3,500 foot decent = crazy HOT brakes and Burning Smells

Where did this impression come from?

No, my newer Prii behaved the same way, one week out of the new car sales yards.

This 'system' is still similar to a traditional automatic or manual transmission, in that the driver is left in final control. If it had the new and improved extra intelligence you assumed, the 'B' position wouldn't need to be built in.

From page 184 of my 2012 Prius Owner's Manual:
* Use engine braking (...) to maintain a safe speed when driving down a steep hill.
Using the brakes continuously may cause the brakes to overheat and lose effectiveness.

 

B mode at the top of the hill would active engine braking all the way down.

 

The hv battery can 'fully' charge with a 600 foot drop. Various models will skew that number wildly, the Prius PHV can take over 2000 feet. If you know the battery will fill, use B mode from the top. The car cannot guess how long the hill is, you can. There may even have been signs.

If you had a normal car, you would use L from the top, not wait for the brakes to fail.

Yes Florida is less than 400 feet tall, but you do not live in Florida.

 

Numerous others have made such comments. Few of them have 3500 foot descents, especially without the air drag of 80 mph interstates (and the forced air disc cooling of that wind speed) to blatantly show them the errors in their beliefs. I have disputed some of them, but it is hopeless to try to respond to all of them.

My single descent from Pikes Peak, in slow traffic that was unsafe to pass, found that low speeds were not sufficient to get adequate engine braking. Brake cooling timeouts were needed.

On a recent descent (Highway 12 down Alpowa grade into Clarkston WA, ~2000 foot drop at 60 mph), I checked the difference between 'D' and 'B'. When the battery filled, engine braking in 'B' shot up to 46xx rpm. Shifting into 'D' caused the rpm to quickly fall to 20xx rpm as the car quickly rolled up above 65 mph. 2000 rpm still represents some engine braking (vs. normal coasting at 992 rpm), but not much.

 

Yes.

Do note that descents on I-70 in Colorado are likely happening at 75+ mph, providing a lot more air drag and a lot more brake disc cooling than your 20-30 mph road. Thus, they will get more elevation change in D mode than you before the brakes are cooked. In B mode, they are golden, though the high rpm engine howl can be frightening.

The slower roads generally don't have as much elevation change as what you did, excepting special cases such as Pikes Peak. And that road has a mandatory brake checkpoint on the way down, where an attendant with an IR thermometer gun selects which cars must take a 30 minute cooling break. When I was there, the guilty party was an F-150 pickup with Florida plates.

 

It seems to be normal. The owner's manual (pp. 136, 137) describes D without any mention of engine braking, and says to use B when engine braking is wanted. The slightly meatier information is in the New Car Features Manual pp. TH-21 and TH-22, where it explains that decelerating in D may involve spinning the engine a bit just to keep the MG1 revs within bounds, it is the B mode that will actually backfeed electrical power to MG1 so that the engine serves as a brake.

B is a bit like downshifting/engine braking in a conventional car, but with a couple of neat (or weird, depending on how you see them) features....

• It is all under control of the HV ECU and there is no risk of overrevving the engine no matter how fast you're going when shifting into B
• The amount of engine braking changes in response to how charged the battery is
• The amount of engine braking changes in response to your force on the brake pedal (I'd like to see a conventional transmission do that!)
• The engine braking force is greatest at moderate to low speeds: assuming you kept the force constant, the amount of electric power generated and needing to be directed somewhere goes up in direct proportion to speed, and at higher speeds will be more than the battery can accept and more than MG1 (the smaller motor) can dump into the engine, so the ECU tapers it off at higher speeds to keep the power in limits.

When you're not significantly decelerating or needing engine braking, driving in B is a lot like driving in D, the main difference being how the car feels when you lift your foot from the accelerator. Even in D, the HV ECU is programmed to apply a bit of slowing force then (like a familiar car), and it's programmed to use a slightly greater slowing force in B (again, feeling like a familiar car except that, just like the eco/power button, it's only a game played with numbers in the computer that reads your accel pedal position).

-Chap

 

That is the only car brake checkpoint I've ever seen. Many downhill grades have turnouts at the top where trucks are required to pull over and check their brakes before descent, but I haven't seen any road or police staff there to oversee or do it for them.

The Cascades hills here are generally less severe than the one that just snagged you. Half the elevation drop and higher air speeds make ours less demanding. But inland where I learned to drive, several steep descents are 2000 vertical feet at 5% or 7%. The truck scars, some of them fatal, generally start about 1000 feet down.

 

The regen brakes only do most (or all) of the braking until the traction pack reaches maximum (80%) charge. Then it's back to wheel brakes.

I thought the same way as you in the beginning. Coming down Trailridge Road, I left it in D mode thinking it would capture more energy. I still braked in bursts and the brakes stayed cool enough not to smell or do damage, but now I use B-mode from the top of all these descents and brake in bursts, as needed.

I remember how critical it was to downshift when coming down from Haleakala--I wonder if that's how all those Maui rental cars got wobbly rotors.

 

There really is something to that; I completely remember when I first got my Gen 1, and I measured the linings right away, and measured a year later, and looked up what they were originally and what the replacement limit is, and could straight-line project out when I would need to replace them ... in 2017.

