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Brake Imbalance


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5 minutes ago, Richard71 said:

Passed an MoT last year, even after the brake down-grade. I asked the tester if the braking was adequate, he answered in the affirmative. But as said, I don't feel they're as good as they should be.

Richard.

Did you see the numbers? I would have expected it to lock up or climb out with what you have done 

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1 minute ago, Richard71 said:

Good idea, though it might be simpler to re-fit a known good m/c which I removed during this venture.

Richard.

Im not thinking the m/cyl is nbg, rather that it is working as normal but cannot supply enough fluid to satisfy the rear calipers. Peter

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1 hour ago, Richard71 said:

Yes Steve, but the front calipers have 4 pistons and the rear have 2 pistons.

Richard.

Ok!

so the front calipers have double the piston area of the rear calipers, thus double the hydraulic braking power, this is good. 

I suggest you calculate the surface area of the new calipers and compare to the OE piston areas, front and back.

the hydraulic ratio, between master cyl area and ‘slave’ area is a major determinant of braking power

what does the pedal feel like ?

you should also consider what brake pads you are using and the coeff of friction

Steve

ps a decade ago i spent months trying to track down the cause of poor brake performance on a tr6, i had several issues including contaminated disks/pads which greatly reduced the coeff of friction.....

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I like Steve’s idea: compare std and new 
Did you add discs to the rear? If so, a simple comparison of crossectional area is not possible.

Waldi

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Hi Richard,

the FORCE on the TR6 master cylinder is the same, it is transfered by the fluid between 1st and 2nd piston.

Due to the sketch the secondary piston is smaller - so with the same force the presssure to the rear must be higher.

But I don't own a TR6, this is only science.

Is there a piston in your PWDA (with seals)?

You measured the pressure the same moment with 2 gauges or in two steps by moving one gauge?

 

The balance bar brake with two cylinders is only good to get a 2 circut brake instead of a 1 circut brake.

To adjust more pressure to the front or rear is only hearsay and story telling.

The balance bar is too short and you make the angle of the force sidewards worse.

You better not adjust anything that moves the push rod out of line to the pistons and pushes the pistons sidwards.

The balance bar is needed because the way of travel is different for te two circuts.

Without you would always brake only on cylinder, you cannot adjust this lasting.

 

To really adjust anything you have to move the hole cylinders sidewards, not only the balancing point on the bar.

And you divide the braking force 50/50 (or 48/52 it you absolutely want) on two cylinders and reduce the brake pressure seriously.

This is the clou on the cylinder with two pistons inline and fluid between: you have the same force on both pistons, not the half on each!!!

 

Allow me to tell you I already explained you 2 times why your Wilwood brake calipers is worse than the original Girling.

But obviously you don't want to believe. And this is why I'm now already out again.

I hope this helps, Ciao, Marco

Edited by Z320
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1 hour ago, Z320 said:

so with the same force the presssure to the rear must be higher.

But I don't own a TR6, this is only science.

 No Marco - look at the diagram again. There is a spring between the two pistons, not a solid link. The second piston is free to move independently and is driven by the pressure in the front brake circuit. Note that the bore either side of the secondary piston is the same. It cannot have different pressures either side of it. 

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Richard, Can the poor braking performance be improved by a bedding in routine. 60 mph, and brake really hard to 20 mph ( do not stop or lock-up). Repeat several times.

Peter

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4 hours ago, RobH said:

 No Marco - look at the diagram again. There is a spring between the two pistons, not a solid link. The second piston is free to move independently and is driven by the pressure in the front brake circuit. Note that the bore either side of the secondary piston is the same. It cannot have different pressures either side of it. 

Hi there,

there is s spring between the two pistons to hold them in position,

as Rob points out there is no SOLID link betwenn the, brake fluid is the link and this follows hydraulic rules.

The sketsh is from the ATE brakes manual, mostly the same like a TR master cylinder, but the illustration quality to work with.

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The pressure is the same ob both sides, as Rob points out, sorry me for any confusion.

To have more pressure "ps" the secondary pistion has to have a seal in the wider bore and on the other end in the smaller one.

Ciao, Marco

 

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Richard, you haven’t specified what you feel is wrong with the brakes. Is there imbalance with one end locking prematurely or is the pedal pressure required higher than before? I put Wilwoods on the front of my 4A initially with the smaller of the 2 piston sizes that Richard Good was offering. I later changed to the larger piston calipers. When I rebuilt the car 20 years ago, I fitted ventilated discs with the standard calipers with spacers in them. I was never happy about that modification so I switched to Wilwoods. I did change to EBC pads which lightened the pedal a bit. My wife approved. When I built the car, the regulations required that if you switched from left to right hand steering you had to install dual circuit brakes, so I installed a TR6 pedal box and master cylinder. The pedal pressure is certainly higher than our modern cars but that enables better modulation most of the time. It needs a firm but not excessive push to lock the wheels. Perhaps you should look at alternative pads front and rear. 

