1971 Experimental F750 Racer

I never really cared much about aerodynamics, however I do know that a faired bike rolls a lot further than an unfaired one from high speed. Friends use handle bar fairings which extend to cover their hands. I've never really believed in them, however if you stick your hand out or put you head up at 100 MPH there is a lot of breeze there. I do also know that as you approach 100 MPH it takes a lot of horsepower from there to reach 110 MPH or more. Subject to the 'law of diminishing returns' ?

It was mentioned in a previous discussion here that Peter Williams had spent a lot of time getting the aero right on the JPN Racers - the white ones with red and blue flashes.
It was also mentioned that a good fairing was said to be worth about 15 hp - or was that 15 mph ? - significant if your engine was down on speed/power to the opposition.
What wasn't clear was how a 'good' fairing differed from just a fairing...

Wonder where the windtunnel they used was ?
And how complex the testing they did was...

Rohan said:

It was mentioned in a previous discussion here that Peter Williams had spent a lot of time getting the aero right on the JPN Racers - the white ones with red and blue flashes.
It was also mentioned that a good fairing was said to be worth about 15 hp - or was that 15 mph ? - significant if your engine was down on speed/power to the opposition.
What wasn't clear was how a 'good' fairing differed from just a fairing...

Wonder where the windtunnel they used was ?
And how complex the testing they did was...

Click to expand...

I think I've mentioned before that I put JPN fairing on my BMW racer. It went slower down the straight at Phillip Island. If I ever get down there again I'll take the 900SS half fairing and the JPN and try them back to back.

How complex is aero....how long is a piece of string?? I read somewhere that Carcano (Moto Guzzi designer in the 50s) was a bit surprised to find oil splashes on the front mudguard (fender) the only place that it could have come from was the rocker gear of the horizontal cylinder. This was later confirmed by wind tunnel testing, that on an unfaired or partially faired motorcycle there can be (not will be) forward air movement towards the front wheel.
In his book PW talks about the wind tunnel testing, and the effect of the letters on the back of his leathers. The testing was done at the MIRA wind tunnel at Nuneaton in the winter, which means that it was bitterly cold and very uncomfortable. The tests seem to have been thorough, for instance reducing the height of the screen in 10mm steps. It clearly worked, which I suppose must be the ultimate criteria!
The 1971 F750 racer is clearly the first step in the trip that ended up with the multi tube framed bike.
Its a very interesting book
cheers
wakeup

I think a lot of what we do on our short circuits involves accelerating up to 100 MPH in rapid bursts. It is not so often that we get to places like Phillip Island. I only rode there two times many years ago and I didn't experience the wind lifting the fairing. That must be a beautiful experience, and so must be hitting a seagull. I think if there was a head wind there , my bike would stall.

Hi
Frontal area is very important and that is why Norton Manx's and other singles still kept winning in the 1950s.
See Phil Irving's Tuning for speed regards the effects of frontal area and the effort (power required) to move forward, I am sure the current engineers are more than up to speed where the losses are to speed.
Mark

Hi speed 120+,in public scares me so hi speed aero dynamics as seen in all the other bee line cycles in the world Is Not the path ole sloppy Joe Ms Peel has led me into. Peel should have power to blast me to the speed I seek plain naked. Acceleration is my candy and turns add on top the bee line G's. Peel may get active aero at the rear where it really maters to me and has nothing to do with streamlining and on purpose adds some drag on leans then out of the breeze for the straight aways. I really would rather the leaned rear/bike would stay on the surface for more acceleration and sharper turns as the leaps cut the pull on wrists and must wait doing nothing [but not loosing momentum plus enduring engine vibe onset] til touch down again to get it on, similar to yoose guys wanting-waiting to get back on power, early as can ... wait for it ... ... .... ... ...
after the apex and straightening up.

Canaga assures me Peel natural inhaled engine will reach 150 naked but I want the blower ability to twist frame further like pulling back on a spear gun sling ready to let a spike of G force fly where ya aim it, no tire spinning type traction needed. When a rubber band goes slack it don't spring & rebound it just stops at its relaxed state. feel the G's to get launched....

There are much more efficient and easier ways to get aerodynamic coefficient down than by lowering a bike by 2".

Racing with dust bin fairings would have been interesting.

Rohan said:

It was mentioned in a previous discussion here that Peter Williams had spent a lot of time getting the aero right on the JPN Racers - the white ones with red and blue flashes.
It was also mentioned that a good fairing was said to be worth about 15 hp - or was that 15 mph ? - significant if your engine was down on speed/power to the opposition.
What wasn't clear was how a 'good' fairing differed from just a fairing...

