How to test fasteners on installation of wind screen

I recently purchased an after-market wind screen. I wanted something taller than my stock screen on the NC700-S. My concern was how the taller screen should/could be fastened to the bike's mounting bracket.

CONCERN

The after-market screen utilized the 'well-fasteners' that are fitted on the short Honda stock screen. These fasteners work by swelling up the rubber sleeve that surrounds the embedded nut behind the bike's mounting frame.

There has been much discussion on this forum elsewhere as to whether taller screens should not rely on these well-fasteners. Some riders have replaced the stock fasteners with steel nut-washer-bolt assemblies. There is also some caution expressed about the risk of over-tightening the fasteners, which might lead to cracking of the screen.

CALCULATIONS

As a practicing structural engineer, I endeavored to calculate (Excel and Mathcad) the wind pressures that could be expected on the windscreen at various speeds. I devised a rudimentary test to validate the calculations, where the one variable hardest to estimate would be the drag coefficient (Cd) of the screen.

TESTING

The same test could also be used to test the forces on the fasteners. I wanted to test the fasteners for excessive vehicle speed (above the speed limit), without actually exceeding the speed limit.

My apparatus consisted of a small FM-Radio antenna, which telescopes. The antenna was fixed to the handlebar clamp, and extending to touch the back of the windscreen at about mid height. When the bike is in motion, with wind pushing against the windscreen, the windscreen will flex backwards and cause a contraction of the antenna. I would ride up to speed, note the maximum speed, then come to a stop. A fine scale ruler would measure the resulting gap, which corresponds to how must the antenna was compressed.

A separate apparatus consisted of a fisherman's scale, used to measure the weight of fish. Inside my garage, I applied the fisherman's scale at the top edge of the windscreen, and pulled back horizontally. After applying a pre-determined force (10 lbs), I later observed how much the antenna compressed. The two points (force versus antenna location) were not coincident, but I could scale the lever-arm lengths to find the expected deflection at the top edge of the windscreen under that applied force.

OTHER SCREENS

Most screens are racked at 30-degrees off of vertical. This angle various a bit between manufacturers, but not by more than +/- 5 degrees. As such, these calculations are transferrable to other marketed screens.

Most screens have a similar horizontal curvature (as viewed from the sky, looking downwards). As such, the coefficient of drag would be similar amongst other marketed screens. The only exception is if a large NACA vent is featured on the windscreen.

My windscreen does not have a louver (laminar lip) above the upper lip of the windscreen. Other windscreens which do include a windscreen should not use these calculations, since the drag coefficient could be substantially different.

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MY RESULTS

Dimensions of the after-market windscreen

9 in = Width along top edge
14 in = Width along top fastener row
13 in = Width along btm fastener row
14 in = Height from top fastener row to top screen
3 in = Height between top and btm row of fasteners
1 in = Height between btm row and btm edge of screen

Deflection and resulting pullout force from riding against wind

140 kph = Vehicle speed into static air.
3 mm = Horizontal deflection at antenna device
6 mm = Horizontal deflection at top of windscreen
13 lbs = Pullout force on each of two btm fasteners.

This

5 lbs = Horizontal pulling force between top edge of screen
13 lbs = Pullout force on each of two btm fasteners.

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CONCLUSIONS

The above forces occur for steady wind at the noted vehicle speed. Gusted winds are approximately 40% higher pressure over the coincident steady winds. So, the calculations should be amplified by 40% higher to represent driving past a large truck.

Wind pressure scales to the square of the wind speed. Example:
100% = Steady pressure from vehicle speed at 140 kph (baseline)
150% = Steady pressure from vehicle speed at 170 kph
210% = Gusted pressure from vehicle speed at 170 kph

Applying a horizontal force of 5-lbs at the top edge of the windscreen, will result in a reaction of the two bottom fasteners to have the same pullout force (13-lbs each) as experienced when riding at 140 kph (steady wind). This same force would arise in 100 kph (steady wind) that is expected to gust to 140 kph.

Applying a horizontal force of 10-lbs (210%/100%) at the top edge of the windscreen, will correspond to the expected reaction forces of the two bottom fasteners experienced from gusting winds while riding at 170 kph (steady wind).

So, what speed should I test my screen for? Well, building's are designed to resist wind pressures with a factor of safety of (1.50 ~ 150%). If I validate that the windscreen can endure 150% higher pressure than expected, then an appropriate factor of safety has been incorporated. A test of 170 kph vehicle speed, would correspond to a safe operating speed of 140 kph, with wind gusting allowed for.

