BCC 4 pot caliper upgrade

[Einar]

Hello – does anyone have any experience or thoughts about these upgraded brake calipers available for Vitesse. Will they improve brake performance significantly, or what…..?
http://www.britishclassiccarparts.com/triumph-vitesse-2l-mk1-brake-caliper-kit-6981-p.asp

[JohnD]

The major advantage of a 4 pot (or more) is a long thin brake pad that can be cents red further from the axle, so exerting a greater leverage and more braking power for the same effort.   But unless you can find a way of running wheels that have a significantly greater diameter this won't be effective in a Triumph.

Properly set up and adjusted Triumph brakes are perfectly good and you should be able to lock up al wheels - not a good idea but the ultimate test - at almost any speed.   Brake mods that can be useful are to combat fade through overheating if that is your diving style.   "Hotter" pads - I use Mintex 1155 - or vented discs.

John

[Steve AKA vitessesteve]

As John says.

+ Bigger calipers will contain more fluid - result more fluid needed in the master cylinder and or greater pedal travel to get the same volume to move.  Will you need to change the master as well?

How much more pad area do the  4 pot calipers actually press on the discs?

[Clive]

Years ago somebody did the calcs, and it was a relatively small difference. about 10%, but that is a vague memory.

I had the princess callipers on my old spitfire and a std GT6 brake disc. effective enough, but no noticeable difference to the GT6/vitesse callipers I have on my car now (these are vented, so no difference to std unless used very hard for a period of time as fade seems to be non-existent)

I would say using decent pads will be the easiest and luckily cheapest option. However, I notice you have a Vitesse 6. Is it still equipped with the type 12 callipers and smaller disc? If so I think you can fit the larger 2 litre sized disc to your hubs, and either get the GT6/vitesse calliper mount brackets and callipers, or fit princess callipers(or new aluminium copies)

[Einar]

Yes, I have a Vit 6, but everything is upgraded to 2l Mk1/Mk2 spec . – and fitted with 16PB calipers. The brakes are not bad – but I just want to improve them, if possible  - beeing more responsive, sort of …. Mintex 1155 is fitted. A Powertune brake servo is also fitted.

[A TR7 16V]

Quoted from JohnD

a long thin brake pad that can be cents red further from the axle, so exerting a greater leverage and more braking power for the same effort.  

Oooh, a physics question. Sorry if this is late in the discussion, but I just can't resist these.

True, but it's only a part of the truth. Another part of, in effect, spreading the piston out around the disc, is that the total area of the pistons can be much greater, even double, without their outer edge being further from the centre, which would force you to use bigger discs and wheels. And from hydraulics 101 the force on the pads is proportional to the area of the pistons (for a given hydraulic pressure), and the braking effort is proportional to the force on the pad (Amontons' first law of friction), more piston area gives more braking effort. There is the drawback, however, that a bigger piston area generally means a bit longer travel on the pedal. But if the discs are flat, and parallel, that won't be much.

It generally also increases the pad area too. But, interestingly, the area of the pad has no effect on the braking effort as such (Amontons' second law of friction), though it can affect fade. What a bigger pad can mean, however, is that you can use a material with a higher coefficient of friction (CofF), pad to disc, which would normally wear more quickly, for the same pad life. It's also interesting to apply Amontons' second law to the situation of wider tires and greater grip.

I've fitted the big Princess 4 pots to several TR7s as a very nearly direct replacement - same discs and wheels, etc., but you have to move one of the mounting holes on the calliper about 1/4 inch outboard, or the pads hang over the edge of the disk. They have (IIRC) about half again the piston area of the TR8 callipers, and the improvement in braking is significant. The improvement over TR7 callipers, which are the same as HA viva ones, just with a servo, is vast. There's less fade as well, but (IMO) not as much less as vented discs.

The effect on lengthening the pedal on the TR7 is normally countered by fitting the 4 speed rear brake cylinders, which are a smaller diameter, and effectivly shorten the pedal back to where it was. That will also have the effect of moving the brake balance forward, but if you also put wider, stickier tires on as well as improve the front brake effort, that' also has the effect of moving the limit of brake balance forward anyway, so you have to move the actual balance forward again, just to keep up. Otherwise, if the balance is too far back, the rear wheels lock first when hard braking on a clean dry road, and...

Think about it this way: If you were to increase the front brakes and CofF, tire to road, (or raise the CofG) to the point where you could reverse wheelie on a dry road, any back brake effort would be too much, as the back wheels must always lock before the fronts.

