brake bias migration

[boxermotor]

Morning all,
as I am new to this forum I apologise in advance if my question is not relevant or it has been dealt with already.

I am involved in a one-make single seater series with carbon brakes. Apparently brake bias migration in the pedal release phase is an issue regardless of master cylinder combination etc. It means that brake balance is not consistent: it tends to move to the front at the end of braking i.e. when you do not really want this to happen.

It is hardly predictable although it seems related to friction and histeresys in the master cylinders. The pedal assy has been thoroughly checked and aligned and friction in the ball joints has been virtually eliminated.

I wonder if there is a simple solution to get rid of the issue. Also, what sort of system do F1 teams use to control brake bias?
Many thanks,
Boxermotor

[flynfrog]

Are you sure its not a suspension setup issue?

[olefud]

Carbon brakes are known to vary CoF with temperature. If your brakes are ending up with the fronts at a different temperature than the back it could vary the bias. Measuring peak temps should provide some indication of what’s happening.
Modulating cooling to even out the temps might help. Caveat; carbon brakes tend to like to operate at fairly high temps.

[Jersey Tom]

Apparently brake bias migration in the pedal release phase is an issue regardless of master cylinder combination etc. It means that brake balance is not consistent: it tends to move to the front at the end of braking i.e. when you do not really want this to happen.

It is hardly predictable although it seems related to friction and histeresys in the master cylinders.

And how have you come to this conclusion of all these things going on? What measurements have you taken and what data have you collected?

[Blanchimont]

What series is it you race in and how does the brake system look like exactly?

Have you flipped the two master cylinders for the front and rear? Does the problem still occur?

[marcush.]

I go with JT here.
First you Need to establish a measurement proceedure to be sure your theory is valid and indeed the reason for the shift of bias.
that would start with temperature characteristics ,measurement of line pressure when the discribed things happen and and maybe even applied pressure coming from your Driver as well...
How many times we have found the pedalbox or the Balance bar simply being a piece of junk ...These items have to bear huge loads and maybe you find what you are looking for in this area...
Very often one thinks too complicated ...
I´d start with a thorrow look at components ,System and Installation ...maybe have a gopro camera installed Monitoring the deflection of the pedal area in the test ...or and the front and rear Suspension .... you may be quite surprised how much give there is sometimes ...causing unwanted reactions ..who knows.
...measure and document before concluding and Fixing Problems that maybe somewhere else...

[RR98ITR]

You guys are making me laugh! (You can laugh at me soon enough!)

As described in the scholarly journals "brake bias migration" or "brake bias shift" is indeed real. Well, that's what I gathered from what I read - see RCE's "Technical Spotlight" from April 2003 and Aug 2004. The bearing/trunnion balance bars and pull-type MC's and spherical at both ends type MC's from Alcon and AP at that time were/are supposed to reduce variance from set bias of 12-25% by 75%.

On the acceptance that there was measured but uncited evidence to back the contention (and products) I've gone tandem with a prop valve on my own RC to get a knee'd rear brake force trace.

Scott

[Tim.Wright]

Interesting problem. Sounds like a front caliper could be siezed or siezing. When was the last time the braking system was overhauled?

Another thing could be if you are running some wildly different brands, pad or disc material on the front compared to the rear. Even if the sizes are correct, different brands might have different seal friction or different dependancies of µ over temperature.

Could also be a rear disc is a bit pissed and is knocking the pads off before the fronts have time to release pressure.

Are you absolutely positive that its only on brake release? A simple check would be to run temp sensors pointing at the disc and compare the front and rear response rates.

[Jersey Tom]

You guys are making me laugh! (You can laugh at me soon enough!)

As described in the scholarly journals "brake bias migration" or "brake bias shift" is indeed real. Well, that's what I gathered from what I read - see RCE's "Technical Spotlight" from April 2003 and Aug 2004.

RCE is pretty far from a "scholarly journal." More like casual periodical.

Doesn't matter what you've read, doesn't matter what you've heard. Everything needs to be proven out yourself. Especially when there are many things which can fool you into thinking the wrong thing, best practice is to just start making a list and start checking boxes of what you know and don't know for sure. Being methodical like that, usually the core issue will make itself apparent.

Curious what this one make series is that uses carbon brakes.

