Evening All. Can anyone tell me if the radius arm ends (part no BM112/3) changed in diameter up to but not including the introduction of the ruby twin mounted type? I am rebuilding a front axle from various parts using an original chummy axle beam (XL part number) and using radius arms (with 1A part no.) Conversely, did the hole in the beam (in which the radius arms are fitted) change in diameter? The reason for the query is that the arms are an unusually sloppy fit in the axle beam whereas they are normally a very snug fit and often a b****r to dismantle.

Location: Farnham

If the front axle and the radius arm end part numbers didn't change until the two bolt fixing (which seems the case from looking at my Spare Parts Books) then the fitting in the axle wouldn't have changed.
Can't see why wear would be here- maybe someone opened them out after having difficulties dismantling

Tony.

Location: Malvern, Melbourne, Australia.

Hi Tony. No sign of axle hole or radius arm end being fettled so it will remain a mystery. Ive always found them a very snug fit in the past.

Location: Farnham

Hi David, unfortunately, if you only have the one (early) axle and the one pair of radius arms available, we don't really know whether its the holes which are too large or the pins on the radius arm ends which are too small??

Ian Mc.

Location: Shropshire

Quite. On acquisition the chummy had a ruby axle but with pre ruby arms. I built up an 'XL' axle assuming the arms would be the normal tolerance fit. Unfortunately I dont have any other arms to hand to compare.

Location: Farnham

I have to say, David, unless the Ruby axle was knackered, I would have kept that and acquired some Ruby radius arms to suit! I have never checked the hole sizes, maybe the Ruby axle holes for the "main" taper pin are a little smaller than the standard axle and the radius arms that you have were taken down to suit?? (pure speculation !!)Were the radius arm ends a good fit in the Ruby axle?

Ian Mc.

Location: Shropshire

David, there is a minor point here that early Rubies which used the light radius arms none the less had the "low chassis" with rather less rear spring camber than your Chummy. I believe (no doubt someone will correct me if I'm wrong) that the chassis nosepiece was altered to retain the standard king pin caster angle when the rear of the chassis was effectively lowered. Consequently the shaft of the radius arm that passes through the axle is at a slightly different angle to the pre Ruby version. I think the forging has a different 1A number on it.
With the light radius arms,all this means is that you will have to rotate the radius arm slightly in the axle beam to get the right caster angle. You might find this makes the fitting at the ball end a little more awkward but I think you'll manage. Probably worth checking that both the radius arms at least have the same number on the forging.
Of course with late Ruby Girling radius arms and axle on an early car you can't rotate the radius arms so, if you just bolt it on, it tries to twist the front spring.
Regards, Stuart

Thanks chaps. Unfortunately I no longer have the ruby axle as I swopped it for other bits so direct comparison is no longer possible. I have one other spare early arm which is a similarly less than tight fit. I can only assume the axle holes may have been over fettled in the past although there are no obvious witness marks.

Thanks for all the comments

Location: Farnham

Stuart, and others

Are you saying if like in my case, I have a later semi-girling front axle with the heavier radius arms under an earlier uncoupled chassis, that the castor angle will be less than ideal?

The steering on this car is not that great. I know I am up for a rebuild on the S box, however am wondering if I have other factors at play as well.
With huge lift (6") trials rear springs under it the steering onroad is diabolical. With 2" lift springs car is tolerable but not great.

I had borrowed a castor measuring gauge at one point but in the midst of back axle rebuilds did not get around to using it. Maybe I should do so.

I recall about 6 degrees being ideal in race or fast road A7's

Any comments folks?

Location: Christchurch, New Zealand

Yes, exactly so. This is a function of the camber of the springs as well, of course. There are various fixes if the late front axle and radius arms appeal. Some people lower the radius arm ball joint, I understand. Obviously this involves removing its rivets and bolting on a plate to lower it.
Funnily enough if you use "flat" springs all round as per Ulster or Nippy the geometry works out about right!
The Austin Service Journals are said to quote a caster angle of five degrees. I took this info from the 750 MC "Companion", being too lazy to drag out my photocopied service journals to check.
Massively cambered rear springs will obviously bring other handling problems. I think the thing would be to decide what spec of springs you are happy with then think of ways to adjust the caster to suit.
I would certainly expect the steering with your present set up to be worse than the average Seven.
Regards, Stuart

Thanks Stuart
The car is currently tolerable as it is back in medium height spec for back country long distance travel.

In my case I use the sports model-ish for a variety of uses and so change the configuration of suspension and front axle to suit. Well the latter I will, when I get around to rebuilding the genuine, I think, dropped axle that I have...

For trialling I have gone to some lengths to ensure that nothing drops below the chassis line in the middle of the wheelbase. Thus dropping the radius arm ball down, whilst I have done this before on a Chummy, is not what is desirable here.

Is there another way to shift the bottom of the later front axle forward? I guess I could crank and extend forward the later arms, but that seems like a lot of work... Any other ways?

Location: Christchurch, New Zealand

To continue to use the late radius arms, the only other mod I have seen suggested involves cutting and welding the radius arms to alter the angle (which won't be a huge amount obviously) then packing the ball forward. I've never tried this. It occurs to me that, for trials,it might be easier to give up the advantages of the heavy arms and just fit early arms when you change to the steeply cambered spring? Perhaps one could fabricate an addition to the early arms to stop them rotating using the upper hole in the axle?
Regards,
Stuart

Good idea Stuart
The other advantage will be an improvement to my steering lock. I have relieved the current R arms and S arms however with the earlier R arms I should be able to gain more lock again.

Location: Christchurch, New Zealand

I intend to use the heavy radius arms on a short chassis, with a cambered front spring. Can you give any idea how much I need to lower the pivot? I presume it would be advantageous to gain additional camber for a fast road/sprint car? If so, how much is enough?

John

Good tip I received was to get a metre+ of 1/2 inch solid bsteel rod, pop one front hub assembly off, insert the long rod, lower the corner down so the axle is parallel side-to-side and then you can easily measure the rearwards tip of the kingpin. A long rod makes this all a bit easier and accurate. Not so simple if the kingpins are tight or if you have Girling hubs.


Off to bed for an early start tomorrow. My last week flying 757s.
Soon to start a 737 course. Yuk!

John, my very elementary trignometry suggests that a 5 degree increase in caster will drop the radius arm ends roughly 2". Not to be taken as a precise dimension! Of course, both front and rear spring camber will affect the actual amount by which you might wish to alter the existing caster angle, having determined its current state by the method Rob suggests. Not being a suspension designer, I tend to stick to Austin's published figure of 5 degrees.
Martin Eyre's article on front suspension overhaul in the 750MC " Companion" makes some useful points in its concluding paragraphs. I'm sure you have a copy!
Regards, Stuart