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#1
12-14-2019, 05:20 PM
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Physics question
My wife and I ride our bicycles to the top of a local hill.
I have a bike with 26" tires Her bike has 24" tires I weigh 175 lbs She weighs 120 lbs We both start coasting down this hill at the same time. Invariably my speedometer reads faster. I would think it would be the other way around.
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 Master of Contemplation 
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#2
12-14-2019, 06:31 PM
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What's the +/- error percentage of the devices. Likely one or both are not quite calibrated. I agree, yours should show a slower speed, unless your way ahead of her at the bottom.
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            Kimber          
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#3
12-14-2019, 08:23 PM
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Are you coasting side by side? Is one of you actually going faster or do you arrive at the same time?
The bike with the smaller tires will show faster because the smaller wheels turn more/faster than the larger wheels to cover the same distance of travel. I'm assuming similar speedometers driven off of the tires.
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 April 13, 2016 -- The day Richard was speechless. May your hands always be busy, May your feet always be swift. May you have a strong foundation When the winds of changes shift. May your heart always be joyful, and may your song always be sung, May you stay forever young! --Bob Dylan
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#4
12-14-2019, 08:26 PM
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Master of Contemplation
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#5
12-15-2019, 01:24 AM
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shouldn't the speedometer be calibrated for the different wheel size ? The RPMs would be different but the speed should be the same.
__________________
 “We are all capable of believing things which we know to be untrue, and then, when we are finally proved wrong, impudently twisting the facts so as to show that we were right. Intellectually, it is possible to carry on this process for an indefinite time: the only check on it is that sooner or later a false belief bumps up against solid reality, usually on a battlefield.”—George Orwell, 1946 "The race may not always be to the swift nor the victory to the strong, but that's how you bet." - or - that's how the smart money bets...attributed to Damon Runyon
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#6
12-15-2019, 09:28 AM
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Master of Contemplation
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#7
12-15-2019, 09:34 AM
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__________________
Hi I'm Tom. Darned if I can remember anyone else's name.   MDCGA The Road Goes On Forever... To Ride is Life... In search of the Doof 43. . . .         ....... . .  Colorado Motel Wreckers 2012 ... Midnight Riders of The Beartooth 2013...   
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#8
12-15-2019, 11:34 PM
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Quote:
__________________
“We are all capable of believing things which we know to be untrue, and then, when we are finally proved wrong, impudently twisting the facts so as to show that we were right. Intellectually, it is possible to carry on this process for an indefinite time: the only check on it is that sooner or later a false belief bumps up against solid reality, usually on a battlefield.”—George Orwell, 1946 "The race may not always be to the swift nor the victory to the strong, but that's how you bet." - or - that's how the smart money bets...attributed to Damon Runyon
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#9
12-16-2019, 09:22 AM
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It's made for motor vehicles, so if you install wider and taller tires on your Hyundai beater you know how fast you were going when the local constabulary pulls you over for driving drunk.
 What we need to know is how these speedometers measure the rotation, mechanical connection or magnetic sensor and is there a way to calibrate them to the wheel size.
__________________
Hi I'm Tom. Darned if I can remember anyone else's name. MDCGA The Road Goes On Forever... To Ride is Life... In search of the Doof 43. . . . ....... . . Colorado Motel Wreckers 2012 ... Midnight Riders of The Beartooth 2013...
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#10
12-16-2019, 12:36 PM
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sure, taller tires have a greater radius but why does width matter? (on tires)
__________________
“We are all capable of believing things which we know to be untrue, and then, when we are finally proved wrong, impudently twisting the facts so as to show that we were right. Intellectually, it is possible to carry on this process for an indefinite time: the only check on it is that sooner or later a false belief bumps up against solid reality, usually on a battlefield.”—George Orwell, 1946 "The race may not always be to the swift nor the victory to the strong, but that's how you bet." - or - that's how the smart money bets...attributed to Damon Runyon
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#11
12-16-2019, 01:04 PM
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I get an electronic speed reading off a display screen [ electric motor assist bike ]
The reading seems to be taken off the pad of the disc brakes in front. Seems to read the rotations of the disc. [ same diameter on both bikes ] Probably needs an adjustment calibration of the computer.
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Master of Contemplation
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#12
12-16-2019, 05:39 PM
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>>> Example: 225/60R16, The tire is 60% as tall (sidewall area) as the tread (footprint) is wide. So if I was to replace the 255/70R18 tires on my SR5 Tundra with 275/70 R18 TRD tire size, my speed would be 2 1/4 MPH faster than the indicated speed of 65 MPH on the instrument cluster. Because the tire is taller and a larger rolling diameter. (did I do that right?I sux at any math.)
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Hi I'm Tom. Darned if I can remember anyone else's name. MDCGA The Road Goes On Forever... To Ride is Life... In search of the Doof 43. . . . ....... . . Colorado Motel Wreckers 2012 ... Midnight Riders of The Beartooth 2013...
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#13
12-17-2019, 01:27 AM
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so, apparently where I am going wrong is that the last numbers are the wheel radius and not the tire radius. But still, the aspect ratio doesn't influence the rpm. An 18 inch rim still travels father than a 16 inch rim no matter what the tire size is.
No, no, that is wrong. We need to be working with the radius of hub to road to calibrate a speedometer. The accuracy of a cam sensor is still dependent on tire size, then. Right? PS: what oil do you use?
__________________
“We are all capable of believing things which we know to be untrue, and then, when we are finally proved wrong, impudently twisting the facts so as to show that we were right. Intellectually, it is possible to carry on this process for an indefinite time: the only check on it is that sooner or later a false belief bumps up against solid reality, usually on a battlefield.”—George Orwell, 1946 "The race may not always be to the swift nor the victory to the strong, but that's how you bet." - or - that's how the smart money bets...attributed to Damon Runyon
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#14
12-18-2019, 12:59 AM
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Any vehicle speedometer is calibrated to the size of the OEM wheel/tire it was produced/shipped with. Unless the OEM wheel/tire size have been changed both vehicles should read the same speed.
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Tom  "Age is a case of mind over matter. If you don't mind, it don't matter."...Satchel Paige "Mother Nature may dictate I grow older but there ain't nothin' nor nobody can make me grow up." ..Me                
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#15
12-19-2019, 06:33 PM
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I would use à travelling app on your phone to se thé actual speed thon Check each speedo. Speed us calculateur at 1.48 Ft à second. Si if you Knowl thé distance you can backdoor thé math un to se speed.
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Here by Invitation,Thanks for the invite.Dooer of Circles An owner of FORTY THREE screwdrivers Road Wrench    
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