Notices
928 Forum
Sponsored by:
Sponsored by: 928 Specialists

Help slightly stiffening the rear suspension

 
Old 07-11-2019, 06:53 PM
  #31  
GregBBRD

Rennlist
Basic Site Sponsor

 
GregBBRD's Avatar
 
Join Date: Feb 2005
Location: Anaheim
Posts: 9,846
Likes: 0
Received 18 Likes on 18 Posts
Default

Originally Posted by cschou View Post
I only rarely track the car, and it has been more than a year since I last had it on the track. I know it isn't very informative or scientific, but my notion of "feels soft" is based on street driving and personal feel. Regarding understeer vs oversteer, (with limit track time), it tends to slightly understeer with neutral throttle, but easily converts into oversteer as soon as moderate throttle is applied.
Which means you probably don't have a limited slip...or it is worn out.

Stiffening the rear will induce more oversteer. Since the idea of getting around any track (or parking lot course) is to be on the throttle as much as possible, inducing more oversteer under throttle will not make you quicker (nor more comfortable.)

Drive it like it is.....find a limited slip and have it rebuilt, set-up, and properly installed. Once you can get on the throttle and have the front end push (both rear tires driving the same amount should induce understeer), then think about what to change.

For now, take some of the padding out of the driver's seat, so it feels stiffer.
__________________
greg brown




714 879 9072
[email protected]

If the kids keep breaking a crowbar in the sandbox, sometimes it is easier to just give them a stronger crowbar, instead of trying to figure out what they are doing in the sandbox.
GregBBRD is offline  
Old 07-11-2019, 07:05 PM
  #32  
cschou
User
Thread Starter
 
cschou's Avatar
 
Join Date: Jun 2011
Location: Denmark
Posts: 75
Likes: 0
Received 1 Like on 1 Post
Default

Originally Posted by GregBBRD View Post
Which means you probably don't have a limited slip...or it is worn out.

Stiffening the rear will induce more oversteer. Since the idea of getting around any track (or parking lot course) is to be on the throttle as much as possible, inducing more oversteer under throttle will not make you quicker (nor more comfortable.)

Drive it like it is.....find a limited slip and have it rebuilt, set-up, and properly installed. Once you can get on the throttle and have the front end push (both rear tires driving the same amount should induce understeer), then think about what to change.

For now, take some of the padding out of the driver's seat, so it feels stiffer.
I do have a limited slip diff, but it might be worn out - good point. Thanks...
cschou is offline  
Old 07-12-2019, 01:13 PM
  #33  
GregBBRD

Rennlist
Basic Site Sponsor

 
GregBBRD's Avatar
 
Join Date: Feb 2005
Location: Anaheim
Posts: 9,846
Likes: 0
Received 18 Likes on 18 Posts
Default

Originally Posted by cschou View Post
I do have a limited slip diff, but it might be worn out - good point. Thanks...
Our limited slips have moly clutches, which do wear out. Yes, normally it takes many years and lots of miles for them to wear out, but almost all of our cars meet those requirements. One gear oil change with the incorrect fluid will kill a limited slip in a few hundred miles.
GregBBRD is offline  
Old 07-15-2019, 05:39 PM
  #34  
cschou
User
Thread Starter
 
cschou's Avatar
 
Join Date: Jun 2011
Location: Denmark
Posts: 75
Likes: 0
Received 1 Like on 1 Post
Default

Originally Posted by GregBBRD View Post
Our limited slips have moly clutches, which do wear out. Yes, normally it takes many years and lots of miles for them to wear out, but almost all of our cars meet those requirements. One gear oil change with the incorrect fluid will kill a limited slip in a few hundred miles.
Okay, car has about 160.000 km on it. Can I diagnose the diff myself?
cschou is offline  
Old 07-15-2019, 06:55 PM
  #35  
dr bob
Chronic Tool Dropper
Lifetime Rennlist
Member
 
dr bob's Avatar
 
Join Date: Oct 2001
Location: Bend, Oregon
Posts: 18,026
Received 13 Likes on 12 Posts
Default

Easiest LSD test is to raise one side in the rear. Parking brake released, wheel blocks at the opposite front corner so the car won't roll. Gearbox in neutral. Can you turn the raised wheel? If so the LSD is slipping significantly.

In a previous discussion with Greg, we settled on a target around 30 lbs/ft breakaway torque for a properly-functioning LSD. You can test this with a torque wrench on the axle nut, or take a few readings on a wheel lugnut with wrench across center and wrench out from center, average to get an idea of the average. If you are using a click-style wrench, you'll slowly walk the wrench setting up until it doesn't click when you pull slowly on the wrench. Breakaway is the last setting that clicked.

