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More front braking needed
mark.silcock - 8/10/24 at 03:41 PM

I am trying to get more braking up front as the rears are currently dominating proceedings.

Current set up.

Typical locost running 1800 zetec, type 9 gearbox, cortina M16 fronts, escort rear end with 8" drums. dual cylinder pedal box 0.7 rear and 0.625 front.

Questions, the rear master cylinder is pretty much rock solid with very minimum pedal movement, the front has some travel before getting a firm feel. fronts have been bleed multiple times and there is no air in the system. What is the typical amount of travel the smaller front cylinder moves as I can't see how the adjustment bar will ever allow the fronts to be dominant over the rear cylinder that doesnt move?

I have watched and read many guides on setting the bar up and believe it is set correctly to give the fronts more bias but as soon as there is pressure on the pedal the rears are engaged before the fronts.

Going for a smaller front or larger rear will just make the difference in pedal travel larger will it not?

Thanks

MikeR - 8/10/24 at 03:55 PM

Its been a long time since I looked at this but you need to consider piston size on the master cylinder and at the wheel.

Can you increase the piston size at the rear (so it needs more fluid). I've got a vague memory its the opposite to what I think is logical hence the suggestion.

gremlin1234 - 8/10/24 at 04:29 PM

is the balance bar that you have 'properly "free"' or does it bind as it swivels?

mark.silcock - 8/10/24 at 05:14 PM

"Can you increase the piston size at the rear (so it needs more fluid). I've got a vague memory its the opposite to what I think is logical hence the suggestion."

Do you mean the rear wheel drum cylinders? Wouldn't they need to go smaller?

"is the balance bar that you have 'properly "free"' or does it bind as it swivels?"

Yes it is free and when bleeding the systems I have been disconnecting the opposite cylinder from the pedal box so I dont have to bleed both ends at the same time. So the travel witnessed is independent of the bias bar.

Andrzejsr - 8/10/24 at 08:26 PM

quote:
Originally posted by mark.silcock
.. as soon as there is pressure on the pedal the rears are engaged before the fronts.

..


In garage that might looks like that , but the rear (bigger bore) MC even if it is "catching" first , that does not mean it gives more force to rear caliper (or drum) . Fron being smaller diameter use longer travel giving more pressure-force on front.
But its all to piston areas front vs rear.

brakepower.com can provide some stsrting point calculations.

In my track car I have also front smaller diameter than rear MC, and bias bar regulations during trackday works - I can lock front or rear tyres first depending on bias bar adjustment.

Did You tried the "old method" - car up, one person in cockpit pressing brake pedal, other person turning front wheel - first one adding pressure until other one cant turn the wheel with hands, then second man goes to rear wheel and checking if can turn (should be possible but with resistance) rar wheel . That should be good safe starting point .

adithorp - 8/10/24 at 09:09 PM

Your choices are
Smaller slave cylinders in drums
Larger master cylinder on the rear
Smaller master on the front
Adjust the ballance bar.

JAG - 9/10/24 at 07:39 AM

quote:

the rear master cylinder is pretty much rock solid with very minimum pedal movement, the front has some travel before getting a firm feel.


This is normal. M16 calipers have 54mm diameter pistons and the piston retraction, off-brake, is about 0.25mm. That is a lot more fluid consumption than the rears. Those wheel cylinders will be 19.05 or 20.64mm diameter and still less than 1mm of movement. So the master cylinder operating the front brakes will always travel further than the master cylinder operating the rear brakes.

Unfortunately, anything you do to move the brake bias from the rear to the front will increase this difference.

A Tandem master cylinder would be a better solution - unless you intend to change the brake bias there is no reason or advantage for a twin master cylinder and balance bar set up.

mark.silcock - 9/10/24 at 08:58 AM

quote:
Originally posted by Andrzejsr
quote:
Originally posted by mark.silcock
.. as soon as there is pressure on the pedal the rears are engaged before the fronts.

..


In garage that might looks like that , but the rear (bigger bore) MC even if it is "catching" first , that does not mean it gives more force to rear caliper (or drum) . Fron being smaller diameter use longer travel giving more pressure-force on front.
But its all to piston areas front vs rear.

brakepower.com can provide some stsrting point calculations.

