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paging disc brake ratio experts
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Author:  gmader [ Sun Jan 03, 2010 3:10 pm ]
Post subject:  paging disc brake ratio experts
Hi all,

So, I have Andy F's Viper disk conversion on my front brakes, and now, ford Cobra R calipers and rotors on my ford 8" rear. I am using one of Richard Ehrenberg's big aluminum mopar manual master cylinders.

Problem: The rear calipers seem to need much less fluid than the fronts, and even with a prop valve, the front brakes don't get enough pressure to really activate.

Symptoms
  • The pedal is hard-- I have bled the heck out of it, and it is very firm

    The car stops in a leisurely manner. It is like there is no initial bite from the brake pads.

    I had bigger calipers (cadillac) on the back at one point, and they had the opposite problem- The front's got too much pressure, and the rears were along for the ride. It stopped great, but wasn't optimum.

    I have the prop valve closed all the way, to minimize pressure to the rear brakes.
So... What are my options?
  • 1. Go with a smaller master cylinder? It would give more pedal travel, but I am not sure that it would increase pressure to the fronts.

    2. Go with dual master cylinders, with a balance bar? This is maybe the best option. I have read a bunch of stuff on this, but to be honest, I haven't see the definitive article or formula for calculating this. Is there a good reference to recommend?

    3. Go with Power brakes? Sleazy, but might do the trick- again, it doesn't really solve the balance problem.

    5. Change out one or more of the calipers to something else? I don't really want to do this. What I have fits well, and should work.

    4. Something else that I haven't thought of before?
Thoughts?

Greg
Author:  dakight [ Sun Jan 03, 2010 4:06 pm ]
Post subject: 
I know about enough to be dangerous, but do you by chance have a residual pressure valve somewhere that might be holding pressure in the rear circuit?

It makes no sense that larger calipers in the rear would give better balance. Smaller pistons on the rear should shift more of the braking force to the front; if it's not doing this then something else is wrong. All the other remedies you've suggested will only mask the real problem. You need to find and fix that first.

In theory you really shouldn't need a prop valve for rear disks if they are sized properly because disk brakes are linear in operation while drum brakes are designed with a servo action such that the braking force is proportional to the square of the pressure. In other words double the pedal force and you will quadruple the braking force at the wheel.
Author:  sandy in BC [ Sun Jan 03, 2010 4:15 pm ]
Post subject: 
What did AndyF say?

I have a stock iron MC and stock Prop v/v on my Valaint.

It has 73 disc on the front and Explorer disc on the rear. Stops fine.
Author:  Joshie225 [ Sun Jan 03, 2010 5:57 pm ]
Post subject: 
That's odd. The only car I've driven with nearly the same setup as yours had too much front brake. Are you actually locking up the back brakes? Did you change anything other than going from GM/Cadillac to Ford Mustang Cobra rear calipers?

If you want a softer pedal it's either larger caliper pistons, smaller master cylinder, different pedal ratio or power brakes. Take your pick.

Dual master cylinders and a balance bar is a great choice for a race car. If you don't need to be able to adjust proportioning as your fuel load or track conditions change it's a lot of money to spend and a fair amount of fabrication.

Screwing in the proportioning valve adjuster provides more pressure to the rear brakes. Your description of how you were operating the valve wasn't clear to me.
Author:  gmader [ Sun Jan 03, 2010 10:09 pm ]
Post subject: 
Thanks-- Here are clarifications.
  • Prop valve is on full decrease setting.
    Only thing I changed was the rear calipers.
    No residual pressure valve. I know that it has been removed.

