Why Are Modern Car Brakes So Sharp?

[M@tt 77]

Some friends were up at the weekend and they'd hired some crappy vauxhall astra rental for the long journey as their car wasn't up to it anyway i had to move it on the drive and i almost went through the windscreen several times despite only the lightest of touches on the pedals. The brakes were amazingly sensitive and would stop you on a sixpence.

 

Now has there been some sort of miraculous advances in braking/calpier/servo or cyclinder design since 205's because i'm sure even if i bled and fitted fully refurbished calipers and discs and pads on a 205 they wouldn't stop me as sharply as that, or prehaps i'm wrong. I've got 307 283mm discs & calipers on my Mi16 with standard pads and excellent condition discs and there's no way they stop as fierrecly as that astra!

 

Any insight much appreciated

Edited January 4, 2008 by M@tt

[tom_m 0]

over servoed, with a heavy reliance on abs to stop jo public hoofing the pedal too hard?

[welshpug 1,657]

its just the way cars are made nowadays, over assisted servo's I believe.

 

so your 205 WOULD stop as fiercely as the Astra, IF you could lean on the pedal hard enough, AND stop it from locking up.

 

mind, I have just been working on my 205's brakes, pumped the pedal before moving it, and noticed in amusement that the accelerator pedal moved to the left when I leaned on the brake...

[Goliath 94]

Im afraid i dont know the answer but i know what you mean, My dads 307 HDI has immensly good brakes, wich is why i thought it was a common mod for 205s but i think they are power assisted brakes so that might be why?!

[Tom Fenton 1,541]

Its purely down to the servo assistance. All modern stuff is like it, and can trigger the ABS easily if you jump on them like an elephant. At low speeds they are harsh but at high speeds there is no denying that they are confidence inspiring when facing the car in front slowed suddenly to 30mph from motorway speeds. Even though a 205's brakes will stop you, they certainly can give you a heart in the mouth moment when you first hit them.

[cybernck 402 1 Cars]

i know what you mean M@tt and i get the same feeling when driving a newish car.

 

i figured out though that they have overassisted brakes and small master cylinders,

which makes the pedal very soft and you can't modulate it very well (until you get

used to it ofcourse). it's something like standard m/c with 306 GTI6 brakes (i know

the sensation of upgrading to 23mm cylinder was the same as what we're discussing).

 

furthermore, these cars we talk about here are sitting pretty high, have soft suspension

qith long travel and also chunkier tyres that flex more (all compared to 205 GTI), all of

which makes the weight transfer and sensation of braking quite different than it actually is.

 

as someone said, try pressing your brake pedal suddenly and hard (not progressively

and feeling the resistance) and you'll see that your brakes are very fearce too!

[M@tt 77]

cheers very much

 

i'm not sayign i want to do it however from a purely technical point of view is it possible to over-servo a 205 ie wind the servo assistance up some or is it purely down to the existing build/design of the existing servo.

 

To be honest the servo is one of those things on a 205 i've always looked at like a bit of a black box, i understand what its supposed to do and the effect to some extent, but never really about the innards of how it actually works

[Tom Fenton 1,541]

In order to increase the servo assistance you'd need to make the diagphram area that the engine vacuum acts on larger. There is no way of doing this to the standard unit, you'd need to graft one from another car onto the 205 pedal box. Probably achieveable if you set your mind to do it but not really worth the effort.

[cybernck 402 1 Cars]

theoretically , it would be easier to achieve the same effect by fitting a smaller master cylinder.

[Rippthrough 98]

You could probably get one of the bigger servo's from a 307 or the like to fit, but he main problem with large amounts of servo assistance is that pedal feel is abysmal.

[jackherer 543]

over servoed, with a heavy reliance on abs to stop jo public hoofing the pedal too hard?

 

For ABS to work properly in an emergency you need to jam the pedal down hard and research showed people just weren't braking hard enough in accidents for the abs to work fully so now they are over servo'd and some cars have 'electronic brake assist' as well for the same reason.

