Wildguzzi.com
General Category => General Discussion => Topic started by: jbell on March 13, 2017, 06:35:43 PM
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I've seen to put front wheel on a scale and repeat with back wheel but I think you'd have to know the weight bias for the correct weight. Would using two scales, one each for front and back and then roll the bike up on the two work or would all that weight on the edge of the scale bugger it up? There are 440# scales on eBay for $30.
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Oh man, I'd like to have a buck for every time this one's been brought up. I could probably buy myself a new henway. :grin:
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Stop by any truck stop with a Cat scale or when riding in the countryside most gravel pits, feed mills or scrap yard will have a scale. There is a farmers Coop a mile from my house that leaves their scale on 24/7. There are scales out there no need to buy your own.
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Local landfill will have a scale also.
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Local landfill will have a scale also.
Sent from my SM-G900P using Tapatalk
An engineer member here says those scales at that weight won't be accurate. And Darren is pretty smart.
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Weigh the front, weigh the back and add the two together.
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An engineer member here says those scales at that weight won't be accurate. And Darren is pretty smart.
Makes sense.
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Oh man, I'd like to have a buck for every time this one's been brought up. I could probably buy myself a new henway. :grin:
I did a search before posting and very little showed up. Being cheap, I'll weigh it one wheel at a time and add.
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I "think" if you weigh 1 wheel at a time the other wheel should be elevated to the same height as the wheel on the scale is at so it's not tipped and loading the weight toward the lower wheel.
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I "think" if you weigh 1 wheel at a time the other wheel should be elevated to the same height as the wheel on the scale is at so it's not tipped and loading the weight toward the lower wheel.
Now that's well reasoned. :thumb:
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I "think" if you weigh 1 wheel at a time the other wheel should be elevated to the same height as the wheel on the scale is at so it's not tipped and loading the weight toward the lower wheel.
Yup, agreed.
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Put bike on shoulders.
Climb on bathroom scales.
Note weight.
Take bike off shoulders.
Note weight.
Deduct weight 2 from weight 1.
Easy! :wink:
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Put bike on shoulders.
Climb on bathroom scales.
Note weight.
Take bike off shoulders.
Note weight.
Deduct weight 2 from weight 1.
Easy! :wink:
there is an easier way if you have a boat
some kid did that in 200AD
(http://www.zsbeike.com/imgs/A/A07249/a07249.0153.4.png)
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Put bike on shoulders.
Climb on bathroom scales.
Note weight.
Take bike off shoulders.
Note weight.
Deduct weight 2 from weight 1.
Easy! :wink:
Your a cheeky bugger Muzz :grin:
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Makes sense.
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It does and it doesn't. I have once stepped on our local landfill scale, and it showed my weight accurate to kilogram (and they weigh 40 tons trucks).
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It does and it doesn't. I have once stepped on our local landfill scale, and it showed my weight accurate to kilogram (and they weigh 40 tons trucks).
Yeah that's true, I drive what we call B Doubles here in Australia that can weigh up to 70 tonnes and I stood on the weigh bridge the other day and it got my weight within 1.5 kg. Although I wasn't happy with the number.
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I've seen to put front wheel on a scale and repeat with back wheel but I think you'd have to know the weight bias for the correct weight. Would using two scales, one each for front and back and then roll the bike up on the two work or would all that weight on the edge of the scale bugger it up? There are 440# scales on eBay for $30.
The sum of each tire is accurate......Ever see cops using portable scales on a truck? If you don't level it the weight will be off slightly...
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Muzz has the most accurate method.
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Why not do it like Johnny Cash did to build his Cadillac? :boozing:
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if you had a level ramp on either side of a scale, just roll it over from one tire to the other.
Add up both, walla. :thumb:
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Why couldn't you just put the center stand on the scale and balance it?
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Elevating the wheel not on the scale a couple of inches etc will have no significant effect on the weight reading when you add the two up.
