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I noticed that my rear spring perches are fairly uneven as I was trying to tune in my ride height and noticed the side that I needed to lift was already much higher on the adjustment threads than the one on the left. The gas tank is half full in this photo. Is this difference from the variance in spring rates, or the weight of the fuel, or possibly the suspension binding?
Well, there is ~5mm in aduster difference to be "even" height because the rear right is heavier than rear left, and the front side to side difference in weight play _some_ too in how corner heights play..plus a small amount of spring sag and tolerances..
Thats neat, but I see the same on my car.
My spreadsheet says turn the right rear up 2.2 turns.
Different spring rates can cause that to happen and presumably more so as they age and get tired albeit the fronts are probably more likely to do that. I understand that the stock springs are very accurate or at least they are when new. A full tank of fuel weighs about 70kg so fuel status can make some difference
Bottom line the desirable situation is even weight balance but most do not have access to corner scales. If you can find a way to get the wheel loads measured that would give you a better starting point.
I suspect most 928's on the road have a pre-dominant diagonal axis load wise. If you adjust solely on the basis of ride height you could end up way off corner balance wise. I take my 928 to the police inspection yard for yearly inspection- their test rig actually measures wheel weights- mine was out a bit when it was inspected a few weeks ago.
Height adjustments are interactive corner to corner. Best/safest way to resolve the issue is to check the corner balance, monitoring wheel weights as you adjust the spring positions. Else there's a risk that you'll have the car level at rest, but two opposite corners carrying more that the other springs.
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A SoCal owner hurried the installation of his new springs and Koni shocks to make a fun drive near my home in Glendale. Keith W had his '82 pretty well settled and was leading a train of cars at a brisker pace than I could with rev limiter issues. The car with the new suspension was shifting and yawing side to side noticeably from behind on transition between pulling and trailing throttle, and much more severely on braking. Driver tried to keep up with Keith around a limited-visibility bend in the road, ended up hard on the brakes and into the mountain.
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Advice: Get the corner balance at least close. Set the two fronts at the same adjuster positions, two rears at the same adjuster positions. Move the two rears the same number of turns up or down to get to target ride height. Then move the two fronts the same number of turns up or down as needed to get to the target ride height. While there are differences in car weight left to right, they will normally be pretty close. Put 150# of sand bags in the driver's seat, and half a tank of gas in it before final height adjustment. If one end of the car measures level but the other end doesn't, no amount of spring adjustments will cause both ends to be level at the same time. Focus on the same adjuster heights side to side on an axle. Drive the car to settle things, and do the adjustments again. Rinse, repeat, until the car is settled at you target heights. Then full alignment without lifting, and the corner balancing if it's still needed. It's a good idea to check corner balance if the car doesn't get to target height and level pretty easily. Remember that the suspension needs to be fully settled to get accurate corner weights.
another thing to consider if you drive the car into a driveway at a 45 degree angle and always come in the same way,
the suspension will take a set to one side and possibly cause a lean.
The cure ,
drive it onto a driveway with the other front wheel leading with max deflection,
then stop and let the car sit their for a few mins,
this will let the suspension take a set to the opposite way
If you read the WSM you will see that it suggested to keep both adjuster collars within a turn of each other.
As others have pointed out an unseated spring will cause some of these issues
Thanks for the feedback everyone. I'll get the car up on stands this weekend and inspect all the suspension bits to make sure all the springs are properly seated and all the suspension bits are installed properly. I'll make an appointment to get the car on some scales as well, I know a guy who can help me with that.
Follow the guidance on getting the ride height set and settled with perches even side to side. Then decide if you need to go any further. You'll want to have the car aligned if the front height has changed. You'll find that you are 90%+ of the way there.
I was able to get the car up in the air and give the suspension a good inspection of the front and rear components and discovered a couple of the springs were not properly seated in the upper cups. I was able to get them properly seated, and then I set the height settings equal left and right per the suggestions above in preparation to take it into have it weight balanced.
On a related note, should I loosen all the suspension bolts and then re-torque with the wheels on the ground? I remember doing this on my Audi's but I am unfamiliar if this is required on the unique 928 suspension.
Not sure about 'all of them' but the front lower control arms should be tightened on the ground.
Is that referenced anywhere in the WSM? I was going through the suspension installation and alignment sections and could not find anything that referenced this technique, but my common sense tells me that anything that pivots with suspension travel should not be in tension at the rest position.
Is that referenced anywhere in the WSM? I was going through the suspension installation and alignment sections and could not find anything that referenced this technique, but my common sense tells me that anything that pivots with suspension travel should not be in tension at the rest position.
Michael,
The note about tightening is in the part details dialogue supporting the image of the front suspension. My interpretation of this is that this procedure only applies if the lower arm has been removed the logic being about where the rear bush sits in the clamp, the front bush being set when the joint was made in the factory- that and the fact that the same part is used either side so the assumption logically is that the arm sits horizontal when the ride height is at the factory specified value. If you did not remove the arm then no need to unbolt and re-torque on the deck as it were. Once the lower arm has been mounted and tightened it should not be relocated to the other side either deliberately or unintended.
Similar principle applies to the upper arm if the bushes have been replaced- the end nuts are to be tightened on those when the car is at settled height but not after simply having the car off the ground.
It will be interesting to see if there are different opinions on this.
10-31-2018, 04:11 PM
Uneven rear spring heights
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