Aerodynamic vs. vehicle stabilty

[akk]

re Nebulous reply 22

In my mind caster's primary function is to provide a natural tendancy for the front tires to stay aligned with the centerline of the car with no steering input.. ie. hands off the car goes straight. If the wheel is turned and released the front wheels will again realign to the car centerline. Caster is for hands off driving...

This is symplistic because scrub radius, weight distribution, rear axle alignment, turn banking, cross winds, tire circumfrence, ring gear carrier type, etc......get involved.

However, with the drivers hands on the steering wheel, a car with heim joints and otherwise non rubber elements involved will generally stay on the drivers heading. This assumes that there is no bump steer or roll steer in the steering linkage and front suspension.

With hands on, cross winds are a problem because the center of gravity is not coincident length wise to the center of pressure. Cross winds at speed will cause yaw. This yaw is worse in a car that has roll steer because the lateral area as well as the center of gravity is typically not vertically aligned with the car roll center. I see many cars with roll steer!

Our car is set up with 10 degrees of caster, zero scrub radius, no bump steer and no roll steer. The steering is very light because of the zero scrub radius...it tracks straight hands off the wheel but we always keep our hands on!

Akk

[sabat]

Interetsing reading akk, thanks. Sorry if I missed it, but what class of a car do you run?  Can you post a picture? Thanks, Dean

[dw230]

Akk,

I did make a comment re: parachutes in this thread. I'm sorry I don't understand your question.

DW

[jl222]

Speaking from the perspective of roadsters and other bluff wide track cars....

With a spool in the rear end and the tires rolling, the car is stable!

For a car to spin tires must slide.

If the rear tires break loose due to spinning or tire bounce the car is at the mercy of aero stability, long wheel base (high polar moment of inertia) and driver ability.

At speed a minor side ways movement of the rear end results in rapid opposite sideways movement of the front end. If the driver is not quick enough the front end pulls the car around.

It takes huge aero stability to counter the lateral force of the front tires or traction imbalance of the rear tires. Stability in a bluff body, wide track car will probably never come from aero....keep the tires rolling with, a spool, weight and good suspension. Avoid running if the salt is rougher than your suspension can handle or if there are wet spots.

The above applies to streamliners and lakesters as they are typically longer wheel base giving the driver more time and typically narrower to minimize thrust imbalances. Front wheel drive is inherently more stable because spinning or hopping front tires will not pull the front end around.

Long Live Hot Rods.

Akk
 

  AKK...I agree with ''if the driver isn't quick enough'' and makes me wonder about most Lsr racers recommending ''slow steering'' especially on short wheelbase cars.

                     JL222

[saltfever]

The front tire takes control and changes the center of gravity momentum force to a destabilizing force. . . . If the tail feathers are powerfull enough to overcome the tire force to the left the airplane may recover ...this is usually accomplished with active rudder control.

Interesting post AAk and thanks. I am wondering if the tire force is the dominant force? I agree it starts the course deviation but the CG is now accelerating about a point. Me thinks the rudder authority has to overcome the CG acceleration which is proably the dominant force, no?

The front tires skid and loose all lateral forces, the back tires roll and the car straightens right up. Aerodynamic stability of car should be concerned with keeping the tires rolling at all times!

But in another post you indicated when the rear goes sideways the front is pulled around by the wind. I agree. However, if the front is to regain any steering control to cancel out this "pulling around effect" why would you want to skid the front tires and lose all traction. Don't you want them "rolling"? Also, when the front wheels are turned down-track, their slip angle goes to zero (maybe not zero but you know what I mean) and greatly increases their ability to arrest the "pull-around" affect. Your thoughts?

[gearheadeh]

Speaking from the perspective of roadsters and other bluff wide track cars....

With a spool in the rear end and the tires rolling, the car is stable!

For a car to spin tires must slide.

If the rear tires break loose due to spinning or tire bounce the car is at the mercy of aero stability, long wheel base (high polar moment of inertia) and driver ability.

At speed a minor side ways movement of the rear end results in rapid opposite sideways movement of the front end. If the driver is not quick enough the front end pulls the car around.

It takes huge aero stability to counter the lateral force of the front tires or traction imbalance of the rear tires. Stability in a bluff body, wide track car will probably never come from aero....keep the tires rolling with, a spool, weight and good suspension. Avoid running if the salt is rougher than your suspension can handle or if there are wet spots.

The above applies to streamliners and lakesters as they are typically longer wheel base giving the driver more time and typically narrower to minimize thrust imbalances. Front wheel drive is inherently more stable because spinning or hopping front tires will not pull the front end around.

Long Live Hot Rods.

Akk
 

  AKK...I agree with ''if the driver isn't quick enough'' and makes me wonder about most Lsr racers recommending ''slow steering'' especially on short wheelbase cars.
                     JL222

I would bet that what happens is that as the car first starts to drift, the Amped up on adrenaline driver could over correct the opposite direction too much sending the car into a spin! 

[jl222]

  You mean he would stop the spin in one direction and spin in the other? I think thats the thinking, but I wonder if that happens. I have seen it happen with street drivers that didn't have a clue driving an out of control car, but thats on pavement with more traction.
  Look how far sprint cars have to  turn there tires to keep from spinning out, their flopping all over the place and they definitely have fast steering.

