4 Pot Calipers

[Darren Groves]

Some years back the TSSC Club Shop used to sell a 4 pot caliper conversion (see image). The brake calipers & pads were Austin Princess, but trying to determine what the discs are as they are not standard. HQ don't seem to know and it pre-dates Garth's time there, so he's not 100% sure.

Anyone have these fitted and know where to source the discs?

 

[Pete Lewis]

Club shop do sell 4 pot calipers use same fixing centres and use std discs   bit pricey  page 19 of parts brochure 

 They are light weight ,   the princess caliper weighs very heavy and not exactly stacked on shelves these days 

Pete

 

 

[Clive]

My old spitfire came with princess calipers and std GT6 discs..... I was certain they were GT6, but you have me thinking now. They certainly didn't have any redrilled holes for attachment to the hubs, and were the same diameter as GT6.

I will stick my oar in here. Those calipers are really heavy, and I ditched them for std GT6 calipers, (but spaced, with vented capri discs) If you can get GT6 uprights...much simpler, lighter and just as good. If you find ford calipers, they have spitfire lug spacing. Not sure how discs etc would work easily, but can't be too hard....

[Darren Groves]

The discs that came with this kit are a larger diameter and are thicker than standard....Garth seemed to think they were modified Capri discs supplied by EBC, but EBC say not. Capri discs are easy enough to source and I can get them re-drilled, but finding something off the shelf would be easier.

The kit was so cheap that I'm happy to give it a try, just wanted to ditch the cross drilled discs that came with it as they are a bit noisy I find.

[Clive]

Capri discs are the same size as GT6. The vented capri discs I use fit perfectly with std spaced GT6 calipers. 245mm from memory.

[JohnD]

Me too.  Capri discs, that is.

Given a well.mai mm trained brakevsysyem, centers are the best possible mod.

4 pots, vacuim boosters, pfui!

John

[Darren Groves]

14 minutes ago, clive said:

Capri discs are the same size as GT6. The vented capri discs I use fit perfectly with std spaced GT6 calipers. 245mm from memory.

So in theory then a GT6 disc will be fine with the Princess Calipers? Spacing might need sorting?.....If that's the case, will a GT6 Disc fit a Herald hub?

[Clive]

Yes!

[thescrapman]

A princess calipers will not bolt to a GT6 upright, you need type 12 Spitfire calipers mounts to replace the type 16 ones.

more recent disc can be sourced from a Citroen Xantia I think.

or for bigger a Nissan something.

[Pete Lewis]

Surely to take care of the increase in  displacement volume you now need a bigger diameter mastercylinder  or face a low pedal

So up goes the pedal load input ...  whats to gain ??  Same reason they went from 5/8" on type 12/14s  to 0.7"  with type 16s

Pete

[hugh]

Used hispec ultralite 4 pots with the princess spacing, fits spitfire with Capri 2.8i discs redrilled for spitfire pcd uses same pads as lotus Elise so loads of fancy pads available as they are alloy they weigh virtually nothing compared to the standard callipers, and if they stop a 220bhp turbo spitfire there is no need to go silly, they will just fit under most 13" alloys but not steels we used mgf and montego 15" alloys with the above setup, also beware princess callipers with 13" wheels the corners need grinding away for clerance on most wheels

By the way the standard discs and callipers fried in the welsh hills in less than 3 miles with the turbo engine

[Darren Groves]

8 hours ago, thescrapman said:

A princess calipers will not bolt to a GT6 upright, you need type 12 Spitfire calipers mounts to replace the type 16 ones.

 

They're on Herald (Type 14) uprights and fit just fine.

6 minutes ago, hugh said:

 also beware princess callipers with 13" wheels the corners need grinding away for clerance on most wheels

I've got JBW Minilights, it's a close fit but nothing touches.

[hugh]

Like I said most, its a case of try it  and if the wheels don't fit be prepared

[hugh]

I believe that for the gt6 the m16 type fit but you may need the adapter mounting bolts to convert from metric to unf

[thescrapman]

1 hour ago, Darren Groves said:

They're on Herald (Type 14) uprights and fit just fine.

I worded my answer slightly wrong.. ?

Spitfire uprights are fine, some people want the bigger stub axles so get GT6 uprights, which the calipers do not fit. 

My calipers were filled and drilled to GT6 spacing which is the other solution to type 12 mounts.

[dave.vitesse]

The problem with increasing the pad size on existing disc is you generate more heat which can increase brake fade. The best way to go is either larger disc and/or vented disc, then a larger pad area. Though not a Spit or Herald, I used Capri 2.8i vented discs and calipers on the TR7. With a larger master cylinder, servo and change to the rear slave cylinder to match the increase in the front braking.

When I had the Herald Estate with tuned 1500 engine I fitted 2 Litre brakes all round. This did require 2 litre front uprights and rear back plates but to me it was a good option on the Herald.

Just my own experience

Dave

[Colin Lindsay]

16 hours ago, Pete Lewis said:

Club shop do sell 4 pot calipers use same fixing centres and use std discs   bit pricey  

No doubt good stuff, I considered them a few months ago, but I reckoned the price wasn't worth the use I'd get from them. 

[JohnD]

Brakes are not intuitive!   

A "larger brake pad area" does not inscrease braking force, as friction is independent of area.

