Just say'n Driving big truck and Model T Ford,, learned me to stop at the top. I know I'm off topic, But always compelled to pass on a safety thought, where someday it may help another avoid a bad day.
It seems simple enough to me: you keep riding the bike in under its own centre of mass as affected by gravity and acceleration. You do that having developed a feel for how to do it, largely unconsciously: the mystery to me is more the psycho-neurological processes which make that possible.
You are the first one in this post to allude to the way a “modern” brake system is designed to work. Rear brakes set up first at low pressures. Since every new brake application starts at low pressure, the rear brakes, by design always apply first.
Haha! There is so much bullshit in this thread. All line pressure is equalized throughout the system [except when the proportioning valve regulates maximum rear pressure] Rear brakes are set up to operate at the same line pressure as the front! Engineers then use different size wheel cylinders and caliper pistons to match the line pressures to clamping pressures ,coefficients of friction and brake torque etc. Nothing happens in a brake system until all clearances have been taken up , and further pressure is applied I personally believe brake modifications in the hands of idiots is dangerous [some people learn, and some people let their egos make stupid decisions]
Properly designed brakes would have front and rear lock at same time. When vehicle is braking the weight shifts forward, majority of braking is done by the front wheels. Manufacturers used larger brakes and / or different wheel cylinders on the front to account front bias - majority of braking on front - due to weight shift. Also designed for multiple scenarios. Downhill, flat etc. Reality - Front brakes typically lock first, due to weight transfer forward / larger brakes in front.
The last paragraph is the most important here. Initiation of brakes is specific to vehicle. The exceptions to my description would be cars that were designed with cross braking systems. Cars with cross braking use specific master cylinders with zero bias springs between the pistons in the master. There are also stepped bore master cylinders that can either output separate pressures or equal depending on if they are linked with a specific rod diameter. Take a typical master cylinder apart sometime. How is the fluid compressed in the rear most chamber?
Where can I find one of these mythical stepped bore master cylinders that can output separate pressures???? A step bore master cylinder [aka "quick uptake M/C] has a floating piston so all output pressures are equal. The quick take-up master cylinder takes up excessive pedal travel by the use of a two-stage bore and piston arrangement. It permits the use of increased brake lining clearances, particularly in disc brakes with low drag calipers. Front brakes lock up first, due to lack of forward weight transfer and greater brake torque at the front. This is due to compromised traction. Poor mans ABS [a skilled driver] can overcome this with feedback
Weight transfer is dynamic based on conditions, as is tire lock up / skid. Exact same brake event on 2 different days can yield a different result, road condition, temperature etc, Weight transfer to front will generate a skid as the mass to stop overcomes the tires ability to hold pavement. Rate of weight transfer to front will vary based on how quickly the brakes are applied. Lack of weight transfer to the front, maybe we're saying the same thing - slower rate of weight transfer to the front ? Tires are as big of a variable as anything. we can go down the rabbit hole for days. If you just gotta be right - you're right man.
I'm certainly no expert but, I have studied this subject quite a bit and I couldn't agree more. Especially the last paragraph. The OP titled the thread with the word Physics which is often miss used. Vehicle dynamics is a complex subject with many variables but is ultimately governed by the laws of physics. Since the majority of brake use is in the forward moving direction with any appreciable momentum it only makes sense that increased clamping force is needed at the front when more weight is transferred there and loads on the contact patches are increased and tires are developing more grip. As stated above brake components at the rear are appropriately sized (smaller) for each vehicle to create the best possible brake balance and to minimize rear wheel lock up during the weight transfer event when there is less weight, load on the contact patches, and less grip. Just my 2 cents, carry on.
https://gus-stories.org/september_1928.htm A "few" decades back the usually pretty savvy Gus Wilson seemed to prefer rear wheel lock up. I don't. I'm thinking some SAE paper of the early 60s might have said the original Corvairs were set up for rear wheel lock up, advocating that being able to steer would make all it OK . I'll have to look harder to confirm that
There is a huge difference between lock up and initial application. The key is that on a dual circuit brake system, there is bias in the two springs in the master cylinder. Some use the combo valve to achieve the same end effect. This isn’t an “every last one of em” statement, but a huge majority. In most cases, you will achieve around 120-150 psi in the rear circuit before the front develops any pressure. From that point on, the pressure equalizes because the force exceeds the spring bias. As for stepped bore masters, browse through parts catalogs looking for dual rear axle hydraulic brakes.
