[johnlear]

I think that is a good modification, even if it is unlikley to make the car actually stop better.

'Fixed' calipers (such as those Brembos) tend to be more rigid than 'floating' style calipers. With 'fixed' calipers the caliper bodies themselves tend to be stiffer (probably not universally the case), and the attachment to the hub carrier is also more rigid (which is universal). These factors tend to reduce both pedal free play and 'sponginess' (compared to an equivalent floating caliper). Both free play and sponginess contribute to a subjective sensation of lesser brake performance, even if there is no objective difference between the actual retardation created by a fixed vs floating caliper (all else being equal).

In my experience, a good 'solid' feeling pedal with little free play or sponginess (not the same things, and with different causes) contributes strongly to a higher subjective confidence in the brakes. Other than the pedal, the MC piston, and disc rotation, in an ideal braking system the only components that should move or flex at all are the caliper pistons (ie. pushing the pads against the discs).

Any other component movements or flexures are going to create either free play or sponginess at the pedal, both of which tend to make the brakes feel less confidence inspiring. In varying degree, floating calipers have signicant internal movements in addition to just the pistons moving, but fixed calipers only have piston movement, so contribute less to pedal free play. Fixed calipers tend to be more rigid and so are likely to flex less than fixed calipers, so contribute less to pedal sponginess.

If the driver has less confidence in the brakes because of excessive free play and / or sponginess, he / she may be less confident to apply the brakes as hard as otherwise they might, so actual braking performance may be decreased because the driver may not feel as confident to 'stand' on the brake pedal as hard as is actually possible (...?). I know this affects me, I feel less confident to brake as hard as absolutely possible if the pedal feel is poor. I also tend to automatically back off slightly when the ABS starts to make the pedal feel like something is breaking (as opposed to 'braking'...).

This is one reaon why 'braided' hoses tend to increase driver confidence when braking heavily, even if they don't actually improve retardation (despite the driver possibly 'feeling' that they do...). The more rigid braided hoses make little difference in normal use, but when braking heavily they expand somewhat less than rubber hoses, and in turn this means the brakes feel significantly less spongy when used aggressively, so there is a subjective sensation of improved braking performance...

Regards,
John.

 

[johnlear]

Also, as they tend to expand under pressure, the actual pressure reaching the caliper is reduced, something that doesn't happen with braided PTFE hoses.

Pressure reaching the calipers is unaffected by any hose expansion, regardless of the degree of expansion. Pressure causes the hoses to expand and as a consequence the pedal travel increases, more so the more the hoses expand, but with no other affect, ie. there is no loss of pressure.

It's my undestanding that all flexible brake hoses expand under pressure, rubber hoses a bit more than 'braid' covered plastic hoses, so pedal travel is somewhat greater with rubber hoses. This is only significant when braking hard with higher system pressures. Rubber hoses are pretty good for normal use, because they are internally reinforced with woven kevlar 'braid' that is quite resistant to expansion.

Regards,
John.

 

[johnlear]

That is not true: brake pressure will be exactly the same only the stroke of the brake paddle will be a bit longer.

Agreed, must have been near simultaneous posts...

Regards,
John.

 

[johnlear]

Have you actually tested this yourselves? I've done so and there is a drop in pressure at the caliper using OEM hoses of a certain age, admittedly on bikes but the principal is the same.

Admittedly no, I haven't actually tested this, but physics says it must be correct. If we pump X pressure into a rigid walled pressure vessel, the pressure will be X and the vessel capacity will not increase. But if we pump X pressure into an expandable pressure vessel the pressure will still be X, but the capacity will increase as a result of the vessel not being rigid in expansion.

The pedal travel will increase because it isn't the variable capacity of the hose that limits pressure input and output (both X pressure), but rather it is retardation rate and possibly tyre traction, ie. the driver pushes the pedal as hard as needed to achieve the fluid pressure required to acheive the required braking affect, regardless of hose expabsion or pedal travel.

There will be a very transient loss of pressure caused by hose expansion. The expansion will cause a reduction in the rate at which fluid pressure changes, but it will be only a very slight 'delay' mostly because it takes longer to move the pedal a greater distance. The ultimate pedal travel will be greater, but the ultimate fluid pressure will still be much the same for a given effort on the pedal pad. Hoses with significant expansion will tend to 'cushion' the rate at which pressure rises and falls, but not reduce the peak pressure.

Note that fluid pressure is the same throughout the entire hydraulic circuit operating a particular caliper, because the fluid is for all intents and purposes incomprssible.

