Author Topic: Mk4 2021 Jazz Brake failure  (Read 11064 times)

Lincolnshire Rambler

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Re: Mk4 2021 Jazz Brake failure
« Reply #30 on: February 11, 2024, 08:49:31 AM »
Itís braking by wire - just like we have accelerator by wire -all electronic control with a fail safe mechanical link . You canít have the level of hybrid motor and generator tech efficiency without electronic braking . It needs the computer speed and calculation power to be able to blend seamlessly the power delivery or braking needs . The conventional discs brakes are there for vehicle stability dynamics as for the majority of braking force the car is using the driven wheels

Kenneve

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Re: Mk4 2021 Jazz Brake failure
« Reply #31 on: February 11, 2024, 05:13:14 PM »
Probably a silly question, but Iíll ask anyway.

Assume travelling at 30+ mph in B or D mode and you realise your brakes have failed.
You instinctively reach for the electric handbrake and switch on.

What happens next?

Does the handbrake come on instantly, with full power, locking the wheels? (ABS?)
Or is there some gizmo in the ECU, that applies them  progressively according to road speed?

I presume the second answer in correct, otherwise we would be in a load of trouble. :o

ColinB

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Re: Mk4 2021 Jazz Brake failure
« Reply #32 on: February 11, 2024, 07:57:26 PM »
If the "brake feel simulator" is the gizmo identified on slide 20 of the slideset as "The stroke simulator [which] provides a typical pedal feel to the driver", then I wonder what it's failure actually implies? Does it mean the car has no brakes, or does it mean that it feels to the driver as if it has no brakes because he's lost the normal pedal feedback? Slide 33 does seem to imply the brakes are still functional, but without the feedback or any servo boost. Probably not something to experiment with.

Lincolnshire Rambler

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Re: Mk4 2021 Jazz Brake failure
« Reply #33 on: February 11, 2024, 08:21:31 PM »
My understanding is that applying the handbrake should stop the car as in an emergency stop- it shouldnít lock the wheels as the antilock function should stop that ?
The brake pedal is just a position and speed  of pressing down on it -thatís all the car needs to figure out how much braking force to apply . However for driving that doesnít give any feed back so thatís done electronically via the mechanical brake simulator- maybe best seen as you feather off the brake to come to a smooth halt !
The failsafe function of the hybrid brakes is to press the pedal as it will still bring the car to a stop once ! It has to have a failsafe  if the computer fails or any part of the electronically controlled item.
I saw a you tube on a Toyota hybrid expert explaining their hybrid braking tech - state of the art like Honda s !

Lord Voltermore

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Re: Mk4 2021 Jazz Brake failure
« Reply #34 on: February 12, 2024, 09:32:47 AM »
My, possibly simplistic ,understanding is that in the event of a  sudden major electrical failure of the systems clever tricks the traditional  hydraulic elements in the system will still be able stop the car .But maybe not as efficiently.   . The electric handbrake will probably also still be  working .

If the foot braking fails completely due to  a catastrophic loss of hydraulics    you should still have the electric handbrake  as an independent emergency back up.  No different  to the olden days when conventional hydraulic brakes could also  potentially fail  ,leaving you just the mechanical handbrake.   

Its extremely unlikely that the electric  handbrake and  the main hydraulics would both  fail at the same time.
If they do you have have probably offended the fates    ;D
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Kremmen

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Re: Mk4 2021 Jazz Brake failure
« Reply #35 on: February 12, 2024, 10:45:24 AM »
That needs to be in the 'famous last words' alongside

Custer - where did all those Indians come from
Let's be careful out there !

Jazzyone

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Re: Mk4 2021 Jazz Brake failure
« Reply #36 on: February 12, 2024, 04:05:17 PM »
If you apply the electronic handbrake when moving it stops you immediately with a big jolt. Iíve tried it at 5mph slowing down coming to traffic lights ( with no one behind me!). There is no progression itís full brake lock so I wouldnít want to try it at speed youíd feel like you were going to rip the back end off the car! But if that was your only choice?
I was thinking about this recently when watching the Cupra car adverts on TV. In one part the car swings round like an old fashioned hand brake turn. So a bit of CGI I think, itís just not possible.

Steve_M

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Re: Mk4 2021 Jazz Brake failure
« Reply #37 on: February 12, 2024, 05:57:48 PM »
As per the owners manual:

Jazzik

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Re: Mk4 2021 Jazz Brake failure
« Reply #38 on: February 12, 2024, 06:11:43 PM »
If you apply the electronic handbrake when moving it stops you immediately with a big jolt. Iíve tried it at 5mph slowing down coming to traffic lights

When you do it once a month at 50mph on an empty road (so... with no traffic behind you) you keep your brake discs shining... ;D
If nothing goes right, go left!

Kenneve

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Re: Mk4 2021 Jazz Brake failure
« Reply #39 on: February 12, 2024, 06:40:32 PM »
As per the owners manual:

Is there a difference between pulling up the handbrake switch and holding it, compared with pulling up  the handbrake switch and let go?
You probably need both hands to hang on to the steering wheel,  :o :o

coldstart

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Re: Mk4 2021 Jazz Brake failure
« Reply #40 on: February 12, 2024, 07:46:46 PM »
As per the owners manual:
Thanks Steve_M!

It's info like this that makes this forum so valuable for me!
(the manual contains a lot of valuable information that is easily overlooked)

Lincolnshire Rambler

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Re: Mk4 2021 Jazz Brake failure
« Reply #41 on: February 12, 2024, 07:54:37 PM »
My guess is you need to hold it up in normal driving to quickly do an emergency stop ? If you lifted it up and released it it probably wouldnít do a full on stop as it may register an unintentional operation ? (No doubt kids or teenagers would want to give it a go because they can !)

Lord Voltermore

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Re: Mk4 2021 Jazz Brake failure
« Reply #42 on: February 13, 2024, 09:32:13 AM »
I too had missed this bit about holding the switch up continuously applies all 4 brakes  using the electric servo.
 If  footbrake pedal suddenly went to the floor and you had no brakes  this  might give an alternative that will still be working.   If you have the time and presence of mind.   

I had felt the quite significant jolt if I apply the handbrake a fraction before I have come to a complete stop.

In view of this new (to me) info I have just been out and tested for myself  what happens if you apply the handbrake at a higher speed.   Unlike when you are stationary  it only continues to apply  the brakes while you are pulling on the switch.    For a fraction of a second the  braking effect is quite slight  - enough to notice if for instance a curious passenger flicks the switch , but not enough to potentially lose control of the car.   If you continue to keep the switch up  the car progressively brakes harder ,quickly coming to a controlled emergency stop.

I quickly found I could control the amount of braking quite effectively   just by varying how much I was pulling on the switch.  Not something you should do habitually of course as you are not in complete control of the steering etc,  but  I would recommend   trying it out somewhere quiet. 

 Dont let the idea of complex braking systems and electric switch handbrakes put you off buying a Mk4 Jazz.  They are common on  modern cars from all makers.  ;)
« Last Edit: February 13, 2024, 10:36:45 AM by Lord Voltermore »
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Kenneve

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Re: Mk4 2021 Jazz Brake failure
« Reply #43 on: February 13, 2024, 10:49:52 AM »

I quickly found I could control the amount of braking quite effectively   just by varying how much I was pulling on the switch.  Not something you should do habitually of course as you are not in complete control of the steering etc,  but  I would recommend   trying it out somewhere quiet. 
  ;)

Now Iím confused, I was under the impression that the handbrake switch was simply a spring loaded on/off switch, with no means of varying the amount of braking effect.
I have my handbrake set to auto release, when moving off and in normal driving, use it when Iím likely to be stopped for some time, ie traffic lights etc.
So , how do I vary the amount of handbrake effort under those conditions?

Jazzik

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Re: Mk4 2021 Jazz Brake failure
« Reply #44 on: February 13, 2024, 11:37:35 AM »
We've all heard of rusting brake discs on hybrids.
What I wrote in my post of February 12, 2024, 06:11:43 PM is NOT a joke! By occasionally using the handbrake "trick" you keep your brake discs free from serious rust problems. (Expert by experience ;)).
If nothing goes right, go left!

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