Aerodynamic vs. vehicle stabilty

[maj]

A2 after returning from my first Bonneville trip in 08 to watch the GP bikes at our Phillip island track i was very much noticing aero,
on a wet track with the sunlight in the right angle the air around the bikes was quite easy to see , and most noticable was the way the rear tail acted like an air dam on the sides of a car , air displaced by the rider /bike seemed to be pulling from over the top more than the sides, and gave a 45 deg angle of contained mist from the tail down around the rear tire , sort of confirming the wear seen at the rear of a belly fairing on the salt caused by rear downforce ?? or was it likely more engine torque ??

And had a bad experiance here at Lake Gairdiner in 04 when i lowered the front too much and was rubbing the front fairing on the front guard between 216 and 175 mph when not on power (trying to decellerate without crashing ... big tankslapper)
i put it down to frontal downforce as soon as the torque trying to lift the front was removed..  

Are my assessments anywhere near the ball park from your experiances ?

[Constant Kinetics]

Just from reading the first page of this thread, i'm curious if a longer swingarm would work. The rider could slide his/her weight backwards to get traction for building speed and slide then forward at high speed somewhat changing the center of gravity as aerodynamic advantage  overtakes mechanical advantage, essentialy using your body as a dynamic ballast. Of course, you would need just enough streamliner to break most of the wind around your body to be able to pull forward at 200mph+, and lots of chin-ups. Think it might work?

[Constant Kinetics]

  On a front wheel drive streamliner do I want an open diff or a spool?

      My instinct would say diff just so you can correct if you need to. An air-locker might not me a bad idea, your steering isn't affected in the pits and staging area, you lock for your run, and if there's any trouble you can unlock and try to recover. A detent could be put on the steering shaft automatically unlocking if the wheel is in any other position except straight ahead, so it's not something to forget in a panic situation.

[donpearsall]

A2, in your photo, the rider and bikes only relationship to an airfoil is the top is very cambered. As you can see, the airflow is scattered and turbulent over the top. That is a stalled airfoil as Saltfever stated. Stalled means no lift. There is no smoke on the bottom, but I am betting the airflow is also detached and turbulent there.
In the photo, the smoke is deflected UP, meaning there is downward pressure. You have seen front lift as a a result of rear lift? That does not make sense from a mechanical point of view. I would like to see the data on that. What would account for rear lift?
Lastly, do you have wind tunnel data to show amounts of positive or negative lift on motorcycles?
Thanks
Don

[Interested Observer]

A2,
Just to clarify one aspect of what seems to be an impending discussion, if and when you cite windtunnel “lift” numbers for a vehicle, are those direct load cell readings or has the weight transfer due to drag already been backed out of them?  What is standard windtunnel practice?

One would suspect that, particularly for rather “standard” motorcycles, actual aero lift/downforce might be of secondary magnitude.

[saltfever]

You have seen front lift as a a result of rear lift? That does not make sense from a mechanical point of view. I would like to see the data on that. Don

Don, I agree it would be extremely useful if we could see the data from the scales.

A2 said "front lift associated with negative rear lift (or rear downforce)" What he meant was as you push down on the rear the front will come up. I believe this could be correct under two conditions. We all know a body rotates about its CG. In acceleration (as in drag racing) you know the effect . . front comes up and rear goes down. But we are discussing aero forces (the bike is stationary in the wind tunnel with no acceleration to react on the CG). The possible effect here is the CP becomes the dominant force not acceleration. If the CP is rearward of the CG, as it should be, the bike "might" rotate about the CG and the rear will go down and the front will come up. But to what degree is this effect? That is why the scale data would answer a lot of questions.

You raised another issue to consider. Picture a bike on a front and a rear scale. If you lift the back upward what will happen to the front scale? You have removed some weight. You have made the bike lighter. The front scale will show less weight in a ratio to the bikes weight distribution about the CG.

[akk]

I have got it!!!?!

After deep thoughts about safety, fairness, close racing, advertising revenue, spectator excitement .... you know... the NASCAR mentality.

Assuming I have support from you guys, I propose the following:

1. Each competitor will be issued a class standard parachute.
2. The parachute must be deployed at the 1 mile mark and pulled the entire length of course.
3. Upon qualifying and backups the competitor must submit a timing ticket with verification of proper chute deployment and submit the chute for verification.

Advertising for the event sponsors will be available on each parachute. Parachutes will be sized differently by class. (we want to make sure that Streemliners are faster than Lakesters and so on down the line.

Just think how safe the competition would be!

LOL Akk

[Rex Schimmer]

IF you have a motor cycle in a wind tunnel and have scales under each wheel and IF the motor cycle's configuration does not develop any aero lift, positive or negative, then the readings of the scales when summed have to equal the weight of the motor cycle. When the air velocity is increased and the pressure differential between the front and the back of the motor cycle becomes a large force can be summed into a "center of pressure" on the front of the motor cylce that represents the total combined pressure applied at a single point against the front of the motor cycle. The position of this force will be above ground level and if you sum moments about the contact patch of the rear wheel, which will be the front scale reading multiplied by the wheel base and subtract the moment of the frontal aero force times it's distance above the level of the scales this summation must equal zero. This calculation will give us the amount of the front aero force or if you know the amount of the aero force (drag) you can calculate the amount of weight that will be moved from the front tire to the rear tire but in all cases (again with an aero configuration that does not generate any type of aero lift positive or negative) the sum of the scale readings will equal the weight of the motorcycle.

Rex

[jl222]

I have got it!!!?!

After deep thoughts about safety, fairness, close racing, advertising revenue, spectator excitement .... you know... the NASCAR mentality.

