ABS and a little acceleration

[unclepoo]

I was driving our Auris hsd (2011) to the airport last week (had a lovely time in Corfu by the way) and the road was dry, perfect conditions. I was braking for a round-about from about 30mph to say 15mph. Whilst braking I crossed a sever joint in the road with the front tyres and the ABS cut in for a fraction of a second....but the alarming thing was that the car felt like it actually accelerated for that fraction of a second, not just the normal abs lack of braking I`ve experienced in my other cars etc. Tyres are almost new all round, about 2 months old. Brakes are in good order

Has anyone else felt this happen to them...? is this normal..? or am I worrying over nothing..?

[timberwolf]

Yes, It is normal. Worried, no, but to be aware of it, yes, so that one does not freak out when it occurs.

The braking system detects a lack of traction, this causes the ECU to switch from regenerative to friction braking, apparently to protect the Hybrid Drive System. The switch takes a fraction of a second but we humans are pretty sensitive to changes in de-acceleration and if our fight/flight mechanism kicks in our perception of time can change such that a fraction of seconds seems a lot longer.

With my Prius Gen 2, when braking over manhole covers, it feels like the car "let's go" and becomes floaty. I don't know what the later cars feel like.

As there have been recent discussions about tyre pressures above the recommended values, it should be noted that too high a tyre pressure will reduce the tyres contact with the road and therefore loss of traction is more likely to occur.

[unclepoo]

Thanks for the reassurance.....I`m thinking the new tyres are not as good as the old ones. We never had this on the old tyres.  I`ll check the pressures tomorrow just to be on the safe side too.

[Cyker]

Tyres is very possible; I had some Cooper CS2's on my Yaris and the car was far less sure-footed than the Continental PC5's it had before; I was triggering the ABS all the time when braking, esp. on lightly ?debrised? surfaces, and the car felt like it was understeering round a lot of corners and even roundabouts, which was something I've never experienced before, and I'd have to take them at far lower speeds than I'm used to.

I changed them early for some Dunlop Street Response 2's and the car was back to its normal self with far stronger braking force and zero understeer - As it should be!

 

What tyres did you change from and to out of curiosity?

 

[unclepoo]

Not sure of the make, but a budget brand......may have made a poor choice 

[Cyker]

Was just curious as there's nothing like real-life comparison rather than these artifical tyre tests you see :)

Well not to worry too much, just drive a bit more cautiously; Accelerate a bit gentler, brake a bit earlier, corner slower, be a lot more careful in winter etc.

If they're a budget brand they'll probably wear out faster so you can change them sooner :lol:

 

[unclepoo]

I`ve checked the pressures in all the tyres this afternoon.....cold, they were all over the place..all were over 40psi   tyre fitting monkeys !!!

So reduced the pressures to 38 fronts and 36 rears. Car rides and drives much better.

Their looks to be much talk about running higher tyre pressures....well I`d say don`t go silly, you may regret it with an accident 

[Cyker]

Youch, sounds like someone needs their equipment recalibrated!

40psi is about the highest you'd want to go on normal tyres on a normal car, certainly not much more over! My personal rule of thumb is 3psi above the maximum listed in the manual.

 

Good catch!

 

[johalareewi]

Perfectly normal and loads of threads about this from a few years ago.  The gen3 Prius had a software mod to (try and) reduce the noticabilty of this.

It is to with the switching from regen braking to friction braking and ABS.

As you apply the brakes, your body anticipates the decceleration.

When the regen/friction/ABS kicks in the decceleration changes and your body detects this.

You think you are suddenly accelerating and it can seem it is for a long time but the reality is, there has been a change in decceleration for a very short period of time.

If you ask your passengers, they probably don't notice because they are not applying the brakes so have no knowledge of what is about to happen (if that makes sense).

This is how those off road simulators (and suchlike) work.  You are convinced you body has been through the works but the reality is you haven't actually gone anywhere or done anything.

 

[PeteB]

The mind is easily fooled; I remember my first car with cruise control.  The first few times I switched it on I could have sworn the car was accelerating, but of course it wasn't and the speedo confirmed this.

The Gen 3 Prius braking thing was no worse that 'ordinary' cars I'd had with ABS, when going over a wet manhole cover or something, the split second the wheel locked and the ABS kicked in felt just like the effect on the Prius did.  Since the Prius additionally has the regen braking swapping to normal brakes, the media got excited about it.

The mod did make it less noticable, but I really thought the media over-egged this, and it wouldn't have bothered me if Toyota had left it as it was.

[YarisHybrid2016]

There is a particular road junction that has a certain combination of slope, white line paint, and small loose stones that often causes the ABS to activate, and when it does the car feels like it's not decelerating anymore.

The transition from regen to normal brakes is imperceptable, but the car suddenly feels like it isn't slowing, causing an immediate reaction to brake a bit harder.

Maybe relevent: when the car was new, the point where normal brakes activated could be felt by a slight, sudden improvement in braking action, but now I find that transition to be much smoother.

[PeteB]

I had my first car with ABS in 1992, and where I lived at the time there was a very shiny manhole cover just before my driveway, so I regularly experienced the ABS kicking in when it was wet as I slowed to turn in, even when braking quite gently.  I'm so used to it I'm hardly aware of it now and just brake slightly harder or release and reapply depending on urgency.

[mickburkesnr]

On 12/06/2016 at 10:37 PM, Cyker said:

Youch, sounds like someone needs their equipment recalibrated!

40psi is about the highest you'd want to go on normal tyres on a normal car, certainly not much more over! My personal rule of thumb is 3psi above the maximum listed in the manual.

 

Good catch!

 

I had the loan of a car from a friend once and I could not understand why the car was always wandering to the right. After 30 minutes driving I took it to the petrol station to check the tyres. The left hand front was 45psi, the right side was 18psi.

I never borrowed a car from him again.

[Anthony Poli]

1 hour ago, mickburkesnr said:

I had the loan of a car from a friend once and I could not understand why the car was always wandering to the right. After 30 minutes driving I took it to the petrol station to check the tyres. The left hand front was 45psi, the right side was 18psi.

I never borrowed a car from him again.

That was after 30 minutes, i wonder what the low side was before you set off

[mickburkesnr]

1 hour ago, Anthony Poli said:

That was after 30 minutes, i wonder what the low side was before you set off

This is a good point, however I was a good 4 stone lighter back then haha