Should I relocate the crossmember pivot points? (I have searched)

[pjo046]

Well, first of, I have searced the forums and found much information on the subject. If I get the mod done, it is not me performing it but John at baddog z parts as I am buying a new crossmember from him(My is bent, so I needed a new one. Turns out the whole frame may be a bit bent....)

 

Anyhow, what I want to know is: Is this something worth doing on a primarely street car? I have gotten Ross C's coilovers, with 175 springs in front and 200 springs rear. I am going to lower the car as much as I can without sectioning the struts. I read on several of the threads I found when searching that relocating the pivot points is only for racers etc, and that a street driver won't notice the difference. Is this so? Am I perhaps better of just buying a set of bumsteer spacers?

 

My previous Z(260Z 2+2) I lowered 1.5 inches with the MSA lowering springs.

I did not install bumpsteerspacers on it or modify it in any other way. Well, that car drove much worse after I lowered it. It would chase alot on the highway etc.

 

So basically, what do you recommend I do? Keep in mind that if I do get the pivot points relocated, I won't have camber plates, adjustable control arms or anything like that to correct any errors in the geometry that may arise.

 

Please advice on this one. Also if you know a very detailed description to make sure the new pivot points are located on the correct place horisontally, please let me know, as that wasn't described much in the threads I found.

[fastzcars]

I'm sure JohnC could probably explain better than I, but I don't think you'll notice bump steer on the street. Maybe on a LONG bumpy freeway/expressway simicircle onramp would bumpsteer rear it's ugly head. I wouldn't relocate the pivot point for a street car. save the money.

[johnc]

What will be the car's ride height when you're all done? FYI... ride height is typically measured at the front and rear of the rocker panels.

[ezzzzzzz]

I'm struggling with this too. I have a spare crossmember that is ready to mod but wonder if it's worth it. The idea of the spacers is to relocate the steering knuckle back to it's original relationship with the LCA. It does not fix bumpsteer persay. A quick synopsis... the LCA moves through a given arc as it travels from full droop to full compression. The tierods also follow a given arc through this movement. No bumpsteer would exist IF these two arc followed the same path. Bumpsteer is when the tire is pulled in or pushed out by the tierod arc because it does not follow the LCA arc. It can be small or larger movement dependent on this arc relationship. There is bumpsteer in all stock cars to some degree. Modifications can increase the bumpsteer. Back to the spacers. Lowering the car moves the LCA into a more compressed postion. Bumpsteer is more apparent at the out edges of movement so you may have induced more bumpsteer in lowering. The spacers move the tierods down so they are now in a more neutral angle with the LCA and close to the factory setup. The question is how much spacer do you need. That is depedent upon how much the LCA moved from it's normal position. The lower the car the longer the spacer. The problem now is the tierod will probably contact the stock 14" wheel. Another option is bending the tierods straighter to account for the drop. That way well do more to eliminate bumpsteer than anything but it's an experiment at best. Do you bend, grind or put bigger wheels on? Your call here. Relocating the LVA pivot point is intended to recreate the factory drop angle of the LCA. Spacers would then not be required. Bumpsteer is not corrected by either method alone but the intent is to acheive the factory bumpsteer they deemed allowable. I run 17" wheels so I'll probably go with spacers to test the waters because it's just easier.

[JMortensen]

http://forums.hybridz.org/showthread.php?t=103886

Just moving the pivot up 3/4 or whatever won't fix it, but as ezzzzz says neither will getting a spacer that is 3/4" thick or 15/16" thick. In fact, you're basically doing the same thing. That said it would probably be good enough for a very agressive street driver.

[pjo046]

I am aware that it won't remove the bumpsteer. But, it will reduce the bumpsteer at least. Or, I could be mistaken bumpsteer for something different. Like I said, my last Z handled much worse after being lowered. This is what I want to avoid. The question really is: Should I relocate the pivot points or get spacers? I am aware of that both these methods do the same thing. But I have heard people say that relocating the pivot points is the better option of the two. And also heard some say that only racers should bother with relocating the pivot points.... So which is it?

 

As for my ride height, it will be as low as I can with the setup I have. Stock top mounts, unsectioned struts, illuminas front and rear and MM coilovers.

So I don't know how much I can lower the car from stock ride height and still have sufficient suspension travel. 2.5"? 3"? As mentioned, my springs are 175 in front and 200 in the rear, so it's not the hardest setup which means I will need more travel than if I had say 250 in front and 275 rear.

[JMortensen]

I am aware that it won't remove the bumpsteer. But, it will reduce the bumpsteer at least. Or, I could be mistaken bumpsteer for something different. Like I said, my last Z handled much worse after being lowered. This is what I want to avoid. The question really is: Should I relocate the pivot points or get spacers? I am aware of that both these methods do the same thing. But I have heard people say that relocating the pivot points is the better option of the two. And also heard some say that only racers should bother with relocating the pivot points.... So which is it?

It does the exact same thing. If you move the pivot out to get more camber then that is another thing entirely because the increased camber will help traction in corners, but moving the pivot up 3/4" or putting a 3/4" spacer on the outboard end does the same thing. Putting a spacer in is a lot easier.

 

As for my ride height' date=' it will be as low as I can with the setup I have. Stock top mounts, unsectioned struts, illuminas front and rear and MM coilovers.

So I don't know how much I can lower the car from stock ride height and still have sufficient suspension travel. 2.5"? 3"? As mentioned, my springs are 175 in front and 200 in the rear, so it's not the hardest setup which means I will need more travel than if I had say 250 in front and 275 rear.[/quote']

Probably about 2" is as far as you want to go.

