Lowering front end, caliper now hitting top wishbone control arm

[ardiesse]

Warning: long post.

I've been watching this thread, but avoided commenting - until now.

The theory behind mounting the top balljoint under the upper control arm, and also the racing HQ extended-stud top balljoints, is to improve the suspension geometry. By making either of these changes, you are raising the roll-centre of the front suspension. This is a good thing.

The Holden engineers designed understeer into both the front and rear suspension of Holdens.
Front first: At stock ride height, the inner pivots of the top control arms are somewhat higher than the upper outer pivot. This places the roll-centre of the front suspension slightly below ground level. That's Effect Number 1. Effect Number 2 is what happens with camber as the front suspension goes into bump: the camber goes more positive, because the top control arm swings outward as the suspension goes into bump. This is a bad thing for handling, because as body roll sends the outer front wheel into bump, the front suspension adds more positive camber to the amount produced by body roll. The result is understeer, and lots of it. Google "HD Holden Bathurst 1965 images". It's frightening.
Rear next: Have you noticed that the rear spring shackles in early Holdens up to the HR are underslung? That causes the rear spring to slope down toward the front, and produces (you guessed it) designed-in understeer. Body roll combined with the down slope of the rear springs makes the rear axle steer the back of the car in towards the centre of the corner, which equals understeer. Another consequence of underslung rear spring shackles is to raise the rear axle's roll centre, to very slightly above the centreline of he axle.
Front and rear together: A Holden's front roll centre is slightly below ground level; its rear roll centre is slightly higher than the centreline of the rear axle. When you rule a line through the rear and front roll centres, the line slopes down towards the front. That is a recipe for understeer (Who'd have thought it?).

In the late '80s and '90s I had an HD X2 Premier, and I started mucking about with suspension and braking components to improve both. The first thing I did was to fit HG discs, calipers and steering knuckles along with 14x6 wheels. Now the HG steering knuckles are effectively a 1" drop stub compared with HD/HR hardware, but I didn't appreciate what that meant in practise. The car sat too low at the front, so I spaced up the front springs to restore the ride height. And the car handled worse than before. Why? Because of the geometry of the upper control arm. Raising the ride height made the top control arms slope downwards, which increased the car's tendency to understeer.
Slightly chastened, I bought a book called "How to Make Your Car Handle". It suggested measuring the camber as a function of body roll angle, so I pulled the front springs out of the X2, reassembled the front suspension, used a jack and a couple of blocks of wood to dial in body roll, and a trammel and plumb bob to measure camber. The HKTG stub axles on an HR front suspension at standard ride height were bloody awful. HD/HR stub axles were better, but by no means brilliant.
Then in another book, there was a passing mention about how the engineers made handling improvements to the XU-1 Toranas by dropping the top control arm inner pivots about an inch. I figured I'd give it a go. So I dropped the front suspension out of the X2 and set to work with an angle-grinder and welder. Lowering the top control arm mounting points sounds easy, but it took quite some reworking of the shock towers in the front crossmember. And when I did this, I discovered that when the front suspension went into bump, it produced negative camber. Hooray! So now the front suspension counteracts body-roll-induced camber changes.
I couldn't use HKTG steering knuckles (ride-height troubles), I couldn't use Torana steering knuckles (steering axis inclination incompatible with HD/HR suspension), so (shit eh?) the best equipment to use is that which was designed for the job: HD/HR steering knuckles. But they do take Torana discs and calipers. And again I made the wrong choice: LX discs and those bloody awful single-piston PBR calipers. After a couple of years trying to get them to work right, I said, "Bugger it, I'll go back to a proven, but low-tech solution: XU-1 discs."
With XU-1 discs and calipers on an HD/HR balljoint front suspension, and lowered top control arm mounts, the car handled way better and had half-way decent brakes (the X2s had 2-1/4" rear brakes). I got an extra 10000 miles out of my front tyres with the suspension mod too.

My two cents' worth: I think you're pushing the envelope maybe a bit much. The fact that your calipers are hitting the top control arms at full lock and full bump means, quite simply, that they're too big for the front suspension to cope with. Drop stubs only make the interference problem worse, and you run the very real risk of making the car handle badly and wear out your expensive front tyres by using components that your front suspension wasn't designed to take. Sure, the suppliers will sell you all the parts, and yes, they will go together; but the devil is in the details.

At the very least, pull the front springs out, set the camber to zero at your desired ride height, put a hydraulic jack under the crossmember, and run the front suspension from full bump to full rebound and measure how the camber angle changes. If the camber goes positive as the front suspension goes into bump, that's a sign of danger.

Rob

[eKay]

Thanks Rob,
Appreciate your input into this. It's an issue I inherited and something I should have left "as is" for now and i have no doubt thrown it into disarray, unaware that the parts installed are not matching to the original engineers report, which has made it all the more difficult. The springs I took out, whilst clearing the caliper created the handling that was dangerous at highway speed judging by your comments. The inner pivot point was so much higher than the outer, it was clearing above the caliper, but causing terrible roll over bumps. The lower springs have bought the two pivots closer to level but now the new problem.

I've semi digested most of what you have written and need to mull over the rest and how to get this thing at least driveable in the short term. I'm pretty sure I own the book "How to make your car handle" but had forgotten about it. Hadn't even crossed m mind to refer to some of the books I had.

Thank you for your thoughts.

Mick

[Blacky]

This book here is a good read too - Chassis Engineering by Herb Adams.