ERA cg location--or who cares?

[speed220mph]

During the course of instructing my High Performance/Racing Chassis class at the local community college, I frequently use my ERA Cobra as a project car. Some projects include bump-steer checks, cross weights and determing cg location. The laser Hunter alignment rack and Longacre digital scales make these projects much easier than using a carpenters square, level, plumb bobs and chalk lines.

Checking cg height is the most difficult simply because the front of the car must be jacked up and supported on its wheels with scales underneath. Fun.

Here's what we found about my 427 S/C ERA with 1/4 fuel load. (To be really accurate the tank should have been full or empty):

Weight distribution was 1384/1322 lb for a 51/49 front-to-rear weight distribution. With me behind the wheel, weight distribution changed to 49/51. Fill the tank and weight on the rear wheels goes up further and weight on the fronts goes down because of the cantilever effect.

About where the cg is: With a 1/4 tank of fuel, it's 44.2 inches behind the front wheels on a 90.5 inch wheelbase.

Now for the fun part--finding cg height: The car must be jacked up, the suspension restricted from moving, and all four corners weighed to get the numbers. Then a fairly complicated formula must be used. No easy task, but we did it.

Cg height came to be 18.62 inches above the ground with an empty car except for about 5 gallons of fuel in the tank. All things considered, not bad for a car with an FE engine. How do you get cg lower? One answer: Lower the car. Not easy considered the already restricted ground clearance. An all-aluminum engine would help as would relocating the battery lower, then to the rear while you're at it.

Oh, the ERA has really good bump steer. This is noticeable while driving over undulating or rought surfaces either in a turn or on a straight. Likewise, it eliminates the nasty transitional stuff such as while entering a turn. No steering-wheel corrections are needed.

Now that you have this information, what do you do with it?

[rdorman]

Really interesting information. My car is 45/55 distribution. When I was designing the brakes, I had to guess at the CG height. I guessed at 17". Seems I probably was not that far off.

Being the educated one let me ask you something, is the ERA front steer? Either way, how do you dial in more ackerman?

Here is another, for a bias ply tire of OEM sizes (245/60, 275/60), what are good camber curves?

One more. Does your ERA have a Jag rear end and did you determine the roll center height?

Thanks

Rick

[NeedAntiVenom]

Speed,

Using a small block would lower that center of gravity.

Great info! As far as what to do with it - verify it with behind the wheel empirical testing!

Seriously, will having the CG forward of mid chassis make the vehicle understeer very slightly?

Jim

[speed220mph]

Got a lot of questions here, but I'm about to head out the door. The sun is shining!! I'll give an depth answer to your questions after the sun has gone or the clouds build into thunderstorms.

[EarlsflyinCobra]

I thought I got away from CG and all that Stuff when I gave up building my Airplane and started building my Cobra?????? Guess not........................

[speed220mph]

Earl: You always have to deal with a cg. Mine keeps getting lower. All I have to do is eat more and excercise less.

rdorman: ERA's use front steer, i.e., the steering rack is forward of the front-wheel centerline. As for changing Ackerman, you shim the steering gear forward or rearward. As for more Ackerman, you don't want it; you want less. It was designed in the olden days, like for hay wagons, before chassis engineers realized that tires developed slip angles while cornering, which requires steering to be more parallel. Full parallel means the front wheels would steer at the same angle. Ackerman is where the inside wheel turns at a greater angle than the outside one, which works fine for forklift trucks, but not perfomance cars.

How do you change Ackerman/parallel steering? By moving the inner or outer tie rods end forward or backward in a level plane. This is best done by relocating the rack. Moving it forward moves the steering toward a more parallel condition and vice versa.

Tires? I like the Yokahama 235 front and 295 rear 15's, although they are radials. As for camber curves, bias-ply tires require less camber change for cornering. Also, the wider the tire, the less camber change is required. As for exactly what the camber curve should look like, I don't know because this would require extensive testing which I haven't done with the tires and the Cobra.

Need: Any lighter component that replaces a heavier one above the cg will lower the cg. Aluminum heads and manifolds accomplish this. And, you bet, more weight on the front tires will induce understeer if nothing else is changed. Add in some more rear bar will move it back toward oversteer. It's all about making the tires work harder or less hard. And there are transients to consider, also. The subject of vehicle dynamics is quite involved, but it's all about the front to rear tire slip angles--more in the front is understeer and more in the rear is oversteer. When front/rear slip angles are the same, handling is neutral. Simple!