Graham

[JohnD]

Delighted to have a physics ally on the board, Graham!
As the above posts reveal, and you demonstrate is not true, there is a persistent belief that larger brake pads mean better braking.

But others persist, such as Einer's fitting 1155 Mintex pads, and a servo.   1155 is an example of a 'hotter' or harder brake material, that will provide optimum friction at a higher temperature, so that if you drive fast and use your brakes a lot, you are lesslikely to suffer from 'fade', as standard pads overheat and go beyond their working temperature.     Every manufacturer will provide a range, usually of three from standard, hotter, racing.   The Montex ramge happen stop be called, for no known reason, 1144, 1155, 1166.
The downside is that the hotter material may be outside its working temperature when cold, so that when setting off on a cold morning, you may need to brake earlier for the corner at the bottom of your road.    But I've never found this to be so, and the combination of 1155s and vented from discs has abolished fade for me, even when racing.

A servo is another item of myth and magic.  It does NOT provide better braking, it merely allows you, with your gammy knee, to apply less pedal pressure for the same brake pressure.     But only up to a point.  The servo can only apply pressure to amplify your effort up to the limit of the vacuum that drives it.  After that it's up to you, so at the limit you must press as hard as you ever did.
Most moderns have a servo, but that is because it allows the manufacturer to fit a large master cylinder, with small travel that drivers favour, but without the excessive pressure that would otherwise demand.

But I must take issue with you Graham.  There is no absolute need for multiple pot calipers to have larger area, although the reason you give, lower wear, is of course true.
And Steve, multiple piston calipers have no need either for the piston areas to be larger that a single piston, demanding changes in the master cylinder.  They usually have quite small pistons, for exactly the reason I gave, so that they can actuate narrow pads further away from the axle.

John

[Dogsbody47uk]

If your problem with the 1155 pads is lack of response when cold, try 1144s. I find they bite just fine when cold and are a good pad for road use. Cheers, Dave.

[bxbodger]

I had mintex pads for a while, and found them somewhat wooden.....i changed them for Jurid pads, so much better, a really noticeable improvement in feel. Regarding four-pot calipers, my Metro, being a mk2, has four-pot calipers and vented discs as standard, later, K series powered, metros, were fitted with single pot swing calipers and  solid discs, presumeably much cheaper to make, and certainly much easier to look after, and I can honestly say that there's no noticeable difference between the two types.....

[nang]

TBH, I fitted Princess calipers to my 2500 TC many  years ago. Still got them but they don't really make much improvement on a road car. Mind you, I'm a commercial driver and drive all vehicles as if I have no brakes. (eek)
Tony.

[A TR7 16V]

Quoted from JohnD

But I must take issue with you Graham.  There is no absolute need for multiple pot calipers to have larger area, although the reason you give, lower wear, is of course true. John

I actually do think they have to be bigger in this case, but only because it applies to the situation where you're keeping the same disc while replacing the two pot calliper to increase brake effort. The size of the disc then limits how far out you can move the centre of pressure, to the extent that you can't use a four pot calliper with pistons any smaller than (I estimate) two thirds of the diameter of the two pot ones, or the brake effort must be less, not more.  This is partly because the effect of the position of the pressure centre on the brake effort is linear but that of the piston diameter goes by its square, so it very quickly becomes dominant.

I admit I'm assuming in the above that you don't want to move the outer edge of the four pot pistons closer to the edge of the disc than the two pot ones were. But, because I don't think I understand all the safety implications in doing that, I'm sure I don't want to. Even if you think that distance should be in proportion to the piston diameter, you can't reduce it by more than about a third (with pistons two thirds the size). And a third of it is bound to be small in comparison to the distance between the pressure centre and the centre of the hub.

I also assume that the centre of pressure for the two pot calliper is the same as the piston centreline, and for the four pot, is on a line joining the centres of the pairs and equidistant from them. I have thought about thinking about what happens if that doesn't line up with the centroid of the pad, but decided against it. Again, I think the error in that must be small compared with the distance from the hub centre, so its effect on brake effort will also be small.

