higher speed problem

[Automan]

Hey guys:

Now that i have really gotten use to the car, i have been lately seeing how high i can get measuring off on the QTR mile (approx) using my speedometer. Didnt get very high. here is why.

As i hit 80 miles an hour, my steering wheel really started to vibrate.
At 90 miles an hour, the front end seemed to raise and it went loose. Meaning that i couldn't keep it straight. the steering was loose.

A couple of facts. I havnt gottin an alignment. did everything through measurments.
Before the london show, during a detailed cleaning of the chassis, i put the driver side front tire on the passengers and vise versa. There had been 3000 miles on the origional positioning.
I have had problems with my left side tire rubbing on bumps and on ramps. have tightened the coil overs on the driver side alot more then the right to countermeasure that problem.

So i looked under the dash. my flaming river column is mounted to the cowl mount bracket by a muffler clamp and a piece of rubber where the clamp touches the column. now there is space above the column so actually the column is just sitting on the clamp. The upper part of the column is probably touching the opening to the dash. Under the hood is the normal A&C setup with the muffler clamp and bracket along the footwell. That area is tight. i can move the steering wheel left to right.

With my front end, i was really thinking on replacing the front shocks because of the left rubbing problem before. Maybe the shocks are dead and thats why it lifted.

How did you guys mount your steering column under the dash to the cowl bar.

Any help, would be great.
Thanks
Automan

[Hotfingrs]

Contact Brent at Flaming River, tell him what you have, and get the right mount. They have many different mounts for different mounting places, so I'm sure you will have options as to where to mount. BTW tell him I said Hi.

[Hotfingrs]

After you get your column mounted, be sure to get an alignment. You'll be amazed at what a good shop can do for these cars.

[dcdoug]

Have the balance checked on your wheels too. The vibrations sound like a wheel that is out of balance and you only really notice it at higher speeds. Better yet to have the balance checked while the wheels are on the car.

Also, make sure your corner weights are balanced too.

[PNJSNAK]

Automan:
I did not use a Flaming River column. I did the modification to the GM column and used the bracket on the column to mount to the bracket holes in the cowl bar. I even removed the key on the column body filled the area and it looks like the column key was never there. The key is on the dash. The sterring column is rock solid. I used the same support method on the top of the foot box with the pipe clamp but the clamp has rubber over the steel.
Get the alignment done, the place I use has a Hunter electronic machine and they use the settings from a 76 Mustang II. It makes a big difference in handling and cornering.
When ever I get up over 100 the front end sure does get light, kind of scary actually.
That is why I have my fun under 80 MPH, most fun is 0-60, stop light to stop light.
Jim

[Tommy]

Originally Posted by Automan

I have had problems with my left side tire rubbing on bumps and on ramps. have tightened the coil overs on the driver side alot more then the right to countermeasure that problem.

I don't know how to say this without sounding like I'm critiquing your mechanic skills. But I'm genuinely concerned about your safety, so here goes. ... Cobras are a handful to drive when they're set up right. They can be especially dangerous when they are not set up right. Adjusting the coilover settings to resolve a body clearance problem is not a good solution. Driving the car above 80 MPH with an uncertain steering system (i.e., loose column and amatuer wheel alignment) is not wise. There are three areas on a car where things have to be right - things that affect steering, things that affect stopping and things that can cause a fire. If you are not absolutely certain that you've got those things right, I strongly suggest you call on a skilled mechanic to inspect your work before your next attempted high speed run.

[bobcowan]

Before replacing any parts, spend the time and money to set up the suspension properly. You front and rear suspension is a good design, and should handle well at speeds well above 100 mph.

Start by setting ride height. I like 3.5" front and 4.5" rear. Disconnect the sway bars, and be very meticulous about it. Changing the height at one corner will affect the other corners. SO you spend a lot of time goi ng round the car checking and adjusting until everything is perfect.

Checking corner weights is great if you have the equipment. When the corner and cross weights are perfect, so is the ride height.