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Commercial scales usually have to be annually calibrated and have the calibration certificate posted. Check the certs at your local scales to see at what accuracy they are calibrated.
The heavy load cells we buy to use on our material handling equipment are usually accurate within +/-2% of load in middle 60% of their range (with other factors compounding that accuracy level) and the lower range may be greater than a persons weight. One at each corner and the error can compound. Plus when you consider the load cells on those scales are ones with large ranges, a bike is going to be way down at the lower end of the scales range, well unless weighing a popular make of American bikes that is.
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Put bike on shoulders.
Climb on bathroom scales.
Note weight.
Take bike off shoulders.
Note weight.
Deduct weight 2 from weight 1.
Easy! :wink:
Hey Muzz
Good advice , that's how I weigh our dogs. Mind you they only weigh between 4 and 6 kg's.
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Just a caution, wheying your bike could affect it's appearance ...
http://www.ergo-log.com/acne-whey.html
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Measure the air pressure in the front tire and note. Use two straight edges, one laid on each side to the tire contact patch parallel to each other. Measure the distance between then and note. Take the two straight edges, one laid in front of the tire contact patch and one behind the tire contact patch parallel to each other. Measure the distance. Multiply the tire patch width times the tire patch length times the tire pressure. This will give you the weight of that end of the bike. Then do the same for the rear. You will have the front and the rear weight. Add them together for the total weight. This will get you very close. I learned this on an afternoon kids show. They Clown was teaching kids how to calculate the weight of an automobile.
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Reminds me of an old and very corny joke ..
How do you weigh a whale ?
Take it to a whaleweigh station .
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Why couldn't you just put the center stand on the scale and balance it?
Because most bathroom scales don't go as high as 500 lbs.
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Please let us off the hook now JBell. Were you joking from the get go ? Just put the front wheel on the scales with a piece of wood under the rear the same height, then the same for the rear. Add the two. THAT REALLY IS IT !!! If you stand on a set of scales on one foot you still weigh the same, just on both feet, each one takes half the weight, but you had me sucked in for ages as well...
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Go out to your regional airport , find an FBO that gives pilot lessons for a private pilot certificate. Get to know a mechanic, preferably one that has an IA ( inspection authorization ) get him interested. In the old days and now some aircraft share engines. He should have calibrated load cells, the kind you can roll an aircraft onto. Hold the bike up, not influenceing it. And take your readings, front and rear. Hop on bike, have your assistant hold bike up, take your readings.Buy him some beer. Now you can calculate your c/g. Easiest way I know of.
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Take it to the airport and weigh it at an airfreight shipper. No charge, if you want to know how much to airfreight it .
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Weight each point that touches the ground. Front weight, rear wheel, and sidestand. Add them up. Block up each point that is not being weighed, so that you keep the bike level, if you want accuracy.
So yes, just add up the weights, try to keep the bike level.
And yes, there is a VERY good chance that you have a nearby grocery store that has a scale that I designed.
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You all except for one or two must work for the gov't......just take something simple and complicate the simple out of it... :grin:
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I work for the government. :shocked:. I retired from working for an airline that that did airfreight. Guess what I use to do? :grin:
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Weigh bikes?
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Y'ep, cars, trucks and other freight. 5000 to 6000 lb. Of papaya to Japan. Make the airway bill too. Check for Hazmat req.s and paperwork per FAA regs.
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Please let us off the hook now JBell. Were you joking from the get go ? Just put the front wheel on the scales with a piece of wood under the rear the same height, then the same for the rear. Add the two. THAT REALLY IS IT !!! If you stand on a set of scales on one foot you still weigh the same, just on both feet, each one takes half the weight, but you had me sucked in for ages as well...
No, Huzo, no joke and no sinister motives. I was thinking you'd have to know the weight bias front/rear to calculate the one wheel at a time method. As so often happens, I am wrong again. I'll still have to get a 3-400# range scale but will give it a go on my bikes, enquiring minds want to know.
