Roll Center...What is desired

[tube80z]

From my own experience testing my 240Z with below and above ground roll centers (yes, I did back to back testing at Buttonwillow but not exactly for roll center positioning). Keeping the roll centers above ground made the car more responsive at turn in and the car would take and hold a line in high speed corners (above 80mph). It was easier and more predictable to drive at the limit.

 

Good info. For what's it worth my car rarely gets over 80 MPH and never in a corner. At most course I run the average apex speed is 45 MPH or less. Which may be why I like this.

 

Cary

[tube80z]

Cary, I know your car that you're building is completely tube framed, all custom fabbed stuff. I know you are also planning on extremely stiff springs. That would counter the roll, and make the roll center not so critical I think. But in a "normal" Z chassis with "usual" spring rates, the roll center makes a bigger difference than it will on your car. We all know that you can't put 800 in/lb springs in a stock tub because the tub itself becomes the spring. With springs not playing as critical a role in countering roll, then RC becomes another way to deal with the issue.

 

I think it depends on what you do with your car. John had good info below on what he did on an actual racing course versus what I do in SOLO II. The tube frame car will have an underground front roll center too. But the 1 inch below ground RC is on my unibody Z car.

 

It has a cage that ties into the stuts. When I got the car it had above ground front roll center and some weird spring choices. It was fast but was hard to drive. I gave up trying to make a three wheeler work and decided to try some advice I'd seen on the FF underground about using spring rates that were some portion of corner rate. I started with 400 lb springs, AD inserts, and GC camber plates. The rest of the suspension is monoballs in front and the delrin bushings in the rear, altough I want to change that to monoballs too. Front roll bar is 15/16 and rear is 5/8. Nothing too out of the ordinary. I guess I do have one thing that is a little different. I droop limit the front suspension, which has a major effect on the roll center.

 

I have tried up to 600 lb springs on this car and it seemed like a good direction to go. Part of the problem is I can't lower the car enough to isolate for the spring change to say that's exactly what it is (I'm assuming people know that you have to change ride height to isolate spring changes). A lot of this will depend on tires too as you have to take into account their spring rate to determine the actual wheel rate.

 

The one thing I do a lot of is change chassis rake to balance the car. And this does change the roll couple. So I agree that it has a major influence on your car. So we are in definite agreement that this is a key tuning tool. I just happen to play in a different speed range with my toy. Hence my advice is for SOLO II. I'm just offering it as something to think about.

 

So what are normal Z spring rates these days?

 

Cary "who only wishes he could afford to play on real tracks"

[johnc]

I gave up trying to make a three wheeler work and decided to try some advice I'd seen on the FF underground about using spring rates that were some portion of corner rate.

 

And I think that's where some confusion comes in when reading Race Car Vehicle Dynamics and some of Carroll Smith's books. All of those experts are used to working with "real" race cars like Formula Fords. Many of the suspension topics ARE about Formula Ford suspensions. In those kinds of cars you don't want any roll at all and you're working with fully adjustable and optimized suspension geometry.

 

Sometimes its very difficult to translate the expert's recomendations onto a strut equipped, cost limited, 30 year old chassis. Occaisionally its down right impossible without rebuilding the chassis into something that looks like a mutant Lola/Van Dieman/VW Bug.

[Jolane]

I also meant to thank Tube280Z earlier for the Auto-ware link. There is a lot of good reading there!

This has definitely turned into a good discussion. Thanks to everyone who listed info about RC's. The comparison that JohnC listed definitely helps in my situation since this will be a street car primarily. Maybe I am going overboard, but that is just what I do.

This topic really stemmed from wanting to make my LCA's length adjustable with Heim joints. While I was over buying the Heim joints I noticed an oval track (dirt) car front suspension, which got the thought process working. I have been considering building my own front crossmember, and welding it onto the body. I need a R&P though, which is where the problem started. I like the Sweet/Appleton/Woodward R&P from the standpoints of mounting, cost, availability, and option of power (if needed in the future). They are rather short though at 18 1/4" pivot to pivot, where as the stock R&P is ~24".

This is exactly where this all started. What happens if the LCA is longer...etc?

This then snowballed into thinking about the T/C rod. What if it was flipped to the front, like a 280ZX? Now it creates clearance for the headers, and I can lengthen it/move the pivot to match the LCA length. It also could allow for more tire space during turning, if needed. Cost is rather low also using "stock car" parts.

I guess maybe this is overboard, although I would like to build a new crossmember to better match the engine mounts. Problem is mounting the stock R&P, plus there is not a power option.

I also feel (unproven) that the stock unibody chassis has some rather poorly designed areas. For instance, the method used to attach the crossmember doesn't make much sense. It seems that the two bolts on each side effectly act as hinges, not really contributing to the chassis torsional stiffness. Welding the crossmember in should add a good bit torsional stiffness to the assembly, turning the crossmember into a torque tube. I don't see a reason not to weld the crossmember in.

Maybe the best thing to do is just use the stock crossmember. It just seems that a scratch built crossmember could be beneficial on other fronts.

Thanks Again,

Joshua

[260DET]

Yeh, excellent discussion.

