Brake caliper flex

[sailaholic]

From a material point of view, high tensile sliders will work no better as they are no STIFFER ie the will flex the same. Stronger pins will resist PERMANENT deformation ie bending.

Aluminum alloys can be extremely sensitive to heat treatments but again it effects strength rather than stiffness unless your getting phase changes in the alloy.

Sent from my GT-I9100 using Tapatalk 2

[daevilone]

It was mentioned earlier in the thread but ignored.

Neither group 2B or 2F have restrictions on brakes.... So go get some better calipers.

[manga_blue]

As I said, I wish I could but it's not permitted. In their infinite wisdom CAMS decided to create a whole new set of classes for Supersprints which have the same names but entirely different vehicle regs from the racing classes.

So 2B supersprints bears no resemblance to 2B or 2F racing. Both the NSW (by reference) and Vic (by accident) MX5 clubs use the 2B supersprints brake regs for their modified classes, at least as far as calipers go.

Class 6 - Modified Road Registered Normally Aspirated Mazda MX-5’s 1601 to 2000cc. (As per classes 1b or 2b in NSW Supersprints)

5.1 Brake linings and rotors are free provided that the diameters of the rotors are not increased.

6.6.5 
Modified - May
 use 
brakes 
(rotors,
 callipers,
 pads,
 ABS,
 etc.) 
interchanged
 between
 any
 series
 OEM
 specification.

[greenMachine]

Darting?

That sounds mechanical to me, does not seem that a flexing in the caliper would produce a sudden transition that I associate with 'darting', flexing I would expect to conjure 'weaving' ... probably accompanied by some rather choice adjectives in either case

Just to add to Manga's post, the regs also say if it is not mentioned as allowable, it is not allowable (so the silence on the calipers means they can't be changed).

[manga_blue]

If you have, I would do the test shown in the vid, but with two dial guages - one on the inside as far out radially as possible, the other outside as close to the stub axle as possible. That will quantify the problem, but interpreting it is another matter as I suspect that there will always be some twist there.
I would do a full overhaul of the calipers, including new sliders (unless yours are nearly new and in perfect condition) and new seals in the piston, making sure that the pistons and bores are in top shape, and that all the right greases are being used.

Scarlet and I just tried to do this test with verniers since my dial gauge disappeared years ago, probably into my nephew's toolbox. As far as I can tell there's about 0.2-0.3mm deflection at the hubside edge of the caliper fingers. This is much less than the amount of taper showing on the pads. I guess the wear pattern is the result of a combination of caliper flex and other factors, and probably more the other factors than anything else. So I'll follow GM's good advice and do a rebuild of the old calipers.

From what your saying it sounds like it could be a few small things combining to make life suck.
Sliders very slightly bent, when the heat is increased during braking your getting binding ???
Do you have an adjustable Bias control, see where I'm coming from ?? Too much front bias maybe.
Darting under brakes can also be from suspension concerns, binding bushes???? Front and/or rear.

Terry, I'll replace all the sliders as well.
Bias control is fitted and I've been tweaking that for a while - you can mitigate things a bit with that.
I've already turned down the suspension bushes by 0.5mm (they were binding) after Fulcrum fessed up to getting the interference wrong on their early batches of off-set bushes.
Have also adjusted rake so rear is no more than 6mm higher than front and stiffened rear shocks relative to fronts. These last two gave the best improvement.
Sorry to take you off on a wild goose chase, I'm open to any further suggestions about setup though.

cheers
Phil

[Tony]

Slider pins have been mentioned, but what about the holes in the caliper bracket that slides on the pins? Could the holes have worn out of round thereby giving additional freeplay?

[manga_blue]

Yeah, I know, you're right Tony, the sliders are only half the story but I don't want to think about that.

[tbro]

What happens with these now is they start to pull up OK when you first squeeze them but as you increase the pressure they can go feral. The car might dart left or right a meter or two, usually in the middle of the braking zone. I normally like to put an outside wheel over the fog line as I run up to the turn in point. I can't do that now because I need margin for error. If they start to dive then I'll steer it towards track centre before turn in. As well as that I'm starting to brake abut 10m before the newbies. All up it's costing me 1 - 1.5 secs per lap. After a few sessions the outer rim of the rotors is glazed and the pads are tapered.

This has been getting worse for a year and it was doing my head in trying to find it. A whole lot of detail suspension set up changes mitigated the behaviour a bit.

