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Triumph Torque  /  Herald/Vitesse  /  Brake cylinder bore sizes
Posted by: mikeyb, October 22, 2008, 5:31pm
Been messing about with the Herald's brakes today and have been wondering about correct/optimum sizes of master cylinder and rear wheel cylinder bores.

The car was fitted with a 5/8" small reservoir master cylinder and 0.75 (3/4") rear wheel cyls.
It started life with drum brakes all around, but has been converted to Spitfire discs at the front.  It does not have a servo.

I fitted new 0.7 (11/16") wheel cylinders.  I expect that fitting a smaller bore to the rear should increase braking force at the rear.

I am also contemplating fitting a 3/4" master cylinder (a la GT6/Vitesse), if my theory is correct, then I would expect less pedal movement but more foot effort required.

Anyone disagree?

What's the optimum combination of wheel/master cylinder?

cheers

Mike!

Posted by: CharlieB, October 22, 2008, 5:56pm; Reply: 1
Smaller bore wheel cylinders will give less braking force not more (Force = Pressure x Area).
You're right about the master cylinder.
Posted by: thescrapman, October 22, 2008, 6:42pm; Reply: 2
Quoted from CharlieB
Smaller bore wheel cylinders will give less braking force not more (Force = Pressure x Area).
You're right about the master cylinder.


That is what we thought, then we started thinking about elephants in stilettos.

10 ton elephant walking across a wooden floor exerts 10 tons spread across 4 large flat feet. Floor survives as the pressure is quite low across the soles of the feet. Put the elephant in stillettos and the floor gets damaged as it has exerted a much higher force onto each square inch of floor.

I think we both wished we had listened in Physics.

:-(

Colin
Posted by: KevinR (Guest), October 22, 2008, 6:51pm; Reply: 3
Quoted from thescrapman
That is what we thought, then we started thinking about elephants in stilettos.


Wrong thread, see http://club.triumph.org.uk/cgi-bin/forum10/Blah.pl?m-1224537406/s-37/#num36

Although I didn't look for long enough to see if they were wearing stilettos or not
Posted by: Freebird, October 22, 2008, 7:24pm; Reply: 4
I agree with CharlieB, I don't think the elephant in steletos analogy is the same as what happens in the wheel cylinder.

The elephant has a mass x gravity (force, measured in pounds for us old fogies), the steletos a small surface area (Square inches), therefore it exerts a high pressure on the floor (PSI).

In the wheel cylinder, there is a set pressure (PSI) given by the size of the master cylinder and how hard you stand on the middle pedal. This pressure is applied to the area of the slave cylinders (Square inches), so the bigger they are, the more force you get. The downside is the volumne of the slave cylinders is also bigger, so you will get more pedal travel than for a smaller bore version.

This is why you match the master and slaves, which is why you asked the question. Unfortunatly I don't actually know the answer. Not much help - sorry.
Posted by: CharlieB, October 22, 2008, 7:33pm; Reply: 5
With the Elephant example the force is constant (weight of elephant) so the pressure on the floor is dependant on the area of the foot/stilletto (Pressure = Force/Area) Large pressure over small area or small pressure over large area.
The brake system is the other way round, you have a given pressure in the brake line & the force on the pad is dependant on the area of the piston (Force = Pressure x Area) Large area, large force or small area, small force. With a larger piston you also get less travel.

Edit: Freebird got there first!
Posted by: Freebird, October 22, 2008, 8:40pm; Reply: 6
Charlie

We must have been typing in unison!

Mikey

Triumph would have done a lot of homework, so I suggest you fit standard Spitfire gear all round if you have already got Spitty discs.

Glen.
Posted by: mikeyb, October 22, 2008, 8:42pm; Reply: 7
Ok, but what combination was standard?
Posted by: Freebird, October 22, 2008, 9:19pm; Reply: 8
That I don't know, but the suppliers will.

Another reason to stick to a standard setup is it's very important to get the front to rear balance right, you don't want to be going through a hedge backwards!
Posted by: Rarebits, October 22, 2008, 9:48pm; Reply: 9
A disc braked (type 14) Herald should have 3/4" rear cylinders, with a 5/8" master cylinder.
Most Spitfires had 5/8" wheel cylinders, though later Spitfire 1500s went to 0.7" cylinders. This was concurrent with the switch to a dual master cylinder for LHD cars. Home market cars got the larger slaves for some time before the dual brake master cylinder,
Cheers,
Bill.
Posted by: junkuser, October 22, 2008, 9:51pm; Reply: 10
Spitfire IV
Master Cylinder    5/8"
Rear Wheel Cylinders  3/4"
Posted by: junkuser, October 22, 2008, 9:59pm; Reply: 11
Sorry Bill, I was typing.

