Instability, high speed handling?

[jeromio]

I'm having some problems. My car just doesn't seem very stable - at least, not as stable as it used to be.

 

My baseline is the car with the old 14" rims, front coilovers, new front carts and poly bushes, the old warn out, unmod'ed rear susp. with blown carts and soggy springs.

 

I have done several things, any of which could be contributing to this sensation. I relocated the front control arm pivot points up by 15/16" and out by 1/4". I also sectioned all 4 struts. I have new bushes, carts and coilover springs all around, 150lb up front and 175lb rear.

 

The front still has the stock sway bar, rear still has none. The brakes are all new.

 

With the 17" rims, the car sits up a bit higher than it used to - perhaps an inch. Also, the contact patch is a tad narrower - the old 215/60/14s were fatter on the pavement than the new 235/45/17s.

 

The car has been aligned, and it tracks straight. I have measured the toe front and rear using a tape measure and it seemed good - clearly this is not the best way to measure. I'm considering buying some toe plates. There's not really any kind of a shop out here that I trust, I'd like to have the data and the ability to tweak the settings myself.

 

Anyway, there seems to be something amiss. I used to have no qualms about going over a 100 mph, but now it just seems too twitchy.

It takes diligence to keep it straight on the highway going fast because any road irregularties or crosswinds will move it around and the steering is very touchy - more so than I remember before this latest chassis work. Corrections must be very slight. But, If I yank the wheel, there almost seems to be a delay before it reacts, and then the rear wants to step out. It's hard to describe, but it just doesn't seem natural. (It could be that my speedo is reading slow and I'm going faster than I think I am, but it couldn't be that far off)

 

I plan to get a rear sway bar - but this will be a project since I need to mount it behind the CV axles. Have to find a bar with short arms from the salvage yard. I'm also looking for a front airdam and I'm considering lowering the front another inch.

 

Are there any fairly inexpensive alignment tools that anyone can recommend? Did I buy the wrong tires (Dunlop SP5000)? Anything else I should check out?

 

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240Z.jeromio.com

[Drax240z]

Well yes you are probably going a little bit faster than you think because of the slightly higher tire diameter that your new tires have.

 

Old tire diameter: 24.157"

New tire diameter: 25.327"

 

About a 5% difference. You may want to factor that into your thinking if you know your speedo was accurate before.

 

As for high speed stability, it sounds like you have some type of suspension problem happening.

 

Do you have urethane steering rack mounts & coupler, or rubber?

 

As for the air dam, here's my story. I had a MSA front airdam on my 73, and it was cracked at the point that it attached since I had first got it. It wasn't on there very well, and it would move a fair bit at highway speeds. I didn't really notice any problems.

 

Then one day I got around to refiberglassing it, and I mounted it really solidly. I absolutely couldn't believe how much better the car felt at speed. It tracked straighter, ran smoother, etc. Everything was improved by just tightening it up. I felt I could much more safely travel at even higher speeds.

 

I hope you can get this sorted out, as its been happening for a while now... Doesn't sound too safe.

 

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"Nothing is fool proof to a sufficiently talented fool."

 

Richard Lewis - 1972 240z, Powered by a Nissan 2.8L Turbo Inline 6.

Drax240's Turbo Site

Beginners Turbo FAQ & Answers

[Mikelly]

OK, here is one big concern... You tightened up the front suspension bushings, threw new struts and springs in all the way around, and lowered the car... You have old worn out bushings on the rear... The reason your car is twitchy could be because of the new High Zoot V or Z rated tires that are also taller, and tend to hunt for the groove in pavement, but I'd also suspect that all your torque and load is finding the week link which is those worn out bushings in the rear...

You also might have a toe out issue.

Other opinions??/

Mike

 

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http://hometown.aol.com/dat74z/myhomepage/auto.html

"I will not be a spectator in the sport of life!"

mjk

[jeromio]

I guess I wasn't clear: The entire suspension is now completely refurbished. I have new poly bushings everywhere. The only piece of rubber that did not get replaced was the steering coupler (they sent me the wrong one). Front and rear, everything is new.

