tyre coefficient of friction vs speed

[tonyel]

This question has been bugging me since I did a 'driver training day'. It was stated with great authority by the trainer that, contrary to standard practice (and what I had always done) it was more effective to brake harder (ie, more pedal pressure) at the start, when the car was travelling faster, and reducing the pressure as the car slowed. This was because '...it is harder to lock the wheels at high speed...'.
Nothing in my understanding of friction predicts this - although I only know about the standard, simplified 'coulomb friction'. In a road car with no significant downforce - is he correct? And if so, why?

 

[NormPeterson]

Near as I can tell, nothing about a pneumatic tire is truly linear, and the coefficient of friction between the brake pads and the rotors is also slightly temperature-sensitive.

Maybe it was given such emphasis because most people really don't brake hard enough, soon enough when minimum distance to get stopped (or slowed to some speed) is absolutely required. Probably why some cars already have "brake assist" technology.

If you are late-braking from a significant speed in order to negotiate a turn, you would want to be getting off the brakes as you start your steering instead of stepping on the pedal harder.

I'm not at all sure that I've ever followed your "standard practice". Seems better to keep a little of the remaining stopping distance in hand (so to speak) than to rely heavily on the last few feet of it. I've had to increase my braking when conditions changed after I started my braking, but that's not the same thing.


Norm

 

[patdaly]

I would say each vehicle would have a different answer based upon speed and downforce, assuming ABS, some of that would be negated.

I do know this, driving 91-96 Chevrolet Caprices for the past 13 years, at higher speeds the rear is so light that I would be careful with emergency braking.

 

[BobM3]

You'd have two friction interfaces to worry about - between tire and road and also between brake pads (and/or shoes) and rotors (and/or drums). For the brake shoes or pads, friction goes down with temperature (brake fade). So if you are starting with room temperature parts, I'd think you'd need to step harder on the brake pedal as the braking went along (parts getting hotter) to maintain the same frictional force on the pads (or shoes).

 

[jmw]

Our Driver Training didn't have much to say about how to brake, though Cadence braking was the indicated method for emergencies; hard on the brakes and relax when you lose grip then hard on again when you get it back again... repeat until stationary or under control (of course, we now have ABS etc).

Now, was it to avoid overheating on prolonged hard braking? or more effective?
Logic would suggest you should brake as hard as you can without losing grip... but cadence braking seems to be counter intuitive - except... you don't know when you are at the limit unless you break the limit.

JMW

http://www.ViscoAnalyser.com

 

[NormPeterson]

"For the brake shoes or pads, friction goes down with temperature (brake fade)."

Fade is not the only effect, and friction does not always decrease with increasing pad temperature.

For example, many performance-oriented street pad formulations have a lower coefficient of friction when dead cold than when warmed up a bit. This can easily be felt.


Norm

 

[dgallup]

The wheels and tires have a lot more rotational inertia at high speed so you can do more work at high speed in the brakes slowing the wheels without transmitting higher forces through the tire/road interface. Also, you need to back off the brakes as you turn into the corner so you keep the vector sum of the lateral & longitudinal accelerations constant (friction circle).

 

[GregLocock]

dgallup - but the rotational inertia is proportional to the linear inertia, so high speed or low the relationship is unchanged.

I call shenanigans on the instructor's explanation. On the other hand there is no doubt that getting in early is good practice in traffic.

Looking at real data in a real max ABS stop, the main observation is that the deceleration is virtually constant.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tonyel]

The way I see it, there is a straightforward chain - the force created by the friction of the pad on the rotor, multiplied by the difference in radius, must equal the force created by friction at the tyre/road interface. Since (using the simplified friction law) the force is just proportional to the (coefficient) x (normal force) - there is no place in the formula for speed. So it shouldn't make any difference whether you push hard on the pedal at 100mph or 10 mph - your likelihood of locking the wheels should be the same. This does of course neglect things like downforce, friction differences due to heating, and so-on - but I would expect them to be quite small effects.

 

[SomptingGuy]

If the instructor understands the physics behind vehicle dynamics, why is he a driving instructor? More likely he read something in a magazine and believed it.

- Steve

 

[jmw]

Or, just as likely, experience suggests that man and machine are safer following this advise.

However, people being people, they always want a reason or they do their own thing.
This, true or false, was the best explanation he came up with and that too works, most people settle for that.


JMW

http://www.ViscoAnalyser.com

 

[Bribyk]

Physics often has little to do with driving instruction. Hence the fact that you're supposed to reduce your following distance when behind both motorcycles and tractor-trailers. My instructor mentioned that this was because motorcycles stop quicker (true, no explanation given for the truck though). Then I asked if, when I was riding my motorcycle if I could subtract that extra following distance. His answer was "no"...

Brian Bobyk - Hoerbiger Canada

 

[MacGyverS2000]

I believe you meant "increase your following distance", not reduce...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[SomptingGuy]

My driving instructor told me to "brake like banana". Easy on, consistent pressure, then easy off. More for the comfort of the passengers and other drivers than for wringing the neck out of the brake/tyre performance.

- Steve

 

[NormPeterson]

I'd think that reaction to a motorcycle brake light would be slightly slower as well. For the truck, you don't get advance warning about what's happening in front of him.


Norm

 

[GregLocock]

With ABS in a real emregency easy-on is not recommended, in fact emergency assist detects the speed with which you apply the brake and biffs in some more line pressure if it thinks you are a bit hesitant.

Of course Volvo have gone the whole hog with City Safe.

http://blogs.motortrend.com/1005082/car-news/volvo-city-safe-system/index.html

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

Most emergency braking is for bad drivers who can't think ahead. I see more rear-enders than I see car/animal or car/human collisions.

- Steve

 

[jmw]

Yeah, thank you Volvo, I really want the BSOD when in an emergency, I had enough non-life threatening excitement with Citroen's "nine computers" and dodgy programming.

JMW

http://www.ViscoAnalyser.com

 

[LionelHutz]

I'd go with the others who feel the advice was just to get you to scrub off excess speed earlier in the braking cycle so you're not caught going too fast.


"According to Volvo, the system can help completely avoid a crash at speeds of up to 15km/h (9.32mph), while minimizing damage at speeds up to 30 km/h (18.6 mph)."

Well, at least those rush hour stop and go traffic accidents may be avoided. Seems like a lot of R&D and likely an expensive system to "help" prevent a collision up to 15km/h.

 

[dgallup]

Volvo - making texting safe for the masses.