Aerodynamic vs. vehicle stabilty

[PorkPie]

uh don....everything is relative....

...ie stability.....


..Joe

Joe....

I call it no problem.....when you stayed on the bike.......

[isiahstites]

Interesting topic, I will be watching.

[John Burk]

Front wheel drive naturally improves stability . Too bad it's not more popular .
John Burk

[willieworld]

motorcycles     rear wheel drive  front wheel steer  only                          willie buchta

[saltfever]

Most LSR designers use the idea of a "Cp" as a plot of the lateral area of their vehicles and presume that if the CG is forward of the 50% point of this plot, then the vehicle is stable in yaw.  This is simply not true.

First, most symetrical aerodynamic surfaces rotate around the "quarter chord point" or only 25% of the length.  This is called the "yaw neutral point".  Very few LSR vehicles have their CG forward of this.

Help me out here. . .  I haven't had my tea today like Tony. Just where are these two points? Assume a 100" wheelbase. The CP is at 75" from the front spindle(s). Now if the CG is 50% forward of that point it resides at (75" x .50 = 37.5"). So the CG is at 37" from the front of the car? But you say that is yaw unstable?  Why? Is it too far forward?

Now where is this quarter-chord point? You say it is 25% from the CP. Same exampe, if the CP is at 75" the yaw neutral point is (75 x .25 = 19') in front of the CP. On a 100" car that yaw neutral point is at 56" which is a rearward bias! So where do you want the CG to be on a 100" car? 56" is too close to yaw neutral but 38" is unstable? All the sedans I know have the CG in the forward 50% of wheel base in the 45-48% range. What am I missing?

Edit: Since I see Willie is here I made "spindle" both singular and plural in the interest of diversity.   

[saltfever]

Front wheel drive naturally improves stability . Too bad it's not more popular .John Burk

John, 4 wheel drive was run by Gary McArthur No.327 in a roadster for a few years. Then SCTA outlawed 4-wheel drive in roadsters about 3 or 4 years ago.

[akk]

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
 

[nebulous]

AKK
You posted "one" good example of a straight line vehicles dilemma. Please now add and remove castor using your example! Make a simple model with adjustable caster. Then decide what geometry is needed to maintain stability in a straight line vehicle. Jack

[Cajun Kid]

AKK
You posted "one" good example of a straight line vehicles dilemma. Please now add and remove castor using your example! Make a simple model with adjustable caster. Then decide what geometry is needed to maintain stability in a straight line vehicle. Jack

Caster ??  If you ask 10 folks 1/2 will say more is better and 1/2 will say as little as possible.

I have a 53 Stude.. we can go as low s 4 degrees caster and as high as 13 degrees.  I have it set in the middle at 9 degrees for initial testing.

My 1933 Vicky at 150 MPH  wandered all over at 3 degrees, we went to 8 degrees and it drives so straight and easy now ?  But that is a straight axle front end,,, the Stude is tubular A arm Mustang II  set up with adj coilovers...

Charles

[nebulous]

Joe
That is an undisputable accurate statment. This is why designs change ether by science, and or hard work!
New designs should be asked to make enough runs to show stability. Judging a design be by its resemblence to older designs or areo interpretation, is the way to constrict inovation and choke progress!
The very thing that LSR stands for!
 Roadgraders, conbines, tractors, ect, are not cars. Neither are streamliners! Streamliners go fast in a straight line, and are continuing to develop, change and improve, in order to achieve that goal!
2wheel flatbottom streamliners were stable and got a lot of records without tipping over for 7years like the traditional round bottom designs do.(Except Budfab)!Yet they were called motorcycles and forced to change to a known unstable tip rule! Why?
Please, lets not constrict 4 wheel liners in that same way!
Thanks Jack

[nebulous]

John Burk
 You have just stated  "The freedom of choice!" A term, that should help define LSR!
Jack

[1212FBGS]

I'm a little unsure of motive here so maybe you can help me out here Blue.... Why in the he11 would you have a talk with a Senior SCTA official and drop a bug in his ear to require aero stability testing on our vehicles?.... what in the he11 are you thinking? Do you have some ego problem that you feel the need to feel important to SCTA officials and others.... I for one would appreciate it if you would shut your mouth, especially to officials.... You seem to be a sharp guy why don't you spend your time actually building a race vehicle..... I know you are a aero guy but please don't suggest we all install propellers....

[debgeo]

Amen

[landracing]

motorcycles     rear wheel drive  front wheel steer  only                          willie buchta

what if you rode it backwards? We all know the front of the bike has the forks right?

JonAmo

[willieworld]

ive tried that ---but not on the salt-------  maybe this year          willie buchta