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Wind Stalling

Blog Date - 7 April 2020

It were reet proper windy earlier this year wunnit?

As I'm riding along an open road on a particularly breezy day between hanging on and leaning into the wind I have time to re-notice something I've oft observed on such a tempestuous day as this. From time to time during a gust the engine seems to temporarily lack power.

A tree has grown at almost 45 degrees to the right from the effects of persistent wind
Guess which direction the prevailing winds are on this road.

There's a discernible difference between hitting a headwind and being slowed down by it and the slightly unnerving sensation that the bike is about to stall. For quite some time I've pondered this. Is the bike OK? It's happened before on most of my other bikes, they're not all knackered in the same way surely? 

No, I worked out some time ago this is an effect of the turbulent conditions. But WHY? Purely because I'm a geek, a nerd and I need to get out more I watch many science type videos on YouTube. Several of them are about air flow, pressure, Bernoulli's principle, how planes work and other such flatulence research. This filled my geneeus brain with useful information. 

Ren's tent is partially flattened due to high winds at Shell Island campsite
Science and bitter experience means I am well qualified when it comes to wind.

I think, right, yeah, right, OK. I think when it's windy and the wind is at a certain angle to the bike and the airbox and a certain speed this is causing reduced air pressure around the mouth of the airbox. SEEEE! I am a virtuoso genius! 

Please don't ask me to explain the mathematics and scientific principles to you, you're not smart enough to understand anyway (I'm full of poop ain't I). Suffice to say that in a given set of circumstances the rapid airflow of the wind creates a partial vacuum in and around the area that feeds the airbox. This leads to a temporary reduction in power due to the reduced levels of oxygen available to the engine.

I will await my Doctorate in Fluid Dynamics.

A simple diagram attempting to show how the wind blows over a motorcycle screen
I have previously demonstrated my aerodynamic skills.

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Reader's Comments

Upt'North said :-
Whilst I also suffer with the wind Ed, I have never felt this phenomenon. Does it help explain why my fence fell over in said windy winter.
I suspect that manufacturers would attempt to get a consistent airflow into the general area of the airbox but many bikes position there filters where they can due to design limitations. On the case of the Pan, the Ford Cortina type airbox sits in the centre of the V and there is an airtube that goes from near there to the front of the bike. This isn't RamAir but it purely puts cool ambient air into the general direction of the airbox. I suspect it has two functions, one to cool the components which may suffer from excess heat and secondly to provide more dense cool air to aid combustion.
In the case of old style cafe racers and early race bikes that had grille covers placed over the intake tubes, could you please explain Dr.Ed how they found enough air whilst going forward at 150 mph.
RamAir was introduced to feed the air directly into the airbox at high speed which in turn had a good effect on BHP at said high speed. This presumably because they wouldn't be able to capture the same high volume of air without such an expensive delivery system. Either that or it looked cool when parked up at Matlock Bath.
07/04/2020 09:03:01 UTC
Ren - The Ed¹ said :-
The notion of Ram Air was a kind of turbo effect. At high speed the air is being forced into the airbox due to the pressure at the front of the bike at speed. Of course this could bugger up air mixture because at say 70% throttle and 5000 revs and 30mph there would be less air at the same revs same throttle but higher gear and higher speed. Thus making carb tuning... difficult.

This of course presumes that ram air makes any difference. Logically it MUST make some difference but how much was always a subject of debate. I suspect the difference was, erm, nominal.

With the carbs facing back? Again logically there might be a reduction in the air pressure at the mouth but I suspect the difference is nominal. I expect a certain Mr Soady will have a much better researched and thought out reference for us to read.

Most bikes I've owned had the airbox under the seat. It's the logical position for an upright single or inline multi-cyclinder motor. The advantage beyond simple design considerations is that area will usually be dry to prevent ingestion of rain and also the area least likely to suck in dust and dirt. If your Pan's air intake is close to the front is there not more risk of ingestion of both rain waters and muck?
07/04/2020 09:36:49 UTC
Ian Soady said :-
Long long ago and far far away I embarked on a BSc in mechanical engineering at the (then) Rutherford College of Technology in rainy Newcastle upon Tyne. A reluctance to leave the pub after long liquid lunches led to my being slung off the course after the first year. One of the things I failed to learn about was fluid dynamics. What I do remember is that a chap called Bernoulli developed a set of fiendish formulae describing how fluids behave in all sorts of circumstances. Use of these led to the development of the modern carburettor - for example the venturi arrangement. Some info in the link.

Most modern bikes use a fairly large airbox so that the carbs / induction tracts are pulling air from a relatively consistent and still(ish) environment. The air intakes for these are usually hidden away under the seat / tank so that external breezes don't affect them too much. As Ren says, you don't want to expose these to varying pressure either because of speed or sideways draughts as it upsets the carburation. People have tried reversing the head on bikes to get a "supercharging" effect but generally with poor results. See pic.

