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Hold on the road

  1. Mar 7, 2008 #1
    When I started taking driving lessons a long time ago, my teacher told me that when putting the gear into neutral it diminishes the car's hold on the road. Is it actually true?
    The tires "hold" the road through static friction, which has a maximum value of mu*N. Does the gear directly affect mu, which should depend only on the contact area and kind of tire and road? Maybe he simply meant that letting the car accelerate downhill on neutral would increase the centrifugal force needed for a fast turn, which might be higher than the maximum friction allowed.
  2. jcsd
  3. Mar 7, 2008 #2


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    The gear has nothing to do with how the tyres hold the road so I have no idea what the teacher was getting at. The only thing going into neutral when you drive does is eliminating engine braking.
  4. Mar 7, 2008 #3

    By 'Hold' your instructor meant keeping the car on the intended route.

    Try this experiment.

    Hypothisis - Tyre grip is not affected by gear engagement during high speed negotiation of road bends.


    1) Find a road with a long tightening bend. Approach it a 60miles an hour and slip into neutral just as you enter the bend.

    When you come out of hosptial and recover your vehicle from the body shop try part 2.

    2) Approach the bend at 60MPH use the appropriate brake, gear change, accelerate sequence.


    Analyse and evaluate the difference betwen the two.

    Did you reject the null hypothisis?

    Is there a statistically significant difference between 1 and 2,

    Is there a clinically significant difference

    Discuss the merits and limitations of different driving protocols, how will this affect future driving experience.


    Listen to the driving instructor

    LoL Dave :rofl:
  5. Mar 7, 2008 #4


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    No it's not true. All this does is disconnect the engine from driven tires, eliminating engine induced acceleration or deceleration.

    If anything, reducing the forward or rearward force at the contact patch on the driven tires, leaves more of the total grip available for cornering. For this very reason, almost all racing motorcycles, some street motorcycles, and some race cars, have slipper clutches that slip a bit during engine braking to reduce the engine braking force on the rear tire(s).
  6. Mar 8, 2008 #5


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    Your experiment involved using the brakes. If you apply the brakes before entering a turn to reduce your speed, that's not the same as entering the turn at constant speed and also not the same as keeping the car in gear. It is the application of the brakes to reduce the speed so you can negotiate the turn that keeps you on the road, not the fact that the car is still in gear.

    Your experiment is a non sequitur.
  7. Mar 8, 2008 #6
    Jeff, I agree with you completely, eliminating the force on the forward/backward axis leaves more space for friction on the radial axis, for turning. But I don't understand why is it important to reduce engine braking in the rear tyres specifically.

    However, consider this hypothesis:
    The gear induces only engine break that inhibits rolling of the tyres, which in turn reduce car speed by friction with the road. This deceleration would still be able to increase the maximum static friction through an increase in the area of contact between the road and the front tyres. Mostly the front tyres are important in turning, and the deceleration creates higher pressure on the front tyres (and less pressure on the rear tyres) which can increase the contact area and also the Normal force.
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