rear disc replacement for drums

http://priuschat.com/index.php?showtopic=8715&hl=

May be technically possible because Euro versions have it, but haven't seen anyone who has done it. Prius mostly uses regen braking, so it may be more trouble than its worth.

 

<div class='quotetop'>QUOTE(dogfriend @ Sep 8 2007, 10:32 PM) [snapback]509360[/snapback]</div>

Well, I am sure that the Euro versions use regen brakes too, so why wouldn't discs work? If you get the same parts as the Euro cars it should be no problem. I bet, however, that you would have to change more than just the drums to discs. You probably will have to switch master cylinders and maybe some ABS wiring, sensors, etc. Maybe the ABS computer is calibrated completely different for a four wheel disc setup...

 

<div class='quotetop'>QUOTE(sanmusa @ Sep 8 2007, 11:15 PM) [snapback]509422[/snapback]</div>

That's why I said it may be more trouble than its worth - the brake system components (solenoids, sensors, etc.) may be/ probably are different for the JDM/Euro versions vs. the North American version. And the Skid Control ECU may be different and probably has different programming as well.

Not a simple bolt on conversion.

I didn't mean to imply that there is any difference in the regen braking between the disc brake model and drum brake model - I meant that the rear disc brakes are not necessarily as advantageous on the Prius because it relies more on regen braking. There is more discussion about this in the thread that I linked above.

 

It might be educational to check the present wear of the rear brake pads. If there is virtually no wear, what is the motivation for making this change? Seems like a lot of time and money to spend for looks only.

 

The rear brakes actuate only in cases of hard braking, and under 8 MPH on normal stops. There is very little stress, very little wear. Based on information available on PriusChat, 100,000 miles of use is not unusual. Admittedly, U.S. purchasers of Prius have gotten a bad deal with several features available in other countries (Rear disks, and DRL to name 2). But, so what? The brakes do the job that they were designed for, and do it well. If you want a useful project, how about a Sunroof?

 

<div class='quotetop'>QUOTE(syclone @ Sep 9 2007, 09:48 AM) [snapback]509492[/snapback]</div>

I wonder if the other countries' Prius w/rear disk brakes have shorter 60 to 0 stopping distances? Bringing down the US' Prius 60 to 0 stopping distance from its mediocre 135+ feet to 120 something feet would be a significant improvement.

 

<div class='quotetop'>QUOTE(Boo @ Sep 9 2007, 11:09 AM) [snapback]509520[/snapback]</div>

Disk brakes do not necessarily produce a shorter stopping distance than drums. Any brake that can lock a wheel provides all of the stopping force ever needed, provided it allows for controlled application. Drum brakes work fine in this regard. The main advantage for disk brakes is better heat dissipation, which allows for longer application before fade sets in. Since the braking system on the Prius is entirely computer controlled, the stopping distance should be the same for disks and drums, until heat fade sets in. Most of the braking on a Prius is done through regen, with no involvement of the friction brakes. This implies that rear disk brakes should be an advantage only on long down hills where regen saturates the battery and engine braking is insufficient for controlling speed, at which point the friction brakes have to do some real work over an extended time. To my way of thinking, this is a very limited set of circumstances considering the cost and effort of switching the rear brakes.

Tom

 

<div class='quotetop'>QUOTE(Boo @ Sep 9 2007, 09:09 AM) [snapback]509520[/snapback]</div>

I would expect that the mediocre stopping distances have more to do with the crappy OEM tires that the Prius comes with. Remember when you brake, most of the stopping power is provided by the front brakes. I would bet that even if you could do the conversion with all the associated electronic modifications and reprogramming you would see virtually no improvement in stopping distances.

 

<div class='quotetop'>QUOTE(qbee42 @ Sep 9 2007, 11:38 AM) [snapback]509527[/snapback]</div>

<div class='quotetop'>QUOTE(narf @ Sep 9 2007, 11:48 AM) [snapback]509535[/snapback]</div>

Oh, there's no way I would personally spend what I suspect to be thousands of dollars to convert the rear drums to disks ... I was really just wondering if the rear disk brakes Prius has shorter stopping distances than our rear drums Prius. I'm sure there must be data out there such as a Japanese auto mag review of rear disk brakes Prius, although such a review wouldn't constitute a comparison test because of the other variables involved (different tires, different tire size, different road test surfaces, etc.).

 

Interesting posts.

Regarding regenerative braking, my understanding is that it is controlled on the front through the HSD. When the brake pedal is pressed, a sensor sends a signal to the computer(s) and the electric traction motor engages in much the same fashion as an electrical generator. There is no such contraption on the rear, so therefore, my conclusion that regenerative braking occurs only on the front.

Braking (up to the point of fade) is a function of a number of factors: friction of the pads/shoes, brake torque (as in distance of the caliper/shoes from the center of the axle) and traction (or lack thereof) of the tires. ABS acts to manage all of the above to allow the car to stop as short as possible while not experiencing lock-up. Change any of the factors, and you change the stopping distance. Given the impact of regenerative braking, I imagine that Toyota engineers ran tests with and without rear disk brakes and concluded that the cost of rear disks vs. improvement from the rear disks was negligible. Given the sales volumes involved and the desired price points, I suspect that Toyota decided to pass on the cost of rear disk brakes for U.S. spec Prius.

If you want to shorten stopping distance within the constraints of the U.S. spec Prius, there are things that can be done, however:

1. Change the front brake pads for ceramic compound pads. Hawk (and I think Akebono) offer ceramic pads for the Prius. In addition to braking performance improvement, the amount of brake dust that gets on the wheels is significantly reduced.

