JPN Fairing aero discussion

OK, how about we start a thread on the JPN fairing and it's aerodynamic effects?


A few years a go I was chatting to a tuner who had built a BMW classic race bike and he mentioned how it had been the fastest bike through the speed trap at Spa on the Bikers Classic event. This was against japanese 4s including Kawasaki Z1000 based bikes.

Needless to say I thought it would be a good idea to fit the same fairing to my BMW race bike but after one race at Phillip Island I was disappointed in that it went slower! Now, Phillip Island is very prone to wind blowing up or down the main straight and it can be very hot or very cold on consecutive days , so I admit that one race meeting is not a very scientific way of judging the effect of the JPN fairing, but it consistantly went about 200 rpm slower in top gear than the previous year. ( That's 4 or 5MPH slower).


I went home and looked at the fairing and decided to try to get an estimate of its frontal area compared to the previous fairing ( a Ducati 900ss top half fairing).

I photographed the bike with no fairing, the ducati fairing and the JPN and in photoshop I converted them to cut out images so I could mearue the total number of pixels.

It came out like this

No fairing : 185680 pixels
Ducati half fairing : 185071 pixels
JPN fairing:197196 pixels.

OK. So with the Ducati fairing there seems to be a slight decrease in frontal area, but that must be down to slightly different body position, my jeans are hanging out a little more on the unfaired shot.

The JPN frontal area is higher by about 6.5%. The question is whether that increased frontal area is outweighed by improved drag coefficient. My feeling is that maybe it isn't.

Obviously I need to do some more testing on a long straight, but I thought I'd get the discussion kicked off.

John

Without getting too involved, its as much the shape at the back of the bike as at the front that gives a lower Coefficent of Drag.
If the airflow gets disrupted into eddies, rather than being recombined into a SMOOTH flow, you could be out in nowheresville.

When you say JPN fairing, which one are you talking about too ?
I have such a lump off the JPN cafe replicas, and I'd think its aero is nothing to write home about.
Rather than minimal shape/size, it sems to have gone more for weather protection, and maximum size...
BMs at least did some aero work on the shapes of the back of the seat and mudguard, and even taillight ?

Rohan said:

When you say JPN fairing, which one are you talking about too ?
I have such a lump off the JPN cafe replicas, and I'd think its aero is nothing to write home about.
Rather than minimal shape/size, it sems to have gone more for weather protection, and maximum size...
BMs at least did some aero work on the shapes of the back of the seat and mudguard, and even taillight ?

Click to expand...

It's a replica of one of the race fairings, not the road one, here's another view of it. I think it might be a '74 fairing.

As Matt would say, its those cylinder heads sticking out the side, they spoil the Norton aero ?!

Don't know, looks tidy enough, great bike.
Do you fit in entirely inside the fairing when tucked in and down to it ?
Does your shin block the dellorto ? Carb bellmouth ??

Do the rules require it to have a front mudguard ?
Air going under there could suck 10 mph out of it ?

P.S. Those slot(s) cut for the brake lever (and clutch lever ?) look a litle precarious ?
If the bars move anywhere, could block getting full lever movement.
Would scrutineering allow those ?

Rohan said:

As Matt would say, its those cylinder heads sticking out the side, they spoil the Norton aero ?!

Don't know, looks tidy enough, great bike.
Do you fit in entirely inside the fairing when tucked in and down to it ?
Does your shin block the dellorto ? Carb bellmouth ??

Do the rules require it to have a front mudguard ?
Air going under there could suck 10 mph out of it ?

Click to expand...

I do fit right inside, have a look at the photo in the OP. But the Ducati fairing is a very close fit to my shoulders. The JPN is a bit bigger there.

The bellmouth is further in than it looks, my shin doesn't block it.

I can remove the mudguard. Interesting point. I'll try it.

Also, does that fairing have an opening in it, behind the front wheel, for some air to flow over the motor and into inside the fairing.
If not, it could be presenting a blank wall to the oncoming air, rather than slicing into it.
And having some air exit at the back, to recombine with the externally flowed air.
Just a passing though - what do the BM racers do there.

The BM roadbikes had a lattice/grill there, to flow some air through..

Rohan said:

Without getting too involved, its as much the shape at the back of the bike as at the front that gives a lower Coefficent of Drag.
If the airflow gets disrupted into eddies, rather than being recombined into a SMOOTH flow, you could be out in nowheresville.

Click to expand...

+1

The base drag can contribute more to the overall drag than the frontal area. Base drag results in a low pressure region behind the body that creates a "suction" effect. Tailoring the edges of a fairing can reduce base drag. I have put curved vanes on the blunt back of my enclosed trailor to duct air into the base region ...this has resulted in about a 12% improvement in fuel economy ( reduced drag).