So, for the uneducated.... Would a guy that is wanting to run a drum- drum setup, shoot for the wheel cylinders with a larger bore or smaller bore in the front? Would those be matched to the master cylinder or smaller? (Considering the right ratio for the pedal etc. is already correct.) Thanks, Root
Assuming all drums are the same diameter, and all shoes the same width, you would want the larger bore wheel cylinders on the forward axle. As for master cylinder bore size, I prefer the smallest bore that allows for safe pedal travel. If the pedal travels to much (after proper bleeding and adjustment), step up in size. The smaller the bore on the master, the less effort it takes to achieve full pressure (at the cost of more pedal travel)
I'm thinking the vintage of Gus' story might mean it was talking about non-hydraulic brakes. I'm thinking initial adjustment doesn't affect basic single circuit hydraulic brake wheel-to-wheel application since all 4 wheels receive equal hydraulic pressure). 1928 chevy - look to be mechanical, rod operated. Maybe two wheel only ! ? The 1936 manual describes when adjusting the 4 wheel mechanical brakes , initially set all four to drag very slightly , and then after some road testing slightly slack off the brake that pulls harder. http://chevy.oldcarmanualproject.com/shop/1925/25crm092.html 1928 Ford - mechanical until after 1948?? 1928 Lincoln - 4 wheel mechanical 1928 Plymouth and chrysler. 4 wheel hydraulic 1928 Studebaker bendix mechanical 1928 Packard - bendix mechanical Pierce Arrow - mechanical until at least 1933
You are referring to " I personally believe brake modifications in the hands of idiots is dangerous [some people learn, and some people let their egos make stupid decisions]" I stand by this comment ^^^^ Below is a classic example of somebody chiming in with "Their almighty superior knowledge" and get it totally wrong. Let's hope for the sake of other members, that they realize he is only trying to win an internet argument ,so please don't take this as gospel The springs inside the M/C ^^^^have no influence on line pressure or clamping pressure at all. In theory ,if the springs were removed or swapped around the line pressure or clamping pressure would remain exactly the same. The springs will affect the pedal effort slightly. The purpose of these springs is to help return and locate the pistons [they use 2 different stiffness springs so the stiffer spring can locate the floating piston position more positively] Line pressure [or working pressure as quoted] is a factor of fluid mechanics. "Pressure is Pressure" and "Volume is Volume" they shouldn't be confused. There is no co-efficient of friction until ALL the clearances of the friction surfaces have been taken up, AND further pedal pressure is applied resulting in clamping pressure [and line pressure which can be measured easily] Usually there is more clearances in drum brakes, so more volume is needed to take up clearances before pressure is built up. Usually a small amount of pressure is needed to overcome return springs [but there is no clamping pressure at this point] Even at this point ^^^^ the line pressure is equal in the whole system [because the floating piston in the M/C will meet a point of equilibrium] A quick summary....There is no pressure in the system until the fluid is met with resistance [all clearances taken up] When doing calculations for brake systems on race cars , we use an arbitrary 0.005" crush on all friction surfaces to help calculate Volume. Just remember....."Pressure is Pressure" and "Volume is Volume" And if you're out of your depth, please leave braking systems alone [people's lives are at stake here] @David Gersic
So, I had a conversation with a buddy about drums vs discs. The topic came up of the efficiency of the braking material (pad vs shoe). His point was that on a daily, his drums would stop his truck in the same distance as discs, because his theory was that the tires were the weak link, since it was a non abs system. Assuming at some point, the brakes would lock up. (65 Chevy C10 with stock brakes). Thoughts?
All things being equal, he is probably correct. A properly set-up and sized set of drum brakes will stop the same vehicle in the same distance as a vehicle with a properly setup set of discs....ONCE. The advantage with discs is they are much more effective at maintaining stopping power over repeated brake applications. Drum brake drums expand as they heat up and the pads have to travel further to make contact, eventually reaching a point where the brakes can't adjust enough to apply proper shoe pressure to the drums. Disc brakes don't have this problem, they generally won't lose stopping power until they get hot enough to boil the fluid in the calipers (Some 400 deg. F with dry DOT3 fluid)
What I was taught is that brakes stop a car by converting kinetic energy (the motion of the car) into heat energy (using the friction between the shoes/pads and the drums/rotors) and then dissipating that heat into the atmosphere. Rotors generally do a better job of that than drums. The reason you loose stopping power (brake fade) is that the drum/rotor has been heated beyond its ability to accept and dissipate any additional heat.
