Trying to understand Front wheel geometry 87 S4
11-28-2016, 11:30 PM
#1
Rennlist Member
Thread Starter
Trying to understand Front wheel geometry 87 S4
So, car is all back together and running well..Sort of..
anyway, another topic.
I have never really understood ( or taken the time) the discussions on offset, camber, toe, etc...
That is my project for this week..to read up..
My issue..and confession..
I currently have turbo twist wheels on and have no idea if they are correct for this car. I got a great deal with tires and the bolt pattern fit with no rub.
I rebuilt all of the steering components and it steers well with no leaks..
When I turn slowly like in a circle, the wheels have a tendency to pull the steering wheel into a tighter turn and there feels like a bit of binding or tightness. Kinda like the tire is dragging..It reminds me of how my All Wheel Drive truck pulls in a tight turn..
Does it sound like the offsets are too much?
I will pull the tires sometime this week to look for the offset, just kind of thinking out loud.
anyway, another topic.
I have never really understood ( or taken the time) the discussions on offset, camber, toe, etc...
That is my project for this week..to read up..
My issue..and confession..
I currently have turbo twist wheels on and have no idea if they are correct for this car. I got a great deal with tires and the bolt pattern fit with no rub.
I rebuilt all of the steering components and it steers well with no leaks..
When I turn slowly like in a circle, the wheels have a tendency to pull the steering wheel into a tighter turn and there feels like a bit of binding or tightness. Kinda like the tire is dragging..It reminds me of how my All Wheel Drive truck pulls in a tight turn..
Does it sound like the offsets are too much?
I will pull the tires sometime this week to look for the offset, just kind of thinking out loud.
11-29-2016, 03:06 AM
#2
Rennlist Member
John,
If you have too much offset with Turbo Twist wheels you will be the first 928 owner to have such I suspect!
The problem with TT's, beautiful looking wheels that they are, is that invariably they have too little offset, typically in the range 50mm to 55mm. Many folks drive around on such and advise they have no problems and they are happy then fair enough but Porsche did not design the suspension just to make the wheel fit, they designed it to perform and did a pretty decent job of such.
Most folks seem to think that providing it fits "it works". Some of us think otherwise as seen by a few folks who recently have migrated back to wheels with stock offsets when the "penny dropped". To be fair you probably have to push the car to its limits or close to it to fnd such but then when you are going sideways at 150 mph is no time to figure out "you got it wrong and Porsche got it right"! The 928 is a very forgiving car handling wise and my belief is that Porsche knew what they were doing when they specified such offsets.
As to your observation about feel when driving slowly in a circle the first thing that comes to mind is your alignment- where does it currently sit? When I had TT's fitted with 52mm offset, when pushed hard in autocross the thing did not feel as though it wanted to turn and understeered chronically and the stop watch confirmed this. Whether it was the offset effect or simply my lack of talent behind the wheel not sure but the offset is there for a reason and it is not cosmetics.
I take it you have checked both upper and lower ball joints. By now most originally fitted upper joints on the S4 models are probably shot- have you checked yours? Easy to do- just break the joint and if the ball joint flops around with litttle to no resistance to movement it is likely shot.
Rgds
Fred
If you have too much offset with Turbo Twist wheels you will be the first 928 owner to have such I suspect!
The problem with TT's, beautiful looking wheels that they are, is that invariably they have too little offset, typically in the range 50mm to 55mm. Many folks drive around on such and advise they have no problems and they are happy then fair enough but Porsche did not design the suspension just to make the wheel fit, they designed it to perform and did a pretty decent job of such.
Most folks seem to think that providing it fits "it works". Some of us think otherwise as seen by a few folks who recently have migrated back to wheels with stock offsets when the "penny dropped". To be fair you probably have to push the car to its limits or close to it to fnd such but then when you are going sideways at 150 mph is no time to figure out "you got it wrong and Porsche got it right"! The 928 is a very forgiving car handling wise and my belief is that Porsche knew what they were doing when they specified such offsets.
As to your observation about feel when driving slowly in a circle the first thing that comes to mind is your alignment- where does it currently sit? When I had TT's fitted with 52mm offset, when pushed hard in autocross the thing did not feel as though it wanted to turn and understeered chronically and the stop watch confirmed this. Whether it was the offset effect or simply my lack of talent behind the wheel not sure but the offset is there for a reason and it is not cosmetics.
