Welcome to the Yamaha Star Bolt Motorcycle Forum

Why join our motorcycle community?

  • Membership is free and easy
  • Get technical support and information for your Yamaha Star Bolt motorcycle or search for information before buying
  • Use the member map to find riders near you
  • Participate in our monthly virtual bike show (or at least vote)
  • Get rid of these annoying boxes asking you to register  ;D

More importantly, make new friends!!

Author Topic: Tire Pressure vs Carry Weight  (Read 2207 times)

0 Members and 1 Guest are viewing this topic.

Offline lunkhead

  • Hero Member
  • *****
  • Posts: 1596
Re: Tire Pressure vs Carry Weight
« Reply #30 on: November 25, 2020, 03:17:48 pm »
Oops! fixed it and revised the scale. I wouldn't trust the numbers that I came up with for the non-C-Spec. They were assuming a heavier light weight average and don't jive with a normal curve.
C-SPƎC

Online DrM

  • Sr. Member
  • ****
  • Posts: 428
Re: Tire Pressure vs Carry Weight
« Reply #31 on: November 26, 2020, 02:55:04 am »
Here is a simple way to calculate what the weight distribution is to the axles for items moved or added-on.

1.  First, weigh the item, and estimate where the item's center of gravity (c.g.) is.  For regular polygon shapes the c.g. is usually at the center

2.  For moving an existing item, such as footpegs, just subtract the percentage where the item's c.g. is originally from the percentage where you are going to put the c.g. , and then multiply that by the item's weight.  In the picture attached the footpegs were originally at the 50-50 mark, now moved to the 60 front/40 rear mark. 

Amount to add to the front wheel load (60% - 50% = 10%) x item weight (I'm just guessing 20 lbs) = .10 x 20 = 2 lbs added to front axel.
Amount to add to the rear wheel load (40% - 50% = -10%  x time weight (I'm just guessing 20 lbs) = -.10 x 20 = -2 lbs added to (or +2 lbs subtracted from) the rear axel.

3. For adding a new item, such as a windshield, the item weighs about 4 lbs, and the c.g. is about on the 75 front/25 rear line. 

so 75% of 4 lbs = 3 lbs added to the front wheel,
and 25% x 4 lbs = 1 lbs added to the rear wheel.

Offline Sdaniels

  • Sr. Member
  • ****
  • Posts: 385
Re: Tire Pressure vs Carry Weight
« Reply #32 on: November 26, 2020, 07:26:13 am »
Oh wow...thanks for this, DrM.  One question about adding weight...you used a windshield as an example.  I added handlebars/risers, steering damper, fork brace & bigger front rotor.  Since all of those items are grouped closely together, can I just take the center of gravity between all of them & get an average distribution?  Also, how do you calculate the weight distribution for the bike as a whole when something is added/removed?
« Last Edit: November 26, 2020, 08:20:06 am by Sdaniels »
2015 C-spec

  • Advertisement

Online DrM

  • Sr. Member
  • ****
  • Posts: 428
Re: Tire Pressure vs Carry Weight
« Reply #33 on: November 26, 2020, 11:15:26 am »
Oh wow...thanks for this, DrM.  One question about adding weight...you used a windshield as an example.  I added handlebars/risers, steering damper, fork brace & bigger front rotor.  Since all of those items are grouped closely together, can I just take the center of gravity between all of them & get an average distribution?  Also, how do you calculate the weight distribution for the bike as a whole when something is added/removed?
Grouping: Yes, you can group items and calculate a group c.g., which is generally accurate enough for small items grouped closely together.  In the case of the windshield, you can temporarily attach the mounting hardware to the shield and find the collective c.g. by hanging the assembly by a single string (the taut string will always project through the c.g.).  Do this at a couple of different points on the assembly and you''ll have a good idea where the c.g. lies.  For Handle bars, you could attach the risers to the handlebars, and then find the collective c.g. by the string method.  You can do them all together as one, but then guestimates on c.g. location becomes a little less accurate.

Total weight:  just moving an item doesn't change the total weight, only the distribution of that weight between tires.  Adding (or subtracting) any item would change the total weight by the amount of the item's weight, but what you really want is how much to add (or subtract) from each wheel, which this method will give you.

Offline lunkhead

  • Hero Member
  • *****
  • Posts: 1596
Re: Tire Pressure vs Carry Weight
« Reply #34 on: November 26, 2020, 11:31:01 am »
There should be a link in the link below for a DIY center lift. Maybe Norm can fix it. You can also use any flat center lift and something like a 1/4" wooden dowel at the point where the bike is balanced on it. Measure the distance from the fulcrum to each axle and you get the exact distribution ratio.

https://www.yamahastarbolt.com/technical-library/cheap-diy-motorcycle-lift/

You can also use a bathrom scale on level ground. One wheel on the scale and the other shimmed to the height of the scale. The scale doesn't need to be very accurate for finding the ratio as long as it's consistent.
C-SPƎC

Offline lunkhead

  • Hero Member
  • *****
  • Posts: 1596
Re: Tire Pressure vs Carry Weight
« Reply #35 on: November 26, 2020, 12:32:18 pm »
I think if you're removing weight right over the rear wheel, you might want to step down to a 140 tire. A 50/50% weight bias might work best with 110 140. You could put the battery in a chin fairing. 8)
C-SPƎC

