Alcon TA6 Calipers on R33 GTST - Flexing

[Nismo32]

Hello SAU,

I recently purchased an Alcon TA6 kit through a QLD company who supplies the dog bones/rotors etc. The kit has 356x36mm rotors - aluminium dog bones - TA6 calipers. I used the HEL brake lines I already have. Kit appears to be of high quality.....as much as visually i guess it can be.

What I have noticed after install and pressure bleeding (and pedal bleeding) multiple times is that the calliper is flexing a decent amount when the pedal is pushed - enough that i can push the pedal to the floor as the calliper just keeps on flexing.

Details:

1. OEM master cylinder @ 15/16" (BM44 - non ABS GTST)

2. OEM everything else on brake system - rear callipers etc. Just braided lines (front/rear) and the TA6 front kit

3. The TA6 calliper has ~2% smaller piston area than the OEM 4 pot sumitomo calliper - so no issue there.....i dont think. TA6 is 27/31.8/38.1 and OEM is 40.4/40.4

4. Have checked the calliper flex with car on (booster assistance) and off - same result just have to push harder with car off

5. Have unbolted calliper from the dog bone so its just resting on the disc and tested again - same thing - so flex of the dog bone doesnt seem to be an issue

6. Tried to measure pedal ratio as best i could - looks to be about 4.8:1 with some margin for error - ignoring the booster with 50kg of leg (beeeefy - but with booster probably fairly achievable) thats about 770psi of brake pressure which doesnt seem super high - but maybe it is?? So 75kg of leg = 1150psi.

I guess my question is - is this normal? Alcon distributors over here have been helpful but ultimately have said its normal. I feel like modulation is bad as only the first 30-40% of pedal pressure/travel does anything. After that its a mix of flex....and maybe some extra clamp force.

My old setup - Ksport 8 pots with 12 years of track work on them did not have this issue - pedal was solid. Only reason for change was a hub failure caused the disc to tear one of the callipers to bits.

Thanks SAU - any thoughts/help would be appreciated.

Edited April 29, 2024 by Nismo32

[GTSBoy]

Pedal to floor is not caliper flex. Pedal to floor is hydraulic leaking/bypassing.

You quite possibly stuffed the seals in the MC when you were bleeding. A common mistake the unitiated make is to push the pedal down too far when bleeding with an MC that has been in use a long time. Crud builds up on the MC wall just past the furthest it is ever pushed in use. That crud can be hard enough to damage the seals when you push the piston down past when bleeding.

Might be time for a kit in the MC.

[Dose Pipe Sutututu]

Just watched the video, that's wild!

Could it be there's some tiny air pockets in the caliper causing unequal force when the pedal is pressed?

[Nismo32]

1 hour ago, GTSBoy said:

Pedal to floor is not caliper flex. Pedal to floor is hydraulic leaking/bypassing.

You quite possibly stuffed the seals in the MC when you were bleeding. A common mistake the unitiated make is to push the pedal down too far when bleeding with an MC that has been in use a long time. Crud builds up on the MC wall just past the furthest it is ever pushed in use. That crud can be hard enough to damage the seals when you push the piston down past when bleeding.

Might be time for a kit in the MC.

Hey GTSBoy - I used a pressure bleeder to bleed the kit - didnt do pedal bleeding at first. Issue was still there. When you push the pedal you get the caliper flex - so the fluid is going to the caliper - its not bypassing within the master itself

[Nismo32]

1 hour ago, Dose Pipe Sutututu said:

Just watched the video, that's wild!

Could it be there's some tiny air pockets in the caliper causing unequal force when the pedal is pressed?

To be fair im not sure - i have bled the system a lot of times - the pressure bleeder i use has worked a treat on previous setups on this car. So i would assume its doing the job here also......

I did also bleed with the pedal later on just to be "sure" - didnt make a difference.

[Dose Pipe Sutututu]

14 minutes ago, Nismo32 said:

To be fair im not sure - i have bled the system a lot of times - the pressure bleeder i use has worked a treat on previous setups on this car. So i would assume its doing the job here also......

I did also bleed with the pedal later on just to be "sure" - didnt make a difference.

my bro-science approach tells me there's unequal force being applied from either side, hence the movement. If equal force is being applied, one would assume there would be 0 movement.

 

[GTSBoy]

Yeah, I just watched the video also (I don't generally watch embed videos because I have to enable them in NoScript before doing so).

That's not "caliper flex" in the classic sense. Caliper flex is where the body of the caliper spread because it is not stiff enough to resist being pushed apart by the forced imposed at the piston/pad interface.

That whole caliper is moving. That probably means that the dogbone is flexing, which would point to it being too flimsy and/or simply misaligned so that the caliper has no option but to try to centre itself on the disc.

The pedal sinking to the floor would have to be unrelated.

[Nismo32]

6 minutes ago, GTSBoy said:

Yeah, I just watched the video also (I don't generally watch embed videos because I have to enable them in NoScript before doing so).

That's not "caliper flex" in the classic sense. Caliper flex is where the body of the caliper spread because it is not stiff enough to resist being pushed apart by the forced imposed at the piston/pad interface.

That whole caliper is moving. That probably means that the dogbone is flexing, which would point to it being too flimsy and/or simply misaligned so that the caliper has no option but to try to centre itself on the disc.

The pedal sinking to the floor would have to be unrelated.

I did think of this - i unbolted the caliper from the dogbone so it was just resting on the disc and the result was the same - the caliper spread about the disc. Ill do this again and get some video this time. 

