TopGear.com America high mpg project (regeared, aeromodded diesel Rabbit)

[MetroMPG]

Originally Posted by Deezler

Damn! I accidentally did not save my calculations that led to the previously posted plot.

I'm going to make it so we can copy a "permalink" from the calculator output and then paste it in a thread. Would make it much easier to refer back to a specific vehicle's results in posts.

Blame away. :P

Ha! Already got the t-shirt!

[MetroMPG]

Originally Posted by roflwaffle

.42 sounds right, although according to what I've read that even depends on the wind tunnel, so while it may be .42 compared to a car of that era, iono if Cd measurements age well. Didn't Ford or GM get all hissy w/ Toyota about that?

Yup, and yup.

Maybe the guys will do some A-B-A coastdown testing for us in between dragstrip runs

[roflwaffle]

Originally Posted by Deezler

Damn! I accidentally did not save my calculations that led to the previously posted plot. I will run the numbers again tomorrow with the .42 (and maybe a .3) set of Cd numbers.

I tried to research the Cd, but didn't have much time. Blame away. :P

It's all good! It took me an hour to find some figures when I bought my first beater about four years ago, and it's more of a pain to look for now because of search results for the "new" Rabbit.

[tasdrouille]

Originally Posted by MetroMPG

I wondered out loud about that at one point (here), and Phil suggested the additional losses are insignificant compared to the tire losses.

I depends what you consider insignificant.

The correct numbers are:

Brakes: 6%
Bearings and seals: 12%
Tires: 82%

Assuming 32 psi in the tires, from a couple years back. As you reduce the RR of the tire, by design or increased inflation, the proportion of those additional losses increases.

This is from a paper I have linked in the efficiency paper sticky. I can't remember which one just yet but I'll edit my post tomorrow.

[MetroMPG]

I've added the ability to grab a "permalink" from the calculator results based on form input.

View the calculations / output for Sipster with these values:

Vehicle weight: 1043.3 kg / 2300 lbs
Crr: .01
Cd: .42
A: 1.9 m2 / 20 ft2
Fuel energy density (Wh/US gal.): 37953
Engine efficiency: .37 *
Drivetrain efficiency: .95 *
Parasitic overhead (Watts): 400 *
rho: 1.184 kg/m3

* These are flat out guesses on my part.

[eyesoreracing]

Originally Posted by roflwaffle

Going with LRR tires should add a few mpg, so they wouldn't need to do much in terms of aero to see 70mpg@60mph.

Speaking of low RRC tires, we're planning to use Bridgestone B381s in either 185/65-14 or 185/70-14 (trying to determine what will fit). If anybody has any better suggestions, speak now, or talk **** about how stupid we are later, your call...

-Dave

[bwilson4web]

Originally Posted by eyesoreracing

Speaking of low RRC tires, we're planning to use Bridgestone B381s in either 185/65-14 or 185/70-14 (trying to determine what will fit). If anybody has any better suggestions, speak now, or talk **** about how stupid we are later, your call...

You might check the Consumer Reports tire ratings since they also rate rolling resistance. Since I don't have to deal with much snow and ice in Alabama, I went with Sumitomo T4s and they worked out just fine. Best of all, they were $50/ea. from TireRack.

I jumped my tire diameter up from 175/65-14 to 175/70-14 and saved an extra 5% in the tire revolutions per mile. I have to 'adjust' trip and MPG due to the lower revs but the BSFC data suggests you might use every bit to increase in the final engine to wheel rpm ratio you can get. ... just a thought.

One option would be to get a second pair of wheels to put experimental tires on. This would let you try a pair of 'racing bicycle' style tires on the drive wheels (aka., largest diameter, smallest width, highest pressure) for your mileage run. In my testing, I found the larger diameter 175/70-14 were more stable, probably from the gyroscopic effect, and reduced Prius wander.

Of course there is the 'clown car' approach, four donuts, or compact spares. The smaller diameter would result in more revolutions per mile for a higher indicated MPH and MPG. The car would indicate say 70 mph but be actually closer to 60 mph. <GRINS>

We see this in the Prius community when folks complain about a loss of mileage after putting on new tires. The 'folk lore' is the tires have to 'broken-in' when in fact they just have to wear down the tread, reducing the diameter, again.

The other nice thing about the Sumitomo T4s, they have a sidewall maximum pressure of 51 psi. But I'm out for MPG performance in urban commuting and most Alabama roads are in fair shape, not a lot of patches or pot holes.

Have you considered doing a 4-wheel alignment? More for keeping tread on the tires, the literature indicates suggests it has an impact on rolling resistance. Given the high miles on the 'donor car,' I suspect the wheel geometry may have suffered a little 'road rash wear.'

Bob Wilson

[tasdrouille]

Originally Posted by eyesoreracing

Speaking of low RRC tires, we're planning to use Bridgestone B381s in either 185/65-14 or 185/70-14 (trying to determine what will fit). If anybody has any better suggestions, speak now, or talk **** about how stupid we are later, your call...

-Dave

Hi Dave, the tires looks like a good choice. Go with the 185/70. They will rub a bit, but the heck with it, just grind the inside lip off. For the mpg leg of the challenge, be sure to pump them up real high, as in 60 psi or more.

[Deezler]

Dave, you nailed it; those are the tires you want.

OK, had to check this again. Using the specs provided in Metro's link above:
(btw this is still with the rough gearing estimate of 60mph at 1800rpm). Now using a perhaps more realistic aero target of 0.34 for comparison.

@ 60 mph:

.	mph	rpm	load(kW)	BSFC est.	gal/h	mpg
Cd = 0.42	60	1770.5	11.66	246	0.891	67.3
Cd = 0.34	60	1770.5	9.96	258	0.798	75.2

Cripes! We're missing the target now with stock aero.

But at 50 mph:

.	mph	rpm	load(kW)	BSFC est.	gal/h	mpg
Cd = 0.42	50	1475.4	7.45	265	0.613	81.5
Cd = 0.34	50	1475.4	6.46	273	0.548	91.2

Drive slow and you're golden.

Ok, I saved the spreadsheet this time...

[eyesoreracing]

Originally Posted by tasdrouille

Hi Dave, the tires looks like a good choice. Go with the 185/70. They will rub a bit, but the heck with it, just grind the inside lip off. For the mpg leg of the challenge, be sure to pump them up real high, as in 60 psi or more.

Possible change of plans. We're running into logistical problems with getting the Bridgestones on schedule, and we're a litte concerned they may make the car handle like a Prius.

Our leading alternate is the Michelin X-Radial DT, same 185/70-14 size.

Any opinions on this one? Anybody tried both?

-Dave