Anti knockback springs

[Jeff_8]

Can the AP 4 piston caliper CP4567 be run without the anti knockback springs? I am having the pads drag causing excess heat. I don't know which weight springs are in there, but they are very hard to push back.

Jeff

[DaveW]

Can the AP 4 piston caliper CP4567 be run without the anti knockback springs? I am having the pads drag causing excess heat. I don't know which weight springs are in there, but they are very hard to push back.

Jeff

I removed mine from my Alcons - same reason. No issues because of it. You do then have to pay closer attention to the seals - if they get too loose, then you will have low pedal issues from knockback. In my case that takes a couple of years of hard use before the seals need replacement. If the seals are in good shape, they act as springs themselves, drawing the pads away from the rotors a tiny amount after each brake application to prevent excessive drag. Too tight or too loose, and they can cause issues.

Most of the resistance to your pushing the pistons is likely from seal drag. Typical AKB springs are not stiff enough to resist a manual effort pushing the pistons back.

[DaveW]

Can the AP 4 piston caliper CP4567 be run without the anti knockback springs? I am having the pads drag causing excess heat. I don't know which weight springs are in there, but they are very hard to push back.

Jeff

I removed mine from my Alcons - same reason. No issues because of it. You do then have to pay closer attention to the seals - if they get too loose, then you will have low pedal issues from knockback. In my case that takes a couple of years of hard use before the seals need replacement. If the seals are in good shape, they act as springs themselves, drawing the pads away from the rotors a tiny amount after each brake application to prevent excessive drag. Too tight or too loose, and they can cause issues.

Most of the resistance to your pushing the pistons is likely from seal drag. Typical AKB springs are not stiff enough to resist a manual effort pushing the pistons back.

Another possibly-related issue on brake drag is a pad compound that has a very non-constant coefficient of friction, maintaining excessive brake torque at very low contact pressure. I've seen that on rare occasions. Some earlier pad compounds were infamous for that.

[R. Pare]

Anti-knockback springs are usually only 3 or 7 pound rate, so they are not your hard pushing the piston problem.

[Rick Iverson]

Gents;

Never heard of them. Purpose, function, and necessary?

[DaveW]

Gents;

Never heard of them. Purpose, function, and necessary?

Purpose & function: to prevent pad knockback over bumps. etc., to prevent low pedal from too much pad travel.
Necessary?: sometimes - depends on a lot of things. They would not be there if they were never useful.

[Rick Iverson]

Anti-knockback springs are usually only 3 or 7 pound rate, so they are not your hard pushing the piston problem.

RP;

I believe I have second mod ICPs which you rebuilt for me, fresh on my bench. Is there any compelling reason to consider, or not consider doing this modification?

[Jeff_8]

Anti-knockback springs are usually only 3 or 7 pound rate, so they are not your hard pushing the piston problem.

The smaller pair of pistons move quite freely, but the bigger pair need a harder push.

I'm wondering if the caliper mount is slightly crooked.

Jeff

[mark elder]

I think if it was me, I would pop the pistons out, clean everything up and lub with some nice rubber grease. I would not put the springs back in. The pistons should go back in just with your thumbs (once you have got it past the seal). I would put new seals in too.

Easiest way to get the pistons out is to take the pads out and just pump the pedal. Most of the time, the pistons will come out 'most of the way' before they hit the disc. If you are unlucky, the piston will pop out completely. If so, when you do all this again, slip a spacer of, say, 1/4" plywood in where the pad came out. Brake line pressure can easily be 800psi (vs using compressed air at 150psi (and leaking to boot)).

I also wonder if you are letting the pads get too much of a taper on them. I swap pads (inboard to outboard on the same wheel) after about every 2 races. Yes, OCD.

The other thing to check is if there is free play between the brake pedal and the master cylinder. The MC piston has to be able to come completely back when your foot is off the pedal. Oddly, I have had this more with the clutch than brakes (???).

[DaveW]

The smaller pair of pistons move quite freely, but the bigger pair need a harder push.

I'm wondering if the caliper mount is slightly crooked.

Jeff

...I also wonder if you are letting the pads get too much of a taper on them. I swap pads (inboard to outboard on the same wheel) after about every 2 races. Yes, OCD.

