Brake bias

[747heavy]

That being said brake bias adjustment has little to do with downforce, not everything in an F1 car has to do with downforce. Its mostly to maximize the tires grip all around without locking them.

I´m not quite sure, if we are onto the same thing here.
Not quite sure how you post relate the statement I have made/Question I have ask to Marcus ???
Anyway, as with this statement of yours, is "tire grip" not very dependent on vertical load (downforce)?
So reducing vertical load (in whatever way) would reduce grip, and thereby braking performance or not?
As downforce changes with the square of speed, I would think, that it has a lot to do with braking performance.

But I could be wrong

[Tec5nical]

@ markus

Hi Marcus,

I´m not sure Ferrari wanted to/has retarded braking on one axle with their system.
I think it was a dynamic change in brake bias during braking.

If you don´t mind to share, why would you want to retard (delay) pressure built up in one circuit?
Thanks !!

While i doubt the validity of tht claim i can see on a long straight having tons of rear brake bias at the start of braking and it progressing forwards as rear downforce is decreased to keep from locking the rears or wheel hopping.
That being said brake bias adjustment has little to do with downforce, not everything in an F1 car has to do with downforce. Its mostly to maximize the tires grip all around without locking them. It also if properly set and driver dependent can be used to control turn in agressiveness.
The reason you see so many adjustments is to get the max out of every corner.

Correct, but the main question remains on 'basic how' they change the bias!

[wrcsti]

Correct, but the main question remains on 'basic how' they change the bias!

Thats the easy one. 2 brake systems, 1 for each end of the car. Turning the knob changes the pressure differential between the 2.

747heavy- No my statement was that increasing rear brake does not increase downforce by some magical device.

[747heavy]

747heavy- No my statement was that increasing rear brake does not increase downforce by some magical device.

And that would be correct - IMHO, but where did I claim that it does?

But due to the fast, that downforce decreases app. with the square of speed, but
mechanical load transfere does not change in the same manner, a fixed split in braking force, is not the best option, to make use of all the grip availible.

If you see the optimum brake force distribution, it will be parabel shaped, this will be haed to achieve with a simple front/rear force/pressure split.

[anishhotpunk]

I am doing a research on the braking system of formula 1 car. I am also aware about the 2010 regulations of brake system. now , i have been reading this brake bias forum for 2 days. As some of you said that there is a relationship of aero and speed , cenario of braking situations etc. according to the regulation of 2010. FIA said that there should be " no locking of wheels". secondly no devise should be there on car that will adjust the brake pressure as pre-set up. then why we see locking of wheels speacially of front axle wheels. brake bias is the shifting of brake from front to rear or vice versa according to the conditions and result orientation. but how, the F1 driver shift there brake bias phyically or what is the procedure. what is the static condition. according to me


firstly, we have to consider the weight distribution of the vehicle.
secondly, we have to consider the front wing and rear wing function and the relationship of both to speed of the vehicle. as downforce increase so does the centre of gravity goes further down in the car. when there is maximum downforce the tires will have more mechcanical grip on the track. and the centre of pressure will shift towards one direction but the magnitude will change simultanously and load tranfer tend change place in the vehicle from front to rear or vice versa.


please comment on my post.

i am missing any equations or any other consideration.

[747heavy]

o.k. for what it is worth (probably not much anyway), some random thoughts about your questions / topic.

one thing you should consider is the tire itself, as for an F1 car the front and rear tire is different in size and probably construction as well.
this should lead to an relationship of vertical load vs. the ability to transmit a longitunal force (braking or accelerating/driving).
You need to know, what your tire is able to "handle" under different vertical load conditions.
For different tires (front to rear) this can be different.

IMHO you could sum weight and downforce into one vertical force.
The tire will not "know" how the vertical force acting on it is "generated".
You are correct (IMHO), that this vertical force has an component related to the weight/mass distribution of the vehicle and a component which is related to aerodynamics/downforce.
Both components vary/fluctuate when seen in the time (or distance) domain.

now what influences the ability of the tire to transmit this longitunal force?

some things to look at, may (among others) include.

- tire construction/tire size (diameter and width)
- tire tread compound
- tire pressure
- vertical load on the tire
- temperature of the tire
- diameter of the tire --> think loaded radius
- track/surface condition
- mediums on the track surface (water, oil, sand etc.)
- other forces acting on the tire at the same time (think lateral forces /friction circle etc.
- rpm of the tire/ rotational speed

which of the above mentioned things change during braking? why,how and what does it mean in terms of the ability to transmit a force?

as for the definition of brake bias - I´m not sure I would agree with your definition per se.
I´m sure you mean the right thing, but maybe did not describing it in a "good" way.

