Lets try a Hyosung GT250

[greenjoe]

After what seemed like an endless winter, I thought the bodywork on the Hyosung was almost complete, but a few trips to work and back tells me different. Above 65mph the rear of the bike vibrates, as a lack of a better description, seems to "shutter". At around 75mph it seems to smooth out some what, I'm thinking because the vibration is so fast. (it's not tire balance) I'm planning on narrowing & reducing the height of the tail section, any other suggestions are welcome.

[gil]

Do you have any better Pictures, can't make out the design of the aero panels.
I Have a V-star 250. Because of the similar engine, i'm interested in seen what you have come up with.

[greenjoe]

Sorry about that, I have a hard time resizing the pics.

[sendler]

Originally Posted by greenjoe

"shutter"

Not sure why. The Vetter tailed bikes I rode with are smooth as glass. Even in truck wakes. Can you cut the nose off and try it to see if it caused by the front or the rear?

[greenjoe]

The nose and tail can be taken off separately, but I've ran the nose without the tail and it does fine.

[Grant-53]

Without seeing the material up close I can only guess that there is some flexing going on in the flatter sections just behind the rider. I don't know what the internal bracing looks like or the degree of curvature of the skin material. I would suggest looking at diagrams of older aircraft fuselages with cloth skin.

[euromodder]

Originally Posted by greenjoe

Above 65mph the rear of the bike vibrates, as a lack of a better description, seems to "shutter".

The aerodynamic term is flutter.
Happens to the best of 'm.
(Just about all aircraft companies have had trouble with flutter. Uncured, it can shear of tails, wings, ... so you might want to take it seriously.)

At around 75mph it seems to smooth out some what, I'm thinking because the vibration is so fast.

Likely.
And likely to kill the structure - both tail and bike if the tail is mounted sturdily - when not sorted.

I'm planning on narrowing & reducing the height of the tail section, any other suggestions are welcome.

Narrowing it won't help much unless it's really far too wide - I don't expect you made it much wider than the rider ?

Reducing the height will help a lot as it'll also reduce the bending moment - less weight situated closer to the fixing point on the bike will lead to reduced vibrations.

Reducing weight will lessen the effects of the vibration, but it's two-fold as weight can also dampen vibrations.

Make it smoother in the aerodynamical sense.
Don't overdo the angles, the top deck is too steep anyways.
Cut the boattail off at the rear, rather than make the angles too steep in order to make a pointy end - you don't really need the pointy end.

The theoretical optimum only works best at 0° wind direction, but you're almost never going to go straight into the wind.

Round off top, bottom and front edges.
The bottom edge seems like a hard straight ridge.
Curving it to the inside will likely help - if not with the flutter, then with side-wind issues.


There's a big gap between the windscreen and front fairing, towards the rear fairing.
That literally sucks, and creates a relative vacuum where you sit.
This alone could introduce vibrations through the rider, onto the steering bars. Happened to me with a flyscreen on a BMW R1200R roadster.
You've probably seen windscreens and fairings with holes - they relieve pressure by letting some air into the "vacuum" - or spoilers atop windscreens.

[euromodder]

Originally Posted by sendler

Not sure why.

Most likely : uneven aerodynamic loading.

The Vetter tailed bikes I rode with are smooth as glass.

They're not as tall.
The opening for the rider is kept rather small.
The bottom of the streamline is curved inwards.


On a sidenote :
Craig's site has a pic of someone who's building what looks like an enclosed Piaggio MP3 (twin front wheel bike)



Interesting.

[greenjoe]

Thanks Euromodder,, everything you said make sense. And yes, there is a vacuum in the rider area. I've started to modify the rear to make it "better" but will probably have to build another front and rear to get it right.

[sendler]

Originally Posted by euromodder

The aerodynamic term is flutter.
Happens to the best of 'm.
Round off top, bottom and front edges.
The bottom edge seems like a hard straight ridge.
Curving it to the inside will likely help - if not with the flutter, then with side-wind issues.

Good Eye. The top and bottom edges are too knife edged. Any slight bending will make a sharp bite into the air with no give over the top and bottom.
.
And the tail may get too narrow, too soon, to avoid separation. It might be better to use a wider tail angle and then truncate the length where you want it to keep from getting too long to park. 11 degrees per side for 22* total included? You will gain some useful luggage space by making it with a wider angle. Craig Vetter really did a nice job in designing his tail with generous rounding of the top and bottom.

Originally Posted by euromodder

There's a big gap between the windscreen and front fairing, towards the rear fairing.
That literally sucks,
You've probably seen windscreens and fairings with holes - they relieve pressure by letting some air into the "vacuum" - or spoilers atop windscreens.

My Honda CBR250R scoops air with the fairing and sends it up the inside of the wind screen past the top triple clamp. Many newer bikes such as the new Ninja650 feature a wind screen that is spaced off of the cowl to take air up the outside and the inside surfaces of the screen. It is also a nice design feature as it makes the screen easily adjustable for height.