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Old 06-05-2008, 05:44 PM   #21 (permalink)
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I think it kind of interesting that the smaller tire there was a pretty big jump until about 45 psi while the bigger tire was at a lower pressure of around 35. I would of thought that it would of been the opposite.

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Old 06-05-2008, 09:10 PM   #22 (permalink)
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Quote:
Originally Posted by dcb View Post
re: variance. Did you mark your starting position in two dimensions?
No, I didn't. It's possible (likely) I was off laterally a little from run to run.

On the starting hill, I moved into position each time by sighting down a wall beside me that was perpendicular to the roadway, and creeping the car until my line of sight was straight down the wall. Then I stopped completely and released the brakes when the coast was clear.

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Is there any reason to think that the brakes might not release evenly?
I wondered that too. I suppose it's possible.
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Old 01-20-2009, 10:23 PM   #23 (permalink)
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To put this into perspective for a metrompg.com post I'm working on (not up yet), here's the Firefly coasting results shown to scale:

click to zoom...

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Old 01-21-2009, 12:00 AM   #24 (permalink)
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Regarding Lazarus' post on the larger tire having less difference in distance per PSI level:

The larger tire has significantly stiffer sidewall area, which reduces rolling resistance. Obviously it's 40-60mm wider (manufacturer's measurements vary), but that may not win-over the loss of friction and warpage associated with tall/thin sidewalls.
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Old 01-21-2009, 12:24 AM   #25 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
What I find so interesting is the level of variation between runs, even with such a simple test.
Maybe the lube cooled off. There is a huge difference in coasting speeds in my truck w/ a 50F difference, and given the operating range of greases/oils I imagine there could be a pretty big increase in drag relative to the change in Crr at those speeds as stuff cools.
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Old 05-12-2009, 08:01 AM   #26 (permalink)
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MetroMPG:
What pressure are you using for every day ?

Thank you.
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Old 05-12-2009, 03:23 PM   #27 (permalink)
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50 ish.
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Old 05-12-2009, 04:17 PM   #28 (permalink)
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distance

Darin,my thoughts are that if the car had been tested from some initial velocity,to a lower velocity,or stop,that you might have seen a greater spread in the data.Evidently,the starting force,which must be overcome just to get a car moving is remarkably high,compared to once its rolling.This is a reason for the traffic light synchronization,as "stops" eat a lot of energy.---------------- I think this effect is cutting into your numbers,although I realize the added complexity and variability of the coast-down.My suspicion is that your RR performance is actually much better than the numbers reflect.Your test does reflect a measurable effect so I say it's useful to all,kudos to you and !muchas gracias!,Phil.
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Old 05-12-2009, 11:08 PM   #29 (permalink)
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I agree, a "normal" coastdown from some moving speed would have made for a better test, Phil. No argument there.
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Old 08-02-2010, 04:17 PM   #30 (permalink)
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Thanks for posting and running the test. Has anyone had a blowout from running pressures exceeding the tirewall max?

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