I take it you have checked both upper and lower ball joints. By now most originally fitted upper joints on the S4 models are probably shot- have you checked yours? Easy to do- just break the joint and if the ball joint flops around with litttle to no resistance to movement it is likely shot.
Rgds
Fred
11-29-2016, 08:03 AM
#3
Rennlist Member
Thread Starter
Thanks Fred that clears the mud a bit.
I haven't done much of anything with the suspension yet, ( Outer tie rods). I have been waiting for the second oil analysis to decide where the budget needs to go. First analysis is getting sent off today. Hoping for creamy, but expect chunky.
This is deer season so not much getting done on car in the next couple weeks.
I haven't done much of anything with the suspension yet, ( Outer tie rods). I have been waiting for the second oil analysis to decide where the budget needs to go. First analysis is getting sent off today. Hoping for creamy, but expect chunky.
This is deer season so not much getting done on car in the next couple weeks.
12-03-2016, 10:49 AM
#4
Pro
John,
If you have too much offset with Turbo Twist wheels you will be the first 928 owner to have such I suspect!
The problem with TT's, beautiful looking wheels that they are, is that invariably they have too little offset, typically in the range 50mm to 55mm. Many folks drive around on such and advise they have no problems and they are happy then fair enough but Porsche did not design the suspension just to make the wheel fit, they designed it to perform and did a pretty decent job of such.
Most folks seem to think that providing it fits "it works". Some of us think otherwise as seen by a few folks who recently have migrated back to wheels with stock offsets when the "penny dropped". To be fair you probably have to push the car to its limits or close to it to fnd such but then when you are going sideways at 150 mph is no time to figure out "you got it wrong and Porsche got it right"! The 928 is a very forgiving car handling wise and my belief is that Porsche knew what they were doing when they specified such offsets.
As to your observation about feel when driving slowly in a circle the first thing that comes to mind is your alignment- where does it currently sit? When I had TT's fitted with 52mm offset, when pushed hard in autocross the thing did not feel as though it wanted to turn and understeered chronically and the stop watch confirmed this. Whether it was the offset effect or simply my lack of talent behind the wheel not sure but the offset is there for a reason and it is not cosmetics.
I take it you have checked both upper and lower ball joints. By now most originally fitted upper joints on the S4 models are probably shot- have you checked yours? Easy to do- just break the joint and if the ball joint flops around with litttle to no resistance to movement it is likely shot.
Rgds
Fred
If you have too much offset with Turbo Twist wheels you will be the first 928 owner to have such I suspect!
The problem with TT's, beautiful looking wheels that they are, is that invariably they have too little offset, typically in the range 50mm to 55mm. Many folks drive around on such and advise they have no problems and they are happy then fair enough but Porsche did not design the suspension just to make the wheel fit, they designed it to perform and did a pretty decent job of such.
Most folks seem to think that providing it fits "it works". Some of us think otherwise as seen by a few folks who recently have migrated back to wheels with stock offsets when the "penny dropped". To be fair you probably have to push the car to its limits or close to it to fnd such but then when you are going sideways at 150 mph is no time to figure out "you got it wrong and Porsche got it right"! The 928 is a very forgiving car handling wise and my belief is that Porsche knew what they were doing when they specified such offsets.
As to your observation about feel when driving slowly in a circle the first thing that comes to mind is your alignment- where does it currently sit? When I had TT's fitted with 52mm offset, when pushed hard in autocross the thing did not feel as though it wanted to turn and understeered chronically and the stop watch confirmed this. Whether it was the offset effect or simply my lack of talent behind the wheel not sure but the offset is there for a reason and it is not cosmetics.
I take it you have checked both upper and lower ball joints. By now most originally fitted upper joints on the S4 models are probably shot- have you checked yours? Easy to do- just break the joint and if the ball joint flops around with litttle to no resistance to movement it is likely shot.
Rgds
Fred
Don't forget the Ackerman angle.
All steering mechanisms are designed to decrease the toe as more lock is applied, so as to have each wheel rolling as close as possible on its own arc. It's a compromise of sorts so you always get one wheel scrubbing a bit while the other one rolls. If the Ackerman angle is not set correctly (during wheel alignment), it can result in one wheel either dragging (resisting the turn in) or 'rolling in' ie pulling the steering harder into the turn.
http://teflonmike.craywiki.co.uk/0_L...Steering01.htm
HTH