KING PIN ANGLE BIG 7

[Dennis Nicholas]

BIG 7 AXLE
I notice that on page 182 of the Austen Seven companion that the Official Austin Company "Austin Seven Journal October 1937" states that the swivel pin angle is 0 degrees.
I am just checking my big 7 axle for alignment.  With 5/8" rods in the king pin eyes I noted that they were very slightly out of alignment......as also were the 1/2 inch bars vertically in the radius arm bolt holes., but 1/2" and 5/8" bars seemed in alignment at each end  I clamped the king pin eye at one end horizontally in the vice and using a Stilson with extended tube (motor bike fork tube) on the other end I mightily heaved to untwist (I nearly broke a rib when I pushed my body against the tube to assist the twisting!!). It is surprising just how bendy and elastic an axle is........however it must have done the job in the end and twisted just over the elastic limit as the bars are now as near as I can see in alignment, as can be seen in the photo below.

   

The next bit I notice is that the 'swivel pin' (king pin) angle certainly is not 0 deg. as per the Austin Journal for the normal 7 axle.  Using a protractor with spirit level attached I make the angle 82.5 degrees, inwards at the top on each side.

   


Does anyone have the info on the BIG 7 axle or know where I can find it.  I must assume that the angle of the stub axle will eventually give the correct CASTOR angle of the wheel.......we shall see. (I have an original old dunlop castor camber wheel aligning tool so should give old degree measurements instead of any modern fangled degrees   )

[Parazine]

Hi Dennis,

I went through exactly this on my Big 7 some years ago, I thought that I had a bent front axle but the problem turned out to be a bent radius arm. However, I carried out the checks that you have been doing while I had the axle apart.

Make sure you know the terminology, there is a good synopsis here: https://www.lespaulsmotors.co.uk/4-wheel...ment-faqs/

The correct figures for the Big 7 are:

Castor angle: 5 degrees
Camber angle: +1 degree
Swivel pin angle: 6.25 degrees
Toe in: parallel to 1/8"

These are from the "Austin Service Journal", page 202, October 1937.

The Swivel axles should have an appropriate "set" on them to give you the correct camber angle, this is not adjustable.

I found that, in common with many 1930's cars, the Big 7 did not have enough castor angle, producing an unsettling lack of directional stability. Apparently, this was done to make the steering lighter at the expense of driveability! With a pre 1936 Austin 7, the castor can be adjusted by making sure the radius arms are set correctly but on the Big 7, this is not possible as they are fixed. I made up a hardwood wedge, inserted between the spring and the chassis to try and force the axle forwards at the bottom and increase the castor angle, which was partially successful. I have heard that some owners have made up a special drop plate to put the radius arm ball joint down lower but I'm not sure that is necessary. In the end, I got the car to handle reasonably well, given the extra weight although by comparison with a Vintage Austin 7, Big 7's are never going to feel good!

Good luck!

[Stuart Giles]

Interesting to see that Austin factory spec. for the front axle. When I reworked the big Seven front axle on my special, I set the camber to 0°. I also fabricated the radius arm setup in the photo below which allows proper adjustment of the castor angle -the 4 tubes are joined together at, and use a 1/2" rod end as the pivot on the front crossmember. BTW I was  getting a fair bit less than 5° castor when the car had the factory radius arms on it.

[Parazine]

I couldn't measure the castor angle very accurately but it was between 0 and 2 degrees on the Big 7. Whether there was something worn, spring settlement (it was a new spring although I doubt an Austin Motor Co part) or something else bent I don't know. It could have been a bend in the chassis, the car had suffered a fair wallop at the front at some time. I changed radius arms and the axle beam so I was confident that they weren't bent, the axle was measured on a surface table with bars.
I know a lot of 'thirties cars suffered from the lack of castor problem, I had a 1937 Rover 14 at one time which was dreadful to drive, no castor and 2 1/2 inches play in the (non adjustable) steering box!
I like the adjustable radius arms, that would have been a much better solution to my problem!