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No, Huzo, no joke and no sinister motives. I was thinking you'd have to know the weight bias front/rear to calculate the one wheel at a time method. As so often happens, I am wrong again. I'll still have to get a 3-400# range scale but will give it a go on my bikes, enquiring minds want to know.
Ok then Jbell, that's a different kettle of kippers then. The two wheels are indeed taking different amounts of weight to a more or less degree, but together they still add up to the same as the total weight.
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Take it to the airport and weigh it at an airfreight shipper. No charge, if you want to know how much to airfreight it .
thats what i have done , accurate to the ounce ...fill your tank so you get the real wet weight ...done deal
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Using the single wheel method I get 1120# with gas aboard.
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Is that with your gear as well RK?
I've seen Classic Racers being weighed one wheel at a time
On scales and the other wheel was being elevated.
Another option would be a Post/Parcel distribution centre, I
was a Postie when I got the 1150 and weighed it at work.
Maurie.
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Nope, it's the bare trike with all fluids filled. No luggage or me.
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Is that with your gear as well RK?
I've seen Classic Racers being weighed one wheel at a time
On scales and the other wheel was being elevated.
Another option would be a Post/Parcel distribution centre, I
was a Postie when I got the 1150 and weighed it at work.
Maurie.
Hope you got a mealie out of it. Was it at 2.30 mid week or 3.30 Mon/Fri ? Was also postie (35 years)
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Mealie??? Ah ......... No - I didn't have the priveledge of
Being wrapped in cotton wool and treated like a Princess
:whip2: :grin: Did a few years for a Contractor and then
a few as full time Casual for Aus Post, kinda rare for them
to hire Little Red Riders with Mail dying and Parcels going
Apeshit. Only really got the spot ' cause they got a new
Manager who was an Angel. All over a couple of years back.
Maurie.
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Nope, it's the bare trike with all fluids filled. No luggage or me.
Wow - substantial. I figured you'd built for longevity, but
that's still a bit of a surprise.
Maurie.
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Surprised me, too. I used a gauge thicker than necessary square channel throughout the main trike section, and a cast iron differential punkin. It is probably 200# heavier than it needs to be. I was after a smooth ride and the ability to stick to the road in a sidewind. That part is working out. The unanticipated side effects are that the loaded ground clearance isn't quite what I'd hoped for, and the rear brakes don't make me happy. I'll do better next time. :weiner:
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You did very well this time sport. :bow:
Maurie.
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Bought two $10/330# scales at WalMart. Set them at wheelbase with 2x4's in between to fill the gap and a little ramp up to the first one. First up, my 860GT came in at 470#. Next, the K75RT came in at 540#, both according to spec. When backing the K75 back down, a loud crack from the first scale and it is buggered up. Tried to reset and compensate for the difference in a known weight (me). 850 T3 came in at 475# plus throw in another 20# for mufflers for 495#. Not sure about this one, will get another scale and try again. Very surprised they came in what the manuals said, expected them to weigh heavier. First I tried with a plank across both the scales but that produced varying results and went to tires directly on the scales.
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Bought two $10/330# scales at WalMart. Set them at wheelbase with 2x4's in between to fill the gap and a little ramp up to the first one. First up, my 860GT came in at 470#. Next, the K75RT came in at 540#, both according to spec. When backing the K75 back down, a loud crack from the first scale and it is buggered up. Tried to reset and compensate for the difference in a known weight (me). 850 T3 came in at 475# plus throw in another 20# for mufflers for 495#. Not sure about this one, will get another scale and try again. Very surprised they came in what the manuals said, expected them to weigh heavier. First I tried with a plank across both the scales but that produced varying results and went to tires directly on the scales.
Hats off to you, jbell! Actual instrumentation acquired!
Now you can test theories of the importance of having both wheels on the same level, even without buying another scale to replace the broken one. I hesitated to weigh in (ha ha) when the discussions were just theoretical, but now I'll present my own theory, the Sine/Cosine Theory.
