AP Racing Brakes
Discussion
Also, I'm not an expert on brake systems, but my engineering knowledge would dictate that a larger master cylinder would push more brake fluid and the surface area of the piston is also bigger so would provide more pressure with less effort, hence you would have a bigger cylinder on the front to apply more braking force to the front... Unless It works in completely different way when it comes to brake systems.
Hi Pete
I hear what you say but 'the people' have been talking too are top technical on this without mentioning names. The change I am making is putting more pressure on the front pads and less on the rear so I don't see how the rears would locking up, it surely would be the fronts. There are some aspects of this that I don't want to discuss openly as it will cause problems. From my experience the performance of the brakes on my car are not where they need to be and therefore I will be trying to find a solution. I do have an adjustable brake bias system fitted also which allows on the fly adjustment to fine tune. Thanks for your concerns.
I hear what you say but 'the people' have been talking too are top technical on this without mentioning names. The change I am making is putting more pressure on the front pads and less on the rear so I don't see how the rears would locking up, it surely would be the fronts. There are some aspects of this that I don't want to discuss openly as it will cause problems. From my experience the performance of the brakes on my car are not where they need to be and therefore I will be trying to find a solution. I do have an adjustable brake bias system fitted also which allows on the fly adjustment to fine tune. Thanks for your concerns.
No problem Roj I understand what you're saying, at least it sounds like you have top advice on this, but I guess the Factory's warning should apply to anyone just taking a cursory look at this thread, don't mess with the sizes of the brake cylinders if you don't know what you're doing!
Here's an as yet untested top tip that someone gave me that might help ( if it works!) as a cheap self test to see what your brake bias is: put the car on 4 axle stands, get something that you can press the brake pedal with that can be wedged in place so it keeps the pedal pressed. Then put a torque wrench on a wheel nut and see what setting it takes until you are able to turn the wheel slightly, use it on all 4 wheels and take an average for the front and back and then you can work out what the percentage from front to back is (obviously you don't want the pedal pressed as hard as possible otherwise you'll never turn the wheels but as long as the pedal is pressed at a constant rate then this should give a rough guide for you).
The other thing is to make sure you use a wheel but in the same position for each wheel and turn the wheels forward on both sides.
Here's an as yet untested top tip that someone gave me that might help ( if it works!) as a cheap self test to see what your brake bias is: put the car on 4 axle stands, get something that you can press the brake pedal with that can be wedged in place so it keeps the pedal pressed. Then put a torque wrench on a wheel nut and see what setting it takes until you are able to turn the wheel slightly, use it on all 4 wheels and take an average for the front and back and then you can work out what the percentage from front to back is (obviously you don't want the pedal pressed as hard as possible otherwise you'll never turn the wheels but as long as the pedal is pressed at a constant rate then this should give a rough guide for you).
The other thing is to make sure you use a wheel but in the same position for each wheel and turn the wheels forward on both sides.
Edited by chucknorris on Thursday 14th May 10:02
Top tip Pete, some of the mechanics used to do that but I'd forgotten about it. Also I checked the build manual this morning and there is a specific measurement for setting the correct brake bias on an Ultima ( I assume this is a dry setting). It relates to the amount of screw thread that should be protruding on the right for right hand drive cars.
Yes that's the factory setting for a 'starting' point after the first IVA mine is set up pretty much 60/40 to the front but the bias bar is more that the 21 or 22mm that the factory suggests.
I'd love to know if the torque wrench method works, i may try it later to see if it does and the fact that I know the bias is correct from Monday's IVA will help prove if it works or not
I'd love to know if the torque wrench method works, i may try it later to see if it does and the fact that I know the bias is correct from Monday's IVA will help prove if it works or not
chucknorris said:
Also, I'm not an expert on brake systems, but my engineering knowledge would dictate that a larger master cylinder would push more brake fluid and the surface area of the piston is also bigger so would provide more pressure with less effort, hence you would have a bigger cylinder on the front to apply more braking force to the front... Unless It works in completely different way when it comes to brake systems.
not trueLogic would tell you that a larger bore size in a master cylinder should produce more pressure but that is actually not true. Because of fluid velocity and fluid movement in most cases the smaller the bore of the master cylinder means the more pressure you are going to have at the caliper assembly or the wheel cylinder assembly. Whereas if you have a larger bore master cylinder it is actually going to produce less pressure, its actually going to give you a harder pedal feel as an end result.