Wonder where the windtunnel they used was ?
And how complex the testing they did was...

Click to expand...

I would suggest it is 15 mph. Although, aerodynamics increases in importance and advantage as the speed increases, so 15mph is not a 'constant'. Putting a fairing on a moped clearly won't make much difference, but on a 200bhp bike, it will be a heck of a difference.
In the days before speed cameras, I learnt that my GSXR would just about do an indicated 150mph with me fully prone. I then learnt that folding the mirrors increased the speedo reading by 5mph.
So when I got home, I took the mirrors off. Obviously.
Irrespective of the speedos accuracy, the difference in speed (+5mph) is significant I think. If such a small thing can add 5mph, then the difference between no fairing and good fairing, must be 15mph+.
Having said all that, when I raced in BHR under the Dresda Banner, Degans always told me that the top half fairing was what made the difference to a race bike. Bottom half fairings, in his opinion, were advisable to fit as they gave good crash protection!

Nortons themselves did some serious aero experiments. Remember the Norton Silver Fish ?



Ray Amm did some serious speed record breaking on this.
And it was banned from roadracing, after some early forays - forward vision was not too good, the rider was so prone it was hard to see ahead.

BTW, Nortons logo 'The Unapproachable' was put to the test on this thing.
Apparently the silver fish was hard to get into, and at some event the rider was struggling and wriggling to get on board.
A spectator was heard to remark "this Norton really is unapproachable", which had the factory guys in stitches forever after...

BTW, Unless the track was long and fast, the aero advantage didn't really come into play, so it was no instant success in shorter circuit races.
They weren't too sorry when it was banned ?

There are much more efficient and easier ways to get aerodynamic coefficient down than by lowering a bike by 2".

Click to expand...

Nethertheless, Nortons tried a lower manx norton, back in the factory race team days.
Their version of a 'lowboy manx' took a different track - some years before the dommie version, and on a different track.
It had a considerably beefed up lower fork crown - and no top crown on the fork, AT ALL
This lowered the bike by a good few inches, without a fork top crown to obscure the riders view as he lay literally flat on the tank.
But this position was very uncomfortable, and the rider had to lift his head a little to actually see anything.
It was faster, but so uncomfortable it wasn't raced.
Can't actually find a picture of it, the dommie lowboys have obliterated it from history... ?

In the 70s there were very specific FIM rules concerning streamlining, things like having the full circle of the front wheel visible from the side, maybe the rear wheel as well, width of handlebars, width of fairing, flatness and angle of nose (of fairing!) and so on. Sadly all these changes were brought in as a result of the FIMs blanket ban on full streamlining in the mid/late 50s.
In his book (again, that book) PW talks about motorcycles being considered as a "bluff object" in aerodynamic terms. The drag of a bluff object is determined mainly by its frontal area, which is why PW was so keen to reduce it. He held the approach of Carcano and Moto Guzzi in very high regard with their focus on weight reduction, streamlining, handling, where even on the faster circuits (like Spa) the Guzzi 500 single weighing about 250lb with about 40 bhp, was competitive with Gileras 500/4 which probably weighed another 100lb with 60 bhp or thereabouts. The Guzzi would have required a very committed rider though! I read something, where after practice at a Grand Prix, Carcano offered one of his riders another 2 bhp, or a weight reduction of 2lb but not both. The rider selected weight reduction. On race day the bike was revealed with the frame devoid of paint and 2lb lighter! To me this reveals two things about Carcano, one he was a very pragmatic designer, and two he knew how much paint weighs!
Obviously PW couldn't do a great deal about lightening the engine/gearbox, and was restricted with aerodynamic possibilities, he had to do what he could. Frontal area was one of those things.
cheers
wakeup

For a given frontal area and velocity, a streamlined body will have lower resistance than a bluff body. Maybe PW and others felt they hit a wall with streamlining but I think not with their future endeavors.

I seem to recall being told that dustinbin fairings were disallowed for safety reasons (stability in cross winds). Is there any truth to that?

Dances with Shrapnel said:

For a given frontal area and velocity, a streamlined body will have lower resistance than a bluff body. Maybe PW and others felt they hit a wall with streamlining but I think not with their future endeavors.

I seem to recall being told that dustinbin fairings were disallowed for safety reasons (stability in cross winds). Is there any truth to that?

Click to expand...

