There are some really bright people here I want some solid opinions or experiences. '68 coupe with Vette Brakes adj. spring and smart struts. I am using 16" wheels and P255/60-16s. I have lowered the car about 1" in the front and 1 1/2 in the rear to make it level and improve the angle of the half shafts.

When I put the torque down I get strong squatting. Not unusual in these cars in general but with the lower car and 16" rims I get a rediculous amount of squat. I know I am losing hook and generally making the car handle wierd on power because when the back goes down the frotn goes up.

After looking things over with the car sitting at ride height the trailing arms are lower at the front than at the rear when viewed from the side. They are essentially going down hill toward the front of the car. In drag race four links this indicates the power is trying to drive under the center of gravity causing a squat. This assumes the CG is in a nominal place etc.

I am wondering if anyone has moved the front bolt hole up in the chassis to control the squat. Anyone? If I move this about 1/2" to 3/4" I think I can tune the squat out without any other changes. If I was running the factory ride height and tires I would probably be a lot closer to level.

Opinions and experiences welcome!

Mark

 

I don't think you will get the reaction you think by moving the piviot point of the trailing arm on a vette. Mainly cause you have no rear end housing trying to rotate to apply lift or pull down to the chassie. The trailing arm have no torque applied to them other than up and down movement from the weight of the car.

 

Theres a lot going on back there.I dont know much but it would seem that if the nose of the diff. is trying to push up the back of it is pulling down on the tail of the car.I think Twin Turbo is back on the forum ,I bet he can answer this question for you

 

There are 2 very powerful forces at work here. The first is the tendency of the pinion to climb the ring gear. It is approximately equal to the engine's torque times the transmission first gear ratio, times the rear end ratio. This can easily be 3,500 ft-lb in the dynamic situation of launching on a sticky surface. (It is important to realize that this torque is transmitted to the chassis by the near rigid mounts of the differential case [an infinitely far forward instant center], and not by the trailing arms).

The second is due to the horizontal force generated at the tire contact patch, which is equal to the above torque divided by the radius of the tire, which will be approximately 3,000 lb (car will launch at close to 1 g). To calculate the moment about the c.g., multiply this force times the height of the c.g., and this contribution will be ~4,500 ft-lb.

Add the two together and the result is approximately 8,000 ft-lb of total nose-up torque at launch. To react to this torque, the forces at each axle must change by ~1,000 lb over the 8 foot wheelbase of the C3. In other words, the vertical force at the front tires is reduced by ~1000 lb, and the vertical force at the rear is increased by ~1000 lb.

Now divide that by the spring rates (not considering shock absorber contributions or "stiction" friction) the front may rise by ~2 inches with 500 lb/inch springs (500 lb at each tire times a control arm tire:spring ratio of ~2:1), and the rear may squat by 1 1/2 inches with a 330 lb/inch rear spring (2 sides).

Ok, so what may happen by raising the pick-up point of the trailing arm?... First, no torque is transmitted by the TA, but the force generated by the tires is applied along the line from the wheel bearings to the pick-up point. If this point is moved by 1 inch upwards, it will move the reaction line where it passes the c.g. by 3 inches as 1:3 is the ratio of the trailing arm length to the distance to the c.g. Recall that the tires are generating approximately 3,000 lb in our example, and that this force line has changed by 3 inches (1/4 foot); therefore, the change in moment at the c.g. is 750 ft-lb, and this is but a small fraction of the total moment of 8,000 ft-lb causing the car to squat.

Although this idea sounds good, the payoff appears small, and the cornering handling may suffer. To fix the squat problem, use stiffer springs to minimize the squat, or alternately, install a solid axle with a 4-link or ladder bar suspension. Similarly, if the differential carrier were 4-link mounted, it could be used to minimize squat, but the changes would become extremely difficult.

 

I am not claiming to be an expert but...

"horizontal force generated at the tire contact patch" tires generate a torque about an axis (twist) not horizontal force. A tire is trying to turn the counter clockwise and a suspension point is trying to stop it with a force in the opposite direction. How and where this force is distributed is the game.

"To calculate the moment about the c.g., multiply this force times the height of the c.g.," not the height of the cg but the distance between the imaginary line from the contact patch (tire) through the pivot point to where it passes the cg. If it passes below cg then the force will rotate the car clockwise (squat). If it passes above the cg it will rotate counter clockwise (lift rear). By moving the pivot it should influence the lift/squat characteristics.

If it is a solid axle four link or our set up the physics is the same. If your trailing arm is not transfering torque lets remvoe the bolt and see what happens? The car will sit static nicely but on torque application the trailing arm will twist in the frame pocket until it hits something then it will start to move forward. This forward push is what I think we need to re-direct.

 

Mark couldnt you try to prove your point by putting a spacer between the top of the rear leaf spring and the bottom of the differential?This would raise the car and level the swing arms without changing the spring rate.Then do some launches and see if its any different.Just a thought.

 

This is an interesting idea! I may try to play with it over the weekend to see if raising the ride height and/or lowering the diff could change the angle. Currently my trailing arms are pointing solidly downhill toward the front of the car.

 

The above method is a good test, but it is actually no different than simply shortening the trailing arm hanger bolts - running the nut up the bolts by 1 inch raises back of the car (and therefore the TA pivot) by approximately 1 inch. It uses the same method, only the spacer is harder to do.

And Mark, if you don't want to listen to engineering analysis, don't ask for it.

 

Your right I guess I tried to over engineer that but my point was it would be easier to raise "car" than to raise the "pivot point".