wind and engine performance

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kbailey
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wind and engine performance

Postby kbailey » Sun May 21, 2006 1:30 am

Yesterday I rode about 60 mi in a 30-40mph (48-64kph) cross wind. The wind was blowing exactly perpendicular to my direction of travel. I was cruising about 70mph (112 kph) on some rolling hills. I could swear that the wind just sucked power from my engine. the entirety of the effects are hard to describe. The engine seemed to run a bit rough like someone trying to clear their throat. I would say it required a 10% increase in throttle position from normal. I didn't figure my fuel economy so I don't have that number.
Is this a common phenomenon? Does wind blowing across the air intake lower the pressure enough to really starve the engine for air that much? I have ridden in cross winds before but never really noticed this. I would be interested in others' experiences.

As a side note, for the second and decidedly last time I have ridden on a gravel road. That just scares the crap out of me.

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Ibsen
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Postby Ibsen » Sun May 21, 2006 1:54 am

That is completely normal. I have in fact seen the power loss, or more correctly the top speed loss, calulated somewhere, but I can't remember where.

Back in the 70's and 80's at least, when most bikes were naked, test reports in magazines always stated one top speed normally seated, and one with the rider flat out over the fuel tank.This is the figures from a test of the Kawasaki KZ400 twin, back in 1978:

Mean top speed 97.65 mph
Best one-way 102.40 "
Mean normally seated 85.90 "

As you see, with the rider normaly seated the top speed drops almoast 12mph. If you then add a 30mph head wind, the drop will be maybe 5~10 mph more.

So what you observed was correct, the engine needed more power to obtain the same speed in the wind you were riding in.

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Vatch
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Postby Vatch » Sun May 21, 2006 8:04 am

Do you have the factory air box/filter set up or individual pods for each carb? A lot of wind blowing in could make the engine run lean. My experience with such cross winds, which are common in central Wisconsin, are that I'm slowing down anyway as ride down the road leaning to one side or another in an attempt to ride in a straight line. Not sure if you're using a fairing or windshield, but removing mine seem to have an effect. In my experience, cross winds and higher speeds do not seem to mix well with cb650's.
'99 1500 Drifter

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Chris
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Postby Chris » Sun May 21, 2006 4:35 pm

It's been windy here this weekend too. I've done close to 100mi. between yesterday and today and I can tell you that 30mph gusts make it loads of fun to try and go in a straight line, especially for a newbie. It takes alot of work for the engine to push my 200lb body through a headwind and the crosswinds have no problem tossing me from side to side. I'd imagine that the more aerodynamic riding position and faring of a crotch-rocket would probably help a little, but in the end wind is wind. I doubt it's affecting your engines performance other than that it has alot more work to do. Ever try to pedal a bicycle against the wind? It's miserable. The only good wind is a tailwind.
1980 CB650c

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DammitDan
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Postby DammitDan » Mon May 22, 2006 11:06 pm

I always thought that the effect of a strong wind blowing directly into the intake would cause a super-lean mixture (since you're essentially creating a blower effect), which would cause a "jumpy" sensation on the throttle at higher RPMs due to gasoline starvation... The only way to maintain speed is to lay on the throttle, which causes the engine to "jump over" the super-lean boggy spots. That's why pod filters are really not very good for these bikes, since they're right out there in the open with no cross-wind protection.

Another effect of the super-lean condition at high RPMs is dangerously overheating the engine. Overheating would cause the same symptoms you described, as well as threaten an engine sieze if it got too hot (not much fun at 70-80 mph).

cb650
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Postby cb650 » Tue May 23, 2006 7:23 pm

When I put pods on mine I left the vent hose to long and at speed the vacume/wind would cause the floats problems.

Dont ask how long it took to figure that out!!!!!!!!


Terry

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Chris
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Postby Chris » Wed May 24, 2006 2:08 am

Wow, learn something new every day :? I guess I'll stick with the stock airbox in order to avoid such phenomenon. I'm too cheap to spring for those cool assed pod filters anyhow. I wonder what the difference is between pods and those fancy valve cover breathers for car engines? They might be more cost effective. Nah, first things first... it'll be a while before my bike's in good enough shape to start tricking it out. Possibly even years.
1980 CB650c

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Ibsen
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Postby Ibsen » Wed May 24, 2006 2:23 am

The breather filter is exactly that. They are used on the end of the breather hose from the valve cover when you remove the air filter box. Pod filters, are made for using on the carbs. And I agree, pod filters might look cool, but they won't add much power to the bike, and you will most likley have to rejet the carbs as well. For normal road use, I would stick to the stock air box.


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