Cobra Geometry Challenges!

[DocDirk]

I’ve been having trouble getting much “return to center” on my front steering, and it feels a little too responsive to steering wheel input. Since I blew out my original rack and I suspected the pressure was causing resistance, I reduced the power steering pump pressure from 1000psi to 800 psi without any change in this problem. I suppose I have neglected some geometry that’s adding to the problem. I had the front end aligned professionally last year. I had an appointment for a professional inspection and analysis by an experienced chassis/wheel alignment but the man forgot to remember he had today off......maybe later this week I guess.

Today I went out and measured the car at several locations and I’m wondering how to make sense of what I’ve found:

Legend: RF = Right Front LF = Left Front RR = Right Rear LR = Left Rear

Body edge wheel well from the ground: LF = 26 1/4" RF = 27 1/4"
LR = 27 1/4" RR = 28"
I have inspected and found that the body contours meet the frame identically from side to side, so I suppose that’s not just an installation difference.

Frame Corners measured from the ground: LF = 4" RF = 4 1/4"
LR = 4 3/4" RR = 5 1/8"

Ground to bottom of front outer lower suspension arm:
LF = 7 1/2" RF = 7 1/2"

Ground to bottom of rear axle tube/housing closest to brake:
LR = 10 3/8" RR = 10 3/4"

Poor-man's alignment figures measured by me:

Camber: LF 2.4 deg neg. RF 2.4 deg. neg

Toe-in: Measured by center tread to tread comparing the front and rear measurements on the front tires with steering at midpoint:
Distance between center fronts of tires: 60"
Distance between center rears of tires: 60 3/8” (!)
They also appear toed out to visual inspection.

Caster: I understand the principle, just have no idea how to accurately measure it without some equipment I don't own, but by visual inspection it is present and significant.

The shocks are all at fullest spring extension, symmetrically mounted without spacers or differences in length.
Inspecting the changes made to the A arm attachment in the alignment process I found a 1/8” shim behind the front on the Left side and a ½” shim (wow!) behind the front on the Right side.

So, if these are anywhere close to real numbers, then it seems I have a problem with the toe-in to start with. Might a toe-out such as I have cause the front end to wander more than desired? I don't feel that happening when in a straightline cruise, but perhaps it would make a difference in the steering response?

With regard to camber, from the articles I’ve read, I understand the Mustang II front ends needed about 1/2 deg. negative camber. Might more than this tend to cause the front end to be more aggressive in turning? Might that cause a resistance to the normal tendency to return to center?.

I cruised around today and found the car to be fairly steady in a straight line with a very small tendency to drift right but then the roads here are centrally high-bermed to allow runoff so that might just be the road. On a full left or right 90 deg. turn if I let the wheel go after the turn there is some tendency to return partly toward center, but not much. Certainly a LOT less than I experience in my daily driver (which, granted, has a much longer wheelbase - (1995 Lincoln Town Car) The cobra just feels like it wants to turn right away and just keep on turning. My WCC has a 96” wheelbase.

With regard to the wheel well heights there doesn’t seem to be any asymmetry of body-on-frame installation. I don’t understand how the left and right rear axle housing heights can be so different on a solid axle car? Front tires are 265/50/15 Cooper Cobra Radials, the rears are 295/50/15's. Front tire pressure is 28 both sides, rear 30 both sides (determined by temperature readings across the tires after driving).

Well by posting this I’ve probably RE-achieved the esteemed lowest ranking in auto understanding once again… With all that screwed up geometry, maybe it's a miracle the thing even goes straight and corners AT ALL!!

Regards to y'all and thanks for any input.

Dirk

[ZOERA-SC7XX]

Even a little toe-out will make the car want to wander, or 'hunt' left and right (back and forth). You can measure your toe by using a string from outside of your rear wheel to the outside of the front wheel (both front and rear edges) at spindle hight. The difference between the leading edge and trailing edge of the front tire will be the toe-in (or out) on that side of the car. Do this on both sides for the total. If your rear wheels are wider than the front, then just start the rear measurement minus the difference from the front. My measurements and adjustments on my cars and trucks come out near perfect. It's the way it's done at the speedways.
As far as your 1/2 degree neg. camber, that should not make the steering easier. The more negative camber, the heavier the steering will feel.
One more thought. A Cobra is small enough not to require power steering. Mine has a modified Mustang II manual rack. I hope this helps.

