touchy feely

[LoBelly]

I need help with feeling

My brakes are very effective but dont really give a lot of feedback.

I think i read in a past Snaketales that this could be due to the incorrect assembly of the master cylinder&booster combo.

My vague recollection was that the result was that more brake resulted from pushing the pedal further rather than pushing the pedal harder - which is pretty much how my brakes are reacting now.

They stop really well but its just too easy to lock up.

Did I just imagine the article? Does anyone have any advice.

(have jag brakes and what looks like falcon master cyl & booster)

thanks

LoBelly

[Rob. Smith]

G'day,
I remember something about blocking the front port outlet off.. on the ford master cylinder to make the two individual front brake circuits into a single circuit ( this is the set-up on my RMC and it stops fine ) I got my info from an old, old issue of snake tales and it was confirmed by Norm Thoroughgood, a long time pitcrew manager for Grice racing and one of those old school blokes that just happen to know all the part numbers and specs. for brakes for the past 30 years with books to confirm it !
Smithy

[Rebel1]

LoBelly, I can confirm what Rob has said for the RMC.

Single line to the front brakes then branching to two lines at the front cross member.

There has also been some in-depth discussion regarding the front brakes over-powering the back brakes. By that I'm saying that all the braking seems to happen from the front with the back brakes doing very little. The end result is locking the front wheels without much retardation and lack of feel.

Some folks have resorted to fitting nissan front calipers which although 4 pot calipers use much smaller pistons. This allows the back brakes to contribute more to the stopping of the vehical. This is not an easy mod as the bolt centers of the nissan are different to that on the Jag uprights.

I also read somewhere ( I think from a UK source) That the fitting of the earlier Jag Calipers ( 3 piston I think) is a much easier fix.

However, hopefully some of the others using the Jag gear may post with better suggestions of a fix.

I'd like to get rid of my front calipers mainly from a weight reduction/handing aspect. The Jag front calipers sure are heavy.

Cheers

[LoBelly]

thanks for the feedback guys

It might be that I just have to learn to drive around it and stop comparing what happens in a cobra to what I'm used to in sedans

lotsa fun learning

LoBelly

[Aussie Mike]

A lot of the stuff I've read about getting Cobra brakes working right is band-aid fixes and I find that a little disturbing.

I would strongly advise against blocking off one port of the master cylinder and converting the system to single circuit. Apart from being illegal under the ADR's the whole reason for having dual brake circuits is safety. If you pop a seal on a caliper or burst a line on a single circuit system you have no brakes at all. At least with a dual circuit system you will still have front brakes or rear brakes in the event of a failure.

The problem with many Cobra brake systems is badly matched parts. Brakes are simple friction and hydraulics. In it's simpelest form you are turning mechanical effort from your foot into clamping force on the discs to create friction and slow the car. If you add a brake booster to the system you are effectively adding extra mechanical force at the pedal. If this is making the brakes touchy without the feel at the pedal then you probably have too much mechanical advantage in your system.

There are some typical figures I have at home of what a good pedal pressure should be. I think it was around 50KG but I'll check and get back to you.

So you take this 50KG your foot applies to the pedal and multiply it by the pedal ratio (probably around 5 or 6:1 for a boosted system). This gives you 250-300KG of force on the brake booster push rod. The brake booster has 1 or 2 diaphrams inside that are pulled in by engine vacume. There's a one way check valve to hold the vacume in the booster so you still have boost when your engine has low manifold vacume ie accelerating or stalled. The surface area of the booster diaphram or diaphrams can be quite large so it can add quite a bit of extra force on the master cylinder when you press that pedal.

I believe the force the booster exerts is also modulated by the amount of pressure applied on it by the pedal. This could be a fault with yours.

Your master cylinder is a dual circuit design meaning that inside it there are 2 pistons, one in front of the other. front to rear brake bias is often controled in these systems by either having different bore sizes i.e. the front piston will be smaller than the rear and feed the rear brakes with their smaller pistons. Some have a proportioning valve inside them that uses a heavy spring between the pistons to absorb some of that pedal pressure and give the desired pressure difference the calipers need from front to rear.

The problem with a lot of our cars is that they are built from donor parts. A MC from one car and calipers from another all bolted into a car that weighs much less and has a different front rear weight bias. It's highly unlikely that all these parts will suit each other as the components did in their parent cars.

So if your brakes a locking with only light pressure either your pedal ratio or the booster are giving you too much mechanical leverage. You shold perhaps look at removing some of the mechanical advantage by changing your pedal ratio or booster size. Alternatively you could change your hydraulic leverage. A smaller MC bore will give more ratio between the MC and the caliper pistons. A larger bore MC will give a smaller ratio and less pressure at the caliper.

Brake proportion is the other problem that gets people in trouble. The back brakes lock up before the fronts or the front brakes are doing all the work and the rears do nothing. Ideally you want the fronts to lock shortly before the rears. This will keep the car tracking straight under hard braking. I've heard of some racing drivers that set their brakes to lock the rears before the front so they can back the car into a corner and get it to turn in but that's not for road cars or us mere mortals.

Done right the brake proportion needs to be designed into the system either by using the right mix of piston sizes front and rear or a stepped MC (or seperate MC for front and rear circuits). The proportioning valve should be an aid to fine tuning the system but the system needs to be in the ball park in the first place.

It can be an expensive proposition buying a set of calipers with right size pistons however there are some reasonably priced adjustable proportioning valves from Wilwood and Tilton. One can be incorporated into the rear or front line to drop some pressure to those calipers. These are basically a spring loaded piston that absorbs some of that line pressure. Varying the preload on the spring varies how much pressure they absorb. Wilwood quote between 100 and 1000 PSI for theirs.

Get your pedal pressure issue sorted first and then you can chase your brake proportion when you can feel what's going on.

Cheers

*edited for spelling

[Murphy]

Could not help but to reply to another Mike Murphy.

[Aussie Mike]

Nice to meet another Mike Murphy. Good to see there's more than one of us that's a Cobra nut.

Cheers

[Gav]

Mike, I think the only thing you have forgotten, was to quote who it was that invented the modern braking system..
Yeeesh!

[Rob. Smith]

Aussie Mike is right...my mention of making two lines for the front into one is RTA approved but more importantly it has been engineer calculated to correct specs with the correct Ford master cylinder.PBR 1" bore. We measured master cylinder volumes,slaves, brake line volumes,caliper volumes,disc "wiped" area dimensions and brake pad surface areas (my engineer,now partially retired,) argued and won with the RTA that these specifications were above any safety standard requirements and the simple fact that the brake system was designed to work efficiently for a 2 ton car and was tested a total of six individual times and produced highly acceptable results every time. The system in my car is good ! I'm a fussy dick and this system looks OK to me. I have put 280 Ks on the car before rego and about 18 after...I will be looking for lighter calipers in the future along with "disc hats" on the front end instead of disc/bearing/studs. I've got peg drive jobbies.
Don't be scared of brakes...they are pretty basic when you know all the facts.... just don't take shortcuts.
Good Luck... !
Smithy.

[David Hodgson]

Originally Posted by Rebel1

I also read somewhere ( I think from a UK source) That the fitting of the earlier Jag Calipers ( 3 piston I think) is a much easier fix.

Thats correct Les, Chris whom you spoke to in CBR re radiators, tooled around with Willwood rears and the like, but settled on Jag 3 spot for the front and Jag for the rear, fix the bias problem but does not fix the weight issue.