So, with my fish scale, I applied a force of 10-lbs at the top edge of the windscreen. Simultaneously I noted the status of the fasteners. The fasteners were not slipping - Good! This validated the installation of my fasteners to handle wind gusts that could arise while riding at 140 kph.

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Have you considered the combined force of your 140 kph motion through the air along with the force of an oncoming truck at 140 kph on a 2 lane road? I don't know how the science works out, but I do know that my Madstad windshield flew off at 55 mph and hit me in the face using the stock well nuts. The rubber well nuts had pulled through the holes in the bracket after being on for ~ 15 miles at 55 mph. I have since replaced them with Dale's metal well nuts and haven't had any issues since.

Another thing to consider is that the well nuts are not exactly a failsafe reusable fastener. I had one partially invert upon reinstallation, and I didn't immediately notice. It still accepted the screw thread, but the rubber part was not secure in the casting hole. If you have an improperly installed or damaged well nut in one of the lower two positions, you now only have one single well nut that is resisting all of the tension caused by the air load on the windscreen.

To me, a screw and nutplate is both stronger, and resistant to errors and damage. You can buy 5mm (stock screws) or 6mm (tall screen screws) nutplates from HONDABIKEPRO, or the Honda Tall Screen comes with 6mm nutplates.

TomInOregon,

Dale's Metal plate only conforms to the 700-X model bike bracket; it does not fit the 700-S. Further note, some after-market screen manufacturer's make larger screens for the 700-X than their option for the 700-S. Thus, forces would be higher on such screens for the 700-X relative to the 700-S model.

I might imagine that your "opposed traffic" scenario could generate wind forces in the order of my maximum test case - similar to 170 kph with amplification for gusting. Fortunately, opposed lanes are rare for highways.

When I applied the corresponding 10-lbs test load to the upper rim, the whole screen warped excessively, despite the fasteners holding (at least during this controlled static test). Importantly, the screen warped even around the base in the vicinity of the fasteners. Conceivably, repeated warping could serve to wiggle the rubber mounted well-nuts side to side.

1) This could lead to gradual and eventual withdrawal of the fasteners. (This is consistent with some forum members reporting of gradual loosening of the fasteners.)

See report by ziggie:
http://nc700-forum.com/forum/nc700-mods/1106-installing-calsci-windshield-2.html

2) This excessive warping could also lead to cracking of the acrylic screen at corners (where stress concentration is accentuated). (In treybrad's case, I also suspect that failure may also be attributed to non-conforming curvature of the screen to the mounting assembly.)

See report by treybrad:
http://nc700-forum.com/forum/nc700-mods/883-my-calsci-windshield-16.html
http://nc700-forum.com/forum/nc700-mods/883-my-calsci-windshield-17.html

3) Misalligned holes made during the manufacturing of the after-market screen can cause either cracking of the acrylic screen and/or premature withdrawal of the fastener.

See report by Rennie
http://nc700-forum.com/forum/nc700-mods/1106-installing-calsci-windshield-2.html

I might add that if one was to use well-nuts for such enlarged screens, then the following precautions should be exercised:

1) Ensure screen's holes are perfectly alligned with the bike's mounting bracket. Do not accept a screen where the holes are misalligned; this would force the well-nuts to be leaning on edge, greatly reducing their pull-out resistance.

2) Ensure the well-nuts are not over-tightened. Over-tightening could result in premature or partial withdrawal. Some members elsewhere suggested an installation torque of only 1 ft-lb, which corresponds to the typical torque of turning a screw driver. This is what I have applied on my bike.

3) Do a load test. I would suggest something as simple as pulling the top edge of the windscreen horizontally back (towards to seat) with a force of about 10-lbs. This generate a deflection on the top edge of about 1-inch, for a CalSci NC700S screen. The CalSci NC700X screen would result in much higher deflections for the same force - between 2.0 and 2.5 inch. (I would rather the test cause catastrophic failure in my garage then at speed on the highway.)

4) Periodically check the fitting of the well-nuts. A close-up visual inspection should be adequate. Ensure that no gap has developed at the well-nuts or behind the screen. At the same time, also pull forward the bottom of the screen. Place fingers through the vent space beneath each screen, one hand at each of the two bottom row of fasteners. Pull forward with 10-lbs of force each.