[Drewd11]

Just as clarification, and OP, correct me if I'm wrong, but it seems he's referencing a time dependent migration of brake bias. He mentioned release phase, meaning late in the braking phase, which would not necesarily be a component, pad or disc issue.
Thinking about time dependent bias migration then, most particularly front-wards migration, sounds like temperature dependent migration to me: since front favouring brake bias is common in single seaters, front brakes may simply be getting hotter than the rears and getting closer to their optimal temperature window, leading the brake bias to migrate forward.
Under consideration then, as the other posters pointed out, more data is required. If brake temperature data shows a significant difference at the end of the braking phase between front and rear, then you maybe should look at your brake cooling and see if when temperatures are more closely aligned the problem goes away.
Anyway, as the other posters pointed out, a little data goes a long way in this case. Until you have data, all we can do is provide theories. Laser temperature sensors aren't expensive: see if you can rig something up to get disc temperatures. Alternatively, you can try a bunch of different things based upon the theories people throw up in the forum and see if one helps, but its a needle in a haystack.

[RR98ITR]

RCE is pretty far from a "scholarly journal." More like casual periodical.

Doesn't matter what you've read, doesn't matter what you've heard. Everything needs to be proven out yourself. Especially when there are many things which can fool you into thinking the wrong thing, best practice is to just start making a list and start checking boxes of what you know and don't know for sure. Being methodical like that, usually the core issue will make itself apparent.

Curious what this one make series is that uses carbon brakes.

I know...sadly both RCE and RT have become less interesting over the years. About the other stuff, I really hate to go in this direction, but...there's an old Calvin & Hobbes:

Calvin: The more you know, the harder it is to take decisive action.
Once you are informed, you start seeing complexities and shades of gray.
You realize nothing is as clear as it first appears. Ultimately, knowledge is paralyzing.
Being a man of action, I cannot afford to take that risk.

Hobbes: You're ignorant, but at least you act on it.”

Scott: guys always telling me that their race victories prove their theories..."Ack!"

[Jersey Tom]

Just as clarification, and OP, correct me if I'm wrong, but it seems he's referencing a time dependent migration of brake bias. He mentioned release phase, meaning late in the braking phase, which would not necesarily be a component, pad or disc issue.
Thinking about time dependent bias migration then, most particularly front-wards migration, sounds like temperature dependent migration to me: since front favouring brake bias is common in single seaters, front brakes may simply be getting hotter than the rears and getting closer to their optimal temperature window, leading the brake bias to migrate forward.
Under consideration then, as the other posters pointed out, more data is required. If brake temperature data shows a significant difference at the end of the braking phase between front and rear, then you maybe should look at your brake cooling and see if when temperatures are more closely aligned the problem goes away.
Anyway, as the other posters pointed out, a little data goes a long way in this case. Until you have data, all we can do is provide theories. Laser temperature sensors aren't expensive: see if you can rig something up to get disc temperatures. Alternatively, you can try a bunch of different things based upon the theories people throw up in the forum and see if one helps, but its a needle in a haystack.

To me the first question is, "What makes you think the bias is changing?" I.e. is this coming from good pressure transducer data, or... [cringe] purely from subjective driver feedback.. or what? And are we talking brake bias as in pressure split or actual braking torque or some other measure?

Gotta get some basic information first, otherwise - as you allude to - its all speculation.

[boxermotor]

Wow! Thanks a lot to you all for the prompt replies.
I have been investigating this problem for more than one full season now, with the help of disc IR sensors, pressure transducers front and rear and master cylinder displacement pots. The migration is clearly visible also when the car is standing in the pits: the bias is fairly consistent through the "pressure ramp" phase. Then during release, the rear master cyl returns quicker than the front. The bias moves to the front causing front wheel locking at medium to low speed.
The front master cylinder is usually smaller than the rear in order to get the bias adjustability range right, although there is still migration even with frt and rr master cyl the same size.
New discs/pads reduce pedal travel hence migration. Worn brakes increase pedal travel hence migration gets worse. New discs front and "bad" discs rear require more bias to the rear and migration is slightly reduced. As a general rule, the more bias to the front, the worse the migration.

The car is a... single seater with a 3.5 engine

[gixxer_drew]

Just throwing this out there but aerodynamic CoP shift can be really really quite large, could that be another factor?

[boxermotor]

...and sorry about my poor explanation. For "brake bias" I actually mean the hydraulic pressure ratio:
front pressure / (front + rear pressure)

Therefore I believe the issue is in the balance bar assy or hydraulics system...
And on top of that, according to the tech regs nothing can be changed on the brake system apart from master cylinder size