Live axle diff angle

[nilocp]

Not sure if I have asked about this before, but I am ready to weld brackets on axle case. Any one know what angle the diff should be to ensure the UJs get 'worked 'properly?

CP

[flyerncle]

Try locost cos the Haynes aint got no rigid axle,normally diff nose sits horizontal in a car.

[H-L-Smith]

Originally Posted by nilocp

Not sure if I have asked about this before, but I am ready to weld brackets on axle case. Any one know what angle the diff should be to ensure the UJs get 'worked 'properly?

CP

Are you putting a live axle in the Haynes (Gibbs) Roadster? If so, a complete answer to your question needs to have some more detail. If you're using the earlier Locost design, which already has a live axle, then the simple answer is that 2-3 degrees static inclination of the U-joints is a generally accepted, auto industry standard.

If you're modifying the Haynes Roadster to accept a live axle instead of the Sierra IRS then there's more to consider.

1) Viewed from the side, your transmission output shaft and your differential input (pinion) shaft should be parallel, or as close to parallel as possible if some practical circumstance prevents them from being exactly parallel. In most cases, the transmission output shaft is higher than the differential pinion shaft and there is a slight downward angle of the prop shaft connecting them (angle A).

2) Viewed from the top, an ideal design puts your transmission and pinion shafts directly in line with each other. Sometimes the differential pinion shaft is offset and not exactly in the middle of the rear axle while your transmission shaft usually runs right down the centerline of the car. In that case, you will have a small angle in the horizontal plane (angle B) unless you've slanted your engine/transmission to match the offset differential (very rarely necessary and not usually done).

3) Typically, what's called the true joint angle (TJA) is the critical design parameter because large angles cause unacceptable vibrations on the road. TJA is calculated from Angles A and B above:

TJA = Square Root [ (angle A) squared + (angle B) squared ]

Where TJA is 0.5 degrees minimum and a maximum of 3 degrees.

4) Generally, under acceleration the desire is to have the the angle of the universal joints be as close to zero as possible. Since most live axle designs allow some rotation of the differential under acceleration (3-4 degrees is a good assumption) then you adjust the static angle of the differential to account for that rotation when accelerating while still aiming at getting the U-joint angle as close as possible to zero. A good assumption is 1-2 degrees upwards static setting, but zero degrees may work depending on the installation angle of your engine/transmission in the chassis. That downward angle is what you're trying to match under acceleration since that gets you zero inclination of the U-joints. A typical downward angle of an engine/transmission in a donor vehicle is 4-5 degrees, but yours will depend entirely on your mounting scheme in the Locost.

The first time through, this may seem like a lot, and it is. However, if you do a little research, it becomes clearer. If you go with a well-known live rear axle design such as upper and lower trailing arms and a Panhard rod for lateral location (the original Locost design) there is a lot of information out on the Internet or in books already. Adjustability is a nice thing to have, if possible.

I hope this helps.

Cheers,

[nilocp]

Well thanks for that detailed reply!!

It is actually a standard early MK Locost chassis.

The concensus seems to be to have the brackets parallel to the diff mounting face.

But thanks anyway

Colin

[flyerncle]

Interesting and concise answer,spent 10 days near Caslsbad in Escondido lovely part of the states.