[pjo046]

Oh, and I'll use 17x7 rims with 225/45 - 17 tires.

[pjo046]

Ok. So if I move the pivot point out a bit to get more camber, that helps traction. But then again, that gives me more bumpsteer...

:-/

[johnc]

Well, that car drove much worse after I lowered it. It would chase alot on the highway etc.

 

That's not bumpsteer. What you felt was a common issue with Zs when large wheels and tires are installed and/or the car is lowered. The alignment needs to be corrected to reduce this tendency.

 

Bumpsteer is a physical steering change when a wheel goes up in its travel as a result of a bump or roll.

[ezzzzzzz]

Referring to my previous post, bumpsteer is a change in toe setting as the tire moves from compression to droop. The affect (and the overall toe change) can be anywhere within this vertical movement. Let's say the inner tierod pivot points were exactly where the inner LCA points were located horizonally and vertically but physically forward of the LCA pivot point by 5". If the outer LCA balljoint and outer tierod were also in this identical relationship then both tierod and LCA would move in identical arcs from compression to droop. There would be no bumpsteer because the tierod would not move the tire inward or outward as the tire's vertical position shifted. This only applies when going straight! If the pivot points are not in the same plane both inner and outer and/or the distance between the pivots points is not identical, both inwards and outward, then the tierod will push or pull the tire (bumpsteer) even if the steering wheel is held stationary. The greater the differences in any of the three planes the greater the bumpsteer effect. Bumpsteer is typically greater to further you turn the wheels. You can measure this effect by checking the toe setting at static ride height, then jacking up the front so the tires are hanging freely and then with several good friends sitting on your nice straight hood to force the tires into the wheelwells. The toe will have gradually changed to some degree at the farthest most points. That is bumpsteer. The only way to minmize it is to get the car on an alignment rack, pull the springs, measure the existing bumpsteer and then start moving steering racks, relocating pivot points, etc. to find a sweet spot where it is negliable or gone completely going straight or turning right or left if you race on a particular track with only the applicable turns (think NASCAR). It isn't worth it for a street car and most track cars. Install spacers and expect alignment issues to become more prevalent as tires get wider and traction improves.

[pjo046]

So how about adding a pair of these? http://www.zcarparts.com/store/merchant.mvc?Screen=PROD&Product_Code=23-4190&Category_Code=PSDC10

 

Would that help in any way do you think?

[ezzzzzzz]

They would certainly help with lateral movement of the LCA. This affects the caster more than anything else. Movement of the suspension, back or forwards, would attribute to bumpsteer. If the stock bushings are in good condition then this would really be negliable except when smacking bumps, potholes or roadkill at speed. This specialized item would attribute more to transmitted noise and vibration than anything else on a street car. It is only one simple part of the whole that creates a complex mathmatical formula.

[JMortensen]

What those do is keep the car stable under braking. When you brake hard the TC bushings compress. On a stock Z you can feel the front end squirm under really hard braking. That's the bushings compressing unevenly. One side compresses a bit and that makes the wheelbase shorter on that side and so the car turns (just a little bit) in much the same way that a skateboard turns. Then the other side compresses and the same thing happens in the other direction. With the G machine setup you linked to the nylon/aluminum doesn't compress at all so you don't get that twitchiness from the bushings. It uses a bushing in the back end of the TC rod. If you get that setup save your old rubber bushings and put the best 2 in the back. They are just there to allow the movement of the TC rod, so you don't need a poly bushing back there that will bind up the TC rod's movement.

[johnc]

TC rod bushings are always a compromise.

 

On a smooth race track under braking you want a stiff TC rod bushing. Once you're not under braking you want a compliant TC rod bushing to allow free suspension movement.

 

On a bumpy race track under braking you want a compliant TC rod bushing to allow the tire to "give" for/aft a bit when hitting a bump at the limit of lockup. Without that give the tire can skip over the bump and lock up. Depending on driver prefernece and skill, its sometime better to accept a little squirming under braking then to have to deal with intermittent front wheel lockup.

 

As I said, its all a compromise.

[pjo046]

So I should stay away from the polyurethane T/C-rod bushings that I got with my bushing kit? Because they will be to stiff to allow enough movement? Can that have been some of the problem on my previous Z?

 

I must say this thread has become quite informative for me!!

[JMortensen]

Read this: http://forums.hybridz.org/showthread.php?t=87758

[pjo046]

Ouch.... So I should definately use rubber bushings on the side that is away from the strut, and perhaps polyurethane on the other side.

 

Or, I could get a set of these:

http://cgi.ebay.com/ebaymotors/Datsun-240Z-260Z-280Z-Tension-Control-Tie-TC-Rods_W0QQitemZ7996706444QQcategoryZ42609QQrdZ1QQcmdZViewItem

 

And use some rubber in between to reduce the noise.

[johnc]

But wait...

 

I ran poly TC rod bushings for over 10 years without a failure. I know a number of other 240Z racers and autocrossers who have similar experiences. The link above regarding the failure was on a 280ZX which has the tension rods going forward (so that they are true "tension" rods).

 

Although it won't account for all failures, a good many of them are caused by over torquing the TC rod nut.

[JMortensen]

There was one guy in that thread who said the same failure happened on his father's 72 (and resulted in the car flipping), and I seem to remember another thread where a couple guys had similar failures on the Z. I think poly in front and rubber in rear is probably OK too, but I'd prefer to have the less friction of the rod end or G Machine setup, but that's just my preference.

 

The only insight I can offer on your (or others') racing experience is that you're probably using 1/3 of the suspension travel compared to a stock Z because of your stiff springs and struts. A stock Z with poly is going to want to move that TC rod quite a bit further when the car hits a bump.