In principle this should be possible as the up!'s rear stub axle is shared with the Polo Mk4 (2002-2009). However, whether the ABS systems for Polo Mk4 and up! are compatible with each other, I wouldn't like to guess

 

IMO having had two Gti's over the last six years, now, it's not necessary for road use, only for track & competition. The standard Gti brakes are very good. Yes, the rear drums need cleaning out once a year, but apart from that they're very good.

 

Has anyone attempted a full brake upgrade

If you want to stop quicker then upgrade the tyres first.


Thanks. UP SI

 

If your ABS kicks in under deliberate heavy braking on a dry road in good condition, your brakes are already just right. As per Up Si - it's all about tyres.

 

Cosmetically, discs vastly outperform drums, reason enough to consider such an upgrade, perhaps?

And I suspect in demanding conditions, eg in mountain passes, discs provide more consistent braking performance than drums?

 

The back end of the Up is pretty light and needs minimal braking, their main contribution is to stabilise the car , if they are too powerful its quite possible for the rear to overtake the front in hard braking scenarios, best thing to do with the drums is to service them properly every year ......

 

My last MOT (TÜV in Germany) says brake power distribution:

76% at the front and only (!)
24% at the back

…are drums at the back for an UP OK ?
-> Yesss ✅

Maybe a ‚Load UP‘ with often ‚heavy gear‘ in the back or a track tool (…because of simplyfied maintance) would be a good reason to change to disks in the back…

Greetings, Dirk

 

Many cars have a load mechanism for the rear, often just measuring the the angle of the rear axle line to the body. It modulates the hydraulic pressure to the rear, and allows more pressure to rear if the rear is loaded up. I don't know if a small budget car like an Up! would have one. A car that does have one should probably be recalibrated if you change static ride height.

For public road use, the rear brakes don't do a lot. Just tug the handbrake and see how easy it is to lock the rear tyres - you don't want a lot of brake force at the rear unless you like going sideways a lot. OK for parking though. In the end, brakes are all about turning kinetic energy into heat energy, so if you go for a brisk drive try touching any part of the front brakes (ow!) and compare that temp to the rear. That shows the difference pretty well.

 

Many cars have a load mechanism for the rear, often just measuring the the angle of the rear axle line to the body. It modulates the hydraulic pressure to the rear, and allows more pressure to rear if the rear is loaded up.

Most implementations of this I’ve personally experienced are pretty simple. There’s a restrictor valve in the hydraulic line which feeds the rear brakes connected to the rear suspension by a lever and spring. As the ride height changes, the lever opens or closes the restrictor to alter the fluid flow to the rear brakes accordingly. No electronics or clever control 👍

 

I've never heard of one that is electronic - hence why I said "mechanism" - but I assume it might exist.
Does any such system exist on an Up! though?

 

Fair enough. As you mentioned ‘measuring angles’ and ‘modulating hydraulic pressure’ I thought you were inferring there was some sort of clever control over the system.

I've not seen this either in the flesh or in the docs for an Up!. It's probably taken care of by the ESP/ABS system.
In fact, one of the datasheets alludes to ABS having some control over the rear brakes as it says:

 

If ABS fails and you brake so hard that the front tyres are locked, there's nothing left that modulates hydraulic pressure, so it seems reasonable to assume you will also lock the rear tyres if you keep pressing hard on the brake pedal.

 

Pads and fluid, maybe lines, is all i'd do as a starting point; changing the rears to discs is mostly cosmetic?
As said, i think the abs deals with the balance (it must?).
The handbrake is disappointing on mine!

 

I think mechanical load compensators to adjust brake force to the rear wheels died out with the introduction of ABS, a much more accurate method of preventing rear wheel lock up.