Since the point of the discussion is improving the brakes, you don't want to go anywhere near as small a factor as 0.66 anyway – that's where, I'd say, you are absolutely guaranteed to get no return at all on your investment. So the pads on any 4 pot calliper you would use will, I would say, necessarily be larger than those of the two pots you replace – but only in this context of keeping the same disc. For example, with pistons 0.707 times the two pot one, the area of the four pot pad, relative to that for the two pot calliper, will be roughly 0.707 * 1.414 plus 0.707 times the distance between the pairs of pistons, and as 0.707 * 1.414 already equals 1, it must be larger.

But even with pistons 0.707 the size of the two pot ones, which keeps the same pedal length, I estimate you'll get at most 10 percent improvement in brake effort (it must be very much less than 30 percent, unless the diameter of the two pot piston was half that of the disc [LOL]). Actually, I think it's nearer to 5 percent, but I don't have numbers to firm that up. And while 10 percent improvement in the front brake effort is not nothing [yes it is] I don't think it's enough to justify the cost of replacing the callipers.

So despite the effects on calliper weight and pedal length, if upgrading to four pots on the same disc, I'd be inclined to look for a calliper with pistons a little bigger than 0.707 times the two pot ones. At 0.8 you get 28 percent increase in brake effort from the increased pressure on the pad.  You might also get  few percent from moving the pressure centre, assuming you keep the outer piston edge the same distance from the disc edge, as I would be inclined to. And a third more front brake effort sounds much more like a reasonable return on the investment to me.

Graham.

[Einar]

Hi all and thanks for the input - especially Graham. I did not follow everything, but if I got the conclusion right this is not the route to follow for improving the brakes in a quick and simple manner. As I said – the brakes are good, I just want to further improve them, and hence considered these calipers…..

[A TR7 16V]

Actually, they may well be a really easy way to a really good upgrade. The problem is that the ad doesn't give specs, and I don't have specs for what you've got on currently.

Without those, I for one couldn't tell you if they are any better or not, and if they are, by how much.

With those specs, I think it's relatively easy to work out how much additional brake effort you would get, and whether they are likely to affect the pedal, etc.

The discussion John and I are indulged in is more than a bit esoteric. To you, the pad size will be what it is, and why it is won't matter. I'm just betting that, if they are a worthwhile upgrade, the pad will have at least as big and area as what you've got, and conversly, if the pad is smaller than what you got, they wouldn't be a worthwhile upgrade. But it's the two sets of specs that allow that to be worked out.

Graham

[Einar]

OK, so the specs I (you) need are piston area of the BCC 4 pot and the ones currently on the car? But is the pad area needed? – that area does not matter, or maybe it does…? I do my best to follow your input and advices 😎

[A TR7 16V]

All I need is the piston area or piston diameter and the distance from the centre of pressure to the centre of the disk when the calliper is fitted, for both the new calliper and what you've currently got fitted. The centre of pressure should be near enough to the centroid of the pad (the point where any straight line passing through divides the pad into two equal areas). That should also be near enough the centre of the two pot piston and halfway between the two centres on one side of the four pot.  

Assuming your current callipers are a standard type, maybe someone here has that data already.

No, the pad area is largely irrelevant to the brake effort, though it will have an effect how quickly the brakes fade - the bigger it is, the more it will take to overheat them. Also, the bigger they are, the longer they will  tend last. But I'd like to know what their areas are, because I'm like that.

Graham

[Einar]

I’ll try to get these specs of the 4 pot.  The data on the 16BP currently fitted can be measured. I’ll post a reply when I’ve got them. Thanks.

[Einar]

Graham,
Here is the specs provided by BCC – they also included the OEM specs. The OEM diameter is correct, and I would then also assume the other dims are correct.

BCC piston diameter - 38mm (4 per Caliper)
OEM piston diameter - 54mm (2 per caliper)

BCC brake pad friction area - 3675.58 mm^2
OEM brake pad friction area - 3024.06mm^2

Disc effective Radius BCC - 111.985mm
Disc effective Radius OEM - 112.50mm

Hope you can do some calculations based upon this input
Thanks in advance
Einar

[A TR7 16V]

I can do the effect of the upgrade from the change in area from those, but I would need the radii to the centres of pressure to get the effect of that moving outward. though I can make a rough estimate of that effect, by assuming it moves by the change in radius of the pistons.

I'll post the results in a little while, as I'm currently bored to frigging tears.