Some folks like to "tune" a too-stiff LSD with a friction modifier added to the differential lubricant. You can do the same by tuning a blend of NS and non-NS oils. If this has been done before, you'll want to flush the old oil out with a few changes. Reality is that most LSD's are done by 100k even in "normal" street driving. Every bend in the road and every corner you go around causes slip in the clutch packs and some wear. Using the wrong oil accelerates the wear. Similar to auto-trans clutch pack elements, solvents will quickly damage the LSD clutch plates.
dr bob is offline  
Old 07-16-2019, 04:56 PM
  #36  
cschou
User
Thread Starter
 
cschou's Avatar
 
Join Date: Jun 2011
Location: Denmark
Posts: 75
Likes: 0
Received 1 Like on 1 Post
Default

Originally Posted by dr bob View Post
Easiest LSD test is to raise one side in the rear. Parking brake released, wheel blocks at the opposite front corner so the car won't roll. Gearbox in neutral. Can you turn the raised wheel? If so the LSD is slipping significantly.

In a previous discussion with Greg, we settled on a target around 30 lbs/ft breakaway torque for a properly-functioning LSD. You can test this with a torque wrench on the axle nut, or take a few readings on a wheel lugnut with wrench across center and wrench out from center, average to get an idea of the average. If you are using a click-style wrench, you'll slowly walk the wrench setting up until it doesn't click when you pull slowly on the wrench. Breakaway is the last setting that clicked.

Some folks like to "tune" a too-stiff LSD with a friction modifier added to the differential lubricant. You can do the same by tuning a blend of NS and non-NS oils. If this has been done before, you'll want to flush the old oil out with a few changes. Reality is that most LSD's are done by 100k even in "normal" street driving. Every bend in the road and every corner you go around causes slip in the clutch packs and some wear. Using the wrong oil accelerates the wear. Similar to auto-trans clutch pack elements, solvents will quickly damage the LSD clutch plates.
Thanks for the test-procedure description - sounds easy enough. I will get that done! I seem to remember being told that the LSD effect is different between the earlier boxes and later ones. Just to be sure, the 30 lbs/ft is for a late box? (S4/GT).

Last edited by cschou; 07-16-2019 at 04:59 PM. Reason: more info
cschou is offline  
Old 07-16-2019, 05:09 PM
  #37  
dr bob
Chronic Tool Dropper
Lifetime Rennlist
Member
 
dr bob's Avatar
 
Join Date: Oct 2001
Location: Bend, Oregon
Posts: 18,026
Received 13 Likes on 12 Posts
Default

The LSD breakaway should be about the same for early or late clutch-style LSD. That's a new setup number, so yours will almost undoubtedly be less just from normal driving wear. How much less is the question at hand. You'll get an objective measurement to consider and decide if it's OK for the driving you do.
dr bob is offline  
Old 07-16-2019, 08:10 PM
  #38  
GregBBRD

Rennlist
Basic Site Sponsor

 
GregBBRD's Avatar
 
Join Date: Feb 2005
Location: Anaheim
Posts: 9,846
Likes: 0
Received 18 Likes on 18 Posts
Default

Originally Posted by dr bob View Post
The LSD breakaway should be about the same for early or late clutch-style LSD. That's a new setup number, so yours will almost undoubtedly be less just from normal driving wear. How much less is the question at hand. You'll get an objective measurement to consider and decide if it's OK for the driving you do.

It's a little bit more involved than that......

The "initial break-in" torque specification is 7-25 ft lbs., for a late model limited slip, with one friction disc on each side and brand new clutches (They can be adjusted up near there 25 fl lb range, with different thickness steel clutch pieces.. New friction clutches are sprayed with Moly, which ends up having very tall peaks and valleys. As the "peaks" wear down into "hills", the surface area increases and the available friction area goes up....to a point. You might "see" torques over 30 ft.lbs. Once the "hills" wear down and all become "plains" the discs are thin enough that they have lost some dimensional thicknesss and the pressure creating the friction goes down.....less limited slip activity. When the "plains" finally wear completely flat, the fiction discs "smear" (gall) and although the breakaway torque goes up, the release points become very unpredictable and will release and grab all over the spectrum, making the car extremely "different" through the same corner, on consecutive laps.. Very good drivers can "cope" with this. Most drivers run "out of talent" very quickly and will have "agricultural experiences".
__________________
greg brown




714 879 9072
[email protected]

If the kids keep breaking a crowbar in the sandbox, sometimes it is easier to just give them a stronger crowbar, instead of trying to figure out what they are doing in the sandbox.
GregBBRD is offline  
Old 07-16-2019, 08:58 PM
  #39  
Carl Fausett
Developer
Rennlist Member


Rennlist
Site Sponsor

 
Carl Fausett's Avatar
 
Join Date: Dec 2001
Location: Horicon, WI
Posts: 6,694
Likes: 0
Received 11 Likes on 5 Posts
Default

Pics that show what Greg was talking about. You can see galling, and how the pads have worn down to the wear indicator strip. Not unlike the wear indicator on your tires, when the friction disk wears down to the base plain, its time for new ones. Like all clutches, they are a considered a consumable item on your car.



