Dropped crossmember 2"- what are the effects on camber?

[Guest firebern]

Hey,

 

I dropped my crossmember by 2 inch to fit my V8 under the hood and I was wondering what effect this ad on the camber of my wheels.

I imagine the botom of the wheels being more outward and the top inward. I was thinking of buying the GReddy camber bushings to adjust this. Anybody feels that would not work?

[pparaska]

Ouch. 2" will REALLY change the roll center and camber curves. The most you would be able to raise the pickup points of the control arms would be maybe 1".

 

I'd think you took away negative camber, since the arms are probably quite angled down from the wheels to the pivot points on the crossmember? This is the opposite of what you said (bottom of wheels more outward, top more inward).

 

Was this done for oil pan reasons?

[Guest firebern]

Having the lower part inward is worse.

Yes this was for the oil pan. I didn't think it would make such a big difference.

 

What would be the result of this when cornering?

[johnc]

Did you put spacers between the crossmember/frame mounting point? If you, did, you didn't "lower" the crossmember, you actually "raised" the car. Camber changes are the least of your concerns.

[jeromio]

I disagree. It's very possible to lower the crossmember. One could actually lower it so much that it drags the ground. But this would have no affect on ride height (unless you put casters on the crossmember )

 

The car is supported by the wheel which is supported by the strut. The arm merely locates the strut relative to the chassis. In theory, if you had some kind of super strong strut and top mount point (isolator), you wouldn't need the control arm at all (obviously that wouldn't be very workable).

 

But, by lowering the crossmember, as Pete mentioned, you have altered the geometry. Since the arms now angle downward from the strut to the crossmember pivot point, the distance between the strut bottom and the frame is now effectively shorter. That increases positive caster (wheels like this: /).

 

And in a corner, as the strut compresses, the arm will bend up, further reducing the length of the lower part of the triangle which increases the angle of the outside, vertical part of the triangle and substantially increasing positive camber. This will cause your outside tire to scrub the sidewall and reduce it's ability absorb lateral force and to grip the road.

 

IOWs: it's not good.

[pparaska]

John, I've seen many people make the mistake of thinking that lowering the crossmember raises the car. jeromio is correct - the struts hold up the car and the frame rails. The frame rails hold up the crossmember.

 

I'd think this will handle horribly.

[Guest firebern]

Yes it handles like shit.

 

He`s rigth, cause of the strut it does not raise the car withouth modifying camber.

 

I am driving the car now but it doesn`t feel rigth. I just wanted to know exactly how this changed camber and its effect on the ride from a theoratical point of view.

 

I am working on the suspension next so I will fix this. I think I can shave off 1/2 inch off the spacer and maybe gain an inch with a camber bushing.

[jeromio]

Did MikeSCCA have to lower his crossmember? Perhaps you need a different oilpan?

 

I don't think the camber spacers are going to fix this. If you really must lower your crossmember, consider re-drilling the arm mount pivot hole. You can move it both up and out I did this on my car to alleviate bump steer and to give my car some more negative camber. I moved mine up by 15/16th and out by 1/4. My crossmember is in the stock location. That 1/4 inch really boosted the camber. Maybe too much. The car grips the road insanely. Which translates to extremely excellent cornering, but also some darti-ness and it desperately follows any irregularties in the road. In your case, 1/4 inch or slightly more might get you back to a more normal configuration.

[Modern Motorsports Ltd]

wouldn't a 'bump-steer' (I know that names not completely correct but we know what they are) spacer help out his scenario assuming he wants to keep his Xmember lower? shaves 1/2" from frame, left with 1.5" drop and a 1" or thicker bump steer spacer is a simple job right? then a relocation of his inner pickup point to optimize it all and things should be a LOT closer to being correct??

 

I'll be doing a similar thing to my ZX with ~1" xmember spacers.

[johnc]

Oops! I stand corrected.

[johnc]

Erik Messley and I modeled a 2" crossmember drop on a computer simulation of the 240Z front suspension. Here is what got spit out:

 

1. The effect is the same as lowering the front of the car over 4", only worse.

 

2. The front roll center gets moved below ground level a significant amount (assuming no corrsponding rear suspension drop). This will cause the front end to "skate" on corner entry. Its like driving on ice. Because of the low roll center there is little load transfer to the outside tire so it doesn't "bite" into the turn.

 

3. The built in, OEM, camber curves are upset. The front suspension actually gains positive camber in bump instead of the factory (and much more desireable) negative camber gain.

 

4. The inherent bumpsteer in the 240Z is exacerbated. You would need bumpsteer spacers between 3 and 4" in depth to correct the problem. But, bumpsteer spacers of this depth would require a 19" wheel diameter to clear.

 

A better alternative would be the correct oil pan or cutting, dropping, and rewelding the center of the crossmember.

[Guest firebern]

Holy smokes, great stuff!

 

I'd love to simulate a lot of different

stuff on a computer. Are you a mechanical engineer?

 

Sounds like I am in for big mods. I can easily change the oil pan but I'll have to redo my mounts to lower the engine some more. I don't trust cutting and welding the crossmember, I cannot believe that it would be as solid as originally. Drilling new pivot points for the control arms is not a bad idea.

 

We discussed about this subject in the past and some guys have told me that a 1994-95 intake is 1 to 1 1/2 inch lower, but these are not easy to find cheap in Canada(junk yard). Plus I'd need to know if that's the lower intake and/or upper intake.

 

I didn't think that was going to be such a problem. Tonigth, I'll take a look at how much I can shave off. The hood is the problem.

 

Fire.

[Drax240z]

Firebern, if you want to fool around with 'what ifs' for your suspension on a computer, there is software out there. One good one that I've used in the past is at:

 

http://www.performancetrends.com/rc.htm

 

Its great for seeing the effect of roll center changes with mods to suspension geometry. We used this in part to design our FSAE car's geometry. Of course we also had another $32,000US suspension program from machine dynaimcs inc. given to us on a trial basis, so we did use that a bit too. But for the price, the performance trends stuff is good. (free) They have lots more stuff at:

 

http://www.performancetrends.com/