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Discussion and questions related to the course Suspension Tuning & Optimization
I just swapped from fixed dampening coil overs with 400 lbs/in springs at all four corners to two way adjustable coil overs with 500 lbs/in front 550 lbs/in rear springs. I used the manufacturers recommended bump & rebound settings to start. Based on the lateral load transfer distribution sheet my LLTD would only have a very minor change in the overall balance of the car with the new coil overs installed. However, during the first test at an autocross this weekend the car oversteered during trail braking on corner entry very easily. This never happened with the old coil overs. The temperature was cold and it was hard to get heat in the rear tires, but after a few runs and some heat in the tires, rear grip on corner entry did not get any better. I gradually adjusted the dampening softer throughout the day which felt better eventually with 4 clicks softer at all four corners in bump and rebound. However, it occurred to me today that although the LLTD did not change much the effective stiffness at the wheels, especially the rear, significantly increased (about 15-25%) since I didn’t make any changes to my sway bars with the installation of stiffer springs.
Therefore, my actual question is, could the disproportionate increase in rear stiffness at the wheels compared to the less significant increase in wheel stiffness at the front wheels cause or contribute to my oversteer problem?
Amplifying info: 2011 STI running 24mm front ARB, 22mm rear ARB. Autocross venue is rough stadium parking lot and it was slightly damp from a previous days rain with temps in the 50s F.
Thanks for any help you can offer! Just trying to not chase my tail as I learn to tune dampeners.
Yes, depending on the vehicle even a relatively mild increase in overall roll stiffness can have a big effect, worsened if you have trouble bringing the tyres up to their working temperatures.
Thanks for the response! What I keep getting my head wrapped around is, if I understand the course correctly, the LLTD will capture changes to how/where the load is distributed and therefore the balance/handling characteristics of the car. Perhaps I’m just underestimating the magnitude of a 0.5% change in LLTD, but the oversteer was very easily induced and quite the surprise compared to the cars handling on the previous coil overs. I understand that the bump and rebound settings are going to play a significant part in how the car handles, however I also want to avoid chasing my tail by using dampening tuning to cover up a possibly poor spring and anti roll bar set up. (Hopefully I’m making some sense) Specifically, next event I’m going to plan on running the rear bar full soft which will move my LLTD forward by 1%, but also reduce the effective wheel stiffness at the rear. I plan on starting from the manufacturers recommended bump/rebound settings and reevaluate the car’s handling to see if it’s more stable. Ultimately I’m trying to avoid deviating too far from manufacturer recommendations, because otherwise I assume I’ve probably done something wrong.
Ok, apologies for the convoluted train of thought. I guess I’m trying to ask, should I be looking to target a specific roll stiffness or wheel stiffness in addition to tracking my LLTD?(mechanical grip car, 2.5Hz target wheel rate) My spring rates are nearly dead on for my 2.5 Hz wheel rate.
Thanks for reading this far, and I appreciate any help you can offer.