__________________
Questions? Tech Support?
Call 920-485-0928
International 001-920-485-0928
Email: [email protected]
Website: https://928motorsports.com/
Instructions: http://www.928motorsports.com/install.php


Carl Fausett is online now  
Old 07-17-2019, 05:35 PM
  #40  
cschou
User
Thread Starter
 
cschou's Avatar
 
Join Date: Jun 2011
Location: Denmark
Posts: 75
Likes: 0
Received 1 Like on 1 Post
Default

Bob, Greg and Carl, wow, great info, thanks! I haven't had a chance to measure on my car yet, but will consider my route from there when I do... ��
cschou is offline  
Old 07-18-2019, 12:46 PM
  #41  
76FJ55
Addict
Rennlist Member
 
Join Date: Nov 2008
Location: Grapevine, TX
Posts: 1,061
Likes: 0
Received 1 Like on 1 Post
Default

Originally Posted by GregBBRD View Post
Drive it like it is.....find a limited slip and have it rebuilt, set-up, and properly installed. Once you can get on the throttle and have the front end push (both rear tires driving the same amount should induce understeer), then think about what to change.
from this statement are you indicating that installing a limited slip will aid in "both rear tires driving the same amount"? my understanding is that an open dif causes the left and right to see the same torque with the limit of applied torque equal to the traction limit on the tire with less grip. A limited slip creates a torque bias increasing the available torque for the higher grip tire above that of the lower grip tire based on the torque transferred across the differential through the clutch pack. this seems counter to your description above. Am I mistaken in my understanding of the functionality of open vs limited slip?
76FJ55 is offline  
Old 07-18-2019, 01:35 PM
  #42  
FredR
Addict
Rennlist Member
 
FredR's Avatar
 
Join Date: Jun 2005
Location: Oman
Posts: 5,998
Received 9 Likes on 9 Posts
Default

Originally Posted by 76FJ55 View Post
from this statement are you indicating that installing a limited slip will aid in "both rear tires driving the same amount"? my understanding is that an open dif causes the left and right to see the same torque with the limit of applied torque equal to the traction limit on the tire with less grip. A limited slip creates a torque bias increasing the available torque for the higher grip tire above that of the lower grip tire based on the torque transferred across the differential through the clutch pack. this seems counter to your description above. Am I mistaken in my understanding of the functionality of open vs limited slip?
Greg is correctly stating what happens- if the LSD unit is doing its job the grip generated on both rear wheels will outstrip the cornering grip afforded by the front wheels and car will understeer. With an open diff and particularly so in lower gears, when you get on the throttle one wheel spins and in the bends the rear end will tend to step out [oversteer]. Thus when on the limit the stock 89 when fitted withthe optional LSD unit understeers if the LSD is working correctly and once that has been ascertained then the owner can move forward making whatever changes he wants knowing that the starting point is optimal.

Inducing oversteer by having the rear wheels slipping excessively will do nothing for the stop watch. Induce a slight over steer with both rear wheels gripping invariably gets the car round the bends optimally and this is where adjustable roll bars kick in- stiffen the front bar and understeer is induced, soften the front bar and a tendency to oversteer can be induced if there is enough front wheel grip to start with. .
FredR is offline  
Old 07-18-2019, 02:07 PM
  #43  
76FJ55
Addict
Rennlist Member
 
Join Date: Nov 2008
Location: Grapevine, TX
Posts: 1,061
Likes: 0
Received 1 Like on 1 Post
Default

Originally Posted by FredR View Post
Greg is correctly stating what happens- if the LSD unit is doing its job the grip generated on both rear wheels will outstrip the cornering grip afforded by the front wheels and car will understeer. With an open diff and particularly so in lower gears, when you get on the throttle one wheel spins and in the bends the rear end will tend to step out [oversteer]. Thus when on the limit the stock 89 when fitted withthe optional LSD unit understeers if the LSD is working correctly and once that has been ascertained then the owner can move forward making whatever changes he wants knowing that the starting point is optimal.

Inducing oversteer by having the rear wheels slipping excessively will do nothing for the stop watch. Induce a slight over steer with both rear wheels gripping invariably gets the car round the bends optimally and this is where adjustable roll bars kick in- stiffen the front bar and understeer is induced, soften the front bar and a tendency to oversteer can be induced if there is enough front wheel grip to start with. .
Fred,
Thanks for the reply. I was not concerned with the resultant under steer over steer end result. I was focused on the "both rear wheels driving the same amount" portion of his post as captured in quotes. This is where I believe the misinformation to be. If you are talking diving force, an open diff will always produce the same driving force at the wheels. A limited slip is designed to transfer more force to the wheel with the greater grip which equates to uneven driving force at the two rear wheels.
76FJ55 is offline  

Thread Tools
Search this Thread

Contact Us - About Us - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: