Engine Bay Brace and the Front Won't Turn In

[260DET]

On the 280ZX which used to handle great but now on the same cold semi slicks has lost nearly all grip on turn in after fitting a triangular brace from strut top to strut top and back to the reinforced center of the horizontal firewall seam. The difference, with nothing else changed, is spectacular. It was a circuit track test day for the auto so we were concentrating on other things but it was like driving on ice at the front, with cold tyres anyway.

 

Everything is adjustable on this Z, including rebound and low and high speed bump. Any suggestions where I should start? 

Edited March 23, 2016 by 260DET

[74_5.0L_Z]

The structure of the front of your chassis was probably flexing significantly prior to adding the front bracing.  A flexing chassis acts like a spring in series with the springs on the struts.  By stiffening the chassis you effectively raised the front spring rate.  If the car was neutral before the added bracing, the car will now understeer.

 

So, either decrease the rate of the front springs, go to a smaller diameter front sway bar, or increase the rate of the rear springs. 

[260DET]

That makes sense, thanks.

[johnc]

Load transfer in front also happens quicker.  You can also increase front track to see if that solves the problem.  I had similar issues with my racing 240Z and increasing front track by 19mm fixed the problem.

[RebekahsZ]

Hate to sound stupid, but could you just take the brace back off?  Maybe warm the tires up?  Switch the tires from front to rear?  I had a set of tires age-out over the winter a few years ago, and they were terrible simply from age.  New tires and I was good-to-go!

[260DET]

Going through the log book reminded me that the car has a history of sensitivity to low speed bump adjustment at the front and understeer. Easy place to start. Front track is at the max already John but I like the idea of trying changes that will potentially benefit the car overall. My first thought was tyres RZ but they were OK a couple of weeks ago so trying reversible suspension changes first seems to make sense here, rather than get a new set of tyres just yet.

[Chickenman]

A very simple change that you can make is to replace the front sway bar mount bushings with rubber, if you are using Poly. This gives a progressive roll rate, letting the front end take a set and then stiffening up. I use to do this all the time on my car when I ran Autocross or Hillclimb. Worked very well on cold slicks or DOT compounds. Usually for high speed Road Race tracks, I would put the Poly frame bushings back, but on some tight tracks with slow corners, the rubber bushing just gave the car that initial " Bite " on corner turn-in.

 

It is a very fine tuning method, so you do have to make sure that you have your spring rates and sway bar sizes well sorted. Autocross and Hillclimbs are a bit of a specialized form of racing as you are always dealing with cold fronts on a  RWD car. 

[Chickenman]

^ Rubber bushing thickness plays a big part in how the above setup works. You don't want a thick, squishy, and soft rubber bushing. Generally something from around 1/4" to 3/8" thick works well for rubber bushes.

[260DET]

The Z's front ARB has rubber bushed links, it would be quite easy to adjust their resilience either by slackening the retaining nuts or replacing the bushes. The sort of thing that can be done at the track too, good idea.

[260DET]

 Softened the front ARB up a bit, not a lot, and the understeer is gone. In fact it's a bit oversteery now so some of the ARB change will be reversed. It certainly happens that once the body is stiffened up small suspension changes can make a big difference.

[260DET]

Routine check showed that the four M8 brace bolts to the firewall had loosened up quite a bit after some track work, obviously there is body movement between there and the area forward of the firewall. I just hope that tightening them does not result in another handling change.

[260DET]

Pic of said engine brace.