Alcon TA6 Calipers on R33 GTST - Flexing

[Komdotkom]

The most important thing to do is to ensure that the pistons are fully back in the bores so that you can't get air bubbles forming behind them. If you've got AKB springs then you'll need to make some timber wedges to hold them back which is pretty simple - but you'll need wedges for bleeding anyway to stop the pistons moving up the bore.

I'm a big fan of bleeding calipers from the 'bottom' because bubbles like to go 'up'. Since you're going to have the calipers off the car and mounted to some sort of fixture (unless you've got 5 sets of hands) I'd suggest that you bleed it by introducing fluid from both directions and with the caliper in both orientations. Get a barb for the end of your braided line and go to the chemist and get a 100ml syringe to force the fluid through with. You can use any old shit fluid for this (*new) because you'll displace it once you bleed the system properly later on with SRF or similar. Probably worth cracking the transfer pipe too just to be sure.

Once you've got brake fluid everywhere and no more bubbles stick the caliper back on the car and bleed the fluid up from the caliper to the MC.

[Nismo32]

26 minutes ago, Komdotkom said:

The most important thing to do is to ensure that the pistons are fully back in the bores so that you can't get air bubbles forming behind them. If you've got AKB springs then you'll need to make some timber wedges to hold them back which is pretty simple - but you'll need wedges for bleeding anyway to stop the pistons moving up the bore.

I'm a big fan of bleeding calipers from the 'bottom' because bubbles like to go 'up'. Since you're going to have the calipers off the car and mounted to some sort of fixture (unless you've got 5 sets of hands) I'd suggest that you bleed it by introducing fluid from both directions and with the caliper in both orientations. Get a barb for the end of your braided line and go to the chemist and get a 100ml syringe to force the fluid through with. You can use any old shit fluid for this (*new) because you'll displace it once you bleed the system properly later on with SRF or similar. Probably worth cracking the transfer pipe too just to be sure.

Once you've got brake fluid everywhere and no more bubbles stick the caliper back on the car and bleed the fluid up from the caliper to the MC.

Thanks for that Komdotkom

Does anyone by chance know what the thread is on the end of the hard line of the GTST front brake system before it goes to the braided line - will need to get a fitting to put in there as you say.

Could you do this with the caliper on the car? Just get the pistons all the way back with a spacer or even some clamps? Im running a 36mm disc and 18mm pads - the pistons are almost as far back as they can get but i think with some old pads and a wedge I can make sure. Just easier if the caliper is mounted to something.

[Komdotkom]

Stick the caliper in your vice with soft jaws, that way you can turn it upside down simply.

I think the factory lines are 10x1mm but I'd just take the whole thing down to your local Pirtek/MSCN and get them to sort it out.

[Komdotkom]

Did you get this sorted out?

I'd be interested to hear what you think of this brake upgrade, I'm currently looking for a new setup. With the Yen on it's knees I'm tempted by an Endless kit at the moment.

[Nismo32]

Nah never got it sorted. I have clamped all the pistons fully back using g-clamps. Pressure bled, vacuum bled, tapped it with a mallet, sung to the gods of deceleration.....no difference.

My only real conclusion is the brake pressure from the car is to much for the calliper. 

So I would definitely not buy these callipers. Endless would be a better option especially if it doesn't have so much lightening done to the calliper by ways of material removal.

As a side note the new D2 1 piece calliper does exactly the same thing as the Alcon. So there are other callipers that have the same issue. If anything the D2 was worse.

[MBS206]

It's really interesting how much it's moving left to right, and doesn't appear to be actually spreading.

For this to be moving left to right, without flaring, means unequal forces being applied until equilibrium hits in one way or another.

 

Reasons it could be unequal;

Piston sizes left to right are different sizes. Only minute differences.

Pistons moving on different planes. That is, are they moving directly straight at each other, or slightly out of alignment.

The fluid is restricted getting to one side.

 

Some tests you could do. Pull the bleeders on the left and right, fit each side with pressure gauges. It will allow you to make sure pressure is equal (and really, it should be as fluid flow is so damn low, so fluid flow dynamics shouldn't come into it).

