The curious case of the Atom 4 brakes

[Winmoz]

One of the benefits of being a club member is access to the Members only section of the forum the Club will continue to create and commission content for its members.

IanG’s work on the issue regarding the APs, and the subsequent focused dialogue with the factory, are a great example of how marque and club can work together to improve the owner experience for all.

A personal example- my AP rear brakes now feel like an upgrade over my old Sport Brake rear set up, as the new ‘bleed’ and piston set process was carried out when the car was upgraded to R power spec. The pedal is firm with ‘bite’ from the top of its travel.

Clearly there are still questions regarding the application of the APs and we, as the Club and as individuals, will continue to post on both sides of the £30 paywall, as we seek to advance the enjoyment of ownership of the vehicles.


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[Preseh]

An update after the Bedford track day... My pedal is now back to where it was before the brake bleed Will chat to the factory on Monday and see what they say.

[speedmachine]

One of the benefits of being a club member is access to the Members only section of the forum the Club will continue to create and commission content for its members.

IanG’s work on the issue regarding the APs, and the subsequent focused dialogue with the factory, are a great example of how marque and club can work together to improve the owner experience for all.

A personal example- my AP rear brakes now feel like an upgrade over my old Sport Brake rear set up, as the new ‘bleed’ and piston set process was carried out when the car was upgraded to R power spec. The pedal is firm with ‘bite’ from the top of its travel.

Clearly there are still questions regarding the application of the APs and we, as the Club and as individuals, will continue to post on both sides of the £30 paywall, as we seek to advance the enjoyment of ownership of the vehicles.


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Wait wut? R spec power? Are you talking atom 4R? Did I miss something here

[John Scherrer]

No, you didn't miss anything about Atom 4R !

Winmoz got his Atom 3 upgraded to 350 and sidepods ..

[anx10us]

An update after the Bedford track day... My pedal is now back to where it was before the brake bleed Will chat to the factory on Monday and see what they say.

Good to meet yesterday, I'm exactly the same, I had a service, brake pedal had little to no travel, felt very different (in a good way). Now my brakes are back to the same, lots of travel and don't inspire confidence..

Will be interesting to see what they say ..

[reg]

The theory is that the dust seals are too aggressive for the master cylinders. If you look at the AP application guide these calipers aren't listed for single seaters etc. If there was a servo, I doubt the seals would be an issue. If the next level AP's were fitted, the same. What the factory are doing (so I am told) is to pump the pistons out beyond the point where the seal is at its limit. Then push the piston in a little. After this there is enough give in these seals to stop them from dragging the pistons back and create a long pedal.

I know others have changed the master cylinders and had good results. There is also a bit of a problem with the pedal box flex. So far mine are the decent after the service, but if this turns into a regular thing, i.e. to have to piss about with a bleeding process then I'll have to do something else.

[plip1953]

The theory is that the dust seals are too aggressive for the master cylinders. If you look at the AP application guide these calipers aren't listed for single seaters etc. If there was a servo, I doubt the seals would be an issue. If the next level AP's were fitted, the same. What the factory are doing (so I am told) is to pump the pistons out beyond the point where the seal is at its limit. Then push the piston in a little. After this there is enough give in these seals to stop them from dragging the pistons back and create a long pedal.

I know others have changed the master cylinders and had good results. There is also a bit of a problem with the pedal box flex. So far mine are the decent after the service, but if this turns into a regular thing, i.e. to have to piss about with a bleeding process then I'll have to do something else.

When the brake pedal is released the caliper pistons will stop pushing against the rotors, but should not retract by any more than dictated by the trueness of the rotating rotor. So for example, it the rotor runout (and hub trueness/runout can also play a part) is, say, 0.05mm (which would normally be an acceptable value) the caliper pistons would necessarily be forced to retract by that amount, but essentially not by more than that. And if anti-knockback springs are fitted (which I believe is not routinely the case with AP CP7600 family of calipers), rotor and pad might actually remain in contact, albeit under very light pressure (a typical AP anti-knockback spring is rated at 4lbs/ft).

Master cylinder type and sizing should have no bearing on the matter, although other characteristics of brake pedal feel have been described as improved by the fitting of larger diameter MCs both front and rear. Pedal box flex, or rather flex in the way the pedal box is mounted, is not insignificant, but can be reduced/eliminated by the fitting of additional bracing.

[reg]

There is negligible run out on my discs. The main point was the piston seals are too aggressive. The 7600 is a basic caliper.

Applications:
- Performance Road Front.
- Factory Big Brake Kits.

It’s annoying to have to spend time on something which was so perfect with the old car.

[plip1953]

There is negligible run out on my discs. The main point was the piston seals are too aggressive. The 7600 is a basic caliper.

Applications:
- Performance Road Front.
- Factory Big Brake Kits.

It’s annoying to have to spend time on something which was so perfect with the old car.

The piston seals in question are purely to prevent dust ingress and therefore potential contamination to the area of the main hydraulic seal. They are passive items and have no role in dictating the way the caliper pistons move. If they are acting in anything other than purely passive role the simplest thing to do would be to get rid of them.

The 7600 family of calipers are relatively basic, but perfectly capable nonetheless. They can produce good retardation rates (up around the -1.3g to -1.5g kind of territory (which is excellent on semi-slicks) and feedback I'm getting from drivers is that the pedal feel and reaction inspires confidence, although that will partly be down to the chosen pad compound and personal driving style preferences.

[reg]

Nope. But you can choose to believe that the 7600’s are fit for purpose. Obviously well proven by all of us who now have to go through ‘special’ bleeding processes and modifications to make the calipers work as well as they are supposed to do.

[plip1953]

Nope. But you can choose to believe that the 7600’s are fit for purpose. Obviously well proven by all of us who now have to go through ‘special’ bleeding processes and modifications to make the calipers work as well as they are supposed to do.

Let's agree to differ for now. I have the evidence to support my assertions and I'm more than happy to share it with you and any others that may be interested. And I believe the opportunity to do that in person will arise in just two weeks time :-)

[reg]

Sounds good to me

[robfitz]

An update after the Bedford track day... My pedal is now back to where it was before the brake bleed Will chat to the factory on Monday and see what they say.

Exactly the same after 2 days at Anglesey, or more accurately by 10:30 on the first morning!

Worth trying different pads and fluid? Or just removing the dust seals?

[speedmachine]

An update after the Bedford track day... My pedal is now back to where it was before the brake bleed Will chat to the factory on Monday and see what they say.

What did they say?

[reg]

An update after the Bedford track day... My pedal is now back to where it was before the brake bleed Will chat to the factory on Monday and see what they say.

Exactly the same after 2 days at Anglesey, or more accurately by 10:30 on the first morning!

Worth trying different pads and fluid? Or just removing the dust seals?

How depressing. I know others who have changed master cylinders and this seems to be working. I'll stand by my original comments that these AP calipers are not really meant for non servo systems. Either way, it's a bit rubbish. I'll guess I will find out next week on track.