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

[johnl]

My 147 has been suffering from an excess of 'free' brake pedal travel. It's not that the brakes need bleeding, they don't. It's to do with the rear brake callipers. There is movement in the caliper, i.e. it's possible to manually move the caliper body in and out by about 0.5mm (or so) on the slider pins, because the piston retracts that far. This means that the piston has to be moved this amount before the pads make proper contact with the rotor, which equates to unwanted pedal motion.

Multiply this by 2, because there are two rear calipers, and the brake pedal travel (before the brakes start to work) is excessive. This may not be a significant issue for many drivers, but it means that the brake pedal moves substantially below the height of the accelerator pedal, which makes 'heel / toe' braking action quite awkward. Add to this the additional pedal motion that is a result of the rubber hoses expanding under heavy use (normal) and it's not a good thing.

This seems largely to do with the handbrake mechanism on the Lucas calipers. To assist heel / toe braking I have already bent the throttle pedal shaft so that the pedal pad is statically closer to the floor, and significantly below the height of the brake pedal pad when the brake is not being used (this is not all that easy to do since the shaft on the throttle pedal is unnecessarily thick and rigid). But, it's still not enough, the throttle pedal pad is still too high when the brake is being used, and there is still that icky free play.

So, experimentally I adjusted the piston rotation in the rear callipers until it was quite difficult to slide the caliper body over the pads as the body was fitted to the caliper bracket (i.e. with the caliper bracket fitted to the strut with the pads already in position against the rotor surfaces). Only a small rotation of the pistons was needed. This resulted in some significant pad drag as the rotor was turned by hand, but not a great amount.

So, now, the brake pedal motion is a lot less and the brake pedal pad does not move below the static height of the throttle pedal pad. Heel / toe is now much much easier and more consistent (because brake pedal travel is now more consistent as well as less). Additionally, the brakes feel far more responsive and more confidence inspiring, the pedal feels so much firmer than before. I now no longer have to make sure that the edge of my shoe does not catch the edge of the throttle pedal when braking firmly (when not doing heel / toe, note that my pedal pads are closer together than standard, to facilitate heel / toe).

I'm not sure how long this will last. The pads will wear, and the issue will probably reappear in time. However, the improvement is so great that having to adjust the piston rotation every few weeks (or however long) will be worth it. Since the pads are now significantly dragging (but not heavily dragging), the rear pad wear will probably increase to some degree, but again the improvement is so great it will be worth it (unless pad wear rate increases a lot more than I expect it will).

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I wonder if there is a rear caliper that does not suffer from this problem that might be substituted for the Lucas caliper? I know that there is an ATE brand caliper used in earlier model Alfas that uses a different handbrake mechanism (and is a dual piston design as opposed to the single piston in the Lucas caliper). I vaguely recall reading of some owners of earlier model Alfas swapping these for the Lucas caliper because the Lucas caliper handbrake mechanism works much better. I wonder if the swap can be done the other way? I'd be happy to trade a weaker handbrake for a better pedal action...

Regards,
John.

[johnl]

Further thoughts on sliding pin single piston rear calipers;

Looking at a typical sliding pin / single piston front caliper, the pins usually seem to be a close fit with the bushes in which they slide. This means that the caliper body is not fee to move relative to the bracket, other than to slide in and out on the pins.

Looking at a typical rear sliding pin / single piston caliper, the pins seem to be a fairly loose fit in the bushes. This means that the caliper body is free to move in and out, but also has some small degree of freedom to move in other axes. Take the wheel off, grab the innermost part of the caliper body, try and move it longitudinally (toward the front and rear of the car). The caliper body will most probably wiggle to a significant degree. Now try and move it up and down, again it will most probably wiggle significantly. The caliper body will 'wiggle' with vibration and with 'impacts' due to road bumps etc.

With either the longitudinal or vertical caliper body wiggling, the motion will tend to push the piston back into the caliper to some degree.  This cannot be a good thing because if the piston is pushed back into the caliper then upon brake application the piston must be pushed back out again, which equates to 'free' motion at the pedal.

What can be done? Can we re-machine the pins and bushes for a much closer fit, as front calipers seem to commonly have? Perhaps, but why are the pins so loose in the first place? Is there some not particularly obvious reason why they are engineered this way, maybe to do with how the handbrake mechanism works?

