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Author Topic: Tire Pressure vs Carry Weight  (Read 2212 times)

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Offline DrM

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Tire Pressure vs Carry Weight
« on: September 21, 2020, 10:48:19 pm »
Using the two tire pressure specifications given for the Bolt for 198 lb carry weight (33 psi front, and 36 psi rear) and 445 lb maximum carry weight (36 psi front, and 41 psi rear), I constructed a graph for a carry weight (rider, add-ons, and passenger) from 100 lbs to a max of 445 lbs.  FYI.
« Last Edit: September 21, 2020, 11:12:49 pm by DrM »



Offline west.c.ridez

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Re: Tire Pressure vs Carry Weight
« Reply #1 on: September 22, 2020, 01:55:00 am »
Thanks DrM!


Offline Pir8

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Re: Tire Pressure vs Carry Weight
« Reply #2 on: September 22, 2020, 10:35:41 am »
Is tire pressure to weight a linear equation?  Pressure often moves as a square; see Boyle's Law. 

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Offline DrM

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Re: Tire Pressure vs Carry Weight
« Reply #3 on: September 22, 2020, 11:09:58 am »
Is tire pressure to weight a linear equation?  Pressure often moves as a square; see Boyle's Law.
Maybe in some parallel universe.

Boyle's Law: the volume of a gas at constant temperature varies inversely with the pressure exerted on it. (P₁ V₁ = P₂ V₂)

That's a linear system (straight line)

Offline Pir8

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Re: Tire Pressure vs Carry Weight
« Reply #4 on: September 22, 2020, 11:22:25 am »
Well, it's P varies with the inverse of V. So unless one axis uses x^-1 you'll get a hyperbola.  Either way, the graph you made is likely a decent approximation considering the small variations and multiple variables we're dealing with. 

Though, according to Dave Moss, we're both wrong: https://www.youtube.com/watch?v=XaIuWKfnGEI

reference for hyperbola:
https://en.wikipedia.org/wiki/Boyle's_law

Offline DrM

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Re: Tire Pressure vs Carry Weight
« Reply #5 on: September 22, 2020, 12:54:45 pm »
Well, it's P varies with the inverse of V. So unless one axis uses x^-1 you'll get a hyperbola.  Either way, the graph you made is likely a decent approximation considering the small variations and multiple variables we're dealing with. 

Though, according to Dave Moss, we're both wrong: https://www.youtube.com/watch?v=XaIuWKfnGEI

reference for hyperbola:
https://en.wikipedia.org/wiki/Boyle's_law
Well, Boyle's law is stated for an open system (Weight is constant, volume free to change).  In our case volume is mostly constant, so weight (stress in the tire) has to change.  But the biggest constraint is that there are only two reference points (per tire) given by Yamaha, which only makes a unique straight line (for each tire.)  We would need three points (per tire) to make a unique curve. So that's the best I can do with the data given.  Anyway, like you say, it's close enough for our purposes.

As far as the video goes -- for all he legitimately talks about contributing factors, in the end, he adjusts the pressure in the tire until it feels good to him.  That might make him happy, but it's not what an engineer would do, and there really isn't any hard criteria to measure against in the video. 

Offline lunkhead

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Re: Tire Pressure vs Carry Weight
« Reply #6 on: September 22, 2020, 02:58:33 pm »
The stock Commander II rear is rated for max 761 lbs @ 41 psi. If the weight bias is 40/60% front/rear, a 450 lb rider would only load the tire to 600 lbs @ 41 psi. That's 160 lbs light for 41 psi but a 200 rider with a 250 lb passenger sitting right over the axle would be about right for the tire's rating.

You really need two charts. One showing a higher pressure front/lower pressure rear for single heavy rider and lower pressure front/higher pressure rear for a heavy passenger.
C-SPƎC

Offline DrM

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Re: Tire Pressure vs Carry Weight
« Reply #7 on: November 12, 2020, 12:57:31 pm »
I saw a bathroom scale for $41 at Amazon.com that weighs up to 550 lbs.  That's right 550lbs.  So I thought to myself why not buy it and also measure my actual weight distribution front and rear on the Bolt?  So I bought it and weighed my bike at the wheels (separately and independently.)

I did some more calculations on Tire Load (lbs) versus Tire Pressure (psi) for front and rear tires separately.  Also factored in Yamaha's maximum vehicle load and spread that to the tires using 60% to the rear tire and 40% to the front tire and using Yamaha's stated tire pressures at that load of 41psi rear and 36 psi front. 

Turns out the OEM Bridgestone tires are not the load limitation, but rather (apparently) the bike suspension, which, as pretty much everyone agrees, is pretty crappy.   I also ran calculations for my own bike and was surprised to see that the maximum passenger load I could sustain using Yamaha max load is 156 lbs - given the mods I have already installed. 

