Brake bias

[autofetish]

-1968 dodge charger
-Front brakes - viper 4 pot calipers
-Rear brakes - reconditioned standard drums
-Standard drum brake Mopar proportional valve
-Mopar performance master cylinder no servo

The car was a true drum brake all round car. I fitted the big viper 4 pot calipers.

To reduce braking efficiency to the rear I installed an adjustable brake bias valve. This valve was fitted in the line to rear brakes.

Little did I know that 90% of off the shelf brake bias valves will only bias to a max of 57-60%

Even with the valve fully tightened the car is still doing handbrake turns.


I have found a brake bias valve that will bias to 100% but before I buy this am I missing a trick ??

Thanks in advance Will

[morgan]

OOh sounds familiar. Mine is still sitting in the box.
Same problem - rear drums are wayyyy keener than the wilwood-4-pots at the front. When cold its hilarious - arrive at a set of lights as if you are doing a handbrake turn. Better when everything is warmed up...

I plan to fit the bias and the line lock when I have the engine out this spring. I should have started that really...

[autofetish]

The thing I don't get is that even with the Bias fitted and the bias valve screwed all the way in
(Which is not fully closed like you would think 60% close - Read the small print)

I'm still locking the rears immediately

It's so bad I have isolated the rear brakes and its been fronts only for a while


I sat and read posts all night going back to 2015 and nothing like this. Time to strip the rear drums maybe I put the leading shoe in the back ??

[Dave999]

I would suggest you go for the narrowest rear cylinders you can get

7/8 or smaller


http://www.doctordiff.com/7-8-small-bor ... nders.html


and you need a master cylinder designed for a disk/drum car with a bigger bore (the one you have is probably fine)

less peddle travel but more effort

if its too big there will be no feel to the brakes, its like brakeing with blocks of nylon instead of brake pads

if its too narrow there will be massive peddle travel loads of progression but ultimately scary due to the distance the foot needs to travel towards the bulkhead.

I can't find the number but there is a mopar cylinder for disk brake cars which is biased something like 70% front 30% rear

something like that combined with the narrowest rear cylinders you can find will certainly help

if you use the drum brake master you get 50:50 when the weight distribution and momentum of the moving car dictates that is so so so... not-good


other options

10 inch (60s B body) or 9 inch rear drums (A body, Aussie A body, dakota with a bit of fiddling about)

9 inch used on dakotas wrong centre bore i think for older diffs

back plates may need drilling to fit as on either a dana or mopar axle that is not your usual muscle car fare

9 inch rear drums with wide 5 PCD for our era cars from Australia SA and canada but you need to use back plates off a 9 inch US car

if you go 9 inch use as much 70s stuff as possible 60s shoe mounting springs are rubbish and the self adjuster and hand brake set up is not as good as the 73-> stuff. however both work in early and later drums

rockauto for parts get wagner or raybestos

dave

[autofetish]

I hear you Dave and thank you.

What i don't get is loads of B- Body cars have had a set of front disks stuck on the front with an inline bias and fine.... Most don't even have the bias!

Surely if this really is my issue swapping the bias valve with one that will go to 100% bias is the easiest option ?

Let me strip the drums tonight and get some pictures

[Dave999]

if you can find one that does that....

its basically a line lock or a tap

most are a tiny hole a spring and a bit of rubber washer
some have screw or lever to push the spring harder

the pressure has to rise to a level to overcome the spring to allow fluid to pass through a tiny hole (hole size controls how fast the rears activate)
before the rears come on.. its a delay switch based on pressure and nothing more

you need one on all disk/drum set ups becasue

1) drums use a tiny amount of fluid to produce the movement necessary to apply the brake shoes to the drum and when adjusted normally its pretty instant due to the front shoes energising the back shoe
2) disk calipers use an oceans worth of fluid, the more pots you have the worse it gets, and although the pads are kicked back into the calipers by rotor flex vibration loose bearings and the slight springiness of the dust seals its still takes a load of brake fluid to move them. they also operate most effectively at much higher peddle pressure i,e you can stand on that peddle in a manner that would have totally locked drum brakes up and still have no lockup and dramatic slowdown at the front

what you have is a situation where the pressure needed just to move the front pads into contact with the rotor say 1% of brakeing is a pressure that has already half applied the rear shoes.

that coupled with the fact that the rears on 60s mopars are duo servo brakes means the front shoe touching the drum causes the rear shoe to be pushed round the backing plate so that it jams itself with double the force of the front shoe into the rear of the drum.. they self energise by riding the C shaped end of the shoe's support up and round the stop on the back plate. note how the adjuster and everything just self centres and floats in the drum the only thing pinned down solid is the shoe stop. when the brakes are applied the front shoe is pulled off from that stop by the drum which drives the rear double quick upwards and outwards

so your aim is to keep the rears off for as long as possible
or
reduce how much PUSH they get for a certain peddle movement to give your front pads a fighting chance of getting to the rotor before tha back brakes self energise and send you looking for the nearest ditch.

