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Old 04-14-2008, 08:37 AM   #1
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Red face Spun on Rolling Resistance calcs...

Hi folks,

Did some rough and ready rolldowns this morning to try and get a handle on what my RR is. Figure the accuracy wasn't that great. After doing all the crunching, I'm seeing a Crr of 0.032 for my voyager....

Now I can't find it specifically spelled out anywhere, is the Crr for a vehicle 4x the Crr of each tire or not? I see things like typical Crr of LRR tires is 0.006 to .010 and typical Crr of a car on asphalt is 0.030 but nothing spells out whether this is a car on sucky HRR tires or whether this is a result of adding up 4 tires worth of RR.

I know, stupid question... thanks for hints...

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Old 04-15-2008, 12:08 PM   #2
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clue me.
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Old 04-15-2008, 12:16 PM   #3
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well dont forget as the tires heat up they get harder and change RR. same with the fluids in the drivelines, grease in the bearings, etc
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Old 07-31-2008, 09:17 AM   #4
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Hmmm still not getting this right, now figuring 0.012 Crr for whole vehicle... rigorous testing would be nice, so would a road that's both smooth, flat and quiet.
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Old 07-31-2008, 10:05 AM   #5
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You have to divide the mass of your car + mass of each wheel by the numbers of wheels and multiply the result by 9.81 m/s2.

Can you explain how did you get the result?

I believe the answer to your question will be.. yes! It seems that Crr for a vehicle is the sum of all Crr's. You got 0.032. You car has 4 wheels, then 0.032/4 = 0.008, lying between 0.006 and 0.01 which represent Crr values for low resistance tires, as you said. I presume you don't have LRR tires. Do you drive with overinflated tires?

off-topic (I went from 6th to 7th... )
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Old 07-31-2008, 10:44 AM   #6
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Thanks, actually forgot which calculation I used the first time, doh, this time I was going off that .pdf linked off the wikipedia rolling resistance page. I've got Marshal/Walmart 791 touring tires made by Kumho, which I believe were of similar construction to the Kumho 795 which was reckoned to be lower RR, and they've long had all the rough edges knocked off, and are inflated to 40psi. 205/70R14

Took 25 secs to coast down from 40kph to 30kph, mass is 1636kg, Cd is .43
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Old 07-31-2008, 12:33 PM   #7
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Ahhh, think I've figured it out, just check tire pressure, I was at 42, from doing it at a gas station pump I guess. I may have been at 38 on that first calc.

Also the calc sheet I used there I think was for bikes, and divided by 2 instead of 4, so that would put me at .006 per wheel. Within the variables of the experiment, timing inaccuracies and possibility of slight grade or wind assistance, and increased tire pressure over first result, it seems like a fair result. Oneofthesedays when I get a roundtuit I'll find a spot where I can do some runs in each direction and video the speedo, so I can get good timings without taking my eyes off the road so much.
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Old 07-31-2008, 04:47 PM   #8
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Well done then. I'm a physics student and I find these matters quite interesting, testing to get good results a prove the theory that lies behind them.
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