When drums or discs heat up they expand! Drums expand away from the shoes.[this causes the dreaded brake pedal to the floor feeling] Discs expand towards the pads as well as increasing in diameter But the caliper is at a fixed distance from the spindle centerline so increase in diameter doesn't affect brake torque. [and the expansion in thickness is minimal compared to diameter] @Roothawg Drum brakes produce greater brake torque than disc brakes [for the same clamping pressure] because of the greater friction surface area. This is why electric brakes on trailers and park brakes are usually drum brakes. [less clamping pressure is required] Discs require greater clamping pressure , which is why the swept area of a caliper piston is far greater than the piston area of a wheel cylinder. Disc brakes can also run closer clearances between friction surfaces,so less caliper piston travel is needed. So consequently the M/C piston travel between disc and drums is very similar. [it is at the other end where the Fluid Mechanics/Bias is calculated] @X-cpe and @FrozenMerc are both correct about brakes only removing kinetic energy. It is actually the tyres that stop the car! It is very easy to swap which end of the car will lock up first by merely changing the tyres [lock up only happens when the available traction is exceeded] Note: Anybody that is Savvy with brake designs always starts at the tyre footprint, then works their way back to the brake pedal.
Some really good info here. Makes me appreciate even more the folks racing pre-war cars with only rear mechanical brakes
I haven’t read all this yet. It’s early, and this coffee has not kicked-in. I’ve seen some comments about weight transfer, and that’s where a lot of this stands. Pascal’s law: pressure on a confined fluid is applied equally in all directions. This means that every part of the closed hydraulic system will see the same pressure. Proportioning valves and different size wheel cylinders just change the rate or timing of which that pressure is applied. You want the front to apply first, because the weight balance moves forward. A rail dragster only has rear brakes because the static weight balance is biased extremely to the rear. I think an interesting story: pickup trucks are brake-biased towards the front because their unloaded static (non-moving) weight balance is to the front. When antilock brakes first came out, they tried them on pickup trucks. The antilock system did little more than reduce flow rate to the back brakes. We had service trucks back then with very heavy utility beds. With a load, those trucks would hardly stop at all. When you unhooked the antilock, it was like night and day, stopped hard! That’s just one example of weight transfer effecting braking.
I've sure gotten some great laughs about some of the theories stated in this thread. If many of them espoused here were true the old single pot masters would never have worked, or at least not worked well. Besides a proportioning valve used on modern dual master cylinder systems, brakes in the single pot got one piston, and a T fitting sent that fluid to all four wheels equally.
https://www.jalopyjournal.com/forum/threads/tech-week-57-fargos-brakes-101.1229211/ Some light reading.
Obviously no "ego" coming from your end.........right? Why don't you stick to discussing the technicalities rather than attacking someone every time you post. Trendz has posted lots of very good information in the past and helped a lot of people. If you disagree with him then post what you think is wrong and explain why, but the personal diatribe isn't helpful to anyone.
Root, @X-cpe and @Kerrynzl already addressed this, but I'll add one other thought about the difference between drums & discs, and that is the issue of feel, and controllability; and that self-actuated drums have quite poor feel at the limits of tire adhesion, they transition quickly from almost to the point of lock up, to past it, quicker than most drivers ability to detect it and modulate foot pressure to keep it right there. Discs are much better at that, so when coming to a stop quickly is dire, in the heat of an emergency stop discs will perform better. I will add that non self-actuating drums may work better at this point, but I have no experience with them, all my drum brake experience has been with self-actuating, so there is that. Self-actuating action reduces pedal effort required to stop, but they also reduce the ability to control the braking action at the limits of grip.
Suppose you were riding a bike and tried to stop suddenly with just the front brake, or hit a railroad tie. Is the bike's rear tire just going to sit there without complaint? No; the bike is going to flip, rotating about the slowing or stopped front tire. Now suppose that you're driving at 30 mph and left-front tire suddenly stopped in its tracks. Like the rear wheel of the bike, wouldn't the rest of the car want to continue moving, if there was any way to move, when left-front tire is planted? Surely the car would rotate about the left-front tire as the car dissipated its energy through the tire-to-road friction of three scrubbing tires. It could flip, like the bike, or it could try to spin horizontally about the stopped tire. Now suppose that you locked up both front tires but not the rear tires. Unless the front brakes and the weight distribution of the car were balanced, the rear end is going to want to come around, vertically or horizontally, because its kinetic energy leaves it no choice.. (Note that front brakes designed to lock up first is different from their being designed to dissipate more heat than the rear brakes.)
My opinion on drum brakes vs disc brakes: drums typically have more surface area and can work very well. Most manufacturers switched to discs, because of cost. Discs have few moving parts. The main advantage of disc brakes is heat dissipation and less brakes-fade. My first car had drums and when they got hot is was dangerous. I had an off topic car capable of going fast with tiny discs. Brake fade can happen with disc brakes too. Guess how I know.
I just struggle with the whole traditional vs reliability issue. I know period correct looks the best, but when it comes to safety, I tend to err on the side of caution. I just like to know the facts to make an informed decision.
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