Disk brake sizing questions
 

I took a look at the front disk pads on my Cobra today and they don't look good. The most notable thing is uneven pad wear on both pads and both brakes. I pulled out a set of dial calipers and the caliper mounts have a 1/4" angle as compared to the disk surface. That explains the uneven wear! I wish I could remember the company I bought these from as I'd let everyone know so they could avoid them. With that poor quality of product, I'm guessing they're out of business now. It's been too long and I don't remember :(

So I'm starting research for some new front disk brakes. The Cobra is a CRL with MII type suspension and uprights/spindles. On the front, it currently has Granada rotors that are 11" and GM intermediate calipers that are single piston. On the rear, it has TBird disk brakes that are 10 1/2" rotors and single piston calipers. The brake effectiveness has been OK but it could stop better.

From a budget perspective, I'd like to keep the new brakes to the front ones only, if possible. In looking at the Wilwood MII brake options, there's some with an 11" disk and some with 12" disk that should be compatible with the 16" rims on the car.

So questions:

1. Will the combination of 12" front disk with 10.5 rear disk work without any odd quirks? The brake system does have an adjustable bias valve.
   
2. Will the 12" front disk be worth the extra money over the 11" disk for street driving? This isn't a track car but does get used from some spirited driving on occasions.

I did look back at old threads on brake upgrades but couldn't find answers to these specific questions. Thanks in advance for any insight you guys can provide.

If you use 15 inch wheels, make sure the caliper will clear the ID of the wheel with the 12 inch rotors. Other than that you have a good basic parts selection. It should provide you with excellent service.

I upgraded the brakes on my CR a few years ago and it made a world of difference. Had the same set up as you but maybe the master cylinder didn’t match up well with my combination. (Larger bore-more volume, smaller bore-more pressure). I didn’t want to break the bank which you can do with big brake kits so I went with the 12” Wildwoods in front and the Cobra rear setup. The cobra rear is not available but it ‘s just a bigger disc with an adapter to move the GT caliper out. The car stops great now.
As long as the rear axles had to come out, I also upgraded the axles and carrier to 31 spline and c-clip eliminators.

If the pedal feel and stopping ability of your current setup is OK for you, then new parts with the same geometry should be fine too. In other words, if you can lock the front brakes in an emergency stop now, getting a larger disc will not help you stop any better. On a track where the brakes are used heavily and cooling is a major consideration, bigger brakes make sense, but not for "occasional spirited driving." . . . During heavy braking the front brakes provide about 70% of stopping action. As long as you have a brake bias adjustment, I see no need to change the rear brakes from a performance perspective.

Joel,

There is an SPF document that chronicles the evolution of the braking systems they have used over the years. Your choices compare very favorably to their more recent offerings.

Forum doc size restrictions (31Kb) preclude attaching the doc to this post. If you would like a copy, PM me with an email address and I will send it to you.

16” Compomotives, GM calipers, Granada rotors lots of us have this.
Your GM calipers are 70s Chevelle 2 13/16” bore size D52s.
They use a 1 1/8” bore master cylinder with booster if you want.
Gut and plug the proportioning valve and add a Willwood valve for rear brakes.
New brackets from Helix or upscale Howe are not to expensive.
Make sure your MC bore is correct and calipers are the bigger bore D52s.
You can adjust for rear size, these are good brakes when set up right.

For an upgrade these Wilwood 2 piston calipers and Corvette 1" master without booster.
https://www.wilwood.com/calipers/Cal...0Dual%20Piston
https://www.amazon.com/Corvette-Cast.../dp/B075NM35SL

Joel,

I stumbled onto your Lambo Miura project thread.

Absolutely Stunning from concept, to design to the actual build would fall embarassingly short of the mark. I was blown away! That is a scratch build from the get go and you build everything including the aluminum body. Your creativity, vision and craftsmanship is right off the scale!

WOW! Very, Very Impressive!!

Thanks Ed!!

