Braking at high speed vs low speed.

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  • #26
Ranger Mike
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The original question is -
1. forces at play determining wheel lock up at different speeds.
2. is it easier to lock up the brakes at 30 mph vs. 130 MPH

Typical passenger cars with hydraulic master cylinder and wheel cylinders, drum / disc brakes has a mechanical linkage at brake pedal to the Master Cylinder of 6:1 ratio. This means you push the pedal with 100 lbs. leg force and the master cylinder squeezes the caliper and brake pads at 600 lbs. force on the rotors.

Assumption- Once the brake system is locked up ( the rotor can not turn anymore) you now have all tires not rotating. Tires are sliding on one small contact patch per tire. You have effectively lost control.

Forces effecting vehicle braking
Aerodynamics –the faster you go the more effort it takes to overcome aero drag. This works for you when braking.
Tire grip- any slip angle advantage due to down force will go totally away once you lock the brakes.
Again let us look at the mechanics of the situation. You have managed to motivate a race car to 130 MPH having overcome aero drag et al..
If we apply the brakes and manage not to lock them we have to deal with the forward momentum being reduced by the brake system. In this case momentum energy is converted to heat caused by friction of the tires in contact with the pavement working thru the brake rotors being pinched by the brake calipers as actuated by the wheel cylinders via the pedal on the master cylinder. This HEAT is the big difference between a 30 mph stop and a 130 mph stop.

it takes 996,424 (lb/ft) energy to slow down a 1760 pound formula car from 130 mph to 0.
It takes 53,064 (lb/ft) energy to slow down a 1760 pound formula car from 30 mph to 0.

Now the brake system has to deal with 18 times the energy and it does not do it well. Momentum energy changed to heat energy can heat things up in the brake system to 1000 degrees F.
Heat reduces the ability of the brake system to “ lock up”. The coefficient of friction of the brake pads is severely reduced. The calipers squeeze the brake pads but the pads just can not slow the rotors.. They are cooked. Pads just can not cope. And..the brake fluid can boil. The higher quality racing brake fluids are very expensive but have a 600 degree boil point. Cheaper convectional brake fluid can turn to a gaseous state at the caliper/ brake pad interface so you got no pedal...very spongy. These factors contribute to what we in racing call Brake Fade.
Now evolution in racing has brought about super good brake fluid and ceramic hi tech brake pads and rotors and cooling ducts to counter this problem. But the fact remains you have to deal with 18 times the energy and its not that you need to hit the brakes “ harder” to brake from 130 MPH...it is that the brake system itself is not functioning at any where near the effectiveness as it is at 30 MPH for obvious reasons.

At 30 mph the brake pads and rotors are pretty cool. Brake fade is nil in that the system can easily absorb the energy without boiling the fluid or causing brake pad fade. So you can lock up the brake system easier at the lower speed. Or something like that and I'm out of beer..
 
  • #27
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The original question is -
1. forces at play determining wheel lock up at different speeds.
2. is it easier to lock up the brakes at 30 mph vs. 130 MPH
...
Now the brake system has to deal with 18 times the energy and it does not do it well. Momentum energy changed to heat energy can heat things up in the brake system to 1000 degrees F.
Heat reduces the ability of the brake system to “ lock up”.
The brake system only needs to deal with 18x the energy if you *don't* lock up. Assuming you lock up quickly (before the car has slowed much) then the rest of the car's kinetic energy goes into the tyre/road and not the brake system, so the discs/pads don't heat up and the friction doesn't change.

A quick back-of-the-envelope calculation shows that energy needed to be dissipated to stop 4 wheels rotating from 130mph is about the same as needed to stop the entire car from 25mph. So locking the wheels at 130mph is not going to generate any more heat *in the brake system* than a normal stop from 25mph (or thereabouts). And as I said before, the rest of the KE from the car (the 18x factor you mentioned) goes into melting the tyres onto the road.
 
  • #28
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Hello, I am new but I have some technical knowledge of vehicles that I can share that might assist you in finding the answer. The mechanical advantage of the brake pedal is applied to the brake booster, a vacuum is stored on the engine side of a diaphragm and atmospheric pressure is allowed into the cabin side of the diaphragm when you apply the pedal. You would then have 14.7 psia on one side and -5.3 psia on the other, depending on the size and number of diaphragms you would multiply the pressure by the square inches. Then the hydraulic system generally has a .5" piston inside the master cylinder which would force fluid against about a 2" piston in the caliper if it's just one piston, thus multiplying force once again, the drums wheel cylinder has a relatively small piston area about equal to the master cylinder. The smaller piston will travel farther than the larger piston. You will have a wedging affect taking place on a vehicle with duo-servo style shoes used on larger vehicles, and no wedging affect on non-servo drum systems used on smaller vehicles, which also affects braking force. Then you have the size of the friction surface. In a typical disc/drum system we also use a metering valve which builds pressure on the rear drum system first to allow them to engage because they have to travel further and to keep the vehicles rear end from trying to pass the front end. A proportioning valve is used on disc/drum or disc/disc, it allows the pressure to the front brakes to increase or overcome rear brake pressure as braking force increases past a given pressure. The brakes tend to work better once they are warm but start to outgas as the temperature gets too high. The fluid can also begin to boil. Then you have weight transfer and tire grip. I believe the explanation for why a brake rotor will lock up easier at 30 than 130 is an object in motion will tend to stay in motion, as the speed of the rotor increases it's energy increases, thus it takes more energy to stop it. Much the same way a flywheel hybrid system has more energy storage at 50,000 RPM than it does at 20,000 RPM. I tend to think of rotational mass the same as torque so I would like to say the rotor has more torque at 130.
 
