Trial by Trunnion

Ah, that life were simple! I am helping son, Erik, in getting his TA chassis ready for building up a vintage racing special. We understand that the TA is a superior racer because the overall handling is improved by the use of bronze trunnions at one end of the each of the four road springs. These trunnions are used instead of the normal shackle arrangement. The advantage is that although the effective length of the spring can change as its camber alters when the wheel passes over bumps (the main leaf slides in the trunnion bearing when this happens) the spring is located much more positively in a lateral direction, and the resistance to twist is also greater. The result is much improved road-holding.

Alas, when we examined the rear spring trunnion tube, we found that it was a cobbled, ancient repair where someone had used a piece of tubing with a slot cut into it and secured by a hose clamp to the cross member (Picture 1)

Picture 1 – someone’s idea of a rear trunnion tube repair!

Yep, we really wondered just what we had gotten involved with at that point. But all was not lost. An Internet search led us to the UK’s Brown & Gammon’s web site. Sure enough, they showed a trunnion tube repair item for the TA. When it arrived, we were impressed with the obvious quality of the pair but equally depressed when there were no directions for installation.

I contacted B & G as well as several experts on the TA and found that no one could provide any help. Oh, there were some suggestions such as “measure twice and cut once,” but that was of little comfort. I couldn’t even get anyone to tell me if the cross member extended through the chassis rails or if the thing was, in fact, hollow. I know, I know . . . it should have been clear to me that it was hollow. I was still intimidated, but finally Erik said to drill a small hole. Wow! Was I pleased to find that the cross member was, indeed, hollow. Now it was just a matter of making a mark and cutting off the faulty bit that still stuck out.

Once the cut was made, I found that the new piece did not just slide in because there was so much crud on the inside of the tube. A bit of work with an electric drill and a hone soon smoothed things up so the new piece would slide right in. At this point, I refitted the old spring (yes, yes new ones will be used in the final fitting), made sure that it was parallel to the chassis side and then had the exact distance to weld in the new tube. The deed is done. I am sure that a professional restorer would not have been daunted, but this rank amateur was in a deep quandary but things do seem to work out in the end.

Dick Knudson

Picture 2 – the starting out point for the trunnion tube replacement
Picture 3 – lining up the trunnion tube repair item in order to gauge where the cut should be made
Picture 4 – the deed is done, there’s no going back now!
Picture 5 – measuring to check the location of the trunnion repair is correct so that the spring is parallel with the chassis side rail throughout its length
Picture 6 – showing full length of spring; the measurement at the front of the spring where it pivots on the pin should be the same as the measurement taken in picture 5
Picture 7 – shows the repair section now welded in position
Picture 8 (which really should be picture 7!)
Picture 9 – the end result with both repair sections now welded in position
Drawing of the rear suspension arrangement

One thought on “Trial by Trunnion

  1. Erik Benson says:

    Hi Dick !. . . . The TA , and TB chassis are more suitable for racing , not only because of the sliding trunnions. It is largely due to the grade of steel used before WW2. . .there is a much higher carbon content. . .this gives more ‘spring’ in the performance of the chassis.-( you could say it performs a bit like a torsion bar ) Proof is. . the reinforcements welded into the post war TC chassis to stop it cracking in mid section. However. . .you have to be REALLY seriously using the race car to see, or feel the difference.
    Cheers, and well done to the young uns for following the ‘T’ path . . .Erik – ( an older one !! )

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