The Sine/Cosine Theory holds that physical problems of this sort can be solved by appeal to either the sine or cosine function, without any messing around with vectors or other actual mathematical analysis. The Theory says that as some relevant angle varies, the required measurement (a force, perhaps) is related either to its sine or cosine, whichever seems more appropriate.
Here is my analysis of bike weighing using the Theory. If the bike is weighed with two scales while it is on the level, the front scale (e.g.) will show weight W1, which is usually about 50% of the total bike weight. That's the first proposition. The second proposition is that if the bike were rotated upward 90 degrees and stood on its rear wheel pointing straight up in the air, the weight on the front wheel's scale (if it could be positioned under the wheel) would be zero.
Now the Theory comes into play. Since the cosine of zero degrees is 1, and the cosine of 90 degrees is 0, we use the Cosine codicil of the theory, and assert that the weight on the front wheel is equal to the cosine of the angle in question, times W1. Simple, right?
Thereby I calculate that if the front wheel were on a bathroom scale 2 inches higher than the rear, and the wheelbase were about 58 inches, as is true for your T3, the angle in question would be sin-1(2/58) = 2 degrees, and the needed cosine would be cos(2 degrees) = .999. Therefore your T3's front wheel should weigh .999 times what it did when you had the bike level.
You may need a somewhat more accurate scale to check my theoretical prediction with only a 2 inch difference in wheel elevations. Nonetheless I take pride in having offered a specific value rather than a loose generalization.
You could test my theory more accurately by seeing what the front weighs when it is elevated 2 feet. The theory says the weight should be cos[sin-1(24/58)] = .91 times what it weighed when measured on the level. That would amount to about a (1 - .91)*(475/2) = 21 pound difference.
The Sine/Cosine Theory is the certainly the most sublime and universally applicable of all physical theories, though it is only hit or miss when it comes to accuracy. I eagerly await your empirical results.
If the measurement at a 2 foot elevation agrees with my theory, it would seem that no one needs to worry about a 2 inch elevation difference. This in turn would mean that none of us need to run out and buy a second scale to weigh our bikes. Scientific theorization coupled with empirical research will once again have worked in tandem to the benefit of all mankind!
Moto
P.S. To weigh the front wheel at a two-foot elevation, place the wheel on a bathroom scale sitting on an automotive hydraulic jack. Then raise the jack until the top surface of the scale is 24 inches above level, and note the weight it registers. Make sure to have someone hold the bike so it doesn't fall over.
M.
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Hats off to you, jbell! Actual instrumentation acquired!
Now you can test theories of the importance of having both wheels on the same level, even without buying another scale to replace the broken one. I hesitated to weigh in (ha ha) when the discussions were just theoretical, but now I'll present my own theory, the Sine/Cosine Theory.
The Sine/Cosine Theory holds that physical problems of this sort can be solved by appeal to either the sine or cosine function, without any messing around with vectors or other actual mathematical analysis. The Theory says that as some relevant angle varies, the required measurement (a force, perhaps) is related either to its sine or cosine, whichever seems more appropriate.
Here is my analysis of bike weighing using the Theory. If the bike is weighed with two scales while it is on the level, the front scale (e.g.) will show weight W1, which is usually about 50% of the total bike weight. That's the first proposition. The second proposition is that if the bike were rotated upward 90 degrees and stood on its rear wheel pointing straight up in the air, the weight on the front wheel's scale (if it could be positioned under the wheel) would be zero.
Now the Theory comes into play. Since the cosine of zero degrees is 1, and the cosine of 90 degrees is 0, we use the Cosine codicil of the theory, and assert that the weight on the front wheel is equal to the cosine of the angle in question, times W1. Simple, right?
Thereby I calculate that if the front wheel were on a bathroom scale 2 inches higher than the rear, and the wheelbase were about 58 inches, as is true for your T3, the angle in question would be sin-1(2/58) = 2 degrees, and the needed cosine would be cos(2 degrees) = .999. Therefore your T3's front wheel should weigh .999 times what it did when you had the bike level.
