Brake force, positive feedback loop?

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Caito
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Brake force, positive feedback loop?

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Hi all!

I have wondered a bit about the driver applying force to the brake pedal. It is often quoted that drivers apply roughly 150kg to the brake pedal.

Is it correct to assume that the driver's body weight acceleration will go partly to the brake pedal and the seat belts?

What I mean is, when you press the pedal, and you're doing 1G, your body will experience a 1G force too. If you rested entirely on the pedal, the 1G*driver_weight would get additionally applied to the brake. This will cause more force and brake acceleration, hence, positive feedback.

Do you believe there is some of this in play? Or should it be assumed that it is pure muscle strength applying 150kg with one leg?

Bonus question: is it true that drivers can't lock at 300kph+? I've read that force is limited by the system itself due to materials and compliance. Cause I don't think I see many drivers locking at the beginning of the braking phase.


Thanks all for the input!

Cheers,
Caito
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Tommy Cookers
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Re: Brake force, positive feedback loop?

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as long as the 150 kg driver is producing 1g deceleration the seat and harness could be removed without detriment

the brake pedal sees 150 kg regardless of where the 150 kg came from

no feedback loop can exist as the load path across the brake master cylinder is complete with or without driver restraint

piast9
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Re: Brake force, positive feedback loop?

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In my opinion only the inertia of the leg plays any role. Yes, the inertia of the whole body pushes drivers forward but they are stripped down with the seat belts and even if they weren't they would have to support that weight with their leg muscles anyway.

EDIT: Additionally, I don't think that this force is exceptionally high for an athlete. I bet you can stand on one leg or even jump on one leg. In that case your whole body weight + force needed to accelerate you to make a jump goes through the leg.

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Big Tea
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Re: Brake force, positive feedback loop?

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Surely this is a product of peddle length movement and pressure, so a longer peddle or shorter crank could move this figure to where ever it would want to be? Failing that, is the dia of the master cylinder regulated? This would also alter the required force would it not?
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sosic2121
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Re: Brake force, positive feedback loop?

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I have this issue when driving 2012 Citroën C4. Force required to break is just to low and breaking force makes me push the break stronger. I have to push against the wheel and with other leg to realise the brake. Really annoying. Luckily it's not my car. I really hate that car :oops:

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Big Tea
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Re: Brake force, positive feedback loop?

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sosic2121 wrote:
Mon Jun 20, 2022 1:53 pm
I have this issue when driving 2012 Citroën C4. Force required to break is just to low and breaking force makes me push the break stronger. I have to push against the wheel and with other leg to realise the brake. Really annoying. Luckily it's not my car. I really hate that car :oops:
If you ever get a chance, drive a 1970's era Citroën DS lux. You will never complain about no-feel breaks again :twisted:
Rest your foot and you headbutt the screen
When arguing with a fool, be sure the other person is not doing the same thing.

sosic2121
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Re: Brake force, positive feedback loop?

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Big Tea wrote:
Mon Jun 20, 2022 3:59 pm
sosic2121 wrote:
Mon Jun 20, 2022 1:53 pm
I have this issue when driving 2012 Citroën C4. Force required to break is just to low and breaking force makes me push the break stronger. I have to push against the wheel and with other leg to realise the brake. Really annoying. Luckily it's not my car. I really hate that car :oops:
If you ever get a chance, drive a 1970's era Citroën DS lux. You will never complain about no-feel breaks again :twisted:
Rest your foot and you headbutt the screen
I've owned BX, it's pedal has no travel. But it's perfect. Later I had a problem with "normal" cars. I have owned/driven many French cars, but c4 is just stupid :roll:
Hard to find anything good about that car.

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Big Tea
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Re: Brake force, positive feedback loop?

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sosic2121 wrote:
Mon Jun 20, 2022 6:57 pm
Big Tea wrote:
Mon Jun 20, 2022 3:59 pm
sosic2121 wrote:
Mon Jun 20, 2022 1:53 pm
I have this issue when driving 2012 Citroën C4. Force required to break is just to low and breaking force makes me push the break stronger. I have to push against the wheel and with other leg to realise the brake. Really annoying. Luckily it's not my car. I really hate that car :oops:
If you ever get a chance, drive a 1970's era Citroën DS lux. You will never complain about no-feel breaks again :twisted:
Rest your foot and you headbutt the screen
I've owned BX, it's pedal has no travel. But it's perfect. Later I had a problem with "normal" cars. I have owned/driven many French cars, but c4 is just stupid :roll:
Hard to find anything good about that car.
Not BX, they were fine (but misunderstood) First time I drove a DS* my wife had a box of cakes on her lap. For about 30 seconds then she was wearing them and the rest was on the (distant) dash. She still swears I did it just to annoy her.

*The one De Gaule had in (film) day of the Jackal. Wonderful cars, If you know what they are.(https://www.youtube.com/watch?v=0XGeb9uBFPo) at 3 min
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NL_Fer
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Re: Brake force, positive feedback loop?