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The OE TR6 caliper has two pistons with a Diameter of 2.125" and a total area of 6.68 sq insthe Wildwoods have four of 1.62in diameter, or total10.18sq jns.

This is a 50% increase and needs to be matched by a similar increase in Master cylinder diameter, else you will have press much harder for the same braking.

JohnD

 

 

 

 

 

 

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I think you have that backwards John.

The increased caliper area will give an increased braking force for the same pedal effort, but a longer pedal travel because more fluid has to be moved.  Using Richard's figure of 800psi fluid pressure:

800psi on 10in^2 gives 8144lbf  on the rotor whereas the original caliper gives only 5344lbf.

To reduce pedal effort to raise the 800psi or to raise a higher pressure for the same effort you need a smaller master cylinder but the travel will become even longer. 

Edited by RobH
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4 hours ago, RobH said:

I think you have that backwards John.

The increased caliper area will give an increased braking force for the same pedal effort, but a longer pedal travel because more fluid has to be moved.  Using Richard's figure of 800psi fluid pressure:

800psi on 10in^2 gives 8144lbf  on the rotor whereas the original caliper gives only 5344lbf.

To reduce pedal effort to raise the 800psi or to raise a higher pressure for the same effort you need a smaller master cylinder but the travel will become even longer. 

I agree , bigger piston area in the calipers = stronger braking but longer pedal travel

bigger master piston area = less power but less travel

Edited by Steves_TR6
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July 2020 Richard posted he has

Wilwood 4 piston calipers on the front and 2 piston calipers in the rear, all pistons are diameter 1.38".

This is / was a aviable Wilwood caliper / piston diameter.

Currently (?) Wilwood offers (also?) the 140-15912 set mit 1.62" pistons.

ACTIO = REACTIO, this means: the force on the pads is only 1 OE Girling piston (not 2) or 2 Wilwood pistons (not 4).

This is why a sliding caliper (German: "Schwimmsattel", mostly used on economic cars) work with only 1 piston very well.

Sadly our TR solid mounted calipers need pistons on both sides of the disc.

The calculation is:

- 2 Wilwood piston 1.38" are 2.989 sq in

- 2 Wilwood piston 1.62" are 4.120 sq in

- 1 OE Girling piston 2.125" is 3.544 sq in

This is why the Wilwood caliper with 1.38" pistons brakes 16% worse than the original TR calipers.

And the caliper with 1.62" pistons brakes 16% stronger.

On the rear axle a disc brake with 2 pistons possibly brake worse than a drum brake because the disc brake do not self enforce. This is not 1:1 to compare.

I hope this helps.

Ciao, Marco

Edited by Z320
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8 hours ago, RobH said:

I think you have that backwards John.

The increased caliper area will give an increased braking force for the same pedal effort, but a longer pedal travel because more fluid has to be moved.  Using Richard's figure of 800psi fluid pressure:

800psi on 10in^2 gives 8144lbf  on the rotor whereas the original caliper gives only 5344lbf.

To reduce pedal effort to raise the 800psi or to raise a higher pressure for the same effort you need a smaller master cylinder but the travel will become even longer. 

Gosh, yes, Rob!   Doh! 

 In my enthusiasm to introduce some factual argument in place of opinion I let my enthusiasm run away.    Thank you for the gentle correction!

John

 

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  • 3 weeks later...

An update on my poor brakes....

A couple of weeks ago I decided to remove the overhauled original m/c & servo and re-fit the TRW offering which worked well with the standard calipers. Took the car for a test drive and the brakes were even worse, so I attempted another re-bleed but couldn't get all the air out. At the end of my patience I decided to walk away from the problem and give the car to local garage who are familiar with these cars (garage proprietor owns a TR4). One of the mechanics called me to say the servo is garbage, I then gave them the reconditioned originals, they fitted them, spent some time bleeding the system and I collected the car this afternoon and I'm happy to say the brakes now perform as they should, so at long last a good result and I still have the unused Mintex pads up my sleeve if I fancy an extra bit of bite.

Thanks to all of the above posters for your input, advice, information & help.

Richard.

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1 hour ago, Richard71 said:

An update on my poor brakes....

A couple of weeks ago I decided to remove the overhauled original m/c & servo and re-fit the TRW offering which worked well with the standard calipers. Took the car for a test drive and the brakes were even worse, so I attempted another re-bleed but couldn't get all the air out. At the end of my patience I decided to walk away from the problem and give the car to local garage who are familiar with these cars (garage proprietor owns a TR4). One of the mechanics called me to say the servo is garbage, I then gave them the reconditioned originals, they fitted them, spent some time bleeding the system and I collected the car this afternoon and I'm happy to say the brakes now perform as they should, so at long last a good result and I still have the unused Mintex pads up my sleeve if I fancy an extra bit of bite.

Thanks to all of the above posters for your input, advice, information & help.

Richard.

Back to what I said and the only way is to pressure bleed it glad you got it sorted 

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