Suspension/alignment gurus come in

[JonW]

I just bought a ‘65 Chevy Biscayne. It’s been heavily modified - 454 engine, lowered, wider than stock tires. When I hit a dip in the road and the suspension compresses, it goes every way but straight on the rebound. It’s very squirrelly. I didn’t even get it on the highway to see how it tracks. I’m afraid to at this point. I know I need to try to find an old school alignment guy who can do more than align it t to specs, but I’m not well versed in alignment and suspension. Any thoughts on what to look for so I can not appear to be an idiot?

[Matt Harwood]

I suspect what you're experiencing is the Ackerman angle getting wonky. Ackerman is the relationship between the tie rods and the spindles and on lowered cars, that gets pushed all out of whack. Essentially the steering box and tie rods remain in the same place they were when the suspension was stock but the connection point on the spindle is higher relative to stock (because the suspension is lowered and therefore the body is closer to the ground and therefore the steering box is closer to the ground). It depends on how the car was lowered, but essentially you're now moving the tie rods through a different part of their travel. Since the range of travel is an arc, the distance between the steering box and spindles has been shortened somewhat and gets even shorter when you go over a bump and compress the suspension. It's easy enough to set up a static alignment to keep the car going straight, but as the suspension compresses, the already shorter tie rods pull the spindles inwards more than they should, causing the squirrely handling. It can be especially pronounced while turning a corner where the left and right wheels have to also travel a different radius.

 

Here's a quick diagram of your tie rods (red) as viewed head-on from the front of the car. You can see how the relationship between the tie rods and their attachment point on the spindle can drastically affect steering (the blue arcs represent the travel of the tie rod end on the spindle). On a stock setup, Ackerman is arranged so that the tie rods are moving through the widest point of their radius pretty consistently. But you'll also note that as the suspension compresses on a lowered car, the tie rods pull the wheels inward significantly more than they do on the stock suspension as they move through the upper areas of their travel (the blue arc).

 

What's the fix? Probably some creative aligning, perhaps some spacers on the spindles to move the tie rods back down to an angle closer to horizontal (or whatever stock spec is), or even different spindles. For instance, "drop spindles" mostly avoid this problem because they simply move the location of the wheel relative to the suspension rather than lowering the entire suspension as with shorter springs. There might even be brackets that relocate the steering box lower on the frame, but I'm not sure whether that would require modifications to the steering column. Or you could put it back to stock ride height, but I'm guessing you don't want to do that. It'll be a little twitchy no matter what, but a good alignment shop and the right components can mostly alleviate the problem. When I lowered my 5.0 Mustang and went racing, alignment was critical and I machined some spacers to drop the steering rack a bit to put the tie rods back into a more friendly relationship with the spindles.

You might also check to see if they installed a quick-ratio steering box when they lowered it (or--God forbid--a modern rack). That can add to the touchiness. Putting in a slower steering box will at least reduce that over-sensitive unpredictability.

Hope this helps!

 

[JonW]

Matt, thanks for the good info.