them.
Now is when you remove the front hubs. Take the time to clean all the bearings, adjuster nuts and washers completely and then dry them with air. Make sure to keep them in order so they remain matched with their original position and bearing race.
In my case, the rear trailing arms include aftermarket “slip fit” stub axles from Tom’s Differentials. That means all I have to do is remove the half-shaft and the flange retaining nut and it all slips apart easily for service. If your car still has the original press-fit arrangement and you decide to service the bearings, you’re going to need some special skills, tools and some extra time. It can be done at home if you do your homework. None of this is required to do the Wilwood brake install, so you can plan accordingly.
After inspecting and re-packing the bearings, reassemble everything in reverse order. Pay close attention to the markings on the new rotors and calipers because they are position specific. After making sure the bracket threads are clean, you can now slip the new calipers in place with the pads already installed in them. Simply drop them over the rotors and torque them to the caliper brackets using the new supplied bolts on the rear and the originals on the front. Wilwood recommends a little red Loctite 271 on the threads for insurance. Next you will be installing the new stainless brake lines, which feed to the center of the caliper bodies through 90° steel fittings (use some Teflon tape on the threads). Route the lines to clear all components while allowing for component movement. Turn the front wheels lock to lock to check for any contact and reposition as necessary. At this point, it’s time for bleeding and the best advice I can give you is to go slow and that “gravity bleeding” works great on ’Vettes.
Once you have it all together, give everything one last look over before installing the wheels. There should be no interference problems with most 15-inch wheels, but inspect closely to be sure. There’s nothing more important than your brakes, so if there is anything that doesn’t seem right, stop and find out why.
You will want to drive slowly for the first few miles to check operation and then look things over one last time before starting the recommended Wilwood bedding procedure for the rotors and pads. The idea is to make progressively faster low-speed hard stops from 5 through 20 mph to check the operation. Next you really start to work them to create some initial heat and then let them cool while driving at moderate speeds. After that, you’re ready to put them to the test!
Braking it all Down
I did exactly that and headed back out to my testing venue. But there was one big change that I hadn’t anticipated, nor could I control. The initial testing was on a relatively cool and clean surface with 85° ambient temperatures, but this time was right in the middle of a Texas heat wave with 40 straight days well over 100°. My asphalt test area had essentially turned into a slick gooey mess! But in the interest of trying to stay consistent we set things up and started making passes.
The first thing I noticed was that the brakes had a great feel, but it took a little practice to get accustomed to the increased front braking power. Whereas before I had to really stand on it hard to get them to lockup, this time there was a finer line between maximum stopping without sliding and whizzing right on by the cones! At first I thought the new system was just too much for the tires or that I just needed more practice, but that’s when I noticed the shiny wet streaks in the asphalt. You could literally smear it with your fingers, but I continued testing and guess what? I was able to do pretty well in spite of the conditions.
•130 ft., 9 in. - Good feel but sliding at the end.
•129 ft., 6 in. - Nearly identical.
•115 ft., 5 in. - Finding the “sweet spot”.
•109 ft., 4 in. - Maybe the pads are getting “bedded”?
•107 ft., 8 in. - Bang! Best ever!
•109 ft., 11 in. - Still working great!
•109 ft., 3 in. - Did it again!
When it was all over, I was very impressed with the performance of the Wilwood kit. You had to be there to witness the road conditions, but even with that going against it, the worst stops were at least five feet shorter than the times when I locked them up previously. In the first tests I hit 109 feet once and could never repeat it; this time I did it multiple times and even managed one at 107 feet.
Averaging all the tests, we knocked over seven feet off and even if we throw out that first 151 foot example, we still reduced it by about three feet. While maybe not dramatic looking on paper, remember back to the technology being used on new ’Vettes to beat these results. These are real world examples of the varying road conditions out there and I actually think it was neat to test them under poor conditions and now I know I can haul this thing down cleanly anytime I want to. These improvements only multiply when you double the speed and/or work your brakes hard on a road course. And surely, stickier tires would allow you to generate some serious G’s.
The quality of the Wilwood pieces is extraordinary. Everything fit as intended right out of the box and there were no parts store runs looking for little odds and ends. If you are looking to upgrade your current brakes to take advantage of modern tires on your 15-inch wheels, then this kit will do it. You can also get all the features of the Wilwood calipers with only four pistons on the front if you prefer to remain stock sized.
If you’ve moved up to 17-inch or larger wheels, they have kits with larger rotors that should really apply some abuse to your seat belts when you slam them! I know people who had to buy caliper cores to be rebuilt and then paid to ship the heavy monsters back and forth, or have gone through several sets of “lifetime guarantee” calipers from the local parts stores or have just fought leaks forever.
The Wilwood system can put all of that to rest quickly with 21st century, state-of-the-art components that work! Lightweight, improved design, more stopping power, great looks, and they fit under your current 15-inch wheels!
What else could you want?
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