I would say that brake bias is
>> the split/ratio of braking force between the front and rear axle, as a percentage of overall braking force <<. (measured/seen at the tire (contact patch))

There are other definitions as well, as some people refer to the brake line pressures in the front and rear brake circuits.

This split/ratio/bias can vary from front to rear (or viseversa) during braking, but don´t have to.
There are many/some (race)cars which have a fixed ratio.

But, as you have correctly established, the vertical load on the tire changes during braking, therefore it would make sense to alter brake force at the tire as well, to account for this change.

How can we do this?
Now there are a varity of possible solutions, before we look at them in more detail, it is maybe worth, to have a look at our braking system, it´s components and it´s functions.

In very basic terms we have one brake pedal onto which the drivers exerts a force and four wheels, which will need to transmit this force onto the "ground".

In most race cars we have a "balance bar" which connects the brake pedal with two mastercylinders (M/C) (normally one M/C for the front axle and one for the rear axle).

The balance bar enables us to change the geometrical ratio/leverage between our brake pedal and the two M/C´s.
It is one component in the brake systems which can alter the "brake bias" between front and rear.
Normally this balance bar is driver adjustable, so the driver can adjust the force split between the front and rear M/C.

For now lets assume we have a symmetric force split.
For the sake of an example lets assume a pedal force of 500N and a pedal to M/C ration from 3:1, which means we have 1500N at M/C level, evenly split means 750N go to the front M/C and 750N go to the rear M/C.

The next component is the master cylinder which transforms our force into hydraulical pressure inside our brake system/brake lines. (P=F/A)
Which means that an M/C with a smaller diameter piston will transform our force into an higher pressure.

So for a given force, which comes from our pedal, we can get different pressures in our front and rear brake circuits, by using different size M/C´s. (second part which we can use to change our brake bias).

Now we have two hydraulic circuits (front&rear) in which we have pressures which are related to the force acting on our brake pedal. (note: we can put valves into each brake circuit which change (or limits) the pressure in relation to other parameters &/or variables)

the next part would be the brake caliper with it´s pistons, which transforms our hydraulic pressure back into a force. F=P*A
This means that for a given (line)pressure a caliper with a larger piston(area) would "generate" more force.
So we have another option to influence our brake forces front to rear, by choosing our caliper(piston) size.
You will see that some/most race cars(bikes) use larger calipers on the front then on the rear.

Then we have our brake pads which will "clamp" the brake disc inbetween them.
Here the friction coefficient of the pad plays a important part.
Similar to the CoF of the tire which finally transmits the force to the ground.
The CoF of the brake pad/disc combo will depent on the temperature (and some other things, such as rotational speed etc) and can change quite dramaticly during braking.

Next we have our brake dics and its diameter.
For a given force, tangential to it´s radius, a larger dics (radius) will generate a larger moment around the centre/axle.
From this we can conclude that dics diameter is another parameter with which we can alter our brake force distribution.
The max. disc diameter is normally limited by the permitted rim size, which atm in F1 is 13" and quite small compared with other race series/cars.
Sometimes technical rules limit the max. permitted disc diameter, even if there would be sufficient space inside the rim/wheel to fit larger diameter disc´s.

So now we have a (braking)moment around the axle and our tire need to transmit it onto the ground.
Here, again, we have a friction force which is generated by the tire/ground interaction and via a "lever-arm" (the loaded tire radius) turned into a moment.

The above is just a very generic discription of the brake system, leaving out a lot of details, but should show where we can attempt to modify our brake force distribution.

The last point´s should show, that even for a equal brake pressure distribution between front and rear circuit, we can have a vastly different brake force at the tire, if we use different piston sizes in our calibers and/or different diameter brake disc´s.
Therefore the sometimes computed (in DAS) brake pressure split between front and rear circuit is not allways representative for the true brake(force) bias, in terms of force at the contact patch of the tire.

Now let´s look at what you could do to change the brake bias, and what we should consider.

If we look at an non-downforce car under braking, we will see a load transfer towards the front axle, which increases (temporary) the vertical load on the front tires and reduces the load on the rear tires.
Most downforce dependent cars will follow the same trend and show a increase in the ability of the front tires to transmit braking force.

Therefore, our limiting factor (also from a stability PoV) is normally the rear axle, which will see a reduction in "brake ability" during braking.

One approach is to stay under the lower limit, and just "limit" the brake pressure/force towards the rear.
This is often done with some valves in the rear brake circuit.
The downside of this approach, is that we "give away" potential braking performance early in the braking phase, while we still have sufficient vertical load on the rear tires.


