87S4 Rear right wheel chirps on rights turns
09-23-2012, 02:14 PM
#1
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Thread Starter
87S4 Rear right wheel chirps on rights turns
On right turns on pavement I hear the right rear wheel chirp as it spun. Car has LSD. Previous 928s with LSD (86.5 Auto with '89 tranny) did not do this.
Normal? No service history on differential fluid changes. Time to change?
Normal? No service history on differential fluid changes. Time to change?
09-23-2012, 02:35 PM
#3
Chronic Tool Dropper
Lifetime Rennlist
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Lifetime Rennlist
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It would be a hell of an LSD if it's actually tire chirp going around slow corners. The clutch-type LSD setups are ambi-directional, so any LSD-induced tire chirp is as likely in left turns as in rights.
With gearbox in neutral and parking brake off, chock a front wheel and lift the opposing rear wheel. Try turning the rear wheel. If you have a torque wrench, get a "breakaway" torque reading, the torque it takes to get the LSD to let go as you pull gently. If you can attach to a lugnut, get a reading with the wrench outward from the lugnut, and a second reading with the wrench across the center of the wheel. The average of those is breakaway torque. A 'new' value might be as high as 25 lbs/ft (not very much), while a car with 100k of in-town use might be almost nothing. Lubricants in the final drive affect the friction in the clutches, so if you are using a high-performance synthetic you may also loose some friction. Conversely, many conventional oils are too sticky, and end up with a graphite or molybdenum 'friction modifier' to solve chattering problems. Bottom line though is that the LSD, even when new and at full rated lockup, has nowhere near the friction needed to chirp a tire turning on dry pavement. I'd be looking at new wheel bearings, after confirming that the cause is not related to service brake dragging or loose bits in the parking brake. Even then, if the rotor and stub axle are moving enough in a turn to make either brake drag and squeek, the only possible cause is a tired wheel bearing. In my experience anyway.
With gearbox in neutral and parking brake off, chock a front wheel and lift the opposing rear wheel. Try turning the rear wheel. If you have a torque wrench, get a "breakaway" torque reading, the torque it takes to get the LSD to let go as you pull gently. If you can attach to a lugnut, get a reading with the wrench outward from the lugnut, and a second reading with the wrench across the center of the wheel. The average of those is breakaway torque. A 'new' value might be as high as 25 lbs/ft (not very much), while a car with 100k of in-town use might be almost nothing. Lubricants in the final drive affect the friction in the clutches, so if you are using a high-performance synthetic you may also loose some friction. Conversely, many conventional oils are too sticky, and end up with a graphite or molybdenum 'friction modifier' to solve chattering problems. Bottom line though is that the LSD, even when new and at full rated lockup, has nowhere near the friction needed to chirp a tire turning on dry pavement. I'd be looking at new wheel bearings, after confirming that the cause is not related to service brake dragging or loose bits in the parking brake. Even then, if the rotor and stub axle are moving enough in a turn to make either brake drag and squeek, the only possible cause is a tired wheel bearing. In my experience anyway.
Last edited by dr bob; 09-23-2012 at 04:51 PM.
09-23-2012, 04:31 PM
#4
Rennlist Member
Thread Starter
Took it for a ride - this new to me S4 spins the tires in first. None of my previous 928s did that so easily. On right or left turns starting from a stop you can hear the opposing tire spinning. Maybe they are crappy tires. But, I do love it. Puts a grin on my son's and my face. I am going to wear these tires out...
Dr. Bob I will do the test you suggest.
Dr. Bob I will do the test you suggest.