Assuming I have support from you guys, I propose the following:

1. Each competitor will be issued a class standard parachute.
2. The parachute must be deployed at the 1 mile mark and pulled the entire length of course.
3. Upon qualifying and backups the competitor must submit a timing ticket with verification of proper chute deployment and submit the chute for verification.

Advertising for the event sponsors will be available on each parachute. Parachutes will be sized differently by class. (we want to make sure that Streemliners are faster than Lakesters and so on down the line.

Just think how safe the competition would be!

LOL Akk

   

         JL222

[Seldom Seen Slim]

Well -- speaking of motorcycles in the wind tunnel -- Dave Salazar and I have decided that we'll put both of the SSS fast bikes in the tunnel sometime soon -- but probably only after the '10 season is done.  By then he'll (hopefully) have all of his instrumentation in place and we'll be riding in glory from all of the records we got this year.

And so, when we do go to the tunnel, I'll see if it's okay to release lots of the data we gather.  That way ALL can see what happens, what gains, what losses, and all of that stuff.  I'll write a full report on it -- and it'll be available for all to read.

Later --
Jon a/k/a SSS

[saltfever]

. . .  Dave Salazar and I  . . . . I'll see if it's okay to release lots of the data we gather.  That way ALL can see what happens, what gains, what losses, and all of that stuff.  I'll Jon a/k/a SSS

OK, by whom? Do you mean ok by Dave or A2? You and Dave bought and paid for the data. A2 is very careful with proprietary data as they should be, but you are the owner of the data and should be able to do what you want with it.  

[Seldom Seen Slim]

Yes - I know that, but maybe our crew chief should get a vote, too, before I give away the store, so to speak.

[A2WindTunnel]

 

. . .  Dave Salazar and I  . . . . I'll see if it's okay to release lots of the data we gather.  That way ALL can see what happens, what gains, what losses, and all of that stuff.  I'll Jon a/k/a SSS

OK, by whom? You and Dave bought and paid for the data. A2 is very careful with proprietary data as they should be, but you are the owner of the data and should be able to do what you want with it.

Slim is being respectful by saying that because he knows that A2 is careful with proprietary data and since this will be his first visit to the wind tunnel he doesn't know the normal protocol. I'm Dave Salazar (General Manager @ A2) and I would be running the tunnel/test for Jon.  That will be his data and he can post it if he would like.  And I’m sure that there might be some speed that he finds that he doesn't want anyone to know about, so he might not post all of his findings. It’s totally up to him.  Since it is his money, you guys should thank him for #1 going through the process of testing and #2 sharing some data with you guys about what he finds. Most people wouldn’t take the time to do that. 

[saltfever]

Hi Dave. Sorry about my disconnect and thanks for introducing yourself. You bring up a good point that I have always found difficult to resolve. You have customers that are paying, not only for your facility, but for your expertise. Before you built A2 you had a lifetime of experience. However, undoubtedly you have gained more knowledge from former customers and their experiments. As a professional you have to protect that intellectual property gleaned from others. However, as a customer I am paying for as much of your grey matter as possible.  I would want all your knowledge focused on my problems at hand. I would think finding the right balance is quite difficult.

Your postings to this forum are always interesting. I respect your protection of customer’s IP. You have done a good job.  Thanks.
 

[A2WindTunnel]

Wow, there has been a lot written since I left work yesterday and I’m too busy to get to all of them specific to A2.  Don't get my analogy wrong, I didn’t say the rider is an air foil, but does resemble the same shape (cambered top).  The round smooth sexy curves you see and that are pleasing to the eye are not surfaces that typical of creating downforce.  Those rounded shapes tend to create lift (in most cases).  You are correct when saying that air is separating off the riders back, but that doesn’t automatically mean that there should be downforce (this is not an airplane).  There is also a pitching moment and if the front of the bike has lift then there could be a transfer to the rear making it negative along with the separation off the riders back. The top shape is not the only component creating lift, front fender shape, air trapped between the front fender and lower fairing creates pressure differences...ect.. many variables to consider and not as simple as looking at a picture.  To give you an idea here are some # of a motorcycle simular to the one in the picture

Lbs @ 190 mph (standard NACA day: sea level)
Drag  = 283 lbs
Front = +134 lbs (lift)
Rear  = -112 lbs (downforce) 
 
Rex answered most of the questions but, before each run there is a wind off zero of the balance which takes the weight of the object (car or motorcycle) in addition to the balance and zeros them out.  Then as the tunnel is turned on and once the air is up to speed and stable the balance is just measuring the aero effect of the object very precisely.  Imagine standing on a scale and someone zeros it and then hands you a 20 lbs weight, the scale would then read 20 lbs with you standing on it.  A wind tunnel balance measures in 6-components to give a drag, front lift, rear lift, front side force, rear side force, pitching moment, rolling moment, and yawing moment to give a better picture of what is happening to the entire system (vehicle).  There are many interactions that happen and there might be a change in the front that could affect the rear and vise versa.

Example: NASCAR Cup series recently went from a rear wing back to the traditional spoiler.  This gave the teams more downforce, but where?  Most would assume that the change was to the rear so the rear was the only component that gained downforce (This is incorrect).  By adding a rear spoiler the center of pressure was moved forward and there was a significant improvement on front down force from the wing allowing the cars drive better in the corners because the balance (front to rear) was improved by increasing the front percent (aero).  I realize LS racers don’t turn but it is still important to have balance and not an unstable condition where the front is lifting (due to aero) and the rear is pushing down (due to aero).  As some of you know, cars can have enough lift that they will become airborne.

Here are some more pictures that help illustrate