The other is to change the rear brake pressure/force during braking to follow the load transfer.
This could be done on a time base or based on other parameters such as long. G-force &/or suspension travel/movement.

An example would be a brake porportion valve coupled between the sprung mass (body) and the suspension (e.g.wishbone), as found on some road cars.
When the car pitches under braking (front dives/rear lifts)the brake pressure going to the rear gets reduced.
Other variations include a mass(pendulum) against a spring/damper unit to vary the pressure in relation to long. accel. or other forms of accel. sensitive valves.

As you have allready mentioned in your post, the basic formula for long load transfer is:

dW = (m * h * a) / l

* with dW symbolising the total weight transfer due to an acceleration a (m/s²),
* a total vehicle mass m (in kg),
* h the height is the height of center of gravity,
* l is the wheelbase.
(borrowed from F1technical )

and you have allready mentioned that a change in CoG height will affect the load transfer.
IMHO the change of CoG height during braking, is a secondary effect when we talk about an F1 car, but it´s there.

Other secondary effects (IMHO) you could consider are
- change in wheelbase, due to kinematic effects (and/or steering input in combined cornering/braking situations)
- movement of fuel inside the tank, both in terms of change in CoG height and for/aft weight distribution
- change in loaded tire radius, both in terms of CoG height and momentum arm for brake torque

other factors worth to keep in mind.
- change in CoF in the brake pads due to temperature and rotational speed changes.
- stiffness of the brake system (flex in the system)
- temperature effects on the stiffness of components (think brake caliper flex and the ban of MMC for brake calipers)
- compressibility of hydraulic fluid in relation to temperature (boiling point)
- hysteresis in the brake system
- effect on aerodynamic drag on overall braking performance (low speed vs. high speed braking) - not transmitted via the tires.
- effect of downforce related vertical load in terms of possible brake force distribution ( high vs. low speed braking, --> higher speed --> higher DF--> enables higher long. G (decceleration). higher decceleration (log.G) means more load transfer towards the front
- effect of track inclination on braking performance (uphill vs. downhill braking)
--> change in track inclination in the braking zone (braking over a crest)
- effect of head or tail wind (at different parts of the track)

So long post - sorry mods
maybe some food for thought for your project

I think you should differenciate between fixed brake bias (but maybe changed for different corners (low speed vs. high speed)), adjustments such as balance bar position
and full dynamic brake bias changes, which alter the bias "automatic" depending from different parameters.
IMHO - the first step would be to calculate the optimum brake bias for different conditions (brake force parabel) and then look a possible ways to achieve it, or to come as close as possible towards it.
Here, we may need to consider limitation which are imposed by the technical reglement, such as ban of electronic &/or active systems, or the condition that the brake pressure left vs. right in one circuit needs to be equal.

[747heavy]

some example values:

[Tim.Wright]

To add another couple of things that 747 touched on;

The primary reason you typically see braking split to give more torque to the front is because of load transfer. The front axle has a higher vertical load and can therefore support more braking effort.

Now the aredynamics mean the vehicle is capable of braking harder at high speeds, this results in more load transfer which means the front axle can support still even more braking effort. This is why you often see the drivers changing the bias between corners. A high speed corner requires a more forward brake bias than a low speed corner.

Then there is the argument of stability. If you have both the front and rear tyres operating at the optimum slip ratio, there is no lateral grip available on the rear axle to stabilise the vehicle. So to keep the car stable you want the front axle to saturate before the rear.

Anyway, with any argument, it all comes down to the tyres. To get a better understanding of the requirements of the brakes, you need to do some reading on tyres.

Tim

[anishhotpunk]

this is a picture of "brake bias" in formula 1. but i can't understand the mechcansim of it.and how drivers adjust them. and do all teams uses the same device.?

thanks
anish

[747heavy]

as for how the driver adjusts this device:

I would venture to say, he pulls back on the silver lever, to get out of the detend, and then moves the lever up.

Looks like ~3 possible positions to me, seeing the detends on the other side, and the fact that the second "notch" it´s wider then the first one.

not in a position to comment on the rest of your questions.

[Crazy Bored]

Assuming that iRacing is correct in their FW31 simulation, in that particular car adjusting the brake bias was not limited to only one pressure for the entire time. It has the ability to select a base brake bias, a peak brake bias, and the point where the brake bias begins to shift towards the peak brake bias. In the simulation this point is measured by % of pedal travel since most people have a brake pedal with a potentiometer, rather than one that uses pressure. I'm sure it would work the same way in the real car but instead they would choose a certain amount of force on the pedal for when the brake bias begins to shift towards peak.

