Technical Rear brakes conversion

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Technical Rear brakes conversion

Snails

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Hi guys, fairly new around here, been browsing for a while and decided to ask your opinion on something I've been thinking of doing for a while now but things got in the way.
I have a 2008 Grande Punto 3 door with a 1.3 MultiJet and 90bhp with the 5 speed gearbox, no mods whatsoever.
I want to convert the current drum brakes on the rear to discs. I'm doing it for aesthetic reasons and not looking for any performance gains.
Got 17'' wheels a while ago and it bugs me how ugly the rear drum looks when wearing these:
fiat-punto-17-alloy-wheels-x4-71805176-404-p.jpg

I've never seen a GP with brake discs on the rear up on a lift to check on the possible difficulty level of the entire job so my questions would be:
1. Is it a simple bolt on job as the real axle is the same on both versions or I will need to change the axle too?
2. Is the EBD software different in operating the drums vs. operating discs? If it does then I will need to modify the software too I guess.

Thanks! :)
 
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OK, I'm going to start the ball rolling here and shout "never run with scissors"!

Seriously, modding brakes was always a bit dodgy in the 1970s but these days, there's more computing power in the braking system than there was on the first PCs!

The basic legal requirement, when type approving a car is that the rear brakes must NEVER lock up before the fronts under ANY circumstances. That means fully-loaded, empty, uphill, downhill, dry, wet or icy road, with-or-without a trailer...EVER. Obviously, if you're braking and your back wheels lock, you spin. It's like pulling on the handbrake and doing a handbrake turn. Great if that's what you wanted to do and were expecting to happen, not so good if it catches you unawares!

Now, it is certainly possible to engineer the system to limit the pressure to the rear discs so that they don't do that. You (if you're a car manufacturer) can also mess about with the formulation of the pad material and the sizes of the disc and caliper pistons to achieve this.

However, today, we have ABS thrown into the mix AND your car might even have Electronic Stability Control. If it has either, it will also have "Electronic Brake Force Distribution". This is the modern way of making sure the back brakes don't lock up first. If it senses imminent lockup-on a rear wheel, it can use the ABS pump to modulate the line pressure to that wheel to reduce the chances of it happening. This all happens without the driver even knowing. It is NOT the same as an "ABS Intervention" where you actually get a wheel locking and the brake pedal pushes back at your foot, this is what happens before it gets to that stage.

...or at least, it does if the hydraulics of the system respond the way the software expects them to! Drum brakes will react to changes in pressure in a different way (at a different speed) to discs, and the ABS controllers will be programmed on the assumption that they're dealing with drums. ESC just adds another whole level of complexity into the equation.

The worst thing, is that just simply bodging a pair of discs and calipers on, you could probably drive the car quite happily and never have a problem until the day you end up in an emergency situation and have to stand on the brakes. Then it all goes pear-shaped, swaps ends, and you (or worse, the deceased's estate!) has an awkward conversation with your insurance company!
 
Thank you for the detailed input Avocet, you do make a valid point.
Honesty I thought that running discs with a drum ECU would cause the braking efficiency to drop because of the different ways drums and discs work. I didn't think (and still a bit unconvinced) that the GP ABS pump ECU (no ESP) is so complicated that it would apply braking for for each wheel individually but rather on each axle separately.
If indeed it's necessary to swap the ABS pump ECU then it would mean installing a yaw sensor, otherwise I would get an brake system error in the dash and it wouldn't work at all.
Seems it might get more expensive than I thought...
 
All ABS systems these days modulate the pressure to each wheel separately. I think some of the early ones (say, late 1980s, early 1990s) would limit (say) both rears to a pressure just below that which was causing the rear wheel with the least grip to lock.

ABS doesn't need a Yaw Rate Sensor to function, but ESC does. Do some of the quicker Puntos have rear discs? (Maybe the Abarth)? If so, swapping the entire braking system (inclusing ECU) might be a way round it, I guess?
 
Yes, ABS doesn't need a yaw sensor but if I'm doing this safely and responsibly then I need to do a complete swap. The 1.9 MultiJet has rear discs and the 1.4 NA petrol I think but not sure.
Putting a ECU with ESC and not having the yaw sensor would render the entire system useless, the lack of the sensor would cause it to generate a fault and it wouldn't work at all.
The ABS pump ECU from the 1.9 would work best I guess since it's also a cast iron block and the weight over the front axle is similar (15kg difference between 1.3 and 1.9).
 
... if I'm doing this safely and responsibly then I need to do a complete swap
Thought about this all day, since I'm looking to upgrade the rear breaking system I could very well upgrade it in the front too, my current discs are on the go anyway and are due for a change this spring.
A complete breaking system "transplant" would be the best way to go.
I'll post details, prices and pics when I'll do this in case someone else is interested in this kind of conversion.
Thanks again!
 
I definitely agree that a complete swap is the ay to go, but crikey, would it involve a lot of work! I don't know if the writing loom is present for the Yaw Rate Sensor on all cars, for example, and these things seem to be sensitive enough to need the actual original "teisted pair" wires (if that's what it uses) or screened cables, perhaps. I know the mounting of the YRS is absolutely critical too, so it would have to go in the same position.

The other thing I wondered would be whether things like the electronic throttle control would use the same code for both the diesel and your car? Obviously, that would be part of the ESC code too, and similarly, whether the steering rack gearing is the same for both cars.
 
Do some of the quicker Puntos have rear discs? (Maybe the Abarth)?
All GP sport versions have rear disk brakes.