[Barnsnake]

You clearly need to get your car on an alignment machine and find out what the actual current settings are. Caster provides the "self-centering" and "weight" to the steering. You'll probably want four degrees or more with power steering.
If your measurements are correct you have too much toe-in and WAY too much Camber for a street car.

[bobcowan]

Check tire pressure.

Disconnet any sway bars you might have. No need to remove them, just disconnect or remove one of the connecter links.

Carefully set the ride height. That can be a tedious process, as a 1/4" change in front ride height may change the rear by 1/8". You have to go round and round making small adjustments until it's perfect.

You will need toe plates of some kind. You will most likely need to jack up the corner to adjust ride height. When you let it back down the suspension won't settle properly. No need to spend big bux here. I use two cheap floor tiles and put grease between them. Some people use plastic garbage bags. When you let the car back down on your "toe plates", turn the steering wheel back and forth a few times to settle the suspension.

You should take all your measurements off the frame, not the body.

Get an alignment tool, or take it to a shop. You can't use levels and guess work, it just isn't accurate enough. Here's the specs I use, and I think my car drives down the road as easily or better than a factory car.

Caster 2.5*
Camber -.05*
To In - 1/16" total.

Now you can connect the sway bars again.

[DocDirk]

Thanks for the replies! I'll work with the frame essentially then, at least at first. Forgive me for not knowing how to do the ride height adjustment, I'll look it up on the web. Any hints for this amatooor?

You're right, I'll have it in the shop for an alignment while the pro and I both watch and discuss the changes and the proper goals for this car - I appreciate the specs, they've been hard to find. Would that be generic for most all cobras or should I pursue Mustang II front end specs?

Thanks!

Dirk

[COBRANIP]

Like Barnsnake said, Caster has the greatest influence on a steering wheel wanting to "self-center" and I agree 4* being a good ball park figure.

With regards to ride height adjustment that is simply a matter of turning each spring platform (assuming you have coil over springs) to raise or lower each corner of the car. Look at your shocks and the body is probably threaded...like it is a large screw...on one end where the spring rides on a "platform" (or nut).
Your Manufacturer should have recommended settings (as measured from the ground to the bottom of your frame).
This makes it eaiser: http://www.pegasusautoracing.com/pro....asp?RecID=817
If you plan on doing any "performance" driving I would suggest that you consider having a shop that has scales set ride height (with your weight in the drivers seat) so the car is properly balanced.
Ultimately, balance is more important than perfect symetery in ride height.
Good luck!

[TINNMANN]

First I would check if the suspension is square to the center line of the car. This can be done by lifting the car up and hanging plum bobs from suspension and marking the points on the floor then adjust the wheel centers so they are square. I then fastened a piece of electrical conduit to the quick jacks and marked the center. Then set the ride height using the frame to floor do not use any part of the body. I would suggest that you buy or borrow an alignment gauge such as a Longacher camber caster gauge or a Fast track both are under $200. I have the Longacher and I have used it 5 times to change the alignment settings so it has more than paid for itself. If Your car has a solid rear end it is critical that it is square IRS is not as sensitive.I would suggest a good starting point would be 1 deg camber (top of tire tipped in) and with power steering about 6 to 8 deg caster (the only way to measure this is with a gauge) The more caster the better the steering will center itself after a turn. Then using the conduits fastened to the quick jacks run string down each side about 6 inches outside the body. With the weight on the wheels you can measure the toe. You should start at 1/16 inch in at the front (1/8" total) This should get you into the game. If you have IRS you can set the camber and toe as well. I went to get my car aligned and when the tech didn't seam to understand what I wanted and he told me that it would cost at least $200 that is when I got the Longacher tool.
Cheers Greg

[Excaliber]

As it regards power steering, I'm actually considering that for my ERA. The G forces are so high in very tight cornering at speed that is extremely difficult to maintain effective control of the steering wheel. This is especially true when your running road race slicks. I've had the car sideways and sliding and it takes immense effort to counter steer in that condition, at speed and with slicks. It is particularly problematic when your coming out of a corner and need to quickly take a new line.