Pressure is measured in a "Force per unit area" fashion. For example, Newtons per Square Milimeter.
So, you apply say 50Kg of force to the brake pedal, which has a lever ratio of say 3 to 1 (pedal pad where your foot pushes is 3x futher away from the pivot than where the master cylinder rod attached) so you get 150kg or roughly 1500Newtons of force pressing the master cylinder piston inwards.
If that master cylinder has an area of 100mm2 that's 15N/mm2
if that master cylinder has an area of 50mm2 that's 30N/mm2
So a smaller master cylinder creates a higher hydraulic pressure.
The trade off is that to move the same volume of fluid, you must move it further. In effect, the work done at the brake pedal is "Force x Distance". And that work can be extracted as a lot of force over a small distance, or less force over a larger distance.
A typical modern passenger car uses a vacuum brake booster, that typically roughly doubles the drivers pedal input. (so driver pushed with 50kg, brake booster adds another 50kg for a total of 100kg) This enables modern brake systems to have high pressure AND a short travel pedal.
So, you apply say 50Kg of force to the brake pedal, which has a lever ratio of say 3 to 1 (pedal pad where your foot pushes is 3x futher away from the pivot than where the master cylinder rod attached) so you get 150kg or roughly 1500Newtons of force pressing the master cylinder piston inwards.
If that master cylinder has an area of 100mm2 that's 15N/mm2
if that master cylinder has an area of 50mm2 that's 30N/mm2
So a smaller master cylinder creates a higher hydraulic pressure.
The trade off is that to move the same volume of fluid, you must move it further. In effect, the work done at the brake pedal is "Force x Distance". And that work can be extracted as a lot of force over a small distance, or less force over a larger distance.
A typical modern passenger car uses a vacuum brake booster, that typically roughly doubles the drivers pedal input. (so driver pushed with 50kg, brake booster adds another 50kg for a total of 100kg) This enables modern brake systems to have high pressure AND a short travel pedal.
It's also worth noting that "static" brake bias is almost irrelevant.
The IVA test requires the front brakes to ALWAYS lock before the rears (to avoid cars spinning from locked rear tyres) but depending on the deceleration level at the point of locking, the amount of rear bias required to do this changes! On a clean dry surface, you can brake very hard without locking brakes, and hence there will be a lot of forwards weight transfer because there is a lot of longitudinal decceleration. Hence, you will need to wind your brake bias forwards to prevent the rears locking first (the brake system puts a higher percentage of force into the front brakes)
But in the wet say, or on slippy roads, you can't get to the same high level of decceleration, and hence you could wind the bias rewards (less deccel = more weight on rear tyres) and get a better overall stopping distance.
Passenger cars often use mechanical bias valves that sense the position of the rear suspension and dynamically limit the rear brake pressure, or these days it's all done electronically in the ABS system. The aim is to have all 4 tyres kept on the limit of grip during a brake event to get the shortest possible overall stopping distance.
The IVA test requires the front brakes to ALWAYS lock before the rears (to avoid cars spinning from locked rear tyres) but depending on the deceleration level at the point of locking, the amount of rear bias required to do this changes! On a clean dry surface, you can brake very hard without locking brakes, and hence there will be a lot of forwards weight transfer because there is a lot of longitudinal decceleration. Hence, you will need to wind your brake bias forwards to prevent the rears locking first (the brake system puts a higher percentage of force into the front brakes)
But in the wet say, or on slippy roads, you can't get to the same high level of decceleration, and hence you could wind the bias rewards (less deccel = more weight on rear tyres) and get a better overall stopping distance.
Passenger cars often use mechanical bias valves that sense the position of the rear suspension and dynamically limit the rear brake pressure, or these days it's all done electronically in the ABS system. The aim is to have all 4 tyres kept on the limit of grip during a brake event to get the shortest possible overall stopping distance.
Hi Pete
Please let me know if the torque wrench works. I will try it Monday or Tuesday next week.
Dom you are quite correct but I think I covered that in an earlier post hence 0.625 front and 0.7 rear.
Other technical points noted but some of it a bit over my head, as I said earlier I am taking top technical advice on these changes along with my own experience.
Its nice to know that those posting on here are prepared to stand up and say if they have concerns especially safety ones. Thanks guys.
Please let me know if the torque wrench works. I will try it Monday or Tuesday next week.
Dom you are quite correct but I think I covered that in an earlier post hence 0.625 front and 0.7 rear.
