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Rotoflex Rear Suspension TUNING, The LO-TEC Way!


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REAR SUSPENSION TUNING, THE LO-TEC WAY

By John R Davies (Cheers)

 

     The worst feature of the Triumph swing axle and variants is the astounding amount of positive camber that can occur. Positive camber means that the top of the wheel is further from the car's centre line than the bottom. When you are going round corners, positive camber is BAD!

  

     In a corner, the only thing that stops you flying sideways off the road is the grip of the tyres. For maximum grip, as much as possible of the tread should be in contact with the road, and in an ideal world the wheel would be always vertical on the road. However, because a tyre must deform even slightly, some negative camber is desirable. That is, the wheel top should be slightly nearer the car centre line than the bottom. Then, as the tyre deforms, all the tread    contacts the road.

 

     But in most cars, the suspension allows the wheels to move up and down and no one has been able yet to design a suspension that do so and does not allow some camber change. By having springs so stiff that the suspension hardly moves at all, Formula One racing cars come closest to this ideal, but at the expense of an impossibly rough ride. In our real world, with roads less than smooth and a general dislike of being shaken to death, we need softer springs and a suspension that moves up and down a bit.

 

     The standard Triumph swing axle suspension is particularly bad at camber angle change in movement. Can this be improved? There are many ways of doing so, but how can you decide what might be best, without a series of expensive experiments on your car?

 

     How can you decide between a spacer under the spring, different wishbone mountings, or drive shaft lengths? Which to use, alone or in combination? You need a lot of time, money or a degree in mechanical engineering. But here is a way to evaluate any possible modification of your suspension and assess the likely effects with some graph paper and cardboard on the kitchen table.

 

    You need the paper and cardboard to make a model of your rear suspension. Not a real model, but full size and in only two dimensions, which is quite enough for measuring camber. The graph paper needs to be about 600 x 460mm (2 x 1 .5ft), much bigger than the usual A4 pad, but tape some sheets together, or buy it by the roll at a stationer�s.

 

     You will also need to know the precise dimensions of your suspension, and the modifications you want to make. Many of these numbers are available from your workshop manual, or from club magazines (Ref: 1), but unless you are certain that your car�s parts are original, it will be best to get under and get measuring.

 

Measure or know for certain:

 

1/ HALF the length of the spring (eye centre to eye centre) (520mm/20.5")

 

2/ EITHER (Non-Rotaflex)

          Length of drive shaft, U/J centre to tyre centre.

And: Height of vertical link, from drive shaft centre to spring eye.

OR (Rotaflex)

        Height of Vertical link, wishbone eye centre to spring eye centre (232mm/9.1")

And: Wishbone length, chassis bracket eye to vertical link eye centres. (235mm/9.25")

(The dimensions given are for my own Rotaflex Vitesse, for you to try out this method.)

 

jd1.jpg

 

Making the model.

     This is how to make a Rotaflex model. Other variants can be made in the same way. Cut two strips of cardboard, and join them together at one end with string, a paper clip or other means of allowing them to pivot on each other, like the hands of a clock. Mark them so that the distance, from the pivot point to the marks is the same as the actual lengths of the wishbone and vertical link.

 

     On your graph paper, draw a vertical line, parallel to the right hand side of the paper. This is the centreline of the car. Draw another line at right angle to this, about a third of the way down the paper, and measure along it HALF the spring length. This marks the NEUTRAL position of the spring.

 

     Using a piece of string as a compass, draw an arc to show how the spring moves from bump (up) to droop (down). Remember that the spring is bolted to the top of the differential, which is four inches wide, so that this centre part of the spring hardly moves. Centre your "compass" two inches along the spring line away from the centre line.

 

     Mark the limits of spring movement. Unless you have fitted wider tyres or other modifications, in which case me e these as well, use 50mm of bump and 66mm droop.

 

     Now draw a vertical line 285mm(11.2") to the left of the centre line and a point on it 158mm(6.2") below the spring neutral line, to mark the normal position of the wishbone bracket.  Look at the diagram if this is not clear.   Please note: these dimensions are correct, I think, for the Rotaflex Vitesse. GT6s may be different, though I doubt it. Your model is now complete!

 

Using the model.

     To operate the model, place the cardboard vertical link/wishbone assembly on the paper, and fix the inboard end of the wishbone to the wishbone bracket point with a drawing pin. Set the top of the vertical link on the spring movement arc, and move it from bump to droop. As you do so, you will see that the vertical link is not always in a vertical position. In fact, it will change, from slightly negative camber (top of wheel in. bottom out) in bump, to wildly positive (top out, bottom in) in droop. The wheel is always parallel to the link, so this camber is the same as wheel camber.

 

     Use a protractor, that semicircular piece of plastic that you measured degrees with in school geometry to measure the camber angle. Line up the straight edge of the protractor with the edge of the "vertical link", and read off the angle from the vertical lines on the graph paper. You may have to slide the protractor up and down the "link" a bit to find a good place to measure, especially when the angle is near zero. Remember that negative camber (good) has the top of the wheel, or "link" nearest the centre line.

     

       The instability of the swing axle is due to the amount of positive camber that it produces, and the change in camber across the range of movement. Remember, when cornering hard the car leans outwards and the outer wheel is in bump (up) while the inner wheel is in droop (down). Very confusing for the poor car! The Rotaflex version is little better.

     

       Now that we have a model, we can experiment and find the best modification, at no cost and without dirty hands! You will find that changes in the suspension are easily simulated. Remember that raising the spring and lowering the wishbone bracket are the same in terms of this model, although the first will do more to lower the centre of gravity of the car. Measure either change along the vertical line for the wishbone bracket.

 

 

By manipulating the model, some very interesting figures result:

 

Table 1. Wheel camber from bump to droop.

 

Different spacers under spring.

 

                                         Bump     Neutral      Droop

Normal position                    -1           +2            +8

25mm spacer (1")                   -1         +1              +5

50mm spacer (2")                  -0.5        +0.5            +2

               

(All figures are in degrees.   "+" = positive camber.  "-"  = negative camber)

     

      Table 1. shows that a two-inch spacer gives the least camber change from bump to droop. However, a spacer this tall is too big to fit without modifying the floor of the rear body tub to accommodate the higher spring. Let�s look at the effect of changing the wishbone bracket:

 

Table 2. Wheel camber from bump to droop.

 

Different wishbone brackets.

 

                                  Bump     Neutral          Droop

Standard bracket              -1            +2             +8    (as above table)

"Racing" bracket                -2           -0.5           +3.5

Eickoff 'A' bracket                0             +1            +3.5

Eickoff  'B' bracket              +1            +1            +1.5

 

Differences from standard bracket

 

"Racing"           =25mm    (1") down. 5mm (0.2") out

Eickoff "A"          = 44mm   (1.7") down

Eickoff "B"          = 66mm   (2.6") down

 

jd3.jpg

 

     These three designs of wishbone bracket all lower the eye below the standard position. The �Racing� design also pushes the eye out by 5mm. The Swedish engineer, C.H.Eikhoff described the other two designs in an article in the TSSC publication "Turning Circle" (Ref: 2). He predicted, from theory and detailed engineering drawings, both the bracket position for the least strain (plunge) on the Rotaflex doughnut (Eikhoff A), and for minimum change in camber (Eikhoff B).

 

     Without boring you further with tables, it is sufficient to point out that Eickoff was right, at least about camber change. His "B" bracket does produce the minimum of camber change, but the camber always remains positive. The "Racing" bracket on the other hand imposes more change, but mostly stays in negative camber.

 

     Further experiments show that the combination of a 25mm spacer to lift the spring and the "Racing" bracket produce the optimum position:

 

Table 3. Wheel camber from bump to droop.

 

                                                                                      Bump    Neutral     Droop

25mm spacer under spring PLUS "Racing" bracket         -1             -2            +1

 

Observation with a jack under the car, and testing on the track confirm that this does happen.

 

Further experiments.

 

     As you can see, it would be easy to make and modify a similar model for cars with the swing axle, by making a cardboard drive shaft and changing the length. Different spring lengths can be modelled. I am sure that even swing spring cars may be modelled, but I have not done so. Would anyone with such a car like to give it a try?

 

jd2.jpg

 

Above image shows how to accurately place the rotoflex bracket to factory specs.

 

References

1 / John Thomason "Suspensions sussed". Courier August 1993, p31,

2/ C.H.Eickoff "Rear Suspension Talk", p9. Turning Circle Feb.1989

(Both published by the Triumph Sports Six Club of the UK.

       Back copies or reprints may be available)

 

*side note by Dave*... I may or not agree with what is described as optimum! I'd personally like -2 deg of camber static, growing to -3.5 over the first 1 inch of bump....which the racing bracket is most likely to give. (DISCUSS?)

 

Please ignore wonky tables, you can make sense of it I am sure? I will learn how to use the table command setup (oneday)

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