Switching to rear disk brakes??

<div class='quotetop'>QUOTE(DanMan32\";p=\"89772)</div>

I'm not your secretary... sigh.

The prius, like most cars, has EBD. It distributes braking force based on the load in the car. IE, if you've loaded the car down, more force can be applied to the rear brakes. It's just how EBD (electronic brake distribution) works. Follow?

So if there is no regen on the rear, and it's established that there isn't, how could EBD work without using the physical rear brakes?

 

<div class='quotetop'>QUOTE(Tideland Prius\";p=\"89793)</div>

It has *nothing* to do with the requirements of braking, and everything to do with the weight of what you've got in the car. If it's just you in the front seat, then you're probably right.

If you've got passengers in the back, or perhaps, 300 pounds of top soil, you'd better believe that those rear brakes are going to be kicking in to help. And the Prius probably has a very low threshold before the rears are going to come on, as it's maximum cargo weight is 1000 lb (I believe).

 

<div class='quotetop'>QUOTE(Tideland Prius\";p=\"89796)</div>

You could well be right.. But I think with any kind of load in the rear you're going to have some rear drum action. It's just the nature of how EBD works...

 

<div class='quotetop'>QUOTE(blkc5\";p=\"89800)</div>

Amen.

And to quash the inevitable ... Euro requirements DO NOT state that 4 wheel discs are required.

 

<div class='quotetop'>QUOTE(seasalsa\";p=\"89731)</div>

Just the explanation I've sought from Service Techs at my dealer's shop. They gave some simplistic answer like "The computer takes care of that." They DID report, however, that they had yet to change either disc pads or linings in the 4 years of Prius service they experienced.

 

Why is it that people want me to research THEIR suppositions? They make claims, provide no reference, and when I ask for it, make ME go look for it.

 

<div class='quotetop'>QUOTE(DanMan32\";p=\"89861)</div>

Because you're providing very little in the way of hard facts to back up your suppositions. I'm still waiting to hear how the rear brakes aren't being used with EBD.

 

Not only does Dan often document his sources and findings (usually Toyota Service Manual), his frequent accurate contributions make him an extremely credible source of information.

What I believe he's saying is rear brakes do use EBD, however they're not activated during regen as on the classic. Everything I've read supports this as well, including the longevity of the shoes. If the rears activated nearly every stop, shouldn't they wear faster?

blkc5, the thread wandered from the original topic because most of us aren't willing to invest the significant cost of a retrofit into something that has (arguably) no benefit. I have yet to see any data that Euro or Jap cars with 4whl disks have significantly better braking performance or lower rate of brake related accidents. Without data, this is a mod just for fun or looks. To say every performance car has them also isn't valid, this isn't a performance car, and the friction brakes are almost a backup, secondary system.

We still haven't heard of anyone who had brake fade yet.

 

<div class='quotetop'>QUOTE(gschoen\";p=\"89889)</div>

First of all, the friction brakes are what stop you below speeds of 8mph, so they are activated nearly every stop. Good luck stopping without those backup, secondary systems.

Second, EBD shifts power to wherever depending on LOAD. Try this simple experiment if you like... throw 500 lbs of topsoil in the back of your car. Disable your rear brakes. Stop from 40mph a few times. Enjoy.

 

Jonny, you're grasping at straws.

Friction Brakes ARE what stop you below speeds of 8 mph, meaning Front AND Rear - which has nothing to do with Regeneration, Fade, relative F/R Brake Wear or anything else we've been talking about here.

EBD does shift braking around, and in SOME circumstances of HEAVY braking either one or both rear drums may be used, but the same is also true of the front. It uses what it needs.

The issue is, under normal braking, which makes up 99% of the use of the brakes, are the Rears used more than the Front?

The answer, according to all the evidence I've read, and others have read and quoted is - NO.

If the Prius can slow down with only front regenerative braking, it does so.

If it needs to apply mechanical brakes, it does so, but it uses whatever it needs, front, rear, both.

If you have evidence that the Prius regularly uses front regenerative braking and rear mechanical braking simultaneously, please provide it.

 

<div class='quotetop'>QUOTE(Jonnycat26\";p=\"89902)</div>

First of all, the friction brakes are what stop you below speeds of 8mph, so they are activated nearly every stop. Good luck stopping without those backup, secondary systems.

Second, EBD shifts power to wherever depending on LOAD. Try this simple experiment if you like... throw 500 lbs of topsoil in the back of your car. Disable your rear brakes. Stop from 40mph a few times. Enjoy.
[/b][/quote]

Your concept is correct, but how EBD functions is not. There is no "load" sensor as you imply. EBD is the process of using software to provide additional hydraulic braking to the rear brakes during conditions where less than the optimum amount of rear braking is occuring based on the design of the brake system, specifically the resulting balance curve and gain of the system. This is determined by speed and deceleration, and where possible by steering angle, not directly by load. Load is an implied input as too much rear braking will result in ABS activation. An additional feature is that brake force can be controlled for each rear wheel, allowing for additional brake force to be applied to the outer rear wheel and less to the inner rear wheel when braking in a turn. This minimizes inboard rear wheel skids and also uses the tractive force at the outboard rear wheel to help balance the vehicle and keep the inboard wheel planted.

The concept of EBD is used to counteract the undesireable affects of proportioning valves. Prop valves result in too little brake pressure to the rear wheels under many conditions to ensure that the vehicle is not rear biased, which results in the possibility of a rear initiated skid. EBD is an enhancement of dynamic proportioning, the software process of proportioning rear brake pressure based on speed and deceleration versus a mechanical valve.