You may need a somewhat more accurate scale to check my theoretical prediction with only a 2 inch difference in wheel elevations. Nonetheless I take pride in having offered a specific value rather than a loose generalization.
You could test my theory more accurately by seeing what the front weighs when it is elevated 2 feet. The theory says the weight should be cos[sin-1(24/58)] = .91 times what it weighed when measured on the level. That would amount to about a (1 - .91)*(475/2) = 21 pound difference.
The Sine/Cosine Theory is the certainly the most sublime and universally applicable of all physical theories, though it is only hit or miss when it comes to accuracy. I eagerly await your empirical results.
If the measurement at a 2 foot elevation agrees with my theory, it would seem that no one needs to worry about a 2 inch elevation difference. This in turn would mean that none of us need to run out and buy a second scale to weigh our bikes. Scientific theorization coupled with empirical research will once again have worked in tandem to the benefit of all mankind!
Moto
P.S. To weigh the front wheel at a two-foot elevation, place the wheel on a bathroom scale sitting on an automotive hydraulic jack. Then raise the jack until the top surface of the scale is 24 inches above level, and note the weight it registers. Make sure to have someone hold the bike so it doesn't fall over.
M.
EXCELLENT! could you make a youtube video of that?
especially, The second proposition is that if the bike were rotated upward 90 degrees and stood on its rear wheel pointing straight up in the air
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Moto, about the only thing I remember of algebraic functions is the old nerd cheer:
Secant, tangent, cosine, sine, 3.14159 gooooooo Spartans. and yes, it has served me well over the years
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Bought two $10/330# scales at WalMart. Set them at wheelbase with 2x4's in between to fill the gap and a little ramp up to the first one. First up, my 860GT came in at 470#. Next, the K75RT came in at 540#, both according to spec. When backing the K75 back down, a loud crack from the first scale and it is buggered up. Tried to reset and compensate for the difference in a known weight (me). 850 T3 came in at 475# plus throw in another 20# for mufflers for 495#. Not sure about this one, will get another scale and try again. Very surprised they came in what the manuals said, expected them to weigh heavier. First I tried with a plank across both the scales but that produced varying results and went to tires directly on the scales.
Action not talk gets results :wink: Some of you need to get out in the real world more often........ :laugh: :laugh:
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EXCELLENT! could you make a youtube video of that?
especially, The second proposition is that if the bike were rotated upward 90 degrees and stood on its rear wheel pointing straight up in the air
The (complete) second proposition is wrong, as I realized in an actual experiment.
M.
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Y'ep......take something simple and make it complicated because we can. :grin: :grin: :grin: :grin:
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The (complete) second proposition is wrong, as I realized in an actual experiment.
M.
you recorded it right? put on youtube OIDH. :thumb:
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you recorded it right? put on youtube OIDH. :thumb:
(http://thumb.ibb.co/fcBo6F/dt100528.gif) (http://ibb.co/fcBo6F)
(click on the image to see all 3 panes)
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(http://thumb.ibb.co/fcBo6F/dt100528.gif) (http://ibb.co/fcBo6F)
(click on the image to see all 3 panes)
Great Dilbert, thanks.
I made paper records of my measurements in two experiments, last night and this morning. But I can't get photobucket's link copying command to work, so won't be sharing them right away.
I redid my experiment this morning to get more accurate results.
I'm going down the rabbit hole again on this one. I think maybe I'll put this aside unless anyone but me is really interested.
In short, the cosine of the angle I indicated before won't do it. A different angle calculated from the actual balancing point of the bike as its front wheel is raised is needed instead. The bike doesn't balance when the front has been raised 90 degrees because its center of mass is above the line drawn between the two axles. This is high school stuff, really, a little bit fun to play with but no great contribution to knowledge!