The 3/4" rears seem to be standard on Australian early Spits according to brake parts manual but this doesn't mean that is what Standard specified.

No rear lockup on my Spit using the 3/4"s
Posted by: Richard B, October 23, 2008, 8:36am; Reply: 12
Quoted from thescrapman


That is what we thought, then we started thinking about elephants in stilettos.



Please, please, please do not let Ferny read this thread; there are images I really do not want him to conjure with......  :o

Posted by: KevinR (Guest), October 23, 2008, 10:02am; Reply: 13
This article makes good reading.
It explains a lot about the relative bore diameters of the master and slave cylinders.

http://www.hotrodheaven.com/tech/brakes/brakes1_index.htm
Posted by: mikeyb, October 26, 2008, 5:53pm; Reply: 14
Bugger, having invested in a new 5/8 master cylinder and pair of 3/4 wheel cylinders, i've just realised that the Herald actually has Vitesse rear brakes at the back - so the question now is

"What size rear wheel cyls do Vitesses have?"
Posted by: Richard B, October 26, 2008, 6:13pm; Reply: 15
Quoted from mikeyb
Bugger, having invested in a new 5/8 master cylinder and pair of 3/4 wheel cylinders, i've just realised that the Herald actually has Vitesse rear brakes at the back - so the question now is

"What size rear wheel cyls do Vitesses have?"



According to Rimmers Vitesse 2litre "Wheel Cylinders (3/4" Cyl) - (NI) - GWC1208"

According to James Paddock Vitesse 1600  "GWC1110 REAR WHEEL CYLINDER 1600"

;)


So not sure what size 1600's had but it was different to 2 litres.
Posted by: Richard B, October 26, 2008, 6:15pm; Reply: 16
ps This might help Rimmers GT6 listing;

Quoted Text
Wheel Cylinder;
Mk1 -
To KC7278 (" bore) - (2) - GWC1208
From KC7279 (5/8" bore) - (2) - GWC1202
Mk2 (all models) (5/8" bore) - (2) - GWC1202
Mk3 (to KE/KF20000) (11/16" bore) - (2) - GWC1110
Posted by: mikeyb, October 26, 2008, 6:53pm; Reply: 17
Thanks, I never thought to look at Rimmers (expecting just part nos).  

Right, looks like I may need to try a 3/4 master cyl then!  
Posted by: mikeyb, October 26, 2008, 6:56pm; Reply: 18
BTW the reason I'm agonising over all of this is that the car's brakes were pretty much on the floor.  Having overhauled calipers and fitted new master and wheel cyls, (5/8 & 3/4) the brakes are still well down the pedal.  I'm 100% sure that there is no air in them now!
Posted by: Nick Jones, October 26, 2008, 7:19pm; Reply: 19
Quoted from mikeyb
BTW the reason I'm agonising over all of this is that the car's brakes were pretty much on the floor.  Having overhauled calipers and fitted new master and wheel cyls, (5/8 & 3/4) the brakes are still well down the pedal.  I'm 100% sure that there is no air in them now!


Flexihoses! - 1 or more gone squashy.  Clamp them off at the inboard end and see what happens..... took me ages to figure that one out.  Local garage and the Triumph 'specialist' I was working for at the time were baffled too.....

Nick

Posted by: mikeyb, October 26, 2008, 7:53pm; Reply: 20
hmmn, others have suggested that . . .  but they are Goodridge Steel Braided - tell me if you think different, but I don't reckon they go squashy?
Posted by: junkuser, October 26, 2008, 8:40pm; Reply: 21
Rear wheel cylinder size is dictated by the weight distribution of the car and the ratio of breaking efficiency front to back.  
The cylinder sizes are thus more related to the car they are on than the car they came from.
The Vitesse would have more wheight over the front wheels than a Herald and thus could well have had smaller rear wheel cylinders to avoid rear lockup but I really don't know.
Assuming the front brakes from the Vit are the same as used on early Spitfire then the Spit rear cylinders should balance out OK.
Posted by: Richard B, October 26, 2008, 8:45pm; Reply: 22
It's not loose wheel bearings allowing the pads or shoes to be knocked back?  :-/
Posted by: mikeyb, October 26, 2008, 9:00pm; Reply: 23
Quoted from junkuser
Rear wheel cylinder size is dictated by the weight distribution of the car and the ratio of breaking efficiency front to back.  
The cylinder sizes are thus more related to the car they are on than the car they came from.
The Vitesse would have more wheight over the front wheels than a Herald and thus could well have had smaller rear wheel cylinders to avoid rear lockup but I really don't know.
Assuming the front brakes from the Vit are the same as used on early Spitfire then the Spit rear cylinders should balance out OK.