Friction = Coeff.of friction (for the materials concerned) x Force pressing them together.

Increase the area, and unless you do something else to increase the hydraulic force in the caliper, then the force perunit area will be LESS and the friction foprce will FALL.

But you are acting over a larger area, so more force, but the result is exactky the same as with the original, smaller pad!

 

A smaller master cylinder will allow you to exert more force, at the expence of greater travel.

If your driving style puts more heat into the brakes than the original designers expected, and your problem is fade, then vented discs, or else a 'hotter' brake material will do the trick.     4- and even 6-pot calipers allow ther brake material to be positioned further out from the brake disc centre, so that the same friction force exerts a greater braking moment on the wheel, but that is one raosn why wheels with greater diameter andlow profile tyres are the mode in competition cars.

John

[yorkshire_spam]

I have a fairly standard brake set-up on the 1500 Spitfire apart from the M1144 compound pads. 

As far as I can tell my limiting factor/bottleneck in braking is not the brakes, it's the adhesion/grip of the tyres. My setup provides good progressive bite, square braking, reasonable front/rear balance. If I push the pedal hard enough I can lock the fronts.... at that point really what I need is grippier tyres, not more braking.

[JohnD]

Yes, YS, tyres are always the limiting factor, but if you can do as you say that's a good test for a well functioning brake system.

John

[Colin Lindsay]

On 7/11/2018 at 1:12 PM, yorkshire_spam said:

I have a fairly standard brake set-up on the 1500 Spitfire apart from the M1144 compound pads. 

As far as I can tell my limiting factor/bottleneck in braking is not the brakes, it's the adhesion/grip of the tyres. My setup provides good progressive bite, square braking, reasonable front/rear balance. If I push the pedal hard enough I can lock the fronts.... at that point really what I need is grippier tyres, not more braking.

That's not the brakes then, that's driving style. Once you lose grip you've lost control completely and no amount of servos, uprated brake pads or multiple calipers is going to make any difference. 

[dougbgt6]

On ‎7‎/‎11‎/‎2018 at 12:41 PM, JohnD said:

A smaller master cylinder will allow you to exert more force, at the expence of greater travel.

All very well but, you will need thighs like a sumo wrestler. I tried a slightly smaller master when I had brake problems, only a 1/4" smaller but increased the travel by 3" and I really had to stamp pretty hard to get original effect. 

Doug

[JohnD]

Doug,

Sorry to hear that.    The extra travel is inevitable, and the extra braking force is also inevitable, with the same foot pressure application.   It's just physics, no need for sumo thighs, something must have been at fault in the installation.

Do a bit of reading on hydraulics, eg, 

"Hydraulics 

The master cylinder translates the pedal movement into fluid movement. The bore size dictates how much fluid is moved for any given pedal pressure, and is directly related to the main brake component - the caliper. Bigger diameter or multiple pistons will need more fluid displacement to make them work properly. This generally means a larger bore master cylinder is needed for any given application. However, as this component can be very costly to replace, most folk stick with the standard one. It will work OK, but means that on the multi-piston calipers there is more pedal travel than normal. The good news is that a smaller bore means higher line pressure for any given pedal pressure - less driver input for same braking effort, and better ‘feel’. 

 

'Feel', incidentally, is the sensation of what the brakes are up to, and is an important part of our thought process as to how much pressure we need to apply to the pedal in any given situation. 

 

For information sake, the Mini master cylinder has a 0.70” bore. The old Cooper 997 had a bore of 0.750” largely because of the pathetic size of the discs - a great deal of pressure was needed to slow the car down but would be ace on a four-piston caliper set-up. The old Austin 1300 also had a 0.750” master cylinder. Both of these types are desperately rare now though. Some misguided folk have actually fitted a clutch master cylinder in place of the brake one as it also has a 0.750” bore. This is EXTREMELY fool-hardy as it has no internal return feature, so could leave the brakes on all the time once applied! 

 

For race Minis, going to a 0.875” bore master cylinder is best. There isn’t one that looks and fits just like the Mini one, it necessitates going to a master cylinder with separate fluid reservoir and making a suitable push-rod. No problem though as Girling and AP make a cylinder that will fit the stud pattern, a standard Mini push-rod can be easily modified to suit, and the reservoir is easily mounted to the bulk head. And is surprisingly cheap - less than a new Mini split-system one in fact! 

 

When trying to get the best master cylinder bore size for application, you need to remember that the amount of hydraulic pressure produced at the pedal is INVERSELY proportional to the master cylinder bore. So if you are locking the brakes up too easily, you need to INCREASE the bore size. Consequently if you are standing on the pedal, pulling of the steering wheel and gritting your teeth together to lock the brakes, a smaller bore is the order of the day. "

(From https://www.calverst.com/technical-info/brakes-how-they-work/)

JOhn

 

[Darren Groves]

Car's back on the road after being poorly for a couple of weeks, so can finally report back on the brakes. I decided to fit a larger 0.7" bore M/C  (Vitesse/GT6), got all the air out the system after a bit of a struggle by using a Vizibleed (no helper available), leaving the cross-drilled discs on for a bit and will revisit that later and try some Vitesse/GT6 jobbies. Certainly the best/most reassuring the brakes have ever felt in my 20 years of ownership, so has been a worthwhile upgrade.....