Reversing - Renault Z.E. Forum
 
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post #1 of 9 (permalink) Old 02-14-2014, 05:11 PM Thread Starter
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Reversing

Ever since I got my Fluence I've been aware of a slight scrubbing (probably not a good description) noise coming from the rear when reversing - especially on the first outing of the day, although I could be wrong about this.

It sounds as if the (rear?) brakes may be being lightly applied, and the sound is not present when moving forward at a similar speed.

It appears to me that this could be a deliberate feature that may be designed to, say, guard against the car suddenly shooting backwards in the event of injudicious use of the accelerator pedal, or to give the otherwise underused rear discs a very slight workout to keep them in decent condition.

Has anyone else noticed something similar or have any other ideas what might be going on?
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post #2 of 9 (permalink) Old 02-14-2014, 08:24 PM
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RE: Reversing

Rust on the rear pads. When the handbrake is released it leaves a shiny part. When you reverse, the rust is rubbed off. Or something like that.


NOTE: I meant rear discs not pads. The pads are rubbing the rust off the discs.
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post #3 of 9 (permalink) Old 02-15-2014, 04:44 AM
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RE: Reversing

Sounds very much like you're hearing rusty discs to me, too. Never heard that on mine, though they were rusty when I got the car. Depending on your driving, you may be barely using the brakes as the regen does much of the braking for you, so the discs get barely used.

When I first got mine I was concerned about the amount of rust on the rear discs, due to it standing. I used the handbrake for a while when stopping to try to cut the disc back a little. It is not a top-hat type disc, so you're safe to use the handbrake to slow the car, which has the desired effect. The warning beeps at you but that's not important.

It is important to make sure there are no bands of rust appearing on the discs. Take a look. Over time these will exaggerate themselves because they wear back the pads which then increasingly do not touch the disc at those points. A few good, deep applications of the rear pads will help wear these away before they become a problem. My Grand Espace was like this when I got it a few years back, and much as I have tried to wear the rust back it has now taken over and needs new discs, which I'll have to do sometime this summer.

Due to changes in legislation (no more asbestos in the pads) the discs have to be made soft. So the pad and disc wear together, which can created self-perpetuating problems like pitting and rusting, but at least it has been dropped as an MoT failure point so long as the brakes are up to the required efficiency.
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post #4 of 9 (permalink) Old 02-15-2014, 05:03 AM
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RE: Reversing

I seem to remember sometime in the dim and distant past reading that under braking pads are forced onto the discs but require forward motion of the car to "throw them" off once the brake is released. This could explain why you only hear the noise when reversing.
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post #5 of 9 (permalink) Old 02-15-2014, 07:27 AM
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RE: Reversing

Yes. Unless it is a design that specifically withdraws the pads (and some of the attempts to improve hybrid efficiency do do this, but clearly there is no such equipment on the Fluence to do this) then the 'standard' design of brake calipers is that the brakes are pushed onto the disc and simply return only if they get pushed back. There is a little compliance in the pressure on the pad, so there is a little self-return but essentially you're relying on the discs rotating and touching the pads enough to push them back slightly.

When you spin an undriven wheel jacked up off the ground then it would spin for a long time if there was no such effect. Instead it should rotate on its own for no more than a turn or two, because the pads should be in slight contact with the discs. You'd find the brakes would be slower to respond and more grabbing if the pads opened up and came away off the disc.

A little bit of brake friction noise is no problem. Also, you might hear a 'click' from the brakes when reversing and then changing direction and braking again. This is simply the callipers being pushed against their stops, which reverses when you change direction.
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post #6 of 9 (permalink) Old 02-15-2014, 09:35 AM
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RE: Reversing

I assume we can't reverse as fast as drive forward, but a) what stops us from doing so (a limiter of some kind?) and b) how fast in reverse before this cuts in? I mean, we haven't got any actual gears.....(well, I suppose we have kind of one gear) ?
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post #7 of 9 (permalink) Old 02-15-2014, 09:58 AM
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RE: Reversing

And I assume that none of us has actually tried it yet, Chris - and lived to tell the tale. Of course, it could explain where the missing Fluences have gone to......
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post #8 of 9 (permalink) Old 02-15-2014, 11:50 AM
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RE: Reversing

Control is purely electronic and it will happily throw in the currents to drive the motor to destruction if it was programmed to do so. Limited to 84mph, which is around 11,000 rpm IIRC. No reason the motor could not be safely instructed to run at 84mph in reverse, then, from a mechanical engineering point of view.

Not sure if I could be bothered to find out its top speed in reverse, but if I happen to be in a sufficiently empty car park in the future maybe I'll have a quick 'squirt', see if I hit the reverse limiter!
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post #9 of 9 (permalink) Old 02-15-2014, 06:44 PM Thread Starter
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RE: Reversing

Thanks for all the suggestions - most helpful.

I'll have a closer look at the discs and try to give them a bit of a workout if required.
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