The function of holes in the windshield ST1100?

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Was at a regular motorcycle gathering last night where I got the following question.

What is the function of the holes in the windshield on the Honda ST1100 Pan-European?

Haven't thought about what the holes are for but something that affects the airflow... What is the exact explanation from the manufacturer for these two holes?

IMG_1326.jpeg
 
They are 'nostrils' to let the pressure that builds up behind the screen escape.

Explanation. When something this size punches its way through the air , the air gets pushed out to the sides and it flows much faster. The slipstream - that flows past closest to the edge of the screen - tries to get back to where it was and it swirls back behind the screen creating an eddy - but also pressure on the inside of the screen. You can see this happening if you look at a rock sticking out from the river bed. The water behind the rock is either stationary or it is flowing upstream. (Canoeists use these for a bit of a rest when tackling the water in fast flowing rapids).

You might also notice as you are driving along that the fly or the bee that just manages to escape being hit by the winscreen, often overtakes your right arm and knocks itself out on the inside of the screen.

This also happens on the ST1300 - but the screen isn't fixed at the bottom like it is on the 1100. So the pressure behind the screen can escape at the bottom to be swept up the screen and into your face - unless you fit one with flip-up lip which directs the airflow over your helmet,

The same thing happens behind your back - often giving the sensation of being pushed forward. It tends to happen on both sides of you - wind swirling in from the right hand side to push forward behind you, then from the left. Alternating from one side to the other. This is of little consequence if you ride solo, but if you put a top box on the back riding solo, it can make the back end feel very unstable. The ST1300 has a mount which allows the top box to slide left and right - to provide a little movement to reduce the effect that it has on the bike. Fitting a pillion between you and the top box helps smooth out the air flow, so the eddy happens long after you have got out of the way. But you will feel it if you ride behind a van or lorry / truck.
 
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jfheath said:
They are 'nostrils' to let the pressure that builds up behind the screen escape.
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That's already covered by the slot below in the black garnish plate...

Those "cat eyes" in the '96 onward screens actually only serve the pillion...
By redirecting/calming the airflow at the sides of the rider's helmet, a pillion suffers a lot less buffeting on her head...

I experienced that myself while an acquaint rider gave me a lift on his (back then pretty new) '96 ABS-II/CBS over a stretch of motorway and was astonished how calm it is back there, while the pillions on my '92 ST (with the old, straight cut wind-screen) frequently complained a bit...
Since then I upgraded all my ST's with the "cat eye" style wind-screens (mostly taller ones made by SECDEM, suiting my size, whilst still looking very OEM)

www.bullster.com

High-protection windscreen HONDA 1100 ST PAN EUROPEAN - 1995/2002

www.bullster.com www.bullster.com
 
Extra pressure relief at higher speeds. The nose duct/hole wasn't enough, causing rider fatigue from the slight-forward riding position and the low pressure behind the windscreen also pulling him forward. I had the slots on my 95.
 
jdmccright said:
Extra pressure relief at higher speeds. The nose duct/hole wasn't enough, causing rider fatigue from the slight-forward riding position and the low pressure behind the windscreen also pulling him forward. I had the slots on my 95.
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What surprises me about this windshield is how comfortable it is to sit behind it, but I have a vent on top of my helmet when it is open it is not noticeable on the ST1100. On the other hand when I ride the R1250 GSA my head gets very cold, almost uncomfortable with the vent open. But you have a point with the original handlebars behind the windshield, fast driving on the highway is noticeable on the shoulders and neck where you think another windshield would be better?
 
I have to think way back when I had my 95 and stock handlebars and windscreen, before I added handlebar risers, 1996? I could definitely feel it in my neck and shoulders on long, highway speed rides. Adding risers did more than a windscreen would I think. It raises your sitting position more upright and moves you away from that low pressure area. It does also increase buffeting over your helmet since your a bit more in the slipstream, but that's a small tradeoff for neck and shoulder comfort IMO.

I don't farkle my bikes much, but handlebar risers (or helibars), heated grips, and TPMS are three must-haves for me.for the record I'm 5'11", 32-inch inseam. I have an aftermarket windscreen now (no slots) bought in 2018 after a minor crash. I shows it's as a National Cycle oversize model, but height is unknown. Also have a Corbin seat. My eyes are just about 3" above the top edge and I can ride without any discomfort.
 
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Almost have to include what AI describes in this. Copied and pasted here:

AI-overview

The holes (or slits) in the windscreen of the Honda ST1100 Pan-European are primarily designed to reduce turbulence and air buffeting for the rider. They are an aerodynamic feature that helps manage the pressure difference between the front and back of the screen.

Primary Functions:
  • Reduce Vacuum/Buffeting: As the windshield moves through the air, it creates high pressure in front and a low-pressure vacuum behind it. This vacuum pulls the high-speed air down into the rider's cockpit, creating turbulent "buffeting". The holes allow a small amount of air to pass through, filling that vacuum and resulting in a smoother, less tiring ride.
  • Prevent Airflow Stall: The vent prevents the airflow from stalling over the top of the windshield, providing a more consistent laminar flow over the rider.
  • Increase Comfort: By breaking up the vortex (swirling air) created at the top of the screen, the holes reduce wind noise and head fatigue on long-distance trips.
  • Manage Crosswinds: Some designs, particularly in aftermarket screens, help stabilize the bike in crosswinds by reducing the sail effect of the large screen.
While some aftermarket screens may include these, they are a known feature designed to make the large, touring-focused Honda ST1100 windscreen more comfortable for the rider.
 
jdmccright said:
I have to think way back when I had my 95 and stock handlebars and windscreen, before I added handlebar risers, 1996? I could definitely feel it in my neck and shoulders on long, highway speed rides. Adding risers did more than a windscreen would I think. It raises your sitting position more upright and moves you away from that low pressure area. It does also increase buffeting over your helmet since your a bit more in the slipstream, but that's a small tradeoff for neck and shoulder comfort IMO.

I don't farkle my bikes much, but handlebar risers (or helibars), heated grips, and TPMS are three must-haves for me.for the record I'm 5'11", 32-inch inseam. I have an aftermarket windscreen now (no slots) bought in 2018 after a minor crash. I shows it's as a National Cycle oversize model, but height is unknown. My eyes are just above the top edge and I can ride without any discomfort.
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It feels very comfortable with the original handlebars and the windshield I have now, thinking about how calm it is for the upper body and especially the head.

I have had typical Adventure motorcycles where it has been like sitting in a storm behind the windshield on the highway at high speed. On the ST1100 now, this is what really surprised me.
 
Compared to windscreens of the time (looking at you GW) I'd hardly call the ST's large. It's just enough to keep you mostly dry in a light rain if you tuck in. Maybe due to the bike's higher CG, it might have had more effect in crosswinds. I dunno. IMO, the whole bike is a sail in crosswinds, that puny windscreen is only a small factor.
 
My Vetter Windjammer II or III fairing had two round ports in locations similar to the above's cat eyes. They also have a cover that was easily opened. And closed if you had the reach. I've always preferred tall windscreens and the vents worked well to keep rain from being pulled into the inside of the windscreen.

That Vetter was one heavy fairing. But it was rock solid.
 
Mostly the holes on an ST 1300 with a California Scientific windscreen let the rain hit you in the face. Dont ask me why I carry a wide roll of duct tape.
 
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