 

With the calipers off the car, have both sides retracted as far as possible, measure very very accurately (think micrometre) distances between opposing pistons in four different spots per set of pistons.

Firstly, on each opposing pistons pair, the measures on for different spots should be equal. If it's not, and especially if one spot appears to actually get closer compared to another, the pistons aren't aligned, and now the forces left to right won't be equal, as they're the forces fighting each other.

 

You can also pull all the pistons out, and very very accurately measure the diameter of them. If they're not equal left to ight in the pairings, and especially if the area of the pistons on one side is different to the area on the other overall, it'll all be out of whack.

 

My bet, is shit QC in making sure the pistons move in the same plane left vs right. Some minor out of square machining, and nothing will line up, things will want to move around.

 

[Komdotkom]

While all of the above makes sense, we're not talking about some Ling Long calipers of AliExpress; Alcon would have to be one of the top 5 caliper suppliers in the world and these are not their budget units. If there were manufacturing defects in the caliper causing this issue I'd be very disappointed.

Having re-watched the video a few times I think it's dog bone/mount related. The caliper moves independently to the rest of the suspension and brake assembly which suggests to me that it's not mounted rigidly. I know this sounds stupid, but have you got enough thread on the retaining bolts or are they a poofteenth too long and it's not super tight? For axial movement to be occurring without deforming the caliper that can be the only answer.

Edited June 4 by Komdotkom

[GTSBoy]

Yes, you can very clearly see from more than one angle that it is the whole caliper moving left-right, not deformation of the caliper. The top moves, the bottom moves. Equally.

It should be possible to see where the movement is occurring. Just need to look more closely.

[Nismo32]

Hello,

The dog bone wont have much to do with this - the calliper applies a load about the disc - the outside of the calliper is more flexible than the inside by virtue of it not haveing the big torqued down bolts, additional metal structure on the caliper itself, and mostly not being cantilevered. So the outside of the calliper is pushing away as its more bendy and brining the inside with it as an equal pressure on both sides of the caliper looks to cause more flex on the outside - which makes sense.

I have done this same thing with the calliper unbolted from the dog bone - same thing happens. Although id have to watch more carefully to see how much more the outside moves than the inside in this case.

Im guessing there is a point in the pedals travel where the pressure is not actually increasing in the system - its just volumne displacemnt - and because the outside of the calliper is more flexy, it receives that volume.

Have gone back to Alcon again - but beyond a resolution there I would stay clear of this calliper and possibly any of the lighweight type of callipers - especially if you have the smaller 15/16" master cylinder.

Cheers.

[Dose Pipe Sutututu]

Out of curiosity, does your car have ABS?

*** Ignore, you've posted it a non ABS BMC

[GTSBoy]

But the caliper is clearly NOT FLEXING. The whole bloody thing is moving. Both sides are moving relative to the disc.

[Nismo32]

9 minutes ago, GTSBoy said:

But the caliper is clearly NOT FLEXING. The whole bloody thing is moving. Both sides are moving relative to the disc.

Hey GTSBoy.

What would be the reason for that - if the entire caliper is moving to one side because of the dog bone being flexy - what would actually cause that? If a caliper has no physical flex as the pressure on both sides is even - then why would it move at all? Keen to get thoughts - not having a go. I just cant see the mechanism that would cause the entire caliper to shimy to one side.

[Komdotkom]

Setup a dial indicator on the dogbone and pump the pedal. If that's not moving then the caliper is moving in the radial mounts.

If you think about it, the movement you are seeing (3-5mm) is miles above the elastic limit of the caliper material, so you'd be seeing cracks if this was a flex issue.

[GTSBoy]

52 minutes ago, Komdotkom said:

Setup a dial indicator on the dogbone and pump the pedal. If that's not moving then the caliper is moving in the radial mounts.

If you think about it, the movement you are seeing (3-5mm) is miles above the elastic limit of the caliper material, so you'd be seeing cracks if this was a flex issue.

This ^.

There is massive movement somewhere. If the caliper was flexing you would see it spreading out at the bottom, as if there was a hinge through the top, above the pad pins. That's not what we're seeing.