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Old 03-13-2011, 01:15 PM   #121 (permalink)
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Originally Posted by NachtRitter View Post
so if the tires were under-inflated to begin with (28psi seems really low) and then they are pumped up, then naturally the ride height will increase... doesn't mean the diameter of the tire increases though.
The diameter does change - when measured from the contact point through the hub to the top) .
That's what lifts the vehicle when the tyres are pumped up.

What doesn't really change due to inflation pressure is the tyre's circumference.

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Old 03-13-2011, 05:24 PM   #122 (permalink)
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Originally Posted by euromodder View Post
The diameter does change - when measured from the contact point through the hub to the top) .
That's what lifts the vehicle when the tyres are pumped up.

What doesn't really change due to inflation pressure is the tyre's circumference.
...true for the "unloaded" circumference, but NOT true for the "loaded" circumference.

...simple test: (1) measure the circumference of a tire; (2) paint a white spot on that tire; (3) drive exactly 5,280 ft measured mile at a slow speed and video record the whole distance; (4) review the video, counting exactly how many "tire revolutions" occur. I'll guarantee the number is greater than the mathematical number derived from dividing the tires' circumference-in-feet into 5,280...the video "count" will be close to 3-4% higher than the mathematical number...and that difference is because the "loaded" circumference is SMALLER than the "unloaded" circumference.

...unless you're driving on fuelly-slicks, circumference won't "grow" too much during normal driving...but it actually does at high speeds with high profile sidewall tires.
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Old 03-13-2011, 11:28 PM   #123 (permalink)
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Originally Posted by euromodder View Post
Signs or traffic lights never stopped anyone from running them.
Short of a physical barrier, some people will run them no matter what the council puts up.

They're not the people that councils should be considering when putting up signs.

Having everybody stop wastes everybody's gas, so it shouldn't be done.

In Europe we most often use the Yield-sign .

The only difference to a stop sign, is that it doesn't oblige you to stop if stopping isn't necessary.
Oh, but lawyers here in America don't like the idea!

If you drive through a yield sign and cause an accident, it's a prima facie case. (Obviously you didn't yield.) In other words, there isn't anything for them to litigate. God forbid the idea should catch on and become popular...
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Old 03-14-2011, 01:28 PM   #124 (permalink)
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Quote:
Originally Posted by euromodder View Post
The diameter does change - when measured from the contact point through the hub to the top) .
That's what lifts the vehicle when the tyres are pumped up.

What doesn't really change due to inflation pressure is the tyre's circumference.
I could be wrong, but I am fairly certain that the diameter and the circumference are very closely related... changing one changes the other...

You are right of course; the distance between road and hub changes depending on the tire's pressure... though I wouldn't call that the tire's "diameter". That is what I was trying to get at... Doubling the measure from road to hub is not the tire's diameter (and therefore not a way to measure the overall circumference); and the "natural" diameter of the tire won't change noticeably between 28psi and 35psi.

However, the weight of the vehicle will cause the tire to flex / compress at the contact point; the lower the pressure, the more flex / compression (and smaller "real" circumference).

Quote:
Originally Posted by htvfd460 View Post
With my ride height, it only increased by less than half of the tires height as the pressure was increased. I measured the tires height on mine. ...
So actually, only the distance from the ground to the hub changed, not the overall tire diameter... which is not the same as saying the tire diameter changed by 1.5", but rather it is saying that the distance from ground to hub changed 0.75" (according to the statement "With my ride height, it only increased by less than half of the tire's height").

Quote:
Originally Posted by htvfd460 View Post
...
I was focusing on the required energy to beable to stop the vehicle when it comes to an increased diameter of the tire. The location of the force changes when contact is placed further out. ... The braking distance is significantly increased and brake fade comes about faster. Glazing of the rotors if from the surface of the rotor at contact with the pads creating enough heat to make the metal in the rotor maliable enough to essentially smooth it out. .... None the less placing the position of the surface of where the resistive energy is located increases the amount of torque required to slow down or stop it.

...
Putting the above together, what I'm reading is that inflating the tires from 28psi (which seems to be under-inflated even according to the vehicle's tire inflation placard, though I don't know for sure) to 38psi (which is probably nearly correct according to the vehicle's tire inflation placard) will change the distance from road to hub by about 0.75". The result is that "The braking distance is significantly increased and brake fade comes about faster. Glazing of the rotors if from the surface of the rotor at contact with the pads creating enough heat to make the metal in the rotor maliable enough to essentially smooth it out."

It would be helpful to know what the proper tire inflation pressure is for the vehicle. I would suggest that the "road to hub" distance changes less when starting at the tire pressure recommended by the vehicle's tire placard then going to the tire's sidewall max than if starting with an under-inflated tire and going to the vehicle recommended pressure. That is, if one started at 34psi (assuming that's the recommended pressure) and went to 40psi (assuming that's the sidewall max). I'd guess (complete WAG, I admit) that it'd be less than 0.5" difference "ground to hub" distance between recommended inflation and sidewall max inflation.