[KRISKARRERA 2 1 Cars]

Old cars' brakes have been sharp too. Autocar moaned that the brakes of the early 405 Mi16 and CX Turbo were badly over-servoed.

[Jer309GTi 3]

But to be fair, the early Mi16 didn't have a servo Dunno bout the CX. The old Volvo 340's brakes were known for being over servo'd, mine certainly were!

[jackherer 543]

CXs have completely powered brakes but no servo as such. They are quite sharp but less than the DS that preceded the CX.

[tom_m 0]

CXs have completely powered brakes but no servo as such. They are quite sharp but less than the DS that preceded the CX.

 

all hydrospastic citroen's are fully powered like that aren't they... it took me ages to get used to the xantia i'm driving at the moment the brake peddle is more of a switch

[Dan Ingram 1]

I drive a brand new mini quite a lot and the brakes in that a seriously over servo'd but if you lean on them a bit for more than 5 minutes spirited driving they seem to fade very quickly. They definitely fade alot quicker than the brakes on my 205, possibly because of the weight there trying to stop.

[jackherer 543]

all hydrospastic citroen's are fully powered like that aren't they... it took me ages to get used to the xantia i'm driving at the moment the brake peddle is more of a switch

 

They all work in the same way, a valve that releases high pressure hyraulic fluid to the calipers, but Xants have a MUCH more civilised pedal than a CX!

[KRISKARRERA 2 1 Cars]

Yeah the early 405 had that hydraulic thingy didn't it. Nice and reliable

[Dave G 0]

better without in my opinion, less to go wrong providing your brakes are working well.

[Rippthrough 98]

Yeah the early 405 had that hydraulic thingy didn't it. Nice and reliable

 

 

Ain't it just, good job the handbrakes decent

[snillet 1]

The citroëns i actually like, the old types like DS/CX/BX where there´s a direct connection to the brakevalve from the pedal, but on XM/Xantia and newer they´ve doned a strange thingie where there´s a litte spring between the pedal and the valve that makes the pedal travel in a similar way that a conventional brakepedal does.

 

However that in my opinion makes the brakes harder to control, most cars with that spring construction have ABS though , heavily relying on ABS on newer cars it seems....

 

I belive that there was also a tred introduced by the japanese manufacturers during the 80:s to with that pedal-feel. If you drive a corolla from 1985, it has that sharp an overassisted feel to the pedal, most 80:s japs i´ve driven has had that feel.

 

I really loved the brakecontrol on the Saab 9000 turbo i had for a while, it was precise and you could easily control the amount of braking power before the ABS did cut in. Saab was very very good at some things when they still were "saab" and not GM:ed.

 

So now i´m a bit off topic....

 

Point is that i belive the trend began with the japs.

[Anthony 1,003]

...but Xants have a MUCH more civilised pedal than a CX!

Crikey, and I thought the Xantia brake pedal was bad... dread to think what the CX one is like if the Xantia is good by comparison!

[jackherer 543]

The citroëns i actually like, the old types like DS/CX/BX where there´s a direct connection to the brakevalve from the pedal, but on XM/Xantia and newer they´ve doned a strange thingie where there´s a litte spring between the pedal and the valve that makes the pedal travel in a similar way that a conventional brakepedal does.

 

The spring doesn't make all that much difference really, people (such as tom_m above) still think they are too sharp.

 

Basically Xants/Xms are a compromise that leaves no one happy, just another phase of peugeot killing citroen.

[jackherer 543]

Crikey, and I thought the Xantia brake pedal was bad... dread to think what the CX one is like if the Xantia is good by comparison!

 

It takes about two weeks before you feel confident in a CX, after you have got through that stage they are amazing. I don't just mean the brakes either, the steering is entirely hydraulic too.

[Alastairh 47]

It takes about two weeks before you feel confident in a CX, after you have got through that stage they are amazing. I don't just mean the brakes either, the steering is entirely hydraulic too.

 

And the steering?

 

That worries me