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 Ritation under braking

 Created by: Air2air
Orig. Posting Date User Name Edit Date
Nov 23, 2015 12:56PM Jemal  
Nov 23, 2015 09:14AM RedRiley  
Nov 21, 2015 09:05PM Cup Cake  
Nov 21, 2015 01:13PM Spank  
Nov 21, 2015 12:43PM Dan Moffet Edited: Nov 21, 2015 12:54PM 
Nov 21, 2015 12:19PM Air2air Edited: Nov 21, 2015 12:23PM 
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 Posted: Nov 23, 2015 12:56PM
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US


I can only think "rotation".... as in your back wheels rotating around your front!

I use the 5/8 rear cylinders with the late 8.4 disk brakes (under my huge 'hideous' 13 inch wheels!) on my 66 S, and it has really nice balance.  You CAN lock the whole thing up, but it does not do so unexpectedly!

It's not as easy with a Mini, but you might try some larger diameter rear brake pipe - install downstream of the proportioning valve.  I used larger metal pipe to fine tune the bias on my K5 Blazer when I custom-built a 4 wheel disk brake system for it.  That change alone 'softened' the rear brakes to balance the front -to-rear bias...

 Posted: Nov 23, 2015 09:14AM
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US

I'm stuck on trying to figure out what ritation means. Wink

 Posted: Nov 21, 2015 09:05PM
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When I had this happen twice it was the brake proportioning valve. I looked at the brake proportioning valve after the first occurence but there was nothing obviously wrong so I changed all the rear brake components. When it happened again I put a rebuild kit in the proportioning valve which solved the problem. I believe S disk brakes require 5/8" rear slaves.

The power of accurate observation is commonly called cynicism by those who have not got it. G.B.S. Sarcasm is the lowest form of wit. Oscar Wilde

//www.cupcakecooper.ca/

 Posted: Nov 21, 2015 01:13PM
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What I have chosen to do is use 1/2" rear wheel cylinders on all of my cars when it comes time to replacing them and this is the setup I use on my LeMons cars, from the 10" wheeled, 7.5" 4 piston vented rotor saloon to the 15" wheeled, 8.4 mg metro vented rotor moke to the 14" wheeled, 2 piston non-vented rotor America. I have substituted 5/8" when I didn't have 1/2" ones immediately available. 

I'm not saying this is the best way to go. But it has worked out for me very very well and keeps the back end planted beneath the car when hard braking in a straight line.

The other thing I've noticed is the "In a straight line" thing is VERY important. If once you've committed to a turn you find yourself needing to stab the brakes, the back end may want to step out because of the sudden weight transfer to the front of the car. If your rear brakes lock up at the same time as you are already turning, it compounds this natural FWD car tendency and the back end can come around REAL fast.

Brake in a straight line and throttle through the turns. If the back end starts to want to rotate out, stomp the throttle (Don't Lift!).

 Posted: Nov 21, 2015 12:43PM
 Edited:  Nov 21, 2015 12:54PM
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CA

You might want to look at this.

//www.minimania.com/Upgrading_Mini_Brakes

Note in the table section "Drums to Cooper S Discs" the list of components in the right column mentions

"Swap Brake Balance Valve for Cooper S type."

"Fit disc brake Mini rear drums with built-in spacer and longer wheel studs."

"Fit Cooper S rear slave cylinders and shoes."

Your problem may lie in the balance valve (AKA proporioning valve) or the wrong combination of rear cylinders and shoes. Too strong a cylinder pressing on a stock shoe may make it grab.

The reason it always kicks to the same side may be your car's balance and wheel loading. It will have more in front than back because of the power plant (Minis are nose-heavy) but braking really exaggerates it on any car.  it should have also have equal wheel loading side to side - thats WITH the usual load (meaning you as driver). So, if your car is balanced empty and you get in the left side (assuming your car is a lefty), the left wheels now have more load than the right, and more traction. When you apply the brakes, weight transfer moves to the front wheel , leaving the right rear with the the least load and traction. It brreaks loose first. The front left will have the most weight and best traction, also tending to steer left.

Kieth Calver goes into the vagaries of proportioning as well as rear cylinder size, and he certainly knows what he's talking about! Note where he says "if they indeed work at all".  He also discusses split systems, including the diagonally split format.

Side note: bigger vehicles like MInivans (Chrysler-sized) have brake proportioning that is load-sensing - variable according to vehicle loading. So as you load up the back, proportioning is adjusted to make the rear brakes wheels work harder.

 

.

"Hang on a minute lads....I've got a great idea."

 Posted: Nov 21, 2015 12:19PM
 Edited:  Nov 21, 2015 12:23PM
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I've read that some of you guys are having the rear end come around on hard braking.  

1. S brakes front, Greenstuff

2. Probably the original 998 rear slaves, haven't changed them out.

3. New pads/shoes/rotors after 2 years.

4. Rears adjusted well.

In panic stops the rears have always locked and rotated to the right, long before I did the pads/shoes/rotors.  I don't believe I've ever locked up the front.  Is it correct that swapping to 11/16" rear slaves is the fix?  

That's what this Calver article says.