Bump Steer Correction

[MD]

Whether on the street or track, when a car is lowered typically it will need a little or a lot of bump steer correction. To date I have not seen  what I consider a safe and appropriate way to do this on a transaxle vehicle. It is tempting to heat and/or weld the steering arm on the upright but how much does that affect the metallurgy?  I don't have the means to find tha tout.

I am presently building up a 75 racer and sooner or later I am going to come up to this problem (again). I am looking for a better solution than I have used in the past-if there is one.

For the sake of the discussion I am attaching a possible solution that I have been pondering upon for a while. The designations of "X" in the photo indicate fixing points of the modification.

Appreciate your feedback.

[vin sharp]

The factory GroupA cars had the roll-pin that sets the position of the steering arm/stub axle removed, then the stub pressed out and the arm rotated UPWARD to it's new position, shrunk/pressed back in and the new pin position drilled and set back in.

[MD]

Thanks Vin.
I have considered that before as well and it is a good idea. The only snag is that for amateurs it is problematic to know exactly where the bump steer correction should be and may require some trial and error. Using this procedure could be quite onerous. For example one of the lowered Alfettas I built required a drop connection of around 80mm from the original steering arm location. That's bloody huge but it's where it needed to be.

So perhaps a blend of the two. Install a calibration trial plate to establish the desired position followed by the factory lock pin revision?

[vin sharp]

Yes, you must use a toe/travel calibration plate and double dial-gauges to get any idea of what's going on.
I do recall some years back a (very successful)  Targa competitor in a 105 bought a UK produced "bump-steer' kit. When fitted it lowered the steering arm by quite an amount, then checking with a calibration plate it seemed to make no sense at all. The kit certainly added bump steer!  He rang and asked me my thoughts. I suggested that the factory always went upward with the arm to correct the faster-shortening top arm relative to the lower, given that the stub axle steering connection is located in between the heights of the upper and lower arms. This worked out better than the lowered version when he dummied it up and checked with the gauge. 
IF the steering rack/box was located on exactly the same horizontal plane as the lower arm, and the steering connection to stub at the same height as the lower ball joint, all would be near perfect. However, we have a mid-mounted steering-to-stub height, which tracks at an arc somewhere between the different shortening rates of both the lower and top arms. This is why the plate gauge is so useful to find a (narrow) sweet spot between the two.

[MD]

All good info Vin. I am sure the guys appreciate it.
Looking back at my Alfetta sedan racer that runs upside down ball joints, it makes a conundrum of its own. Anyhow it was resolved with the set up illustrated in the photo below.

On the surface of it, the set up looks correct and it is tough. It actually stiffens the upright (hopefully the spindle is up to the forces  ) However I was always uncomfortable with what all that welding did to the metallurgy. Submitted if there is reader interest.

Cheers, MD

[Colin Edwards]

I invested in one of these to understand and calibrate suspension kinematics on sons Formula Ford Duratec.

https://www.longacreracing.com/products.aspx?itemid=2448&prodid=7239&pagetitle=Precision-Bump-Steer-Gauge-with-Billet-Plate

A very handy widget.  Sold it for about 75% of its purchase price.  Takes much of the trial and error out of minimizing suspension deflection induced toe changes.  A very good investment!

[Duk]

Definitely not an answer for the factory TA type upright, but if you could use 105 uprights.
They use a bolt on steering arm, that is lower on the upright to begin with, so any custom made steering arm would need less bending to correct bump steer.

Comparing the 105 upright to a 75 upright, the balljoints are the same distance apart (well, the machined surfaces that the nuts work against are). The 105 top balljoint bore is larger that the TA one.
The stub axle is about 10mm lower on the 105 upright.
And the stub axle is also larger in diameter and longer, on the 105 part.

[Colin Byrne]

Another option for measuring bump steer that I've  used in the past, is to lift the car on stands after all the static settings have been checked then with the steering locked, and suspension spring removed, attach a laser pointer to the hub of the front wheel. With a big piece of cardboard set up in front of the car you can trace the path of the laser with a marker as you move the suspension through its travel.
With a few different color pens you'll get a good idea of how the bump steer has been effected as you adjust steering arm locations

[Mick A]

Jeff Micklem did a video of this on his youtube channel "Home built by Jeff" when setting up the steering for his Ferrari engined 105.

https://www.youtube.com/watch?v=9Ss7Su4bkoM

Cheers.