So I was on the crapper the other day reading GRM (yes I had to share that, dont you think its the best reading position?) and the article on brake bias really got me thinking.

if you havent read it, its a follow up to a previous article where the main point which may be familiar to many of us that ULTIMATE braking force is determined solely on the tires. however, they follow up that point that also the front/rear brake bias plays a strong factor in braking performance and stopping distance.

the article talks of achieving an elusive "balanced" bias, and having too much rear bias makes the rears lock up and the car becomes unstable. HOWEVER, what is bugging me is that too much front bias also is detrimental to brake performance because it simply doesnt utilize the potential rear braking force and will cause longer stopping distances. common and popular examples of this are "big brake" upgrades and upgrading front pads and not rear pads. they also mention that FWD and pretty much all passenger cars are setup heavily front biased anyway.

so, whats bugging me is that I have always considered the way to go for setting up our race cars (which are constrained to stock sized rotors and calipers, and only brake pads, lining and fluid changes) is not to bother with putting any strong performance pad in the rear, and get the best pad for the front. but following this design, I have just increase front brake so that the car is even MORE front biased, and according to the article, have actually made the stopping performance of the whole vehicle less.

now, i know a greater reason to buy performance pads is for superior heat range over stock, off the shelf pads. but among performance brake pads which can more or less handle the demands of thermal control in racing and track driving, is finding the pad with the greatest amount of grip going to be best?

also, should upgrading the rear pads be more looked into? who else has actually experimented with different rear pads and combinations and found actual different performance differences?

and if anyone from GRM is reading this, why does david wallens article on adding big brakes to the Civic immediately follow the brake bias article when its clearly stated the addition of such devices may(?) be detrimental to brake performance? seems like then you are trading off the benefits of thermal mass to ultimate brake performance?

lastly, brake proportioning valves ARE LEGAL in IT, has anyone experimented and installed them in their car? perhaps stock proportioning valves from other vehicles with different ratios may provide a cheap alternative to redesigning the whole brake line system.

 

As much as I like GRM, magazines make their money by selling advertising space. It is not entirely unheard of for a magazine to pimp the products of its advertisers. So I am always very careful to consider the source of information that comes from magazines.

Friction materiel makes a huge difference in brake effectiveness. I personally really like Porterfield products, but Hawk and other manufacturers are equally good I am sure.

Front brakes do the majority of the braking on any car, and on front wheel heavy cars like Hondas this is even more so. With so little weight over the rear wheels, the rear brakes just can not contribute that much stopping power. On the other hand, a car that is tail heavy like a Porsche gets more usage out of its rear brakes.

Many "tuners" slap the biggest rotors and calipers they can fit under their wheels. There is a penalty to be paid for all that extra rotational mass and unsprung weight.

In CSP Solo2, I am allowed to swap my '87 CRX Si's iron rear drums for 84-87 CRX HF aluminum drums. Not only does this save me weight (rear disk brakes on most Civics and CRXs are heavier than drums and do not provide any extra stopping power over drums), but the aluminum drums disapate heat nearly as well as disks.

A brake bias valve is a great tuning tool, you can move the bias forward a bit to stop your rear brakes from locking up.

Scott

 

You also have to consider whether the best setup for braking is the best overall.

The optimum tire pressure for braking is probably not the same as the optimum for cornering.
Zero camber front and rear under full braking would probably shorten braking distance, but would play havoc with handling.
If all four tires are at the limit of adhesion when you brake at the end of the main straight, would a little bump, or oil, or dirt unstick the back with dire consequences?

No answers here, only more questions.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tyson &raquo;</TD></TR><TR><TD CLASS="quote"> but following this design, I have just increase front brake so that the car is even MORE front biased, and according to the article, have actually made the stopping performance of the whole vehicle less. </TD></TR></TABLE>

i got up off the pot with the same thoughts after reading that article. currently i run aem pads front and rear on my daily driver/weekend warrior, but switch out the fronts for gt-sports for hpdes. i immediately starting thinking i should search for a grippier rear brake pad for events (i don't even think they make gt-sports for the rear), and toyed with the idea of an adjustable proportioning valve, although i believe that would leave to re-plumbing the lines, which i don't think i'm up for right now. it would be nice if their was an easy oem switcheroo, but i doubt there is one without some customization involved.

i almost want to experiment with say a 'race' type pad in the rear and an oem equivalent in the front, just to see if that swings the bias rearward significantly.

as it is, i can't remember the last time i locked up all four tires, street or track, in a straight line. that can't be good for the front rotor and tire temps - and over the course of a session, or a race, that could have a serious effect on tire wear and brake fade.

 

I don't often post here, but my input was requested on this thread...


a bit of background:
I've raced a '88 crx si in ITA for 3 years
my first two years were with rear discs but a drum proportioning valve, my third year was with the proper valve. I run hawk blue pads in front, and $10 pads in the rear.

I was absolutely astounded at how much of a difference the valve made. By my estimations, with the drum valve, I was getting far too much rear braking, causing the car to get a bit squirrely in corners, and so on. After doing nothing more than swapping the valve (really nothing more, not even an oil change) between seasons 2 and 3, I instantly lost 1-2 seconds per lap, consistently. the car is much more stable and predictable in corner braking

Now on to the subject at hand. Ultimately, like Tyson said, braking is limited by tire traction. The key is to prevent all 4 wheels from locking up, while still braking to the limit. The reason we typically run far less rear brakes, is because when you get on the brakes, the car will pitch forward, taking weight off the rear wheels, and essentially taking traction away from the rear wheels, reducing the amount of braking the rear wheels can do without locking up. The front wheels, with all the added weight thrown forward, plus the weight of the engine et al, have more traction with which to brake.

The setup for ideal braking, (and nothing else) would be to set up the car with identical static weights at all four corners, and some sort of front-rear anti-roll bar, that would keep the car extremely level, and with the same weight on all 4 wheels. Thusly, you could put the same amount of braking force into all 4 wheels without locking up one or two, because with the same weight on each wheel, there would also be the same traction on each wheel (in an ideal situation)
I suppose one could also do this through higher spring rates and shock valving in the front, also, rather than a front-rear anti-roll bar, though maybe not quite as precise.
This concept also applies on a comparison of right-left tire braking in corners, as in a corner the car's weight is thrown towards the outside wheels, so the outer wheel has more traction with which to brake

But you're saying you prefer a stiffer suspension in the rear of the car to add a bit of oversteer for cornering. So that makes it more difficult. It would be hard to set up a practical roadrace car that enjoyed equal or near equal braking force on the front and rear wheels.

Since I am an IT racer, and extremely limited in what I can set the car up as, I am happy to all but ignore the rear brakes...

Hopefully this helps someone, and if there's any technical errors in what I've said please point them out to me

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tyson &raquo;</TD></TR><TR><TD CLASS="quote">get the best pad for the front. but following this design, I have just increase front brake so that the car is even MORE front biased, and according to the article, have actually made the stopping performance of the whole vehicle less</TD></TR></TABLE>

I've yet to read this article, but it sounds interesting. The reason why it's good to get a better pad for the front is because in reality with your stock brakes, it’s still not the tires that are the limiting factor in braking performance. With stock pads after braking hard a few times, you'll start fading and then your brakes will become the limiting factor. The main advantage of upgrading your front pads is to reduce brake fade. If the stock pad is able to lock up the front wheel the first time you brake hard, upgrading to a better pad will in theory not reduce the distance on this first try. However, do this 20 times and the upgraded pad will no doubt perform better by being able to hold the tires as the limiting braking factor for a lot longer before it (the pad) eventually becomes the limiting factor.

Chris, did you have to do any changes to the lines? From what I understand by talking to those who have taken a CRX equipped with rear drums and converted to rear discs, it's quite a hassle to get the bias right. This is mainly due to the fact that adjusting the brake bias controller actually adjusts the front left/right rear and front right/left rear wheels together, in essence it adjusts it diagonally. The solution to this was to re-configure the lines so that adjusting the front/rear brake bias from the controller actually did just that. Was this different on your car?

