rear wishbone attachment

[campmi]

Help Members!

'65 Seven, a number of years in re-build, Cortina differential,rear wishbone attaches to diff in marginal (unsafe,I think) way. Diff has 1 x 5 plate welded to underside with a threaded nut(more heft than a nut) welded to it. Wishbone is attached to diff through a heim joint head welded to the apex of the wishbone with an aircraft grade bolt. Will this pull apart or otherwise rupture during stress of racing, autocrossing, track days. Any suggestions for robust fix.

 

Thanks for any help,

 

Mike Campbell, Abbotsford, British Columbia, Canada

[Peter Carmichael]

This is rather specialised and beyond most of our experiences, hence no response. It is also difficult to understand your description because you don't detail the orientation of the components.

 

By the sounds of it, the aircraft grade bolt is in single shear which isn't ideal. I presume because of the rebuild the bolt has been renewed.

 

The orientation of the heim joint is important. (I had to go net search to see a picture of a heim joint. How is it different to a normal spherical rod end?) 99% of us are running some sort of compliant bush at this joint which takes out the peak loads.

 

You need an engineer, not a bunch of car nuts.

[deanteam]

You seem to have a mixture, Cortina axles were used as orginal on Series3 from '68. But what you describe and Peter's discussion sounds like the 'A' frame as it is normally refered to. The orginal set-up was through flexible bushes and bolt into a welded bracket on to the differential casing. There is an option with a sherical bearing which appeared a bit later, but sevens are notoriously different one to the other.The best book to describe the early sevens is by Tony Weale. If you don't have the book I could fax the revelant pages if you e-mail number.

[deanteam]

Just another thought you could contact John Watson who has some records of the early sevens and might have some history.

[R2D2]

I guess the "heim joint" and the "rose" joint are terms from the same era. I think the original spherical joints were discovered in WW 2 on a crashed ME109 fighter. They were copied in the UK and known as rose joints and the information was passed to the USA where they are still known as heim joints.

 

Clearly a '65 car would have had a Standard Axle fitted originally with a conventional A frame and the usual rubber bush problem.

 

Lotus never fitted strenghtening plate on this axle and both axle and chassis failures were quite common as the Series 2 cars had relatively weak chassis compared to Series 1 cars.

 

I believe that the early Series 3 Lotus chassis were not too much different to Series 2 cars but did get better with time as Mike Costin gradually improved chassis strength and stiffness.

 

The strongest genuine Lotus Chassis was the Twin Cam SS, which I think was the basis of the original Caterham in the late seventies/early eighties.

 

It was quite common to fit the Mk1 cortina axle but I am not sure that this produced a total fix. I am syre that half shafts and diffs were better but the "twist" produced by the A frame still caused problems of oil leakage and deformation of the rear banjo. The Lotus Cortina fitted with an A frame certainly caused problems.

 

I can't see why a properly sized spherical joint should be a problem and this solution was certainly used on Cortinas in the early sixties. I think that the bracket fitted to the axle should be properly engineered and be designed so that the bolt is in double shear. I would also use tapered alloy spacers between the bracket and the joint to ensure that bolting forces were correctly transferred into the "ball" and to be sure that there was enough angular capability to stop the body of the joint fouling on the bracket.

Obviously the diameter of the rod end needs to be reasonable to take tensile loads and it needs to be lockeinto the correct plane.

 

Denis Ortenburger's book " The Legend of the Lotus Seven" has some pretty good pictures of modified axle arrangements and DSK engineering in the States built quite a few successful racecars including a BDA with turbo so I think they have a good idea of the detail needed.

 

If you are really concerned then new A frames for Series 2 cars are probably available from Redline (if current Caterham units don't fit) and fitting the correct bracket to the Cortina axle would be really easy. You could then use a standard Live Axle bush (buy several they wear out quickly) or a Powerflex. (Powerflex now make the correct length bush).

 

I would still worry about twisting the axle and I would fit a plate similar to current Ital axled cars. Photos of this are in either Tony Wheale's or Ortenburger's book.

 

I still think that a five link arrangement, as per the Series 1 is probably better as it does reduce twist but it needs quite a few chassis mods to fit an possibly does devalue an S2 car.

 

[campmi]

Many thanks to all who replied to my query regarding rear wishbone attachment. for your infomation the heim joint (rose) is in the horizontal orientation and the aircraft hardened bolt passes vertically through it and attaches to the differential. I will use an automotive engineer to design a cradle which attaches to the differential, with a horizontal bolt holding the wishbone to the diff.

 

I will do my best to uphold the reputation of the Lotus Seven in vintage races here in the Pacific Northwest of Canada and Washington, Oregon, and California next year.

 

Regards....Mike Campbell