Norton Commandos had a fairly large airbox with air intakes underneath and I discovered that replacing this with K&N type filters meant I could never get a decent idle.

As I remember it, the term Ram Air was used by Suzuki(?) to describe the design of the head finning on one of their 2 stroke twins rather than anything to do with carburation.

Posted Image
07/04/2020 10:19:31 UTC
Ren - The Ed¹ said :-
Suzuki 2 strokes from "back in't day" had a sort of cover over the cylinder head fins to encourage some kind of cooling air flow. But many other vehicles the term ram air means... well I'll let Wiki describe it better.

The BMW 310 has a "reverse" set up but they aren't claiming it's for ram-air, more to do with design considerations.

The stalling from the wind is only occasional and during high side winds. There will be just one specific angle where the wind whips around under my leg, causes a vacuum under the seat and the pressure drops, maybe only a little. Or something like that.

As for Bernoulli - I can grasp the principles but as for the maths, I can't be arsed. Although if this lockdown goes on much longer I might make the effort... I'm starting to have arguments with inanimate objects. What's worse is I'm losing the arguments.

07/04/2020 10:52:22 UTC
Ian Soady said :-
That wikipedia article describes ram air as being a tapered inlet tract (M Bernoulli again) - nothing to do with pointing it in the direction of travel that I can see. We used to call them velocity stacks back in the day...

Link is wikipedia for Suzuki ram air.

Don't I remember someone here talking about the effect of side winds on the ambient air pressure pipe? Bob was it?
07/04/2020 11:40:51 UTC
Ian Soady said :-
Velocity stack
07/04/2020 11:41:15 UTC
Upt'North said :-
This isn't my Pan, but a borrowed picture.
The filter box can clearly be seen along with the air pipe which sits to the right of the air box when sitting on the bike. It must be well hidden Ed, I've never found it wet or mucky under there.
Posted Image
07/04/2020 12:39:24 UTC
Ren - The Ed¹ said :-
Upt', I struggle to imagine that small flexi-pipe is the air intake for an 1100cc engine?
07/04/2020 06:45:10 UTC
Ren - The Ed¹ said :-
Ahh, here we go...
The entry is the 2 curved ports to the right of the image.
Posted Image
07/04/2020 06:50:37 UTC
nab301 said :-
I seem to be exercising my pea sized brain these days more than I do in a normal 8 hr work day.... Ian mentioned large airboxes on modern bikes so after doing some research in my own library the typical size is ten times engine displacement!. Also I never considered Bernouilli's theorem in that the faster air travels the lower its pressure , so that if the slipstream is rushing past carb bellmouths at speed there will be less pressure than static air and less airflow into the engine. Pressurised airboxes are mentioned Kawasaki ZX9r as the example. Airbox pressure going from 980mbar at idle to 1050mbar at 170mph. Again the author uses the Bernouilli equation, a column of air travellling at 170 mph dropped to a standstill , the rise in pressure is .6psi at 80mph the rise would be .15psi or roughly 1% rise. Not much apparently but as the author mentions compared to a poorly designed intake the difference between poor and well designed could be 10%. Then there's the Helmholtz resonator equation....
I can't say I've noticed any of this temporary lack of power in gusting winds on any of my bikes but especially not on my Enfield which has the dreaded external filter with no air box. However as the Enfield barely does 50mph , this might have something to do with it.
07/04/2020 07:45:21 UTC
Snod said :-
I ran my TRX850 very lean for a while and during that time it would barely keep running in a strong sidewind, it seemed the air was forcing its way into the airbox (underneath the tank) and leaning it out even more. I fitted larger jets which mostly solved it, and then over time the bits wore out and it got richer and richer until it's just not bothered any more. Some bikes can definitely be affected.
07/04/2020 08:38:41 UTC
Ren - The Ed¹ said :-
I quote "980mbar at idle to 1050mbar at 170mph" - 170mph!!! I suppose the benefits to we mere mortals ain't that much then. As you say, a good DESIGN counts much more than just a hole at the front of the bike.

If you're not experiencing the subtle power difference in high winds... you're night riding in high enough winds! Ug hurgh! You tell 'em, Snod.
07/04/2020 09:39:29 UTC
Bill said :-
Your airbox is a resevoir of air so when you open the throttle it uses that air and it replaced via the airbox intake if the engine is affected by the wind it would have to cause the airbox to be replenished slower than you are using it. Is the intake not under the seat as the CBF250 so in a relatively sheltered position? It's hard to imagine the wind restricting the flow. The next test will have to involve flow paint as used on F1 cars. When my local Mercedes F1 team have finished making CPAP devices perhaps the would be interested in testing your theory:-)
07/04/2020 10:43:11 UTC
Bogger said :-
Fluid dynamics. Whaaaaat??

LJK Setwright eat your heart out.

08/04/2020 12:29:53 UTC
Ross said :-
"Bogger said :- ...
LJK Setwright eat your heart out."