 

Just one possible point, lowering a Z will lower the center of gravity (C of G), a good thing. That same lowering will lower the roll center, which may or may not be a good thing. Assuming that any such lowering uses the stock suspension pivot points.

 

So lowering the car may still show up in testing as an improvement, due to lowering the C of G and not to the changed roll center.

 

And Jolane, yes, the attachment of the front Xmember to the body would appear less than ideal The only improvement I could think of, short of a major rework, was to use hardened steel flat washers under the bolt heads. They being much less likely to flex and deform than the stock type washers.

[tube80z]

And Jolane, yes, the attachment of the front Xmember to the body would appear less than ideal The only improvement I could think of, short of a major rework, was to use hardened steel flat washers under the bolt heads. They being much less likely to flex and deform than the stock type washers.

 

The weak link is on purpose. This is what lets the motor exit out the bottom of you really stuff the car. Welding in the crossmember will make the front a lot stiffer but if you get in a serious accident the motor will be coming through the firewall. It's your choice.

 

Cary

[Jolane]

Cary,

That is a good point. I never thought about the whole engine issue in a head on. I wonder though if this is effective after a V8 install anyways, considering that:

1. The engine is wider than stock, so it is bound to go through the firewall with the heads, instead of down the tunnel.

2. With the method most use to mount the engine/tranny, I wonder if the tranny holds everything in place, or does the tranny mount shear.

3. Was the cross member ever really designed to drop out.

 

I have not seen any serious Z car accidents that involved head on collisions. It seems to me that the reason it is mounted this way is:

1. for manufacturability, to raise the engine assembly through the bottom of the car (if that is really how they assembled it, assuming that is the easiest)

2. to transmit the load of the suspension/engine/etc into the body effectively. The frame rail box also seems weak and would probably collapse (parallelogram) with any significant load. At least the method puts a shear load into the inner fender.

A real disadvantage to welding is that now I am stuck with whatever crossmember I use, unless I cut it out (a lot of work compared to unbolting).

Thanks for the thought! Yet something else to ponder now..

Joshua

[JMortensen]

Cary, to answer your previous question, most track Z's seem to run between 250 and 400 in/lb springs. Weren't you planning on 800 or 1000 in/lb springs? Or am I just imagining things?

 

Joshua, why not add another brace in front of the engine? Especially easy if you're going V8, cause there is plenty of room. Plus you'd have to crush that brace before you even GOT to the engine.

 

I was in a pretty good head on in my first Z. I got rear ended by a lady who never even saw me. She was going ~50 in a Firebird. Hit my Z which was stopped making a left hand turn, I went into the oncoming traffic and hit an Acura Legend that was going ~35 head on. I figure the combined speed of the head on was probably 50 to 60 mph. The front caved to the core support, and then the frame rails crushed about an inch more. The frame rails tweaked a little bit to the driver's side. That was it. In the back the Firebird's low nose hit my right rear tail light area and pushed the car in about a foot. The tail light lense broke on the right side. None of the glass in the passenger compartment broke, including the hatch glass, since the Firebird hit below the hatch. It was pretty amazing. All the taillight bulbs were still on, and my blinker was still blinking when the cop was taking the report. He asked "Did you signal that you were turning, and were your lights on?" and I just pointed to my car and said "They still are and the blinker is still blinking." It was crazy...

 

So IMO Z's hold up in crashes pretty well.

[260DET]

The weak link is on purpose. This is what lets the motor exit out the bottom of you really stuff the car. Welding in the crossmember will make the front a lot stiffer but if you get in a serious accident the motor will be coming through the firewall. It's your choice.

 

Cary

 

Interesting point. IIRC the 280ZX has a similar setup but with three bolts per side instead of two.

[tube80z]

Cary' date='

That is a good point. I never thought about the whole engine issue in a head on. I wonder though if this is effective after a V8 install anyways, considering that:

1. The engine is wider than stock, so it is bound to go through the firewall with the heads, instead of down the tunnel.

2. With the method most use to mount the engine/tranny, I wonder if the tranny holds everything in place, or does the tranny mount shear.

3. Was the cross member ever really designed to drop out.

[/quote']

 

I believe Mercedes Benz developed this and gave it to the industry as a safety feature. It wouldn't be hard for a manufacture to create a much stronger joint that was easy to manufacture.

 

I think a V8 would be fine as long as it mounts in a similar fashion. I don't think the width matters in this case. If you look at a guard rail along the freeway the end has a brace so that if you run into it the rail bends a large section rather than spearing through the windshield. This works in a similar fashion. Fortunately not many of us ever test this out.

 

Cary

[tube80z]

Cary, to answer your previous question, most track Z's seem to run between 250 and 400 in/lb springs. Weren't you planning on 800 or 1000 in/lb springs? Or am I just imagining things?

 

650 was my upper limit. That would be 1.5 times the corner weigt on the EMOD car. To get there will require more development work on the shocks I think.

 

Last year I ran 1 to 1.2 times the corner weight on the unibody car. Right now it has 400 lb springs all around from an event we ran late last year where it was cold.

 

Cary