2. Change the tires to get a larger patch that contacts the road and get a more sticky rubber compound. Going to something like 205mm tire width results in about a 10% increase in tire width (over the base Prius tires); adding tires with stickier rubber compound will further improve stopping distance. This modification (wider, stickier tires) does come with a penalty, everything else being the same - reduced gas mileage.

My personal experience with my 2007 Touring edition involved swapping the OEM pads for Hawk ceramic compound pads and changing the stock wheels and tires for 10 lb forged wheels (Motegi Tracklites) and Michelin Pilot Exalto A/S tires in the 205/60-15 size. The net effect of the change was to drop 60-0 stopping distance by ~10 feet and best part of all, I got ~1 mpg improvement (there is an improvement in how the car handles cross winds, but that is impacted by the suspension modifications that I have made, so no claims in that regard). Since there is no free lunch, the improvement came at a cost of about $1,800. Given that I was able to realize a good reduction in 60-0 stopping distance while not suffering a mileage hit, for me it was worth the expenditure. I was able to offset some of the expense by selling the OEM wheels, tires and pressure sensors, so the hit to the wallet was somewhat mitigated.

Hope this helps.

 

<div class='quotetop'>QUOTE(apriusfan @ Sep 9 2007, 02:14 PM) [snapback]509636[/snapback]</div>

Thanks! How much did it cost to change to the ceramic front disc pads?

 

In addition to changing to discs in the rear one would need to make allowance for the changes needed to incorporate the mechanical parking brake.

 

<div class='quotetop'>QUOTE(Boo @ Sep 9 2007, 11:27 AM) [snapback]509642[/snapback]</div>

If memory serves, the parts cost was something like $80. If you swap the pads when the car is new, it is a plug-and-play effort (no requirement to push the pistons back into the calipers). Otherwise, you need to loosen the bleed screw on the caliper and then push the piston back into the caliper before installing the pads (still not a major repair effort).

 

<div class='quotetop'>QUOTE(qbee42 @ Sep 9 2007, 08:38 AM) [snapback]509527[/snapback]</div>

Tom, et al, So far as I can tell, the battery charge status should have no impact on regen ability, as any "excess" energy would be shunted away from the battery and simply dissipated, kind of like what happens to excess energy produced by nuclear plants at night (though hopefully, soon to be captured by PHEVs).

 

<div class='quotetop'>QUOTE(MPG > HP @ Sep 9 2007, 05:22 PM) [snapback]509706[/snapback]</div>

There is no place to shunt it. It's not like the dynamic brakes on a diesel/electric locomotive, where they have a great big heater to dissipate the excess electricity. The Prius can only do regen braking while the battery stays within charge limits. The ECU will let it go a little high, and then burn off the excess charge by spinning the ICE, but that only buys a little extra time. Trust the electrical engineer here; the state of charge (SOC) presents a hard limit.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Sep 9 2007, 04:58 PM) [snapback]509761[/snapback]</div>

Out of curiosity, if the SOC gets close to a threshold (say 95%) and regenerative braking is activated, can't the computer disengage regenerative braking?

 

<div class='quotetop'>QUOTE(apriusfan @ Sep 9 2007, 04:36 PM) [snapback]509685[/snapback]</div>

apriusfan,

Thanks again for all your good advice. I wasn't previously aware of the development of ceramic compound brake pads. Very interesting to me because, among other things, I had to replace the rotors on my prior car because of the abrasion from metallic brake pads. In any event, here's a short informative article on ceramic brake pads, from TireRack.com:

"Why Ceramic Brake Pads?

We want our vehicle's brake system to offer smooth, quiet braking capabilities under a wide range of temperature and road conditions. We don't want brake-generated noise and dust annoying us during our daily driving.

To accommodate this, brake friction materials have evolved significantly over the years. They've gone from asbestos to organic to semi-metallic formulations. Each of these materials has proven to have advantages and disadvantages regarding environmental friendliness, wear, noise and stopping capability.

Asbestos pads caused health issues and organic compounds can't always meet a wide range of braking requirements. Unfortunately the steel strands used in semi-metallic pads to provide strength and conduct heat away from rotors also generate noise and are abrasive enough to increase rotor wear.

Since they were first used on a few original equipment applications in 1985, friction materials that contain ceramic formulations have become recognized for their desirable blend of traits. These pads use ceramic compounds and copper fibers in place of the semi-metallic pad's steel fibers. This allows the ceramic pads to handle high brake temperatures with less heat fade, provide faster recovery after the stop, and generate less dust and wear on both the pads and rotors. And from a comfort standpoint, ceramic compounds provide much quieter braking because the ceramic compound helps dampen noise by generating a frequency beyond the human hearing range.

Another characteristic that makes ceramic materials attractive is the absence of noticeable dust. All brake pads produce dust as they wear. The ingredients in ceramic compounds produce a light colored dust that is much less noticeable and less likely to stick to the wheels. Consequently, wheels and tires maintain a cleaner appearance longer.

Ceramic pads meet or exceed all original equipment standards for durability, stopping distance and noise. According to durability tests, ceramic compounds extend brake life compared to most other semi-metallic and organic materials and outlast other premium pad materials by a significant margin - with no sacrifice in noise control, pad life or braking performance.

This is quite an improvement over organic and semi-metallic brake materials that typically sacrifice pad life to reduce noise, or vice versa."

 

<div class='quotetop'>QUOTE(apriusfan @ Sep 9 2007, 05:11 PM) [snapback]509771[/snapback]</div>

When SOC is near max (~80%), or if a hybrid component is just too hot to charge the battery, then friction brakes will be used. This is where the difference between drums and disc brakes would be noticeable. I can notice it after regen-braking down the big hill where my parents live. At the bottom, I have to make a left turn and then down a short slope where I have to step on the brakes harder to come to a stop. CAN-view will show no regen, as well.