Qusetion by: Nick Retief Answer by: Ronald HairbottleI recently read an expert’s response to a suspension-related query (Flexible Patrol) in your July ’11 issue. I’m attempting a similar conversation (shackle reversal and lengthening) to my Cruiser but as mentioned by your expert the lengthened shackle upfront has resulted in wandering and instability. I inserted a 5 mm spacer between the diff housing and the leaf springs to compensate for the caster change and it’s helped tremendously.
My question is: would the rear diff also need a similar adjustment or does the rear suspension not matter much? Also, does the diff and prop shaft alignment have to be exact or will the universal joint make up for mild discrepancies? I’ve noticed a slight vibration between 2 300 and 2 600 rpm, could this be caused by the rear diff’s alignment? Could you provide me with the diff angles of the front and rear prop shaft? Finally, what are the benefits / procedures behind a shackle reversal?
THE EXPERT’S OPINION:
As the rear wheels are fixed in place, a caster change is irrelevant for the rear axle. However, what is important is the angle of the pinion in relation to the transfer case flange. If this angle is correct it will ensure that the u-joints turn in unison and don’t pick up any unwanted vibrations.
With mild suspension lifts the u-joint angles are generally within a safe operating range and one can therefore keep the standard u-joints at both ends of the driveshaft. In other words, if the angle on the transfer case output flange is 0° then the angle on the diff pinion flange must also be 0°.
With spring-over suspension systems the pinion angle can be 1 to 2° lower than the transfer case angle. This will help counteract the effects of axle-wrap – an unwanted rotation of the diff assembly as torque is applied to the wheels. Fortunately, this is not a problem for vehicles with coil spring suspensions or spring-under setups.
The operating angle of the u-joints becomes too acute when overly excessive (4” or more) suspension lifts are used – particularly for SWB 4x4s. When this happens one needs to use a double-cardan joint by the transfer case. This joint will halve the operating angle of the standard u-joint.
A shackle-reversal modification is easily accomplished if certain rules are adhered to, the most important being caster. The front mounts have to be long enough to keep the caster angle positive and between 3 and 6°. If the front mounts are made shorter than the original shackles, then the rear tubes have to be moved up and mounted through the chassis. If you don’t get the caster angle correct it will result in wandering and a slow return to centre. You can use longer shackles, which will net more wheel travel and lift, but they’ll also impede your clearance. What’s more, if you have too much caster, as a result of the longer shackles, you’ll create shimmy problems.
The most significant benefit of a shackle reversal modification is the added clearance, softer ride (wheel moving backwards when engaging an object), easy caster adjustment and less strain on the chassis mounting points. If you’d like to know more on this subject please feel free to contact us.
TAC 4×4 Traction
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