Experiment: coast down distances (rolling resistance) @ various tire pressures

[Lazarus]

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.

[MetroMPG]

Originally Posted by dcb

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.

Is there any reason to think that the brakes might not release evenly?

I wondered that too. I suppose it's possible.

[MetroMPG]

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...

[Christ]

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.

[roflwaffle]

Originally Posted by MetroMPG

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.

[noxman]

MetroMPG:
What pressure are you using for every day ?

Thank you.

[MetroMPG]

50 ish.

[aerohead]

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.

[MetroMPG]

I agree, a "normal" coastdown from some moving speed would have made for a better test, Phil. No argument there.

[SoobieOut]

Thanks for posting and running the test. Has anyone had a blowout from running pressures exceeding the tirewall max?