Brake Pedal Feel

[bennyboysvuk]

Can anyone explain to me why different brake pedals feel different when you push them? I mean the difference between the rock hard feel of a pedal in say, a TVR Tuscan compared to the longer push you might get in a Mercedes before there's any braking force at all. I'm talking about pedal feel when all is correct with the car and there's no air in the lines and disks and pads are fresh.

My assumption is that it's the brake servo that is just set up differently, but how does that work? Can it be adjusted for preference?

[MagicalTrevor]

I don't know the definitive answer but the brake lines are metal (copper?) for the main part inside the car, once they get to the calliper then there is a short section of rubber hose. The rubber hose expands under pressure from the compressed fluid inside and that gives a more spongy feel. This is why people often replace the rubber sections with stainless steel braided hoses that stop this happening.

It should also be noted that when the rubber expands it becomes more porous (?) and allows small air bubbles inside which build up over time.
I also believe that the fluid can boil or become affected by heat through heavy braking (I.e. on the track) so the fluid can be changed for higher DOT rated fluids such as DOT5.1

I replaced all the above on my Clio and it made a fantastic change to brake performance and feel (along with new discs and race pads).

I'm sure that servos will be calibrated differently too with the TVR either having no servo (like an Elise) or a much weaker servo than a big heavy Merc.

I'm not an expert... The above may be completely wrong

[bennyboysvuk]

Thanks for the reply.

With those two cars there's a huge difference between how far you push the brake pedal before serious pressure builds up. The TVR feels like you push the pedal 1cm and it's firming up and the car's slowing down. Whereas the Mercedes' pedal hasn't firmed up by this point and even a further 1cm push doesn't bring the braking into effect.

[Mastodon2]

Speaking of servo calibration, that can absolutely destroy the braking in a car. I drove a Renault Clio Campus for a little while and it had almost no resistance in the brake pedal at all, it felt almost completely electronic. You pushed the pedal down and got nothing for the first 2cms, then about 90% of the total braking force was applied in an instant - grabby brakes are probably not such an issue for the average buyer demographic of a base spec Clio, but for a car enthusiast they felt absolutely rubbish.

[Martin350]

I'd say it was mainly the strength of the servo, if fitted.

The size of the master cylinder internally can also have an affect as this determines how much fluid is moved in relation to the brake pedal movement, as can the length of the pedal from its pivot point.
So can the size and capacity of the calipers and the piston diameters.

Brake pad material will also have an influence on pedal feel.

Basically, it's a combination of lots of things.

[davepoth]

More assistance usually means less feel - any springing medium in the brake system (a servo generally will use both rubber and air) reduces feel.

As you've noted, many modern cars have very "grabby" brakes - bad for anyone who wants to be progressive with their controls, but for the person on the street it means the brakes feel powerful.

[speedtwelve]

Often soggy brake pedal feel is just down to the system needing to be bled. I keep on top of this on my Chimaera, and the pedal feels like it's set in stone, but with very progressive action and feel when you do press it (like the Tuscan mentioned). On my Corrado VR6, however, I changed the fluid, bled the brakes, fitted braided steel hoses, yet the pedal always felt a tad unresponsive.

I, too, hate 'grabby' brakes as fitted to modern shopping cars. No feedback, difficult to modulate, and have you face-pressing the windscreen if you so much as fart.