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+--- Thread: Radius arm comparison (/showthread.php?tid=3473)
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Radius arm comparison - JonE - 31-10-2019
I've been trying to locate radius arms in the index cards. Can anyone tell me which section, as couldn't find from first search in the obvious places. There might be some different name for them.
I've been comparing what I think is a 1931 arm with a 1937 Girling ended one, and finding it difficult to see ANY difference in stub angle that I would assume existed. However, the earlier arm is marked 1A-4024, which is noted the same as BM112 in the early Ruby parts list, but may thus not conclusively be earlier than 1934 i.e. it could be that both have "low frame" angle.
Has anyone got an obviously early arm (provenance etc) which has similar 1A 4024/4025 markings, or are they marked as per the parts list BM112 and 113?
Has anyone compared arms and found the angle difference obvious when viewing in the correct, sideways-on plane?
RE: Radius arm comparison - Robert Leigh - 02-11-2019
If you look at the parts lists in the archive they appear to confirm what I have always assumed that the angle remained the same as previously when the low chassis (early Ruby) came in. The same part nos are given for the arms and the machined forgings on the end of them. In the past I have selected and fitted replacement radius arms randomly apart from left and right selection, and I have not had any problem.
Robert Leigh
RE: Radius arm comparison - JonE - 02-11-2019
Robert, so if that is correct, we infer that the dropped position of the ruby radius ball directly addresses the 1 degree difference in nosepiece + the reduced camber (i.e. low) springing?
Have you ever used the later machined forgings though? i.e. did the Semi Girling line up fine too?
What I can't get my head around is that the lower camber ruby springing surely results in the axle being higher up in relation to the chassis which thus drops lower. And yet the ball goes down...
RE: Radius arm comparison - Robert Leigh - 02-11-2019
I have to admit that I have never tried to get my head round it all: if it works, accept it! When I had an RK fabric saloon it was fitted with a semi-Girling front axle and the later radius arms. With a high compression head and an SU carburettor it went quite well and I always managed to keep it on the road, and the later brakes although still uncoupled were a distinct advantage. I often set the handbrake part on before double declutching down to second when I was in a hurry!
Robert Leigh
RE: Radius arm comparison - AustinWood - 02-11-2019
Having bought that car from you Robert, I can confiirm that the brakes were very satisfactory.
Semi Girling on the front of split braked cars works well. It avoids the difficulties of front-back compensation.
RE: Radius arm comparison - JonE - 02-11-2019
The fact that the geometry seems to be viewed as slightly "magical" though is exactly the reason it's interesting! But there has to be a formula with the variables. That thus might suggest that A. the axle is no different and the radius arms ditto, and the spring was the same as first specc'd? Or B. that the radius arms are different, the axle is the same, and the spring is less cambered?
Some old forum postings that are useful for discussion:
(Alan) "reduced camber front spring? If so, what has happened is this: Flattening the spring effectively lowers the chassis relative to the axle. That means in a normal fitted position the back end of the radius arm will sit too high. You can just force them down to fit, but as you say, this twists the spring and loads up the shackles. A wedge fitted between the spring and chassis will correct the twist situation, but it will give increased castor angle. This might be no bad thing, but excessive castor does have negative effects, like heavy steering away from centre for example. The other way to do it is to lower the front end of the radius arms, either with special end forgings- like an Ulster, or by drop links commonly found in specials. Either way, the geometry stays correct.
(StuartU) early Rubies which used the light radius arms 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.
The latter needs some comparison of the arms from '23 up to early '34 Ruby as might contradict Robert's finding. But as Stuart replied to another posting about a similar situation (early chassis and late axle and arms):
"(Remember) it is a function of the camber of the springs as well. 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...
RE: Radius arm comparison - NBS - 03-11-2019
In the Austin Seven Companion M.Eyre makes reference to Grasshopper radius arms. Does anyone have details of these or know how they differ from semi-Girling axles?
RE: Radius arm comparison - Stuart Giles - 03-11-2019
(03-11-2019, 08:45 AM)NBS Wrote: In the Austin Seven Companion M.Eyre makes reference to Grasshopper radius arms. Does anyone have details of these or know how they differ from semi-Girling axles?
IIRC they differ in being underslung like the earlier "sports" radius arms, but also have the Girling style bolt fixing through the axle beam.
RE: Radius arm comparison - JonE - 03-11-2019
I found one in Ruairidh's excellent Pinterest database of images (below)... wonder if anyone has a better shot?
Incidentally, I also found reference to 2013 VSCC guidance for competing referring to to low frame Ruby chassis being eligible for "accurate Grasshopper and late Nippy replicas". What's the latter all about?? Has anyone got a real late Nippy on a low frame please??
IMG_5260.jpg (Size: 41.88 KB / Downloads: 205)
IMG_5297.jpg (Size: 59.69 KB / Downloads: 204)
RE: Radius arm comparison - Stuart Giles - 03-11-2019
As the saying goes: a picture is worth a thousand words