For a good, unassuming empirical investigation of how to weigh your bike with a bathroom scale, see this youtube video by a central-casting Aussie with his sheila lending a hand:
https://www.youtube.com/watch?v=_z7SUUYGRqU (https://www.youtube.com/watch?v=_z7SUUYGRqU)
He demonstrates empirically that weighing each wheel separately on a bathroom scale without worrying about bringing the other wheel up to the same level works well enough. A pleasant viewing experience.
Moto
P.S. Now photobucket's working for me. Here are my two "lab" notes, and photos of my two experimental subjects. I'll leave it at that, for now.
(http://i127.photobucket.com/albums/p135/motocomo/IMG_20170324_194122432.jpg)
(http://i127.photobucket.com/albums/p135/motocomo/IMG_20170324_194342231.jpg)
(http://i127.photobucket.com/albums/p135/motocomo/IMG_20170323_112857756.jpg)
(http://i127.photobucket.com/albums/p135/motocomo/IMG_20170324_111312981.jpg)
M.
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Someone post a pix of the Mr. Nevada tee. :grin: :grin: :grin:
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Someone post a pix of the Mr. Nevada tee. :grin: :grin: :grin:
? Care to be more direct, Tom?
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If memory serves me right. Chuckie post the pix of the tee. :laugh:
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If memory serves me right. Chuckie post the pix of the tee. :laugh:
Just describe it for us.
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Nah....I'll let Chuckie post the pix. :wink:
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Nah....I'll let Chuckie post the pix. :wink:
Pretty coy. You a teenage girl?
Say what's on your mind.
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Nah....I'm laptop set on auto-response. I'm sitting on my owner's coffee table. You a girl?? :grin:
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Nah....I'm laptop set on auto-response. I'm sitting on my owner's coffee table. You a girl?? :grin:
You're a troll.
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Now....now....don't get your panties in a wedgie. Take some time to take them out of your butt crack and take a chill pill. Maybe you'd like to stop looking at yourself in the mirror. :grin: :grin: :grin:
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Someone post a pix of the Mr. Nevada tee. :grin: :grin: :grin:
I'm sure you are asking for the one that says "For every problem there is one unexpected solution" rather than the one that says "I have always relied on the kindness of strangers"
jdg
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I'm sure you are asking for the one that says "For every problem there is one unexpected solution" rather than the one that says "I have always relied on the kindness of strangers"
jdg
Ah. Well, in that case, never mind. I'll cancel the Vendetta Special tickets I bought on Alitalia last night.
Mr. Nevada is known to me only as the butt of countless character-demeaning references on this site. I supposed I was being called a fat, doughnut-gorging nincompoop. Which is ok if the one doing the calling says what he means. Context is important. Though Tom will never get an Emily Post award on my recommendation, I guess we can stop the Pee Wee Herman exchanges -- "I know you are, but what am I?" -- if that is what he was intending.
I thought I would go back to the original question while waiting for a clarification of the intended message, so this morning I plotted the implications of my two ideas of how to model the problem, and realized a basic error, which I will now explain, for any who may be revisiting this thread for something other than low comedy.
My first idea was to model the motorcycle -- remember the motorcycle? -- as a point mass on a weightless stick pivoting on the rear axle. With this model it is true that the weight measured at the mass is proportional to the cosine of the angle the stick makes with respect to the horizontal. (This corresponds, of course, to the line drawn through the axles of the motorcycle.) The formula for predicting the weight in this model is weight1 = weight0 * cos(alpha), where alpha is the angle from the horizontal, and weight0 is the measured weight at zero elevation of the front wheel.
After a first set of hasty measurements that disconfirmed model 1 I went back to the lab (my workshop) and did much more careful measurements on the second test subject (shown above). I measured the wheelbase and the position of the front axle when the bike was balanced fore-and-aft by using a very lightweight plumb bob made of thread and a small eye screw, and taking my measurements over to the side of the bench with a try square. To weigh the front I used an electronic postage scale that I believe is accurate, and carefully measured the height of the scale's platform, to the nearest millimeter. I found it hard to get a reliable weight measurement, mostly because the bike's handlebar could rotate, so I took three measurements at each elevation setting. These are the data that I showed in the message above.
