TC Steering – the drag link

I have read a number of very interesting articles on TC steering and the Bishop cam steering box over the years.

I hope this short article on setting up the drag link will help others to avoid the nightmare scenario of the drag link parting company with the Pitman arm (drop arm) as mine did on a left-hand bend some years ago – leaving me without any steering.

There are two points to consider when setting up the drag link. To ensure:

  1. the peg in the steering box is in the middle of the worm when the wheels are in the straight-ahead position. (The point of minimum play)
  2. there is sufficient rotational clearance on the drag link at each end of the steering travel. It was a failure to set this up properly that caused mine to “drop off” the Pitman arm.

Assembly

Before attempting to set up the drag link, ensure that both ends are in good order (the ball is not oval, etc.) and they are assembled correctly.

The N/S drag link end is identical to the track rod ends. Consisting (from left to right) of a dished screw end, dished plug and spring.

The ball fits between the screw and plug, and as for the track rod ends is fully tightened then backed off between ¼ and ½ turn before being locked in place by a split pin.

The O/S drag link end is different. This consists of four parts, (from left to right) the dished plug with a plain end, dished plug, spring, and screw with a plain end. On my drag link, this is different from the track rod ends in that it has only a single slot in it, rather than the two slots of the track rod end. Additionally, the slot cut into the body of the drag link end is longer. The screw is fully tightened and then backed off up to ½ turn as with the N/S drag link end.

When assembled, both the springs are on the O/S of the ball. This arrangement provides a spring “buffer” between the wheels and steering box for both left-hand and right-hand movement of the wheels.

Centralising the steering

  • Jack up the car under the centre of the front axle to maintain the springs in the fully loaded position and remove the N/S wheel.
  • Disconnect the steering box end of the drag link and loosen both the lock nuts.
  • Turn the steering wheel from one extreme to the other ensuring the peg hits the end worm in the box (sometimes the left-hand travel is restricted when the Pitman arm hits the sump). Counting the number of turns.
  • Turn the steering wheel back to its central position. (If unsure, remove the top cover from the steering box and check the position of the peg in the worm). A patient and willing volunteer is a useful “special tool” at this point to hold the steering wheel in position!
  • Reconnect the drag link and set the front wheels to the dead ahead position by rotating the drag link shaft to change its length. Like the track rod, the opposite ends have different handed threads.
  • When correctly set, tighten the lock nut on the steering box end and continue with the next stage. (At this point you no longer need your probably now very bored volunteer).

Setting the rotational clearance

The axis of rotation of the Pitman arm and the wheel on the king pin are different. To accommodate these differences the drag link is forced to rotate about its longitudinal axis as the steering moves from full left to full right lock. If the relative positions of the slots in the two drag link ends are not set correctly, then at one extreme the drag link ends will bind on the two steering balls, tending to twist the ball out of the drag link end.

Normally, there is insufficient clearance for the ball to “pop out” of the drag link end. However, over time both the ball and the drag link end will wear until, as with me, the drag link will twist itself of the ball – helped by gravity in the case of the O/S end. The wear was particularly evident on the slot in the O/S drag link and on the dished plug that fitted into the spring.

At this stage you can demonstrate this effect:

  • Fully tighten the lock nut on the O/S drag link end.
  • Go to the N/S of the car and twist the drag link full clockwise.
  • Twist the N/S link (wheel end) fully counter clockwise and pinch tighten the lock nut.

You will notice that as you turn the wheels in one direction the free rotational movement on the drag link will increase, while turning the wheels in the other direction will actually force the partially locked N/S end to turn on the shaft.

To properly set up the relative positions of the two drag link ends:

  • With the lock nut tightened on the O/S drag link end, pinch the N/S lock nut tight.
  • Move the steering to the extreme left and rotate the drag link back and forth until the outer casing hits the balls, noting the clearance.
  • Repeat with the steering at the extreme right.
  • Rotate the N/S drag link end on the shaft until the full left/full right clearances are approximately equal and fully tighten the lock nut.
  • Personally, I go through the above procedure a couple more times, probably more out of the fear of finding myself heading towards an oncoming vehicle rather than out of necessity.

Conclusion

The other components of my steering are in relatively good order and with the drag link set up as above, I can typically achieve about ½ to ¾ inch play in the steering wheel; I think that by TC standards this is acceptable.

I hope this article has not scared everybody into running out to the garage to check the clearances on their drag links, but considering the possible effect, I would recommend doing it.

Paul Ireland

Ed’s note: Publishing Paul’s article has given me the opportunity of showing a recently taken picture of his car (TC9701). His attention to detail in setting up the drag link has obviously paid off since he did the job a few years’ ago and he has covered thousands of miles in the car since then.

6 thoughts on “TC Steering – the drag link

  1. Geoff Piddington says:

    Back in the 60’s there was a modern type drag link marketed for the TC. I have only seen one in recent years and the owner had no idea that it was not original or its source – pity.

    • Colin Boucher says:

      I cross loop two zip ties under my drag link and over the Pitman arm as a bit of extra security after setting up rotational clearances. I had one drop off as well once…lucky it was slow traffic. Not a nice fee;

    • Opentopman says:

      Hi Chris, where did you source rose joints? What quality joints do you use? Did you use your existing Track and Drag Rods, or replace the whole steering system?
      Regards
      John Wylie (Opentopman)

  2. Chris Parkhurst says:

    John they are quality stainless steel rose joints which I think have a 3/ 8 thread this screws into a threaded cross shaft. I got them from Terry Patchett I think about 5 years ago.
    They are readily available from most race car spares suppliers you should have little difficulty in sourcing them but if you need help come back to me .
    Chris

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