Other technical points noted but some of it a bit over my head, as I said earlier I am taking top technical advice on these changes along with my own experience.
Its nice to know that those posting on here are prepared to stand up and say if they have concerns especially safety ones. Thanks guys.
Roj, I ran out of time today, but I'm definitely going to try it tomorrow...I'll let you know.
Max: much respect for that full explanation, that sounds like you are well clued up on the subject....however 😖 ... I'm curious about two things, have I got it right that the factory suggest putting the larger cylinder on the front, if so, why would they do that if by your calculations it gives less pressure (if I have that wrong and its the smaller cylinder on the front, then I'm a total goofball)
Secondly, I still can't get my head round the figures (and I'm not doubting you for a minute I promise) but with the larger cylinder, surely the fact that you're pushing a larger amount of fluid through the same size pipe as the rear brakes, that would then give you a higher hydraulic pressure in the front...( again that is a totally irrelevant question if I've got it wrong about the bigger cylinder being on the front...abuse is deserved if I have that wrong) 😉
Max: much respect for that full explanation, that sounds like you are well clued up on the subject....however 😖 ... I'm curious about two things, have I got it right that the factory suggest putting the larger cylinder on the front, if so, why would they do that if by your calculations it gives less pressure (if I have that wrong and its the smaller cylinder on the front, then I'm a total goofball)
Secondly, I still can't get my head round the figures (and I'm not doubting you for a minute I promise) but with the larger cylinder, surely the fact that you're pushing a larger amount of fluid through the same size pipe as the rear brakes, that would then give you a higher hydraulic pressure in the front...( again that is a totally irrelevant question if I've got it wrong about the bigger cylinder being on the front...abuse is deserved if I have that wrong) 😉
chucknorris said:
Roj, I ran out of time today, but I'm definitely going to try it tomorrow...I'll let you know.
Max: much respect for that full explanation, that sounds like you are well clued up on the subject....however ?? ... I'm curious about two things, have I got it right that the factory suggest putting the larger cylinder on the front, if so, why would they do that if by your calculations it gives less pressure (if I have that wrong and its the smaller cylinder on the front, then I'm a total goofball)
Secondly, I still can't get my head round the figures (and I'm not doubting you for a minute I promise) but with the larger cylinder, surely the fact that you're pushing a larger amount of fluid through the same size pipe as the rear brakes, that would then give you a higher hydraulic pressure in the front...( again that is a totally irrelevant question if I've got it wrong about the bigger cylinder being on the front...abuse is deserved if I have that wrong) ??
think of it like standing in the snow... if you stand on a big door with large area it doesnt sink very far... but if you step off the door in your boots you sink deep in the snowMax: much respect for that full explanation, that sounds like you are well clued up on the subject....however ?? ... I'm curious about two things, have I got it right that the factory suggest putting the larger cylinder on the front, if so, why would they do that if by your calculations it gives less pressure (if I have that wrong and its the smaller cylinder on the front, then I'm a total goofball)
Secondly, I still can't get my head round the figures (and I'm not doubting you for a minute I promise) but with the larger cylinder, surely the fact that you're pushing a larger amount of fluid through the same size pipe as the rear brakes, that would then give you a higher hydraulic pressure in the front...( again that is a totally irrelevant question if I've got it wrong about the bigger cylinder being on the front...abuse is deserved if I have that wrong) ??
to push the door down in the snow to the same depth as you in the boots it takes alot more weight ie pressure
dom
Yep, its all about leverage. Think of the smaller master cylinder as a bigger lever, you can apply more pressure at teh cost of more pedal feel. I actually reduced the size of the master cylinder on my old vw to make the pedal feel softer but more travel and more control (no servo!).
However because there are separate circuits and master cylinders on the ultima for front and back its about getting the ratio right between the master cylinders. IE putting a bigger master cylinder on the front will create more pressure on the front in relation to the back for a given pedal pressure, BUT will create a stiffer pedal and LESS pressure on the braking system with the same pedal effort as a whole because you have reduced leverage.
Make sense?
However because there are separate circuits and master cylinders on the ultima for front and back its about getting the ratio right between the master cylinders. IE putting a bigger master cylinder on the front will create more pressure on the front in relation to the back for a given pedal pressure, BUT will create a stiffer pedal and LESS pressure on the braking system with the same pedal effort as a whole because you have reduced leverage.
