Upgrading rear brakes - ditch the drums!

I noticed the gen 3 Prius comes with disc brakes in the rear... I'm jealous!

I'd like to get rid of the wimpy, old school drum brakes in my gen 2. Really, what are these relics of automotive history doing in a space-age car?

What are my options here? Aftermarket? Has anyone tried to fit the gen 3 rotors on gen 2 wheels? I realize there will have to be some black magic used to fit the calipers, but that doesn't worry me.

Speaking about worries, should I worry about ABS or traction? Are there sensors integrated with the brakes here? I haven't torn the drum apart enough to check...

Thanks!

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I'm not really sure... but I think I remember seeing somewhere that the rear drums were part of the regenerative system? I too had this thought when I first bought my Prius, like wtf!? why didn't they use superior disk brakes for all 4 wheels? I just accepted it, and now I really don't care, the car stops when I need it too. so I don't think about it too much now.

The rear drum brakes are not part of the regenerative system; neither are the disk brakes.

There really isn't much need for disk brakes on a Prius. Much of the routine braking is done regeneratively by MG2, so you don't have the usual issues of fade.

In a panic stop, drum brakes can stop the wheels just as well as disks. The big advantage of disks is heat dissipation, so they can provide longer continuous braking before exhibiting fade. While this can be an issue with the Prius, it isn't very likely.

The Gen III Prius has rear disks. Mostly this is a marketing tool, since most prospective owners think it an advantage.

Tom

This article points out how much harder it is to use a disc brake as a parking brake.

Besides price, I think that is the major reason for rear drums.

What's the difference between drum and disc brakes?

There is on my Prius, otherwise I wouldn't be asking.

Thankfully my driving consists of more than panic stops

Ah great, so you know why I'm interested in discs then!

Likely? This isn't an exercise in statistics. But if you must, let me spell this out:

P(maestro needs disc brakes) = 1

Ok, that being said, can you answer my original question?

Thanks in advance.

While the rotor does contract as it cools, that contraction is on the order of 1/10 of 1% of its total width. (assuming a very generous 100 deg C of cooling; see: [ame=http://en.wikipedia.org/wiki/Thermal_expansion]Thermal expansion - Wikipedia, the free encyclopedia[/ame])

If you compare the movement of the brake's piston to that thermal contraction, you're talking about a difference of >>100x (assuming the piston moves >10% of the rotor's width, a conservative estimate).

Hmm, now that you actually put numbers into the problem, it doesn't seem so significant, now does it? This is why four wheel disc brakes are available on such a wide range of cars. Drum brakes are relegated to the cheapest of the econoboxes.

You do raise a good point, however. I'll have to swap the parking brake assembly in addition to the main caliper... dunno if it's separate or integrated. More research to do...

In Europe, the Gen II Prius came with rear disc brakes, so presumably it should be possible to convert using all OEM parts.

What you need to do is to find a parts diagram of the rear axle for a European car and compare to the US car... then figure out where to get the parts.

Others have answered. But I am curious. Why do you need disk brakes? In all of the posts on PC, I have yet to read of any situation where a Gen II Prius had inadequate brakes.

Tom

How long have you owned your Prius? You don't seem to know a whole lot about how the brake system works. I have never seen ANYONE complain about drum fade here. Regen braking does most of the work. If you are coming down a long, steep grade, then you should have the car in B mode i.e. the jake brake.

The brake ECU controls line pressures to caliper pistons and wheel cylinders individually. You will need to investigate whether or not the ECU is the same in North America vs. Europe. Otherwise you could go to a ton of work and cost, only to find that the rears either lock up, or are completely ineffective.

The swap isn't too hard at all and I have done it on a couple of other cars like a civic. It's just about getting the parts: rear hubs, rotors, calipers, parking brake cables, brake lines, brake pads

Actually the European version of the gen2 Prius has disc and drum rear brakes, disc for the foot brake and drum for the parking brake to give the best of both systems.

Maybe he's racing his Prius at his local race track? If that was the case than drum brakes would certainly be inadequate.

please elaborate, or better yet open er up and give us a picture!!

In the business this is known as a "drum in hat" rotor.

With respect to the ABS, the question will be whether the current rear hub units can be used (which probably incorporate the rear wheel speed sensors).

Hah!

To be honest, the thought did cross my mind. But it just wouldn't work with the amount of lag induced by the engine cycling. If I could keep the engine on all the time, however...

The reason for the need is that I've been towing a teardrop trailer and I don't want to wait until I experience brake fade to do something about it. I've done some moderate hills and have smelled cooking brakes in the rear (even though I drive like a granny), so I know they're working at their limit.

One does not learn about the internals of a car by owning it. I'm starting my research here.

Just because you haven't seen Bigfoot doesn't mean he does not exist

Thanks, but this doesn't have anything to do with my drum brakes. "B" mode only does so much to slow one down; the car can still accelerate in this mode.

Now here's an interesting tidbit! Where did you get this information?

There is a risk that the brake system is "tuned" differently in the cars with rear drums. Less likely but still possible is that there are different master cylinders and reservoirs used due to the drum brakes requiring slightly more fluid volume on application while there is more fluid drop with wear.

Google "electronic brake distribution" regarding the earlier mention of individual electronic brake control.

I would think that it would be cheaper and safer to add brakes to your trailer, that trailer seems to be putting a lot of stress on your rear suspension. I would actually keep the axle currently on the trailer and try to buy or build another axle with brakes in front of the current axle.

You can have your drum brake shoes relined with semi-metalic linings. I used to get mine religned with Velve-Touch linings on cars and motorcycles back in the pre disc brake era. They had vastly improved fade resistance. Velve-Touch had almost none of the drawbacks of some brands and acted pretty much like normal brakes except they didn't fade when they got hot.

Here is one source of semi-metalic rear brake shoes for a Prius.
Premium Brake Shoes
I have no experience with this company. If I were doing it, I would find a good performance brake shop and have them reline my shoes with
Velve-Touch material

Disc brakes became practical only when "power boosters" became available.

Without a "power booster," the disc brakes required a lot of leg strength to apply.

Drum brakes, however are known as "dual servo self actuating." That means the front and rear shoes are "locked" as a unit by springs, the hydraulic wheel cylinder and thed pivot pin. When the hydraulic cylinder expands the unit, the friction of the front shoe "twists" the entire assembly, forcing the rear se against the drum.

I remember this from Smokey Yunick who advocated the old tried and true drum brakes vs. Jan Norbye preference for the newer disc brakes in an article that was published in Popular Science in the early 1960's.

The genIII has disk brakes because of the skit/traction control. The gen 3 will apply braking pressure to the rear brakes (i think indipenedntly left/right) to help steer the vehicle in slide/cornering. It's the only vehicle on the market with larger rear disk than front disk.

If you get this mod functional with miles tested, I'll be curious of the same upgrade for towing reasons.