Tech Time – Brakes (Part 1)
By Kev Rooney
Note – This was originally published in the UKK’s Gazette and is reproduced here with their permission.
Many of you are running standard brakes on your cars but many do uprate to discs or put in stronger axles, which normally involves a brake swap. Once installed you'll notice you need greater pedal effort or that the pedal travels much further before gripping.
A braking system design from scratch is a very complicated procedure, including calculations based on acceleration / deceleration graphs, vehicle weight / brake ratios and coefficient of friction calculations.
Yes, you need friction, firstly between your pads / shoes and discs / drums, but most importantly between your tyres and the road. It's ok stopping your wheels dead but with no tyre grip you won't stop at all! So your brakes can actually be too efficient, often caused by having too large a braking surface area for the weight of the car or sometimes by the weight transfer caused under heavy braking, a problem often experienced in the early 70s on jacked-up cars.
The whole hydraulic operation is based upon Pascal's Law that the pressure applied to a confined fluid is transmitted evenly in all directions. The force on the brake linings is equal to the piston force of the master cylinder, if the master cylinder, disc brake cylinder and wheel cylinder are all of the same diameter.
To work out what size master cylinder is required with the larger bore brakes, you'll first need to work out the original configuration of braking capacity. If you're using a 0.518" master cylinder (0.625) and the drum brakes cylinder, 4 off, are also 0.518" bore and the useable master cylinder stroke is two inches this is how you work it out;
The master cylinder fluid movement is - pr2h
3.142 x (.3 125 x .3 125) x 2" = 0.614 cubic inches
The movement of the 4 wheel cylinders is found by: -
Master Cylinder fluid movement x pr2 = h
0.614 x 3.142 x (.3 125 x .3 125)< = 0.5 inches
So there's even fluid displacement and travel on all 4 cylinders. Leave the back cylinders as they are and fit discs to the front, the cylinder bore is now considerably more, say three inches. So, the same amount of fluid will be displaced but this means the actual travel of the calipers will be reduced. There's only half of the master cylinder displacement (0.614 ¸ 2 = 0.307 to operate 4 off 3-inch pistons.) Because of their larger operating area they don't need as much travel as they have increased operating pressure due to the greater bore area.
To find how much movement there is: -
pr2h x 4 = 0.307c.i. or h = 0.307c.i. ¸ 3.142 ¸< (1.5 X 1.5) = 0.10856 inches.
Or simply put about 3/32" compared to 1/2".
To work out the increased master cylinder bore requires taking into account swept area of disc brakes, friction requirements etc. It's far simpler to let the manufacturers do it for you. Generally a 1/8" (.125) increase in master cylinder bore will upgrade if you've fitted discs to the front of a drum braked car, and an increase of 1/4" if you've fitted discs all round. This can be complicated by pedal ratios as well but you'll have to suck it and see.
In the past I've thought well, the cars weight hasn't changed so surely it should still stop ok, even if it accelerates three times as fast. The problem with this is velocity. The faster it gets to a set speed the more momentum there is.
The brakes will operate but then friction comes into play with the added velocity and overcomes the operating range of the brake linings. A servo will reduce pedal effort, making it easier to push even harder, but once the brakes are hot then the grip fades and it's not pleasant, trust me, I've been there!
With the case of over sensitive brakes on the rear, load sensitive check valves can be installed, as per Renault 5, Mini, Fiesta etc. or even manually set units are available from people such as Demon Tweeks or Merlin Motorsports.
Bear in mind that many yanks, especially 70s and 80s models have residual pressure valves. These devices keep a small amount of pressure in the line to prevent the need for excessive pedal travel to get the brakes to operate. All in all a very complicated and, of course, dangerous subject, and one where a slight mistake could be fatal. If in doubt ASK!
I know of someone who spent nearly eighteen months slowly but surely replacing all his brake components to try to cure a spongy brake pedal. The fault was eventually traced to the front calipers being fitted upside down! Remember the bleed nipples need to be on the top, compressed air rises!
I hope this article gives a small insight into the amount of study the manufacturers go into so we can go and screw it up!
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