Make sense?
i've got it now (doh!)the smaller cylinder requires it to be compressed further compared to the larger cylinder in order to move the same amount of fluid into the caliper. This means it takes less effort to push the front brake cylinder compared to the rear....so why does the factory fit the smaller cylinder to the rear and screw most of the brake bias to the front to compensate. Would it not make sense to fit them the other way round and then the bias could be more neutral to get the same 60/40 to the front....
ARRGH!...Now i'm considering doing something that is definitely not recommended!
ARRGH!...Now i'm considering doing something that is definitely not recommended! WHat you care about is the longitudinal force your braking system can generate at the tyre contact patch (ie the bit of the tyre touching the road)
This is not just dependant upon the size of the mastery cylinders!
Step 1: Driver presses on brake pedal - lets say with 50kg (typically that would give you a 1g stop in a normal passenger car with brake servo etc)
Step 2: Brake pedal leverage ratio (between 2 and 3 to 1 usually) multiplies drivers foot force, and applies that to the master cylinders, lets say now 150kg. (if you have a brake balance bar and seperate front and rear brake cylinder, the amount of force that each cylinder gets is controlled by the position of the pedal input point. If it is exactly in the middle of the two cylinders each cylinder gets 50% of the force. if you moved it towards the front cylinder, that would take a large proportion of the force, and the rear cylinder would get less. For now, lets say the balance bar is centred, so each master cylinder gets half the load, for 75kg on each (or roughly 750 Newtons)
Step3: The Master cylinder converts the axial load applied to them into a pressure in the brake fluid, depending on its diameter (and hence surface area of the piston in the cylinder). Lets say we have a master cylinder with a 100mm2 piston area (11.4mm diameter). So, with 750N being applied, that's 7.5N/mm2 (which is 75bar, or 75 times atmospheric pressure!)
Step 4: Being hydraulic and hence incompressible, the brake fluid transfers that pressure to all parts of the system, including the backs of the pistons in the brake calipers. Let's take a 4 pot caliper, with 40mm pistons. Each piston is about 1250mm2 in area, so that's a total of 5000mm2 of piston area. Now we do our pressure sums the other way around: 7.5N/mm2 x 5000 = 37,500N (or 3.75 tonnes!!!) That is the clamping force applied to the brake disc by the caliper.
Step 5: The brake pads: Loaded with the 37.5kN force press into the surface of the brake disc spinning beneath them. If we take a steel brake disc, and typical normal road brake pads, we would expect a dynamic co-efficient of friction (ie, when the disc is sliding between the pads) of typically 0.30 which means 30% of the clamping load is generated as a force opposite to the radial direction of the spinning disc. In this case, that's 0.30 x 37.5kN = 11.25kN.
Step 6: The brake disc: We now have a figure for the force opposing the spinning disc, and this force can be assumed (for the purposes of simplicity) to be applied at the "average" diameter of the brake pads on the spinning disc. For example, say you have 300mm outside diameter discs, and pads which are 50mm "wide" we an assume the force is practically applied at a radius of 125mm ((300mm - 50mm)/2). This enables us to work out the brake torque applied to the disc. In our case, we have 11.25kN at a radius of 0.125m, which is a brake torque of 1.4kNm (1400Nm).
Step 7: Getting that force to the ground: Depending on the effective diameter of the wheel/tyre, that brake torque, which is around the centre point of the wheel rotation (the wheel centre/bearings) acts down where the tyre touches the road. If we have say 235/40/18 tyres, they are 645mm diameter, or a radius of 0.3225m. We know the brake torque is 1.4kNm, so at that radius this is a longitudinal decceleration force of about 4300N, (~430Kg).
Of if we assume that all 4 "corners" of the car have the same brakes and tyres etc) the total brake force is 4 x 430kg, which is 1720Kg. If our virtual car weighs lets say 1200Kg, assuming our tyres were sticky enough, that would generate a vehicle deceleration of 1.43g. (in reality, at 1.4g deccel, the rear tyres would most likely be unable to transfer that kind of load and lock up)
So, you can see that you cannot consider any single part of the brake system in isolation. It is, in effect, a complex system of levers, where component sizes, including wheels and tyres, make a big difference to the brake force for any given wheel.
The chances are, that the Ultima has a smaller rear master cylinder because it has rear brake calipers with a lot less piston area than at the front. And being rear/mid engineed, it can obviously use it's rear brakes more than a normal car with the engine at the front (and it's stiff, and has a low Centre of Gravity, meaning less dynamic weight transfer forwards during decceleration etc)
;-)
This is not just dependant upon the size of the mastery cylinders!