I did also use a digital caliper when fitting the kit - the misalignment of the disc to the caliper was <0.5mm. The shims I have are 0.5mm so i wasnt really able to improve alignment. Seemed pretty good off the bat really.

I tell ya - this has me thoroughly confused

[Nismo32]

25 minutes ago, Dose Pipe Sutututu said:

my bro-science approach tells me there's unequal force being applied from either side, hence the movement. If equal force is being applied, one would assume there would be 0 movement.

 

This would be the only reason the dogbone would flex - pushing/pulling it based on unequal load. Not sure how to check for this besides putting a gauge on each bleed nipple and giving it the berries to see what pressure it reads.

[GTSBoy]

Take the pads out, one side at a time, and see that the pistons on the padless side move properly when you press the pedal. Just to make sure that they're all doing the right thing. You can restrain 2 out of 3 pistons on a side if you need to make sure that one of them is moving. You might have too because if there is one pistons that is much easier to move than the others, then it will and they won't. Same (but opposite) with one that is much stickier than the others. You might find that one side is not working properly.

To be clear, I do not think the caliper is spreading. It seriously only looks like it is moving side to side. To spread it has to bend around an axis formed by a line through the bridge bolts (basically the line between the two caliper halves along the top. (I'd hate to think that a brand new Alcon caliper would be that bendy.) That implies the piston related problems I speculate above.

[GTSBoy]

Both sides of the caliper properly bled? I know it can be hard to convince yourself yes/no to that question.

[Dose Pipe Sutututu]

17 minutes ago, GTSBoy said:

Both sides of the caliper properly bled? I know it can be hard to convince yourself yes/no to that question.

I reckon there's a cheeky air pocket somewhere on the side facing the body

[Murray_Calavera]

Any chance you have access to a vacuum bleeder? 

(something like this)

https://www.supercheapauto.com.au/p/toledo-toledo-brake-bleeder-and-fluid-extractor-1-litre/540782.html?gad_source=1&gclid=Cj0KCQjwir2xBhC_ARIsAMTXk848QmzMiDD5_EO8X8AgI11x2F7mmv9Ty9hDU8PZPOcDBm2yNyZDVVsaAtHaEALw_wcB&gclsrc=aw.ds

I've had great success with this bit of kit, it's never let me down and its bailed out a few mates that couldn't manually bleed a stuck air bubble out. 

[Nismo32]

37 minutes ago, GTSBoy said:

Take the pads out, one side at a time, and see that the pistons on the padless side move properly when you press the pedal. Just to make sure that they're all doing the right thing. You can restrain 2 out of 3 pistons on a side if you need to make sure that one of them is moving. You might have too because if there is one pistons that is much easier to move than the others, then it will and they won't. Same (but opposite) with one that is much stickier than the others. You might find that one side is not working properly.

To be clear, I do not think the caliper is spreading. It seriously only looks like it is moving side to side. To spread it has to bend around an axis formed by a line through the bridge bolts (basically the line between the two caliper halves along the top. (I'd hate to think that a brand new Alcon caliper would be that bendy.) That implies the piston related problems I speculate above.

Good plan. I will do that for sure. havent actually checked to see that all things are happening as they should on each side of the caliper!

And yes, i agree, it seems super odd that a caliper can be so bendy. Im just struggling to find an alterante explanation.

[Nismo32]

58 minutes ago, GTSBoy said:

Both sides of the caliper properly bled? I know it can be hard to convince yourself yes/no to that question.

Yep - i went from furthest away to closest to MC - both sides of each caliper - then came back and did it again to make sure.

[Nismo32]

18 minutes ago, Murray_Calavera said:

Any chance you have access to a vacuum bleeder? 

(something like this)

https://www.supercheapauto.com.au/p/toledo-toledo-brake-bleeder-and-fluid-extractor-1-litre/540782.html?gad_source=1&gclid=Cj0KCQjwir2xBhC_ARIsAMTXk848QmzMiDD5_EO8X8AgI11x2F7mmv9Ty9hDU8PZPOcDBm2yNyZDVVsaAtHaEALw_wcB&gclsrc=aw.ds

I've had great success with this bit of kit, it's never let me down and its bailed out a few mates that couldn't manually bleed a stuck air bubble out. 

I do - but have better results with the pressure bleeder. I could try the vac bleeder and see if it gets any other bubbles out. Will see if i can find it.....got relegated a while ago!!

[Komdotkom]

Looks to be  a two part problem to me. Air behind pistons on one side of the caliper and a flexible dog bone. I know all the cool kids run alloy dog bones for the added lightness but I prefer steel because I've had problems with an alloy kit before (chattering).

This sounds extreme but I'd force all the pistons right back in the bores, bleed the caliper on the bench, re-install and bleed the system again. Assuming there are no AKB springs in these?

[Dose Pipe Sutututu]

37 minutes ago, Komdotkom said:

This sounds extreme but I'd force all the pistons right back in the bores, bleed the caliper on the bench, re-install and bleed the system again

sounds like a good solid plan actually

[Nismo32]

2 hours ago, Dose Pipe Sutututu said:

sounds like a good solid plan actually

Agreed. I will do this....although not sure how to bench bleed a caliper. Will have to find some gear or use the little vac pump I have.

There are AKB springs in these. Although I feel like they are only on the large piston....although I could be wrong there. So can't push them all the way back.

[GTSBoy]

3 hours ago, Nismo32 said:

although not sure how to bench bleed a caliper

Buy a very large syringe and some small bore PVC hose.