Graham

[A TR7 16V]

I make the change in area to be less than 1 percent, which is probably much smaller than the effect of errors in or rounding of the measurements of the piston diameters. For example, taking the OEM ones as 54 mm diameter, for the four pot's to be exactly the same area, its pistons would want to be 38.178 mm diameter, rather than 38 mm. So, if their areas are the same, the volume of hydraulic fluid needed to move them the same distance is the same, and there should be no effect on the length of the brake pedal.

There will, however, be some effect of moving the pressure centre. However, I don't know either their actual values or their relationships to the effective disc radii (if they were to be the same thing, then there's no significant improvement whatsoever – less than half a percent 112.5/111.985 = 1.0046). And without those values, the best I can give you is a range.

Assuming that the largest possible value for the radius of the pressure centre for the four pots is 112.5 (and I think it must be less than that) and it only moves 8 mm from the change in radius, the minimum change in brake effort would be 7.6 percent. So I think you must get at least that much increase.  

I struggle to see minimum possible values for the radii to the pressure centres. So it's a bit hard to estimate the most you could get. But if the outer edge of the pad were at the effective disc radius (111.985 mm) and the width of the pad were the square root of 3024.06 sqr mm (55 mm), the pressure centre radius would start at near enough 84.5 mm. Allowing that it moves 8 mm for the change in piston diameter and 0.5 mm for the change in effective disc radius, that gives (84.5 + 8.5)/84.5 = 93/84.5 = 1.101. So assuming those assumptions are correct, the maximum you could possibly expect is 10 percent increase in effort. There might also be some small additional effect from the edge of the four pot's piston being nearer the edge of the pad, but I'd expect that to be around 0.15 times whatever that selvage is. So a small fraction of not very much, i.e. bugger all.

So, these callipers should give between 7.6 and 10 percent improvement in effort for no effect on peddle length.

They will also give either a longer pad life (and be less prone to fade) or allow a higher coefficient of friction (which will give more improvement in effort) for the same life (and, perhaps, fade), because of the greater pad area. If, for a first approximation, you take 3675.58/3024.06, you get 1.21. So, if you can get a pad that gives the same life as the OEM's, you could get something like another 20 percent improvement in brake effort for no more pressure on the pedal. You may also be able to compromise, and get some improvement in effort from the pad and some increased resistance to fade. But, I have to admit, we're now well in the area of speculation as far as I'm concerned.

So, if it were all down to moving the pressure centre, I'd see 8 to 10 percent as a marginal improvement. If you can also gain from the increase in pad area as well, then 25 to 30 percent sounds much more reasonable.  

However, I still think I'd go for a larger four pot piston. With a four pot calliper with 40 mm diameter pistons, you get 10 percent more brake effort from that, and maybe 6 or 8 percent from moving the pressure centre. And that's clearly more than nothing. If you can also get say 25 percent from the ability to use a higher CoF pad (a four pot with 40 mm pistons should have a bigger pad area than one with 38 mm pistons), that sounds like a significant improvement to me.

I suppose there's a worry that, with the improved brakes and tires, you've moved the balance of braking on a clean dry road too far back. I know that sounds the wrong way around, but it's not: if you brake harder, without the front wheels locking, more of the car's weight is transferred forward off the back wheels; and because the maximum force of friction that can be applied before a wheel locks is proportional to the downward pressure on it, which is less, they lock sooner; even if the back tires are improved as well – which means the balance, at the limit of braking, has moved backwards. That would require that you reduce the back brakes a bit, e.g. by using smaller diameter slave cylinders at the back.

It is possible to work out what the effect on brake balance is. But, to do that, you need the height of the centre of gravity of the car. And that's not so easy to find.

There is another effect that I can't assess, partly because I forgot to ask for data but mostly because it's outside my area of ken, and that's unsprung weight. I'm guessing that these new callipers are made from a much lighter alloy than the OEM ones. And with the same total piston area, they probably won't have a very much larger volume. In which case, they are very probably a good deal lighter, and thus reduce unsprung weight. However, I've no idea if that's an issue that bothers you at all.

Graham.

[Einar]

Wow – thanks for a comprehensive reply. Summarizing the calculations and your recommendations, I may search for a 4-pot with larger piston area. The point you make about the balance of braking may worry me, and need to be looked further into. There are various threads about modifying the brakes, but they need some further modifications than just simply swap the calipers. I may go along such a route – time will show 😎

Thanks again for your effort and input 😀

[Nick Moore]

Einar, have you looked at Canley Classics' four pot caliper conversion? Their kit includes larger 254mm vented disc and new caliper mounting brackets. The advantage of the larger disc is that the pads sit further from the hub and provide greater leverage. The disadvantage is that standard 13" steel rims no longer fit. I'm not sure what the piston diameter is, sorry.