 

Also, I have a 1980 5sp matched to a 4.11 rear. The 5sp has it's same speedo gear in it, so, my figuring was that the change in tire diameter would be offset by the difference in a 3.9 vs the 4.11 (about 5%).

 

I agree that toe could be an issue. This is why I want to order some toe plates. My understanding is that they allow for a much more accurate measurement of toe.

 

Before the new tires, when I spun out in the rain, Drax mentioned that rear toe could be a problem which worries me a great deal. There's no quick fix for that. I do know that the car was hit in the right rear before I got it. You can see the bondo on the inside of the right quarter. I should probably jack the car up and check the hole thing for square.....

 

 

 

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240Z.jeromio.com

[Kevin Shasteen]

A poly-bushing steering coupler is going to remove some of that slop; now, whether all of that slop can be contributed to the old steering couplet remains to be seen.

 

All I know is that when I received by poly-bushing set the first bushing I attacked was the steering coupler-I couldnt believe how mushy my original rubber coupler was. I could actually squeeze it w/my hand!

 

Now that you have new bushings everywhere except at the steering coupler-whatever weakness that old coupler represented before is now multiplied & will result in front end wondering. Every nook/cranny in the road & every gust of wind is gonna send you from side to side & demand an immediate steering wheel correction.

 

I'ld correct the old steering coupler factor w/the new one as soon as you get it. Then once that factor is no longer a factor you can redirect your efforts to your suspension geometry/rear toe, ect. First things First/get rid of the known's before you attack the unknowns!

 

Kevin,

(Yea,Still an Inliner)

[Guest Anonymous]

I have done several things, any of which could be contributing to this sensation. I relocated the front control arm pivot points up by 15/16" and out by 1/4".

 

I am thinking that the increased negative camber is making the car feel "twitchy". Positive camber aids in straight tracking - that's why OEM's put it in there. Neg. camber gets you a LOT of road feel, not what the public wants usually. My 280Z feels like the steering is disconnected when I drive on the street with my track setting of -3.5, but she turns in nicely on-track. And since race tracks are usually smoother than public streets, I tend not to notice the down side there. Plus, I'm gripping that wheel like my life depends on it, 'cause it does.

 

 

 

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John

78 280Z

Coilovers, cage, blah, blah, blah

[pparaska]

Sounds like either toe out and/or the rubber steering coupler. A worn out one gives a ton of play.

 

How much camber do you have? If it's less than a degree, I'd think that it would have the numb feeling that John was referring to.

 

Go for toe first, if waiting on the coupler. Just try making up a trammel bar and adjusting it yourself. I made one out of a piece of aluminum tubing I bent up to go under the pan, etc. with adjustable ends (nuts attached to the ends with bolts that screw in/out) that I use on the rim lip at spindle height.

 

But you can make a simple trammel bar from a piece of 2x4 with some plywood pieces on each end to extend up to 1/2 tire height. Mark the tire at the front, roll the car, and measure the toe.

 

Anyway, I'd set the toe to have a bit of toe-in and see if that helps.

[jeromio]

These are good comments. I wouldn't expect the steering puck to have that much of an effect. The one I have now seems okay - I could bend it with my hands, but only slightly - the Poly one is of course, much stiffer. (I ordered and installed these front bushes many, many months ago and just never followed thru on returning the incorrect steering isolator).

 

Anyway, I guess I need to take care of that.

 

Strictly by eyeball, the front and rear camber look identical at the current ride-height. I'd love to get like a digital inclinometer to get an accurate reading.

 

I did consider making some jigs out of metal to check toe, etc. I was looking at some pictures and they do seem pretty simple.

 

 

 

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240Z.jeromio.com

[Drax240z]

You may want to check out "How To Make Your Car Handle" by Fred Puhn. Lots of suspension info in there, including procedures for making up your own tools to check everything. Its a pretty darn nice book if that type of stuff interests you.

[TimZ]

Do you still have the old 14" wheels/tires? If so, try putting them back on temporarily, and see if the twitchiness gets any better. The lower aspect ratio 17" tires can definitely be more prone to following grooves in the highway.

 

I agree with everybody else, as far as checking your toe-in, and other alignment settings.

 

Also, from your description, I doubt that this is causing your current problems, but (I'm risking being accused of heresy here )contrary to popular belief, moving the front control arm pivot point up and out by some pre-determined amount (15/16" ands 1/4", in this case) will most likely NOT cure your car's bump steer characteristics.

 

Change it? Definitely.

Make it better? Maybe.

Make it worse? Maybe.

 

You car's bump steer characteristics are highly dependent on it's individual suspension geometry, and in all three dimensions. The ONLY way that I know of to acheive a known good bump steer curve is to directly measure it, and keep readjusting the geometry until it's what you want. This is a huge pain in the ass, and the curve is amazingly dependent on the geometry. Two brand new factory vehicles can have markedly different bump steer curves. If you've ever tried tuning this, then you know that you haven't got a prayer of getting it right by moving it a set amount once, and quitting.

 

That said, there are still two very good reasons to relocate the pivot point -

(1) You can get more negative camber

(2) You can acheive a much better camber gain curve on a lowered car (this is the amount that the camber changes with suspension travel - bump steer is the amount that the toe changes with suspension travel - they are not the same thing).

 

Sorry to rant, but I keep hearing this mod referred to as fixing the bump steer curve (probably because that's what JTR said it does ). It's greatest value is in acheiving a more desireable camber gain. It will change your bump steer, but not necessarily make it better. Don't let anybody tell you otherwise...

 

Exact same story for the 'bump steer spacers', BTW.

 

Oh - one more thing - you really need to change the front and rear sway bars as a matched pair, unless you have an awful lot of experience as a suspension tuner. Increasing the stiffness of the rear bar will increase your car's tendency to oversteer - you are already complaining about the rear steeping out, this will only make it worse. Also, all else equal, the shorter the arm on the bar the stiffer it is.

[jeromio]

I've noticed that bump steer - at least the sensation of the car jerking to one side when hitting a pothole or bump, has lessened. It might still be there, but it's an improvement.

 

I've sold the old rims.

 

I don't have a rear bar on there at all right now. The reason I'm saying I need to find a short armed bar is because I need to mount it to the uprights, below the mustache bar. It's my understanding that the stock bar, mounted ahead of the arms, will interfere with the CV halfshafts. Cutting the stock bar's arms shorter would cause it to be way to stiff. I'll be fabbing up some new uprights ala Terry Oxendale's or Darius's cars.

 

I would like to get a thicker front bar - the one I have currently is the puny stock 71 240 unit.

 

 

When I first re-did my front suspension, and lowered the front end by about an inch (this was with the old 14" rims so the car was much lower overall), I noticed that the top speed went from an indicated 105 or so to almost 115 (this is with the 4sp).

 

I've had the car in it's current form up to 110, with room on the tach to go faster, but it doesn't feel comfortable. I am getting used to the way the car behaves now, but I do think it could be made better. I haven't had a chance to properly measure toe - hopefully this weekend.

 

Besides MSA, can anyone recommend a vendor of airdams for 240Zs?

[BLKMGK]

Hrm, I'm wondering about the relocation of the front mounting points. That could account for much of this - 15/16th sounds high to me for some reason, I thought JTR said a lesser amount and not to move it out or in? There could be some whacky htings going on there.

 

As fo airdams - if you're ever up my way I've got a brand new one in the box here from MSA I've not got th ename of their supplier too so going direct might save some cash - don't have the name handy in front of me at the moment though so ask me later and I'll provide.