Front brakes are Spit, rear are Vit - Vit calipers/discs are larger (same as GT6), think this would take into account heavier engine.  

Quoted from richardb
It's not loose wheel bearings allowing the pads or shoes to be knocked back?


unfortunately not!

The car has been devekoped/modified over the last 18 years for rallying, that's why its not quite to standard spec!
Posted by: CharlieB, October 26, 2008, 9:10pm; Reply: 24
Quoted from mikeyb
Front brakes are Spit, rear are Vit.......

Might be better to fit smaller rear wheel cylinders to restore balanced braking?
Posted by: Richard B, October 26, 2008, 9:25pm; Reply: 25
Quoted from mikeyb

Front brakes are Spit, rear are Vit - Vit calipers/discs are larger (same as GT6), think this would take into account heavier engine.  



Thats the Vitesse 1600 brake spec then. Type 12 (or 14?) Calipers & nine 8'' Drums or am I talking bullshit again?  :B

What size M/C did the 1600 have?

ps yes I was Vitesse is 8'' not 9'', 2000 & Stag is 9''
Posted by: junkuser, October 26, 2008, 9:30pm; Reply: 26
Was thinking early Vit had 7" drums.    Wrong!
8" drums would require less cylinder diameter to achieve same breaking effect.
The rear axle loading on a Herald would be greater than on a Spitfire so there is another variable to throw in.
Posted by: mikeyb, October 26, 2008, 9:41pm; Reply: 27
Quoted from CharlieB

Might be better to fit smaller rear wheel cylinders to restore balanced braking?


brake balance seems fine, it's pedal travel I'm trying to eliminate.
Posted by: junkuser, October 26, 2008, 10:01pm; Reply: 28
Was the hole in the upper end of the brake pedal lever a neat fit on the clevis pin?
Is there full movement of the master cylinder?
If the rear brakes are correctly adjusted and there is no air in the system then deteriorating hoses could be the cause but these would most likely give a spongey feeling to the brakes.
Will be interesting to hear what you discover.
Posted by: Nick Jones, October 26, 2008, 10:18pm; Reply: 29
Quoted from mikeyb
hmmn, others have suggested that . . .  but they are Goodridge Steel Braided - tell me if you think different, but I don't reckon they go squashy?


I think you have that covered then.... dunno what else to suggest.  5/8 master should be ok with Spit calipers - not sure the rear wheel cylinders make much difference on that score.  Not normally too hard to bleed either, gravity usually does the job on my Vitesse.

I would say clamp off various wheels to see if you can isolate the problem, but thats a challenge with braided hoses....... :-/

Nick
Posted by: mikeyb, October 27, 2008, 6:28am; Reply: 30
The only other thought that has come to me is to look at the pedal box, ithe accelerator has been modified to get rid of the organ pedal, but the whole lot feels skewed to the right to me  . . . or are all Heralds skewed this way?
Posted by: ferny, October 27, 2008, 7:27am; Reply: 31
All Heralds go a long way to the right.


So the brakes actually work fine, it's just the pedal travel which is the problem? If so it must be related to the pedal. You say it's been modified, so maybe the position has been changed and now it needs a longer through? I assume all MC pushrods are the same length. I notice Canley's have an adjustable one listed with a part number and no price. Could be an idea... Won't be hard to make.
Posted by: thescrapman, October 27, 2008, 3:23pm; Reply: 32
Quoted from ferny
All Heralds go a long way to the right.


So the brakes actually work fine, it's just the pedal travel which is the problem? If so it must be related to the pedal. You say it's been modified, so maybe the position has been changed and now it needs a longer through? I assume all MC pushrods are the same length. I notice Canley's have an adjustable one listed with a part number and no price. Could be an idea... Won't be hard to make.


No, the brakes don't work OK, Mike just has nerves of steel, I have underpants of steel, and they weren't enough to hold it all in when I had a go!

It is spongey squish almost all the way to the floor, so it is moving fluid, then it was like the same feeling as when you have cooked your brake pads, massive amounts of push resulting in only a slow deceleration.

Cheers

Colin
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