Issue:
Pantera steering wheel is rock solid on perfectly smooth road at 120 mph. Tight, solid steering. At 65 mph on roads grooved for rain water drainage or uneven pavement, steering is twitchy and nervous, steerng wheel vibrates.
Facts:
Wheel alignment performed, 1/8" toe in, caster within spec, four wheel alignment without defect. No front air dam. Front of car set down 1 1/2" relative to back of car (slopes toward ground in front), measured at rear of front wheel body to pavement and front of rear wheel body to pavement. 32 psi front and rear. 15" rims, stock Campy's. Tire radius measurements: rim to pavement: front: 2 1/2 ", rear 3 1/2". Tires perfectly balanced.
Impressions/Past Experience/Diagnosis:
1982 Lotus Esprit, 4 cylinder, wedge shape with ground effects (air dam front and side body skirts) had same twitchy steering feedback. Vehicle weight: 2,300 lbs. (1,000 lbs. under Pantera's weight.) Was running 8" wide front tires and 10" wide rear tires. Tire radius: 3" rim to pavement tire radius, front and rear. Overactive steering feedback also. So tires gradually deflated: 32 psi, front and rear. 24 psi front and rear. 18 psi front and rear. Final set-up: 8 psi front and 12 psi rear gave the tire tread four inches of tire contact at the outside tire wall at the tread, measured at the pavement. This means the front had a "tread contact patch area of 4" X 8" = 32 square inches. (32L7")" and the rear had a "tread contact patch area of 4" X 10" = 40 square inches (40L7")". The vehicle's weight bias was about 45% front and 55% rear as best I can recall. At speed on rough road, the tire ply absorbed the road irregularities and the ride was buffered by the tire ply. I never hit a pot hole and punctured a tire by a pinch against a rim. At 148 mph the vehicle really squatted down and was stiff to turn. 140 mph speed was for only about 3 minutes, so increased tire temperature from increased tire sidewall deformation didn't reach degradation limits. Driving was comfortable and control was seemingly at optimum.
If, in fact, the Lotus had 45% weight on front and 55% weight on back, the a 32 square inch contact patch on each front wheel and 40 sqaure inch contact patch on back equals 32/(32 + 40) = 32/72 = 44% contact patch on the front tires and 56% contact patch on the rear tires.
I must say, the car, with that specific set-up, drifted with nearly exact front and rear drift, at any speed, through any corner. That is to say, if at the apex of the turn the car was sliding, and at the final curvature of the turn the wheel slide was measured again, then if the front tires were 2" off the protractor line through the curve (i.e.: the perfect, non-slide line through the curve) then, the back tires were also 2" outside the protractor line through the curve.
Pretty close to the ideal set-up. Neutral steering geometry. Controllable. And absorbing road shock through surface irregularity.
Warning:
Sustained manuevers at these high speeds would tend to increase tire temperatures in the sidewalls and potentially lead to tire failure.
Correction Factor:
"Tire contact patch" calculations could be performed to increase tire pressures, such that tire sidewall distortion would be minimized, leading to decreased temperature increases from sidewall distortion, while sacrificing "contact patch surface area" and thus cornering grip. The question is: how long are you going to be doing the high speed driving? If over 7 minutes, better consult your tire manufacturer's guidelines and recommended tires inflation settings. If driving at high speeds for very brief intervals, lower tire pressures may optimize cornering grip, and neutralize excessive steering feedback resulting in a more comfortable ride.
Questions:
(1) What is the weight bias: front to rear on the Pantera with the standard 351-C?
(2) Has anybody done a study as to what tire pressures optimize both the neutrality of the front to rear drift while cornering (during occasional high speed cornering) while deflalting the tires enough to overcome the steering input shock to the steering wheel on uneven surfaces?
(3) Surely nobody is running 32 psig in front and rear as per tire manufacturer recommendations, so what is working well for you guys? Certainly the front tire pressures must be less than the rear tire pressures to compensate for the rear weight distribution factor in the Pantera.
As well, since the Pantera is roughly 50% heavier than the Lotus, tire pressures must correspondingly be about 50% more. Thus, my next question;
(4) Has anybody been running about 12 psi front and 18 psi rear in their Pantera? If so how does it affect your handling?
Current Modification:
A Mini Front Air Dam is being installed on my Pantera at the moment. I believe this will have a dual benefit: (a) hold the front down more firmly at speed and (b) increase cooling at all speeds.
Generalized Question:
Has anyone done anything that reduces the road input to the steering wheel, such that the steering wheel is less twitchy and offers greater stability at all speeds on all road surfaces?
Has anyone else observed a twitchy steering wheel on less than perfectly smooth roads? What have you done that helps correct thesituation?
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