  • #29
48
0
I believe the explanation for why a brake rotor will lock up easier at 30 than 130 is an object in motion will tend to stay in motion, as the speed of the rotor increases it's energy increases, thus it takes more energy to stop it.
I think this is the key, it's not that the *force* required to lock up the wheel is any different, just that because it is rotating faster it takes a longer *time* to lock up with the same force. So if you want to lock up the wheels at 130mph *in the same time* as you can lock up wheels at 30 mph, you will need to apply a larger force. This gives the impression of the wheels being "easier" to lock up at 30.
 
  • #30
141
7
The original question is -
1. forces at play determining wheel lock up at different speeds.
2. is it easier to lock up the brakes at 30 mph vs. 130 MPH

Typical passenger cars with hydraulic master cylinder and wheel cylinders, drum / disc brakes has a mechanical linkage at brake pedal to the Master Cylinder of 6:1 ratio. This means you push the pedal with 100 lbs. leg force and the master cylinder squeezes the caliper and brake pads at 600 lbs. force on the rotors.
Not quite right Mike. The pedal ratio might be 6:1, but you have boosters and hydralics in the mix too. A typical production car is more like 30:1 on pedal to brake force ratio.

Assumption- Once the brake system is locked up ( the rotor can not turn anymore) you now have all tires not rotating. Tires are sliding on one small contact patch per tire. You have effectively lost control.

Forces effecting vehicle braking
Aerodynamics –the faster you go the more effort it takes to overcome aero drag. This works for you when braking.
Tire grip- any slip angle advantage due to down force will go totally away once you lock the brakes.
Again let us look at the mechanics of the situation. You have managed to motivate a race car to 130 MPH having overcome aero drag et al..
If we apply the brakes and manage not to lock them we have to deal with the forward momentum being reduced by the brake system. In this case momentum energy is converted to heat caused by friction of the tires in contact with the pavement working thru the brake rotors being pinched by the brake calipers as actuated by the wheel cylinders via the pedal on the master cylinder. This HEAT is the big difference between a 30 mph stop and a 130 mph stop.

it takes 996,424 (lb/ft) energy to slow down a 1760 pound formula car from 130 mph to 0.
It takes 53,064 (lb/ft) energy to slow down a 1760 pound formula car from 30 mph to 0.

Now the brake system has to deal with 18 times the energy and it does not do it well. Momentum energy changed to heat energy can heat things up in the brake system to 1000 degrees F.
Heat reduces the ability of the brake system to “ lock up”. The coefficient of friction of the brake pads is severely reduced. The calipers squeeze the brake pads but the pads just can not slow the rotors.. They are cooked. Pads just can not cope. And..the brake fluid can boil. The higher quality racing brake fluids are very expensive but have a 600 degree boil point. Cheaper convectional brake fluid can turn to a gaseous state at the caliper/ brake pad interface so you got no pedal...very spongy. These factors contribute to what we in racing call Brake Fade.
Now evolution in racing has brought about super good brake fluid and ceramic hi tech brake pads and rotors and cooling ducts to counter this problem. But the fact remains you have to deal with 18 times the energy and its not that you need to hit the brakes “ harder” to brake from 130 MPH...it is that the brake system itself is not functioning at any where near the effectiveness as it is at 30 MPH for obvious reasons.

At 30 mph the brake pads and rotors are pretty cool. Brake fade is nil in that the system can easily absorb the energy without boiling the fluid or causing brake pad fade. So you can lock up the brake system easier at the lower speed. Or something like that and I'm out of beer..
A subtle difference in the wording, 'locking up at 130mph' vs 'from 130mph'. For heat soak to affect lockup, it would have had to absorb a significant amount of energy, and thus will have slowed the car down significantly. I agree that braking from 130mph, you are less likely to be locking up when you get down to 30mph due to heat soak.

At the moment you touch the brakes at 130mph though, there is no heat soak.
 
  • #31
Ranger Mike
Science Advisor
Gold Member
2,159
225
agreed and yes I did not go to production psi as power booster not used on our race cars..good points though!!
 

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