conversion to 4 wheels disc brakes

The ABS sensor (for sensing wheel rotation) in U.S. models is mounted to the plate that the drum brake shoes and actuating cylinder are mounted to. If the euro-spec disc brakes had a similar provision for mounting the ABS sensor, then that would solve part of the problem. However, there remains the challenge of VSC (Vehicle Stability Control). VSC is a very complex program that contains functions for application of brakes on different corners depending on what the spinning condition requires. The program for drum brakes has a different set of values than the one for disc brakes (due to the differences in stopping power of drums vs. discs). If you change to disc brakes on the rear, you will have to change the VSC program if you are going to not detract from the safety of the VSC system. This is where you start to slide down the slippery slope - change VSC to the euro-spec edition, and the car likely won't start because the computers are out of synchronization (ECU looks for a certain version of VSC software; if it doesn't find that version, it shuts down).

The general rule in the U.S. (with exception for states that have adopted no-fault liability rules) is that the person who hits the other is at fault. Unless there is a specific law/regulation (such as if seat belts were required to be worn but weren't) exempting the fault rule, there is no 'mitigating circumstance'.

 

is the VSC not working together with the abs sensor and then makes its program react on the situation of wheels blocking?
a drum base mai brake different buth it depends on the survice your riding on! or not?..

 

VSC works to stop a spin (such as could occur on ice while turning) of the entire car. To stop such a spin, VSC selectively applies the brake on all four corners depending on inputs from sensors (yaw and steering wheel angle are two that immediately come to mind, there may be others) on the car. If you have swapped the drum brakes for discs and the VSC is expecting drum brakes and applies the rear brakes expecting one outcome, but a different outcome occurs, then VSC has to try again. Time is wasted and the spin may not be stopped.

 

everythings so complex now
vsc, tcs, abs, etc...

regardless the idea of 4 wheel disc brakes sounds good to me
why didn't we get that option.

 

we have it. its now a standard feature on the 2010's

 

Other than 'Cool' factor to make the car looks sportier, drum brakes have little disadvantage on front-heavy car like Prius. Also drums have much longer service life, so if the front brakes can last 100k miles, the drums on Prius may last twice as that.

Yea, drums just ain't look cool though.....esp on Aftermarket wheels.

 

I tend to disagree with the notion that rear discs are not needed on the Prius, especially in winter. I live in very hilly 'back country' where the snow is plowed maybe twice a day, no salt, and only sanded. I have found that the best brake control is achieved with the Prius when going down these hills in 'B' mode with 1 or 2 clicks of the emergency brake, using the brake pedal VERY "gingerly". This tells me a few things: the rear drums are basically doing nothing, the front brakes are far too grabby, and the proportioning between the two is not ideal for winter hills.
I have lived out here for 30+ years. I love my Prius, but it is by far the worst vehicle that I have owned for the winter. Yes, I only use the most aggressive snow tires, and my rear shoes are not worn or out of adjustment.
The ABS is another thing. I am used to European cars, Audi, Saab, and Peugeot. The ABS on the Prius is terrible. It is jerky and violent, too harsh, like the traction control. To me, it feels like somone beating the underside of the car with a sledge hammer! If this car detects the slightest bit of wheel slide, it completely releases the brakes, then abruptly re-applies pressure. Meanwhile, you are on your way to losing control down a slippery hill.
If there was a reasonable source to purchase the Euro spec rear discs, I would convert the car without hesitation, regardless of any speculative software issues. IMO in couldn't get any worse.

 

Prius is not my first ABS car, I came out from earlier ABS-systems from different classes of vehicles (Standard, Luxury and High-Perf) and do find the Prius' ABS is less intrusive. The brake pedal pulses less and quieter too. How effective the Prius ABS is I'll still wait to see and play with, but the fact of ABS brake is it will only help you stop straighter but never keep it shorter, esp on snow surface.

As much as I'd like to have rear discs too, but the norm is for typical FWD cars manufacturers will always put on drums only. Primary is cost reason, but the secondary is for longevity (Service Life). Due to the nature of FWD (much Front-heavy), the brake force distribution is almost always tuned to front-bias.

Disc brakes do provide more grips. However; you can convert to rear-discs all you want, but that's only superfical on our Prius. Unless you are able to re-tune the braking system on the hydralic side which can equalize the force applied on both front and rear brakes, you won't see much effect even on discs too. I can simply do the same by changing the rear brake-shoes with higher grip-materials (high-perf/racing shoes) to make the rear-end slightly more sensitive like it is on disc brakes.

 

Berkshire, I suspect your problem is caused by the regeneration braking. It only works on the front wheels, and on very slippery downhills you wouldn't be applying the brakes hard enough to engage hydraulic braking. So you're using the front braking only (in regeneration mode).

Conclusion, rear disk brakes wouldn't help at all. Now if we had a -lever- to operate the parking brake you could actually -control- the rear braking. The current parking brake pedal is pretty useless when moving. It's too difficult to control.

 

Again, I must be used to European cars, all of which (that I have owned) have had rear disc brakes.
I guess the only real world way to find out about any brake bias issues, software or hydraulic, would be to go ahead and do the conversion. The beauty of this is that all the parts are out 'there'. The ugly is that the parts would be very difficult to research and source out.

