mikeyb Posted October 22, 2008 Share Posted October 22, 2008 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?cheersMike! Link to comment Share on other sites More sharing options...
CharlieB Posted October 22, 2008 Share Posted October 22, 2008 Smaller bore wheel cylinders will give less braking force not more (Force = Pressure x Area).You're right about the master cylinder. Link to comment Share on other sites More sharing options...
thescrapman Posted October 22, 2008 Share Posted October 22, 2008 CharlieB wrote: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 Link to comment Share on other sites More sharing options...
Deleted User Posted October 22, 2008 Share Posted October 22, 2008 thescrapman wrote: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/#num36Although I didn't look for long enough to see if they were wearing stilettos or not Link to comment Share on other sites More sharing options...
Freebird Posted October 22, 2008 Share Posted October 22, 2008 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. Link to comment Share on other sites More sharing options...
CharlieB Posted October 22, 2008 Share Posted October 22, 2008 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! Link to comment Share on other sites More sharing options...
Freebird Posted October 22, 2008 Share Posted October 22, 2008 CharlieWe must have been typing in unison!MikeyTriumph 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. Link to comment Share on other sites More sharing options...
mikeyb Posted October 22, 2008 Author Share Posted October 22, 2008 Ok, but what combination was standard? Link to comment Share on other sites More sharing options...
Freebird Posted October 22, 2008 Share Posted October 22, 2008 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! Link to comment Share on other sites More sharing options...
heraldcoupe Posted October 22, 2008 Share Posted October 22, 2008 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. Link to comment Share on other sites More sharing options...
junkuser Posted October 22, 2008 Share Posted October 22, 2008 Spitfire IVMaster Cylinder 5/8"Rear Wheel Cylinders 3/4" Link to comment Share on other sites More sharing options...
junkuser Posted October 22, 2008 Share Posted October 22, 2008 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 Link to comment Share on other sites More sharing options...
Richard B Posted October 23, 2008 Share Posted October 23, 2008 thescrapman wrote: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 Link to comment Share on other sites More sharing options...
Deleted User Posted October 23, 2008 Share Posted October 23, 2008 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 Link to comment Share on other sites More sharing options...
mikeyb Posted October 26, 2008 Author Share Posted October 26, 2008 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?" Link to comment Share on other sites More sharing options...
Richard B Posted October 26, 2008 Share Posted October 26, 2008 MikeyB wrote: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. Link to comment Share on other sites More sharing options...
Richard B Posted October 26, 2008 Share Posted October 26, 2008 ps This might help Rimmers GT6 listing;Quote:Wheel Cylinder;Mk1 - To KC7278 (¾" 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 Link to comment Share on other sites More sharing options...
mikeyb Posted October 26, 2008 Author Share Posted October 26, 2008 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! Link to comment Share on other sites More sharing options...
mikeyb Posted October 26, 2008 Author Share Posted October 26, 2008 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! Link to comment Share on other sites More sharing options...
Nick Jones Posted October 26, 2008 Share Posted October 26, 2008 MikeyB wrote: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 Link to comment Share on other sites More sharing options...
mikeyb Posted October 26, 2008 Author Share Posted October 26, 2008 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? Link to comment Share on other sites More sharing options...
junkuser Posted October 26, 2008 Share Posted October 26, 2008 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. Link to comment Share on other sites More sharing options...
Richard B Posted October 26, 2008 Share Posted October 26, 2008 It's not loose wheel bearings allowing the pads or shoes to be knocked back? :-/ Link to comment Share on other sites More sharing options...
mikeyb Posted October 26, 2008 Author Share Posted October 26, 2008 junkuser wrote: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. richardb wrote: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! Link to comment Share on other sites More sharing options...
CharlieB Posted October 26, 2008 Share Posted October 26, 2008 MikeyB wrote:Front brakes are Spit, rear are Vit.......Might be better to fit smaller rear wheel cylinders to restore balanced braking? Link to comment Share on other sites More sharing options...
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