The other thing to check is if there is free play between the brake pedal and the master cylinder. The MC piston has to be able to come completely back when your foot is off the pedal. Oddly, I have had this more with the clutch than brakes (???).

Definitely check your pads for taper wear - any more than 1/32" can cause piston binding and drag/heat. I usually check my pads for taper wear every session and flip them if necessary to even out the wear. So if your pads can be flipped that might help.

A lack of free play can cause the port to the reservoir to not become open when you let off the pedal. That will definitely cause issues such as brake pads not retracting.

[Phil Creighton]

We went through a few issues with Formula Atlantic with the DB4 at road America and ended up compromising with 2 lb AKB springs.
We found that even 4lb springs (standard) dragged a bit of top speed with the PFC pads that were dragging a tiny bit as they warmed up. These were the early days of non-asbestos pads so it was something we hadnt seen before.2lb springs were enough to keep the pedal firm.
With floating discs AKB springs are more necessary especially on bigger vented discs. Running a Ferrari 458 GT3 car at VIR World Challenge we had knock back running over the kerbs in the esses and could really do with heavier ones next time. (front discs are 15in x 1.375 so they are heavy!) I dont think I have ever run them on a FF or FC caliper - I dont know about ICP or the alloy FC calipers. Always found that bigger diameter master Cyls alleviated most pedal issues caused by disc and pad knock back.

I have always been a believer in not changing caliper seals too often as they seem to get better once they have taken a set. Doesnt mean we dont pull the pistons and clean them to stop dirt getting dragged back into the seals. We just use brake fluid to lubricate them and always use Castrol SRF for the system.
My 2 cents
Phil

[stonebridge20]

We went through a few issues with Formula Atlantic with the DB4 at road America and ended up compromising with 2 lb AKB springs.
We found that even 4lb springs (standard) dragged a bit of top speed with the PFC pads that were dragging a tiny bit as they warmed up. These were the early days of non-asbestos pads so it was something we hadnt seen before.2lb springs were enough to keep the pedal firm.
With floating discs AKB springs are more necessary especially on bigger vented discs. Running a Ferrari 458 GT3 car at VIR World Challenge we had knock back running over the kerbs in the esses and could really do with heavier ones next time. (front discs are 15in x 1.375 so they are heavy!) I dont think I have ever run them on a FF or FC caliper - I dont know about ICP or the alloy FC calipers. Always found that bigger diameter master Cyls alleviated most pedal issues caused by disc and pad knock back.

I have always been a believer in not changing caliper seals too often as they seem to get better once they have taken a set. Doesnt mean we dont pull the pistons and clean them to stop dirt getting dragged back into the seals. We just use brake fluid to lubricate them and always use Castrol SRF for the system.
My 2 cents
Phil

In 93 for the Runoffs, Stimola had me pull the springs out all together on my DB4.
I didn't reinstalled them since the car never had an issue with a low pedal.

[Rick Iverson]

Definitely check your pads for taper wear - any more than 1/32" can cause piston binding and drag/heat. I check my pads for taper wear every session and flip them if necessary to even out the wear. So if your pads can be flipped that might help.

Dave, is there a specific pattern you flip the pads?

[mark elder]

What I do is I just swap the inside pad with the outside one. Same wheel. I do not swap sides of the car (ie the pads on my left front wheel stay on the left front).

I do the fronts every second race, Dave seems to do it every race. This is for the front.

I am not sure what others do for the rear but I would do them about every 3 races.

I am about to swap pad brands so will be more rigorous (esp. given they are 3x the cost of my Mintex). So my plans are to swap the fronts after every race and the rears after every 2 races.

I would love to hear what others do. Its not a common practice though (well... no-one admits it except Dave and I).

[DaveW]

Dave, is there a specific pattern you flip the pads?

While I am running a car set of pads, I flip them inside to outside on the same caliper. I use well-worn pads plus old steel backing plates (holes drilled in them for less heat transfer to the pistons) as spacers for test and practice. For qualifying & race I use pads that don't need spacers, but I still flip them if the taper is noticeable. I do this any time I take the wheels off so the pads are visible.

When I get home I measure every pad's thickness and taper with a caliper and sort them so I am able to decide which pads to use where & when, and when they need to be scrapped.

The thicker pads in a given car-set (all the same D044 size) go on the front, and the thinner ones on the rear (obviously fronts wear faster).