If reg. braking was the culprit, it would seem that using the 'B' mode would make things worse.
I'm used to having a clutch pedal, so my lazy left leg is actually getting pretty good at controlling the E-brake. I agree, a had lever would be much better for this.

 

I stopped reading posts here when I saw how illogical people were getting.
People posting here seem to think they can regulate vehicle braking better than the ABS system, sure you can.

You guys do understand that if there is any sign of lockup the ABS takes over from regen braking if you let it? ABS can't control the PARKING brake.

 

In general, since most PriusChat members seem to pride themselves on driving economically and striving for the maximum possible mpg, I do not see how rear disk brakes will help. Just another pair of brakes to get rusty from disuse. In any event, the 2004 New Car Features Manual (p. CH-38) has this to say about the issue of front vs. rear brake activity (note that the English is not perfect, probably because this was awkwardly translated from Japanese):

3. Outline of EBD Control​

General​

The distribution of the brake force, which was performed mechanically in the past, is now performed under electrical control of the skid control ECU, which precisely controls the braking force in accordance with the vehicle’s driving conditions.​

Front/ Rear Wheels Brake Force Distribution​

If the brakes are applied while the vehicle is moving straight forward, the transfer of the road reduces the load that is applied to the rear wheels. The skid control ECU determines this condition by way of the signals from the speed sensor, and the brake actuator regulates the distribution of the brake force of the rear wheels to optimally control.
For example, the amount of the brake force that is applied to the rear wheels during braking varies whether or not the vehicle is carrying a load. The amount of the brake force that is applied to the rear wheels also varies in accordance with the extent of the deceleration.
Thus, the distribution of the brake force to the rear is optimally controlled in order to effectively utilize the braking force of the rear wheels under these conditions.​

Right/Left Wheels Brake Force Distribution (During Cornering Braking)​

When the brakes are applied while the vehicle is cornering, the load that applied to the inner wheel decreases to the outer wheel increases.
The skid control ECU determines this condition by way of the signals from the speed sensor, and the brake actuator regulates the brake force in order to optimally control the distribution of the brake force to the inner wheel and outer wheel.​

 

I've read your posts in this thread and just don't understand your position. Out of curiosity, when was the last time you drove your Prius in mountainous, snowy terrain? Also, have you driven the NA Prius with rear drum brakes? IMO you have no valid basis for discussion, and yes, you are correct that the ABS takes over if there is any sign of lockup. I do understand that, but when you are negotiating down a slippery hill, trying to maintain a very minimal speed, the ABS will release brake force, increasing the cars velocity, then suddenly re-apply force (primarily on the front), causing the car to once again slide in an uncontrollable and continuous cycle. This is where experience and 'gut feeling' comes into play. The slightest amount of wheel slide is not the worst thing to occur when trying to maintain control. The car just can not account for all the environmental variables presented to the driver. Illogical? - I think not.

 

I think what your describing as "releasing brakes" is the regeneration system turning off at the 7mph crossover..... Try braking in neutral next time and it'll use your regular brakes - eliminating any sudden "release" of brake force.

 

It always seems strange to me that the US spec models don't have discs at the back.

We have disc brakes all round in the UK. But then we also have EV mode, which you don't have in the 04-09. Strangely enough, the UK owners all moan about the lack of heated mirrors and a lower level of trim....

It makes no sense in today's climate to produce so many variants of what is essentially the same car. I'm sure it costs Toyota more to make these differences than it does to churn out the same basic models...

Anyone desperate to convert to discs should keep an eye on the UK ebay for cars which are breaking - you should be able to purchase a whole rear assy....

Nikki.

 

I didn't read all 10 pages of this thread but it seems that for some reason people believe that disc brakes are better than drum brakes. I do not believe this to be true. Disc brakes are cheaper to make by a long way and are easier to maintain but they are not more efficient than a drum if the drum brake is adjusted correctly. Disc brakes are also bloody awful if you have no servo assist, the mechanical advantage is not enhanced by the wheel rotation as it is with a leading shoe drum brake, this leads to a total loss of the progressive braking that drum brakes give.

Rear brakes on cars do virtually nothing anyway unless you are going backwards particularly with front wheel drive front engined cars, they are there to keep the car straight, not to stop it. For rear brakes drums are a lot better for that reason, they will stay in good condition with little use, disc brakes deteriorate much more quickly. It is also a lot easier to incorporate a parking brake into a drum brake than a disc brake. I don't know if the Prius uses the rear disc or an extra small drum for the parking brake.

Personally I would take drum brakes every day over disc if I had the choice.

It does seem bizarre to me that Toyota has this difference at all, why would they not all be the same?

 

Exactly what I would like to do, stay straight on a slippery hill.

Do you want to swap?

 

I'm quite sure it is the ABS releasing pressure. I thought the regenerative braking 'kicked out' at ~18 MPH?

 

I had a chance last year to drive a Euro Prius while at a work related training seminar in Tremblay, France. To me, the brakes just felt better, more like what I would expect from driving European cars for many years.

I didn't think of checking ebay UK, thank you.