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Open Roads Forum  >  Tow Vehicles

 > Tire pressures matter....lol

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BenK

SF BayArea

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Joined: 04/18/2002

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Posted: 09/25/21 06:25pm Link  |  Quote  |  Print  |  Notify Moderator

There is more to it than just air pressure, though important, it is just part of the whole system

Designed a tire stand for either or both DOT & RMA back while in college. Also working as a tire monkey to both pay for college and support my track/rally/etc days.

Part-time working at a small industrial controls company and one of the full time engineers was having problems with a contract and the owner/boss knew my love of automotive, so he assigned me to help. That turned out to become my project.

Specification was to replace an old hydraulic tire stand they had been using for decades and our controls the best at that time (solid state controllers & power converters). Precision was our big sell.

The old guy already did the calculations and ordered a refurbished GE locomotive traction motor and I had already designed the stand (1" & 1/2" plate with many vertical sections made up of square & C-channel...with tons of gussets).

The drum that tires were in contact with was about 5 foot in dia and had a textured surface. That was directly driven by the traction motor.

The tire/wheel was mounted on a 5 axis arm and each axis had its own electric motor. The tire/wheel could either drive the drum (5th axis), or freewheel on the drum.

They sent us about 5 tire/wheel combos for our testing to their specification. "Weight" (how hard #1 axis would push onto the drum), "Slip angle" (#2, off center of rotation), Accel/Deccel, Speed (#4 and hardest was rate of change they spec'd out) and can't remember the others

Two acceptance tests by 'them' and they showed up with a trailer full of tires & a tire machine to change them.

Their testing stunk up the whole shop/welding/testing/stockroom with burnt rubber smell. So the boss told them to do at night.

Since only part-time, still in college and trying to be a boy-racer on a very limited budget...LOVED that they were working after everyone was gone.

Made friends with them, and they even put my wheels on our machine (they hadn't accepted it yet). Learned tons and loved their new to me, IR heat gun (big compared to the ones we have today)

Another engineer and I even made them a custom adjustable strobe to freeze the image. Cool being able to see the tire sidewall's wave and ripples during high stress at 100 MPH.


Back on topic...

Rim width vs tire section width vs aspect ratio plays huge on how a tire behaves.

Pressure affects the amount of sidewall 'bend' and the rim affects the amount of tire 'bend-back'. Aspect ratio plays with them all.

Searched for what wanted to discuss and here are some images & comments

[image]
This shows the relationship between rim width and tire's flex points (part of the sidewall bend-back)


[image]
This is another image showing rim with and aspect ratio. How the tires 'bend-back' 'rolls over' during cornering or sweeping curves. When that tire straightens back, the wheel to tire will 'wallow' back and forth until normal.


[image]
This image shows how a tire bead fits on various rim widths. From too narrow, just right, to too wide.

All tire specifications will list a range of rim widths for 'that' tire. Too narrow well have more sidewall bend-back and provide the best or highest level of ride quality. Wider will have little sidewall bend-back and will be the poorest in ride quality. Plus, narrower rims cost the OEMs less than wider rims.



So, it depends on more than just PSi, but the rim width vs tire section width vs aspect ratio.


The more sidewall bend-back, the softer the ride (higher ride quality), but it will have a larger slip angle and has to move the wheel back and forth from side to side during corner or sweeping curves.

Less sidewall bend-back will have the smallest slip angle and responsiveness greatly increased, but harsher ride

Why I like wider rims. My 1996 K3500, 7.4L Suburban has LT265/75R16E tires on 10x16 alloys and LOVE it that way. Most think it rides like a truck...which it is and the way I like it.

It also is super responsive to steering input. Steering input happens *NOW*.


-Ben Picture of my rig
1996 GMC SLT Suburban 3/4 ton K3500/7.4L/4:1/+150Kmiles orig owner...
1980 Chevy Silverado C10/long bed/"BUILT" 5.7L/3:73/1 ton helper springs/+329Kmiles, bought it from dad...
1998 Mazda B2500 (1/2 ton) pickup, 2nd owner...
Praise Dyno Brake equiped and all have "nose bleed" braking!
Previous trucks/offroaders: 40's Jeep restored in mid 60's / 69 DuneBuggy (approx +1K lb: VW pan/200hpCorvair: eng, cam, dual carb'w velocity stacks'n 18" runners, 4spd transaxle) made myself from ground up / 1970 Toyota FJ40 / 1973 K5 Blazer (2dr Tahoe, 1 ton axles front/rear, +255K miles when sold it)...
Sold the boat (looking for another): Trophy with twin 150's...
51 cylinders in household, what's yours?...

Grit dog

Black Diamond, WA

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Joined: 05/06/2013

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Posted: 09/25/21 09:02pm Link  |  Quote  |  Print  |  Notify Moderator

Or just a PSA to air up yer tires ifn they’re squishy.
But good explanation Ben. Very informative


2016 Ram 2500, MotorOps.ca EFIlive tuned, 5” turbo back, 6" lift on 37s
2017 Heartland Torque T29

MFL

Midwest

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Joined: 11/28/2012

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Posted: 09/26/21 04:43pm Link  |  Quote  |  Print  |  Notify Moderator

Grit dog wrote:

MFL wrote:

^^^^Lol


Slight lol here too.
I understand tire pressures, load, ratings, stability, etc as much as the next guy. And CR, I feel is pretty darn knowledgeable bout tars.
But like other unrelated tasks, I too feel that harder is generally better when you’re going fast and putting a lot of pressure on your unit!
However max sidewall pressure is not an accurate measuring stick anyway due to the generality associated with it. Proper pressure for the load is, however, a good reference.


True that, one size does not fit all!! The scenario I mentioned, would likely require full max pressure (firm sidewall) on the rears, especially with P or similar tires of a 1/2 T.

Anyway...schools out for CR, if he didn't get it by now, he won't, and after 5 whole years of towing experience! Lol

Jerry





Grit dog

Black Diamond, WA

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Posted: 09/26/21 05:02pm Link  |  Quote  |  Print  |  Notify Moderator

^Yes, correct, even max rated pressure is not “that” stiff on a P, C or XL rated tire if a half ton is really doing work.
I have been known to, on more than a few occasions, air up 44 or 50psi max truck tires to 60-65 psi when carrying heavy loads or big trailers. It works and it does the trick.

I would have never believed the tires would hold up, but I once got upset with my Area Manager because he wouldn’t authorize me getting a 3/4 ton company truck. I had a very nice new 1/2 ton but got a couple projects that required me to be towing pretty heavy, frequently. So I shimmed up a set of Timbrens with a couple extra washers under the bed of that F150 and cranked the little 44psi tires up to 65 psi or so. Enough to keep them from squatting under the weight I was carrying and pulling. Figured I’d destroy those tires in short order. To my surprise, 4 months later I was done with those projects and let about 25psi out of the tires so that it didn’t ride like flintstone wheels and they were none the worse for the wear, other than the tread was wore out at about 25k miles.

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