Switching to rear disk brakes??

"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"

Yes, thank god those Europeans have real wheel disc brakes to handle those scorching 8 mph panic stops. ;-)

 

Someone who drives at 108 MPH and is hopping from brake to gas on the highway probably DOES need four wheel disc brakes. And to have his head examined.

 

During panic stop situations, the friction brakes engage even if the car is traveling faster than 8 MPH.

 

In a panic stop, the front weight bias is further exaggerated, and the rear brakes will be used very slightly, so as to avoid a rear wheel skid. The amount of friction supplied by the rear brakes is therefore intentionally very low. Given identical tires and suspension components, I would be amazed to find any difference in panic stop distance between otherwise comparable Priuses with rear drums or discs.

Speculation that there is such a difference, against the choice by (let's admit it) pretty smart fellas at Toyota, lacking any comparative data, is meritless.

 

My two cents;

I to was a little dissapointed when I learned that the Prius used rear drums but it din't prevent me from buying one.

In my older car (95 Civic) I spent several hundred doallars and a weekend swapping out my rear drum brakes to disc. The difference was immediately apparent with an almost 25% descrease in braking distance tested at Thunderhill raceway's tarmac - However: in that situation, the civic came with incredibly inadaquate rear brakes, a lack of ABS and a 5/8" Master cylinder -producing far to little pressure and a potential for fluid boilover/fade during heavy driving.

Now, as for the Prius, before I purchased mine I rented it twice (2 weekends) and drove the hell out of it, including; going up and down every steep hill I could find in SF, driving it like crazy numerous times over the costal range (hwy 17, 92 & 152) and doing numerous panic stop attempts from 20, 40 & 60mph in a deserted mall (Oakridge) parking lot.

The verdict- The brakes on the Prius never showed any sign of fade even through this rigorous testing and the braking always felt confident and balanced even during 60-0 panic stop.

I should also point out that during some of these tests I had four (4) full sized adults in the car as well as a wooden kitchen island (courtesy of Ikea) taking up the entire cargo area. In the other tests only my girlfriend and I were in the car.

Unfortunately, I don't have solid numbers since this was all done ad-hoc and in parking lots. I am however an experienced import enthusiast and do have some pretty solid experience both in driving and stopping.

I'm 100% certain that disc brakes in the rear would descrease braking distance in the Prius; however, I don't think it would be as substantial as say, in my civic. This is because the Regen, ABS and EBD do a fantastic job of controlling how the brakes are applied to keep the car balanced. During 99% of most driving situations there wouldn't even be a differnce. As for the 1%, there's always technology that can reduce your braking distance but you need to balance out cost vs. function. Even if you had disc brakes in the back you could always ask, "why didn't Toyota put in 11" Callipers with slotted rotars and Organic pads".

The bottom line - It would've been nice to have rear discs, but what my Prius came with is good enough... for now.

 

Personally, I don't think there is anything the VSC or ABS would let you do that would lead to a millisecond difference in stopping time between the rear disc cars and the rear drum cars, assuming they had the same tires.

It's just a complete non-issue.

Even the Drum brakes of today are better than the best Disc brakes of 20 years ago, and all are far more brakes than a car like the Prius would ever need.

The rest is all just ego.

As far as suspension tuning goes, Toyota knows its target audiences. The US likes softer rides. They also like automatics.

You may want a european style car, but Toyota knows you are in the tiny minority.

As far as tires go, we got the mileage tires. Toyota knew that the key to acceptance here was mileage, and they were right on the money.

In other words, Toyota got it right.

The Prius has tons of brakes for its performance. More than enough to be safe and secure, and the ride is acceptable to the American Audience. One of the things you don't hear US reviewers complaining about with the prius is a 'jittery' or 'harsh' ride, which is what you get when you tune a small car (and don't forget, the Prius is a short wheelbase car), for sport.

Toyota knew its initial audience was educated people who value comfort more than sport. They hit it dead center.

They will make sportier hybrids, I'm sure, but the Prius isn't it, and likely won't ever be it.

Sounds like you really don't want what a Prius is after all. Maybe waiting for a sport hybrid is more appropriate for you.

P.S. You could also put a 5 foot high wing on the back that would improve downforce at speeds over 180 MPH. That would make it sportier too, and be just about as useful

 

Brian -

I'm not defending the rear drum brakes. I know disc brakes have inherent advantages as well as disadvantages (mainly weight and cost). The problem is there is no EVIDENCE discs in the back give improvement on this application (Prius), and there is plenty of evidence drums are performing well. If people were experiencing brake fade (still haven't found anyone) that would be a start.