I've fitted the CC calipers to my GT6, along with 15" rims. The calipers are unbranded AP (the pads are for an AP caliper so you can do the maths) and the kits bolted on with no tweaking required. My only concern is that some of the paint on the calipers has flaked off - we'll see how they stand up to road conditions. Oh, and they're not cheap but - how to put this - I don't want cheap brakes!

[A TR7 16V]

If you're just upgrading the front brakes, and only by a bit, I don't think you have anything to worry about in regard to brake balance. I think that the worry comes if you want to upgrade front and back brakes together or if you're going totally nuts with the tires and front brakes as well. My concern is that there are those who think that if you're upgrading the fronts, you should also upgrade the backs more or less equally. That does sound sort of logical, I suppose. But, as it turns out, it's actually dangerous bollocks.  

It's bollocks because, as you brake harder than before, more weight is transferred forward, the front wheels are harder to lock, and the back wheels will lock more easily. It's dangerous because if the back wheels lock first you lose directional stability and probably spin. If it's a TR7, that's "you lose what little directional stability you had, and you will spin".

So, if you are increasing the total brake effort, you always want to increase the front brakes more than the back ones. And if you only do the fronts, you obviously are. It's true that if you go absolutely nuts at the front, you can actually need to reduce the back ones – either that, or lower the cars CofG a fair bit. As I said, if you were to get to the point where the back wheels lift off, any back brake effort is too much. But, roughly speaking, the turnover point, where the back effort starts to need to be reduced, is about half way to where the Back wheels have no load on them at maximum braking (would be exactly half way, if the CofG didn't move, WRT the wheels, under deceleration). And I think that is a long, long way from where you start with a Vitesse or a Herald or can get to with road going tires. That is, unless you stick up a mast and heavy canvas sail, and raise the CofG way up high. But what kind of "forest clearing" is going to do a thing like that.

Graham

[JohnD]

But the OP was working on a Herald (?).

I've long had 4-pot fronts (Princess - that long!) with Ford Capri vented disks, more to inhibit fade than to increase braking power which is more than  enough on Herald, Vitesse, all the small-chassis.
For SofS I went for MGF uprights at the rear, to accommodate modern bearings and 'might as well!', the discs that come with them.
I was already to heed your words, Graham, expecting to need either a limiter valve or two master cylinders, to make sure the back never got away, but it doesn't!     The MGF has single pot, sliding calipers, so maybe loses a very little in friction, but the brakes remain in perfect balance, no need to modulate the brake balance at all!

John

[A TR7 16V]

I think the issue of rear discs vs. drums is more complicated than commonly understood. That's because there's a self-servo effect on the leading edge shoe(s), where the action of the drum on the shoe pulls it into the drum - in effect, a limited amount of positive feedback. I have a nasty suspision that it's also non-linear.

That self-servo effect does make the calculation of the brake effort from a drum rather more complicated than the disc and calliper, so it can be hard to work out if you actually are upgrading or not. I'd probably try to guess what the rear drums are doing from the effort of the fronts and what the balance should be, where known.

But if you put rear discs on, it's fairly easy to work out what the ratio is (unless there's some kind of reduction/delay device in the system; then you're on your own). But working out what the limit is, i.e. the perfect balance, is not so easy.

The interesting question would be, if you did set the brake balance behind the perfect balance for the very cleanest, dryest road you're ever likely to drive on, with nicely warmed new tires, etc., would you know that was the case before you try to brake hard in that extreme case?

I ask that, because for all other road surfaces the perfect balance moves backwards as the CofF is reduced. So, what's safe on a wet surface, in the sense that the fronts will always lock first, isn't necessarily safe when there's more friction.

Graham

[Einar]

Yes, I’ve looked at the upgrade provided by Canley, but as they need 14’’ rims or larger I may need to do some further modifications. Also any 4 pot caliper with 40mm pistons may need further modifications. The benefit of the ones provided by BCC is that it’s just a simple swap – no modifications required.  I think I may consider calipers with larger piston diameter with reasonable modifications required – need to check clearance etc. Anyway, thanks all for the contribution in this thread – I’ve learned much about brakes and their handling. I may also leave the brakes as they are – as they are good…… 🙂