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Do drivers "self servo" the brakes under very high g braking?


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#1 mariner

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Posted 07 August 2018 - 20:34

In the latest Racecar Enginering Richard Divila has an article on braking where he quotes some actual F1 data.

 

At Monaco after the tunnel the cars drop from 290 kph to 95 kph in 1.8 seconds covering less than 50 metres. That is 5g de -acceleration requiring a 144kg pedal load At 144kg it sounds like no power brakes were involved.

 

I think the 5g must be peak as not enough downforce near 95kph to get 5G. So the pedal load of 144g is of short duration. Nonetheless that's is one leg so equal to a leg press of about 300kg at the gym.

 

Now F1 drivers weigh about 70kg so they are , in effect, bench pressing  four times their own weight. An average person should be able to do 2x their weight, over 2.5 is very good. So the 4x factor isn't impossible BUT a modern f1 car has a body layout very unsuited to hard leg presses.

In the old lay down, straight leg days the driver could toggle the knee straight using the shoulder straps as his reaction point. Today his backside is low down but his lower legs are straight out almost at chest height. 

 

https://www.reddit.c...rmula_1_driver/

 

So how can he generate a pedal push equal to leg pressing four times his weight? Is the trick that his legs weigh about 20% of his weight i.e 14kg , or 7kg for one leg and at 5g that is providing 25% of the pedal load.

 

SO , if true he is doing effort worth about 3X body weight ( 144kg *0.75*2 = 220kg/70kg) ), in leg press terms and the leg inertia provides 25% or 35kg.

 

Of course every reaction has an opposite reaction so he still has to modulate the leg inertia force.

 

Maybe I'm over-analysing it and F1 drivers do so much fitness training now they can leg press 4 times their weight in such an awkward position?

 

 



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#2 gruntguru

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Posted 07 August 2018 - 22:09

Nice analysis. The leg inertia is clearly doing part of the job as you show. Perhaps they add some lead weights to their braking foot?

 

Does the 5g decel and 144 kg pedal effort, include aero drag which is probably good for 1g or more at 290 kph.


Edited by gruntguru, 07 August 2018 - 22:55.


#3 kevins

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Posted 08 August 2018 - 06:26

I've often wondered that myself and yes I agree, the G force must be a factor.

 

But, what I believe is happening is this (just my own theory - no inside knowledge to back it up).

 

Looking at a typical brake pedal/master cylinder setup (do not want to embed someone else’s image without permission so link to page)

 

https://www.swraceca...373=15-354.aspx

 

I would think that the 144 kg is measured at a sensor within or just before  the master cylinder, and not under the pad part of the pedal the driver presses on. So, we can clearly see the brake pedal is a lever.  So, say in an F1 car there is 3cm from lever fulcrum point to the  master cylinder piston shaft, and then 6cm to the pad, that means just 72kg 'leg press' (or 144kg like in the gym with 2 legs - still very impressive considering the seating position as OP points out)

 

What you all think?


Edited by kevins, 08 August 2018 - 06:32.


#4 thegforcemaybewithyou

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Posted 08 August 2018 - 09:52

Regarding the lever, you can see it clearly at 30s in the video. The center of the foot contact patch seems to be three times further away from the axis of rotation than the cylinder connector. The 144kg would reduce to 48kg with that ratio.

 



#5 mariner

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Posted 08 August 2018 - 18:44

I have since found a thread on Pistonheads which links to Brembo data  of 930kg PEDAL pressure per lap at Montreal. That is over 6/7 braking zones so it could still come back to the 144kg level at the pedal. One writer there also raises my leg weight question.

 

https://www.pistonhe...f=228&t=1622200



#6 gruntguru

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Posted 09 August 2018 - 04:54

Since when can you add successive applications of the same force and come up with a much larger force?


Edited by gruntguru, 09 August 2018 - 04:54.


#7 Allan Lupton

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Posted 09 August 2018 - 08:04

Since when can you add successive applications of the same force and come up with a much larger force?

The OP used references which lost me completely but were, I think, gymnasium-speak so perhaps that's relevant to the above.

The OP also seems not to understand knee-action which, whether it's a leg or a mechanism, can give a large end-force as the result of a (relatively) small straightning force.



#8 thegforcemaybewithyou

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Posted 09 August 2018 - 13:08

Another way to find the actual force could be this real world telemetry from Caterham from a few years ago. It shows a peak pressure of around 80 bar and that those pressures are not applied for a long time.

If we would know the cross section of the master cylinder, the force can be found easily.

 

https://www.reddit.c...os_courtesy_of/



#9 NotAPineapple

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Posted 10 August 2018 - 20:25

The driver weight does have a significant effect on F1 braking efforts and is included in driving simulators as a mathematically calculated increase in brake effort since the simulators cannot create anywhere near the required braking accelerations to reproduce this self servo effect.

The 5g is is definately not at 95km/h but at the start of the braking phase where about 1g is from drag, 1.7g is from mechanical grip and the rest (2.3g) is from aero downforce.

#10 sblick

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Posted 14 August 2018 - 18:43

When we do manufacturer vehicle work it is always from the pedal that we get the force.  A typical ABS push could be 600-700N so with a 4 to1 pedal ratio at the Master Cylinder the input rod would see over 244kg if I am correct.  Hate math. (600N/9.8)*4?  It is easier in a car where you can "lock out" your knee and have a nice straight line to the pedal.

Interesting subject and something I never really thought about.  You get so used to power brakes and steering.  Would be just as interesting to see how much arm force it takes to turn an IndyCar at speed since they have no power steering. 



#11 Lee Nicolle

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Posted 16 August 2018 - 08:47

These days dont forget the rear brakes are charging the battery. 