    Andy F told me that this MC would work on the viper brakes. He is correct, but I don't think it works well with the rear brakes.. I think that are getting fully pressurized(filled up) prior to the front brakes. I believe that the fluid capacity for the rear brakes is considerably smaller than for the front.
So... I dunno. I am still looking for some sort of worksheet to figure optimum MC to SC sizing, as well as front/rear balance. Any leads?

thanks,

Greg
Author:  radarsonwheels [ Sun Jan 03, 2010 10:48 pm ]
Post subject:  I am switching my same m/c out
Because of the same problem. Go with 7/8" or 1 1/32" bore mc
Author:  andyf [ Sun Jan 03, 2010 11:44 pm ]
Post subject: 
Front to rear bias is important and is often overlooked when people do four wheel disc brake conversions. There aren't any rules of thumb because the actual ratio is dependent on the weight split, height of the CG, tire size, etc. But you probably want the fronts to do about twice the work of the rears. 2/3 to 1/3 split. So the easy way to do that is to have the front pistons be twice the area of the rear pistons. That gives you twice the force up front assuming the line pressure is equal everywhere.

There are a number of books on the subject of brake design. Puhn's book was one of the best but is out of print. Go to Amazon.com and do a search and you'll find plenty of info.

Your conclusions don't really make sense to me but I think that is because you're guessing at what the problem really is based on how the car feels to you. You might be right and you might not be. I don't know how you can tell that the fronts aren't getting enough pressure for example. If you don't have pressure gauge on there you don't really know what is going on. Are you able to lock up the brakes at all? If so, which end locks up first? Do you have a pressure gauge on either line? Ideally you should mount two gauges, one on the front and one on the rear line.

It is impossible to debug these things over the internet as you could have a number of issues including glazed pads, mis-assembly, stuck valve, bad MC, kinked brake hose, etc.

Get the piston diameter measurement first to see if your design is in the ballpark. Then get some pressure gauges hooked up to see what is actually happening. Then go from there. If the designed bias is in the ballpark and the line pressure is fine then you have a mechanical issue somewhere.

Have you disconnected the rear brakes to see what happens? Just block of the rear line and see how it stops. Then block off the fronts and see how it stops with only the rears. I could go on and on but you get the idea.
Author:  Dart270 [ Mon Jan 04, 2010 7:10 am ]
Post subject: 
Hard to know, as Andy said. Pad material can also make a substantial difference. I have not had such extreme issues, but have tried to match piston diams. Screwing the valve all the way out will dial out rear brakes, as mentioned above. I don't think those adj valves can compensate for much of a difference, but are a fine tuning tool. If you are grossly off, then it won't work.

Lou
Author:  2 Darts [ Tue Jan 12, 2010 9:10 am ]
Post subject: 
Friction material on the pad DOES make a big difference. The new ceramic linings have to get some heat in them before they work well. Unless this is a serious track (road racing) car, I'd avoid them. The carbon brakes work much better on less heat than the ceramics. I'd recommend them for a ¼ mile car. For pure street cars, I'd recommend a semi-metallic or even an organic for a trailer queen show car. For certain, there's a helpless feeling that overcomes me when I press on the whoa pedal and nothing happens. Good luck with your fix.
Author:  sandy in BC [ Tue Jan 12, 2010 9:36 am ]
Post subject: 
Greg: When I had similar problems with my mudbogger 4 wheel disc project it turned out to be my pedal stroke was too short. It would work one end or the other depending on pedal preload. I changed the pushrod mount to give me more stoke and solved all my problems.

I found this by removing the pedal pushrod and using a big punch to push the piston by hand.

I moved the pivot point about 5/8" to provide more travel.
Author:  radarsonwheels [ Tue Jan 12, 2010 11:32 am ]
Post subject:  I already said it but-
CHANGE TO A SMALLER BORE M/C

I had the identical setup, with identical poor braking. The pedal is hard but with no 'bite', right?

Won't lock up the brakes no matter what you do?

On mine I had an adjustable prop valve behind the stock '73 up style abody prop valve. With the back line restricted as much as possible the fronts could be locked up with some effort.

On my '73 with stock 7/8" iron m/c the brakes grab perfect.

I had the same M/C as you- the one that leaks if you fill to the top...


Good luck
Author:  gmader [ Thu Jan 14, 2010 2:46 am ]
Post subject: 
Thanks everybody. I have a 15/16 master cylinder waiting for me when I get home from DC this week. I will get everything changed out and bled, and see what it is like.

Greg
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