But there may also be small indirect mechanical affects. If hose expansion causes pedal travel to increase significantly enough, then there may possibly be a small indirect affect on system pressure caused by a change in the pedal geometry caused by angular changes to the push rod, caused by the increased arc of pedal arm swing.

E.g. if the push rod is statically at 180° to the MC piston axis, then a short pedal travel will see little change in that angle and pretty much all push rod force will act in a straight line with the piston movement. But, if the pedal travel is significantly longer (as might occur if the hoses are soft in expansion...), then as the push rod moves it will not move purely in line with the MC piston but also move vertically at the pedal pin. The push rod angle relative to the MC piston will change and so the leverage will change, in some degree.

I suspect this could be why your test with bike brakes saw a pressure loss with old softened hoses relative to new hoses, ie. the brake lever geometry may well have changed when the lever travel increased as a consequence of greater hose expansion...?

Regards,
John.

 

[johnlear]

The linked video (above) explains that OE brakes are specced to be perfectly adequate to cope very well with any reasonable demand in normal road use. I more or less agree with this, but it should be noted that the factory fitted pads will not be rubbish, unlike some cheap aftermarket pads.

A few years back I had an instance of cheap pads fading badly during an emergency stop from about 100kmh or so. The initial 'bite' and braking effect was acceptably OK for maybe 30 metres or so, but then the pads 'outgassed' and the braking effect decreased dramatically, with the car failing to stop as required (would have hit an animal had I not been able to steer around it). I binned those pads and fitted new ones (TRW) then deliberately repeated the 'test', with a hugely improved result, the pads didn't fade and car pulled up in a far shorter distance...

Regards,
John.

 

[johnlear]

I agree that ratio of swept volume is very important, and shouldn't be altered significantly at the risk of making the brakes worse. When substituting floating calipers with fixed calipers, the cumulative area of all the caliper pistons should remain about the same (fixed vs floating caliper).

Most commonly the 'new' fixed calipers will have four opposed pistons compared to the floating caliper having only one non opposed piston (though of course some fixed calipers have more than four pistons, and some floating calipers more than one piston). The multiple pistons in the new fixed calipers should have a diameter that is quite a bit less than the diameter of the pistons in the original floating calipers, in order to maintain a similar total piston area as the original floating calipers.

If the piston area in the new fixed calipers is too large relative to the old floating calipers, then the fluid displacement at the MC piston will increase and cause the pedal travel to become longer, as well as changing the fundamental front / rear brake bias. This should be avoided unless we want to pretty much re-engineer the design of the entire braking system, or suffer less effective brakes...

Contrary to what many seem to believe, increasing clamping force at the pads is not why changing from floating to fixed calipers is generally a 'good thing'. Rather, any significant increase in clamping force is likely to be problematic as it will affect the fundamental brake bias, and so any significant change in clamping force should be avoided...

The actual reasons why fixed calipers are generally superior to floating calipers has nothing to do with increased clamping force, but is actually because fixed calipers:

• Are typically more rigid. A more rigid caliper body reduces pedal travel due to decreased 'sponginess' and free play in the caliper.
• Can accomodate larger pads, so the pad material runs at a cooler temperature and wears less.
• Can accomodate more pistons. Multiple pistons acting on each pad causes clamping force to be spread more evenly over the entire pad area (which becomes a bigger deal the larger the pads are).
• More evenly distibuted clamping force causes the pads to wear more evenly (i.e. reduce taper wear). More even pad wear means the pad faces remain more parallel with the backing plates and so the distance from pad faces to disc faces is more consistent, which means the pedal free play is less affected as the pads become worn.
• More even pad clamping means that the average pad temperature tends to be lower and more even over the pad area.

(Note: If the substitute fixed calipers were to have only two opposed pistons, then they should both be of the same diameter as the single piston in the oiginal floating calipers because in a floating caliper the piston is in effect 'double acting' in that fluid pressure pushes the piston one way while simultaneously pushing the caliper body equally in the opposite direction. This doesn't happen in a fixed caliper, the fluid pressure only exerting force in one direction.)

Regards,
John.

 

[johnlear]

When new brake pads are fitted in place of old taper worn pads (and taper wear is typically 'normal', usually in two planes along and across the pads, typically more so the shorter the pad length happens to be), there will be an improvement in brake feel due to the new pads having friction faces that are perfectly parallel to the backing plates (unlike the old pads...).

And also, when new discs are fitted there will be an improvement in brake feel due to the new discs having faces that are perfectly flat and parallel to each other (and so both faces at 90° to the action of the caliper), i.e. not worn thinner toward the circumference of the disc (and less worn nearer the hub), as is typical for a worn disc.

