tyre coefficient of friction vs speed
tyre coefficient of friction vs speed
(OP)
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?
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?





RE: tyre coefficient of friction vs speed
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
RE: tyre coefficient of friction vs speed
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.
RE: tyre coefficient of friction vs speed
RE: tyre coefficient of friction vs speed
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
www.ViscoAnalyser.com
RE: tyre coefficient of friction vs speed
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
RE: tyre coefficient of friction vs speed
RE: tyre coefficient of friction vs speed
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: tyre coefficient of friction vs speed
RE: tyre coefficient of friction vs speed
- Steve
RE: tyre coefficient of friction vs speed
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
www.ViscoAnalyser.com
RE: tyre coefficient of friction vs speed
Brian Bobyk - Hoerbiger Canada
RE: tyre coefficient of friction vs speed
Dan - Owner

http://www.Hi-TecDesigns.com
RE: tyre coefficient of friction vs speed
- Steve
RE: tyre coefficient of friction vs speed
Norm
RE: tyre coefficient of friction vs speed
Of course Volvo have gone the whole hog with City Safe.
http://b
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: tyre coefficient of friction vs speed
- Steve
RE: tyre coefficient of friction vs speed
JMW
www.ViscoAnalyser.com
RE: tyre coefficient of friction vs speed
"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.
RE: tyre coefficient of friction vs speed
RE: tyre coefficient of friction vs speed
- Steve
RE: tyre coefficient of friction vs speed
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JMW
www.ViscoAnalyser.com
RE: tyre coefficient of friction vs speed
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: tyre coefficient of friction vs speed
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
RE: tyre coefficient of friction vs speed
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.