Quote:
Originally Posted by euromodder
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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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
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. ...
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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
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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.
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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!