Then spend the time and money to get a good alignment. I suggest doing it yourself. You don't need much special equipment or special skills. It is a pretty simple process. It is a tedious job, though. Expect to spend a few hours the first couple of times until you get good at it. Doing it yourself allows you to try differant settings until you find what you like.

Vibration above a certain speed usually means a problem behind the transmission. Engine transmission problems are usually rpm related, not speed. As said above, have all four tires balanced. Could be any kind of suspension issue, or even a drive shaft out of balance.

[Automan]

Thanks guys

DCdoug, what do you mean by corner weights.

Dont worry, i will not be continuing with my speed tests untill i get those items looked at. Everything runs awsome up to 70 miles hour so there wasnt any signal of a problem.

Next steps:
Alignment
Tire balance
New Bracket for the steering wheel.
I will also take a closer look at this countermeasure that i did for the rubbing on the left side coil over.

So its normal for the car to lift at 100miles an hour. your right about it getting loose at that speed. it was scary. How did they manage to get higher speeds? would a front sway bar help?

[Tommy]

Originally Posted by Automan

So its normal for the car to lift at 100miles an hour. your right about it getting loose at that speed. it was scary. How did they manage to get higher speeds? would a front sway bar help?

The fundamental problem of lift at higher speeds is due to the difference in air pressure above the car and below the car causing a net upward force that increases with speed. A simple way to think of it is to imagine the air being packed under the nose of the car as forming a cushion that lifts the nose. The customary way to deal with that problem is to prevent air from getting under the nose by either lowering the nose or installing an air dam. An air dam is a piece of bodywork that attaches to the underside of the nose of the body and hangs down very near the ground. Air dams can greatly improve high speed stability, but they can be a problem on public roads with speed bumps, etc. My car has a modest 3" air dam and it is stable up to at least 140 MPH.

Corner weights refers to the amount of your total car's weight that is on each wheel. Ideally, you'd like to have the same weight on each wheel. If that is not possible, there are other combinations that are better or worse. The two common ways to adjust corner weights are to move things around and to adjust the spring tension on each wheel. If you adjust the springs on one or two wheels just for body clearance, you can throw your corner weights way off and cause the car to handle badly.

BTW, when you get ready to drive your car at high speed, I strongly recommend you sign up for and participate in a track day event on a race track designed for such things. It's a great way to learn more about your car and yourself in an environment much safer that public roads.

[ROBERT GWALTNEY]

here is my column set-up i bolted two pieces of threaded rod to the bracket on the cowl bar then used two muffler clamps one on top and one underneath to hold the column

[Roscoe]

I had a friend with a machine shop weld up a piece of pipe to a flat steel base. Rock solid. Also used an Ididit non tilt column.



Roscoe

[FUNFER2]

As already said, in such a short wheelbase and light weight, the alignment and balancing is extremely,.....important !

Take the car to a reputable,..... shop that has a "Hunter" machine. It makes a huge difference. I can drive at 100 mph + with no shake.

Some cars, even Cadillac, have what's called "Harmonics", where the engine & drivetrain are at a imbalance, creating this affect. Some manufactures like Lexus to Ferrari's, add weight to the chassis to lower the harmonics or even stop it altogether. I have my harmonics at around 63 mph, so I'll slow down or increase speed.

I do not like a rubber vibration joint in the steering shaft, as I like to feel the road beneath me. Same goes for power steering & brakes. Some people like them, some don't. A light weight car can get twitchy and can possibly,...... loose control.

[CobraEd]

I agree with what Tommy said earlier. You gotta be careful with what you have and what you are doing. What you outlined are all major elements that need to be addressed properly.

1) Get a good alignment to include caster / camber and toe
2) Get the corner weights all adjusted properly
3) Do whatever it takes to have the steering secured tightly and properly
4) Have a really astute gearhead go over the whole car for you

These first three elements are major considerations just motoring around the street. At full throttle at 90mph, they can be the cause of a catastrophy.


.