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At 300kph there is very much downforce giving the tyres enough grip to handle all the break force without locking. Than when slowing down they have to release some pressure to prevent locking up.

Since that is all done in 1-2s, there is no time to experience any positive loop.

Billzilla
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Re: Brake force, positive feedback loop?

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I did a Claude Rouelle course in the mid 2000's, and one of the things he mentioned was that one F1 team measured the brake pressure with the car stationary at 900 psi, but when stopping from high speed it was 1,300 psi.

gruntguru
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Re: Brake force, positive feedback loop?

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There is about 12 kg equivalent of leg moving toward the brake pedal. Lets say a 10kg contribution to horizontal force on the brake pedal. At 6G deceleration that's 60kg leaving 90kg for the muscles to push.

Tallies very well with your post Bill.
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Tommy Cookers
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Re: Brake force, positive feedback loop?

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gruntguru wrote:
Mon Jun 20, 2022 11:28 pm
There is about 12 kg equivalent of leg moving toward the brake pedal. Lets say a 10kg contribution to horizontal force on the brake pedal. At 6G deceleration that's 60kg leaving 90kg for the muscles to push.
Tallies very well with your post Bill.
"braking pressure - 900 psi at rest 1200 psi at speed"

something like ? ....
the driver's body is acting as fixed via the seat to the car ... but ....
the driver's legs are in a quasi-independent load path via their hinged mode of installation
ie the legs can develop a push force on the pedal by the reaction at the seat
but are isolated by the seat harness from the decelerational inertia force of the driver's body

so if F1 brake pedals had levers and weights the DF effects on pedal force could be largely cancelled - a driver aid ?

similarly with flying - the pilot's arm weight gives a weak 'servo' effect
(and eg weights on Spitfire control cables were used to give an 'anti-servo' effect)

Mchamilton
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Re: Brake force, positive feedback loop?

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sosic2121 wrote:
Mon Jun 20, 2022 1:53 pm
I have this issue when driving 2012 Citroën C4. Force required to break is just to low and breaking force makes me push the break stronger. I have to push against the wheel and with other leg to realise the brake. Really annoying. Luckily it's not my car. I really hate that car :oops:
all peugeots and citroens are like that, breath on the pedals and your get an emergency stop

gruntguru
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Re: Brake force, positive feedback loop?

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Tommy Cookers wrote:
Tue Jun 21, 2022 9:33 am
gruntguru wrote:
Mon Jun 20, 2022 11:28 pm
There is about 12 kg equivalent of leg moving toward the brake pedal. Lets say a 10kg contribution to horizontal force on the brake pedal. At 6G deceleration that's 60kg leaving 90kg for the muscles to push.
Tallies very well with your post Bill.
"braking pressure - 900 psi at rest 1200 psi at speed"

something like ? ....
the driver's body is acting as fixed via the seat to the car ... but ....
the driver's legs are in a quasi-independent load path via their hinged mode of installation
ie the legs can develop a push force on the pedal by the reaction at the seat
but are isolated by the seat harness from the decelerational inertia force of the driver's body
A correct if slightly overcomplicated analysis.

Simple analysis: Draw a free-body-diagram of the drivers leg simplifying all forces and moments to:
- a single horizontal force at the brake pedal pushing rearwards on the sole of the foot
- a single horizontal force generated at the driver's hip joint and
- a single horizontal acceleration (f=ma) at the CG of the leg.

Force 1 = Force 2 + ma = 90kg + 60kg = 150kg

And yes - strapping a mass to the drivers leg (a heavy shoe or even a heavier brake pedal) would provide some servo assistance. This assistance would be proportional to deceleration (40% in the above case) so not a "drivers aid" in the rulebook sense.
Last edited by gruntguru on Wed Jun 22, 2022 1:49 am, edited 1 time in total.
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J.A.W.
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Re: Brake force, positive feedback loop?

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Mchamilton wrote:
Tue Jun 21, 2022 10:11 am
sosic2121 wrote:
Mon Jun 20, 2022 1:53 pm
I have this issue when driving 2012 Citroën C4. Force required to break is just to low and breaking force makes me push the break stronger. I have to push against the wheel and with other leg to realise the brake. Really annoying. Luckily it's not my car. I really hate that car :oops:
all peugeots and citroens are like that, breath on the pedals and your get an emergency stop
I owned a car which came with the sans-servo 'performance option' of linear brake pedal response,
which when fitted with 'race pads' required quite a hard push, (to be fair, it did feature a sturdy pedal
broad enough to accommodate both feet), but - 'as advertised' - it did offer excellent modulation.

(For regular road use, I fitted 'bitey' pads on one side of the brake discs with 'hard' non-fade pads
on the other, to gain a reasonable balance between leg effort & braking consistency.)

Even the comparatively firm boost-level provided by M-B anti-lock brakes gave me an impression of
vague-response/over-servo'd feel that required some adaptation time to trust as fully, as it happens...
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).