That's my understanding, that dustbin fairings were banned for what the FIM called "safety reasons". To be fair there were some dustbins that were dangerous, to design an effective and safe dustbin either a thorough understanding of aerodynamics is required or preferably unfettered access to a wind tunnel, like Moto Guzzi, and the talent to use it. Have a look at the great Bob Macs Manx with a home brewed dustbin and compare it to a Guzzi, apparently Macs Manx was all over the road in a crosswind, whereas the Guzzis were as straight as a die
I heard a story that one of the English teams, probably Nortons, wanted full streamlining banned because it is very expensive to develop. Whether the story is true or not I don't know.
cheers
wakeup

Dances with Shrapnel said:

For a given frontal area and velocity, a streamlined body will have lower resistance than a bluff body.

Click to expand...

This is where the story gets a bit murky ?
Define 'bluff body' ??
The airflow at the BACK of the bike has possibly more to do with the total drag than what happens at the front, within limits.
Even a 'bluff' body will have a semi-streamlined shock wave ahead of it.

Not bikes, but VW were vastly amused that a 924 Porsche was slower, with the identical engine/drivetrain, than a Golf.
It takes some serious aero knowledge to calculate that one out....






Aerodynamics is not always entirely obvious stuff ?
Or anyone could do it....

Might be an urban myth that one Rohan. I recall it "from the day" as engineers had bunged a 928 engine and driveline in a Golf and that went faster than the 928. Think Mythbusters had a go at something along similar lines, putting the engine and drive arse-about in a 928.

I'm with Dave on 924 curvy-ness vs Golf bricky-ness and says so in here too. Selected models drag coefficients
http://www.mayfco.com/dragcd~1.htm

Side winds on 'so so' dust bins is what the race rule makers stated as too risky. Same thing early land speeders found out the hard way on too subby at the rear streamliners. Same with air racers that only got away with it d/t powerful tractor engines at front not pushing like a cycle. Air pressure steering sucks. Lots of Interesting reviews about aero shapes and frontal area, but not one yet to educate us on the other benefits listed for the lowered compacted frames, such as pilot control and fit, stiffness stability, CoG effects, weight loss or economic cost, all mentioned as important or more important than aero reasons - w/o any references on anyone actually reporting those reasons for the designs they tried. Moderns got their basic size and height figured out couple decades ago so mainly just type shape and material changes since plus engine accommodation. If CoG too low it hinders tip over and hold down, if CoG too high it tips too easy and resists rise back up.

davamb said:

Might be an urban myth that one Rohan. I recall it "from the day" as engineers had bunged a 928 engine and driveline in a Golf and that went faster than the 928. Think Mythbusters had a go at something along similar lines, putting the engine and drive arse-about in a 928.

Click to expand...

Golf GTi 1.6 litre 1975 110 hp, 112 mph
Porsche 924 2 litre 110hp (US Spec) 1977, 110 mph
That engine swap was very widely reported back then, and the comments about the aero.
It doesn't come up under urban myths....
The design originally was going to have it built as a VW..

"While the (924) car was praised for its styling, handling, fuel economy, and reliability, it was harshly written up in the automotive press for its very poor performance, especially in its US spec cars. With only 95-110 hp, rapid acceleration was simply not an option, but the Porsche name carried with it higher expectations. " Wiki

Can't explain Steves cd link differences.
But they don't make sense with the quoted horsepowers and performances ?
Although, the US Spec had rubber bumpers - maybe they killed the aero ?

Rohan said ...."...define bluff body..." or words to that effect. I googled "bluff body" and came up with several pages of links. I had a look at some, but they mostly showed pretty pictures, no definitions.
Then I found a definition, "A bluff body is one in which the length in the flow direction is close to or equal to the length perpendicular to the flow direction." from "Bluff Body Aerodynamics and Flow Separation
Jeffrey Newcamp
Group 4
AME 342
18 September 2002"

There's enough stuff there to keep most people quiet for a while, I'm interested in it and my eyes glazed over fairly quickly. However in the conclusions he says that, if a bluff body is extended downstream, the drag reduces, which is pretty much what Rohan said. In other words, short fat profiles create more drag than long slender ones.
cheers
wakeup

Dances with Shrapnel said:

For a given frontal area and velocity, a streamlined body will have lower resistance than a bluff body. Maybe PW and others felt they hit a wall with streamlining but I think not with their future endeavors.

I seem to recall being told that dustinbin fairings were disallowed for safety reasons (stability in cross winds). Is there any truth to that?

Click to expand...

I think the main reason Dustbin fairing were banned on safety grounds is that they could / would be used as 'battering rams'!
Seriously, racers get damned close in corners as it is, but one knows that if your front wheel touches the guy in fronts rear wheel, he'll hardly feel it, but you'll be off...
With a dustbin fairing, this would be reversed and would encourage, err, 'over spirited riding'!