5) If the screen is noted to have suddenly become loose or wobbly while riding at speed, immediately reduce speed. This can be inspected with the left hand even while riding (when safe to do so), by wiggling the top edge of the screen.

6) Don't speed! - especially not with these over-sized windscreens. Wind pressure on the windscreen is proportional to the square of the speed. So, riding 40% over the speed limit will have the windscreen experience double the wind pressure as that at the speed limit. Of course, this doubles the pull-out force on the fasteners - all the more catastrophic while speeding!

Deflection of only 1"? Holy cow I've seen mine move 3"+ in gusts. Modifications coming.

CaptDL said:

Deflection of only 1"? Holy cow I've seen mine move 3"+ in gusts. Modifications coming.

Click to expand...

The numbers I gave all pertain to the CalSci NC700S screen, which is 18" high overall.

Your bike, NC700X, uses a taller screen from CalSci, so the deflections will be higher for the same applied force of 10-lbs.

If you have the CalSci-medium screen (24.0 inch high), deflection would be 2.8 inch.
If you have the CalSci-large screen (25.5 inch high), deflection would be 3.5 inch.

So, your observation of 3-inch (+) deflection while riding at highway speed, with wind gusts, is as predicted. (Yup, that is ALLOT of deflection.)

http://www.calsci.com/motorcycleinfo/NC700prod.html

Height = Net height for these screens = [overall height]-[90 mm]
Deflection = [pressure] * height^3 / (24*E*I)
Deflection = [force at top] * height^3 / (3*E*I)
Deflection is proportional to the cube power of the net height.
So, one just needs to scale with height^3 for different height screens.

Skeleton, thanks for applying your scientific knowledge to a subject that's close to my heart!! Being a fan of over-engineered solutions myself, I went for Dale's nut plates.

scrumpymike said:

Skeleton, thanks for applying your scientific knowledge to a subject that's close to my heart!! Being a fan of over-engineered solutions myself, I went for Dale's nut plates.

Click to expand...

I'm also a fan of over engineering, my rule of thumb for the bicycle is something is attached well once I can lift the bicycle up by the part. (Plastic water bottle cages often fail this test).

Applying the sale rule to the automotive world has not been feasible, turns out I can't pick up my car, or motorcycle at all with out the aid of a Jack.

So I guess my question is can you suspend the bike from the nut plates?

Sent from my SAMSUNG-SGH-I537 using Tapatalk

So then how would this apply to a frame that brought the screen outward from its original position, i.e. the frame offered by Palmer Products. Would just moving the unit outward also increase drag due to increased flow under the screen?

bduffey said:

So then how would this apply to a frame that brought the screen outward from its original position, i.e. the frame offered by Palmer Products. Would just moving the unit outward also increase drag due to increased flow under the screen?

Click to expand...

The Palmer screen should have LOWER drag (Cd). The forward position of the Palmer screen would have the same advantage as that served by a NACA vent - reduce the back pressure behind the screen.

I might guess the Palmer perhaps has about 10% to 20% less drag than what I could estimate for the CalSci screen. Thus the forces would be scaled to 80% (100%-20%) of what I had stated for the CalSci screen.

I was able the calculate the tension force on the CalSci bottom row of fasteners to be 25 lb each at 140 kph. Should the fastener on the Palmer be 80% of 25 lbs = 20 lbs? No, even less - Good! The geometer of the Palmer is different; also, the leading edge at the bottom is solitary (separate from the fairing), so it would have a much higher positive pressure than the CalSci. The positive pressure would serve to counter act the over-turning mechanism of the upper edge of the screen. Perhaps the bottom edge fasteners each have 15 lbs uplift force at that speed.


http://www.palmerproducts.co.uk/

anglachel said:

I'm also a fan of over engineering, my rule of thumb for the bicycle is something is attached well once I can lift the bicycle up by the part. (Plastic water bottle cages often fail this test).

Applying the sale rule to the automotive world has not been feasible, turns out I can't pick up my car, or motorcycle at all with out the aid of a Jack.

So I guess my question is can you suspend the bike from the nut plates?

Sent from my SAMSUNG-SGH-I537 using Tapatalk

Click to expand...

If you got the beefier M6 plates from Dale and removed the NC's engine and gearbox for a fairer comparison with a push-bike - almost certainly YES!!