If you want my opinion, forget about it and enjoy your new wheels. It's a VERY difficult and costly project. In the end... you only want rear discs for the looks. It doesn't even worth the hasle...think about it ;)
 
Avocet,
There's A LOT of research to be done regarding the entire electrical/electronic part of this.
Considering the car's class I would think the loom is designed to save cost so there's a basic variant in which different sensors and options plug in. Plus mine's a diesel like the 1.9 so it should be pretty straight forward. In theory at least, you never know with Italian cars I guess, some surprises could arise.

cmosfxx,
If we wouldn't do something because it's difficult or expensive we would still live in mud huts. Some people around here still do.
 
What i'm going to say might not be very popular:
-first do the disk conversion without worrying about the electronics.
-you might get lucky and the abs sensors fit in the old plugs.
-worry about the electronics after everything mechanical is ok

Why i'm saying this is because the drum brakes have 3-4 times more surface area than the disk brakes.... Also the brake drums are very effective at what they do, the advantage of the brake disks is that they cool off faster and do not store so much heat.

You might want to find a few GP's with factory fitted rear disks and get they're VINs and use ePER to see if any of them have the same ABS controller module.
 
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What i'm going to say might not be very popular:
-first do the disk conversion without worrying about the electronics.
-you might get lucky and the abs sensors fit in the old plugs.
-worry about the electronics after everything mechanical is ok

Why i'm saying this is because the drum brakes have 3-4 times more surface area than the disk brakes.... Also the brake drums are very effective at what they do, the advantage of the brake disks is that they cool off faster and do not store so much heat.

You might want to find a few GP's with factory fitted rear disks and get they're VINs and use ePER to see if any of them have the same ABS controller module.

You're right! That's not popular! There's a million times more to designing a safe braking system than just the friction area! (and that's BEFORE complexities like ABS and ESC come into it)!

Modern cars with ESC are a bit like fly-by-wire planes. They can do more than earlier vehicles, but without their fancy electronics, are often inherently less safe (hence all the warnings in the handbooks about driving very carefully if one of the warning lights come on). You can even cause accidents by seemingly insignificant things like moving a Yaw Rate Sensor!
 
Replacing iron is easy, the electronic part is the main concern that made me open a thread here in the first place.
I don't need other cars' VINs to check all part codes in ePer, you can navigate by category.
As you can see, there are quite a few different types of ABS ECUs
There's an army of engineers designing the brake system for every specific model and variant, why would any car manufacturer create so many different software variants (another army but this time it's programmers) if stuff works when you just bolt it together?
2qc0zv7.jpg
 
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I will use a drum disc covers and a disc brake clippers covers to make it looks better, but it is the same drum disc that we both hate ?. I have also asked for the same idea ?
 
Replacing iron is easy, the electronic part is the main concern that made me open a thread here in the first place.
I don't need other cars' VINs to check all part codes in ePer, you can navigate by category.
As you can see, there are quite a few different types of ABS ECUs
There's an army of engineers designing the brake system for every specific model and variant, why would any car manufacturer create so many different software variants (another army but this time it's programmers) if stuff works when you just bolt it together?
2qc0zv7.jpg
The software tends to be very model-specific. Different tyre size? Probably needs a slight software tweek. Different suspension rates? Again, another software tweek. With / without Electronic Stability Control? HUGE software re-write.

Sometimes software changes because the regulations change part way through the car's life and the car needs to have its type approval extended, so the software after a certain build date will differ. Similarly, cars with different weight distributions wil lprobably need slightly different values putting into the software for various parameters.

Sometimes it's a simple case of the manufacturer having sourced 9say) a different make or model of sensor on each wheel, so the software just needs a tweak to accommodate that.

It can even get as silly as having to make changes when different wheels are offered. The wheel and tyre will have a rotational moment of inertia. The ABS software will be mapped to take that into account because when the wheel locks, that inertia will determine how long the wheel and tyre take to get back up to speed again after locking.

Imagine that from a particular speed, you start braking and you get a lockup on one wheel, the ABS will kick in and release the pressure to the caliper on that wheel. The wheel will then start to turn again, accelerating back up to speed at whatever rate its moment of inertia and the friction between its tyre and the road surface permits. So if the ABS software is expecting it to get back up to speed in (say) 1/4 of a second) but because you've fitted different wheels and tyres, it takes (say) 1/2 a second, the ABS might already be dropping the pressure again, thinking that because the wheel isn't back up to speed, it must still be on a low grip surface. In fact, it was back on a fine surface but all that had happened was that it wasn't the wheel and tyre that the system thought it was, it was one with more inertia which was taking a bit longer to spin back up to speed!

The result (at best) is that the car doesn't stop quite as well as it could because the ABS pressure modulation is out-of-phase with the wheel accelerations. Obviously at worse, it could be downright dangerous!

Now clearly, there is a fair bit of tolerance in the system - there has to be because the customer still has a choice of tyre manufacturers (even for the same nominal size of tyre) and in some situations, might even be running with snow chains, but the effect is still there and altering wheel inertia is something that the system will only be able to accommodate up to a point. The effect is recognised in Germany, where they are pretty strict on what you can and can't modify on your car. Only wheel and tyre combinations that have been tested to the appropriate standards are permitted.
 
Avocet, I think he meant using something like this:

2pcs-Universal-Car-Auto-Aluminum-Disc-font-b-Brake-b-font-Racing-font-b-Cover-b.jpg
LOL! Yes, I'm sure he did (yuk!) but I thought you were keen to try the actual real disc conversion, weren't you? I'm sure the fake discs would be a lot safer, but I think they look really naff without a caliper!
 
Avocet,
I was, I think I'll give it a try in spring.
I'll be doing it at a friend's shop and I can't take up an entire workstation, lift and all so I'll be doing it outside in the courtyard so waiting for warm weather to come back.

digger58,
That looks awesome, I need that in my life.
 
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