 

Alex, I didn't change any of the hard lines. All I did was take out the stock brake proportioning valve, and replace it with one from a 90 integra non-abs (oops! technically not IT legal, but much more common than the identical IT legal part)

I haven't heard anything ever about the hard lines being different on drum cars vs. disc cars

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">from Chris Sawatsky:
The setup for ideal braking, (and nothing else) would be to set up the car with identical static weights at all four corners, and some sort of front-rear anti-roll bar, that would keep the car extremely level, and with the same weight on all 4 wheels.</TD></TR></TABLE>
The physics of the situation dictate that you cannot eliminate rear to front weight transfer under braking.. Your solution may be better at keeping ideal camber, but it won't significantly affect weight transfer. For the most part, roll occurs because of weight transfer, not the other way around. I am not trying to invalidate the earlier things you said, though..

As pointed out by others in this thread, I would agree that adding in more rear bias has its dangers. The problems of too much front bias seem generally easier to handle (suboptimal braking distances, understeer) than the problems of too much rear bias (higher chance of losing the backend), and in general seem less likely to put your car in a body shop... One thing to think about, though, is if you are too heavily biased to front brakes, you may perceive things as understeer at corner entry, and try to compensate with bars, shocks, springs, pressures, etc. which may end up hurting you in other places.

my background:
I'm a minor physics geek/mechanical engineer without the funds/*****/insanity to do wheel to wheel racing.. I'm working mainly from autocross and theory.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by MechE00 &raquo;</TD></TR><TR><TD CLASS="quote">
The physics of the situation dictate that you cannot eliminate rear to front weight transfer under braking.. Your solution may be better at keeping ideal camber, but it won't significantly affect weight transfer. For the most part, roll occurs because of weight transfer, not the other way around. I am not trying to invalidate the earlier things you said, though..
</TD></TR></TABLE>

I realized that weight transfer cannot be eliminated, due to simple momentum laws and such. However, my understanding of sway bars is that when force is applied to one end (in our example, force on the front end due to weight transferring forward) of the swaybar, it is transfered through the bar, to the other end, to make the forces equal. If the forces are equal, then the effective weight felt by the road, at the point of tire contact, should be equal, or at least more equal than it would be without the front to rear swaybar

 

I havent read the GRM article but i've thought about the subject some time ago after experiencing some funny behaviour on a few different cars as in regards to front and /or rear lock-up. Something to consider when talking about bias. The stock braking system was designed and biased with the stock tires in mind and with a good safety margin towards front bias built in for obvious reasons. Now, add some sticky tires to an otherwise stock car and you add a significant amount of braking force causing the car to dive more and unload the rear tires more. Depending on how soft the stock suspension is you can (and i have) end up with having too much rear bias even with the stock brake setup causing pre-mature rear lock-up forcing the driver to reduce the braking force well short of reaching maximum braking force on the front tires. Now change the front pads to a higher-friction compound and you may balance things out somewhat by swaying the bias back towards the front. This is the main reason why a lot of people end up with high-perf pads in the front and stock or near-stock at the back...

Now this was assuming a stock suspension. Install some stiffer springs and things change once again. You now have more dive control and the rear will not unload as much. So with high-perf pads in front and stock in the rear, you may end-up with too much front bias and early front lockup. Changing to high perf pad in the rear may improve braking somewhat.

I would imagine these are the main reasons some people swear by running hawk blues on all fours. Others swear with blues in front, hp+ in the rear. Some run hp+ on all fours. Still others run hp+ and stock rears. You get the point... It all really depends on car setup and tires used.

Anyway, the point is that rear brakes are also important and maximizing the rear braking is just as important as the front. Ideally you'd want the brakes balanced so that the fronts lock-up just a fraction of a second before the rears. The best and quickest way to achieve that is with an adjustable proportioning valve and some track testing. Or you can forever dick around with different pad compounds, oem prop valves, MCs, etc.

My .02 and hopefully this made sense...

 

weight transfer is independant of suspension stiffness. it is a function of cg height, braking force, and wheelbase. stiffer springs won't change the amount of weight transfer, but the lower ride height typically associated with the stiffer springs will. the better camber control with a stiffer suspension will help too.

nate

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Chris Sawatsky &raquo;</TD></TR><TR><TD CLASS="quote">I realized that weight transfer cannot be eliminated, due to simple momentum laws and such. However, my understanding of sway bars is that when force is applied to one end (in our example, force on the front end due to weight transferring forward) of the swaybar, it is transfered through the bar, to the other end, to make the forces equal. If the forces are equal, then the effective weight felt by the road, at the point of tire contact, should be equal, or at least more equal than it would be without the front to rear swaybar</TD></TR></TABLE>

I'm pretty sure that a front-rear anti-rollbar would actually increase longitudinal weight transfer, since it would change the f/r roll couple (pitch couple?) as it connects the two axles. Edit: there is going to be the load transfer of the unsprung mass (we can't change much there), the sprung mass (indp. of suspension), and the through roll centers, which changes with changes in the roll couple. The last one I'm making the assumption that it is similar for lonigtudinal transfer as it is for lateral. The f/r sprung weight tranfer is going to happen regardless of how stiff your front springs or the f/r anti-rollbar are. The input/output forces of the anti-rollbar may or may not be equal (depends on how accurately you want to account for its torsional strain), but the forces seen at each axle are not equal.

My background is the same as MechE00's (more physics, less ME), so I can't comment as an IT racer (unfortunately). I completely agree with Hracer regarding stopping distances: Hawk Blues shouldn't make you stop any sooner than Hawk HP+s assuming your brake system functions properly (ie you can lock up your race tires). The Blues would bite harder, sooner, but if you are on the brakes hard I would think it's negligible. The fade resistance is the important reason, especially after looking at a set of OEM pads after a track weekend.


Modified by GSpeedR at 12:20 PM 1/13/2004

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by solo-x &raquo;</TD></TR><TR><TD CLASS="quote">weight transfer is independant of suspension stiffness. it is a function of cg height, braking force, and wheelbase. stiffer springs won't change the amount of weight transfer, but the lower ride height typically associated with the stiffer springs will. the better camber control with a stiffer suspension will help too.

nate</TD></TR></TABLE>

Right. But the stiffer springs will reduce the amount of dive or pitch forward which in itself is responsible for some weight transfer. Nah? Imagine if you were to compress the front suspension 2 inches and then raise the rear 2 inches in effect jacking a stationary car. Wouldnt that cause a change in the cg and/or the weight distribution. Unless i'm thinking about it all wrong...

Also, i cannot imagine how a front/rear anti-roll bar (anti-dive bar?) would function since the attachment points would move with the body. What would it leverage on? A typical antiroll bar leverages itself on the body to restrict suspension movement which itself is independant. Know what i mean?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">Right. But the stiffer springs will reduce the amount of dive or pitch forward which in itself is responsible for some weight transfer. Nah?</TD></TR></TABLE>
In reality, yes there will almost certainly be some effect due to that... but this is generally held to be negligible, or at least not dominant (I don't have any real world data to back this up) compared to the the torque due to the force of the tires' contact patches acting about the center of mass of the car. The change in vertical load distribution among the four tire patches that takes place to balance this torque is what is being called weight transfer.

If you have a very softly suspended car with a very high center of mass and a lot of suspension travel, the weight transfer due to pitch/roll may become important.

 

Also, here's an old StopTech article i remembered while reading this. It briefly discusses the effect of increasing front brake torque and how it affects bias and braking ability...

http://www.stoptech.com/whitep...1.htm

In effect, when changing pads to a higher friction compound, we are increasing the brake torque applied at the wheels (we are not increasing the effective braking force generated at the tires) and so effectively changing the front to rear bias...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Chris Sawatsky &raquo;</TD></TR><TR><TD CLASS="quote">

I realized that weight transfer cannot be eliminated, due to simple momentum laws and such. However, my understanding of sway bars is that when force is applied to one end (in our example, force on the front end due to weight transferring forward) of the swaybar, it is transfered through the bar, to the other end, to make the forces equal. If the forces are equal, then the effective weight felt by the road, at the point of tire contact, should be equal, or at least more equal than it would be without the front to rear swaybar</TD></TR></TABLE>

Sway bars DO NOT change the amount of weight transfer from left to right. They only change the effects of the weight transfer (ie. body roll) although, they will change how the weight tranfer is distributed (ie front or rear). So in your example of sway bars going front and rear, you would not be able to adjust the total amount of weight transfer from rear to front under braking, but you would be able to transfer some of the weight transfer from the left front to the right front and vice versa.