However, these settings are not adjustable while driving the car. In the cockpit you can only adjust the base and peak brake pressure in one combined adjustment, by the same amount. I mean, you can increase both by 0.25% or decrease both by 0.25% but not increase one and decrease the other.

This setup is useful because it allows you to optimize the brake bias during hard braking as well as when only light pressure is used. Under hard braking you will be nearer to the peak setting, which tends to have more front brake bias due to the increased load transfer under threshold braking. For the beginning of the pedal travel the bias is more towards the rear to help the car rotate on corner entry, and it simply doesn't make sense to have a lot of front brake bias when the longitudinal g's are low relative to the maximum braking ability of the car.

I don't know what the rules are for braking systems in F1, and I don't know if this system is still used in 2011.

[747heavy]

While I can´t say (don´t know) if the valve, which you show in your photo, has a similar function, here some info´s about possible brake proportion valves, as used in a varity of racing categories.

Just to show one possible way to change the brake bias during braking.

These valves vary/alter the brake bias as a function of brake pressure used.
You can use two valves in series, if you like, to have another digression point.
Similar valves (non adjustable) are used in many/some road cars.
Sometimes these valves are integrated into the master cylinder (in road cars).




some info about use and principal function of these valves (not F1 specific)
http://www.stoptech.com/tech_info/wp_pr ... lves.shtml
http://www.tiltonracing.com/pdfs/98-126 ... valves.pdf
http://www.apracing.com/drawings/cp3550-13cd.pdf


brake proportion valve in older DTM/ITC car (note this car was equiped with ABS)

[high-on-octane]

Some great discussion going on here. 1 thing I've also noticed while analyzing some of the data from some of the better mods in rFactor is that the brakes will have an operating temperature window in which they achieve optimum friction. The theory's the same for real cars though obviously.

It is possible using bias, disc thickness and duct sizes, to control the rate at which the discs on either end of the car heat up during a braking zone.

For example, you could make the front discs heat up faster, thus sending them outside of their optimum temp range before the rears, which means they will fade and loose power before the rears do, sending your effective bias to the rear.

I've seen it in rFactor alot. People complain about constantly locking the rears, so they add forward brake bias. This only makes it worse though, because the extra front brake bias will heat the front brakes faster and cause them to fade even more compared to the rears.

I have a channel set up in Motec's i2 that shows my effective brake bias through a braking zone, and by controlling the rate at which the front and rear brakes heat up, its definitely possible to tune the bias forwards or rearwards over the distance of the braking zone without having a bias adjuster.

[silente]

really interesting discussion.

reading all the posts i have noticed that all of you point on the fact that, during hard braking at high speeds, you normally want more front brake bias because of weigth trasnfer. And i generally agree with this.

But one thing i was also thinking about is that, at high speed, since center of pressure of formula cars is normally more toward the rear axle, you will have more donwforce on the rear tyres than at the front. This, together with the (normally) greater contact patch of rear tyres compared to the front, may push some drivers to use a final brake bias not so heavily front balanced.

In my experience with formula 3 cars, i have seen that normally you stay between 52 and 55 % of braking force at the front, no more.

Of course, F3 has less downforce than a formula 1 and its weight bias is more on the rear (someting around 42% of static weight at the front) but it brakes anyway up to 3 g.

[Sayshina]

Now my question is, am I right in this conclusion?
Drivers should be cautious to not to lock the rears by not over-downshifting. Cuz the resultant engine braking sums up with the disk braking and as the result endangers the rear end's stability.
On the other hand, ideally, it usually is better for the car to have more braking bias in start of braking and transfer it the rear axel as they slow the car down.
As far as I've noticed Lewis Hamilton and Robert Kubica tend to go much later on downshifting during a braking period, but as they start to it, they do it much faster. I wonder whether if in fact they are using engine braking to improve their desired actual changeable braking-bias path (algorithm) they want to, during the braking period?

Under braking, the center of gravity moves forward, but the center of pressure will tend to move forward more. The front wing works much better as it gets closer to the track and is in no danger of stall, while the rear diffuser was already near stall before you increased its angle of attack. Increasing the angle of attack of the entire car will also tend to move the center of pressure forward.

Normally, you'd set your bias for what you want at the front, and use the engine to adjust the rear. But engine braking tends to be fairly harsh. Guys who have raced 2-strokes and gone on to 4-strokes complain endlessly about the engine braking.

In bikes, where you have independant control between front and back, you ride the front going in, and use the rear mostly to adjust your slide. If drivers are bunching their shifts up I'd bet their cars don't want to turn in and they're doing it to force the issue.