Offline Sdaniels

  • Sr. Member
  • ****
  • Posts: 385
Re: Tire Pressure vs Carry Weight
« Reply #36 on: November 26, 2020, 08:30:38 pm »
Appreciate all the info, guys.  DrM, I figured grouping things together wouldn't be quite as accurate as calculating separately.  I was just trying to get a ballpark idea of what shift, if any, would occur with what's been done to the bike.  I like your idea of a center lift, Lunkhead.  Don't have a scale that can weigh the rear of the bike & even if I did, I'd have to get one of you guys to explain how to calculate the ratio anyway  ;D  Hope everybody had a good thanksgiving. 
2015 C-spec

Online DrM

  • Sr. Member
  • ****
  • Posts: 428
Re: Tire Pressure vs Carry Weight
« Reply #37 on: November 26, 2020, 09:46:27 pm »
Appreciate all the info, guys.  DrM, I figured grouping things together wouldn't be quite as accurate as calculating separately.  I was just trying to get a ballpark idea of what shift, if any, would occur with what's been done to the bike.  I like your idea of a center lift, Lunkhead.  Don't have a scale that can weigh the rear of the bike & even if I did, I'd have to get one of you guys to explain how to calculate the ratio anyway  ;D  Hope everybody had a good thanksgiving.
If you ever do get a scale, here are the formulas for weight distribution:

WRear + WFront = WTotal

%WFront = WFront/WTotal x 100

%WRear = WRear/WTotal x 100

Offline lunkhead

  • Hero Member
  • *****
  • Posts: 1596
Re: Tire Pressure vs Carry Weight
« Reply #38 on: November 27, 2020, 12:49:24 am »
40/60 is the same as 2/3 or 1:1.5. Interestingly, the exact same ratio can be found in the tires size and rating, 100/150 and max loads @41 psi, 507/760.

The only pressures recommended for bias street tires are 41, 36, 33 and 29 psi. The SCR calls for 41,41 passenger or not and has a radial on the rear. On the other end of the scale, smaller bikes take 29,29. Most bikes fall in between and use the 33,36 36,41 label like the Bolt. I fill to 35,37 and they leak to about 30, 33 before I check again. I need to work out what pressures I should be running in my Zero with the 90/90-18, 110/90-18 combo and 285 lbs (475 lbs with me on board).

Good read:
https://www.cycleworld.com/almost-everything-you-need-to-know-about-motorcycle-tire-pressures/
C-SPƎC

Offline Sdaniels

  • Sr. Member
  • ****
  • Posts: 385
Re: Tire Pressure vs Carry Weight
« Reply #39 on: November 27, 2020, 09:12:59 am »
DrM, just making sure I read this correct... %WFront = WFront DIVIDED BY WTotal x 100?
2015 C-spec

Offline Sdaniels

  • Sr. Member
  • ****
  • Posts: 385
Re: Tire Pressure vs Carry Weight
« Reply #40 on: November 27, 2020, 09:55:34 am »
Okay, here's what I got.  Finding the point on the frame to balance on I got 30" from the rear axle to the pivot point, 32 1/4" front axle to pivot point.  Added the two together to get 62 1/4", then divided each distance by that & got 52%/48%.  Found out my digital scale is good up to 350lb.  Got 268lb reading for rear wheel, 247lb front.  The math works out to 48%/52% so I screwed something up.  Kind of interesting I get the same percentages, only reversed but I'm trusting the weight measurement more because it seems less likely to have error.  Trying to use a tape measure to distance two points, making sure the front wheel is pointing straight & with all kinds of things in the way seems like it is much more prone for error.  Either way it's much closer to 50/50 than 60/40.
2015 C-spec

Online DrM

  • Sr. Member
  • ****
  • Posts: 428
Re: Tire Pressure vs Carry Weight
« Reply #41 on: November 27, 2020, 10:21:55 am »
DrM, just making sure I read this correct... %WFront = WFront DIVIDED BY WTotal x 100?
Yes, (Wf/Wt) x 100 -- the 100 just converts the decimal to a percentage.

Offline Sdaniels

  • Sr. Member
  • ****
  • Posts: 385
Re: Tire Pressure vs Carry Weight
« Reply #42 on: November 27, 2020, 11:09:28 am »
DrM, just making sure I read this correct... %WFront = WFront DIVIDED BY WTotal x 100?
Yes, (Wf/Wt) x 100 -- the 100 just converts the decimal to a percentage.

gotcha
2015 C-spec

Offline lunkhead

  • Hero Member
  • *****
  • Posts: 1596
Re: Tire Pressure vs Carry Weight
« Reply #43 on: November 27, 2020, 12:15:46 pm »
So according to the weight measurements and subtracting about 2 lbs from the front to get back stock front end weight, a stock Bolt is right about at 45/55%. If my Bolt was 48/52% and would never carry anymore weight on the rear, I would prefer a 140 rear if it was a common size. With only 50 HP and a limited lean angle, a 150 can't get used to it's full potential and is just extra weight. A 140 could make more use of the available tread at full lean.
C-SPƎC

Offline Sdaniels

  • Sr. Member
  • ****
  • Posts: 385
Re: Tire Pressure vs Carry Weight
« Reply #44 on: November 27, 2020, 12:43:20 pm »
So according to the weight measurements and subtracting about 2 lbs from the front to get back stock front end weight, a stock Bolt is right about at 45/55%. If my Bolt was 48/52% and would never carry anymore weight on the rear, I would prefer a 140 rear if it was a common size. With only 50 HP and a limited lean angle, a 150 can't get used to it's full potential and is just extra weight. A 140 could make more use of the available tread at full lean.

I believe that...I scrape pegs on occasion & the rear tire shows about an inch of rubber on each side yet to be touched by asphalt.  I assume a taller profile would be required for the narrower tire?  Avon has a 140/90-16.
2015 C-spec