Rear Disk Brakes...Is there really a need? Talk it over.

A few more thoughts worth maybe a penny or two, or maybe less:

Ultimately, braking power is dependant on traction available between the tire/pavement interface.  The more traction available, the faster the car stops.  You can have brakes from a formula one car on a speedster (now that would be an interesting and mindblowingly expensive fabrication!), but with narrow 165 tires the majority of that braking potential would a waste.  Think of braking on an icy road, you have to very delicately apply brakes due to a lack of traction.

As an illustration, My go fast car is a Porsche GT2.  it has composite carbon ceramic brake rotors.  The rotors have a replacment cost of $5,000, not for a set of four but each!  The brakes on this car are world class incredible.  I can lap the big track at Willow Spring Raceway all day long with no fade, braking from 150 mph on the front straight.  To make use of all of this braking power, and also to put 550 horsepower to the pavement, I have Michelin PS sport cup tires, 245's up front and 315's on the rear axle.  The car has anti-lock hardware and software, but the braking capabilities of this car are so immense, that I have rarely felt the anti-lock engage.  On a wet surface, all of a sudden, all of that braking potential is not usable.

A 3rd component that is a variable in ultimate braking performance, in addition to tires and brakes, is suspension.  Too stiff and the lack of compliance and damping will have the car dancing and tires skipping, too soft and all the weight transfer suddenly and overload the front tires, or the outside tires, or in worst case transfer all weight to the front outside tire.

I think that many blend the concept of braking power and braking feel.  Strong initial "bite" feels powerful to us, power boosted brakes feel strong.  I have been in race cars with brakes that require a lot of leg strength, but clearly can outbrake street cars that just need a tap on the brake pedal.  If you can lock all four wheels, then you have all the braking power you need.  But as the saying goes, "All complex and difficult problems have an easy answer, and that easy answer is wrong."  As our favorite nutcase billionaire, Ross Perot, was fond of saying, "The Devil is in the details" (using a nasally texas accent).

Braking (and acceleration) is too often treated as an on/off switch.  Incremental braking, progessively starting at 1% braking up to 100% and then backing off to 1% take a very educated foot.  If you watch racecars carefully (many spectators line up to watch racecars blast down the straights, but the action is really at the corner entrance and exits)  the drivers that are at the top of the leaderboard are so smooth that you have  a hard time seeing the transistion to braking.  Slower driver tend to slam on the brakes and you can see the cars suddenly dip the front suspension and get out of balance resulting in less precision in setting up for the turn.  There truly is a difference in 75% braking and 74% braking when at the limit trail braking into a fast corner trading off braking traction for cornering traction.

So the driver is the all important 4th variable in braking performance.  When it comes to performance the most critical part in the car in the "Nut" behind the steering wheel.  I try to practice braking skills daily.  After check to see that no one is behind me, and after warning any passengers, I will practice maximum braking at a stop sign or red light in a remote or empty area.  The goal is to stop as quickly as possible with locking up the brakes.  Just like golf, if you don't practice, you will suck at it.  You can have the best brakes in the world, but if you don't know how to best use them, then you wasted your money.  This is why manufacturers have engineered anti-lock brakes in almost all modern car.  Drivers will panic and slam on brakes and the computer will take care of them, no skills required (and all of us should acknowlege that the level of driving skills and attention in this country truly is shockingly horrible).  Get to know how you brakes perform on dry, wet, snowy, clean, dusty, hot, cold conditions.  It may save your life someday.

This is where rear disks are so helpful.  Drums may lock up early, the right rear one stop, then the left rear on the next stop, both on the 3rd stop, then fade and lose power on the next stop, and so on.  For a driver to have confidence, consistency in braking is everything.  Our speedsters have adequate brakes, but just fine for a car with a top speed of 98 mph (per my GPS unit).  If I had a big motor and a 140 mph speedster like some of you folks, I would upgrade wheels, tires, suspension, and brakes.  But I really enjoy the feel of just putting around in my little speedster with 60 year old technology.  It get driven 10 time more often than the GT2.

... and there it is, neatly wrapped up in a nutshell.

BTW, "Pdrvr"-- who are you? You seem like a really good guy to have around. I'd love to address you by name, if you didn't mind horribly.

I second what Stan said.  Who are you?

Troy,

I was only joking and shamelessly plugging our new products.

Can you PM me the best way to get in touch with you, as I have some questions about the Vintage sure wide bodies and someone said you know things....

Thanks

ALB,

The 12.5 disc brake package will fit behind some 16 inch wheels. You would need to take a measurement to know for sure on the wheels you want to run. We do know they will not fit behind a 15 inch wheel. Well at least not with the caliper in place.......

Does that mean a 15" Fuchs will fit over an 11"disc and Wilwood caliper?

Yes

Hi guys.  My name is Young Hong.  I live in Orange County, Peoples Republic of California, living under the regime of Kim Il Moonbeam Brown.  I have a 2008 VS, 1915cc with dual Kadrons, that I bought in November and have been enjoying greatly, at least when I can find it and get it away from my sons, who also love driving it.  I've learned an amazing amount of information from reading this forum and thought it was time that I tried to contribute.  You guys have been so helpful in helping me learn about my VS.

Young--welcome aboard from Dogpatch too. I enjoyed your post and printed it for my reference book.  You know some stuff!

One of the things that hasn't been mentioned is a brake component's ability to absorb and shed heat. Drums are great at absorbing heat, but poor at getting rid of it. In the mid/late '50's when most European car companies were going to disc brakes for competition, Porsche took drum brake development just that little bit further, using aluminum drums (with steel liners), fins on the outside circumference and ducting air for cooling. They did have some success, but soon realized they were at a dead end and made the switch, with discs appearing on the production C series 356's ('63?).

