Front end dances at speed - Power steering?

I am not officially on the road yet so my trips are about 1/2 mile to the corner store and back. I have an 818S, with power steering, 3.5degrees of caster, FFR wheels. As I accelerate the front end gets twitchy. It feels like one small move and the car will jump right off the road. I haven't had a chance to calibrate the speedo but would guess that this is happening over 35-40mph. I can get moderately high in 3rd gear before I need to slow down. I have seen the recommendation of 5-7 degrees of caster when using power steering. Might this take care of the issue or should I be looking at the power steering system? Now that I am thinking about it I should try driving at speed in 3rd gear then again in 5th gear to see if over pressure is a factor. Anyone else have this experience with or without PS?

Start with more caster, to be sure. You might also consider adjusting the PS pressure, which is usually done in the pump with a spring/bypass valve. Other setups use a solenoid that varies based on vehicle speed.

CNC
I don't have anything like this going on. My manual rack was just modified per ffr manual.

The rack has a control valve in it that controls the flow of fluid in the rack.
Here's how it works.
When you put right torque into the steering wheel the valve flows fluid to move the rack to toward a right turn. When the steering wheel matches the rack, equal pressure goes to both sides of the rack.

If the valve over reacts, it will push to far right, then left, then right, then left and so on. You might feel this in the wheel.
The hydraulics are tune not to do this for the intended application .
The front end on the WRX weighs 2000 lbs (WAG) and the front of the 818 is about 800 lbs. So I could see the 818 having this problem.

I would remove the PS belt and test drive to see if the problem goes away.
If this fixes the problem. Then you can change the pressure control spring on the ps pump to fix the problem. I would start with 1/2 pressure.
There is also a damper valve in the rack, which also could be adjusted.
Hope this helps
Bob

Make sure your lower rear lateral links are perfectly parallel. I had the same problem on my 1st 818S. Wayne Presley had me check and sure enough they were not parallel. After setting them correct and redoing 4 wheel alignment car was stable at 140. If they are not parallel they cause toe change on accel and decel.

Check your "Tow Angle" because I suspect it is pitched inward. If you are at zero our a hair outward then I have no clue since I have no experience with the 818.

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Originally Posted by GoDadGo

Check your "Tow Angle" because I suspect it is pitched inward. If you are at zero our a hair outward then I have no clue since I have no experience with the 818.

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I would agree that this is the first thing I would check. However, I think you have that backwards. Increased toe-in will help straight line stability in a rear wheel drive car. Before I went for my alignment, my car was also very twitchy. Turns out my eye ball alignment included slight toe out (it was only for go kart mode anyway)

Stupid possibility but simple to check - check tire pressures.

Henry

You will undoubtedly need more caster. I went through the same process with my roadster when I added power steering. I initially had the manual-steering caster dialed in - about 3 or 4 degrees - when I added power steering. It felt just as you described, darty - like you had to keep a firm grip on the wheel all the time. I can't remember for sure, but I think I dialed in 7 degrees of caster and it was night and day.

Quote:

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Originally Posted by 65 Cobra Dude

Stupid possibility but simple to check - check tire pressures.

Henry

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Not stupid at all. It is easy to overlook the obvious and I haven't checked this. Great technical knowledge guys. Thanks. I hope to have it roadable in time for the Huntington Beach event. Doug