Both new pads and / or discs will contribute to a reduction in pedal free play and a more pleasing and confidence inspiring brake feel, independant of whether or not retardation is actually improved (i.e. much of the affect may be subjective, but well worth having none the less since the car feels so much nicer for it...).

I proved this again to myself last week after I noticed that my brake pedal travel seemed to have become noticably greater than it used to be and the brakes no longer felt to be as 'sharp' or generally working quite as well as I recalled some months ago.

Checking the front brakes I extracted the pads and measured thickness with a Vernier caliper. Both pairs of pads had about 0.25mm taper wear in two planes (i.e. along the length and across the width). Using a surface plate with 60 grit emery glued to it, I sanded all four pads until the friction faces were parallel with the backing plates (outside and wear a mask, don't breathe the dust, it probably doesn't contain asbestos but it can't be good to breathe it in whatever it is...). I then checked the rear pads, which were all perfectly parallel to their backing plates (wearing 'flat' I think due to my modification to eliminate the excessive OE slider pin clearances).

Pads refitted and the difference was immediately apparent at the first brake application, with pedal free play reduced to about half (or less, maybe) and significantly decreased sponginess. The brakes just felt so much more 'together' and confidence inspiring than they had previous to 'adjusting' the worn pads.

I've done this quite a few times to various cars over the years, and the result is always the same, i.e. a quite significant and immediate decrease in pedal free play and sponginess, and a strong subjective feeling that the brakes now perform quite a lot better than before. An added benefit appears to be that 'correcting' pad parallelism every now and again during the life of the pads appears to promote more even disc wear, i.e. less 'groove' wear and the disc faces seem less 'tapered' toward the circumference (given some time the newly flat and parallel pad faces tend to 'flatten' slightly uneven disc wear).

I suspect that this is an affect that helps to convince many people that their new 'upgraded' brakes have a bigger affect than they actually do, i.e. the new upgraded pads are flat and parallel with their backing plates, new discs are nice and flat too, so the pedal feel is greatly improved over the worn out parts that have just been replaced. Just fitting new standard parts may well have a had a fairly similar affect.

This is not to say that fixed calipers are not sigificantly superior to floating calipers, I think they are and a worthwhile upgrade. But, when changing from floating calipers to fixed calipers, the apparent and real improvement may not be entirely due to the upgraded calipers, much is probably the result of nice new flat pads and discs.

I also suspect that fixed calipers very probably contribute to (compared to floating calipers) their pads wearing more evenly with less taper in either plane of wear, so are likely worth it from that perspective alone...

Regards,
John.

 

[johnlear]

Short answer here:
The Brembo 4 pot callipers have different sized pistons to prevent (or reduce) taper wear. That is the case with most multi piston callipers.

The leading pistons are smaller so they push less on the leading part of the brake pad.

Yes. I did imply earlier that muliple pistons acting on a pad would more evenly distribute the clamping force over the pad area, but this was a simplification, for the sake of avoiding lengthy explanation...

I'd expect most multi piston calipers (i.e. floating calipers with two pistons or fixed calipers with two or more pistons per side) would typically use different sized pistons to partially compensate for the the self energizing effect created at the leading edge of the pad. Different sized pistons actually cause the hydraulic clamping force to deliberately be unevenly distributed, in an attempt to compensate for the inherent self energizing effect.

Ideally, the lesser clamping force created by the smaller pistons, added to the self energising effect (which acts most strongly nearer the smaller pistons, and most weakly nearer the larger pistons) will combine to become near equal to the clamping force created by the larger pistons (or thereabouts considering that the self energising effect is not a constant but diminishes as the pad wears).

A geometrically created self energizing effect inherently exists in all calipers. This can be compensated for by using different sized 'staggered' pistons, but this only works to an approximate degree because the self energising effect changes from stronger to weaker as the pads become thinner. Pads wear in a longitudinal tapered pattern (as opposed to radial taper wear) due to the variable self energizing effect, which cannot be eliminated, only partially mitigated, as evidenced by pad taper wear being more or less ubiquitous, only differing in degree with different calipers, despite the number of pistons or their relative diameters.

Some calipers may have multiple pistons of the same size, in which case the hydraulic clamping force will be evenly distributed, but, the self energising effect will also add to pad force at and near the leading edge, so in the end the pad will not be evenly pressed against the disc (other than perhaps transiently at a certain pad thickness).

Having noted this, I expect that multi piston calipers with uniform piston diameters would still most probably press the pads more evenly against the disc than single piston floating calipers or dual piston fixed calipers...

Regards,
John.

 

[johnlear]

Not sure that counts as an "upgrade", more of a 'repair'...

Regards,
John.