Suspension tuning Q
10-09-2008 | 09:41 PM
#1
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From: Moraga, California
Suspension tuning Q
My chart here says that increasing the rear track will reduce oversteer and stiffening the rear bar will increase oversteer. What will be the net effect of doing both? I have a 7mm spacer and one hole left in my rear bar.
Greg H.
Greg H.
10-10-2008 | 02:40 AM
#4
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Joined: Jun 2001
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From: Northern NJ
I'm not terribly familiar with the 993 suspension ... but I don't think the rear is a trailing arm?
If it's a traditional a-arm layout... Increasing the rear track effectively softens the springs... since the tire now has a better mechanical advantage about the pivot of the a-arm. Softening the rear springs gives you less oversteer. We'll say the distance from a-arm pivot to contact patch is 20" and the spring attaches at 10"... and the spring is vertical. The motion ratio is thus 0.5. You add a 7mm spacer (0.28"). Now the motion ratio is 0.493. Wheelrate = Motion Ratio^2 * Springrate. So you square the 0.5 and 0.493 and find the percentage change. This comes out to be 2.8%. However, we'll assume the springs contribute 75% of the roll stiffness and the swaybar contributes 25%... so we'll knock this down by 75%. This is a big assumption here, but I don't know the number for your particular car. This leaves us with a 2.1% change in roll stiffness due to the springs & widening the track in the direction of reducing oversteer.
Widening the rear track in and of itself (maintaing effective spring stiffness), reduces weight transfer and increases the roll stiffness and oversteer. The rear track width is 59.8". Your spacers would increase this to 60.35". A delta of 0.92% (towards increasing oversteer).
So between the above 2 changes, you've got 1.2% of change in the direction of reducing oversteer.
Earlier we assumed the swaybar contributes 25% of roll stiffnes, we'll use that number here. We'll say the activation arm of the swaybar (distance from the bar to the droplink connection point) is 5.75" and your extra hole will decrease it to 5". This is a 13% change, which when reduced by the 25% factor gives you a 3.2% change in the roll stiffness. This 3.2% change will be acting thru a moment arm which is effectively reduced by 2.8%. So now it's really only a 0.4% change in rear roll stiffness due to the swaybar change, in the direction of increasing oversteer.
Based on this, the car will oversteer less after the change. This change is roughly 0.8% LESS oversteer. I know, what the hell is 0.8% less oversteer? Drive the car and find out...there are too many variables to say which is the faster setup. For example, If you assume the swaybar and the springs each contribute 50% to roll stiffness, you end up 3.22% MORE oversteer. You'd need to know this split to come up with a decent number, since it's a big contributor to the calcs.
And now .... back to sleep... this post was typed at 2am so I hope it's somewhat understandable.
If it's a traditional a-arm layout... Increasing the rear track effectively softens the springs... since the tire now has a better mechanical advantage about the pivot of the a-arm. Softening the rear springs gives you less oversteer. We'll say the distance from a-arm pivot to contact patch is 20" and the spring attaches at 10"... and the spring is vertical. The motion ratio is thus 0.5. You add a 7mm spacer (0.28"). Now the motion ratio is 0.493. Wheelrate = Motion Ratio^2 * Springrate. So you square the 0.5 and 0.493 and find the percentage change. This comes out to be 2.8%. However, we'll assume the springs contribute 75% of the roll stiffness and the swaybar contributes 25%... so we'll knock this down by 75%. This is a big assumption here, but I don't know the number for your particular car. This leaves us with a 2.1% change in roll stiffness due to the springs & widening the track in the direction of reducing oversteer.
Widening the rear track in and of itself (maintaing effective spring stiffness), reduces weight transfer and increases the roll stiffness and oversteer. The rear track width is 59.8". Your spacers would increase this to 60.35". A delta of 0.92% (towards increasing oversteer).
So between the above 2 changes, you've got 1.2% of change in the direction of reducing oversteer.
Earlier we assumed the swaybar contributes 25% of roll stiffnes, we'll use that number here. We'll say the activation arm of the swaybar (distance from the bar to the droplink connection point) is 5.75" and your extra hole will decrease it to 5". This is a 13% change, which when reduced by the 25% factor gives you a 3.2% change in the roll stiffness. This 3.2% change will be acting thru a moment arm which is effectively reduced by 2.8%. So now it's really only a 0.4% change in rear roll stiffness due to the swaybar change, in the direction of increasing oversteer.
Based on this, the car will oversteer less after the change. This change is roughly 0.8% LESS oversteer. I know, what the hell is 0.8% less oversteer? Drive the car and find out...there are too many variables to say which is the faster setup. For example, If you assume the swaybar and the springs each contribute 50% to roll stiffness, you end up 3.22% MORE oversteer. You'd need to know this split to come up with a decent number, since it's a big contributor to the calcs.
And now .... back to sleep... this post was typed at 2am so I hope it's somewhat understandable.
Last edited by adrial; 10-10-2008 at 03:01 AM.
10-10-2008 | 03:53 AM
#5
Instructor
Joined: Feb 2003
Posts: 236
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From: Mpls
Make one change at a time. Multiple changes remove your ability determine what did what.
Also, please note that oversteer on corner entry vs mid-corner vs corner exit should be a addressed differently. For example, corner exit oversteer is often actually understeer. The car pushes coming in and the driver instinctually adds throttle coming out but the car is not rotated enough. Eventually the car oversteers from the addition of throttle while still asking for steering input.
Also, please note that oversteer on corner entry vs mid-corner vs corner exit should be a addressed differently. For example, corner exit oversteer is often actually understeer. The car pushes coming in and the driver instinctually adds throttle coming out but the car is not rotated enough. Eventually the car oversteers from the addition of throttle while still asking for steering input.
10-10-2008 | 11:39 AM
#6
Race Director
Joined: Aug 2002
Posts: 17,643
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From: Phoenix, Az
Make one change at a time. Multiple changes remove your ability determine what did what.
Also, please note that oversteer on corner entry vs mid-corner vs corner exit should be a addressed differently. For example, corner exit oversteer is often actually understeer. The car pushes coming in and the driver instinctually adds throttle coming out but the car is not rotated enough. Eventually the car oversteers from the addition of throttle while still asking for steering input.
Also, please note that oversteer on corner entry vs mid-corner vs corner exit should be a addressed differently. For example, corner exit oversteer is often actually understeer. The car pushes coming in and the driver instinctually adds throttle coming out but the car is not rotated enough. Eventually the car oversteers from the addition of throttle while still asking for steering input.
The more I have learned about suspension set-up is that it is very complex. General rules are very general and may have the opposite impact you think based on what is happening where. The real key is to be able as driver to feel what the car is doing at various places in the corner. A car does not just oversteer or understeer in a corner. It can do all those things depending on where you are in that corner and even if it is a fast or slow corner. As a driver you must try to break these up. If not you may change the wrong part of the car.
If you are learning to feel this stuff is very important to make one change and see what the car does. Plus better in on corner can be worse in 2 others making overall lap times increase.