<cue the 1950s documentary music>
Your Brakes And You
You are probably here because you want to know more about upgrading the Prelude's braking system. Before we get into upgrades, you should know how the whole system works. Attached to this post, there is a general diagram of a car braking system. There are a couple of parts you should notice:
- The rotors on each hub. The hub is the part at each wheel which rotates inside the knuckle. The front hub is connected to the axles (FWD) with a big 36mm nut.
- The calipers on each knuckle. The knuckle is the steel part which the suspension members bolt to. The hub spins inside it. It has pickup points for the A-arms and the lateral links, depending on front or rear.
- The ABS modulator, and the hydraulic lines which connect it to the fittings on each caliper. The ABS modulator electronically controls hydraulic braking pressure to each wheel independently, based on the ABS computer.
- The master cylinder (MC), which is mounted on the firewall. The MC is driven by your foot; it is a pump which drives force from the brake pedal into the hydraulic system.
- The brake booster, which is the saucer-shaped round thing also mounted on your firewall. This takes vacuum created by your engine (at idle or when the accelerator is not pressed), and uses that vacuum to reduce the amount of force you need to press the brake pedal. It's a booster mechanism, which allows you to hit the brakes a little bit quicker than without it.
There are other misc. parts, but these are the large ones which you should know to read this guide.
The Golden Rules of Braking
Before we run off and spend money, we need to know our destination. It's vital to understand the most effective strategies. Behold, I give you the cardinal rules of braking:
- THE BRAKES DO NOT STOP THE CAR. YOUR TIRES DO. I am dead serious about this. This is far and away the single most under-appreciated fact of braking. It makes absolutely NO sense to dump $2k on a big brake kit (BBK), and run all-season tires. Buy sticky, high performance tires before you spend a DIME on your braking system. Otherwise, you are pissing away your money.
- For a certain weight/size car, up to a certain point, no amount of money can make your car stop faster. We will get to the explanation of why in a bit, but know that there is only a very small improvement you can make (~10%) to the actual stopping distance of a factory car. Beyond this point, it is simply not possible to stop any faster with more/better parts. We could improve braking without limit if we could change the car's weight or run infinitely large tires, but obviously reality puts strong limits on these things.
- Brake upgrades can really only be categorized into 3 things: (1) upgrades which lower your stopping distance, (2) upgrades which manage heat more optimally, (3) upgrades which improve braking feel. The first one has surprisingly few options for it. The second is what 90% of all braking parts in the aftermarket essentially do, and the last is useful to create a better driving experience (explained later).
- On a street car, you will never generate enough heat to require expensive braking parts (e.g. non-OEM calipers/rotors). The simple reason is that you don't brake often enough (and at high enough speeds) to require fancy/expensive braking upgrades. A lot of people will read this and ignore it, and go out and buy that fancy Endless BBK kit anyways, 'cause it's hella tite yo.
- Upgrading your front brakes (including calipers, pads, and/or rotors) without upgrading the rear will almost certainly result in worse braking distance. The reason why is explained in the very next section: brake bias. If the bias deviates from a certain ideal number, your braking performance gets worse, no matter how good/expensive the brake kit.
The best thing to do is to figure out what you can do to lower stopping distance, since that has the most practical value for your time/money. If you auto-x or track the car, you may
want to then consider heat management, but only if you compete often enough to make it worthwhile.
Before I go on, I want to put in another word about tires. The stock Prelude braking system is very good for an OEM system, and the best thing you can possibly do to improve your braking is buy quality tires. You want a high-performance summer tire for as long as it stays dry in your part of the world. Buying a sticky tire will dramatically improve your braking distance without touching the brake system. There is a reason why all the magazine brake kit reviews choose the most extreme R-compound tires they can fit; you will more often than not see Hoosiers as the test tire (that's not a coincidence).
There's a simple way to visualize this why tires are important: consider the caliper, rotor, and wheel on one corner of the car. Your stock braking system has more than enough power to lock up the rotor and the wheel. If the wheel is no longer turning, your tire will be sliding against the pavement. This is where you need friction, because that is what is actually stopping the car. This is still true for ABS, since ABS uses sophisticated computer controls to keep the tires right at the threshold of sliding; basically it keeps the wheels barely rotating enough so that the tire doesn't slide. You are still limited by the traction from your tires in these cases.
What can go wrong?
The best example I can give you guys of the perils of upgrading things without knowing what you are actually getting is a case I saw of a test done on an RSX (not a Prelude I know, but it makes a good example). There were no less than three
pricey BBK setups that had worse braking distance than a stock car
. You can read about it on Stoptech's website here
. This is what happens when companies sell a BBK which only modifies the front brakes. It throws the bias off, and too much braking force is applied to the front. I strongly encourage you guys to make sure, if you really want a BBK (despite what I wrote here), make sure it comes as a complete kit including the rears.
Lowering Stopping Distance
Keep in mind that, even if you do everything right, you can only lower your stopping distance by about 10%. In theory you could lower it much more, but you would have to do more radical things like dump hundreds of pounds off the car, or run super-sized tires (255+ width), or basically turn it into a track car. If you want to build a track car, this guide is not for you.
