jackb_ms Posted July 30, 2012 Share Posted July 30, 2012 On a 21 there is two positions for the radius arm, a lower one which make the radius arm more or less parallel to the ground and an upper one which makes the radius arms at about 30-40 degrees to the ground. My question is what is the effect of the position of the radius arm on the handling of a car? Jack Link to comment Share on other sites More sharing options...
Paul Deslandes Posted July 30, 2012 Share Posted July 30, 2012 two questions relating to 21 rdius arm positions in one week! Must be a record 😬 If the 21 is the same as the '95 vintage 7, there are two alternative radius arm forward positions. The upper 'ride' setting goes into a threaded boss and has no nut and the lower 'handling' setting is a plain, un-threaded boss so a nut is required with a longer bolt. The higher position apparently gives a slightly more comfortable ride and, incidentally, creates a bit of rear wheel steering with suspension movement, and the lower position has better handling but with much less rear steer effect. If you think about what happens to the radius arm either side when the car rolls, you can see how this might work at either setting. At the higher position the arm is at quite an angle to the horizontal so, as the outside arm front end drops with roll, the back end of the arm will tend to push that end of the deDion tube rearwards making the rear axle add to the steering of the car in the same direction of travel. At the lower setting the arm is horizontal 'at rest' so any angle change results in a much reduced change in fore and aft position of its rear end. Not sure why this results in a change in ride though. Paul Link to comment Share on other sites More sharing options...
myothercarsa2cv Posted July 30, 2012 Share Posted July 30, 2012 I moved to the lower position and thought it was more comfortable! Link to comment Share on other sites More sharing options...
jackb_ms Posted July 30, 2012 Author Share Posted July 30, 2012 Cheers Paul The issue it that I have a bit of bolt stuck in the lower position. I was pondering what was the effect of the position of radius arm and if it was worth the hassle to remove that bit of bolt So basically the in the lower position i should have less "steering" effect Jack PS: On a 21 there is two threaded bushes Link to comment Share on other sites More sharing options...
Paul Deslandes Posted July 30, 2012 Share Posted July 30, 2012 ah! hadn't spotted both threads were from the same source. Interesting that both upper and lower are threaded on the 21. I never did understand why the lower boss on the 7 was plain, other than it probably being a bit cheaper. You do have to drill through the inner skin on the 7 to get the bolt through and attach the nut so do make sure that it is definitely threaded and not just full of crud before you rely on it. Moving to the lower position would solve your problem of removing the broken stud. Paul Link to comment Share on other sites More sharing options...
jackb_ms Posted July 30, 2012 Author Share Posted July 30, 2012 It's the lower position which is buggered. I'm updating the suspension which was abit old... Maybe the solution would be to drill from inside... Jack Link to comment Share on other sites More sharing options...
richard bradbury Posted July 30, 2012 Share Posted July 30, 2012 I changed the position from upper to lower on my 1992 HPC as under full compression the de dion was hitting the chassis. It no longer does this and I feel the handling and ride are better. I used the nut and bolt from the upper position with no probs. Link to comment Share on other sites More sharing options...
6speedmanual Posted July 30, 2012 Share Posted July 30, 2012 In the upper position the geometry of the link introduced roll oversteer. In the lower, roll understeer. Lower handles better as toe-in on roll inproves stability and gets the rear tyres generating some slip angle (and therefore side thrust) without the whole car having to adopt a bigger yaw angle. It's all much more contained and tidy, especially on transitions. The "comfort" in the upper position is from the fact that as the wheel hits a bump its upward movement is accompanied by some rearward travel at the same time. The condition where the difference is most likely to be noticeable is when driving over transverse ridges. Sunken gas pipe repairs might show it, or the approaches to roundabouts with the thickly painted rumble strips. Go test! I don't have the pleasures of such choices as I've the Watt's linkage located deDion. Peter Link to comment Share on other sites More sharing options...
k.russell Posted July 30, 2012 Share Posted July 30, 2012 Like Richard I also had de-dion /chassis contact in the upper position on my 1992 HPC Link to comment Share on other sites More sharing options...
jackb_ms Posted July 30, 2012 Author Share Posted July 30, 2012 Thank you gentlemen It seems I need to have this broken bolt out then Jack Link to comment Share on other sites More sharing options...
6speedmanual Posted July 30, 2012 Share Posted July 30, 2012 You do! Anyone care to hazard a guess as to the toe progression curve per degree of roll for the two settings? I don't. (hic) Peter Link to comment Share on other sites More sharing options...
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