But that can be deceptive too. Really the only thing that regen braking slows down is the frictional wear of the brakes. (Ok, maybe thermal aging of brake fluid too, as hobbit found using test strips, near the end of this page.) Everything else that happens to brakes still happens to brakes.

The slide pins get gummed up. The rubber seal slowly leaches black goo into the brake fluid. Rust happens ....

That really works! It isn't something I have to remember to do on any kind of schedule, because I find my brakes will sound and feel rough right away after any several hours parked in wet weather, and all it ever takes is a couple steps on the brake in neutral and they are silent and fine again. It's like magic. It must just be second nature to every Prius driver who knows about it. (You have to know about it ... there's a recent thread here from a new owner who didn't, and kept going back to the dealer saying "my $@^& brakes are noisy and bad again" and the dealer kept happily charging money to "fix" them. Knowledge is power.)

But braking only clears surface rust off the contact surfaces. Nothing stops the rotors rusting elsewhere, especially around the outer edge, where the vent vanes and passages are a good place for water to linger. Then the rust can eventually spread over the edge, not as light surface rust, but tough gnarly rust that carves away the top edge of the pad. On the inner rotor surface (where it's hardest to see!) the same thing also happens where the vent channels are exposed at the inner circumference, so the inner pad gets carved away top and bottom. It's a vicious cycle since the carved-away pad edges do nothing to slow down the rust, and when I first caught it on mine, one of the inner pads was only touching the rotor for about an inch in the middle.


So, having a Prius really puts a premium on tending to your brakes in all the other ways besides just pad/rotor wear. In a conventional car that wear sort of means you end up looking at the brakes fairly regularly anyway, which on a Prius you might forget to do. But on the Prius it's really worth taking care of these parts, because a lot of them might really last you the life of the car, if you stay ahead of deterioration or rust.

I just recently got introduced to the electroplating shop in town, and learned it's not at all hard to renew the zinc plating when it's no longer keeping the rust off. I took him three old gnarly caliper mounts and one (not as old, but even more rusty) caliper, and was pretty satisfied with what came back (all within his minimum batch price).

(My Gen 1 calipers are cast iron, unlike your aluminum ones.)

I haven't tried this yet with rotors. If I had caught them early before the edges were as chewed up, they would probably be in great shape. (Others do re-plate rotors, it's not just my nutty idea. It might just be a kind of extravagance for many conventional car enthusiasts, but again on a Prius there's a real chance this part can be preserved the life of the car.)

When you plate a rotor, it comes very quickly off the friction surfaces with normal braking, which is fine, as normal braking will keep those surfaces clean, and you've still got the zinc on the rest of the rotor.

-Chap

 

The history on phenolic pistons seems to be pretty interesting. They go back ~ 40 years. A lot of what I read said there were sticking problems early, and the manufacturers have worked out the changes they needed in the caliper bores and seals to address that. It surprises me a little to see you having that problem in a late-model car, but I suppose your steep drive could have exposed them to unusually extreme prolonged heat.

In general, what I read about modern phenolic pistons talks about their advantages. One apparently very significant one is that they are much less heat-conductive than steel, so a lot less of the heat generated in braking gets conducted back to boil the brake fluid. That might have worked in your favor....

There's a Google Books result in a 'classrom manual for automotive brake systems' section on phenolic pistons that's an interesting read. So is this.

For my cast irons, that would be a normal finish. For the Gen 2 and later, aluminum calipers, knuckles, etc., I wonder what Toyota does for a finish? Anodize? Anybody know?

-Chap

 

The problem with using "b" is that your HV bat fully charges in a short period of time, leaving NO REGEN, making use of the marginally sized stock Prius hydraulic brakes. My rotors were cherry red on my gen III coming down Mt. Washington. Had to pull over multiple times to let the brakes cool off.

XT1585 ?

 

^^ it would have been even worse without 'B'.

 

No, see that is the thing. Once the battery is 80%-+ SOC, regen is disabled so it won't continue to charge the battery. "B" position gives more regen (more amps/current) for a shorter period of time. (Because the HV battery gets "full") regen will be inhibited at this point, and ONLY THE HYDRAULIC brake system works after that, until some of the whopping 6-6.5 amp hours is discharged through acceleration or heavy accessory use. I had only made it a mile or so down MT Washington before my regen gave up. Had I used the regular "D" position, it may have helped less, but for a longer period of time, charging the HV battery at a more reasonable rate, possibly keeping the friction brakes cooler over more of the deceleration. Remember, regen amps are usually a fraction of acceleration amps from the battery, otherwise we'd have perpetual motion Prii.... While that would be cool, what really happens is the battery chemistry can't respond by putting metal back on the plates fast enough, plus electrical/Eddy losses in MG2/MG1/Inverter IGBT transistors. So excessive regen makes the battery hot AND wastes energy to have more current than the battery can take. A battery's ability to take or give current is known as it's C-Rate, or Charge and Discharge amps as a multiple of the Amp-Hour rate. Regen with "B" position can get up near 100 amps at times. 5 minutes at 75 amps is enough to fully charge our HV battery, nevermind that only about 40% of the SOC is normally used anyhow.
When the manual says b should be used for steep hills, it is talking about a decline of a few hundred feet, over the course of miles, not thousands of feet of decline.





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....You should NEVER put your Prius in neutral, that is why you should never tow the car in neutral on it's drive wheels. It is said to cause high voltage leakage/damage to the IGBTs in the inverter assembly.