[Tommy]

Automan,
When I reread my last post this morning, I realized I had assumed knowledge that you or some others reading this may not have. It has to do with the weight on each tire of a car. The amount of force a tire can generate to help a car change its direction and velocity is determined by many things, but one of the most important is the downward force applied to the tire by the car. More downward force on the tire allows the tire to generate more grip up to a point. If the driver demands more grip that the tire can generate, it will start to slide and the ability of that tire to help control the car will diminish greatly. Here's an example. Suppose a driver has a car with 400 pounds being supported by the left front tire and 800 pounds being supported by the right front tire. As long as the driver doesn't demand too much from those tires, he may not notice the difference. But in a panic stop when he is trying to get every bit of grip from those tires to slow down the car, the left front tire (with its light load) will lock up before the right front. At that point, the driver is faced with two bad choices. He can let up on the brakes to regain traction from the left front, but not get all the braking action still available in the right front. Or he can stay on the brakes to get all he can from the right front while getting very little help from the left front. As the second option can easily lead to a spin, the first bad choice is usually better than the second, but neither is as good as having balanced the load on the front tires in the first place.

You may be wondering what a change in the spring tension on a wheel has to do with the weight pressing down on that wheel. It's a little difficult to describe in words alone, but think of it like a table with four equal length legs each supporting one fourth of the tables weight. If you were to shorten one of the legs just a little, the table would begin to rock slightly between the shortened leg and the one opposite it. That means the majority of the weight is being supported by the other two legs. The weight of the table is no longer distributed evenly over all four legs. ... Adjusting the spring on a wheel is like lengthening or shortening the leg on that corner. It affects the weight carried by that wheel AND all the other wheels.

[Automan]

thankyou so much Guys:

there is no doubt about it that i will be taking care of all of those items before
another trial. And that trial will be on a race track. its 60 miles away so it will be worth the drive.

I will be using that threaded rod muffler clamp style for the steering wheel. simple but perfect.

Anyone changed thier shocks that came with the kit. If so, what make and number did you use.

Im going to take my cobra to the ford dealer and get them to do a full alignment using the 76 mustang 2 setup.

Thanks again
Nelson

[tcrist]

You can't necessarily us any OEM manufactured car specs. The weight distribution on these cars is different from a manufactured car.

Take your Cobra to a shop that specializes in hot rod or a race car alignment. They know how to deal with custom alignments and check corner weights.

Doesn’t your car manufacture supply you with the alignment specs?

[xlr8tr]

would you set corner weights with the driver (or weights accordingly) in the car?

[john chesnut]

Set it up with the driver in the car. I know that alot of replicas have been built with the old Mustang front suspension. It probably is fine for cruising. But, lets face it, it's not what I would calll a race suspension. I wouldn't drive any car on the race track that wasn't properly prepared, properly inspected, and had the appropriate parts. It's very easy to get into trouble with a shortwheelbase car that has a fairly high center of gravity and alot of power. Have fun but be safe too.

I drive my car almost exclusively on the street so I didn't want to add a spliter or air dam to keep the lift down. So, I designed my chassis in a way that the air is diverted out the side of the car. Air goes through the radiator and out vents behind the wheel. The air can not get into the engine compartment and build up. My car is stable at high speeds. The front end is rock solid. Because of efficient air flow my radiator is only 12 inches by 18 inches. It sits straight up. I have never had a temp issue.

Lots of good info and advice in this thread. John

[FUNFER2]

We race 360 winged sprint cars, and most of the IMCA modified teams have electronic scales one each wheel. We borrowed a set and weighed my cobra, and all four corners were off so much, I could not believe it.

I wanted to lower the stance and also set it as level as I could. I took everything apart and started from scratch. My chassis is a four bar and the heim joints needed replacing. I moved the top front bars down to make them parallel with the bottom. I made a tool out of aluminum that replaces the shocks, and has male & female threads with heim joints, then you twist the tool clockwise or counter clockwise to raise or lower the car. This way, I could adjust the ride height and be able to measure for what spring length I needed.

I replaced the coil overs to QA-1 and went from 12" springs to 10" on the rear. With adjusting the coil overs and using the scales, I was able to set the weight on each corner & front to back almost perfect. It's now 48/50 %, even with a big block.

If you want to borrow the tool just ask, or it's easy to make. It works great.