Rear End Steer

[Barnsnake]

Stop by Harbor Freight and pick up a cheap 18" laser level. Cut a piece of wood (maybe a 1x2) that is the correct length to fit against the bead portions of your rear wheels. Clamp the level to the edge of the board, put it against your rear wheels and project the laser forward. Measure where the beam hits a tape measure placed against your front wheel hubs or brake rotors (wheels dead straight). Comparing the measurements from each side will tell you whether your thrust angle is down the center between the front wheels.

If the thrust angle is correct, check to see if your rear links are "bound up" against each other. "Bound-up" four-links will cause all manner of handling weirdness. Making sure they are all "free" is critical to proper operation.

As a side note, three degrees down on the pinion is pretty high for a setup with no compliant bushings. If you do need to adjust the rear links, reducing that angle some would probably be a good move. It would also reduce your u-joint angles a little.

[treblig]

Originally Posted by Barnsnake

Stop by Harbor Freight and pick up a cheap 18" laser level. Cut a piece of wood (maybe a 1x2) that is the correct length to fit against the bead portions of your rear wheels. Clamp the level to the edge of the board, put it against your rear wheels and project the laser forward. Measure where the beam hits a tape measure placed against your front wheel hubs or brake rotors (wheels dead straight). Comparing the measurements from each side will tell you whether your thrust angle is down the center between the front wheels.

If the thrust angle is correct, check to see if your rear links are "bound up" against each other. "Bound-up" four-links will cause all manner of handling weirdness. Making sure they are all "free" is critical to proper operation.

As a side note, three degrees down on the pinion is pretty high for a setup with no compliant bushings. If you do need to adjust the rear links, reducing that angle some would probably be a good move. It would also reduce your u-joint angles a little.

Jim,
Thanks for the info. On the thrust angle what is common 5 degrees or 10 degrees? Won't this wear your tires becuase they are not straight in-line.


The pinion angle is at 3 degree down at ride height. Based on motor mount location with my manifold angled about 1 to 2 degrees down that's where the tail shaft ends up. I will do as you said with the laser this weekend.

On the "Bound up", there is none. I checked for bind before I installed the shocks.

[vector1]

i think you are jumping the gun on this whole thing. you can't tell anything without an alignment, and you don't want to spend time and money on an alignment until you get everything on it and ride height set, etc. getting everything close at this point is good, but you can't make any determinations as to the setup until the above is performed.

you can set it to ride height and put an alignment on it, which will tell you if the solid axle is toed in/out, where the thrust angle is, and adjust the front suspension. might be a good idea to do before you install the body. btw, an 1/8" turn of a radius rod will be the difference between in alignment or out of alignment, on my machine leastways. you can't see it though.

[Jerry Clayton]

I believe from the info you have given, the responses, your responses to those and your discriptions-----

[Dwight]

Treblig, drove his Cobra over to my shop last week and let me drive it.

I think the upper and lower control arms are mounted to the frame to close together. Making them parallel, not triangulation. If the upper arms were mounted closer to the center of the car, I think it would stop the side to side movement. The rear end moves with the least amount of acceleration or deceleration.
If you look at his picture the uppers are mounted to the frame just above the lower mounting points. Is this to close together to prevent side to side movement?

You might be able to correct the problem with a pan hard bar.

Dwight

[Jerry Clayton]

Dwight

I would like to see more pictures of his frame and for him to be a little more descriptive of how he checked from a hard point of the frame to where on the axle housing--

It appears to me in the pict that the frame is 2 x 4 tube and has absolutely nothing to add to torsional strenght behind the trans mount(and I can't even see that in the photo)

Yes his triangles are very small and change several degrees on any movement of the rear end in relation to frame. In fact they are more like an open sided polygon more than triangulated.

The Shell Vallies I have seen are a very little bit similar but the uppers are almost straight across so rear can't move sideways--they are so restrictive that there is very little drop of the rear when you lift the car.

[xlr8tr]

as mounted, the top links will force the axle to move in and out, rather than just up and down. Look at the pictures above and think about the arc that short arm travels in: under either compression or extension, it tries to pull the axle towards the center of the vehicle. If both sides go up or down at the same time you will be ok, but if only one side goes up or down there is an immediate force trying to pull the axle in towards the centerline. Longer links mounted closer to the centerline would minimize the effect, as the arc is bigger creating less "pull".

[xlr8tr]

from a Car craft article: Rear Suspensions and Increased Traction- Car Craft Magazine

The Application of Power

When power is applied to the pinion gear and into the ring gear of the rear axle, the pinion tries to climb the ring gear. When viewed from the front of the car, the clockwise twist of the pinion attempts to lift the right (passenger-side) rear tire off the ground and plant the left (driver-side) tire. This is the natural reaction of all rear axles to torque input. This also explains why drag racers place a certain amount of preload on the right rear tire to counteract this force. An example of this is the use of an airbag over the right rear axle that preloads the chassis to counteract this torque reaction.

At the same time that the axle is attempting to lift the right rear tire, the body is twisting in the opposite direction, which normally results in the body squatting over the right rear. All of this is the reaction to torque input. The more torque you apply or the more gear ratio you use to multiply the torque, the more twisting effort is applied to the chassis. Drag racers and suspension engineers have collaborated to create very specific ways to explain how all this happens and have also come up with ways to manage the power in a systematic fashion.

My comment based on the bold type above would be, as the body squats over the right (passenger) rear, the short upper link on the right side pulls the axle FORWARDS towards the center of the car, which will result in the car steering to the RIGHT.

[garyjettrike]

I think the top link is far to short and as it is at an angle and only 9"long it proberly only scribing an arc of 6" , wear as the bottom link is scribing an arc of 12",
when one wheel goes up and the other down this will try to twist the axle or pull the links off of the chassis

[Barnsnake]

Thrust angle is where the rear wheels are pointed relative to the front wheels. The center point between the rear wheels should be heading directly toward the center point between the front wheels. That would be a thrust angle of zero. Otherwise the rear is trying to pass the front under acceleration. This may not be the cause of your problem, but it needs to be right to start with.

I'm not sure if referencing the pinion angle relative to the carburetor mounting flange is a good measurement. Manufacturers have cut all manner of angles on those flanges to compensate for driveline angles and acceleration compensation for the fuel level. If possible, try to get an actual measurement of the tailshaft angle. The machined surfaces on the u-joint yokes are generally parallel with the tailshaft. Failing that, checking the angle of the lower crank pulley would be a good reference if you have room. Check it, roll the motor 180, and check again to make sure the pulley runs true. That angle off vertical would be equivalent to the tailshaft angle off horizontal. With solid links on your rear suspension I would expect one degree down to be more than enough to compensate for any change due to torque reaction.

Keep us posted on what you find.