Brake pedal travel, heel / toe issue, a cure?

[johnl]

I've been trawling the interweb trying to find a specific reason as to why calipers fitted with a handbrake mechanism seem to commonly be designed with a fairly loose fit betwen the pins and the bushes (and wobble quite a bit, causing piston push back), while those without a handbrake mechanism seem to have a much closer fit between the pins and the bushes (and don't significantly wobble). And, I've found nothing to answer this question.

This does at least appear to be a design thing, and not a wear thing.

I have found a significant number of forum posts where people are asking about the loose fit between the pins and bushes, seemingly far more to do with rear calipers than front ones. Most complaints seem to be to do with loose pins causing the rear calipers to rattle, not so much to do with the affect on pedal travel (which is the issue I have with it).

Regards,
John.

[Citroënbender]

I've attached a typical integrated handbrake caliper piston image, mainly to illuminate the ability of the "adjuster" to turn independently of the piston, albeit with appreciable friction.

Getting back to my earlier comment about trapped air, there are 12 electrovalves in the brake bloc that might entrain a bubble or two, and also the pressure pump chambers (I believe it runs two divorced chambers to maintain some failsafe attributes). Pressure bleeding from the reservoir down is the first step in reducing this air, then whatever software is available to cycle the pump and electrovalves. The Pug/Cit software is great, it just locks off the wheels not being bled and uses the pump to push fluid through.  I wish they were all so easy. 

[johnl]

Aren't there some parts missing in the photo? I'm mainly thinking there should be a rod with a course pitched thread at one end, which is what automatically alters the effective rod length because the rod gradually rotates on this thread in use, which is what keeps the handbrake mechanism in adjustment as the pads wear down (note that my understanding of the mechanism is rudimentery, at best).

It's this effective rod length that I assume I have altered when I rotated the piston, limiting how far the piston can retract into the caliper body, so the piston 'leans' on the rod and cannot retract (or be pushed) enough to create a clearance between thre piston face and the pad to which the piston abuts.

To the possibility of bubbles in the system, there are no symptoms that I would associate with air in the hydraulics. The pedal isn't squishy in that way. I don't think changing the brake fluid would be a problem (which I keep meaning to do...), but I live in fear of ever having to bleed air out of this brake system...

Regards,
John.

[johnl]

The only explanation I have come across (as to why significant pin clearance is necessary) is that it allows the caliper to still apply an even force to the pads, even if (when) the pads develop some tapered wear, by allowing the caliper body to freely 'tilt' in two axes (other than moving in / out). This makes some sense, but doesn't explain why rear calipers often seem to be built with so much more pin clearance than front calipers.

And, fixed calipers seem to work very well without any capacity to 'tilt'. Though having said that, with a fixed caliper tapered pad wear does result in increased pedal travel, because the taper worn pads can be statically close to the disc only at the point of least pad wear, necessitating more piston travel to fully present the pad face to the disc. In this instance a sliding caliper can be superior to a fixed caliper.

I'd still prefer to have fixed calipers (than sliding) because with reasonably 'flat' pads there is still likely to be less piston movement, and the caliper body is more likely to be more rigid, so the pedal will probably feel firmer under heavy braking (due to less caliper flex with higher fluid pressures).

The best brakes with the best pedal travel and feel that I've ever had on a car I've owned were those on my Nota (clubman type sports car), which were Cortina GT front discs with fixed calipers, and Morris rear drums, all unassisted with a balance bar at the pedal. This set up had minimal pedal travel and very strong braking, albeit requiring a strong pedal application (very light car, if it were as heavy as a sedan it would have needed a booster).

Later I fitted rear disc brakes from a Fiat 124 Sports, which used sliding calipers similar to the 147 rear calipers. These Fiat disc brakes were stronger than the much smaller Morris drums (i.e. more effective rear braking for X fluid pressure, necessitating a master cylinder change in the rear brake circuit as the balance bar didn't offer enough adjustment), but had significant pad knock off resulting in more 'free' pedal travel than with the Morris drums (when the drums were properly adjusted). This meant more braking for somewhat less pedal effort, but really it would have been better to have kept the Morris rear drums, and adjust them manually as needed.

I only changed the rear brakes because I had the Fiat brakes already and I was changing from a Morris rear axle (which kept breaking) to an RX2 rear axle (which was bullet proof). The Fiat brake conversion on this axle was fairly easy, so I went down that road rather than use the relatively huge and heavy Mazda rear drums (that came with the RX2 axle). Besides, the disc brakes looked much nicer...

Regards,
John.

[johnl]

I found this product related to caliper pins and bushes, and the clearance between them:

https://www.ecstuning.com/b-ecs-parts/performance-caliper-guide-bushing-set/003326ecs01-02kt/

Should I think greatly reduce the caliper wobbling on the pins, and I expect lessen piston 'push back' caused by it. Of course it's for VWs and Audis, not Alfas, but it does at least suggest that tighter fitting pins may be worth pursuing.

Regards,
John.

[Citroënbender]

If we took the "cocking" of each caliper to be 1mm to nothing at worst case, it's fair to average this at 0.5mm of clearance or travel before full contact between piston, pads and disc.

With 54mm diameter front caliper pistons and 38mm diameter rear caliper pistons, I then make the total displaced fluid volume 3.424ml.

To displace this volume in a 22.2mm bore master cylinder requires specific travel of approx. 8.9mm.

If your leverage on the pedal is approximately 4/1 (rough guess, not gone out to measure the relay linkage accurately) that's anything up to 36mm of travel before full brake contact is achieved.

