Can I have a few comments on this ?

[Frank Lee]

Hmmmm... the earlier posts make sense to me but I'm not convinced of what they are saying or doing in the "indifference" parts, especially the "laminar" part; there can't be much if any laminar flow on any of those weird bikes. I mean, they could be right, but I'm not understanding what they're putting down. My guess is the things they did altered the center of pressure with relation to the center of gravity more than anything... CP being behind CG leading to much more stability...

[PeterS]

Originally Posted by PeterS

Indifference

Extreme efficiency can lead to extreme problems. In general the cleaner a shape and the closer the airflow over it, the more sensitive it will be to disturbances in that airflow.

OK ,that makes sense ...the cleaner the airflow the more 'side lift' is going to be experienced .If the shape is a bad aerofoil there will be less 'side lift 'but of course more turbulence and drag !

For FFs these problems chiefly amount to sensitivity to turbulence, where the vehicle is noticeably buffeted by turbulence, typically on motorways, at speed, in traffic. Ideally an FF should be “Indifferent” to these disturbances.

Poor indifference is caused when attached (laminar) airflow over a surface changes direction or pressure. Sideways lift generated across a smooth nose may be balanced by sufficient tail area but the sudden cessation, or reversal of the airflow will instantly cancel the side load and this will be felt in the steering or attitude of the vehicle.

Could you translate that Frank ?

This is avoided, on cars as well as FFs, by enforcing separation of the airflow from the surface at chosen points, rather than allowing the separation line to wander about the surface generating unpredictable lift effects.

At the front this is usually done almost immediately, certainly within 500mm of the leading edge. 001 used immediate separation devices on all the leading edges and some of these sharp direction changes, can be seen in the picture.
(on the track | FF Web). 002 had less separation on flat surfaces, as nose lift was not seen as a problem but has clearly visible channels running up each side of the nose to enforce separation of any airflow across the nose surface
(002 | FF Web). The ledge in front of the lights is intended to enforce separation of the flow over the lights and features on all the Voyagers in various forms.
(FJ original nose, plus Production nose, for comparison. | FF Web)

002 however had a minor indifference problem. Strong turbulent side winds could be detected as tail buffeting. The probable reason for this became apparent when wool tufting tests were studied. 002 has no separators on the upper tail for cross flowing air. It's tail ridge is a gentle curve. .
(002 | FF Web)

The production Voyagers are a simple step on from 002. There is more tail area, chiefly due to the use of slides on the head fairing and a complete head fairing. The separation feature on the nose is somewhat more 'styled' and the nose is shorter due to packaging improvements in the basic design. The result was very encouraging with excellent stability and indifference, while efficiency seems as good as any other of these shapes.
(03 | FF Web)

Enforced separation of flow across the tail is just as important as across the nose. 001 and FJ both use separation enforcing edges on the upper edges of the tail. Subsequent study revealed that other vehicles have suffered from cross-flow generated tail lift and buffet and this device is a fairly common solution. The sharp upper edge of FJ's tail may be extreme but some device like this is important.
(Banana and Fat Jogger | FF Web)
FJ's nose shape paid only lip service to enforced separation. The shut line gap of the nose opening panel was supposed to bleed air into the airflow over the nose to provide separation. It's pressurised by the air entering the headlight opening. There is no evidence that this works and indifference was initially so bad that strip separators were retro-fitted, very crudely, to the nose.
(Royce | FF Web)

Indifference was still only moderate in some conditions and a new nose was made with explicit separation features and other changes.
(New Nose | FF Web). This shape improved indifference and flow into the radiators.

Finally the plan view of the vehicle should be considered. If the shape tapers too sharply after the point of maximum width then there may be too much separation towards the tail leading to poor indifference in some conditions. FJ has a slight problem in this respect, whereas the fatter-tailed production shape is markedly more indifferent. Remember the airflow will separate below 2 degrees positive angle of attack, so the tail should be almost parrallel untill the cut-off (Kamm-style) or whatever final separation point is chosen.

By 'separation forcing 'does he just mean a break in the aero shape ? A lump or sharp angle ? The idea being to stop 'side lift'? It's very wordy but seems to be saying bad aero shapes don't get kicked around by turbulent air which makes sense until you look at Allert's bike which he says is good in cross winds to 40 mph ....but he doesn't mention the turbulence you get in traffic on a freeway .

