what causes brake recoil? - PeachParts Mercedes-Benz Forum

sixto · #1 01-19-2007, 03:23 PM

Say you apply the brakes hard to stop a car. In some cars there's a considerable backwards jerk just after the car comes to a halt. In other cars like the 140 there's little to no jerking. Is that a difference in suspension design? Brake materials? Rate of pressure bleed in master cylinder?

Sixto
93 300SD 3.0

TheDon · #2 01-19-2007, 03:27 PM

physics?

kmaysob · #3 01-19-2007, 03:31 PM

abs

justinperkins · #4 01-19-2007, 03:33 PM

When you apply the brakes while moving, the forward momentum of the vehicle pushes the vehicle chassis forward while the brakes are pushing the wheels back. This causes the front suspension to compress and the rear suspension to extend. Once the vehicle comes to a stop, the suspension returns to it's steady state causing the "settling" feeling.

It's been said that when you taking your driver's test, it's pretty crucial to let the tester experience the settling feeling (what you call recoil) at a complete stop, unless you want to fail the test.

That'd be some physics I suppose.

justinperkins · #5 01-19-2007, 03:35 PM

Quote:

Originally Posted by kmaysob

abs

not the feeling he's talking about

Brian Carlton · #6 01-19-2007, 03:54 PM

My educated guess is suspension.

An older suspension design will allow significant compression of the front springs which will return to their unloaded condition rather suddenly after the vehicle attains a complete stop on a hard brake action.

On newer vehicles, the suspension won't compress anywhere near as much and the recoil is significantly less.

Simply look at the suspension travel of a W140 vs. a W126. I'll bet the W140 has 3" travel at the very most...........versus nearly 5" on the W126.

What I fail to grasp is how they manage to give the W140 a significantly better ride quality with much less travel. It's not simply a function of it's greater mass.

I had the SD out last night and the ride impressed me as "miserable" yet again. The W126 finds every single pavement undulation and seems to amplify it before it's delivered to the driver.

Larry Delor · #7 01-19-2007, 05:10 PM

Benz says their suspensions have "anti-dive geometry" - betcha that helps.

Ara T. · #8 01-19-2007, 07:34 PM

Different cars have different weight distribution as well... my mom's Miata is 50/50 and has a lot less of that recoil you speak of. Has a lot to do with the stiffness of springs as well.

Hatterasguy · #9 01-19-2007, 08:01 PM

Slam on the brakes with a W220 car stays perfrectly flat. Air suspension rules!

Yeah Brian annoys me to! My friends W140 with 18in wheels and 35 series profile tires rides better than my W126 with stock tires! MB sure improved the design quite a bit.

justinperkins · #10 01-19-2007, 08:26 PM

All cars settle back, even if a little bit.

t walgamuth · #11 01-19-2007, 10:54 PM

the little roll backwards if you stop aggressively and let off the brakes just right is indeed from the suspension.

an abrupt stop will cause the car to dip in front and raise in back. the springs will compress and also the rubber suspension mount pieces. with the car standing on its nose if you let the brakes off just right the recoil of the suspension is enough to throw the car backwards just enough to be startling.

i guess the tires will compress too and perhaps even wind up a bit on the rims.

i can't do it every time i dont think but i can get her to do it fairly consistantly if i try.

tom w

RobTheMod · #12 01-20-2007, 12:51 AM

Anecdotal:

The Rolls-Royce chauffeurs' school would show you how to make the motion of the vehicle "die" without any such feeling of recoil. It's a matter of releasing the brakes just prior to the actual stoppage of the vehicle; I can't tell you when to release them, just take it to an open space such as a church parking lot on a weekday, and try it. You'll get the hang of it. Think of the idea of braking to 0.5mph and letting the 3800 lbs of W126 "snuff the candle" on the last bit of momentum. People comment on how smooth the ride is even in my Audi 5000TQ with Eibach lowering springs and 17" wheels, because I picked up this trick when Mom taught me to drive...

Ara T. · #13 01-20-2007, 01:26 AM

I thought everyone knew how to do that ^

I figured it out while I was learning how to drive after I got my permit. My dad noted how much more enjoyable it was to ride along. When I did it on my driver's test at a stop sign i almost got failed because the stupid woman didn't think I did a full stop... I made sure *not* to stop smoothly after that...

MBeige · #14 01-20-2007, 03:07 AM

Didn't the w140 have the "soft stop" feature to eliminate the brake recoil? I remember this from ads years ago. Even my female cousin remembers this.

Hatterasguy · #15 01-20-2007, 10:29 AM

Quote:

Originally Posted by RobTheMod

Anecdotal:

The Rolls-Royce chauffeurs' school would show you how to make the motion of the vehicle "die" without any such feeling of recoil. It's a matter of releasing the brakes just prior to the actual stoppage of the vehicle; I can't tell you when to release them, just take it to an open space such as a church parking lot on a weekday, and try it. You'll get the hang of it. Think of the idea of braking to 0.5mph and letting the 3800 lbs of W126 "snuff the candle" on the last bit of momentum. People comment on how smooth the ride is even in my Audi 5000TQ with Eibach lowering springs and 17" wheels, because I picked up this trick when Mom taught me to drive...

Oh yeah I have been doing that since I learned to drive, makes your brake pads last longer too!