I hope this answers your questions, but such answers typically generate more questions.

[rdorman]

My CG has the same issue. I call it 'central tendency'.

I am going to have to make up a paper 'doll', I am still having trouble visualizing ackerman on front steer. For some reason, rear steer is clear as a bell. Mental block.

Lets have some fun with ackerman? Fun!!! I suggest that ackerman can and often is a good thing. Two radius circles, two differently loaded tires, both tires have maximum corning power at the same slip angles.

Slip angle, the difference between the tire actual and apparent path (pointed in one direction but the tread lags a bit behind, a few degrees behind). Slip angle for a given amount of steering input increases as the side load increases. Side load increases with corning force and vertical load(load increase due to weigth transfer).

Sooooooo we have an outside tire doing the majority of the work and dictating the turn radius, the majority of the corning force and getting larger slip angles developed. The inside tire, given parrellel steering, is following a smaller radius turn (this in and of it self is less slip angle assuming the inside tire is not sliding over the), with less cornering force and less slip angle. The tire is not working at its optimum slip angle therefore not generating the most corner force that it can.

So what can we do, limit weight transfer. You run into excessive roll and camber control problems. Weight jack, works great on a circle track. Slow down, ummmmm, not a good one! Increase the angle that the inner tire is turned into the corner? Sure!

Before I even knew what ackerman was, we played with toe out on the front of our formula ford. Basically, 'static' ackerman. Close but not exactly the same. You know what happened? With no other changes, the car turned in GREAT! Corner speed was up as shown through interval times. It was GRIN CITY! Until a point. Then the effect began to back off, braking, especially on uneven surfaces got real exciting.

Fun with ackerman.

Rick

[EarlsflyinCobra]

Tom,
I'm with you on "THAT" CG problem....fighting it constantly!!!!

[Bob Putnam]

As far as the ERA ackerman is concerned, I specifically designed in a little less than what would be ideal for low speed turning. The tires scrub a bit when you're parking or going slowly around sharp turns. I did this to decrease the amount of understeer caused by the outside tire running at a higher slip angle than the inside. Real race cars actually may run negative ackerman, but I figured that having the tires squeal that much would be too embarassing - and hard to explain to non-racing onlookers.

[speed220mph]

rdorman:

I believe you are entering into the zone where you're getting bump steer and Ackerman cornfused. The best thing to do to get this sorted out is read HPBooks "How to Make Your Car Handle". Fred Puhn (yes, that is his real name) explains it very nicely in about a billion words.

But what you're describing about things getting exciting is probably caused by bump steer, or the tires steering as the suspension moves up and down while the steering wheel is held steady. This is not good and can be dangerous. Most cars are set up with some toe out in bump at the front and toe in at the rear for transient understeer. The reverse is bad, very bad. Any bump steer in a performance car should be avoided, thus the undesirable transients.

As for Ackerman being good, I don't like it at all. Just imagine what Ackerman really is: It assumes the turn the front wheels are steering about lies on a line that's projected off the rear-wheel center. This is only the case is the vehicle is not developing any grip, and thus a slip angle. But when speed goes up and slip angles increase, that point moves forward, requiring the front wheel to be steered at the same angle. Of course, this changes depending on whether the car oversteers or understeers. Bottom line when it comes to Ackerman is don't concentrate on it if you're setting up a car. It's best to work at getting rid of bump steer at both ends, particularly at the rear.

Formula Ford, eh. I used to race one, and a Super Vee, late model stock car, CanAm car . . . Hey, they all work the same, except for the Class 7 desert truck and the supermodified on dirt. That's a different world, but it's exciting none the less.

This is a good excercise!

[speed220mph]

Bob:

So you're the guy who made it hard for me to push my car by hand while maneuvering it around the garage. You made the point about Ackerman.

[mickmate]

You figured out a theoretical CG. I'm interested in where that puts your roll center height, assuming it's on the centreline.............Inquiring minds, very healthy mental stimulii!

[speed220mph]

Mick: Roll centers are independent of cg. There's a roll center at the front and one at the rear.

