Tire load and inflation data, and scientific papers

[westyventures]

Since there seems to be a need for more education on this important matter, for myself and many others, I've been prompted to pull up a few resources in order to further our education . This is only what I have so far - I am still waiting for more links from a personal friend who currently designs and tests tires for Kumho. Please feel free to add other links you may find.

http://www.dawsengineering.com/tirepressureatreducedloads.pdf
(heavy duty reading, dedicate a few hours here!)

http://marktg.toyotires.com/file/loadinflationtable.pdf
http://wheel-rim-tire.com/tire-load-range-index
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=72

And last but not least, the most complete Vanagon-specific tire pages:
http://www.roadhaus.com/tires/guideline.html
http://www.roadhaus.com/tires.html

Enjoy!

[randywebb]

thanks - looks interesting
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1986 2.1L Westy 2wd Auto Trans.

[Jon_slider]

here is a comparison of the linear calculation I am using, to data published by VW. My calculation produces higher inflation than VW recommended. Therefore my recommendations are conservative, or "safer"

> http://www.roadhaus.com/tires/OE%20Tire%20Load%20Inflation%20Table.html

for the 185 tire, says 55psi carries a load of 1710, and 40psi carries a load of 1365. This is not a linear calculation. 40/55*1710 is 1244 lbs, so why does VW show 1365?.. It appears they use a factor, in this case they use 9.7% added to the linear calculation.

now a specific example
assume a tire has a load rating of 1765 lbs @ 50 psi.
what would the load be at 40 psi, calculated linearly? I believe that would be 40/50*1765= 1412
based on the 185 tire load inflation, VW would have raised the 1412 by 9.7%, to 1549

therefore I believe that linear calculations will produce an inflation pressure that is higher than VW's recs, by 9.7%

by this I mean, if I want to use a tire rated 1765 lbs @ 50 psi, and I want to inflate to carry 1500 lbs, I could use linear math to derive 1500/1765*50 = 43psi. If I was to enhance the load carrying capacity by VW's 9.7% the 43 psi would carry a load of 1646lbs, but I dont "enhance" the load capacity.

therefore, it seems to me that a linear calculation will produce an inflation that is higher by 9.7%, than what VW used when they applied a factor of 9.7%

In conclusion, a linear calculation is more conservative than one that uses a factor less than 1.

Karl, I invite you to use this example:
a tire rated 1765@50psi,
and tell us what psi to inflate it to for use on a syncro westy with a GVWR of 5512 lbs.

and thanks!
I hope the result of this thread will produce a meeting of the minds on how to calculate tire pressure to use with tires that were not originally issued with Vanagons.

I have a thread going where I explain my methods and offer to calculate pressures for people's tires here:
http://www.thesamba.com/vw/forum/viewtopic.php?t=475666
my goal is to offer an inflation that is safe to carry the GVWR of a Van.
_________________
My Soapboxes: Inflation; Handling; Gearing; Decoupling; Swepco

[westyventures]

"The equation is exponential, not linear as he is assuming. Load is related to the air pressure raised to a exponent less than one. This was derived from a combination of first principals and practical experience. The equation is not published because too many other factors need to be considered. This is why ETRTO and T&RA just publish the tire/load/inflation tables. The equation is not a “secret”, it’s just better if most people have to refer to the table to sure that they get the proper number."

[Jon_slider]

> The equation is exponential, not linear as he is assuming. Load is related to the air pressure raised to a exponent less than one.

as I point out above, it is the load that is increased by the use of an exponent less than one, not the air pressure

therefore a linear calculation will produce a higher inflation than indicated on the tire load inflation table, as I linked to above

> it’s just better if most people have to refer to the table to sure that they get the proper number.

that is a GREAT suggestion

here is a load table
http://www.goodyear.com/truck/pdf/edb_loads.pdf
on page 7 it says that a
245x75x16 LT tire carries 3042 lbs @ 80 psi, and carries 1700 lbs @ 35 psi

my linear calculation for a tire rated 3042@80psi, says 35 psi carries 1331 lbs.

the load table says 35 psi carries 1700 lbs.. so the linear calculation is more conservative, in this case by 369 lbs of additional load carrying capacity MORE than the table value

to put it another way, if I use the load table in this post, and I want to carry 1700lbs on one 245x75x16 LT tire, then I should use 35psi

but if I calculate linearly I would use 10 psi MORE, 1700/3042*80= 45psi.

thank you for the education, it is clear to me that linear calculation produces a HIGHER inflation than the load tables.

it is also clear that a linear calculation produces a HIGHER load carrying capacity than the load tables.

I think it would be a great idea to post some good links for load inflation tables for the tire sizes we use with vanagons

for example, do you know of a load inflation table that applies to the BFG AT KO in 215x75x15?
what psi does it say to use to carry 1500 lbs?

and also, do you know of a load table you can link to that provides a load inflation table for the 245x75x16?
what psi does it say to use to carry 1500 lbs?
_________________
My Soapboxes: Inflation; Handling; Gearing; Decoupling; Swepco

[westyventures]

The point I have been trying to make is that the tires need to be inflated to 'carry' more, or an excess of what the linear number results give - inflating a tire to carry only the max load at that pressure is under-inflating it, because it will be experience more load/weight beyond that max in maneuvers, turns, hills etc.

If I were to run 245/75-16, I would probably start at the usual 40F/45R psi I currently use with the 30x9.5-15 I currently run, and adjust till both the tread lies flat (chalk test) and the van handles the way it should, i.e predictably with no real understeer or oversteer. The 40/45 goes up a few psi when I load the van heavily, like at the 5800lbs it was at on my last western trip.

[Jon_slider]

> The point I have been trying to make is that the tires need to be inflated to 'carry' more, or an excess of what the linear number results give

If linear math produces lower recommended inflation than a Load Table, that is of great concern to me also, and I would like to come to a meeting of the minds with you. Thanks for starting this thread.

What I have seen so far, comparing linear math to Load Tables indicates exctly the opposite, linear math produces a higher recommended inflation than load tables.

I definitely appreciate learning about, and recommend using load tables, but linear math wont hurt you, linear math does not underinflate compared to load tables.

here is a load table
http://cache.toyotires.com/sites/default/files/imce/LoadInflationTable.pdf
that shows the
215x75x15 LT tire rated 1765@50psi would carry 1475 lbs @ 40 psi

If I use linear math to carry 1475 lbs it would result in 1475/1765*50= 42psi

therefore, linear math uses 2 psi more than the load table, for this 215x75x15LT tire.

in all cases, linear math produces higher inflation than a load table

I have not yet found a load table for your 30x9.5x15 LT tire, please post a link if you find one

next time someone asks me what tire pressure to use I would like to be able to say Use a Load Table, and heres a link!

anybody have a link to a load table that covers the inflation loads of a 245x75x16LT tire?

ps, I think I have just realized a flaw in my logic. It starts with my assumption that I can use the door sticker inflation to do a linear calculation of the load capacity of the tire. The linear load is lower than the load table spec vw used. Do not use the linear load as the lookup value on a load table for the new tire. Use the load table spec VW used, to look up the load table spec for your tire.

bottom line is we need to be consistent, if we use the load table for the original tire, we should use a load table for the new tire. do not mix and match linear math with load tables.

The load VW inflated to changed over the years. Pick one you are comfortable with. The lowest value was 1365 load front, 1580 load rear. The highest value was 1480 load front, 1750 load rear. Load table loads, not linear loads.

pps
> 40F/45R psi I currently use with the 30x9.5-15 I currently run

that tire is rated 1965 lbs @ 50 psi.
if you find a load table I would like the link.

according to the load table formula, your inflation carries 1665 lbs per front tire, and 1825 lbs per rear tire, for a total load capacity of 6980 lbs.. more than 1000lbs more than your actual weight, and almost 1500 lbs more than GVWR.

what pressure we inflate to depends first on what target load we want to use.

the GAWR of a vanagon syncro would call for 1433 lbs front load, 1521 rear load.

I found a formula, discussed at the bottom of this post:
http://www.thesamba.com/vw/forum/viewtopic.php?p=5495624#5495624
that attempts to emulate a load table and the results for your 30x tire
formula is
(load target-(0.21*max load))/(0.79*max load/max psi)

for example
if you want to inflate to VW load tables for syncro the load targets would be
1480 lbs each front tire
1665 lbs each rear tire
those wheel loads total 6290 lbs, though a syncro westy GVWR is 5512

for those loads linear math says your 30x9.5x15 rated 1985@50, tire should be inflated to 37f, 42rear

by contrast, those same load targets on a load table call for 34psi front, 40psi rear
_________________
My Soapboxes: Inflation; Handling; Gearing; Decoupling; Swepco