Finally, as to the original question, the only way to understand the potential affects of changing to the rear disc brakes is to get the dimensions of the components and determine the gain of the rear system versus the drums. Unless you are doing numerous high speed stops back-to-back, you will gain nothing with respect to reduced stopping distance. You will also need to determine the stroke and attachment method for the parking brake cables versus the drums to be sure they are compatible. The European version likely has shorter stopping distances but primarily a result of the wider and more tractive tires.

Credibility of my comments? I design and develop this stuff for a living, so you can chose to believe me or not.

 

<div class='quotetop'>QUOTE(blkc5\";p=\"89911)</div>

Just the fact that one set of brakes is disc and one is drum says NOTHING, let me repeat NOTHING about how well they stop in a single emergency stop situation.

If you engineer two brake systems to perform equally they will perform equally. (Ignoring the Fade Issues).

The implication here is that Toyota somehow under-engineered the Drum Brakes they put on the US Prius.

In theory, if you are driving your Prius like a Ralley Car, you might have some comparative fade in rear drums, but for a single panic stop, and every day safety, there is NO DIFFERERENCE.

If you have evidence that Toyota provided less braking capacity in the US, step up and provide it.

This seems to be some weird fixation people have that Disc Brakes are 'better'. They have some characteristics that are good, indeed, but functionally they just stop the wheel from turning just like any other brake mechanism.

If you want to stop shorter, put on stickier tires. Tires are the limiting factor in panic stops in almost every case. The brakes are more than capable of locking up every wheel dead solid a panic stop and the discs will show no difference at all from the drum stopping distances.

 

<div class='quotetop'>QUOTE(blkc5\";p=\"89911)</div>

I am not trying to pick on you directly, but I do need to respond to your comments. I don't believe anyone credible has suggested regen braking stops the vehicle by itself under all conditions. Rather, referencing the braking curves supplied by Toyota, unless more brake torque is required than can be supplied via regen, then regen is what you get until about 8 mph, at which point all braking has transitioned to hydraulic. If the brake torque requested via pedal travel and master cylinder pressure exceeds regen capability, hydraulic braking is "added" to the regen braking. Again, this is very well documented. As for wimpy rear drums, see below.

<div class='quotetop'>QUOTE(blkc5\";p=\"89911)</div>

Fair enough, but this is not a 1970's retrofit. Any changes must comprehend the potential affects to ABS, stability control, EBD, etc. If you were to make the vehicle rear biased, or more front biased, you would sacrifice stopping distance not reduce it. As all the hydraulic braking is done via electronically controlled metering valves (solenoids), the rear brakes are at the mercy of the control algorithm.

There is a huge potential issue evolving in brake systems. We design and calibrate systems based on OEM design requirements. When people jack up the back end, add wide low profile tires, and then add so-called "performance" brakes to their vehicles, the calibrations for ABS, VSE, etc. are at high risk. Stability control cannot function correctly if the system is calibrated to provide a specific force over time to correct a skid and the resulting force is much greater than designed because of the tire and brake changes. Now you have created instability. Does the public understand what many of them are doing? No!

<div class='quotetop'>QUOTE(blkc5\";p=\"89911)</div>

Compared to what? Any comparison data you have for the Prius would be skewed due to the tire size difference in Europe. Compared to any other vehicle, you need again to check the specs of each vehicle. In nearly every case, if a rear drum and rear disc version is offered, the rear disc option is a "premium" brake system. Included in nearly every one of these systems is upgraded tires (either size or traction or both), stiffer suspensions (reduces weight transfer during braking), and often an upsize in brakes. As to your "wimpy rear drums" comment, can you show me that the available rear brake torque or thermal capacity is inadequate for this vehicle? No. Are the rear drums sized such that the linings are wearing out prematurely? No. The brake system is actually over designed with the available regenerative braking. To go to disc brakes adds cost and mass. Then there is the challenge of getting the parking brake to work as required by law with the small rear disc that would be the right size to use. As a designer of these systems, for this vehicle I am quite happy with the 10-20 pound mass savings and about $200 in cost savings. Toyota has chosen to spend money on other content rather than inflate the selling price by providing a brake the owner does not need. Toyota does this for many of their vehicles. Granted, $200 may not mean much to those buying this car now, but when the original sales price target of <$20k was in effect, $200 was a big chunk of savings for the overall vehicle.

 

<div class='quotetop'>QUOTE(blkc5\";p=\"89911)</div>

You're 100% right, but good luck getting anyone to admit to that. Everyone here seems to have Prius blinders on.

Let's change the scenario a little bit too. Let's say you're crusing down a busy road, where the speed limit is 50, and you have a bit of load in your car. And there's a bit of stopping and accelerating. This isn't at all uncommon in NJ. Is regen braking going to stop you each and every time? After 10-15 miles of this, don't you think your brakes are going to start to feel it? Try cruising Route 22 from Union to Bridgewater in NJ and see what happens to your brakes. I'm sorry, but regen alone isn't going to provide more than a fraction of what you need to stop.

 

<div class='quotetop'>QUOTE(blkc5\";p=\"89911)</div>

My two cents, based on my experience with bicycles and motorcycles as well as cars:
in straight ahead panic stops, the front brakes are doing most of the work, ~90% for a suspensionless bicycle. Before anyone shouts apples and oranges, I agree that a heavy, suspended car is not a bicycle or even a motorcycle. I also contend that for a typical creampuff American sedan, the difference between disc and drum REAR BRAKES in STRAIGHT AHEAD PANIC STOP is negligible. Historically narrower, rougher roads in Japan and Europe led to cars with stiffer suspensions and tighter steering. Hence, the early adoption of disc brakes, radial tires, rack&pinion, active suspensions, and so on. (Oversimplified but you get the point.) So, I have no doubt that rear disc brakes are superior during say a high speed turn over cobbestones through a traffic circle. But let's face it, the Prius is no roadster; rear drums were good enough but probably wouldn't sell in Europe.