The three things I can think of which would materially improve stopping distance are:
- Create the ideal F-R brake bias. This is the number one most important goal. There are many ways to do this, and you have to start upgrading/replacing parts to achieve this. More explanations in a minute.
- Run brake pads with a better coefficient of friction, subject to your heat conditions. Race pads are designed to work at high temperature, and street pads are designed to work at low temperature. Putting race pads on a street car is very stupid. You will understand why when we get to the section about heat. In the meantime, improving the friction between the caliper and the rotor puts more stopping force through the wheel/tire. There is a little leeway to improve, but again there is an upper limit on how effective this will be.
- Use aerodynamics to create more downforce on your tires. For a street car this is pretty impractical, but it can make a small difference to do the following things:
Keep in mind these things are only going to make a very small difference (like ~1%), so they should not be a priority unless you have done everything else.
- Lower the car
- Run a smooth skidplate under the front
- Run a diffuser in the rear
- Run a front lip with better than stock aerodynamics
- Run a spoiler with greater frontal area
Quick comment about braking system reviews: Most magazine numbers are the 90mph-0 number, which has the greatest potential to look good. However, for a street car, you rarely are going to be driving 90 (at least you shouldn't be on public roads). The most relevant numbers are usually the 40mph-0 and 70mph-0 numbers. The improvements are less good in these cases.
Getting The Bias Right
Here's where the magic is, and this section has a bunch of math in it. Brake bias
, in a nutshell, is the amount of available braking pressure applied (through the pads) to the front half and rear half of the car, when the brakes are fully engaged. Brake bias is usually a percentage, such as 60% front and 40% rear. To do this section, we are going to use measurements taken from the stock braking system to compute the ideal and actual brake bias. Here is our data:
- Stock front piston diameter: 57.2mm/2.25in
- Stock rear piston diameter: 1.30in
- Stock front rotor diameter: 11.1in
- Stock rear rotor diameter: 10.5in
- Brake pad coefficient of friction: 0.4 (good pads are around this value)
For this part, we will use TCE's Brake Bias Calculator
. For anything not listed here, use the default value. Using these values, we get an OEM brake bias of nearly 75/25 (+-2%). This is very heavily front-biased. Let's see what the ideal bias is.
Ideal bias can be computed using math. The problem is that math approximates certain things, and doesn't take into account cornering loads or uneven surfaces, all of which occur in real life. However, it's a good approximation, so we can start there. Using this info
, I have computed the dynamic weight bias of a Prelude under 1G of braking force. 1G is a lot, but with good tires/pads, you can duplicate this number. Here is my Google Spreadsheet
which computes this dynamic bias. Using stock numbers, an extra 442 lbs is dynamically transferred from the rear of the car to the front under braking, which is bad, because the car is already nose-heavy. Braking always causes this weight shift, so ideally we want to minimize it, which I will get to in the next section. Using this stock data, we see that the ideal front brake bias for the Prelude should indeed be about 75%. In this regard, the OEM setup is quite good.
Improving the OEM Weight Distribution
However, with this setup, rear traction is very compromised. There is a lot of available traction at the rear which we can't use, because so little weight is there under braking. We want to find ways of putting more weight on the rear. By playing with the calculator, you can see that it's possible by:
- Moving the static weight distribution rearward. You can do this by removing/replacing parts on the front of the car, or doing something like a battery relocation to the trunk.
- Lowering the car. This is surprisingly effective.
- Increasing the spring rate. This isn't shown on the chart because of simplicity, but higher spring rates keep the center of mass of the car from pitching forward so strongly under high deceleration. This keeps more of the car's mass in the back, ever so slightly.
If you play with the numbers, it's possible on a street Prelude to get a dynamic bias close to 70/30, depending on what you do. In this case, you can shorten your stopping distance by biasing the braking more towards the rear, in order to take advantage of the extra weight (force) on the rear tires. This eventually leads us to the modifications section, which is where you can choose ways of doing this.
For street cars, the intent is always to lock up the front brakes first. This happens when the total traction available by the tire is exceeded (assuming no ABS in this case). This is done for safety reasons, because locking up the rears first means the tail end gets fishy, which is much harder to control than plowing the front end. This is still true even if you are an experienced driver. So use caution when modifying the brake bias, and don't get yourself into a setup where the rears lockup first (be conservative).
Measuring Actual Bias
For those of you serious about taking action on this stuff, there are tools you can buy which allow you to tweak and measure the brake bias. I don't have links, but you can buy amateur-level force gauges to actually measure the amount of forward weight transfer under braking. If you do that, then you will know exactly what the real weight distribution is, and then you can setup your actual brake bias appropriately. If you are really sophisticated, you can even find ways of digitally logging the force over time, and even correlate that with deceleration G's. Using that info, you could come up with a dynamic weight transfer curve, and use more sophisticated methods
to dynamically control brake bias. However, that's way out of the scope here.
- tzywen.com - Electronic Driving Aids
- StopTech : Balanced Brake Upgrades
- AUTO REPAIR HELP - THE BRAKE PROPORTIONING VALVE