Ooo, there's a name I haven't heard for a long time. Didn't he have a column in the early days of Bike magazine?

I can't say I've ever noticed this 'wind stalling'...I thought there was an element of 'suck' from the engine that would negate that, but that is just an assumption on my part!
08/04/2020 03:05:30 UTC
Ren - The Ed¹ said :-
Bill - unfortunately I doubt popping down to Brackley would be deemed as "essential travel". Also I'm not so sure Mercedes would be inclined to give a scruffy oik from "The North" some of their special flow paint. However being the geeneeoos that I am I shall make some myself. I reckon some kid's water paint from poundland should do the trick.

"LJK Setwright" - I'd never heard of him, I had to google that name.

Ross - you're just not riding in strong enough winds. Ug. Urrrrgh!
09/04/2020 07:53:40 UTC
Bob said :-
Ren, do you remember the post I made about the static air pressure reference tube on my SLR650? The bike started cutting out when a strong sidewind hit.
The pipe had come off the side of the carb robbed the underside of the diaphragm of an atmoshperic pressure reference.
I worked out the same effect was happening to my fairing-less FZR600 and cured it by taping around the edges of the fuel tank to create a cell of still air behind the engine.
A VStrom 650 I briefly owned also suffered to a small extent with it - I guess that bike had an inlet atmospheric pressure sensor in the EFI.
22/04/2020 10:34:22 UTC
Ren - The Ed¹ said :-
I vaguely recall the comment Bob. I'm glad It's not just me imagining things! Interesting call on the various pipes etc to the carbs - it might not be the airbox receiving reduced air pressure but the various breathers etc. This being said... there's no carb on the 500 but it could be an airbox feed or some other random as yet unknown somewhere. I could imagine the manifold absolute pressure sensor has 2 sides "inside" and "outside" and if the air pressure drops on the outside this would effect the computer's calculations.
22/04/2020 12:21:41 UTC
Bob said :-
I've found that the EFI systems on bikes are pretty bum basic to be honest - I'd be surprised if that CB had differential MAP sensors.
That VStrom definitely suffered with this issue, I put it down to the air intake getting pressurised by gusting wind coming it at just the wrong angle.
I haven't noticed the problem on the KLX, perhaps that's because the air intake is well protected due to the bikes off-roadyness?
22/04/2020 02:57:11 UTC
Ren - The Ed¹ said :-
Yeah at least the one on the CBF125 is very basic, I haven't fully explored the one on the CB500X (I hope I don't have to either).
22/04/2020 07:26:14 UTC
Bob said :-
Bikes have an easy time of it compared to cars.
Our old 2005 car has a PCV valve, EGR valve, differential upstream and downstream O2 sensors, MAF sensor and has been a cause of problems over the years, whereas the EFI on my 2012 KLX has no EGR, no PCV only one upstream O2 sensor, no MAF sensor - thank goodness.

I think the newer Euro 4 & 5 bikes will be absolutely horrendous ten years later when things are starting to go wrong, especially as due to the relatively low volumes sold the spares backup could start to become patchy - try tracking down that custom Honda version of the TPS or MAP sensor which is just sufficiently different to the millions of easily obtainable car equivalents.

Because bikes don't adhere to the OBDII (a couple of exceptions) standard, you can't use that £6.99 adaptor of Ebay like you can with your car. I paid £130 odd for a Healtech diagnostic for my KLX, in the past I paid over £300 for a GS911 to use with my BMW. I read that there is nothing available for Enfields, it's dealership only, similarly SWM, Mash, any of the small off brand manufacturers - good luck if you have a problem with the EFI on them (then amplify that sentiment when 10 years have passed and Mash et al are no longer even operating).

All of this is a key factor for me when considering a bike to buy. I'm sold on the idea of EFI, it's been two years since I had to mess with a float bowl or a jet and I don't miss it one bit, but I have to be able to maintain and repair my own bike, so I need a bike with good spares back up and easily accessible and simple EFI.
23/04/2020 04:02:35 UTC
Ren - The Ed¹ said :-
Some of the high end bikes can have as many complex sensors as the complex cars Bob, but yes generally bikes are a lot simpler, mercifully. I've seen the healtech kit, from what I can suss out it would work for both my 500 and Sharon's Z250SL so it may be a worthwhile investment sooner or later. What do you think of the kit? Does it work as well as their site suggests?
24/04/2020 08:12:41 UTC
Bob said :-
The Healtech is good but I'm pretty sure that you'd have to get one for each bike. They sell them as Kawasaki version / Honda version etc.
It enables full diagnostics, you can read all the sensor values and do things like turning on the fan and fuel pump. Fault codes can be read and cleared - very useful.
Agree about the newer top end bikes, that's why I mentioned the Euro 4 and Euro 5 compliance - the EFI had to become more complex to pass the tests.
24/04/2020 03:57:06 UTC

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