[bazzbazz]

Aren't we forgetting any wear & tear movement in the pedal assembly?

Or did I miss that part in the info above?

[Citroënbender]

I deliberately omitted any consideration of that, as it's not impacted by any misalignment of the calipers.  Same for free motion in the master cylinder before fluid displacement starts.

[bazzbazz]

Yes, but we are not talking about that here, the original question was that of the inability to properly "heal-toe" due to free/dead movement in the brake pedal. I just would have thought that would have been the first thing to consider, that's all.

[johnl]

CB,
the amount of "cocking" is important insofar as how much piston push back it allows to happen. Some of the 'push back' will actually be 'pull back'  caused by the hysterisis in the piston seal (a designed characteristic of square section piston seals, eliminating pad drag by slightly withdrawing the piston when system pressure drops to effective zero). Some of the push back seems to be caused by the caliper body wobbling on the loose pins with vibration and impact (potholes / bumps). Rotating the piston to create a tighter clearance (actully zero clearance) between the piston and the inner pad will prevent (or at least substantially lessen) both 'push' and 'pull' back. Some push back may be caused by compliance in the wheel bearings etc, but in this case I suspect this is minimal.

With my front calipers there is some slight manually feelable freedom to 'cock', but there is no easily detectible in / out motion at the caliper body (relative to the caliper bracket, disc). Having said that, there probably is at least some in / out motion in use, but not a lot I would expect. It was different at the rear calipers (prior to rotating the pistons), much more significant 'cocking' was easy to produce by manually wiggling the caliper body, and somewhere between about 0.5mm and 1mm of in/out caliper body movement was easy to feel, and to see. This could also be felt (with hands as the caliper body was pushed / pulled) as a slight 'clunk' as the pads contacted the disc with each in and out push and pull on the caliper body. The great majority of the unwanted pedal travel seems to be generated in the rear calipers, with not much (if any?) at the front calipers.

The free motion at the pedal is now much less than it was, but was initially (prior to piston rotation) still significantly less than your 36mm guessalculation (not that I actually measured it, it was Xmm statically, but often X+ dynamically, and it was not consitent). It was still enough to be a problem for heel / toe, and inconsistency in the pedal feel. There is still some pedal free motion, but significantly less than before, and the pedal feel, consistency and braking responsiveness are quite noticably improved even when just braking 'normally'. The brakes are now more confidence inspring in general.

Eliminating caliper body wobble at the front calipers would probably have some affect on pedal free play, but I think not much because the pins are not all that loose. It's the substantlly greater looseness of the rear caliper pins that I think was creating the great majority of the somewhat erratic free play at the pedal. I think further improvement is probably achievable (over and above what the piston rotation has created), with tighter pin clearance in the rear caliper pins. It would be interesting to fit residual pressure valves at all four brakes...

Apparently there is a bronze bushing / machined pin and bushing kit available for BMWs, which replaces rubber mounted stock pin bushes (the pins are a close fit in the bushes). The reports are that this kit significantly improves the braking performance (pedal feel, motion, consistency, pad wear), but in this case it is a 'race only' modification because the BMW calipers have rubber pin bushes and don't have an effective dirt exclusion device (i.e. rubber bellows), and the bushes are exposed to road dirt, water etc.

Regards,
John.

[johnl]

Aren't we forgetting any wear & tear movement in the pedal assembly?

Or did I miss that part in the info above?

I don't think that pedal assembly wear is a significant issue here, though some probably exists. If such wear were the primary problem here, then the rotational adjustment of the rear pistons would not have had as significant positive affect as it has had...

Regards,
John.

[bazzbazz]

I think you should go and have a look at how much pedal movement free play you have with the pedal movement assembly.

I have yet to work on any 147/156 that doesn't have some.

[johnl]

Baz,
I'd more than half expect to find at least some pedal pivot wear, but I can't see how it could be significant to this since it was a change at the rear calipers affecting an obvious piston clearance issue that has improved things so much (still not perfect though, much improved).

Of course some free pay at the pedal itself is necessary (i.e. at the push rod). If play is zero here then this can cause the hydraulic circuit to be shut off from the reservoir, so, as the fluid heats up and expands the resulting pressure cannot 'escape' into the reservoir, so the pistons move out instead. This causes the brakes to 'auto-apply', and when this happens the fluid gets hotter, the pressure increases, the brakes auto-apply harder, the fluid gets hotter, the pressure increases more, etc. This gets worse and worse until you can feel the car 'losing' power, and to smell very hot pads. Don't ask me how I know this, but it does involve a Honda Accord.

I've seen an identical problem with a friends racing kart when he adjusted the pedal free play to zero (the brake disc went black from the heat, then the brake stopped working entirely because of all the smoke coming off the pads).

While I'm here; it's not 'ideal' that there is now some amount of pad drag at the rear brakes, but it doesn't appear to be causing any problems. If the drag were excessive then I'd expect to encounter a fairly substantial heat build up in the rear brakes, but this doesn't occur. I checked this the other day, after a long stretch of straightish road I gradually pulled over and felt for heat coming from the brakes, the rears were about as warm as the front brakes (i.e. I could hold my fingers on both front and rear discs for a few seconds before the pain began...).

Regards,
John.

[Citroënbender]

I have here, a set of MY05 147 rear calipers and yokes, approx 130K on them. Make me a cheeky offer and you can build up a set to your ideal specs that will readily exchange with the present items. 

[johnl]

CB,
Thanks for the offer. I'd be interested were I not so embarrasingly impecunious at present (just gave all my money to the ATO, who are adamant that it was their money...). Maybe later...

Regards,
John.