In my track car I have also front smaller diameter than rear MC, and bias bar regulations during trackday works - I can lock front or rear tyres first depending on bias bar adjustment.

Did You tried the "old method" - car up, one person in cockpit pressing brake pedal, other person turning front wheel - first one adding pressure until other one cant turn the wheel with hands, then second man goes to rear wheel and checking if can turn (should be possible but with resistance) rar wheel . That should be good safe starting point .


It is more than just in the garage, the rears lock quite easily on the road even with bias set all the way to the front.

mark.silcock - 9/10/24 at 09:01 AM

quote:
Originally posted by JAG
quote:

the rear master cylinder is pretty much rock solid with very minimum pedal movement, the front has some travel before getting a firm feel.


This is normal. M16 calipers have 54mm diameter pistons and the piston retraction, off-brake, is about 0.25mm. That is a lot more fluid consumption than the rears. Those wheel cylinders will be 19.05 or 20.64mm diameter and still less than 1mm of movement. So the master cylinder operating the front brakes will always travel further than the master cylinder operating the rear brakes.

Unfortunately, anything you do to move the brake bias from the rear to the front will increase this difference.

A Tandem master cylinder would be a better solution - unless you intend to change the brake bias there is no reason or advantage for a twin master cylinder and balance bar set up.


What would you say is normal amounts of travel? Just want to make sure the fronts are operating correctly and there isn't something causing extra travel.

mark.silcock - 9/10/24 at 09:02 AM

Realised the rears are actually 9" drums not 8"

Are they oversized?

adithorp - 9/10/24 at 09:28 AM

When your balance bar is at rest what angle is it at? If its square, then it might be locking out (binding up) under load. It should be angled (by adjusting the push-rods) to allow the additional movement in the front master.

Also how close are the push-rods to the sleeve? Too close and again, they can prevent the bar articulating and it bind (I chamfered my tube slightly to aid that movement).

There's a decent description of this on the RallyDesign catalog

mark.silcock - 9/10/24 at 09:42 AM

quote:
Originally posted by adithorp
When your balance bar is at rest what angle is it at? If its square, then it might be locking out (binding up) under load. It should be angled (by adjusting the push-rods) to allow the additional movement in the front master.

Also how close are the push-rods to the sleeve? Too close and again, they can prevent the bar articulating and it bind (I chamfered my tube slightly to aid that movement).

There's a decent description of this on the RallyDesign catalog


Pretty sure I have read that one.

Slight angle at rest with the front further away from the box, light pedal they are roughly square then angled towards the front under heavy pedal.

The pedal box is similar to this, they looked to have changed the design slightly compared to the one in the car.

https://www.compbrake.com/product/universal-floor-mounted-cable-pedal-box-balance-bar-uprated-3-pedal/

JAG - 9/10/24 at 09:54 AM

quote:

Realised the rears are actually 9" drums not 8"


That's not a problem (as such) but it is making the rears over-powered and it is making this situation worse. Could you buy and fit 8" drumbrakes instead?

quote:

What would you say is normal amounts of travel?


You can work it out.

M16's use 2 x 54mm pistons. Each piston will travel approx' 0.25mm before the pad touches the disc. Calculate that volume.

Then divide that answer by the area of your 0.625" (15.875mm) master cylinder piston.

The number you will get is the piston travel at the master cylinder.

[Edited on 9/10/24 by JAG]

mark.silcock - 9/10/24 at 10:06 AM

quote:
Originally posted by JAG
quote:

Realised the rears are actually 9" drums not 8"


That's not a problem (as such) but it is making the rears over-powered and it is making this situation worse. Could you buy and fit 8" drumbrakes instead?

quote:

What would you say is normal amounts of travel?


You can work it out.

M16's use 2 x 54mm pistons. Each piston will travel approx' 0.25mm before the pad touches the disc. Calculate that volume.

Then divide that answer by the area of your 0.625" (15.875mm) master cylinder piston.

The number you will get is the piston travel at the master cylinder.

[Edited on 9/10/24 by JAG]


5.8mm.... hmm feels like more than that but will have a measure in a bit.

