Master cylinder affects brake bias?

[logician]

Can the master cylinder affect brake bias?

I didn't think so. But then a friend told me that using the Mercedes master cylinder would change the bias vs. my current 1970 master cylinder. Then another friend told me the same, and that the master cylinder has a spring in it that affects bias. Presumably that spring affects the piston between the two chambers, reducing the pressure on one chamber relative to the other.

What's the story? Do master cylinders have a built in bias other than 50/50?

-Juan

[ttweed]

Originally posted by logician
What's the story? Do master cylinders have a built in bias other than 50/50?

I don't see how a single cylinder can develop anything but the same pressure in F&R ports. What would a spring on the piston do to change that? Without a modulation valve in one of the lines, or a dual cylinder setup with balance bar, I don't see how you can have differentials in pressure F&R. If I am missing something here, someone please enlighten me.

TT

[RallyJon]

It would be pretty cool if you could get a tandem MC with two different bore sizes, front and rear. But I've never heard of one.

[s_wilwerding]

Read the article in Pano this month about brake technology - it is very well written and explains why master cylinders can affect brake bias.

[RallyJon]

it is very well written

Once you get past the erroneous half-page on weight transfer under braking.

[ttweed]

Originally posted by s_wilwerding
Read the article in Pano this month about brake technology - it is very well written and explains why master cylinders can affect brake bias.

I looked at that article. I don't see any mention of MC piston diameter effecting F/R bias. The caliper piston size difference F&R, pressure limiting valves on the rear, pad and rotor differences are all mentioned, but I see nothing about changing MC size alone to change F/R bias mentioned anywhere. What did I miss?

All that changing the MC piston size can do is effect the total pressure generated in the cylinder for a given stroke and pedal force applied. Given that the F&R brake line ports share equally in the pressure generated in the cylinder, all that can change is the pedal feel and stroke, not the F/R bias.

TT

[logician]

Originally posted by ttweed
I don't see how a single cylinder can develop anything but the same pressure in F&R ports. What would a spring on the piston do to change that? Without a modulation valve in one of the lines, or a dual cylinder setup with balance bar, I don't see how you can have differentials in pressure F&R. If I am missing something here, someone please enlighten me.

The way I understand it, the master cylinder has a single bore, but with two chambers in tandem, one for the front brakes, one for the rear. The brake pedal operates a piston at the end that appiles pressure to the first chamber. The two chambers are separated by a second piston that is floating. So the pressure in the first chamber pushes on that center piston, which applies pressure to the second chamber. That by itself would give you equal pressure in both chambers. But if springs are introduced to apply force on that center piston, that spring force could diminish the force that the first chamber applies to the center piston, and thus alter the relative pressures of the two chambers.

I guess the question is whether the strength on those internal springs are great enough to change the pressure between the two chambers, i.e. bias, or whether they are just for the purpose of centering that floating piston. My friend claimed the latter, and what he says makes some sense.

Here's a picture of a master cylinder:



BTW, I am familiar with dual master cylinder systems and the reasons for brake bias. But I am specifically interested in the operation of the stock designed MC, Mercedes and 911.

-Juan

[Steve@Rennsport]

Tom is right, but I'd add a few things,....

1) A dual MC/balance bar setup (like the Tilton or AP ones) allows one to alter brake bias since you have 2 separate master cylinders; one for each circuit. This is whats found on every race car (F1, IRL, NASCAR, ALMS) so different sized MC's are installed and the balance bar permits fine adjustment of brake bias.

2) Another way is by using a single MC that has differential bores at each end, al la 944 Turbo.

One must be VERY careful about such activities since a car with a bit more rear brake bias than the driver's skill level can handle makes it VERY easy to spin with very little warning.

[logician]

Originally posted by Steve@Rennsport
Another way is by using a single MC that has differential bores at each end, al la 944 Turbo.

That could explain it too. I'd be curious to know whether the Mercedes MC is designed that way.

-Juan

[s_wilwerding]

Originally posted by RallyJon
Once you get past the erroneous half-page on weight transfer under braking.

In what ways was it erroneous? I'm not arguing with you - I'd like to know.

[Steve@Rennsport]

Originally posted by logician
That could explain it too. I'd be curious to know whether the Mercedes MC is designed that way.

-Juan

The 23mm Mercedes MC's all have the same size bores; front & rear.

Brake bias corrections can be made by:

1) Installing adjustable proportioning valve.

2) Installing different compound brake pads (F&R) from the same manufacturer.

3) Changing rear brake calipers, depending on the system in use.

One or all of these measures usually does the job very nicely.

[RallyJon]

I made the same mistake they did, until corrected by Messrs Puhn and Verburg:

Weight transfer vs brake bias

[ttweed]

Originally posted by logician
I guess the question is whether the strength on those internal springs are great enough to change the pressure between the two chambers, i.e. bias, or whether they are just for the purpose of centering that floating piston.

