Remind me why I need power brakes

OK, so I'm working on a light weight car, locost-ish frame, thin sheet metal body panels (think appliance parts), a fairly weak engine, and a solid rear axle. I'm guessing it will weigh around 2200-2400 lbs when finished....it could actually be under that. (Although the stick axle, dakota front clip and heavy L24 make that iffy.)

In a car like that, do I need power brakes? Cars used to get by without them. Is it possible to use the existing brake setup (front disk, rear drum) without having a vacuum booster?

I'm asking because there's not much room on the firewall to mount stuff, and I am already strongly considering putting the brake master cylinders behind the firewall, under the dash. Wilwood Reverse Swing Mount Dual Master Cylinder Pedal Assembly

Can a brake setup like that be safely run without a vacuum booster?

Are you talking about the Datsun? If so, then I don't think you need power brakes at all.

My '67 Le Mans doesn't have power brakes and never did. It probably weighs 3500 pounds and it has 4-wheel drums. My mother drove it for 15 years before it became mine, and never had a braking-related incident. You can easily lock all 4 if you stamp down with both feet.

I kind of like power brakes for autocross because you can modulate more easily with just a little toe pressure. But for a cruiser like the Datsun that's never going to be driven in anger, I wouldn't hesitate to skip it.

[edit] Sorry, I can't add anything to the technical side. Except it seems to me the question of needing a booster or not is determined by the master cylinder and the lever arm of the brake pedal. Anything that can generate enough hydraulic pressure will make the business end operate correctly. I would talk to Wilwood.

You don't. My 4500 lb wagon with four wheel drums doesn't have them and I can lock them up with a good hard push. There are reasons to WANT power brakes, but that doesn't mean you NEED them.

Drums ain't discs. Drums are self-energizing, which means that they'll generate additional braking force once they start to generate any. You can feel this on an all-drum car. Discs require more line pressure for the same stopping force. So I wouldn't recommend running discs on a two-ton car without some assist.

That said, you're basically describing an MGB here in terms of weight and brake layout, and they had unassisted brakes for most of their life cycle.

Keith Tanner wrote: Drums ain't discs. Drums are self-energizing, which means that they'll generate additional braking force once they start to generate any. You can feel this on an all-drum car. Discs require more line pressure for the same stopping force.

I did not know that. Ignore me. Listen to Keith.

Yeah, power brakes weren't really necessary before disk brakes got popular. And disk brakes got popular because they don't lock up so easily even with power assist... self-energization is a double edged sword.

That said, I've driven quite a few full-bodied cars with Strange brakes and no power assist and they're perfectly fine to drive on the street. No idea how well they'd hold up to heavy use, though, given that they are designed to be very light and whoa you down from ~150mph about once every half hour. In that order.

My E30 race car is 2350lbs with 4 wheel disks, running Tilton double overhung pedals with properly sized everything as per Angry's article and a couple phone calls to Wilwood for MC sizing, etc.

Other than missing ABS, it is the best feeling, effective braking system I have driven to date. There is almost no pedal travel due to pad wear, I can feel/modulate everything like I was the princess in the mattress/pea story and it will haul the car down from 155mph to nothing all day long without any issues.

So, done right, no worries. You can't just cobble it together though - everything from the moment arm of the pedal to the size of the pistons, MC bores and so on has to work together or you can end up worse than you started with by a lot.

EDIT: I have not mixed drums/disks so pay attention to Kieth but call Wilwood, Tilton, etc and ask them what they recommend. The Wilwood guys were especially responsive and helpful.

Lotta good info here re: sizing MCs etc., should get you in the ballpark.

Yes, Duke, I'm talking about the datsun replica.

It sounds like the general consensus here is that it can be done safely, but not if I just throw things together; i.e. that I will need to think about it carefully and pick parts that work together.

Which issue was Angry's article in? That would be a good starting point....

I just sent a PM to Angry. I also noticed that speedway sells this little vacuum reservoir http://www.speedwaymotors.com/Single-Diaphragm-7-Inch-Power-Brake-Booster,36759.html

The Abomination doesn't have power anything. The brakes, are Miata disks on all four corners, with a FB Rx-7 master. It will lock up all four corners without any problem and that's running autocross slicks and cheap pads. It does take more pedal pressure than most people are used to, but it's not excessive. Pedal travel is perfect, and like GPS, I love the feel of manual brakes. Threshold braking is a breeze.

I know there are some magic numbers as far as master piston diameter, caliper piston diameter and the moment arm on the pedal. I don't know what they are though.

You might send Curmudgeon a PM. He set up the brakes on the Abomination and they are a pleasure to use.

Keith Tanner wrote: Drums ain't discs. Drums are self-energizing, which means that they'll generate additional braking force once they start to generate any. You can feel this on an all-drum car. Discs require more line pressure for the same stopping force. So I wouldn't recommend running discs on a two-ton car without some assist. That said, you're basically describing an MGB here in terms of weight and brake layout, and they had unassisted brakes for most of their life cycle.

Exactly what he said.

All a booster does is help you press the pedal. Without one you will either have to press harder.... or change the motion ratio of the pedal to get more leverage at the cost of a longer stroke.

My Fiat has drums in the rear and 4 piston fixed calipers up front with no booster and it has the best feeling brake system I have ever put together. The falcon has 11" front rotors and explorer rear drums with a 15/16" bore MC without assist and the pedal requires just a tad more effort than any other car.