Many cars on the road have required a fair push on the brakes, to the point of bending seat frames. But they do stop and dont lock brakes!!

Though with carbon rotors and the very trick pads I doubt the braking effort is huge. 

Though the seating positions they have  in F1 would be a lot harder than a 'correct'  stance. 



#12 Lee Nicolle

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Posted 16 August 2018 - 08:58

When we do manufacturer vehicle work it is always from the pedal that we get the force.  A typical ABS push could be 600-700N so with a 4 to1 pedal ratio at the Master Cylinder the input rod would see over 244kg if I am correct.  Hate math. (600N/9.8)*4?  It is easier in a car where you can "lock out" your knee and have a nice straight line to the pedal.

Interesting subject and something I never really thought about.  You get so used to power brakes and steering.  Would be just as interesting to see how much arm force it takes to turn an IndyCar at speed since they have no power steering. 

Remove power steer and decrease caster. Though would still be driveable with what they use now, just slow down the steering ratio. And have superfit drivers for 500 milers.

Lots of woosy drivers though, back not so long ago power steer and brakes was for the towcar. Still my [and a lot of others] preference. And is also the KISS principle as well. We see it often on so called pro racing where they have powersteering failures. Electric pumps, hydraulic pumps, rack or steering box all have hoses, fittings etc to fail.

I do not know of any real race car with power brakes though, a lot of thought with cylinder sizing and pedal ratio usually makes efforts manageable.

Dont know F1 pedal ratio but  I suspect it is still more than 4-1. With tintops, as well as passenger cars I am used to 6 or 7-1 and between 3/4 to 1" master cylinders.

Some powerbrake systems use up to 1 1/8 masters. And heavy vehicles even bigger because of displacement issues


Edited by Lee Nicolle, 16 August 2018 - 09:04.


#13 gruntguru

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Posted 16 August 2018 - 22:58

Don't underestimate the braking force of an F1. It only weighs 700 kg but the retardation force at top speed is the same as a 3 Ton road car.

 

At the low speed end of the scale the wheels will lock with half that pedal force and the driver still needs to be able to accurately modulate that force so it can't be too low.



#14 Lee Nicolle

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Posted 17 August 2018 - 07:56

Don't underestimate the braking force of an F1. It only weighs 700 kg but the retardation force at top speed is the same as a 3 Ton road car.

 

At the low speed end of the scale the wheels will lock with half that pedal force and the driver still needs to be able to accurately modulate that force so it can't be too low.

THAT is the one thing that F1 does very well. In reality too well now as braking duels seldom happen, and even then it is old cheese tyre v new cheese tyre.

What will they do with 15" wheels and 12 " rotors presumably. Stop so well the driver will black out from deceleration!

Brake modulation happens for every driver. Even at 60k you modulate the pedal to stop on your mechanised pram.

Just not near as much


Edited by Lee Nicolle, 17 August 2018 - 07:58.


#15 gruntguru

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Posted 17 August 2018 - 22:45

You missed my point about modulation which was - if pedal effort required to lock wheels is too low, modulation becomes more difficult.



#16 ray b

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Posted 18 August 2018 - 17:57

could ''they'' ghimic the eltro part to shut off or reduce the electron harvesting to detect or  limit locking the rears

without an illegal anti-lock brake system anti-lock involved ?

 

high weight leg press is eazy

legs are strong far stronger then arms

I could leg press 450 lbs several times at a school gym at 150lbs myself

as a teen

and the 5g transfer should add force also

plus the lever ratio



#17 Lee Nicolle

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Posted 19 August 2018 - 01:30

You missed my point about modulation which was - if pedal effort required to lock wheels is too low, modulation becomes more difficult.

Actually just a bit more thought, I have driven  cars with functional power boosters [eg decent manifold vaccuum] and you can still modulate the pedal.

As well as too small master cylinders which are just a sponge near the floor,, but stop well. Just does not inspire much confidence.

Then there is differences in drivers, some jump on the pedal, other squeeze the pedal and achieve the same distances in a different way.
But you still use brain power to modulate to stop locking wheels. That is why different drivers prefer different pads. 

When you start left foot braking you realise it is a big effort, and many use both feet at different times. Dog boxes you can left foot all the time. Synchro boxes you cannot but those little brake squeezes to get the nose to turn in, as well as keeping the pedal up you can.



#18 sblick

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Posted 22 August 2018 - 15:37

ray b I like your thoughts.  Once the rears on an F1 car went full electric it must have opened the flood gates of ideas for rear ABS.  There must be 50 ways to do it.  Luckily we still see rear wheels lock up occasionally.  Correct me if I am wrong but I thought on most race cars you initially set the balance so the fronts lock first.  Making a case for never seeing the rears lock and thus really no inquisitive looks to why the rears never lock.  With software in this mix I think F1 engineers could easily do rear ABS and get away with it.....for awhile



#19 gruntguru

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Posted 22 August 2018 - 21:44

 . . . But you still use brain power to modulate to stop locking wheels. . .

So you disagree with the premise that increased brake sensitivity makes manual modulation less precise?



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#20 Kalmake

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Posted 15 September 2018 - 15:02

ray b I like your thoughts.  Once the rears on an F1 car went full electric it must have opened the flood gates of ideas for rear ABS.  There must be 50 ways to do it.  Luckily we still see rear wheels lock up occasionally.  Correct me if I am wrong but I thought on most race cars you initially set the balance so the fronts lock first.  Making a case for never seeing the rears lock and thus really no inquisitive looks to why the rears never lock.  With software in this mix I think F1 engineers could easily do rear ABS and get away with it.....for awhile

Standard ECU makes it not easy if you don't want to get caught cheating.