As far as I know, cars with Xenon and LED headlamps have always been equipped with electronic front and rear ride height sensors as part of their automatic headlamp levelling system. These types of sensor are also used on high performance cars as part of their active suspension system. I don't know whether they play any part in the braking system

 

I believe the upgrades for brakes on our cars is a Lupo setup for discs on the rears, for the front you can upgrade to the GTi brakes, but you need the GTi hubs as well, along with the calipers and carriers - the non GTi brakes are setup differently without a carrier.

Probably just easier to upgrade the front pads with something slightly better - its a light car and you can 'over brake' the car in terms of messing with brake bias.

 

My pickup had rear pressure compensation and ABS. Maybe small cars don't use or need load-based compensation, and are just calibrated so the rears only get so much of the available hydraulic pressure.

I'm pretty sure ABS has little to do with the rear brakes - you don't see the rears ever lock and release like the will fronts do on emergency braking. I'd guess it's a state that you'd never want the rears to get even close to, as any slight change (in say the road surface) could destabilise the car.

While the rear brakes do have ABS sensors, I think these are meant as a means to detect the situation where both fronts are spinning under hard acceleration or both fronts are locked - the ABS, nor the traction control, will not work properly unless you can detect that the car is in motion by the fact that the rears are set up to never risk varying from actual road speed.

 

Perhaps the valving is in the abs pump or the distribution block at the rear beam (pretty sure i've seen one there). I've read/heard anecdotally that removing the abs fuse (to disable tc on track) upsets the brake balance which seems to tally with Pete's info. Don't know

 

Agreed. This is the ESP part of the ABS/TC/ESP functionality.
There are a lateral acceleration sender (G200), a yaw rate sender (G202) and a longitudinal acceleration sender (G251 - depending on model) installed inside the ABS control unit. In conjunction with the steering angle sensor, they're used to predict stability issues and maintain dynamic balance by adjusting the brake valving in the ABS pump accordingly. The system is diagonally split front to rear and all wheels are controlled.

 

There must be something that reduces hydraulic pressure to the rears to ensure they can't lock, no matter how hard you press the brakes - whether that is in the ABS block or elsewhere in the pipework, I don't know. I just can't see that rear pressure gets modulated on and off like it is for the front brakes when ABS is doing its thing. I'm just guessing at what the cheapest way is to implement ABS on a budget FWD car, I really don't know the design details, but the logic seems sound....

 

ABS definitely operates on all 4 wheels individually. The whole point of ABS is to obtain the highest possible braking effort on all 4 wheels while preventing the wheels from locking. VW specifically warns that if the ABS warning light comes indicating a fault, the rear wheels may lock prematurely during braking.

Introduction of ABS enabled rear brakes to be upgraded. For example, the Polo Mk3 was equipped as standard with puny 180 x 30 mm drums, but those with optional ABS came with larger 200 x 40 mm drums. A rear brake pressure regulator was fitted to non-ABS versions only

 

Probably right, but while the fronts clearly get modulated to release and reapply hydraulic pressure, I've never heard of the rears doing this.

 

What makes you think only the fronts get modulated?

 

I don't have in-depth technical knowledge of how ABS works for the rear brakes, but I have never seen or heard of the rears getting to the point of lockup and having to be cycled to release them. It would have to be done separately for each rear brake of course. This implies a 4-channel active system, which sounds to me to be more expensive than a 2-channel system - my gut feeling says a manufacturer would save money by just ensuring the rears never get enough pressure to lock them, without having to modulate them as such. I'm it even sure how effective and reliable it would be to modulate the Up!s cheap rear drums.

By modulation, I mean rapid release and reapply from a hydraulic reservoir, not just reducing or limiting hydraulic pressure.

I'm only debating, and not asserting anything here - the only thing I know for sure is that accountants rule!

 

VW ABS systems have always been 4-channel, as far as I know, as seems to be the case on all modern cars.

Reading around, it seems 2- or 3-channel systems were more of a thing on goods vehicles and pick-ups. I'm guessing early ABS systems fitted to limousines and high performance sports cars in the 1970s could well have used a simpler setup in keeping with the technology available at the time