I've found I prefer zero or even slight toe out for auto cross, where the speeds are generally below 80 mph. But the car is a beast to drive that way on the street and becomes unstable and "twitchy" at 80 and above. Downright scary in fact.

[DocDirk]

Thanks for all the great help guys! I'll definitely be getting the measuring device - if only to be able to check and reassure myself periodically that things are the way they should be. Thanks for the link Cobranip. Yes, I have four corner adjustable coilovers. As for the manufacturer's recommendations....mine is a West Coast Cobra - the recommendations consist in a few assembly instruction sheets, a "good luck" and unanswered phone calls

About the spring platform adjustment - I remember cranking the spring platforms on my motorcycle years ago to adjust spring preload for different riding conditions. If I use this method to adjust ride height, it seems it would preload the four corners differently. Might that influence the way the car behaves too?

Questions, questions, questions.....maybe I ought to attend Wyotech on weekends

You folks are the greatest!

Dirk

P.S. I still haven't found out how to make a donation to support this forum.

[bobcowan]

Originally Posted by DocDirk

Forgive me for not knowing how to do the ride height adjustment, I'll look it up on the web. Any hints for this amatooor?

Depends on your set up. A quick scan of the web site doesn't show much, but it appears that coil overs are an optional component. Without seeing your set up, it's difficult to tell how to do adjustments.

Originally Posted by DocDirk

I appreciate the specs, they've been hard to find. Would that be generic for most all cobras or should I pursue Mustang II front end specs?

Thanks!

Dirk

Those are pretty generic specs, and should work with just about any set up.

Caster is a variable spec. Some people like a lot, and some like a little. A lot will hepl the car return to center a lot easier. That also means it takes more work and effort to move it off center. If you have a bump steer issue, more caster will make it worse. I'v played with differant setting, from 1* to 5.5*. I like the low setting much better.

Speaking of bump steer, have you checked yours yet? Kinda hard to do without getting the alignment close. But it should be on your list to do eventually.

[RICK LAKE]

DocDirk I'm am going to give you another aproach to this problem
The car needs to be weighted first in all 4 corners with you in it. I don't know if your motor is square centered in the car or off to the r/s up to 1". All the has a MAJOR effect on the car and handling.
Measuring the cars corners will get you into the LARGE ball park to start. You need to weight the car with you in it. If possible a 49/51 front to rear bias is a good place to start. I found that measuring the frame from ground to corners had my car over 300 pounds off. This was without my weight in the drivers seat. This is another important thing. Tire pressures need to be all the same or running 2 -4 psi in the front tires over the rear ones.
Once the car weight is done( have 1/2 tank of gas in the tank) Lets look at the alignment numbers. On the street with P/S running +.75 camber L/S, +1.0 R/S. Caster should be about +3.0-+3.5 depending on how tight you want the car to handle. Toe is 1/16 toed in. The thrust angle of the car need to be close to "0" as possible. This will make the car accell in a straight line as long as the bushings are in good shape and not worn to have any play.
If you are going to a race track these numbers will need some work. Measuring tire temps to find out what the car needs and wants to turn and drive.
You dropping the pressure in the rack will do little to change the way it works. The problem sound like the gear ratio is changed between rack 1 and 2. As others have said, bump steering also has alot to do with the car turning. This is easy to do yourself with the gauges and board.
If the car is going to remain a street car the spring rates can stay low 250-350 rate if you are looking for track rates, you will need to increase them by about 50-100%. Going to slicks too.
Stay away from wyotech, there basics will do little to help. The best place is to to fine a dirt tracker you can follow and have things explained. Book teaching and real world are not the same, again a large ball park. There are a couple of suspension books that will help. There are also Suspension programs you can use to help setup the car without all the wrench turning to start with.
There is a small thing you said about having the car aligned, Did you ask when the machine was last calibrated?? If the heads are dropped once, it can knockout the measure ments up to 1 degree. This is alot. Rick L.

[DocDirk]

I take it from your several feedbacks that the weight/frame height adjustments should be made before the alignment is done...is this correct? I can get to the alignment shop this week but the tool for adjusting the ride height won't be here until at least next Friday - will the alignment be thrown out if I make the adjustments to corner weight/chassis ride height afterwards?

