High Speed Shaking and Rattling

[mikewags]

I was driving on the expressway the passed few days, and noticed when I hit about 70 - 80mph, my car will rattle a little a bit. Is this caused by the aerodynamics of the front of the car/bumper?

I read this on the Wikipedia Nissan S30 page:

"For instance, simply removing the heavy front bumper and adding a front spoiler corrects the otherwise jittery high-speed handling of the stock version."

How true is this statement?

[gnosez]

You could have any of the following issues causing your problem:

1) tire balance

2) alignment

3) loose steering rack

4) old suspension bushings

5) loose suspension

6) bad ball joints/tie-rods/shocks/spings

7) no front air dam

8) bad front tires

9) car not balanced

Go through the list and check (and correct) as needed.

The S30s need a front air dam to keep the tires in contact with the road at speeds in excess of 70mph. At speeds higher than that the the steering wheel will feel light, as you have all of about 2-inches of rubber touching the ground.

[ConchZ]

Mine did this until I replaced all the bushings in the front suspension and steering. I have the stock bumper and no spoiler. It doesn't have any rattle or vibration at any speed anymore. I used poly bushings. The old ones were shrunken, squished and cracked. Couldn't say if the ride is rougher because of the poly, as it seems the same as it was with the worn out stock bushings. Maybe it'd be softer with new rubber bushings, but the poly seems just fine to me. Before the bushings, I could feel the vibration in the steering wheel. The one bushing I didn't replace was actually the steering coupler. Its rubber and in pretty good shape. I was going to replace it if the vibrations continued, but since they stopped, I kinda like the cushier rubber coupler.

I think you should check for play in your wheel bearings, and make sure your tires are balanced and your front end aligned before digging into those bushings, though. Good luck.

[Zs-ondabrain]

Adding the front air dam WILL give a more responsinve feel at higher speeds as well as give a little more down force on the front end.

My 70' was wobbly as hell at high speeds until I installed the Urethane Air Dam. To correct the inward folding of the air dam at high speeds, I used 1/2" "L" bracket across the front of the lower lip then used the same style L bracket to Hold the lip outwards by screwing that bracket from the frame to the lower lip of the air dam.

This basically helps it hold its shape and give the car down force without folding in. I can now hit an easy 130 MPH without wiggle or loss of steering control. I had already replaced all the rubber with urethane bushings and upgraded to KYB's and Eibach springs but the air dam definately finished it all off.

Dave.

Edited October 5, 2008 by Zs-ondabrain

[Poindexter]

Somewhere Carl actually published the figures on downforce- or the relative lack thereof! I think the lift at speed is around 140 pounds! Versus what should be downforce. I too have the original urethane air dam and have noticed absolutely nothing bad at 100 mph- which is about as fast as the old girl seem to really enjoy going at. No bad manners, no squirrely-ness, no looseness, no shudder, no instinctive "feeling of dread" in the pit of your stomach when a car feels somehow "off" at speed. About the same as she feels at 60.

Of course I've never driven another Z at high speeds so I really have nothing to compare it too- except the various descriptions around here, accurate as they are. I can't imagine what 130 would feel like! I have driven many other cars for long periods of time well above that, not there have been so many cars that I would actually trust (with my life) much above 100. Everything seems to change once you hit that mark, it seems to me.

FWIW, my top speed, legally, on the open road was 162 (measured by GPS) in a heavily modified and chipped BMW 750iL and it felt like there was still another 10-15 miles left in her! Wow.

Edited October 5, 2008 by Poindexter

[d240zx2]

Adding the front air dam WILL give a more responsinve feel at higher speeds as well as give a little more down force on the front end.

My 70' was wobbly as hell at high speeds until I installed the Urethane Air Dam. To correct the inward folding of the air dam at high speeds, I used 1/2" "L" bracket across the front of the lower lip then used the same style L bracket to Hold the lip outwards by screwing that bracket from the frame to the lower lip of the air dam.

This basically helps it hold its shape and give the car down force without folding in. I can now hit an easy 130 MPH without wiggle or loss of steering control. I had already replaced all the rubber with urethane bushings and upgraded to KYB's and Eibach springs but the air dam definately finished it all off.

Dave.

Ditto, Dave.

My '70 was almost uncontrollable at 85 and above, it seemed. I installed the FG spoiler as shown on my avatar, and at 130 it's as solid as any other car at that speed. It REALLY makes a difference.

Frank

[Arne]

After a couple of freeway trips in stiff crosswinds and lots of freight trucks, my wife declared to me that she wouldn't ride in the car at those speeds (65-70 MPH) anymore, until I fixed it. Rather than the big air dam, I added a ducted Spook instead. Made a HUGE difference. The car is far more stable, at much higher speeds. Very solid feeling at up to 90-95, which is all the faster I'm willing to push it on public roads these days. It was a very worthwhile addition.

[mikewags]

What is the average price of an airdam/spook ... and who sells them?

Thanks! Appreciate the advice guys.

[gnosez]

The issue here is to remember that any application of downforce at one end of the car will cause lift at the other end. Adding a front airdam means you need a equal downforce devise at the rear. Add a rear wing and your front airdam better provide more downforce than the wing.

Balancing this all out makes for a more stable and fun car.

[Zs-ondabrain]

What is the average price of an airdam/spook ... and who sells them?

Thanks! Appreciate the advice guys.

The is a page from MSA, it's good for starting out prices, also check your local Craigslist. I found the Urethane Air Dam that MSA sells for $199 for $30 just down the road from me.

http://www.thezstore.com/page/TZS/CTGY/7AD2

Dave.

[Arne]

The issue here is to remember that any application of downforce at one end of the car will cause lift at the other end. Adding a front airdam means you need a equal downforce devise at the rear. Add a rear wing and your front airdam better provide more downforce than the wing.

Balancing this all out makes for a more stable and fun car.

True, although front downforce alone is better than none at all. I've also heard - in connection with the Spooks - that if you're only doing the front, the ducted version is a better choice, as it has less downforce than the un-ducted version. If true, the un-ducted one is definitely best used with a matching rear spoiler.

What is the average price of an airdam/spook ... and who sells them?

I keep forgetting that your car is a 280Z, Mike. Spooks only fit 240Zs, and maybe the early 260Zs.

As for air dams, MSA and Black Dragon are the easiest sources to browse.

[mikewags]

Thanks guys, much appreciated. I will look into this. I still have a 3 cans of my original car paint left so this is a definite upgrade possibility.