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?

 

[SomptingGuy]

Yeah, bull bars on the rear too.

- Steve

 

[jmw]

No matter how many millions are spent sometimes its the little things that cause a cop[uter system to crash... for example, dividing by zero:

http://www.maritimeprofessional.com...puter-is-mightier-than-the-Aegis-cruiser.aspx

JMW

http://www.ViscoAnalyser.com

 

[GregLocock]

"Seems like a lot of R&D and likely an expensive system to "help" prevent a collision up to 15km/h."

Correct. Now think about the longer term implications, as this is the first baby steps in active primary safety (ie the best crash safety system is the one that prevents you having a crash).



Cheers

Greg Locock

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

 

[wrightti]

Apologies for bringing this back from the dead, maybe I have something to add.

Was this a racing instruction day or for general driving?

If it was for general driving I would have to agree with the majority that there is no clear cut reason why a tyre should have more grip at higher speed. But I still think its a good idea on the road to knock off a few kph quickly to give you more time (distance) to react to whats going on ahead.

Also, remembering energy = 0.5mv^2 so the more speed to can take off initially will mean less energy to dissipate should you end up hitting anything.

For an aero racecar on the other hand, you will have a higher grip capability at high speed due to the downforce. This will then reduce with the square of the speed as the car slows down.

Whether this effect is enough to require some modulation on the brake pedal depends on the car obviously. But I learned in an OptimumG seminar some years ago that a modern F1 car (2006 at the time) has a massive brake pedal because the drivers use both feet to brake. Even with both feet on the pedal they can't apply enough force to lock the brakes until the downforce drops off.

Though I'm not willing to put my life on the previous statement, I have seen for myself in some recent telemetry on a GT2 sports car that the driver braked the same way. Hard and fast initially, then holding constant, then decaying gradually.

Regards
Tim

 

[GregLocock]

You are absolutely right, on an aero circuit car the effect due to downforce is far more important.



Cheers

Greg Locock

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