After seeing the Miura project, it's probably more obvious why I'm trying to tightly manage the spending on my Cobra (every dollar that goes into the Cobra doesn't go into the Miura). But alas, the Cobra is going to require some "investment" if I'm going to keep it "drivable". On the first road trip after fixing the gas tank venting, I think the engine spun a rod bearing :CRY:

I haven't published this adverse outcome yet as I haven't pulled the engine to verify, but the rpm variable clack and metal sheen in the oil are fairly telling symptoms. A friend and I checked the whole top end and no major issues found there, the compression is good on all 8, and so there's only one thing left that it really can be. So I'm now collecting up information about all the issues in my Cobra so I can create a "budget" for fixes to make it safely drivable again. I guess it's true that owning and enjoying a Cobra is a fairly constant drain on the wallet :rolleyes:

I am not attempting to tell you how to build your engines but rather offering a thought that might be helpful.

Whenever an engine has run for some period of time without having spun a bearing and then suffers a spun bearing, it is always an oil starvation issue. Sometimes it arises from a high volume oil pump used with a stock or near-stock capacity oil pan. The oil level above the pickup can become marginal if the engine has a slow oil return path to the pan or just poor oil control in the pan.

In the scenario where the pickup is marginally covered with oil, any braking, turning, or acceleration can momentarily uncover the pickup allowing it to draw air instead of oil. The sporadic but repetitive presence of air bubbles in the oil feed to the engine will be a cumulative sort of failure scenario.

The first few times, probably no serious damage. Eventually, the bearing surface is sufficiently damaged that the oil film that usually supports the load on the crank pin is thinner than the physical scarring on the bearing. The first time this occurs, the crankpin grabs the rod bearing insert and rotates it between the other insert and the crank. At this point, the show is over, and the damage is done.

The mechanical failure process is always the same. The cause of the starvation is always an uncovered pickup. Drawing air into the oil system is always less oil in the pan than is necessary to cover the pickup. The fix is always one of the following;

• A larger capacity oil pan, if possible,
• Better baffling of the pickup to maintain oil coverage,
• Use of a dry sump system,
• Use of an oil accumulator.

Even when we attempt to run the largest capacity pan with the best baffling that is prudent, the oil starvation problem can still rear its head. Dry sump systems remove the opportunity to uncover the pickup because of their physical design — but they also cost an arm and a leg.

The last item, using an oil accumulator, is the poor man’s dry sump system for this type of failure. The accumulator retains oil under pressure and is “T’d” into the oil delivery line between the oil pump and the engine. When the pickup is uncovered and draws air, the accumulator sees the drop in oil pressure and begins to discharge oil into the oil engine oil galley to protect the engine bearings.

The accumulators are relatively inexpensive and great insurance for the engine. They also can be used to prelube a collectible car that we don’t start every day. Here is a pretty good YouTube video by Moroso to better explain the protection process; click here => Moroso Oil Accumulator.

In addition to Moroso, Canton offers the original oil accumulator called the Accusump. Both work equally well. You can not go too big, you can go too small. Get the 3 quart gizmo.

On my Hurricane, which I don't own any more, I had Wilwood 12.19" rotors in front with Dynalite calipers. Mustang II spindles. I had 15" Trigo wheels. No rubbing issues.
Keep an eye out on track width with some brake kits. They'll change the track width.

Larry

My car already uses a Corvette master cylinder along with a power booster, not sure the bore size though. Going with the Wilwood D52 calipers could be a good way to upgrade the brakes and save about $500 over the cost of the Wilwood MII 12" rotor upgrade kit. I'm guessing the biggest difference to the brake system effectiveness for street driving would be the 11" rotors versus the 12".

Does anyone have experience to know if the larger rotor size in this situation is really worth the extra cost?

I have the 12" rotors on mine but never tried the 11" rotors for comparison. I do know when I got my new DD it came with 395mm (15.5") rotors and I was suprised by the braking power they had.