This is where discs shine; while you can use semi-metallic shoes, bigger drum assemblies (type 3) and even duct air through the backing plates to improve braking performance, their limitations at getting rid of heat will eventually rear it's ugly head (with sometimes costly/painful results!). Discs (with wheels with openings designed to move air through them and across the rotors), on the other hand, are much better at shedding heat. The question becomes "how much brake do you need?" In a panic stop from 70mph, a beetle's (a valid comparison as not only are they the source for the drum brakes, they weigh close to the same as most Speedsters) brakes will fade somewhere around 30-40 mph. They also don't have the heat soak/shedding capabilities that repeated braking will demand of the components. The first upgrade is front (Karmann Ghia) discs, and now that same car will panic stop from almost as fast as you can get it. A friend's car loaded down with 800 or so pounds of transmission stuff (he'd found some ZF diffs and other trans bits in Mexico) is the only time I've ever heard of anyone discovering the limitations of the Ghia style front discs. This was a panic stop from a 70 or 75mph- he crested a hill on a Mexican highway and there was an accident involving 2 trucks and he said they simply "went away" at about 35 or 40mph, and some fancy driving (thank goodness for wider wheels/tires and suspension mods) and some luck saved his butt.

For most people in these plastic cars, the Ghia front discs and either beetle or type 3 rear drums and semi-metallic pads will be adequate, and yes, I know rear discs will be better (note that I used the word "adequate"). For those constantly tweaking suspensions and pushing the limits of their higher hp cars, rear discs are a damn good idea. And for those with monster hp and much above 100mph capability, vented (and possibly larger) front discs are not only a good idea, they may very well save your life.... Al

Young- Great post, and welcome to the madness!      

ok this thread has become too technical

A percussive solo followed by a replica speedster... fabricate that.

Braking force is dependent on three things (for a given car/tire): leverage, clamping force, and coefficient of friction. The force of friction is only dependent on the clamping force and the coefficient of friction, it is independent of contact (pad) area. The coefficient of friction is dependent on temperature.

Discs:

Larger diameter = more leverage. The farther out from the axle you get with your caliper/pad, the more leverage you have and thus the more braking force. Like using a breaker bar instead of a normal wrench on a bolt.

Bigger = more thermal mass, room for venting. Better heat absorption and dissipation.

Better materials = better heat absorption and dissipation.

If you can control the heat better, you control the coefficient of friction better. That means the brakes can be used more often and with less fade.

Calipers:

Larger = more piston area. More piston area means more clamping force available against the pads. Piston area is also the first method to set front/rear brake balance.

Pads:

Larger = more area to spread heat load, lower wear from larger contact area (assuming all else equal).

Compound: Different compounds have different coefficient of friction vs temperature curves, you need to choose the one that matches the normal driving style of the car. Street pads often have high CoF at low temperatures but fall off quickly at higher temps. This gives them decent braking ability for the infrequent braking (cold brakes) seen in city/highway driving but are prone to fade when pushed hard. Race pads on the other hand, might have poor CoF at low temps but work well at much higher temps caused by frequent track braking. But even race compounds will fade at high temps, hence the need for huge, vented (even fancy carbon ceramic) discs to control the temps. If fading is a problem in panic stops, try a different pad compound first. Better to have slightly less agressive pads during normal driving than to be plastered on someones back bumper because the brakes faded early.

Yes, I suppose if you can lock up the wheels, you have all the braking power you need. But better brakes means you can keep locking those wheels up long after drum brakes have faded to greasy uselessness.

Well... we are in the "Technical General" section. Perhaps you're looking for the "online banter" section on the Miata site?

.

I have 4 wheel disc brakes on my coupe, and although I don't have anything to compare it with, I think it's overkill. I can stop great, I worry about the guy behind me. Plus the fact that I have semi-metallic pads which tend to squeal. (Has anyone else dealt with this?) If I had it to do over, I'd go with drums on the rear, maybe even 4 wheel drums.

This can go on my list of things I've never heard before, "I just stop too fast".

4-wheel drums? You first.

That was my 930 which had much better brakes than the idiot guy behind me

That's my point.

Despite the better brake than the guy right-behind downside, I'd still opt for above average stopping power. To wit, I just dumped my for wheel drums and shoes for discs and pads (Kevin has them and might make you a deal if you're serious ... they are wide-5 pattern which I will miss).

 What Stan said X2. Remember that every car out there these days has at least front discs; the damage to Doug's 930 was caused by the inattention of the idiot behind him and NOT his car having better brakes. In today's traffic with the speeds people travel on congested roadways (almost everyone following way too close!) and at least front discs being standard equipment on just about every car out there, I don't understand why choosing drums all around in a plastic car with no crash protection would even be considered. It's not just foolish, but downright dangerous. In these cars we need all the help we can get. As for the brake squeal, spring for different pads.  Al  

Pull your pads out and bevel the edges by grinding at a 45 degree angle an 1/8 inch around all of the edges. Then buy some good quality high temp grease and apply it to the metal backing of the pad everywhere it makes contact with the metal of the caliper or brake pad saddles. We use a silver colored grease called anti seeze. The squealing is caused by pad vibration and the metal to metal contact, not the compound or any other wise tell.......

I currently have basic 4 wheel disc brakes on my speedster, and they do a good job coming down from freeway speeds of 60-70, however they do not do very good when trying to bring it down from 100. They over heat and fade like crazy. Hell I turned the rotors blue in less then 50 miles. This is the reason I came out with our vented brakes. While they maybe overkill for most of you, I'll never rear end anyone because of brake fade.

Great advice, Kevin; I had to search back and find it to write it down. As for my advice, I don't know what I was up to that night...