tire pressure and caster angle

Neville

Supporter
hi, looking for tire pressure recommendations for 295 50 15 and 235 60 15 radial street tires on my monocoque CAV sreet car, 0044 301 ,1a29 mono b66. just put 4 new tires on and trying to get dialed back in, also front and rear toe suggestions and front front caster settings, been out of commission for a while, any and all help much appreciated, thanks, Neville on Long Island, New York.
 

Terry Oxandale

Skinny Man
Generally, and this is a broad generalization, I run about 10% of the car's weight on the tires. so my car runs about 2100 lbs, so generally, at the track, I run around 21 lbs in each tire. On the street I run more pressure closer to manufacturers specs for better milage. Again, just generalization with the knowledge I've got narrower tires up front, but less weight up front as well.
 
My GT40 weighs 2170 pounds without driver. I'm running Continential Extreme Contact tires. The fronts are 245/40-17 and 285/40-17 rears.

I tested my car on a 200 foot skid pad and measured tire temperatures at three locations to determine the best pressures to run. I ran a few laps, recorded the temperatures and the adjusted (lowered) the pressures until I got an even distribution across the tire. I ended up with 15 psi front and 21 psi rear with the hot (warm) tires. For the street, I use those same pressures when the tires are cold knowing that the pressures will rise under track or performance driving.

Bob Woods
Tornado GT40 in Texas
 

Mike Pass

Supporter
GTD and not CAV but may help

GTD Tyre pressures and suspension settings
Tyre pressures

Front 21psi
Rear 23psi

Front toe in 1.6 – 3.2 mm overall
Front Camber 15’ negative (0.25deg)
Front Castor 3deg 20min positive

Rear toe in 0-1.6mm overall
Rear camber angle 30min (0.5 deg)

Cheers
Mike
 
Hi Neville, What you're really asking for is the complete four corner setup of the car. There is a limit on the amount of caster, camber and toe available on the CAV GT. The first concern is ride height. The second concern is the corner weights. Then you can head towards alignment specs, not before.

Ride Height Front: If you were running a 24.5 dia front tire like a Dunlop CR65 you could attain 4.00 ground clearance like the originals, since your front tires are 26.10 inch diameter there is a good chance of bending the upper ball joint pin in bump at anything less than 4.5 inches clearance between the front of the monocoque and the ground .

Ride Height Rear: Your rear tires diameter of 26.61 works fine. I recommend the rear ride height be .25 inch higher than the front. This puts a small amount of wedge in the car that really helps high with speed stability. So your target minimum is 4.75 inches clearance between the rear of the monocoque and the ground.

Corner Weights: With the front are rear sway bars disconnected and weight added to the car to simulate driver / passenger and fuel load weight you can adjust the ride height with the coil over springs to target height. Once you have that dialed the cross weight number on each axle (front side to side and rear side to side) as close to equal as possible you can go after the alignment specs.

IMPORTANT NOTE: All the Series 100 CAV GTs monocoque or tube chassis ran with inboard suspension pickups that were rubber bushed. This is was great idea as the rubber bushes reduced road noise and vibrations being transferred to the chassis and steering.

However, in every single S100 CAV GT I've worked on, or seen close up pictures of the inboard suspension mounts (front and rear) show that the rubber bushes have "walked" off the initial and correct centered position in the middle of the suspension arm tube that houses it. This is caused by a combination of brake torque forces up front and weight transfer under acceleration in the rear.

The result is an unwinding of the front suspension castor, with toe in reduced or eliminated. These changes will make the car more sensitive to the crown in the road or hunt the ruts caused by truck traffic. Also the steering becomes more snatchy on one wheel bumps (bump steer) and won't self return as quickly after a corner because of the reduced castor.

On the rear suspension the inboard lower reversed arm pickup will wander off the center of the bushing towards the front of the car. This adds toe out to the rear. Also the rate of camber climb is increased in one wheel bump giving a twitchy feeling on uneven roads.

To minimize the possibility of unintended oversteer, and calm down the seat of your pants feel over road irregularities the front and rear bushings MUST BE CENTERED before attempting to align the car.
 
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