hence smaller brakes
or smaller diameter in the cylinder (less force applied when Force=pressure x area. smaller area used to push when you use a smaller cylinder


one thing to check have you got your rear brake shoes in the correct way. you must have the leading shoe at the front. and the smaller syyrface area of friction material it has the less it energizes the rear

one has way more friction surface than the other. one is for the self energising bit and the other is the one that gets rammed by it into the drum. they need to be the correct place towards front or rear

easy to remember Big to the Back.

set them up so that there is just no drag. then back each side off about 3 clicks out of optimal adjustment

then do a load of 50 mph slow stopping so that you get the rears heated well up and bedded in
don't do any reversing and slamming on the brakes that just adjusts them up again.

the cowboy ways are

1) stamp the brakes do a big burnout without the use of the line lock. ie just burn the rear tyres and brakes
2) adjust them up until they show hard to turn resistance and go for a drive until they noticeable bind or get very hot. you can usually smell them.

back em off and drive home

new shoes are very grabby becasue the shoe never matches the contour of the drum. they need to ware in and take a "set"

if you check them now you might find only the middle or the two ends of the shoes show ware...if that is the case that's probably your problem and you can address it by any of the 3 methods above

a true engineer would have shoe shaving tool that is perfectly round that mounts on the axle and can be used to take off the high spots
thats a boring way to do it

or if its just the ends that are grabbing
face mask and file can see you right

also you have to remember that many people drive about in a manner that rarely tests their brakes to point of a ditch finding or facing the on coming traffic rear lockup

one other thing during the 60s all drum brake cars had a residual pressure valve to keep the shoes near the drum when the brake were off
with disks on the front the master cylinder didn't have one for them.

during the 70s 80s and 90s disk brake circuits didn't have one, no need with a single or twin pot caliper at each wheel

recently disk brakes calipers have got so big with so many pistons that the volume of fluid needed to move them all back to the rotor is so big the residual pressure valve has returned in modified form, to allow easy return of fluid while pressure exists but to stop back-flow and piston return when pressure is low enough..
i.e keeps the pads really close to the disk to provide some immediacy with the brakes

how many pistons do you have?

check for uneven ware first see if you have a grabby rear end

Dave

[Dave999]

a lot of B body cars have a set of disks on the front based on the single pot slider off the M J and F bodies.....!
those that don't have a copy of a sumitomo 4 small pot caliper used in land cruisers and hiluxes made by wilwood

or have mopar 4 pot and a special master cylinder suited to that that came off an A body.

mine was horrendous from the factory for rear wheel lockup an A body with a foot of sheet metal cut out from the tail end is a bit lighter at the rear wheels than a dart

i fitted one of these when they were way more reasonably priced and have never looked back

https://www.ebay.com.au/itm/NOS-DISC-BR ... 2281008147

but i have single pot sliders and 11 inch rotors and all this is is a big spring a washer and a tiny hole added into the rear circuit

brakes work better mind and my rear drums are 9 inch 1960s mopar stylee car weighs 1.4 tonne yours will be a good bit more than that

dave

[autofetish]

Dave I have missed you so much massive thanks

Well the driver's side rear drum is black where it's overheated .......

Other side is normal colour but ran out of time to get it off tonight

[Dave999]

are the shoes newish

if just the ends show ware thats yer problem

the pressure applied to the shoe is being applied to the drum only at the ends of the shoes Pressure =force/Area if you only use a fraction of the area of the pad the pressure is huge and concentrated just on the ends of the shoes makes for easy lock up
better to have lower pressure over wider area for progressive friction to slow you


However .....they look like they have seen a bit of use...so you would expect them to have bedded in

try adjusting so you get half of the peddle travel before it stops .

and make sure the front shoe is the one with the short length of lining material.

if they have been binding anyway in normal use just back off the adjusters some

they work pleanty well when not adjusted up as per the book, which assumed OEM shoes that were perfectly round and matched to a drum from the same source. thats not the case anymore

Dave

[autofetish]

Everything was pretty much brand new

[Dave999]

yeah they look ok

try them badly adjusted

i.e wind em out so you can just slip the hub over
and then knock em back a little bit a few clicks.

one shoe at least has been favouring the ends but they are more or less bedded in

you don't want to run them so badly adjusted that you can pop the seals and pistons out of the cylinder. but you can impact their over effectiveness by having the shoes travel just that bit further when you apply the brakes.


tyre pressures ok??

Dave

[Dave999]

or stick a 40 KG sack of sand in the boot?

[morgan]

This is the thing with our Dave. He pursues solutions outside the box...

"Bag o sand" .. hahahaha