Any idea how Craig Vetter's latest bike handles traffic air ?

and Frank ,thanks for the comments .

[Frank Lee]

For FFs these problems chiefly amount to sensitivity to turbulence, where the vehicle is noticeably buffeted by turbulence, typically on motorways, at speed, in traffic. Ideally an FF should be “Indifferent” to these disturbances.

I'm still stuck with the idea that they had a CP ahead of CG, or a high CP, or CG in a bad place regardless of CP i.e. in relation to the tire contact patches, or something because I'm not convinced that a sleek shape is by necessity more sensitive to x-winds/turbulence and it must be intentionally sabatoged to increase stability.

Poor indifference is caused when attached (laminar) airflow over a surface changes direction or pressure. Sideways lift generated across a smooth nose may be balanced by sufficient tail area but the sudden cessation, or reversal of the airflow will instantly cancel the side load and this will be felt in the steering or attitude of the vehicle.

This is just a kinda strange way to say that even if a design has proper tail area to weathervane into a side wind the way we like it to, if the wind suddenly stops or reverses the driver will not be oblivious to it; he'll feel it in the steering and/or the way the bike tilts.

In that last bit he's talking about how much boattailing the bodywork should have when viewed from above i.e. "plan view". And he's saying there shouldn't be any boattailing in that dimension or else it will separate, which I am not believing.

Craig reports good handling dynamics; there's another guy on here that's ridden Craig's scooter too and IIRC he said it was good. Allert reports good stability with his design too. So the bottom line is, if you believe these guys (I do) then you can have a full bodied bike with good manners. BTW Craig's bodywork is very much boattailed in plan view so where does that leave the Voyager guy?

[PeterS]

Yeah ,it's all well explained, kinda, but I find myself at odds with a lot of it . It's in the link I posted on the board BTW .

I've been reading the site Tony Foale Designs, article on motorcycle aerodynamics. which seems at odds with the FF guys .When you don't know who's right it all gets bit confusing ....so that's for helping out .

[PeterS]

Originally Posted by Frank Lee

This is just a kinda strange way to say that even if a design has proper tail area to weathervane into a side wind the way we like it to, if the wind suddenly stops or reverses the driver will not be oblivious to it; he'll feel it in the steering and/or the way the bike tilts.

In that last bit he's talking about how much boattailing the bodywork should have when viewed from above i.e. "plan view". And he's saying there shouldn't be any boattailing in that dimension or else it will separate, which I am not believing.

Craig reports good handling dynamics; there's another guy on here that's ridden Craig's scooter too and IIRC he said it was good. Allert reports good stability with his design too. So the bottom line is, if you believe these guys (I do) then you can have a full bodied bike with good manners. BTW Craig's bodywork is very much boattailed in plan view so where does that leave the Voyager guy?

Here's the Voyager from the rear,it certainly isn't particularly aerodynamic !



and Craig's .Those comments about his handling in turbulence are very welcome .Building a body is going to be a big job and I'd like to get the thing half decent to start with .

[Frank Lee]

I'd take my cues from the guys that have made it work.

[Grant-53]

I'll read up on Hucho. I do recommend the works of Alberto Morelli, Gaetano Cocco and Joseph Katz. Definitely I want my frame and fairing to absorb as much crash energy as possible as well as making me visible to other drivers. Most of my jpeg files are too large to post but I promise to get something graphic online soon.

[PeterS]

I look forward to them Grant.

[Frank Lee]

Building a body is going to be a big job and I'd like to get the thing half decent to start with .

I've come to prefer the rough prototype before diving in M.O., because it always seems there are unforseen changes to make to get it the way you really want it, no matter how much napkin engineering took place first. Look at how cobby Craig's scooter is, but he's changed it numerous times and in the end it'll have very nice fit and finish I'm sure.

[PeterS]

Yep, Craig's scooter is looking like a very good jump in point .Without the big wrinkles from the flat sheets of yellow plastic the drag might be lower too .

A material that seems to offer potential is heat shrink nylon or Hypalon ,the stuff people cover lightweight aircraft with and some people build kayaks from .Kudzu Craft SOF kayak kits

It's very quick , not too expensive and pretty tough.