# Stopping distance: Locked wheel skidding & rolling tyres

1. Nov 5, 2009

### Langmarais

Why is it that a vehicle travels further when skidding with locked wheels than when decelerating with rolling, braked wheels in conventional, non-abs vehicles.

2. Nov 5, 2009

### mgb_phys

Whats the friction in both cases?

3. Nov 5, 2009

### Langmarais

The co-efficient of friction of the road surface is constant/ same in both.

4. Nov 5, 2009

### Staff: Mentor

The point is that it's not!

5. Nov 5, 2009

### sganesh88

For dry roads, the stopping distance is less for the skidding vehicle.

6. Nov 5, 2009

### sganesh88

@Doc Al
Why is that? If we assume that the tire isn't notably damaged by skidding, the coefficient should be almost the same right? :-o

7. Nov 6, 2009

### Lsos

Place a heavy book on a table and gently try to push it. You'll notice it is harder to get it moving than it is to keep it moving. It's because static friction is higher than sliding friction. Same mechanism works in tires....

8. Nov 6, 2009

### Staff: Mentor

No, just the opposite. Otherwise anti-lock brakes would serve no purpose--just jam on the brakes and skid.
No. Once the tires skid, you are dealing with kinetic friction and not static friction. And kinetic friction, being less than static, leaves you with less control of your vehicle. That's why you don't want your tires to skid.

9. Nov 6, 2009

### sganesh88

The main purpose of ABS is to maintain directional control - which is possible only when the wheels are rotating- and not reducing the stopping distance.. But ya. I did the calculations and saw the rolling vehicle will stop sooner. The skidding vehicle screeching through the road makes me think its losing energy faster. Thats not the case though. :(

10. Nov 6, 2009

### mgb_phys

The point of the question though, is that in one case you have rubber+road * weight of car and in the other you have steel disc+brake shoes * force of hydraulics

11. Nov 6, 2009

### xxChrisxx

Your max braking force is still determined by static friction coef*vertical load on wheel though. Once you exceed that you'll lock up.

12. Nov 6, 2009

### sganesh88

Yes. But the only external force is friction. So one can prove that the rolling car will stop faster than the skidding assuming that the friction at pads is high enough to support equivalent deceleration of the wheel..

13. Nov 6, 2009

### rcgldr

Except in the case of overheated brakes (or really bad design), the dynamic coefficient of friction at the brake pads is always enough to maximize braking.

However, on a somewhat related note, it's common for the clutches in most street cars to have inadequate dynamic friction for maximum acceleration from a standing start, and drivers often resort to finding an ideal rpm to drop the clutch and spin the tires to acheive better launches than can be acheived by slipping these clutches when doing acceleration testing. In these cases it's better to slip the tires than the clutch.

14. Nov 7, 2009

### Langmarais

Could you please elaborate and explain the application of static/ kenetic energy theory in the case study as per the original example. I am not a physics expert and would like to understand.

15. Nov 7, 2009

### Langmarais

Could you please elaborate and explain the application of static/ kenetic energy theory in the case study as per the original example. I am not a physics expert and would like to understand.

16. Nov 7, 2009

### Staff: Mentor

The first thing to understand is the difference between static friction and kinetic friction, and that in most situations the coefficient of static friction is greater that the coefficient of kinetic friction. (Consider the sliding book example brought up by Lsos.)

When a tire rolls without slipping, the friction between tire and road is static friction. When you lock the wheels and start to skid, the friction is kinetic friction. Since kinetic friction provides less force, when skidding the tires it takes you longer to stop.

17. Jan 20, 2010

### RichardH

Problem: A car yaws in an anti clockwise motion because it travels around a curved path too quickly and adhension is lost. The car is braking but such that the wheels are on the verge of locking up. The car then rotates from 0-90 degrees as it travels longitudinally down the road so it comes to rest broadside. Tyre marks consistent with heavy braking, and the car rotating relative to it's longitudinal path are left on the road surface. Skid to stop tests are completed to find out the road surface's kinetic friction value.

Q: Why is it that calculations used to determine the vehicle's initial speed use a coefficient of friction value less than the kinetic values taken when skidding in a straight line?? Why does the coefficient of friction decrease as the vehicle is part way through it's rotation??