This graph shows the more accurate measurements on the second subject (i.e., experiment 2) as the set of three dots per elevation setting (nearly indistinguishable in some cases). The red dashed line is the model 1 prediction, which clearly doesn't fit the data.
(http://i127.photobucket.com/albums/p135/motocomo/mcweight_exper2_1.png)
I realized as I did the first experiment that my second proposition -- that the bike would balance perfectly fore and aft when rotated 90 degrees from horizontal -- was false. In fact, it balanced when the bike was rotated about 70.9 degrees from horizontal, because the center of mass was higher than the line drawn between the two axles. So, in effect, the bike was rotated 19.1 degrees further toward its balancing point than it appeared at any given elevation. Adjusting the basic cosine formula for this gave model 2: weight2 = weight0 * cos(alpha + 19.1). Model 2's prediction is shown as the blue dashed line on the plot. It doesn't fit either.
Only after doing all this -- and, I confess, entertaining myself along the way -- did it occur to me that I knew the reason why my procedures wouldn't work. The mechanical concept of a "moment," understood as an integral, is what's needed. The mass of the motorcycle is not really a point, but is distributed along its length. Because the rotational torque generated by the weight is a function of distance from the axle, not all weight can be treated equally in summing up the force. Instead, using f(x) to designate the point mass at any point x, as measured from the axle, the needed calculation for the weight seen at the scale is, I believe:
\int cos(alpha) x f(x) dx,
where \int is the integration symbol. To actually calculate this with a real motorcycle would require numerical integration from a good estimate of that distribution of weight, which would be quite a project to obtain, probably using a dedicated jig and four scales. A well-chosen smooth function might do pretty well too, but the choice of a motorcycle density function isn't obvious, and would fit different motorcycles better or worse.
This whole exercise in the end shows that there is no universal solution for all motorcycles. It also suggests that my first model is a very poor guide. The fact that a motorcycle (not just the test bicycle) would balance pretty far away from 90 degrees means that the change in measured weight as the front is raised from the ground is on a steeper part of the cosine curve than model 1 had it. So the thickness of the scale would matter more than I first suggested.
In the end the best guide is probably the Aussie's empirical project with a real motorcycle that I recommended before
https://www.youtube.com/watch?v=_z7SUUYGRqU
At the end of his 10 minute demonstration the presenter is surprised to find the weight measurement was off by about 2 kg (not measured very precisely) when the front wheel was up on the scale. "Cool bananas!" he remarked. Not exactly as memorable as "Eureka," but, again, a very enjoyable experiment to watch.
Moto
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Pull it Dusty...
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Because most bathroom scales don't go as high as 500 lbs.
I bet the ones at Walmart do.
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Bloody hell Moto - that's some hypothesis - my head now hurts!!!!
I think Muzz got it right on page one of this saga ...... pick the thing up and subtract your weight :thumb:
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use a big fish scale?
(http://static.panoramio.com/photos/original/8357647.jpg)
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That may be to simple. :grin:
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I looked at my earlier analysis and decided that the approach of calculating a torque for half the mass of the bike around a fixed pivot was, well, bogus.
So I did a satisfactory analysis by modeling the motorcycle as the abstraction seen at the top of this page:
(http://i127.photobucket.com/albums/p135/motocomo/IMG_20170325_223035581.jpg)
All the mass (the total weight of the bike) is concentrated at point M, which is attached by a vertical bar to another bar that ends at the front and rear axles. Both bars are massless, and the axles are points. The angle beta is the angle between the center of mass M and the rear axle, which I measured by finding the point where the bike balances on its rear wheel. This, as mentioned before, is not achieved when the bike has been rotated up by 90 degrees, but instead when it has been rotated 19.1 degrees less, which is the numerical value of beta for the bicycle being used to model a motorcycle.
