ok good news, so to my earlier "thought" then...
does it amount to the same thing if....
the new wishbone is placed in position bolts inserted loose, the strut is pulled clear and the wishbone allowed to find its own position as dictated by the rear bush, being vertical/fitted flat it will presumably ammount to the same end result, that being a level wishbone. Then simply do the bolts up as is...? same fing no?
Must say i think i would prefer the car off the jack then the bolts tightened at the true ride hight, but doing so on a drive way is such an almighty pita, would this method surfice?
seems easier to me?
Not keen on that idea as the wishbone does not sit at 90 degress to the rear bush and differs dependent on engine fitted (and hence weight at the front) and suspension setup
agreed, so how is done to the appropriate level of accuracy with a ram?
With a digital level?..... Rule of thumb is the wishbones are horizontal so that the roll centers comply with the original build, we use a ram because we can, just because members haven't heard of this before doesn't make it wrong!
forgive me, roll centers?
Taken from the wim forum (without pictures)... enjoy.
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The roll centre is an imaginary point about which the vehicle rolls. The calculation process that’s followed to find the roll centre varies a little according to the suspension design; this diagram shows the approach for double wishbones where one wishbone is angled to the horizontal. The lines of the wishbones are extended until they reach a common point – ‘A’. A line is then drawn that connects ‘A’ to the centre of the tyre’s contact patch – ‘C’. The roll centre is where this line crosses the centreline of the car – ‘R’.
The amount of body roll that occurs with a given cornering force largely depends on the relationship between the height of the centre of gravity and the roll centre. Raising the suspension roll centre, or lowering the centre of gravity, will decrease roll.
However, while having a high roll centre therefore sounds attractive, it has significant negatives associated with it. In fact, most well set up vehicles run a roll centre at, slightly above, or slightly below ground level.
So what does this mean to you?
Effectively, the only thing you are interested in is the centre of gravity of the sprung mass. Imagine attaching a string to this point and pulling - sideways to simulate a cornering load, fore or aft to simulate acceleration or braking, diagonally to simulate a combination. The attitude a car adopts when subject to these forces is dependent upon how stiff the springs and anti roll bars are, and of course these will be different front and rear. Due to differing roll resistances front and rear, and the fact that weight is transferred diagonally, the car will almost always be operating in what Allan Staniforth refers to as 'skewed roll' - ie. a combination of roll and dive/squat.
The main implication of the roll centre locations are that they are used to calculate diagonal weight transfer, which in turn can be used to derive suspension deflection and individual tyre loads (which in turn influence understeer/oversteer balance).
The traditional rule of thumb was that that the roll centre is lower at lighter end of the car, but many other factors have to be taken into account - spring/roll bar stiffness, tyre sizes, front and rear track, CG location etc., so this rule is by no means hard-and-fast.
Probably more important to make sure that your roll centres don't move about much in relation to the sprung mass, as movement changes the diagonal weight transfer and can lead to very uncertain handling characteristics.
Steady state under constant lateral acceleration:
The total amount of lateral weight transfer is determined by the CoG position and track width.
Some of the lateral weight transfer occurs through the suspension links. This is what I term unsprung weight transfer. The amount for each axle can be determined from the weight on the axle and the position of the roll center for that axle. Obviously the answer can and usually will be different for each axle. It's independent of spring rates etc.
The remaining lateral weight transfer i.e. total minus unsprung is what I call the sprung weight transfer. This occurs because the body rolls and deflects the springs, dampers and anti-roll bars etc. The body is usually stiff enough that the front and rear roll can be considered identical. In a steady state the sprung lateral weight transfer is divided between the front and rear axles in proportion to the roll stiffness i.e. front twice as stiff in roll as the rear means the front sprung lateral weight transfer will be twice as much as the rear. There's a slight subtlety - it is actually the roll moment which is divided in this ratio. If the front and rear tracks are different then the weight transfer will be proportional to roll stiffness divided by track, not just proportional to roll stiffness. But to avoid getting a headache let's pretend the front and rear tracks are always the same, in that case the weight transfer is proportional to roll stiffness.
If you change the springs and/or anti roll bar stiffness to change the ratio of front versus rear roll stiffness then this will have effect of increasing the sprung lateral weight transfer at the end you are making (relatively) harder, and reducing it at the end you are making (relatively) softer. The total is fixed, so if you take 50 lbs of weight transfer away from one end, you will end up adding 50 lb
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Topic: Just Been To WIM (Read 2864 times)