814 Braking characteristics on wet roads

Maximum friction is developed between a rubber tyre tread and the road surface under conditions of slow movement or creep. A tyre's braking response on a smooth wet road with the vehicle travelling at a speed, say 100 km/h, will show the following characteristics (Fig. 8.9).

When the brakes are in the first instance steadily applied, the retardation rate measured as a fraction of the gravitational acceleration (g m/s2) will rise rapidly in a short time interval up to about 0.5 g. This phase of braking is the normal mode of braking when driving on motorways. In traffic, it enables the

Normal braking

Hard braking

Wheel locked braking

Peak rolfing grip

f f

1

V

-Crash stop Normal rolling grip

1 1

I

M

Sliding grip ——j

I

Braking from 100 km/h on smooth wet road with new tyre

Time interval (seconds)

Time interval (seconds)

Fig. 8.9 Possible retardation braking cycle on a wet road driver to reduce the vehicle speed fairly rapidly with good directional stability and no wheel lock taking place. If an emergency braking application becomes necessary, the driver can raise the foot brake effort slightly to bring the vehicle retardation to its peak value of just over 0.6 g, but then should immediately release the brake, pause and repeat this on-off sequence until the road situation is under control. Failing to release the brake will lock the wheels so that the tyre road grip changes from one of rolling to sliding. As the wheels are prevented from rotating, the braking grip generated between the contact patches of the tyres drops drastically as shown in the crash stop phase. If the wheels then remain locked, the retardation rate will steady at a much lower value of just over 0.2 g. The tyres will now be in an entirely sliding mode, with no directional stability and with a retardation at about one third of the attainable peak value. With worn tyre treads the braking characteristics of the tyres will be similar but the braking retardation capacity is considerably reduced.

8.1.5 Rolling resistance (Figs 8.10 and 8.11) When a loaded wheel and tyre is compelled to roll in a given direction, the tyre carcass at the ground interface will be deflected due to a combination of the vertical load and the forward rolling effect on the tyre carcass (Fig. 8.10). The vertical load tends to flatten the tyre's circular profile at ground level, whereas the forward rolling movement of the wheel will compress and spread the leading contact edge and wall in the region of the tread. At the same time, the trailing edge will tend to reduce its contact pressure and expand as it is progressively freed from the ground reaction. The consequences of the continuous distortion and recovery of the tyre carcass at ground level means that energy is being used in rolling the tyre over the ground and it is not

Fig. 8.10 Illustration of side wall distortion at ground level all returned as strain energy as the tyre takes up its original shape. (Note that this has nothing to do with a tractive force being applied to the wheel to propel it forward.) Unfortunately when the carcass is stressed, the strain produced is a function of the stress. On releasing the stress, because the tyre material is not perfectly elastic, the strain lags behind so that the strain for a given value of stress is greater when the stress is decreasing than when it is increasing. Therefore, on removing the stress completely, a residual strain remains. This is known as hysteresis and it is the primary cause of the rolling resistance of the tyre.

The secondary causes of rolling resistance are air circulation inside the tyre, fan effect of the rotating tyre by the air on the outside and the friction between the tyre and road caused by tread slippage. A typical analysis of tyre rolling resistance losses at high speed can be taken as 90-95% due to internal hysteresis, 2-10% due to friction between the tread and ground, and 1.5-3.5% due to air resistance.

Rolling resistance is influenced by a number of factors as follows:

a) cross-ply tyres have higher rolling resistance than radial ply (Fig. 8.11), b) the number of carcass plies and tread thickness increase the rolling resistance due to increased hysteresis, c) natural rubber tyres tend to have lower rolling resistance than those made from synthetic rubber,

Fig. 8.10 Illustration of side wall distortion at ground level

Fig. 8.11 Effect of tyre construction on rolling resistance d) hard smooth dry surfaces have lower rolling resistances than rough or worn out surfaces, e) the inflation pressure decreases the rolling resistance on hard surfaces, f) higher driving speed increases the rolling resistance due to the increase in work being done in deforming the tyre over a given time (Fig. 8.11), g) increasing the wheel and tyre diameter reduces the rolling resistance only slightly on hard surfaces but it has a pronounced effect on soft ground, h) increasing the tractive effort also raises the rolling resistance due to the increased deformation of the tyre carcass and the extra work needed to be done.