So far no stories of anyone replacing the disks with drums. Knowledgeable people guessed it may be possible but very difficult and might adversely affect your safety & stability systems. If you undertake it, good luck and please post your results. There are certainly others who feel like you who might want to try, and those who don't (me) can still learn from your experience.

If you have a bunch of money burning through your pocket and want to make a cool mod, go for the plug in hybrid! You'd get a very real benefit, be the envy of every Prius owner, and unload LOTS of cash! Who knows, it might be cheaper than switching the brakes!

 

Let's stop making comments about regen during panic stops. Regen is clearly not intended for panic stops, that's for hydraulic brakes and ABS to take care of. Regen is to slow the vehicle down under normal deceleration. However, the computers do monitor what your needs are, and will interject hydraulic brakes where and when you need them. EBD is great, but and moderate deceleration, really isn't a factor, and front wheel regen braking by itself should be fine. If more braking is needed, or if the rear wheels are not moving properly, then the skid ECU will apply the needed hydraulic brake force.

Just as a reminder: preventing yaw skids (oversteering or understeering) is a function of VSC (ESC) which is an option, not standard. Providing this feature requires sensing the car's yaw (turning) versus what turn you commanded of the steering wheel.

 

Excuse my ignorance but do the hydraulic brakes have any power assistance and if so is it electric driven or do they just rely on foot pressure?
The reason I ask is I was thinking about comparisons with standard cars of the past and realised that the Prius does not have a gearbox, a starter motor, a generator or alternator. By that I mean not as was on my previous car.

 

<div class='quotetop'>QUOTE(Orf @ Nov 8 2006, 11:01 PM) [snapback]346055[/snapback]</div>

The hydraulic brakes are pretty much the same as any other hydraulic brake system. About the only difference I know is that the fluid pressure level in the accumulator is maintained by an electric pump (you can hear it cutting on and off at low speed) instead of vacuum.

[attachmentid=5642]

 

The European Prius gets some stuff that the US one doesn't, but also vice-versa. We may have rear disc brakes, but we don't have auto-dimming rear-view mirrors, or smart-entry, and a few other things I can't think of. Do you guys have rain-sensing windscreen wipers? I can't remember - we don't.

It's all about what sells in what market. Some manufacturers decide to put discs all-round, so others do to compete - and soon enough most new cars have them.

 

<div class='quotetop'>QUOTE(Tempus @ Nov 9 2006, 12:54 PM) [snapback]346340[/snapback]</div>

Thanks, Tempus, the info you supplied is just what I was looking for.

 

I have yet to see a 2007 model here but.. the electronic brochure on the Toyota.ca site says: " - 2007 Prius - Brakes
Power-Assisted, Ventilated Front/Rear Discs, Antilock Braking System (ABS)
w/Electronic Brake-force Distribution (EBD), Brake Assist (BA)"

Sooo, soon we may be able to compare rear drums with rear disks re stopping distance.

Personally I think switching to the 16" wheels (light weight) and therefore getting a larger tire contact patch would make a bigger difference in emergency stopping distance than going to rear disks, but we can certainly do tests to find out!

As for fade of the rear drums, I've read the rear brakes provide about 10% of the braking force on the Prius, a figure that sounds about right to me based on weight distribution. I doubt you could heat the rear drums up enough to fade with that amount of frictional energy, unless you were racing or something.

 

OK, I'm selling popcorn and beer. This thread is interesting.

 

To heck with stopping faster, I want disc brakes on the rear wheels because it'd be a good way to spend a huge chunk of money to shed a few pounds.

 

I can't for the life of me see how drums are cheaper to produce then disk. There are more components in drum brakes <springs, keepers, bars, etc and the shoes are way more to manufacture then disk pads>. The only way drums are cheaper to produce is that with drums your parking brake doubles as your rear brakes. In disk there has to be an additional braking system for your parking brake assembly. Disks should be cheaper to produce then drums even with the added expense of the parking brake but I don't manufacture cars for a living, I've just worked on them all my life.
-Paul R. Haller

 

How many people in the post have actually expierenced brake fade? The only time I've felt it was doing a few quick 100+ to zero stops in sucession. This was track driving of course. And the car was not a prius, but did have 4 wheel disk brakes. I have yet to feel brake fade in my daily street driving on cars with or without 4 wheel disk brakes.