In part B the abstract motorcycle is elevated at an angle alpha above the horizontal. The applied force (weight) at the front axle F is equal to the total mass (M) times the ratio of the moment arm a = MN divided by the wheelbase, w. (This is a point mass model, instead of the distributed mass moment (integral) model proposed before.)
The page shows calculations of angle and line segment lengths using trigonometry and the given values, namely the wheelbase, w, the elevation, e, and the angle beta to the center of mass. The result is a single equation that gives the predicted weight on the front wheel as a function of its elevation:
m3weight = M / (2 cos(beta)) * sin[pi/2 - beta - arcsin(elevation / wheelbase)]
The following graph shows how well the model does in reproducing the previously obtained measurements:
(http://i127.photobucket.com/albums/p135/motocomo/mcweight_exper2_4.png)
There are some interesting features to the graph. The heaviest dashed line represents the model 3 prediction, which is close to the observed values on the whole. It correctly predicts the observed weight when the elevation is zero, and also the correct front wheel weight of zero when the bike is at the balancing point [which a calculation from the measured angle (19.1 degrees) shows occurred at 703 mm of elevation of the front wheel]. The model doesn't do as well in predicting the weight when the front wheel is 248 mm below the horizontal (hanging off the end of the workbench, resting on the scale), for no reason that I can think of.
The most interesting part is model 3's predicted value when the bike is 90 degrees from horizontal, which is past the balancing point. The predicted weight on the front wheel is negative, about -1.6 kg. Of course I cannot directly measure this negative weight with my scale.
This implication of the model can be understood by bringing to mind that the total measured weight on both wheels together must be equal to the motorcycle's actual weight (as measured previously in a basket, at 11382 g). If the front wheel has a negative weight, then the rear wheel must have a measured weight that is greater than the motorcycle's actual basket weight!
This is easily confirmed by locking the rear wheel, placing the bike on the scale and rotating it to 90 degrees, which takes some forward-directed pressure from one's hand to keep the bike in position, of course. Doing this as carefully as I could, the reading on the scale was about 12.9 kg, or 1.6 kg greater than the bike's actual weight, even though I was very careful not to add any downward pressure.
Where does the extra apparent weight on the scale come from? It must come from my forward-directed pressure keeping the bike from falling backward. Newton's third law says that every force (my hand pressure) is countered by an opposite and equal force. The only available point on the bike through which that opposing force can be applied is at the tire's contact patch, sitting on the scale, and that's where it is coming from. There is a vector analysis that someone who knows what they're doing could perform to make it all balance out, but I could feel I was exerting about the right amount (1.6 kg, perhaps) of pressure with my hand, and I could see the scale measure the extra 1.6 kg of counteracting force in the difference between the apparent and true weights. My son was somewhat impressed, for once. :cool:
In any event, this new model is a good one. It reproduces the data, and makes out-of-sample predictions that are confirmed. I plotted the extra 1.6 kg of apparent weight as its corresponding negative value in the plot, at the red triangle. This is an actual, indirect measurement of the counter-intuitive "negative weight" prediction of the model.
The upshot is still the same as before. Using a scale under one wheel will produce more of an error in calculating the total weight (as the sum of both wheels separately measured) than I figured from model 1, a simple, erroneous application of my so-called Sine/Cosine Theory. Again, this is because the action is occurring in a steeper region of a trigonometric function (now represented as a sine). The error is probably in the range of a pound or two, I would guess, though I don't have any actual motorcycle measurements to base the guess on. The Australian still provides the best empirical evidence.
Moto
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With tongue in cheek,
Unless your after weighing the bike for a racing series to make sure you meet the weight. Just put the bloody bikes front wheel on a scale while holding it upright, write it down and then do the same for the rear. Add the two together and it will tell you about what the heck it should weigh.
Wow, I never knew the math involved in this :grin:
Tom
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Easier to take it to FEDEX at an airport and put it on the scale. This could be too easy. :grin: :grin: :grin: For the Daytona RS, I took the owner's manual and the DMV took the weight from the manual. :thumb: