12.7" AP brakes 'stick'
Discussion
Hello
My Ultima Sport had these brakes professionally fitted 500 miles ago. The one problem I've had, and which I was hoping would dissappear with use, is that both front brakes sometimes stick on (i.e. simultaneously). For example, slowing for some traffic, braking medium hard then releasing the pedal will on occasion (maybe every 5th time) result in the brakes staying on, the retardation remaining constant despite the foot being well away from the pedal. The pads will then remain clamped on and there's two ways of freeing them:
- hit the pedal hard once or twice, this unsticks them smoothly but suprises any traffic behind !
- drive off from stationary against the resistance, there's a bit of squeal, then a noticable unsticking sensation, safer but a bit clutch wearing !
I hope you can at least give me some pointers. For example, I have not as yet had opportunity to heat-cycle the brakes as described in the documentation. But I suspect this is nothing to do with their sticking.
Thanks
Roger.
My Ultima Sport had these brakes professionally fitted 500 miles ago. The one problem I've had, and which I was hoping would dissappear with use, is that both front brakes sometimes stick on (i.e. simultaneously). For example, slowing for some traffic, braking medium hard then releasing the pedal will on occasion (maybe every 5th time) result in the brakes staying on, the retardation remaining constant despite the foot being well away from the pedal. The pads will then remain clamped on and there's two ways of freeing them:
- hit the pedal hard once or twice, this unsticks them smoothly but suprises any traffic behind !
- drive off from stationary against the resistance, there's a bit of squeal, then a noticable unsticking sensation, safer but a bit clutch wearing !
I hope you can at least give me some pointers. For example, I have not as yet had opportunity to heat-cycle the brakes as described in the documentation. But I suspect this is nothing to do with their sticking.
Thanks
Roger.
...this sounds like your brakes are hydraulic locking. I've had this happen to me when the brakes were not properly bled. Have the guys rebleed the front brakes and see if this cures the problem. To properly bleed the system, I always start with the caliper furthest from the master cylinders. Also. Each caliper will have two bleed screws. Bleed from the inner screw first and then the outer. You might watch the guys who bleed them to make sure that they are doing this. I assure you, that once properly bled, you will not have this problem.
Cheers!
-Mack
Cheers!
-Mack
It could also be how the Balance bar is set up. The way to setup the bar is to have it level when you are putting normal force on the pedal, so when it's at rest the bar may not be level. There should also be at least 1.5mm clearance on both sides of the balance tube. These measures are to make sure the bar etc does not bind, that could be your trouble.
Have now spoken to 3 Race engineers, and one of the top men at AP who deals with the F1 Boys. The smaller of the 2 master cylinders will travel further than the larger one, when the pedals at setup right. This makes the smaller cylinder produce more pressure, therefore it must go to the front wheels.
This explains why I could only get 58% on the front of my system, but yet I could get 70% on the rear. Now time to get told I'm wrong, well it's not me best you tell Shuey's Mechanic....
>> Edited by gtrclive on Thursday 20th January 18:59
Have now spoken to 3 Race engineers, and one of the top men at AP who deals with the F1 Boys. The smaller of the 2 master cylinders will travel further than the larger one, when the pedals at setup right. This makes the smaller cylinder produce more pressure, therefore it must go to the front wheels.
This explains why I could only get 58% on the front of my system, but yet I could get 70% on the rear. Now time to get told I'm wrong, well it's not me best you tell Shuey's Mechanic....
>> Edited by gtrclive on Thursday 20th January 18:59
Oh dear, here we go again. And this one is from an AP engineer.
Lots of posts on here say larger cylinder on front. The factory doc shows larger on front.
What gives? My explanation is quite different to this engineers.
We had better find the definitive answer to this so this forum gives the correct answer.
Lots of posts on here say larger cylinder on front. The factory doc shows larger on front.
What gives? My explanation is quite different to this engineers.
We had better find the definitive answer to this so this forum gives the correct answer.
canam-phil said:
Oh dear, here we go again. And this one is from an AP engineer.
We had better find the definitive answer to this so this forum gives the correct answer.
My 2c
I understand the idea is to achieve a similar braking friction per unit contact area of rubber. OK, little more on the front so they skid first but only fraction.
Now the rears are wider than the fronts. However the rears also carry proportionally more weight... so the weight per unit contact area & co-eff' tyre friction etc, front or rear is roughly similar. On the 'edge', in a tight corner, the car should slide nicely sideways, back should not swing out nor should the front slide away...... so far so good?
But again... the rears are wider than the fronts... about 30% wider. So we will need to apply 30% more braking force to the rears than the fronts to achieve (distribute) the same braking force per unit of actual contact area. This means larger MCs on the rear... 30% larger in fact.
Well it figures OK for me... though, I would feel more comfortable with validation by some other smart alec :-)
Also note this is only approximate.. as we know that weight is 'thrown forward' onto the fronts during braking. In fact, if someone knows the location of the cars CoG (height and distance from the tyre contact location), we can calc how much more weight is on the fronts, given a specific rate of retardation and compensate... by applying less pressure to the rears.
Adam.
The size of the master cylinders' piston has a direct result on brake line pressure.
However, it may not work like you would think... higher line pressure will build up on a smaller master cylinder piston to react the force applied by the pedal. A smaller master cylinder will create more brake fluid pressure but will also increase brake pedal travel. The smaller diameter cylinder requires a longer stroke to move the volume of fluid necessary to move the caliper pistons during braking.
Larger master cylinders will create less pressure but will require less travel.
So, I vote for larger master cylinder on the front.
I agree with the reasoning re brake bias ratio but you have to go back to line pressures at some point.
However, it may not work like you would think... higher line pressure will build up on a smaller master cylinder piston to react the force applied by the pedal. A smaller master cylinder will create more brake fluid pressure but will also increase brake pedal travel. The smaller diameter cylinder requires a longer stroke to move the volume of fluid necessary to move the caliper pistons during braking.
Larger master cylinders will create less pressure but will require less travel.
So, I vote for larger master cylinder on the front.
I agree with the reasoning re brake bias ratio but you have to go back to line pressures at some point.
When I was a petulant youth I bought a 1974 Lotus Elite - it had the same problem but turned out to be nothing more than collapsed brake hoses preventing the fluid from returning... This caused the disks to get hot enough to seize the wheel bearings.... I just saw the smoke, thought Lotus and ran....
Dave.
Dave.
Just on a SVA note here, forget the size of your tyres as the bloke at the SVA Station wants to see a Braking split of 60 front and tyres make no difference on his rollers, but yes they may make a differnce in the real world I'll give you that.
PS I too had a SPA Brake Pressure Gauge so could tell exactly what was happening.
PS I too had a SPA Brake Pressure Gauge so could tell exactly what was happening.
gtrclive said:Are you sure that's true Clive? The SVA test takes into account weight distribution and therefore still expects a slightly rearward distribution. I agree that he ignores tyre width.
Just on a SVA note here, forget the size of your tyres as the bloke at the SVA Station wants to see a Braking split of 60 front and tyres make no difference on his rollers, but yes they may make a differnce in the real world I'll give you that.
Bob
BobM said:
Just on a SVA note here, forget the size of your tyres as the bloke at the SVA Station wants to see a Braking split of 60 front and tyres make no difference on his rollers, but yes they may make a differnce in the real world I'll give you that.
Are you sure that's true Clive? The SVA test takes into account weight distribution and therefore still expects a slightly rearward distribution. I agree that he ignores tyre width.
Bob
When they passed my car, he showed the readings to me on the screan, but if the PC did some form of calc's based on wieght then you may be right.
I think you would want to run the smaller M.C. going to whichever you want to have the most pressure and bias. The factory should probably jump in here, but if they run the smaller to the rear/larger to the front then its because they feel the car needs more rear bias not front. I would tend to agree with this based on the weight distribution of the car, C of G height, width of the tires, and actual testing. I must warn, however, that this is with dry conditions. In the wet you might try 50%/50%.
On my older sports, I have equal size masters and my bias bar is adjusted all the way to the rear which gives 40%F/60%R. I run it between 40-45% front and 55%-60% rear. What your going to find is with all of the bias going to the front, the fronts will lock quite easily putting you into a front skid, and the rears won't even be close. 60%front/40%rear is ludicrous with this car. I've tried it.
Cheers!
-Mack
On my older sports, I have equal size masters and my bias bar is adjusted all the way to the rear which gives 40%F/60%R. I run it between 40-45% front and 55%-60% rear. What your going to find is with all of the bias going to the front, the fronts will lock quite easily putting you into a front skid, and the rears won't even be close. 60%front/40%rear is ludicrous with this car. I've tried it.
Cheers!
-Mack
Well done Layada. Seems the AP brakes are plumbed via the servos in the pods, at least the fronts definitely are, I can't see into the pod to confirm whether the rear line goes past or through the servo.
Can I just bypass the servo on the front brakes, leaving the rears as they are ?
cheers
Can I just bypass the servo on the front brakes, leaving the rears as they are ?
cheers
I have the ford brakes, but also get the "sticking" sometimes, so I feel the bias bar could be the problem, Also, interesting point about bias. I have just changed the pads on the rears which were fully worn. The fronts were almost as new! I think that it proves that the rear brakes on an ultima do a LOT of work. More than I thought. I can still lock up the fronts, but never the rears. The fact the pads wear out on the rears show the work they are doing is far more than the fronts...
layada said:
But again... the rears are wider than the fronts... about 30% wider. So we will need to apply 30% more braking force to the rears than the fronts to achieve (distribute) the same braking force per unit of actual contact area. This means larger MCs on the rear... 30% larger in fact.
Layada
Sorry I don't understand this fully.
The retard force is only proportional to the weight on the wheel and the coefficient of friction (dimensionless) and is independent of contact patch area.
The contact patch area is a function of tyre pressure and car weight on that corner so would change anyway when the corner loads up with the weight transfer.
Unless someone can prove otherwise.
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