Step 1: Driver presses on brake pedal - lets say with 50kg (typically that would give you a 1g stop in a normal passenger car with brake servo etc)
Step 2: Brake pedal leverage ratio (between 2 and 3 to 1 usually) multiplies drivers foot force, and applies that to the master cylinders, lets say now 150kg. (if you have a brake balance bar and seperate front and rear brake cylinder, the amount of force that each cylinder gets is controlled by the position of the pedal input point. If it is exactly in the middle of the two cylinders each cylinder gets 50% of the force. if you moved it towards the front cylinder, that would take a large proportion of the force, and the rear cylinder would get less. For now, lets say the balance bar is centred, so each master cylinder gets half the load, for 75kg on each (or roughly 750 Newtons)
Step3: The Master cylinder converts the axial load applied to them into a pressure in the brake fluid, depending on its diameter (and hence surface area of the piston in the cylinder). Lets say we have a master cylinder with a 100mm2 piston area (11.4mm diameter). So, with 750N being applied, that's 7.5N/mm2 (which is 75bar, or 75 times atmospheric pressure!)
Step 4: Being hydraulic and hence incompressible, the brake fluid transfers that pressure to all parts of the system, including the backs of the pistons in the brake calipers. Let's take a 4 pot caliper, with 40mm pistons. Each piston is about 1250mm2 in area, so that's a total of 5000mm2 of piston area. Now we do our pressure sums the other way around: 7.5N/mm2 x 5000 = 37,500N (or 3.75 tonnes!!!) That is the clamping force applied to the brake disc by the caliper.
Step 5: The brake pads: Loaded with the 37.5kN force press into the surface of the brake disc spinning beneath them. If we take a steel brake disc, and typical normal road brake pads, we would expect a dynamic co-efficient of friction (ie, when the disc is sliding between the pads) of typically 0.30 which means 30% of the clamping load is generated as a force opposite to the radial direction of the spinning disc. In this case, that's 0.30 x 37.5kN = 11.25kN.
Step 6: The brake disc: We now have a figure for the force opposing the spinning disc, and this force can be assumed (for the purposes of simplicity) to be applied at the "average" diameter of the brake pads on the spinning disc. For example, say you have 300mm outside diameter discs, and pads which are 50mm "wide" we an assume the force is practically applied at a radius of 125mm ((300mm - 50mm)/2). This enables us to work out the brake torque applied to the disc. In our case, we have 11.25kN at a radius of 0.125m, which is a brake torque of 1.4kNm (1400Nm).
Step 7: Getting that force to the ground: Depending on the effective diameter of the wheel/tyre, that brake torque, which is around the centre point of the wheel rotation (the wheel centre/bearings) acts down where the tyre touches the road. If we have say 235/40/18 tyres, they are 645mm diameter, or a radius of 0.3225m. We know the brake torque is 1.4kNm, so at that radius this is a longitudinal decceleration force of about 4300N, (~430Kg).
Of if we assume that all 4 "corners" of the car have the same brakes and tyres etc) the total brake force is 4 x 430kg, which is 1720Kg. If our virtual car weighs lets say 1200Kg, assuming our tyres were sticky enough, that would generate a vehicle deceleration of 1.43g. (in reality, at 1.4g deccel, the rear tyres would most likely be unable to transfer that kind of load and lock up)
So, you can see that you cannot consider any single part of the brake system in isolation. It is, in effect, a complex system of levers, where component sizes, including wheels and tyres, make a big difference to the brake force for any given wheel.
The chances are, that the Ultima has a smaller rear master cylinder because it has rear brake calipers with a lot less piston area than at the front. And being rear/mid engineed, it can obviously use it's rear brakes more than a normal car with the engine at the front (and it's stiff, and has a low Centre of Gravity, meaning less dynamic weight transfer forwards during decceleration etc)
;-)
Hi Max
Thanks for the technical info which mostly went straight over my head. Thats why I rely on my own experience i.e. feel, and where I don't understand something then speak to people who fully understand the technical side.
Regarding the Ultima the callipers and discs are the same size all round hence one of the reasons for the discussion on the master cylinders.
Thanks for the technical info which mostly went straight over my head. Thats why I rely on my own experience i.e. feel, and where I don't understand something then speak to people who fully understand the technical side.
Regarding the Ultima the callipers and discs are the same size all round hence one of the reasons for the discussion on the master cylinders.
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