Thanks again!

Dirk

[Excaliber]

I would do the alignment last, after corner weighting and ride height and such. However, be advised these kind of adjustments can drive you mad if you strive for perfection. A very illusive goal!

Just changing the TIRE PRESSURE a pound or two will alter the corner weights! This is an incredibly time consuming process your about to enter into. Good luck, you'll need it.

[scottj]

Tire pressures should be set first. Ride height should be set next and maintained during all the other adjustments. Alignment can then be done followed by corner weight adjustments. Changes to alignment and tire pressures can alter the corner weights, possibly by large amounts.

Scott

[Bobcat]

X2 on pretty much everything the others have said . One other "factor" to check is tire circumference . May be splitting hairs , but if you are going to all the trouble to set the car up , do it right . Stagger can introduce some really interesting problems , however , modern radials are pretty good ( I run Avons ), but I still check them .
The boys that did my car set it on a surface plate first , set tire pressures , added weight equal to me in the driver`s seat , set the ride heights and then found the center line of the car . Wheel weights and cross-weights were set next and then last , alignment . Then all weights/diagonals were checked again .
For a street car , I have one diagonal at 50.3% and the other at 49.7% and a front to rear bias of 47.5% / 52.5% with 1/2 tank of fuel , driver`s weight in the car and all fluids in .
Ernie is dead on about tire pressures .... depending on the tire we used , 1 psi was equal to 10 lbs of spring change ... and used as a fine tuning tool at the track.... a tire is just a "black" spring . Took these guys about a day to set the car up.
If you can , find some of the circle track boys who are really serious and have them do it . These boys I went to are dead serious ( Donahue`s old Lola T6 is there ) as is an ex NASCAR Caterpillar Dodge car plus several ex Roush cars . The local alignment shops can leave a lot to be desired .
I can`t believe it`s the same car the way it handles now .

[Excaliber]

I learned about corner weights, ride height, alignment by mostly working on Formula Ford race cars. First time I spent darn near a week because ANYTHING you changed, changed everything, and I wanted perfection.

It took way less time when I settled for "pretty darn close" all around.

[Maurice Butler]

There is another way if you don't have scales to get things in the ball park.
1. measure the ride height under the diff, remove the rear wheels and lower the rear on to a /\ under the diff, making sure it is at ride height. This takes the rear springs out of the equation.
2. disconnect the front sway bar.
3. adjust the front springs so the front is level and at the correct ride height.
4. reconnected the sway bar ensuring there is no tension on the sway bar.
5. refit the rear wheels and remove the front wheels.
6. fit the /\ under the front in the centre of the chassis, and adjust so the front ride height is correct
7. disconnect rear sway bar and adjust the rear springs so the rear is level and at the correct ride height.
8. reconnect rear sway bar insuring there is no preload/tension
9. refit rear wheels and remove the /\ from the front and you are in the ball park

[DocDirk]

Thanks again for the pearls of wisdom on this topic! Maurice, by correct ride height do you mean simply to make sure all four corners are at the same height.... or is there a specific "correct" ride height for optimal performance?

Also should this be done with weight placed in the driver's seat to simulate the driver and take that into account?

Thanks, there's a lot I don't know,that's for sure!

Regards,

Dirk

[Excaliber]

Ride height will vary from car to car and what you want/need to achieve. Full race vs some street use for instance. 5 inches is a good ball park for a street car, I prefer a slight rack myself. Front slightly lower than rear. Some race tracks have a minimum hieght requirement, usually 4 inches or so. Measured from some point on the FRAME, or the diff. Do NOT use any part of the body for your measurments, do use some weight to simulate drivers weight inside the car.

[kitcarbp]

A good point raised earlier is when checking the corner weights and ride height, make sure the REAR sway bar is disconnected on the WCC chassis and always use the drivers weight in the front seat. For those not familar with the rear suspension used on Docs WCC, its a mid-size Ford 9 inch 4 link set-up without a track rod but with a sway bar and modified to use coil overs . (aka '76 T-Bird,LTDII, Lincoln Mark xx) The later WCC vintage are not fitted with a front sway bar which I believe would need to be added to improve the handling. But that'll be another thread.