One thing I have yet to see anybody address, is, as you add better and better front brakes, you need to dial out the rear brakes. The better your braking in the front, the more weight transfer you get to the front from the rear, and the less rear brakes you can use. So it seems to be the opposite of what they are saying, the better front brakes you have, the less rear brakes you need. This is assuming you have the same suspension geometry and you are getting similar or greater weight transfer.

C.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">One thing I have yet to see anybody address, is, as you add better and better front brakes, you need to dial out the rear brakes.</TD></TR></TABLE>
Well, I think jsi mentioned it, but using the less misleading focus of "better tires" rather than merely better brakes. If you can lock the tires up with stock brakes, putting on better brakes with the same tires shouldn't do anything significant to your braking distances or braking forces (assuming brake fade is not playing a part).

Of course, I guess it's somewhat expected that people upgrading brakes for track driving have probably upgraded tires too, but I wanted to clarify things.

 

Re: Weight transfer...

On a solid axle suspension a torque arm *can* effect weight transfer under braking (the same way it can effect weight transfer under acceleration).

Speaking from personal experience, with the simple change of springs (350 lb front increased to 500 lb front) my car gained a ton of stability under braking (same front pads, rear shoes and same tires on all 4 corners). The rear end was no longer trying to pass the front every time I got on the brakes.

Re: You have enough brake if you can lock them up...

Maybe so, but a different pad/rotor/etc could possibly effect modulation at the limit, and if you can control the threshold braking better, then you can stop shorter. Maybe setup A lets the driver use 80% of the braking capacity and setup B allows 90%, my bet is that setup B will get shorter stopping distances, all other factors being equal (in a perfect world).

Scott

 

What I was also trying to say, but not clearly is that let's say you have the following scenario

Say at 50lbs of brake pedal pressure you get

300 ft-lbs of brake torque at the front wheels at the limit of adhesion
100 ft-lbs at the rear wheels at the limit of adhesion

Assume that the system is balanced so that the front and rear wheels will lock-up at the same time with the given 50lbs of pedal pressure.

Now let's say you change the front pads to a higher friction compound. You may end up with something like this at the same 50lb pedal pressure:

Front 330ft-lbs
Rear 100ft-lbs

Now, since the front tires only take 300ft-lbs before running out of traction, obviously they would be long locked up at 330ft-lbs. But the rears are still perfectly at the threshold. Obviously, locking up the wheels is not beneficial to braking performance so you would have to reduce pedal pressure a certain amount to bring the front braking torque back to 300ft-lbs. And so you may end up with 45lbs of pedal pressure and:

300ft-lbs in front
90ft-lbs in rear

The rear brakes are now under-used and so effectively the overall braking torque has been reduced.

 

i read that article and it convinced me to put an adjustable prop valve in my next performance car.

setting the valve untill the rears lock up and the backing off slightly to bias slighty to the front.

anytime you remove weight, change pads, change tres, switch caliper, lines, rotors... whatever; you are "damaging" the factory calculated stock brake bias. it should be tuned for you setup just like you would tune an engine for optimal hp after mods.

as far as the rear pads, the rear brakes dont generate as much heat as the fronts... by using a true track pad in the rear you may never reach ideal operating temps and the "more aggressive" pads may not work as well as a good grippy street pad working within its range.

just more food for thought

 

While I have no experience with proportioning valves, I remember reading that they will only allow you to *reduce* the braking force in the rear. Since I don't know exactly how a bias valve really works, I can't comment on whether what I read was BS or not.
Just thought I'd throw that out there.