Slick

Wonder how, aerodynamically, this would go ?



That half fairing airflows the top half of the rider and much of the bike.
Having skinny legs would probably go faster ?(if the gearing allowed).
Pic selected at random, off fleabay, but it looks tidy...

pommie john said:

Now, Phillip Island is very prone to wind blowing up or down the main straight and it can be very hot or very cold on consecutive days , so I admit that one race meeting is not a very scientific way of judging the effect of the JPN fairing, but it consistently went about 200 rpm slower in top gear than the previous year. ( That's 4 or 5MPH slower).

Click to expand...

Atmospheric conditions have a significant effect on engine performance; air temperature has a significant impact on atmospheric conditions. I lost a race weekend chasing electrical problems which included an inadequate needle jet due to unusually high temperature and low air pressure. As for winds, Daytona is notorious for wind differences between morning and afternoon. This is where a six speed close ratio is handy where you may be able to pull a section of track in sixth in the morning but with a change of winds only pull it in fifth in the afternoon.

If you are into a bit of math, the chart and URL are something hobot rounded up. If you know your top speed and your rear wheel HP, you can use the chart to get a rough idea of your value for Coefficient of Drag X frontal area. (ie which is your appropriate colored line?) For a given fairing configuration, if you know the frontal area (which it sounds like you are on your way to measuring), you can back out the Coefficient of Drag for your set up. As long as you don't significantly alter the frontal area, you can test different side and rear aerodynamic treatments and then measure the impact and calculate the new Coefficient of Drag. The trick is to get a reliable measurement for speed and power- best if you had a wind tunnel but an alternate is testing flat out speed and knowing your rear wheel HP. Even if you cannot get the gearing to peak speed @ peak HP, if you have a dyno curve, the engine rpm will indicate what power you were making at any given rpm. Again, no wind and atmospheric conditions need to be identical if you want to rely on a dyno curve.


http://www.borynack.com/xr650r/Horsepower2speed.htm

Dances with Shrapnel said:

If you are into a bit of math, the chart and URL are something hobot rounded up. If you know your top speed and your rear wheel HP, you can use the chart to get a rough idea of your value for Coefficient of Drag X frontal area. (ie which is your appropriate colored line?) For a given fairing configuration, if you know the frontal area (which it sounds like you are on your way to measuring), you can back out the Coefficient of Drag for your set up. As long as you don't significantly alter the frontal area, you can test different side and rear aerodynamic treatments and then measure the impact and calculate the new Coefficient of Drag. The trick is to get a reliable measurement for speed and power- best if you had a wind tunnel but an alternate is testing flat out speed and knowing your rear wheel HP. Even if you cannot get the gearing to peak speed @ peak HP, if you have a dyno curve, the engine rpm will indicate what power you were making at any given rpm. Again, no wind and atmospheric conditions need to be identical if you want to rely on a dyno curve.


http://www.borynack.com/xr650r/Horsepower2speed.htm

Click to expand...

I'm happy to say that my top speed is well off that chart

Dances with Shrapnel said:

I lost a race weekend chasing electrical problems which included an inadequate needle jet...

Click to expand...

I'm very confused. Is this some sort of racing trick? Electric needle jets

Sorry I couldn't help myself...It all ran together.

pommie john said:

I'm happy to say that my top speed is well off that chart

Click to expand...

Interesting comment.
When you look closely, that chart doesn't seem to have all that good a correlation between horsepower and speeds for other known road bikes either.
If Dave Rawlins naked Commando can do 143 mph on ~70 hp, how does that fit in.
And a Rickman I've been for a spin on isn't there either.
Maybe the curves just need to be projected out for more mph...

pommie john said:

I do fit right inside, have a look at the photo in the OP. But the Ducati fairing is a very close fit to my shoulders. The JPN is a bit bigger there.

Click to expand...

Your head/helmet looks to shick above the level of the front screen. ?
If your helmet is copping the wind blast (and the air not being deflected up above/over it ?) it may be worth trying a slightly taller screen, or tipping that one up slightly - if possible.

Dances with Shrapnel said:

I lost a race weekend chasing electrical problems which included an inadequate needle jet...

Click to expand...

dennisgb said:

I'm very confused. Is this some sort of racing trick? Electric needle jets

Sorry I couldn't help myself...It all ran together.

Click to expand...

heh heh heh
We are all confused, but dws isn't...

Rohan said:

Maybe the curves just need to be projected out for more mph...