Let's pretend that the overall radius actually does change by that 0.5" (which it doesn't; it's only the portion of the tire between the ground and the hub that changes distance due to the weight of the vehicle on it). Let's also pretend that 100% of the mass of the wheel / tire combo is on the outside edge (it should be obvious that it's not). According to the formula for rotational inertia, with every other portion of the equation remaining the same (same rotational speed, same mass), the only change would be the radius, from 28.5" (for instance) to 29". Rotational inertia uses the square of the radius, so that's only 3.5% more rotational inertia (check my math folks). That's right, only 3.5% more rotational inertia in the worst possible case; it's probably less than 1% difference in reality. If you believe your vehicle can't handle 3.5% more braking than 100% stock, it's time to get a different vehicle.

If you're worried about higher levels of torque needing to be applied to slow you down, it's even less... torque increases linearly with additional radius, so you'd be talking about only 1.7% difference in torque (assuming same amount of force needed to stop the vehicle) between 28.5" and 29".

htvfd460... I do hope that you're not suggesting that it would be "safer" to reduce the "ground to hub" distance by under-inflating simply to "save the brakes", right??

If you've got some different data, please do provide!
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Old 03-14-2011, 05:35 PM   #125 (permalink)
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When I researched the explorer recall w firestone tires I found that Goodyear endorsed having 28 psi was optimal for those trucks. My suspension is the same as an explorer. After testing tire pressures I found the the placard's spec of 30 psi was too high, and 28 in the front got optimal wear from the tires. But the GOV'T stepped in and said to change explorers to 30 pis and Ford complied. Currently all the rangers I have dealt with running 30 psi have premature center tread wear for front tires.
With relation to Diameter, from one point of the tire to the opposite side (via center tread) the differences between loaded measurements from top to bottom are different from leading and folowing measurements, therefor the top of the tire to the contact at the road is the best applicable measurement since that is where contact is made) the circumference is altered at center tread by the diameter x pie unloaded. Loaded makes the vehicles front and back measurements for the tire's diameter at 9 o'clock and 3 o'clock measurements different from the 12 o'clock and 6 o'clock measurements.
My ride height change was measured at the position directly above the peak of the tire (12 o'clock) the fender and bedside's wheel lip at the outer most edge and lowest point was the position from witch my measurements were derived. Choosing another location on the vehicle would show similar results but the clearance fro the tires was most important and is the reason mini truckers like to tuck the largest rims possible.
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Old 03-14-2011, 06:55 PM   #126 (permalink)
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2 psi isn't that big a deal. Most gauges aren't even accurate to within 2 psi. Once again I doubt what you say.

Tire gauges - Car Forums - Edmunds

Re: tire diameter: this whole thing about psi and tire diameter and glazed brakes is nonsense.
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Old 03-14-2011, 07:17 PM   #127 (permalink)
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Technically it could be a possibility, but if it was true then going from 45mph to 47mph or something would lead to the same risks/results.
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Old 03-14-2011, 07:51 PM   #128 (permalink)
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The tire gauge thing I take personally. I don't use walmart quality tools. I use Snap-on tools. My electronic tire pressure gauge and inflator/ deflator is accurate to the 1/2 psi. I don't kid around when it comes to someones vehicle and life placed in my work. If I screw up it could be their life. I take that VERY seriously.
(I also know you weren't implying that in any way)
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Old 03-14-2011, 08:34 PM   #129 (permalink)
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Originally Posted by trikkonceptz View Post
They will never be able to ban cell phone from use in cars because of how dependent we have become to them.

Best case scenario, police agencies carry cell jammers in their cars disabling cell phone for x radius around a police car ... that would be funny.
I agree with the jammers in police car but..

Honestly I believe that is why they need to be banned in cars. Absolutely NOTHING usefull is done because we have cell phones that we didn't do before cell phones.

All they have accomplished is to make people lazy and in my oppinion loose their ability to fruitfully communicate and work. The world would probably be a better place if most every conversation I have ever heard on a cell never occured. The useless cesspool of ignorance conveyed in 99% of cell conversations would be no loss to anyone including those on the phone.

The only thing cells have accomplished in terms of work is to reduce our ability to take decent notes and schedual our time so we feel like we did more but in reality just were able to avoid time management and get to BSing & getting *****ed at faster.
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Old 03-14-2011, 08:46 PM   #130 (permalink)
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Quote:
Originally Posted by htvfd460 View Post
I don't kid around when it comes to someones vehicle and life placed in my work. If I screw up it could be their life. I take that VERY seriously.
(I also know you weren't implying that in any way)
In the case of severely underinflating tires to 28PSI, yes a mistake to the south of that mark could definately claim their life and furthermore why would you want to be only 9 PSI above the 75mph blowout mark? There are a variety of reasons why people may not check their tire pressure and a variety of reasons including environmental that could cause a drop of 9psi in pressure. I don't think it wise for car manufacturers to recommend a very low pressure that is not far above the failure pressure of the tire.

Yes, if you drive a ford explorer on 2 year old 19psi tires at 75mph they tend to blow out and that is why the low ratings were eliminated because normal people who aren't checking their pressures often had accidents.

Inflating a bit beyond 28psi is an important cushion for most normal drivers because of how close they were running things to the low pressure failure region of the tires.

Also I strongly disagree with the center wear. I have seen poorly designed suspensions cup the center due to pressure but I have never seen the center wear out prematurely even at rather high inflation pressures.

The only exception was some old Nylon threaded space savers I ran on my 1200lb car. But I had thousands of miles on 100 mile tires.

Cheers
Ryan

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