Lowering steering rack and the effects of doing so

[SpartanZ32]

Hi, ladies and gents...I'm a total newbie to this forum, and not even an engineer. However, I am quite the "tinkerer", and love designing and bulding new things.

So with that being said, I am currently working on a 1990 Nissan 300zx....doing an engine swap (V8). The issue I'm having is that the steering rack is mounted a little too high on the front crossmemer, and it is taking up the space where the oil pan needs to be. I have no problem lowering the rack the 2.5"-3", but my concern is bumpsteer.

Will lowering the rack yield negative results (ie: bumpsteer, etc)? One thing that crossed my mind is that if I lower the rack 2.5", can I "correct" this by adding a 2.5" spacer to the tie rod end where it meets the knuckle? In my head, this will keep the same geometry as factory. Am I right or wrong?

I know this might be hard to answer from just reading my post. I can provide pictures if necessary.

Thanks in advance!!!! Any help will be gratly appreciated.

 

[GregLocock]

The tolerance for meeting the designed bump steer curve is typically 0.05 inches. Your proposed mod is thus a complete redesign of the suspension rather than an adjustment.

Your fix will not result in the correct bump steer curve, although it is not a bad place to start.

I assume the suspension is a double wishbone of some form.

Getting the bump steer curve wrong won't kill you at low speed and low latacc.



Cheers

Greg Locock

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[patprimmer]

Why not notch the sump or raise the engine a little or a bit of both.

Regards
Pat
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[MikeHalloran]

Or push the engine back or move the wheels forward.

DO NOT put a spacer in the tie rod end attachment. That will induce extra torsion stress in the steering arm, to a degree for which it's not designed. It will break. It will break at the wrong time for you. Your survivors will be very sad.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Yes

If fitting a V8 to a car designed for a V6 pushing the engine back a bit is a good idea for several reasons. It may involve shortening the tail shaft, moving the transmission mount cross member, cutting and enlarging the firewall and transmission tunel areas and maybe a much modified sump and oil pick up if it is a font pan sump. Also check out different OEM sumps that might be available for that engine for when it is fitted to different models.

Regards
Pat
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[SpartanZ32]

Thanks for the replies so far. I can do all of those things mentioned, but there is a downfall to all:

- Move the engine back to clear the sump: This will require cutting the firewall, which I don't want to do. I am trying to keep the car as unmolested as possible (other than the new engine...haha)

- I can move the engine higher, but that would require a hood scoop to clear the throttle body. I love the way the 300zx looks, and a hood scoop is not very appealing to me.

- I cant notch the oil pan as much as needed, because the area needed to be notched would be where the crank rotating assembly is. I have found an oil pan that is only 1 13/16 inches over the rack....so this is the best oil pan for ths application.

So basically the only thing left to do is to lower the rack. Is there ANY way that you guys can think of to correct the bumpsteer? (If not, I guess I'll have to look into re-designing the front suspension....maybe newly designed steering knuckes?!?)

 

[thundair]

Dry sump will give the lowest profile



I don't know anything but the people that do.

 

[patprimmer]

Thundair

This application has plenty o room for a normal sump once clear of the cross member. A normal sump can be within 0.060" of the rotating assembly for over half its length. A dr sump cant e closer than that, it just doe away with the an section.

Regards
Pat
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[GregLocock]

Yes, a few hours work with a suspension analysis program or a drafting table will give you the same bump steer as you currently have, with the rack dropped by 2.5"

It may involve changing the rack width, it will certainly need a new steering arm on the spindle, and a tie rod to suit.

Depending on the actual layout there may be compromise, as the new outer toe rod location may need to be where the wheel rim is, for instance.

There are ways around that which again will involve further changes.

This is one of the things I do fairly frequently at work, it is not cheap, it is not easy, but it can be done.

You still haven't told us what sort of susensio it is, or where the rack (in front of or behind the wheel, high up or low down).

No more guessing.







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SpartanZ32]

Ok, no more guessing. Here are the details:

- The steering rack is located behind the crossmember
- The steering rack is mounted directly to the crossmember, and sits approx 2" higher than the crossmember



I plan on leaving the crossmember in its factory location, just lowering the rack approx 2" or so. What if I modified my steering knuckle by welding on a 2" spacer to compensate for the 2" rack drop?? Would that be sufficient in eliminating the bumpsteer?

 

[GregLocock]

No, as I said above, it would be a good place to start from in theory, but in practice as Mike said, you'd end up with a weak and flexible safety critical part.

The location of the outer tie rod is critical to within 0.05", your proposed method gets it somewhere in the ballpark only.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[datstang]

There are many completed V8 300ZX's in existence. Many of their owners hang out on the HybridZ forum. I frequent that site because I have a 5.0L mustang engine in my 1974 260Z. The members there will be familiar with the details of the 300ZX suspension, and will know the modifications required to correctly install the drivetrain without compromising the suspension.

http://forums.hybridz.org/

 

[thundair]

Patprimer

True

I was thinking in relation to Tarmac not cross member

Back to the OP I remember seeing a fitting or shim that you could buy to dial in the last bit of bump steer it fit on the tie rod end

http://www.crazymustang.com/bump-steer-kit-install.php

I don't know anything but the people that do.

 

[SpartanZ32]

That is EXACTLY what I had in my head!!!! You my friend just made my day!

 

[GregLocock]

I would be willing to sign off use of that contraption on closed circuits at speeds less than 60mph up to 0.4g.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

... but not a bumpy circuit.



Mike Halloran
Pembroke Pines, FL, USA

 

[SpartanZ32]

I'm not seeing how this seemingly minor adjustment would be such a risk. I would think that as long as it has the ability to "swivel" and move with the suspension travel, then it would be ok. And all it does is basically pull the knuckle left/right to turn (if that makes sense).

Check this thread out. A company modifies knuckles on drift cars to give them more steering angle. The modifications are similar to that I was thinking...welding an "extension" to the stock knuckle. So far, asll users have been pleased. and one user even smashed his car into a wall twice, destryong nearly all suspension components EXCEPT his modified knuckle.

http://zilvia.net/f/advertiser-spec...s-shop-max-super-angle-steering-knuckles.html

Your opinions would be greatly appreiated!!

 

[GregLocock]

I think I've given my opinion. There again, I only do it for a living.

The Max mod is far superior to the stack of washers approach, //so long as you know how to weld properly to your spindle//.

Again, it is only suitable for circuit use, your road insurance will be void if you start welding the spindle, in most jurisdictions.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

Uh, "all it does" includes reacting the force when you accidentally bump one wheel into a curb or hit a pothole. If you think the force is insignificant, try driving around with your power steering inoperative for a while, and get back to us.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Oh, what is your spindle made from iron, steel or aluminium? Is it cast or forged or fabricated?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.