8.1.6 Tractive and braking effort (Figs 8.12, 8.13,8.14,8.15, 8.16 and 8.17) A tractive effort at the tyre to ground interface is produced when a driving torque is transmitted to the wheel and tyre. The twisting of the tyre carcass in the direction of the leading edge of the tread contact patch is continuously opposed by the tyre contact patch reaction on the ground. Before it enters the contact patch region a portion of the tread and casing will be deformed and compressed. Hence the distance that the tyre tread travels when subjected to a driving torque will be less than that in free rolling (Fig. 8.12).

If a braking torque is now applied to the wheel and tyre, the inertia on the vehicle will tend to pull the wheel forward while the interaction between the tyre contact patch and ground will oppose this motion. Because of this action, the casing and tread elements on the leading side of the tyre become stretched just before they enter the contact patch region in contrast with the compressive effect for driving tyres (Fig. 8.13). As a result, when braking torque is applied the distance the tyre moves will be greater than when the tyre is subjected to free rolling only. The loss or gain in the distance the tread

Fig. 8.12 Deformation of a tyre under the action of a driving torque

(2) Mid-contacl

Fig. 8.12 Deformation of a tyre under the action of a driving torque

(1) Beginning oleonlact

Fig. 8.13 Deformation of a tyre under the action of a braking torque

(1) Beginning oleonlact

Fig. 8.13 Deformation of a tyre under the action of a braking torque

SlipCA!

Fig. 8.14 Effect of tyre slip on tractive effort

SlipCA!

Fig. 8.14 Effect of tyre slip on tractive effort travels under tractive or braking conditions relative to that in free rolling is known as deformation slip, and it can be said that under steady state conditions slip is a function of tractive or braking effort.

When a driving torque is applied to a wheel and tyre there will be a steep initial rise in tractive force matched proportionally with a degree of tyre slip, due to the elastic deformation of the tyre tread. Eventually, when the tread elements have reached their distortion limit, parts of the tread elements will begin to slip so that a further rise in tractive force will produce a much larger increase in tyre slip until the peak or limiting tractive effort is developed. This normally corresponds to on a hard road surface to roughly 15-20% slip (Fig. 8.14). Beyond the peak tractive effort a further increase in slip produces an unstable condition with a considerable reduction in tractive effort until pure wheel spin results (the tyre just slides over the road surface). A tyre subjected to a braking torque produces a very similar braking effort response with respect to wheel slip, which is now referred to as skid. It will be seen that the maximum braking effort developed is largely dependent upon the nature of the road surface (Fig. 8.15) and the normal wheel loads (Fig. 8.16), whereas wheel speed has more influences on the unstable skid region of a braking sequence (Fig. 8.17).

8.1.7 Tyre reaction due to concurrent longitudinal and lateral forces (Fig. 8.18) A loaded wheel and tyre rolling can generate only a limited amount of tread to ground reaction to to

Fig. 8.15 Effect of ground surface on braking effort

Fig. 8.15 Effect of ground surface on braking effort

0 20 40 50 90 100

Fig. 8.16 Effect of vertical load on braking effort

0 20 40 50 90 100

Fig. 8.16 Effect of vertical load on braking effort resist the tyre slipping over the surface when the tyre is subjected to longitudinal (tractive or braking) forces and lateral (side) (cornering or crosswind) forces simultaneously. Therefore the resultant components of the longitudinal and lateral forces must not exceed the tread to ground resultant reaction force generated by all of the tread elements within the contact area biting into the ground.

The relative relationship of the longitudinal and lateral forces acting on the tyre can be shown by

C 30 40 60 HO 100

Fig. 8.17 Effect of vehicle speed on braking effort

C 30 40 60 HO 100

Fig. 8.17 Effect of vehicle speed on braking effort resolving both forces perpendicularly to each other within the boundary of limiting reaction force circle (Fig. 8.18(a and b)). This circle with its vector forces shows that when longitudinal forces due to traction or braking forces is large (Fig. 8.18(c and d)), the tyre can only sustain a much smaller side force. If the side force caused either by cornering or a crosswind is large, the traction or braking effort must be much reduced.

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