Click to expand...

I would like to see the lower end of that chart go down further in MPH and be blown up so I can read my coefficient of drag better.

You'd have to calculate it from that chart.

And aero calcs are pretty approximate for bikes, at the best of times.
If you stick an elbow or foot out, or sit up or lean forward, the drag value will alter - significantly...

This is a chart for human power, on bicycles.
A significant range of values shown there, for different situations.

http://4.bp.blogspot.com/_urSQl6wUA5g/T ... _chart.jpg

I came across something similar for motorcycles the other days, will see if I stashed the link someplace.

The function graphing is just about a one of a kind so mostly just hanging out in its own universe of reference values. Realistic enough for good sense of the differences to expect in power and speed but not that close on actual power to particular speed. If it was Ms Peel had ~4 cd, while just tucked naked
Btw this scale of differences also holds for acceleration ability too - almost regardless of the extra mass added for coverings. Flat trackers coming out of a slide draw l arm in as can feel the drag in the tire slip going that loose. In the Dirt Tracker flim movie in m/c discuss area, a Norton guy that also ice raced lifted his left foot on to bar grip to get out of wind so pure frontal area reduction help.

Did someone mention that trying to describe this subject with just theory and calculators is too similar to engine power calculators but fun bench racing em.
Example being engine dynos just can't dyno very low, usually about 2000 for the physics to kick in, so below 50 mph the low forces are hard to measure, unless you are a peddle biker a drink bottle positioned well is worth 6 watt less muscle to hold hi cruise speed and more at race rates. If ya could just plug in numbers and get a product out, would need wind tunnels, just build and fly it.

Ok, take the sleek faired BMW [with jumbo ears sticking out] could someone calculate how much faster would it'd go if lowered 3"? Ignoring if pilot could fit and crink neck enough to mostly see ahead.

Lip spoilers are ancient way to deflect wind over eye or helmet w/o adding as much or any height/area. Those lever slots are pensive making as we see camera views of them walking around buffeting. Wooshhh....

Somewhere back on this forum I made up a chart showing speed vs horsepower for various bikes where they had been tested by magazines. They had been dyno tested and speed tested by the same mags and there was a reasonable trend shown on the graph with some exceptions which I was questioning ( JPN Nortons for example )

Here's the chart. I was questioning whether the various JPN bikes actually made the speeds that were claimed since from the chart they look like anomalies.

Rohan said:

Dances with Shrapnel said:

I lost a race weekend chasing electrical problems which included an inadequate needle jet...

Click to expand...

dennisgb said:

I'm very confused. Is this some sort of racing trick? Electric needle jets

Sorry I couldn't help myself...It all ran together.

Click to expand...

heh heh heh
We are all confused, but dws isn't...

Click to expand...

The old adage - chasing an electrical problem that was a fuel problem - dennisgb got it.

pommie john said:

I'm happy to say that my top speed is well off that chart

Click to expand...

Go to the formula of drag force or drag power and you can calculate and plot the same; create your own graphs in a spread sheet if you are inclined to do so. Phil Irving's Tuning for Speed
has a simple calculation for the same but it is in scary Imperial Units which at least one individual on the forum chokes on. Plenty of SI references for the same.

In the end it is all academic as frontal area and Drag Coefficients are to calculate/predict power/speed relationships; you already have that if you know you rear wheel HP characteristics. Bottom line is trial and error regardless of whether you know your Drag Coefficient or frontal area.

pommie john said:

Click to expand...

Besides the fact that the JPN's and Motoguzzi appear to be outlier data points, my opinion is that the data points for the two JPN's and the Motoguzzi have what appear to be identical indicated horse power at their respective terminal speeds. JPN to JPN, identical - fair enough but a completely different bike with what I would consider a completely different frontal profile (JPN vs MotoGuzzi) and I have questions. Could be data entry error(s) or some pure marketing BS.

Furthermore, assuming the JPN's were 750's, from my experience and from what I have seen out there with the modern components and ~40 years of further technical development I would say 155 mph, possible to likely but 170 mph, not likely. A Norton at 155 mph is pushing it. Following the trend from the chart, for that same norton to achieve the claimed 170 mph, if you follow the trend, it would have needed to nearly double it's horsepower to around 130 140 RWHP. Assuimng the chart shows indicated RWHP, the faster JPN would have had to halve it's frontal area or halve it's Coefficient of Drag or a combination of both.

On the other hand, maybe there was a 750 JPN that cracked 170 mph. If so, tip of the hat to those who achieved it and I would love to see details on how it was configured (fairing, enginee, frontal area etc).