[fordem]

Someone help me understand this...

I've seen multiple vendors advertising caster corrected radius arms, and I understand why they are needed for the front of the vehicle, but not why they would be needed for the rear of the vehicle.

Caster angle is defined as the angular displacement of the steering axis from the vertical axis of a steered wheel in a car as seen from the side of the vehicle, the front wheels are the ones that steer, the rear typically do not, so in my mind the rear wheels have no caster angle and so do not need caster correction and cannot be caster corrected.

What am I missing?

[Boggomobile]

Hi Fordem,

This is something that I have never heard of but marketers can invent problems so they can look good by solving them.

Is it possible that the "castor correction" is to help the driveshaft and mainly the angle variations of the universals?

Hope this helps......

[Gwagensteve]

Driveshaft angle, coil spring and shock alignment and seating and correcting wheelbase after lift might all be reasons for a “cranked” aftermarket radius arm.

[fordem]

You've given me food for thought, I'm still having difficulty wrapping my mind around the need for correction on the rear axle, but, let me make a couple of measurements this week end and I'll get back to you

[Gwagensteve]

Don’t think of it as caster- there are very good reasons to adjust axle position and pinion angle on the rear of a lifted vehicle and with radius arms “cranking” the arm is the only way.

[fordem]

I've grasped that bit - it's not caster correction per se, what I want to look at is how much of a correction is required - bearing in mind that it is constantly changing as the vehicle moves.

[Gwagensteve]

Ok, some very quick and rough numbers based on a radius arm suspension in phase with the rear tailshaft.

Let’s assume a stock, unlifted car with a 3deg tailshaft angle equal and opposite angles on the flanges (so ideal) and a 750mm long tailshaft.

50mm of lift will put the uni at the transfer at 7deg while the diff stays at 3 degrees. My guess is that would be enough to cause vibration that’s noticeable in a quiet low vibration car like a JB74, and a cranked radius arm would solve this.