Brake fluid is essentially incompressible (when it doesn't have any air entrained, etc.) Once the 2 pistons travel past the inlet ports, they are coupled by the volume of fluid trapped between them, which will act like a solid column. The spring strength cannot have any effect on lessening or increasing the pressure this column of fluid transmits to the forward piston, as far as I can see. It is just there to keep the forward "floating" piston centered in the bore and not allowing it to cock or bind. The column of fluid itself does all the work, and will transmit the identical force that is being applied to it (less the small amount of friction from the bore seals, I suppose.)

Is your friend trying to say that the spring strength is different (relatively) between the 19mm and 23mm Ate cylinders, and this would allow a pressure differential in the two chambers, effecting bias? I don't think that is possible, and even if it was, Ate would design the springs to have the same relative strength between the 19mm and 23mm sizes, no?

TT

[Bill Verburg]

It would be pretty cool if you could get a tandem MC with two different bore sizes, front and rear. But I've never heard of one

They are readily available and in fact used on some of the water cooled Porsches, I see Steve mentioned this already

[Bill Verburg]

Originally posted by Steve@Rennsport
The 23mm Mercedes MC's all have the same size bores; front & rear.

Brake bias corrections can be made by:

1) Installing adjustable proportioning valve.

2) Installing different compound brake pads (F&R) from the same manufacturer.

3) Changing rear brake calipers, depending on the system in use.

One or all of these measures usually does the job very nicely.

Exactly!

[Bill Verburg]

Can the master cylinder affect brake bias?

Not the ones used on a 911, unless you go to a dual master setup

[Bill Verburg]

Originally posted by ttweed
Brake fluid is essentially incompressible (when it doesn't have any air entrained, etc.) Once the 2 pistons travel past the inlet ports, they are coupled by the volume of fluid trapped between them, which will act like a solid column. The spring strength cannot have any effect on lessening or increasing the pressure this column of fluid transmits to the forward piston, as far as I can see. It is just there to keep the forward "floating" piston centered in the bore and not allowing it to cock or bind. The column of fluid itself does all the work, and will transmit the identical force that is being applied to it (less the small amount of friction from the bore seals, I suppose.)

Is your friend trying to say that the spring strength is different (relatively) between the 19mm and 23mm Ate cylinders, and this would allow a pressure differential in the two chambers, effecting bias? I don't think that is possible, and even if it was, Ate would design the springs to have the same relative strength between the 19mm and 23mm sizes, no?

TT

Another exactly!

[ttweed]

Originally posted by RallyJon
I made the same mistake they did, until corrected by Messrs Puhn and Verburg:

Weight transfer vs brake bias

OK, I read the thread referred to above, and the article in Pano, but I am still not sure I understand what the error is that the authors made, exactly. Are you saying that their calculation of weight shift under braking is incorrect? They do say that it is a "zero sum" change, and according to the calculations you make in the previous thread, the car somehow "weighs" more when braking at 1G? How is that possible in a universe where mass must be conserved???

Your calcs in that thread show a force on the front wheels of 944 lbs. and 1527 lbs. on the rear wheels in static conditions. Under 1 G braking your table shows 1773 lbs. on the front and 1112 lbs. on the rear? How is that possible? Where does all that extra weight come from? The car cannot just weigh more all of the sudden because the brakes are applied, can it? It's just the distribution of the weight that changes under the dynamics of braking, no? I'm confused about your calculation. Without any aero downforce involved, the sum of the weights on front and rear must be equal at all times, no?

Or is there some other issue with their explanation of weight shift that isn't apparent to me? If you will let me know, I would be happy to transmit the info to Bob Gagnon, one of the co-authors, as I will be seeing him next weekend in all likelihood, since he usually runs his 964 Cup car in our local region's autocrosses. I am sure he would be interested in correcting any errors, as it appears the article is one in a series that will be continuing.

Thx,
TT

[logician]

Originally posted by ttweed
Brake fluid is essentially incompressible (when it doesn't have any air entrained, etc.) Once the 2 pistons travel past the inlet ports, they are coupled by the volume of fluid trapped between them, which will act like a solid column. The spring strength cannot have any effect on lessening or increasing the pressure this column of fluid transmits to the forward piston, as far as I can see. It is just there to keep the forward "floating" piston centered in the bore and not allowing it to cock or bind. The column of fluid itself does all the work, and will transmit the identical force that is being applied to it (less the small amount of friction from the bore seals, I suppose.)

Is your friend trying to say that the spring strength is different (relatively) between the 19mm and 23mm Ate cylinders, and this would allow a pressure differential in the two chambers, effecting bias? I don't think that is possible, and even if it was, Ate would design the springs to have the same relative strength between the 19mm and 23mm sizes, no?

While the fluid is (essentially) incompressible, some fluid flows out of the MC chambers into the rest of the system, presumably due to the caliper piston movement and also some expansion of brake lines. The fact that your brake pedal has some give to it is evidence of that. Thus the spacing between pistons does change, allowing the springs to compress, and so affect the force on the pistons and bias.

I don't know whether the spring force is enough to affect the pressures in the chambers significantly. But in theory it seems possible, and that is why I'm asking the question, hoping that someone has a difinitive answer.

-Juan

[RallyJon]

Where does all that extra weight come from?

As far as my small brain can get around it, the extra "weight" comes from the friction force between the road and tire.