On an independent SLA type front suspension such as used on all Cobras, it's found graphically by extending a line inboard from the ball joint of the upper control arm through its pivot and beyond, then doing the same for the lower arm. Where these two lines intersect is called the Instant Center. A line is then drawn back from the IC to the center of the tire contact patch with the ground. Where this line crosses the center of the vehicle is the suspension's roll center.

The same is done with an SLA type IRS such as the Jag setup. Typically the front rc is lower than the rear.

By the way, the cg isn't theoretical, it's real. You just can't see it. Same with roll centers. They are points in space. What's theoretical with the cg is it's a point where all the weight of the car could be located to provide the same front/rear weight distribution and weight transfer during cornering, braking and accelerating. But, then there's polar moment . . . another can of worms.

[Bob Putnam]

Speed220,

Hey - I can't take all the credit for the cars being hard to push in tight circles.

The Powerlock doesn't want the car to turn either - and probably contributes about half the extra effort.

Anyway, it's good exercise.

[speed220mph]

Bob,

I've got to give all of you at ERA credit for the great job you've done and are doing with the cars. I haven't driven the car at 100% yet, but close. The car seems very neutral with no surprising transients except for that good old throttle-induced oversteer provided by the 427. Mid Ohio would be a good track for going all out, but maybe I'll get it up to VIR, which should also be good. Need some good runoff areas just in case.

One of the best compliments the car has ever gotten was from an original Cobra owner, a 428 street version. I drove his to check out a chassis problem, which seemed to be something loose in the rear suspension. The car felt unsettled at about 60 mph and higher speeds. To return the favor I let him drive my ERA. His response was, "That's the best Cobra I've ever driven." He's owned more than one original, one being a small block which he currently owns and drives frequently.

[rdorman]

I love Fred Puhns books. His brake handbook has turned my car into one that will barely stop to one that feels like some one just dropped an anchor! You need to shoulder harness under braking just to keep from hitting the dash.

Tom, SAAC at Mid-Ohio Oct. 3, open track! I live about 45 minutes away and will be there!

Rick

[speed220mph]

Great that you've read his books. Fred knows his stuff, having built and maintained the Quasar D-Sport racer, invented the modular wheel and, of course, wrote the handling and brake books for HPBooks. I got to know Fred real well since I was publisher then, so I can vouch for him. Oh, last time I knew Fred he was involved in a nuclear fusion project.

Mid Ohio; I'm a little too far from there to attend. I'll be going to the Z06 affair in Kentucky the middle of September, so that and Spring Fling will have used up all of my "have fun" energy (money) for the year. Great track, though. Used to campaign cars such as an M1B McLaren, F-V, Super V and IMSA GT there, so I am familiar with the Keyhole, Carousel and those wicked turns on the back side of the course. Got a Vette sideways and through the grass on the off-camber, blind corner that unloads the car back there. The track was wet, too. Can't remember the name of that turn.

[rdorman]

Great track to drive! Are you talking about turn five (depending on the keyhole being open) after the tight right hander at the end of the back straight (big hill up downhill left) or the nasty right hander after the esses before the downhill heading towards the Carosel? I was there the year that Rahal won a couple of days after Trueman died. Not a dry eye in the place!

Fred writes some good stuff. Even Carroll Smith said he wasn't going to bother on a brake book because Fred had already written the best one out there and he didn't think he could top it. With a little time you can take Fred's book and put together a brake system that will live through whatever you can throw at it.

Sad thing about Carroll Smith. I met him at the 86 runoffs in Atlanta. After I helped push his trailer out of the mud he autographed my copy of Tune to Win. A couple of years ago my wife put that very same book on the table at our yard sale for $1.00. Best buck I ever spent

'old' racers never die, they just build Cobras

Rick

[speed220mph]

I think it was the "nasty" right hander after the esses. I followed a lot of famous black marks to the inside of the turn through the grass, barely kissing the Armco with the front bumper.

About Carroll, I was working on getting him to do a book I used the working title "High Performance Fittings, Fasteners and Plumbing". Before we could get cranked up on it, HPBooks was sold and those left didn't pursue it. Carroll decided to publish the book himself.

Carroll was a neat guy. You never had to guess how he stood on things.

[SCOBRAC]

Easy way to turn your car in the garage and defeat ackerman and newton at the same time: start engine, turn wheel full lock, dump clutch, bingo.. perfect 180.