So below are my calculations, and a new chart using individual tire load - for your use and or comment.  The green dot on the chart is where my normal riding lies for my bike (no pax).

EDIT (16Nov20): I have substantially revised the chart below from what I previously posted.  Chart now reflects changes suggested by Lunkhead
« Last Edit: November 16, 2020, 01:43:34 pm by DrM »

Offline lunkhead

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Re: Tire Pressure vs Carry Weight
« Reply #8 on: November 12, 2020, 04:25:30 pm »
DrM, you shortchanged yourself by 84 lbs on the rear LI.
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Offline DrM

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Re: Tire Pressure vs Carry Weight
« Reply #9 on: November 12, 2020, 05:57:12 pm »
DrM, you shortchanged yourself by 84 lbs on the rear LI.
It looks that way.  But I'm trying to use that Yamaha stated max vehicle load of 998 lbs and impute that to the front and rear wheels by the 40/60 ratio, which for the rear wheel would be 599 lbs.  Now that imputed 599 lb limitation on the rear wheel clearly isn't the rear tire limitation, so I assume that it is a rear shock assembly limitation of some sort.  Of course, this is just guess on my part, since I don't have any input from Yamaha.

Offline lunkhead

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Re: Tire Pressure vs Carry Weight
« Reply #10 on: November 13, 2020, 12:31:29 am »
GAWR is 686 lbs rear and 322 lbs front at 41/36 psi. You state the front is maxed at 399 lbs (77 lbs too high) and rear maxed at 599 lbs (87 lbs too low). Max total weight (GVWR) is biased more to the rear at 68/32, not 60/40%.
« Last Edit: November 13, 2020, 10:51:21 am by lunkhead »
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Offline lunkhead

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Re: Tire Pressure vs Carry Weight
« Reply #11 on: November 13, 2020, 10:28:29 am »
« Last Edit: November 22, 2020, 08:14:01 pm by lunkhead »
C-SPƎC

Offline DrM

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Re: Tire Pressure vs Carry Weight
« Reply #12 on: November 14, 2020, 10:37:14 pm »
GAWR is 686 lbs rear and 322 lbs front at 41/36 psi. You state the front is maxed at 399 lbs (77 lbs too high) and rear maxed at 599 lbs (87 lbs too low). Max total weight (GVWR) is biased more to the rear at 68/32, not 60/40%.
I'm curious as to where you get those numbers (686 lbs rear, 322 lbs front at 41/33 psi; and 68/32 % distribution rear/front).  Do you have a Yamaha citation, or is that what your bike actually weighs at the wheels?  I ask because my bike actually weighs out rear/front very near 60/40.  I would agree that any rear seat passenger or cargo carried in the rear would go primarily to the rear wheel, but that's not necessarily how every rider would load additional poundage.  For example, a tall, very fat Harley rider might weigh 400 lbs, and on a Bolt would likely distribute that weight 60/40 rear/front or close to it.  Of course a lot might depend on what kind of modifications the bike may have undergone.

Offline lunkhead

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Re: Tire Pressure vs Carry Weight
« Reply #13 on: November 15, 2020, 02:08:38 am »
The max loading specs are inside the right down tube. 68/32% is what they work out to. If you use saddlebags and a luggage rack, the combined load will be slightly behind the rear axle. The sissy bar allows the passenger to lean back so their CG is roughly right over the rear wheel. I used your measurements of weight distribution to plot the minimum tire pressures and Yamaha's specs for the maximum. A light rider with no passenger and a heavy windshield but no other accessories or added weight (58/42%) might need 36/36 psi. A light rider with a heavy passenger/loaded saddlebags and loaded luggage rack (70/30%) might need 41/33 psi. Hopefully, you'll be able to get a pretty good idea what combo is best just looking at the distribution and estimating about how heavy things are.
C-SPƎC

Offline DrM

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Re: Tire Pressure vs Carry Weight
« Reply #14 on: November 15, 2020, 07:57:59 am »
The max loading specs are inside the right down tube. 68/32% is what they work out to. If you use saddlebags and a luggage rack, the combined load will be slightly behind the rear axle. The sissy bar allows the passenger to lean back so their CG is roughly right over the rear wheel. I used your measurements of weight distribution to plot the minimum tire pressures and Yamaha's specs for the maximum. A light rider with no passenger and a heavy windshield but no other accessories or added weight (58/42%) might need 36/36 psi. A light rider with a heavy passenger/loaded saddlebags and loaded luggage rack (70/30%) might need 41/33 psi. Hopefully, you'll be able to get a pretty good idea what combo is best just looking at the distribution and estimating about how heavy things are.
Thanks - that's good information.  Also, I see I made an error using the LI 67 instead LI 71.  I will correct my chart

For readers who want to know, in the picture below are the max loading specs on the inside of the right down tube for my bike: