Suspension & Steering
Ride Height Adjustment
John had some questions about suspension:
1. The Spec Book talks about coloured markings on the springs to differentiate between spring lengths. Where abouts are they?
The colored markings really designate spring rates, rather than lengths. They are hand-painted 1/2" wide paint stripes, usually located on about the second or third coil from the bottom. Use a spray cleaner (409, Fantastik, etc.) to find them. The codes are in the shop manual.
2. I do not know whether the shocks are Bilstein or Boge. How can I tell without removing? (They are black in colour).
Surest way to tell is to clean the bottom of the shocks with your spray cleaner and read the name. You can tell by appearance, but the differences are subtle until you see both.
3. If I have the right combination of springs and shocks then one/both must be worn out. The car has done 32,700ml. What should I try replacing first or should I do both at the same time?
Shocks don't affect ride height, except for VERY minor change when gas pressure shocks lose their internal pressure. At 32K miles, I doubt that you need shocks - although a lot of people like the feel of firmer (i.e. Koni) shocks.
Suspension springs DO NOT "wear out". (Valve springs on some cars can and do wear out through fatigue. Apparently this is not a big problem on 928s.) The only change that normally happens to springs is a small change in free length, which can change ride height. The only detrimental factor in the change in free length is a slight loss in suspension travel, which is not a problem in street driving.
My suggestion is to adjust your ride height and align your suspension.
To measure Ride Height:
1) What and Where.
Front: There is a small flat machined area on the bottom of the cast bracket that holds the rear of the Lower Control Arm, between the ribs. This pad should be 180mm +/- 10mm from the floor. There should be a max difference of 10mm from left to right.
Rear: There is a small flat machined area on the aft bottom of the large u-shaped bracket that holds the lower transverse control link (the large flat blade). This pad should be 173 +\- 10mm from the floor. There should be a max of 10mm difference from left to right.
2) How to Measure.
First, and most important: You CAN NOT measure, jack the car to change the ride height, lower the car and remeasure! You MUST either measure, jack and change, then drive the car for at least three or four miles before you remeasure; OR measure, change ride height WITHOUT jacking the car, then remeasure. The 928 takes several miles of driving, or using special tool 10-222A to pull the front end down 60-70mm for one minute. Yes, they are serious. Yes, if you don't do it one of these two ways, you will screw up your ride height - and the same thing applies to front end alignment. You will need a tool that you can use to measure the ride height points while manipulating it at arm's length under the car. One possibility would be a pair of yardsticks, bolted together near the centers using a wing nut, making a kind of three-foot scissors. Slip this under the car to the machined pad, put one tip on the floor and one tip on the pad, being sure to keep them plumb, tighten the wing nut, pull it out and measure the height.
3) How to Change the Ride Height.
Most 928 springs are adjustable. These have a round, notched, threaded nut around the shocks under the lower spring seats. Ed Ruiz found a tool to adjust these nuts - a "Motion Pro Single Shank Nut Spanner, 08-029" at a motorcycle shop for about $16. (Someone bought one of these tools by mail order from a motorcycle dealer in Pittsburgh (tel. 800-860-0686). They refer to it as an "ATV Shock Tool" part no. P529. It's made by Motion Pro whose own part number is 08-029. Cost was about $14 plus shipping.) Turn the front wheels all the way left or right, lube the nut area well with a penetrating spray lubricant, and turn the nut right to raise, left to lower. You can get to the rear (also lubed) with only a little stretching.
Adjust the height all the way around and remeasure. When you think that you have it right, drive the car for three or four miles and remeasure. If your Spring Struts are not adjustable, the only way to change ride height is to change springs or to disassemble and add a maximum of two spacers to the lower spring seats.
4) After you get the front ride height set, adjust the toe-in. Easiest way is to "string" the car, ala NASCAR. Use any four convenient objects to securely hold two strings that run beside the car, an inch or two from the tires. Warning! The track is rarely the same front and rear, so don't use the rear tires to set your string! Measure the distance between the strings in front of and behind the car, and make these distances identical. Make the distances between each rear tire and its string identical. Make the distance between each front wheel center and its string identical. Straighten the front wheels with the steering wheel. Measure the distance from the front of each tire to the string and the distance from the back of the tire to the string. Try to be very consistent on where you measure from on the tires. Do the simple math to get the difference between the front and back measurements on the front tires. Adjust the tie rods on the steering rack to get the tires almost straight ahead, with the smallest amount of toe-in that you can measure. This will be close enough until you can find an alignment shop that will align your car WITHOUT JACKING IT UP. If your alignment shop can't or won't do the alignment without jacking the car up, find another shop.
4) Personal Opinion
Set your car at the factory ride height. This is where the suspension was designed to work. My car was lowered by the Previous Owner. Raising it to standard height very noticeably improved both the ride and handling. Low may look cool, but it doesn't work well!
Factory specs are 180 +/- 10mm front, and 173 +/- 10 mm rear. You are allowed an additional 10 mm for settling after usage.
Lowest starting points would be 170 mm front, 163 mm rear. IMHO, it looks a little better to go to the minimum on the front, and perhaps 10 mm higher than the minimum on the rear. So, 170 front, 173 rear for
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