OK disclaimer first, since I was crook when that little mongrel moved into my body I've suffered some permanent damage to the grey stuff upstairs, not that their was a lot to start with, so sometimes I'll not make sense and may need to re-post to try and make it clearer.

After rereading this,I'm more inclined to think mechanical wear in the suspension or k frame. Whatever is bent,loose,worn or just misaligned has been slowly getting worse over time.

Phil, I know how fussy you are so I'm thinking your maybe overlooking some of the basics. Put the car up on stands or hoist, grab a torch and VERY slowly look for shiny patches mainly at suspension mounting points. The reason behind this is the car is different under harder braking, leaving me to think that something is moving one way then the next way at the next application. I don't think its returning to the normal position after braking and that's why it can go either way. The k frame and the rear frames I suspect may be loose even if the nuts/bolts are tight. Have you had someone watch the front wheel or even video it under reasonably hard braking, as is the whole wheel moving back, think falcon with worn strut bushes!!!that type of movement. As for the tapering of the pads I'm incline to think that's more to do with the pad material and maintenance( re:caliper overhaul, sliders for straight etc), with 6 spot calipers on the GT3 I have something similar on certain pads more than others.

End advise, it WILL be something simple it always is, its just finding it!!!!

If you want to talk to me pm your number.

Terry

[mitch_f1]

I am actually less inclined to think pad material/caliper maintenance. My front calipers are freshly rebuilt (every seal possible has been replaced), along with brand spanking new OEM sliders. I have also tried 3 different kinds of pads (A1RMS, HP+, RS500), and all 3 have shown this taper (removed the HP+ before it got significant, and the RS500 pads are sitll fairly new, but both showed the beginnings of it. The QFM pads are screwed).

I like the suggestion that someone previously made that it is heat related. This makes sense to me; the situation is virtually unreproduceable when cold, but when hot the molecular structure changes, making the calipers marginally softer. Since cooling is something that is relatively easy to experiment with, that is what I am going to try next.

Has anyone mentioned rotor material? I have RDA rotors, which are the cheaper rotors, maybe there is something in their metallurgy that makes them more prone to this situation? Phil and Jeo, what do you guys have?

[Guran]

If anyone wants to try some hardness testing on used rotors, get them to me and I can do a Rockwell hardness profile using proper gear.

[Jeo]

RDA...

[mitch_f1]

Sounds like that might be a good option Bryan; I might pay you a visit after my exams. I assume that it would trash the rotor, so hence you have asked for used ones?

[manga_blue]

Mitch, I used DBA 4000 rotors for a few years with no problems until right near the end of their life when this problem started and it finished the rotors off with severe surface checking and glazing. At the time I put it down to the fact that thinner rotors were less able to dissipate heat, but all the damage was in the outer half of the diameter. Pads were and still are HP+.

Then I fitted RDAs which were the only things I could source in a hurry out here in the sticks. These failed with radial cracks on the first or second track day. Actually I looked at Val Stewart's RX7 at Wakefield last weekend after she withdrew because of cracked rotors. I assumed they were also RDAs since they had the same slotting pattern (definitely not DBAs anyway). She had also grabbed them in a hurry. They had the same cracks as mine, only much worse. This photo shows one of about 8 cracks on mine.


Note that the photo also shows the type of localised heat problems that I'm experiencing.

I'm back on DBAs again now. At least they don't crack and their surfaces are less affected by the localised heating.

[greenMachine]

I doubt that the source of the pad problem it is outside the upright/caliper/rotor group. Terry's suggestion addresses one of the symptoms (the darting), and it may be that Manga's car has two problems and that Terry's suggestion (which is a good one) will lead to a fix for the darting issue.

However the pad wear MUST (imho) be somewhere in the vicinity of the caliper, as any suspension/subframe issues will be moving the caliper/pad/rotor as a group, not relative to each other. The pad wear is highly suggestive of a movement (or pressure) of one pad relative to the other. I reckon the problem has to lie somewhere between (including) the caliper mount on the upright and the 'fingers'.

A question for Mitch, Manga and Jeo - do you have this problem on both sides of the car, or just one?

Mitch, you look like the obvious candidate to plug in some new OEM calipers as you seem to have done all the maintenance steps ...

[Guran]

Sounds like that might be a good option Bryan; I might pay you a visit after my exams. I assume that it would trash the rotor, so hence you have asked for used ones?

No, not really. The hardness test puts tiny indentations into the surface, less than 1mm diameter, which is vastly less than what you have with slotted rotors. The reason I suggested used rotors is that they will have much more exposure to heating cycles, so the metallurgical change (if any) would be more noticeable.