Switching to rear disk brakes??

Has anyone concidered that fuel economy might have also been a consideration? The shoes on drum brakes retract fully, where disk brake pads are constantly in contact with the rotor. The frictional resistance is very small, but would be present and constant.

 

Japan and Europe get four wheels disk brakes. I assume that fuel economy is just as important a marketing angle in those locations.

 

no bladder in the tank and no thermous and no PTC heaters.

 

I don't think people in Japan or Europe would tolerate tank bladders... Toyota apparently doesn't think Lexus customers will either, since they're dropping them from the Lexus/Highlander hybrids. At least Ford/BMW/Mazda/Volvo can figure out how to do PZEV without a bladder.

 

Ford/Mazda/Volvo is all controlled by ford. bmw.. well.. it's bmw. hehe

 

I think part of the problem with the Prius is the lack of room for a carbon cannister large enough to absorb the hydrocarbons from 10-15 gallons of displace volume of air as the tank is refilled. Probably pretty easy to accomodate on a vehicle the size of a Highlander or Lexus RX400H not so on a Prius.

 

They don't have anything on the RX400h or Highlander... they've abandoned the Evaporative recovery system for those cars.

 

True for conventional cars but untrue for Prius. Fade comes fron continued usage, an occasional panic stop has no fade factor.]

Back to Physics 101, folks! Kinetic energy (momentum and mass) is converted by friction brakes to thermal energy, which must be dissipated before the coefficient of friction is reduced as brake components retain enough of that heat so that heat is no longer created by linings or pads "rubbing" against drums or discs; because (oversimplified) they are as hot as they can get.

This was explained to me by the Transportation Director at the college where I drove an old (1945 International) school bus with 4-wheel drum brakes. I complained that, on a long downhill run into Death Valley, CA, braking was decreasing rapidly even though the pedal felt firm and I stood hard on the vacuum assisted pedal.

Depending on conductivity of the brake components, air carrying heat away from them and the quantity of kinetic energy to be dissipated, a panic stop could become long, indeed. Prius brake drums are relatively heavy, though they are not finned. I made sure I had a long, clear road ahead when I maxed out my 2004 at 108 m.p.h., but did not try a panic stop - don't think I'll do so - hope not.

 

A lot of that has to do with a "returnless" fuel rail system. The long-term reliability issues are still unknown, though I imagine it can't be any worse than a charcoal cannister.

 

Did the brakes have backing plates, making them fully enclosed? Or where they like modern Class 8 drum brakes that are fully exposed, thus allowing better heat transfer?

I would guess that the axle that does the most braking would benefit most from having disk brakes, at least from a maintenance standpoint. Replacing pads is *way* easier than shoes.

Given that the Prius is a FWD car, with the weight bias towards the front of the car, I wonder how much the rear drums even contribute? With a pickup like the Tacoma or Tundra, Toyota is quite firm on using rear drums.

In theory, something like my 2000 GMC Sierra half ton with four wheel disk should have had superior brakes over my Prius. Just the opposite, and for 2005 the GM LD trucks went back to rear drum for reliability and durability reasons.

I can see though if you regularly do mountain driving. Those steep downgrades are where superior heat transfer is most appreciated. Though it doesn't matter what sort of brakes you have, a steep enough downgrade and you had better downshift to maintain control.

 

Thanks for the physics lesson, looks like you proved my point.

(Fade comes fron continued usage, an occasional panic stop has no fade factor.)

 

<div class='quotetop'>QUOTE(DieselConvert\";p=\"89515)</div>

The rest of what you wrote is absolutely true, but you're missing one important bit of information.

The regeneration occurs only on the Prius's *front* brakes. There is no regeneration on the rear. The rear brakes *are* used like normal brakes, and are engaged *ANY* time the car is coming to a stop. You can look this up folks.

Therefore, the Prius's rear brakes *are* subject to fade, just like a conventional car. The front brakes, however, probably won't heat up as much as a regular disc brake would.

An interesting aside: The Escape Hybrid has 4 wheel disc brakes. After an extended run, the writer testing the car washed it. He/She noted that the rear brakes were hot (indicating normal usage) while the front brakes weren't (indicating a lot of regen).

 

I did look it up, and here is what I found:

“For the brakes portion, the brake pedal has a travel sensor attached to it. The amount of travel, as well as the rate of travel change is measured by the brake control ECU. A target deceleration level has been determined for each millimeter of travel, and load, via regeneration, is applied to the motor to generate negative torque to achieve the target deceleration. This is simply loading a generator to create more output, and thus more negative torque, as the level of desired negative torque increases. Once the brake pedal travel has reached a certain point, or the rate of travel exceeds a programmed limit, or the vehicle is below 8 mph, solenoids are opened by the brake controller which allow hydraulic fluid to generate brake torque via the hydraulic brakes.â€

As you can see from this neither the front or rear brakes have anything to do with regeneration and do not have brake fluid available except when under 8 MPH or a panic stop.

Brake fade only occurs after repeated usage consequently they cannot exhibit fade under a single panic stop.

 

As a matter of fact, in New Car Features, one of the highlights of THSII is that unlike the classic, hydraulic brakes are not applied in conjunction with regen.

 

<div class='quotetop'>QUOTE(DanMan32\";p=\"89749)</div>

Front brakes... I'd be *VERY* surprised if this applied to the rear wheels, since the literature I've read indicates that they're always used, whether it's regen or regular braking.

 

I'd like to see your documentation that does NOT pertain to the classic.

There's really no reason to apply hydraulic brakes at the rate of deceleration that regen could supply.

 

DanMan32

Here is a quote from Toyota which supports your position and may be a partial answer to the topic of this thread.

"Paul Williamsen, curriculum development manager of Toyota Motor Sales, said the regenerative braking is the greatest contributor to the HSD’s fuel-efficiency gains. Williamsen, who manages technical training for Toyota dealers and technicians, said the new system exploits the regenerative braking so much better that the conventional brake pads and shoes may never come in contact with their discs and drums under normal driving conditions. This allowed the engineers to decrease the size and weight of the brake components for added fuel savings."