I have no intention of installing power assist on either car.

Toyman01 wrote: You might send Curmudgeon a PM. He set up the brakes on the Abomination and they are a pleasure to use.

Thanks. I just did.

I'll have to pull out my Datsun 910 and Dakota manuals to see what I can find out about their drums and disks, respectively.

Speedway Motors said: The diagram below shows a typical streed rod brake system with the master cylinder mounted on the frame. http://www.speedwaymotors.com/brake-system-selection-service-performance-operation.dlp

While I'm not questioning what Keith or any of the others have said, I had a 1984 Ford Ranger that probably weighed somewhere around what this Datsun replica does, and it had non-power brakes, disc/drum.

Was it the best braking vehicle? No, certainly not. But the brakes were more than adequate.

There are tons of very-high performance brake systems that don't use power assist. Schwartz Performance builds almost exclusively with Wilwood braking systems that don't have power assist and they STOP.

NASCAR Cup cars don't have power brakes, and they weigh over 3000 pounds dry without the driver and go 200mph..

yeah, but they often just use someone else's car as a brake

My '77 F100 has (and has always had) non-power front disk/rear drum brakes. They work quite well. On the other hand, when the booster went out on my '93 Cherokee, it was a whole nuther ball o' wax.

In other words, if the braking system is designed to be sans booster (swept area large enough), it will be fine. If not, it won't take long to find out.

carzan wrote: My '77 F100 has (and has always had) non-power front disk/rear drum brakes. They work quite well. On the other hand, when the booster went out on my '93 Cherokee, it was a whole nuther ball o' wax. In other words, if the braking system is designed to be sans booster (swept area large enough), it will be fine. If not, it won't take long to find out.

the reason the pedal gets hard to push when a booster dies is because of the leverage of the pedal- the pushrod needs to be up closer to the pivot with manual brakes than with power brakes to get the leverage... also, a master cylinder for a power brake application will have a bigger piston than one for a manual brake application.

I agree with Novaderrik. Nascar stock cars do not use any type of power assist. If the drivers can do 500 laps at Martinsville or a race at Watkins Glen and not complain, I think the average street car can get by without power assist as well. I deleted the vacuum booster out of my car a few years ago, made some modifications to get the brake pedal ratio better, and it's all good.

novaderrik hit it on the head: pedal ratio is the secret to non-power brakes. The Abomination is a 6:1 pedal ratio overhung, i.e. the master cylinder pushrod pin is above the pedal pivot point. I'm using the same ratio on the Jensenator with an underhung, i.e the master cylinder pushrod pin is below the pedal pivot. Both cars stop quickly and the modulation is easy.

I like manual brakes because if for some stupid reason the engine dies it doesn't mess with the pedal effort which can be REAL important. If the engine croaks in a screwy maneuver, the last thing I want is a rock hard brake pedal that doesn't do squat.

It's not just pedal ratio, it's also hydraulic sizing. They're basically interchangeable - they're just ways of multiplying pedal pressure.

Keith Tanner wrote: It's not just pedal ratio, it's also hydraulic sizing. They're basically interchangeable - they're just ways of multiplying pedal pressure.

the booster allows you to run a much bigger bore in the master cylinder relative to the calipers or wheel cylinders, which is really the benefit of the booster... but when the booster dies, you are still trying to move that much more fluid with the same pedal ratio, and it doesn't work so hot..

just going from a 15/16" bore like i used in the manual brake setup in my Nova to a 1 1/4" bore like i would have used had i been running power brakes represents a HUGE increase in the amount of fluid that gets pushed thru the lines for a given amount of master cylinder stroke, which translates into a much harder pedal without the boost.

Keith is right, master cylinder bore sizing is another way of achieving the desired leverage ratio. But swapping master cylinders has its own set of problems, the biggest ones in my case were the expense of swapping cylinders compared to modifying the pedal ratio and the possibility that I couldn't find a small enough bore cylinder.

On the Jensenator, the stock pedal ratio for the booster was 4.5-1. It wasn't really feasible to go with a small enough bore master cylinder to make it work with that ratio, 13/16 is pretty much the lower limit of readily available stuff unless you go with some of the racing cylinders.

That presents another set of problems: they are generally single circuit meaning generally you need to use two with a balance bar. That's certainly possible, it's done all the time but if you don't really have the space or fab skills for this then it's best to change the pedal ratio. The Jensen doesn't have the space to run two cylinders without a major rework of the whole pedal setup including the clutch and accelerator stuff, a dual circuit cylinder was the right answer for that problem. So I went with 6.0-1 pedal ratio, this meant redrilling the master cylinder pushrod hole in the pedal and fabricating a flat aluminum bracket to raise the master cylinder, this kept the pushrod straight in relation to the master cylinder bore. Another reason I wanted the dual circuit master cylinder was that it applies the front brakes before the rears, this makes the car much more controllable under hard braking.

When I did the Abomination, the pedal ratio was already 6.0-1. That meant just picking the right master cylinder size, the RX7 master cylinder just happened to be the right bore and available cheap. A BIG improvement on that car was reinforcing the pedal bracket. A tiny amount of flex at the master cylinder mounting becomes a mushy uncertain pedal feel. The Jensen has a cast aluminum pedal box which doesn't flex. The master cylinder adapter plate is a chunk of 3/4" thick aluminum plate that damn sure won't flex, I had to cut it to size and drill 7 holes, not a big deal.