I don't know, but some years ago I came across loose pins in the rear calipers of my Accord (which had a similar handbrake mechanism to the 147 rear callipers). I assumed the pins / bushes were worn (hadn't played with such calipers before, and unaware that this looseness was common in such rear calipers), and as an experiment I inserted some metal shim (curved into a tube) between the pins and bushes to take up most of the clearance, which substantially reduced the 'wiggling' between the caliper body and the bracket. Pedal motion decreased and became more consistent, and the handbrake still worked fine...

Alternatively, could we fit a residual pressure valve into the brake line leading to the caliper? These valves permit free flow of pressure one way, but limit pressure release back the other way. They are available in different pressure settings, i.e. some may retain 2lbs pressure in the caliper, some 10lbs, and probably other settings. So, if there were say a 2lb valve in the line, the valve would always retain 2lbs of pressure in the caliper, to lightly apply the pads at all times and eliminate (in some degree) pad 'knock off' (i.e. piston 'push back').

This would increase pad wear to some degree, and have at least some affect on fuel economy, but if these effects were not too great then it may well be worth it.

It occurs to me that a somewhat similar effect might be had if light compression springs were fitted over the rubber boots that cover the pins. These springs would exert light pressure between the caliper body and the caliper bracket, helping to limit caliper 'wiggling' and the resultant piston 'push back'...?

Regards,
John.

[Citroënbender]

What do you actually like (as in, have no desire to change) about the 147 as it was made?  One gets the impression it isn't much at all. I'm honestly starting to become puzzled by all these changes. 

Surely one "cure" for perceived inadequacies, is a change of car, or a hobby vehicle - like a Seven style project - or even a kart?

With the brakes, bypassing the ABS completely (within its housing, so as not to disturb the interface with ECU etc) might help your pedal feel, it would be an interesting project...

[bonno]

Hi John
Not sure if this helps, as I am not qualified to add much light on Brake Design, but from my knowledge the clearance fit of pins and bushes on the brake calipers, is all to do with coefficient of expansion to cater for the variations in temperature under NORMAL braking conditions. The toe and heel method of braking you refer to is more consistent with racing rather than normal city and highway driving.
Regards
bonno

[johnl]

I suspect that for cars I can conceivably afford, none would satisfy me entirely in every respect. I've never owned a car that I didn't feel a need to change / improve something, and usually quite a few things.

I like my 147 very much, it has a lot going for it, but also it has a number of faults and things that might have been done better, like all other cars, none are perfect. I have a kart already, just no time to take it to the track. I've owned a "Seven style" / clubman type road car too, and I made a heap of changes to that, many more than any other car I've owned (that car was a frantic roller skate!).

I really don't think the issue with the 147 brake pedal travel is anything to do with the ABS, so doubt that bypassing it would improve it. I'm 99% sure it's the fundamental design of the rear calipers with a lose fit for the rear pins, a design feature that seems shared with quite a few other cars.

For instance my Accord had close tolerance pins in the front calipers and loose in the rear calipers, my sone's old Prelude ditto, his current Mazda 323 ditto, my mothers' 323 ditto, and other cars I strongly suspect. Now it could possibly be that these cars all just happened to not wear the pins / bushes in their front brakes while significantly wearing the pins bushes only in the rear brakes, but I tend to doubt it. I recall buying new pins for the rear brake in the Accord, and they were the same OD as the old ones I had assumed must be worn.

My wifes' Saab 95 does have tight tolerance pins in all it's calipers, but the handbrakes are 'mini-drums' inside the brake rotors, i.e. seperate mechanisms to the main rear brakes. We used to have a Cressida that was ditto...

Regards,
John.

[johnl]

Hi John
Not sure if this helps, as I am not qualified to add much light on Brake Design, but from my knowledge the clearance fit of pins and bushes on the brake calipers, is all to do with coefficient of expansion to cater for the variations in temperature under NORMAL braking conditions. The toe and heel method of braking you refer to is more consistent with racing rather than normal city and highway driving.
Regards
bonno

Bonno,
I learnt to heel / toe many years ago driving the Nota I used to own (the above mentioned 'Clubman' style car). Once you've mastered the skill it becomes unthinkingly automatic, you will probably tend to use it quite a lot, and miss it if the pedal layout doesn't permit it, or makes it too awkward / impractical to use (as does a brake pedal that moves excessively...). 

The expansion with heat of the pins inside the bushes can't account for the significant clearance between them (that exists with it at least seems quite a lot of rear calipers). If it did, then the pins in the front calipers would also need a similar clearance, or greater, considering that the front calipers are likely to get hotter than the rear ones. Front calipers don't seem to have a loose fit between the pins and their bushes, at least I have never seen a front caliper that 'wobbles' in the same way as seems common with rear calipers...

Regards,
John.

[bazzbazz]

With the brakes, bypassing the ABS completely (within its housing, so as not to disturb the interface with ECU etc) might help your pedal feel, it would be an interesting project...

I am curious as to what you are trying to achieve here. The ABS system is nonexistent till it is activated by skidding of one of the wheels, as the brake lines just run straight through the unit and its valves as if it was a solid pipe.

[Citroënbender]

Less chance of there being small amounts of air in the system.  I'm never convinced with the monobloc Bosch ABS/ESP type systems that I have got it all out.

[johnl]

Update on this. It's about a week since I manually tightened the clearance between the pads and the pistons in the rear calipers. So far the pedal is still as good as it was the day I did it, with no adverse issues with how well the brakes work. The pedal is till 'high' on application, heel / toe is much easier than previously. The brake pedal in general is much more confidence inspring and pleasant to use. I haven't yet checked for pad wear, but think it should probably be OK.

Regards,
John.

[Citroënbender]

What about the little dogs or pins on the pad back - are they still indexed with the divots in the caliper piston?

[johnl]

CB,
The pistons do each have two "divots" / slots at their periphery (180° apart), but the pads have no corresponding 'dogs', their backs are all quite flat. This is the same as was the case with the old pads that I replaced. Since the pad backs are flat, I assume that the piston slots are intended to fit a piston winding tool (but I find that multigrips work fine).

If the pads were of the type that do have dogs, then fine adjustment of the piston rotation would not be possible, because the piston slots could only fit over the dogs at 180° rotational increments. I found that a quite small piston rotation made a quite significant difference to the clearance between the piston / pads / disc (180° would be a very substantial difference). With the experience I'm having with this now, were I to buy a set of rear pads that did have dogs, I think I'd grind them off (in order to be able to fine tune the piston rotation and associated clearances).

I've also done a similar rear caliper piston rotation adjustment on my sons' Mazda 323, and reduced the pedal travel with some success. This car also had no dogs on the rear pads, and no slots at all in the pistons, and a similar tendency for the rear caliper bodies to wobble on what seem to be slider pins that have an excessive clearance to the pin bushes, causing excessive brake pedal travel.

This morning I took took a rear wheel off the Alfa and checked the caliper for any return of unwanted play / wobble, and, this was much the same as when I first rotated the piston to tighten the piston to pad clearance (i.e. some very slight and much reduced wobble). The only difference is that the disc is now a tad freer to manually rotate, but still a little bit draggy.

Regards,
John.

[bazzbazz]

What about the little dogs or pins on the pad back - are they still indexed with the divots in the caliper piston?

147 doesn't have any locators on the back of the pads? 

[johnl]

147 doesn't have any locators on the back of the pads? 

Not on mine.

Regards,
John.

[Citroënbender]

Nor my spare Valeo rear pads. But I'm sure whatever I fitted last time I did the rears on the '01, did have.

I've always seen them as integral to the self-adjusters working properly. Odd they're not present.

[johnl]

Nor my spare Valeo rear pads. But I'm sure whatever I fitted last time I did the rears on the '01, did have.

I've always seen them as integral to the self-adjusters working properly. Odd they're not present.

My handbrake works very well with no dogs on the pad backs. I've never needed to adjust it as the lever action and effectiveness has never noticably changed since I bought the car, and I assume the adjuster is working as it's supposed to.

My car hasn't had 'dogged' pads since before I bought it, obviously the dogs are not essential for the Lucas handbrake adjuster to work. It would be my guess that where pad dogs are used, with a caliper using the same or similar kind of handbrake adjusting mechanism as the 147 Lucas calipers use, that the dogs are probably redundant...

Regards,
John.