                     JL222

                                       


                             

[krusty]

"Sorry if I missed it, but what class of a car do you run?"             AKK's primary car is the "Contrivance" '23T roadster #920 which currently holds the records in A through E (that's right, all of 'em) Gas Rear Engine Modified Roadster. You know, the one that isn't low to the ground like all the others...        vic     PS: See you on the Salt, Akk!

[akk]

Thanks Krusty....#920 is a GMR...gas modified roadster.

The front tires, when rolling, have great authority for making the car go left or right. When the front tires are skidding they have no directional control of the car left or right. When you lock up the front tires and let the rear tires roll, the rear tires have directional control and will trail the skidding front tires.

Ok....find a smooth assfault parking lot...on a rainy day...no shopping carts, kids or other pedestrians about.....get going say 40 miles per hour straight.... shift to neutral.....hold the steering wheel straight....slam on the emergency brake....the car will spin every time and if you are going fast enough ...stop going backwards!

If you are brave try it the otherway ....backing up....the car will simply stop going backwards without a spin!

Trust me...I have done it...for fun when I was much younger. We use to set our go-Karts up with rear bias on the brakes so that we could brake hard going in to turn and set up a drift in the turn. NASCAR, I am told set, up with forward bias to assure stopping straight?

If I had brakes on only one end, I would put them on the front for Bonneville!!!!!!

[akk]

DW

I was referring to my reply #51 above....a proposal for the land speed equivalent of NASCAR restrictor plates....or high speed tractor pulling...

Akk

[akk]

Re....Fast steering

If some of you guys are specialists on control with feed back loops you might want to correct or add to the following...The last time I performed any calculations on this subject was 40 years ago.

In my opinion a driver, on smooth salt in a car with no vibration and no head buffeting from high speed air, can detect 1 degree of heading error (yaw). In other conditions the number is much higher.

Depending on the wheel base this heading error acting as a steering angle defines a turning radius. The shorter the car the tighter the radius.The faster you go, the faster the car turns or rotates about the center of the turning radius. So, the rougher the driving environment, the shorter the car, the faster you go, the quicker the driver needs to react to input a steering correction. The longer the driver waits the greater the needed input. Some drivers are better than others!

Now...the driver is the actuator, his eyes are the feed back. The actuator has a response time and a response speed(how fast he can steer and how quick the steering is). Put some numbers into some complex equations and graphs can be produced that predict optimum steering quickness for a given speed, driver, wheelbase....All harry stuff!

Generally though... a late response, lost motion, dead band (minimum perceived heading error), too quick of a response ....lead to over steering and can result in oscillating back and forth and total loss of control.

A long wheel base, slow steering, good suspension, head rest padding and a windshield are all pluses when going fast. Not to mention a good driver with good vision and quick responses.

Akk

   

[akk]

Re....the front is pulled around

To clarify...the tire rolling with weight and the coefficient of friction on the salt is what pulls the front end around at spin initiation. After the spin starts and the tire starts sliding aerodynamic forces get involved with the kinetic friction. Spin initiation is the rear kicking out and creating a steering angle.

Akk

[dw230]

Neil,

I got it now. I do not like that idea. I would have to redo all the records. No wait, I could start with a fresh sheet of paper.

DW

[Glen]

"Sorry if I missed it, but what class of a car do you run?"             AKK's primary car is the "Contrivance" '23T roadster #920 which currently holds the records in A through E (that's right, all of 'em) Gas Rear Engine Modified Roadster. You know, the one that isn't low to the ground like all the others...        vic     PS: See you on the Salt, Akk!

The Contrivance 23 T roadster is a unblown front engine car, not a rear engine as you stated.

[jl222]

Re....Fast steering

If some of you guys are specialists on control with feed back loops you might want to correct or add to the following...The last time I performed any calculations on this subject was 40 years ago.

In my opinion a driver, on smooth salt in a car with no vibration and no head buffeting from high speed air, can detect 1 degree of heading error (yaw). In other conditions the number is much higher.

Depending on the wheel base this heading error acting as a steering angle defines a turning radius. The shorter the car the tighter the radius.The faster you go, the faster the car turns or rotates about the center of the turning radius. So, the rougher the driving environment, the shorter the car, the faster you go, the quicker the driver needs to react to input a steering correction. The longer the driver waits the greater the needed input. Some drivers are better than others!

Now...the driver is the actuator, his eyes are the feed back. The actuator has a response time and a response speed(how fast he can steer and how quick the steering is). Put some numbers into some complex equations and graphs can be produced that predict optimum steering quickness for a given speed, driver, wheelbase....All harry stuff!

Generally though... a late response, lost motion, dead band (minimum perceived heading error), too quick of a response ....lead to over steering and can result in oscillating back and forth and total loss of control.

A long wheel base, slow steering, good suspension, head rest padding and a windshield are all pluses when going fast. Not to mention a good driver with good vision and quick responses.

Akk

   

  AKK... I agree with most of what you say but in addition to the eyes I would add BUTT
           I have said for years I would rather have just front brakes than just rear brakes I've told the story before about my cousins
rear brake only stude and solid suspension, he was giving it a test squirt at El Mirage [not on track] and put the brakes on ''sh...t no brakes'' well they were working but the tires were just slideing.
  Anyone try to go down a real steep hill on a motorcycle using rear brakes only? Same result as above.

    JL222