Will put 8" drums on if it will help sort the balance out.

JAG - 9/10/24 at 11:05 AM

quote:

5.8mm.... hmm feels like more than that


Did you allow for two calipers on the front axle?



If you can fit the 8" drumbrake that's probably the best solution.

The smaller brake in a smaller drum will reduce the rear axle brake force - it's the same overall effect as increasing the front brake output.

Drumbrake torque output is proportional to the drum diameter, the hydraulic pressure and the co-efficient of friction between the brake shoe and the brake drum.

You can't change the hydraulic pressure easily and the co-efficient of friction between the brake shoe and the brake drum is fixed.

The advantage of this method is that you don't have to change the hydraulic set up


[Edited on 9/10/24 by JAG]

JAG - 9/10/24 at 11:10 AM

FYI

My car has:

Front: M16 caliper

Rear: 8" drumbrake with a 19.05mm wheel cylinder

(I have a Tandem Master Cylinder)

[Edited on 9/10/24 by JAG]

mark.silcock - 9/10/24 at 11:38 AM

quote:
Originally posted by JAG
quote:

5.8mm.... hmm feels like more than that


Did you allow for two calipers on the front axle?


good spot

11.6mm

MikeR - 9/10/24 at 12:05 PM

Daft question - instead of changing to 8" drums, could you remove (carefully) a percentage of the brake shoes so less force could be applied?

Not sure how you'd remove a percentage safely but if someone knew that might be easier than changing backplates, drums, new shoes, slave cylinders, remaking hard lines etc.

mark.silcock - 9/10/24 at 12:21 PM

quote:
Originally posted by MikeR
Daft question - instead of changing to 8" drums, could you remove (carefully) a percentage of the brake shoes so less force could be applied?

Not sure how you'd remove a percentage safely but if someone knew that might be easier than changing backplates, drums, new shoes, slave cylinders, remaking hard lines etc.



MikeP001 - 9/10/24 at 02:15 PM

Isn't the larger master cylinder usually on the fronts? The bias bar is biasing pressure rather than movement, larger on the front results in more pressure at the slave side on the fronts as pressure/motion is transferred there from the bias bar. if I remember my hydraulics (double check me, I may well not!).

JAG - 9/10/24 at 02:39 PM

quote:

could you remove (carefully) a percentage of the brake shoes so less force could be applied?


This won't work because the co-efficient of friction is not related to the contact area.

Also it could actually make the brake more powerful IF you remove the wrong part of the lining material.

Drumbrake torque output is related to friction level but it's also related to the geometry of the shoe and position of the lining material on the shoe. If you modify the lining material position you could make the situation worse not better.

It's all part of the Stepney-Acres Drumbrake Geometry calculation: Drumbrake Geometry Calculations

adithorp - 9/10/24 at 05:11 PM

quote:
Originally posted by MikeP001
Isn't the larger master cylinder usually on the fronts? The bias bar is biasing pressure rather than movement, larger on the front results in more pressure at the slave side on the fronts as pressure/motion is transferred there from the bias bar. if I remember my hydraulics (double check me, I may well not!).


No, you've hot it the wrong way round.

MikeP001 - 9/10/24 at 06:03 PM

Thanks! I'll be sure to re-read my texts if I need to make any changes...

Fred W B - 10/10/24 at 06:28 AM

You don't mention what pads you have in the front. Can make a make a big difference

JAG - 10/10/24 at 11:23 AM

The other alternative, which should have occured to me earlier, is you could fit larger diameter front discs.

A larger diameter front disc, plus spacers to space the caliper away from the wheel centre, will give you more front axle brake force and should also fix the problem.

I see KitcarDirect sell some 265mm discs:

265mm Solid Cortina Discs

coyoteboy - 10/10/24 at 11:43 AM

quote:
Originally posted by MikeP001
Isn't the larger master cylinder usually on the fronts? The bias bar is biasing pressure rather than movement, larger on the front results in more pressure at the slave side on the fronts as pressure/motion is transferred there from the bias bar. if I remember my hydraulics (double check me, I may well not!).


This might be your problem, if that's how you have it set up?