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Old 04-21-2018, 09:28 AM   #31
fubar929
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I hate to bring science of physics into this discussion, but 36 psi is still 36 psi regardless of what weight is on the tire. It makes absolutely no difference if there is weight on the tire or not when you're inflating it.
Assuming my Super Duke weighs 570 pounds and has a 50/50 weight balance, inflating the tire requires lifting 285 pounds several inches off the ground. Who do you think does that work, if not the compressor?
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Old 04-21-2018, 09:45 AM   #32
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Both of you are missing the point.

If I inflate my tire to 36 psi, measured however the bike is sitting, the pump can't tell the difference of how much weight is on the tire. Whether the tire changes from 36 psi to 36.5 psi between no weight on it and weight on it is beside the point. Pumping a tire to a given pressure is all that matters to the tire pump.

And whatever lame arguments you come up with to try to convince me otherwise won't work.
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Old 04-21-2018, 09:55 AM   #33
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Originally Posted by fubar929 View Post
Assuming my Super Duke weighs 570 pounds and has a 50/50 weight balance, inflating the tire requires lifting 285 pounds several inches off the ground. Who do you think does that work, if not the compressor?
Scott's right that the pressure a compressor can deliver has nothing (zero, not even a little) to do with the weight resting on the tire. I'm glad he pointed out this misconception.

Let's say your Super Duke weights 5,000 lbs and is squishing down the tires to the rims. A compressor would still fill 36 psi into the tires in their squished state. With the tires solidly squished, you'd be pumping into a smaller volume and would reach 36 psi sooner. The tires at 36 psi would remain mostly squished under the 5,000 lbs weight.

BTW does a Super Duke really weigh 570 lbs? I thought it'd be 470-ish lbs wet.

Last edited by Gary856; 04-21-2018 at 11:00 AM..
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Old 04-21-2018, 03:16 PM   #34
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Both of you are missing the point.
I don't think I'm missing that point. I realize that all the compressor sees is pressure over time. Since it does more work one way than the other the pressure vs time curve must look different in that case.

Could that be relevant to the compressor being able to fully inflate a tire or not? It could be that it's more prone to overheat in one scenario than the other. Though if it needed a little cooling off time one way, it might need it the other way too.
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Old 04-21-2018, 06:41 PM   #35
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Whether the tire changes from 36 psi to 36.5 psi between no weight on it and weight on it is beside the point.
I don't think so. I think that's a reason why we normally discount the weight of the vehicle when we inflate it, and a problem for the idea that it matters to the inflator. I don't think I can even measure the pressure difference. Obviously there are pressure or weight ranges where you can. But street bike operating weight and pressure, not so much.

Yes as I've explained you're doing more work (in the physics sense) by lifting the bike. And you've increased the potential energy of the bike in an easily measurable way. And all that energy came from the compressor.

But the potential energy added to the bike is probably small compared to the energy required to inflate the unloaded tire, and doesn't matter. I'm still curious about fubar929's results though.
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Last edited by davidji; 04-21-2018 at 06:43 PM..
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Old 04-22-2018, 08:21 PM   #36
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So, I let all of the air out of the 190/55-17 rear tire on my Super Duke 1290 and inflated it with my Moto Pumps Mini Pro compressor. Once with the bike resting on the side-stand and once with the bike on a rear stand so that the rear tire wasn't in contact with the ground. Ran both tests until the pump's gauge read 36psi (which translated into 31psi as measured by a gauge I trust). The pump body wasn't warm, but I let it cool down for ~10 minutes between tests anyway. Recharged the bike's battery and ran the side-stand test a second time just for good measure.

Rear stand inflation time: 7:44.75
Side stand inflation time: 7:57.68
Side stand inflation time #2: 7:56.49
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