Front-wheel Toe-in or Toe-out

Front-wheel Toe-in or Toe-out

Toe-in is the amount by which the front-wheel rims are set closer together at the front than at the rear with the wheels in straight ahead position when the vehicle is stationery (Fig. 26.16A). Alternatively, toe-out is the amount by which the front-wheels rims are set farther apart at the front than at the rear (Fig. 27.16B). Therefore in Fig. 27.16, toe-in =Tr-Tf and toe-out = Tf-Tr.

Fig. 27.16. Steering toe-in and toe-out.

Toe-in or toe-out compensates for movement within steering ball-joints, suspension rubber bush-joints, and any slight deflection of the track-rod arms or suspension arms when the vehicle is in motion. The objective of the non-parallel stationary alignment of the front steered wheels is for the toe-in or toe-out to be taken up when the vehicle is moving, so that both wheels run parallel under normal driving conditions.

Toe-in neutralises the cone rolling effect of front wheels caused by camber angle. The amount of toe-in for any vehicle, therefore, is primarily determined by the amount of camber angle. Toe-in is usually 2 to 4 mm. Toe-in turns to zero as the dynamic forces develop to deflect steering linkages and take-up slight clearances. Excessive steering linkage looseness, however, allows the wheels to toe-out under dynamic loads. Toe-in and toe-out causes excessive tyre wear. Toe is adjusted with threaded sleeves on linkage members called tie-rods. Booth sides must be adjusted to hold the steering wheel centred as the vehicle goes straight.

A rear-wheel-drive steering and suspension layout is shown in Fig. 27.17. While driving, the propelling thrust pushes the suspension cross member and body forwards, which is resisted by the road reaction on the tyres. This produces outward twisting of both the wheels so that the suspension wishbone rubber bushing distorts causing the front wheels to diverge. To counteract this tendency the wheels are initially provided with a toe-in so that under driving conditions the wheels run parallel.

Fig. 27.17. Rear-wheel drive steered wheel reaction due to forward motion

Fig. 27.18. Front-wheel drive steered-wheel reaction due to forward motion.

 

A front wheel drive steering and suspension layout is shown in Fig. 27.18. In this case the driving traction is imparted to the front wheels due to which the stub axle assembly is pulled forward the suspension cross member and body. The wheels twist inwards due to the natural reaction and so distort the suspension wishbone rubber pivot joints in a horizontal plane causing the front wheels to converge. To counteract this converging tendency under driving conditions, the wheel track is provided with a toe-out, which is equal to the amount of distortion in the horizontal plane expected when the vehicle is being driven. This is an oversimplified presenta­tion. Considering other variations and combinations in steering and suspension geometry, front wheel drive vehicles sometimes have zero toe-out.

Toe-out-on-turns.

When a vehicle takes a turn, the inside wheel moves faster than the outer wheel because the former has to negotiate an arc with shorter radius than the latter. This action causes the wheels to toe-out-on-turns because of difference in their turning angles. This relates to the diverging of the front wheels in the forward direction when the stub-axles are rotated about their king-pins (Fig. 27.19). The difference between the angles of turn of the front steered wheels provides the value of the toe-out in degrees. The amount of toe-out on turns depends upon the track rod arm length, its angular set and the track width, and is obtained by the inclination or set of the track rod arms.

Fig. 27.19. Toe-out angle on turns.

To measure the toe out angle on turns, the front wheels are rolled on to radius or turntable gauges in the straight ahead position and then the pointer of each gauge is set to zero. The outer road-wheel is then turned through 20 degrees as indicated on the turntable scale, and the opposite inner wheel angle is observed on its scale, which generally should read between 20.5 degrees and 23 degrees. The toe-out angles on each lock should be approximately equal and it is more important than the actual amount. A vehicle having excessive tyre wear but with a correctly aligned track in the straight ahead position should be checked for the angular toe-out of the inner wheel with the outer wheel set at 20 degrees. Any error between the readings on both sides may be due to a bent track rod or arm, etc