[Owen]

Ahhh, I can't remember the direct maker of my air dam....Kaminari maybe? It;s the same urethane one that MSA sells, I'm sure they have plastic/glass ones too. If the airdam moves, maybe a switch to a front end thats entirely one piece is the way to go?

 

I may be wrong here, but wouldnt increasing caster (towards the back) improve your straight line stability? Turning the wheel at a stop would become more difficult though.

 

What about changing the steering knuckles?

 

Owen

[jeromio]

I think having an airdam would be key. Hate to pay the MSA markup - can't really afford much of anything since I plarnked all that dough on the new wheels/tires. Might be fun to try and make one, but I have visions of creating $50 worth of a big messy, fibery glop pile.

 

So, BLKMGK, this airdam that's in the box, are you saying that yer not gonna use it and therefore you're willing to sell it real extra cheap to a a fellow (impoverished) HybridZer??

 

My son wants me to put a rear spoiler on there too. Says it would "look really cool and make the sportscar go faster". But then he only just turned 6, so, I don't think he has much experience to back that up.

 

I like the looks of the 280Z units alot more - the ones that also replace the bumper. But then I'd have to deal with relocating the blinkers.

 

At any rate, I would definately be interested in the numbers or preferably websites of any vendors.

 

Thanks.

[Drax240z]

quote

My son wants me to put a rear spoiler on there too. Says it would "look really cool and make the sportscar go faster". But then he only just turned 6, so, I don't think he has much experience to back that up.

 

I'm laughing my ass off over that! Very well put!

 

I am sure you will see an improvement with the airdam, I sure did! Oh yeah, the rear ones help quite a bit too from my experience.

[BLKMGK]

I dunno' about "super cheap" but if you think the MSA price is high wait till you eat the shipping and handling charges on one of those puppies! I'm NOT going to be using the front spoiler and most likely NOT going to be using the rear wing I've got. I don't recall price to be honest but I could pull out an MSA catalog and weep if you're serious.

 

Oh, urethane spoiler was made by XENON

[pparaska]

TimZ, thanks for posting that bit about bumpsteer and having to tune the location of the pivot. I do believe that this has to do with the factory design being a bit hosed and yes there are factory tolerance stackups and cancellations that will make each car different.

 

BTW, I made my pivot points adjustable for just this reason. I can adjust them from 5/8" to 7/8" above the stock location. Independent of that adjustment, I can adjust them out 1/4" to 3/4". These adjustments are entirely independent of each other. They are locked down positively as well. If I need to move the range around I could easily do that a bit too.

[Mikelly]

This is a good case of typing before I have had my first cup of coffee for the day... Other responses and my overlooknig the critical points you made in your initial post are spot on. It IS that little bushing at the steering linkage that is causing you problems...I had one of those Break off in a car once, first Z I ever owned. I couldn't drive the car the 6 miles to my house because the steering was basically gone. Thank God it gave way while I was in a parking lot.

 

On my crossmember I plan to do a stepped approach with the unit Jamie has in Richmond.. Drill several holes. I won't be moving the point outward since I'll be running the adjustable control arms I'm selling.. Bumpsteer has caused many people to lose hair over the years, particularly in the Zcar arena!

 

Mike

[jeromio]

Does anyone not have the steering isolator in place? What about just having a solid linkage?

 

My steering wheel vibrated so badly with those old rims that now, with the new ones, it just feels eery. The wheel just sits there. Maybe a solid connection would get me back to what I'm used to .

 

Seriously, I don't have any kind of an isolator in the steering of my truck and that's not harsh or anything.

 

Perhaps I'll try replacing the current rubber unit with some steel spacers to see how it feels.

[pparaska]

There are urethane and ABS (solid connection) couplers available to replace the stock rubber one. Check MSA, etc.

 

I've used an old nylon coupler for years, and it does transmit lots of road noise into the wheel, horn button area and really transmits every little pebble to the hands on the wheel. But the entire Z is an intense vehicle to drive, always requiring attention. Especially with wide tires, short steering knuckles, bumpsteer, etc.