www.tdiclub.com

Economy - Longevity - Performance
The #1 Source of TDI Information on the Web!
Forums Articles Links Meets
Orders TDI Club Cards TDIFest 2016 Gone, but not forgotten VAG-Com List Unit Conversions TDIClub Chat Thank You




Go Back   TDIClub Forums > VW TDI Discussion Areas > TDI Fuel Economy

TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

Reply
 
Thread Tools
Old July 4th, 2005, 00:22   #31
Ernie Rogers
Veteran Member
 
Join Date: Oct 2001
Location: Pleasant Grove, Utah
Fuel Economy: 49/ 45/ 40
Default Re: Low Rolling Resistance Tires...Which is Best?

Hi, guys,

Okay, so I got pretty philosophical last time. This time, I will try to be more practical. Using the "New" Jetta for a model, I tried calculating the changes in mileage you might get by making some changes with the tires. Here are the results:

Case A: Normal Jetta with Mich. Energy tires, Crr = 0.009
--> 47.5 mpg at 60 mph. (38.2 mpg at 70 mph.)

Case B: Bigger tires, change diameter from 24" to 25"
--> 48.1 mpg at 60 mph.

Case C: Raise pressure from "normal" 30 psi up to 40 psi.
--> 51.7 mpg at 60 mph.

Case B+C: Increase tire diameter and raise pressure.
--> 52.2 mpg at 60 mph.

Case D: Buy cheap tires, more resistance, Crr = 0.012.
--> 42.9 mpg at 60 mph.

So, you can see how your choices affect mileage. I bought some cheap tires last fall, returned them for the satisfaction-guaranteed refund a month later, then paid more for the best I could get. I feel better, but I am still paying for the tires.

This forum has an earlier post on how to measure your own rolling resistance-- a good way to settle any questions. My Michelin Energy tires (25.6 inch dia.) at 40 psi were found to have Crr = 0.0065.

Ernie Rogers
Ernie Rogers is offline   Reply With Quote
Old July 5th, 2005, 11:44   #32
Ernie Rogers
Veteran Member
 
Join Date: Oct 2001
Location: Pleasant Grove, Utah
Fuel Economy: 49/ 45/ 40
Default Re: Low Rolling Resistance Tires...Which is Best?

I'm recommening this post for reading by Darthsurplus.
/Ernie Rogers

Quote:
More philosophical thoughts about rolling resistance--

Let's assume that it is true that the rolling resistance is proportional to the amount of flexing going on in a tire. One measure of the amount of flexing would be the angle formed between the flat contact patch and the tangent to the tire tread face just outside the patch. Let's call this angle "alpha." (If you want to draw a picture, make a circle, then draw a flat spot on it for the contact patch. Alpha is the angle between the original circle and the flat (chord) line, or it's half the angle you get by drawing lines from the ends of the patch to the center of the circle.)

So, then it should be true (our assumption) that the rolling resistance is proportional to alpha:

RR = F alpha,

where F is the "flexing constant" of the tire.

As mentioned before, the load supported by the tire is equal to the tire pressure times the area of the contact patch:

L = P A

This time, we need to further describe the contact patch-- the area of the patch is equal to its length times the tread width, well roughly so:

A = W C

Where W is the width of the contact patch and C is the length, the chord across the circle. Now, I want to invoke a little geometry. The formula for the length of the contact patch is:

C = 2R sin(alpha), or C = 2R alpha

for small angles, that is for a small contact patch. Putting all this stuff together gives a relationship between the load L and the tire pressure, tread width, tire radius R, and alpha:

L = P W 2R alpha

Way up at the top we had a formula for the rolling resistance and now we have a formula for the load. Dividing one by the other gives a formula for the rolling resistance coefficient:

Crr = RR /L

Substituting,

Crr = F /(P W 2R)

Alpha was in both equations and cancelled out, that's cool. This is an interesting formula, but we have to be very mindful of F, the flexing constant. This "constant" depends on the construction of the tire. There is experimental evidence that it Might be true that F is proportional to the tire width, which is just about the same as W. This means that a tire's rolling resistance coefficient Might not depend on how wide the tire is. (But, remember, the aero drag DOES depend on tire width.) The only other new variable to talk about is the tire radius, R, or the tire diameter, 2R. Now, we have an expanded very interesting relationship for rolling resistance and tire properties:

Rolling resistance is proportional to one over pressure, and one over tire diameter.

Provided that we are talking about tires of identical construction except for the tire diameter.

Nothing has been said here about "dynamics", how the tire and vehicle behave when the car is accelerating, turning, and hitting bumps. (We have lots of those in Pleasant Grove, Utah.)

When choosing tires, dynamics considerations can affect choices too.

Ernie Rogers
Ernie Rogers is offline   Reply With Quote
Old July 5th, 2005, 11:49   #33
Ernie Rogers
Veteran Member
 
Join Date: Oct 2001
Location: Pleasant Grove, Utah
Fuel Economy: 49/ 45/ 40
Default Re: Low Rolling Resistance Tires...Which is Best?

About the cases A through D in a post above--

Darthsurplus reminded me that I overlooked a detail. The results I calculated only included tire properties. The effect of larger tires is slightly greater that calculated because the engine efficiency goes up also.

Ernie Rogers
Ernie Rogers is offline   Reply With Quote
Old July 5th, 2005, 21:42   #34
DarthSurplus
Member
 
Join Date: Mar 2004
Location: Indiana
Fuel Economy: low?
Default Re: Low Rolling Resistance Tires...Which is Best?

I wonder a little about the L = PA assumption. It seems a little too tidy to be real.

Something you all need to consider before you air your tires to the max and beyond:

Even though the formula for the tangential (horizontal) friction force vector does not include surface area, the realities of life do. Road and tire anomolies reduce the coefficient of friction in various regions of the contact patch. Lowering your tire pressure, and thereby enlarging your contact patch, provides you with more chances to find regions of good traction when pushing the limits. People have slidden off the road in wet but otherwise manageable conditions because their tires were inflated to the maximum number on the sidewall, but the vehicle was not fully loaded. Please consider the risk of becoming a slide-off when you choose to fully inflate your tires.
DarthSurplus is offline   Reply With Quote
Old July 6th, 2005, 02:53   #35
vwxyzero
Veteran Member
 
Join Date: Jul 2004
Location: Los Angeles, CA
Default Re: Low Rolling Resistance Tires...Which is Best?

” Case B: Bigger tires, change diameter from 24" to 25"
--> 48.1 mpg at 60 mph.”


And over-sizing your tires does what to your odometer reading?
A false idea of your actual MPG achieved?

vwxyzero is offline   Reply With Quote
Old July 6th, 2005, 07:13   #36
DarthSurplus
Member
 
Join Date: Mar 2004
Location: Indiana
Fuel Economy: low?
Default Re: Low Rolling Resistance Tires...Which is Best?

Quote:
And over-sizing your tires does what to your odometer reading?
A false idea of your actual MPG achieved?
Come on. You aren't paying attention.

Ernie posts all of these ideas, calculations and data, the samples of which, that I've observed, do nothing to detract from his claim of being a physicist who experiments with fuel economy. He writes like a nerdy engineer and he plans experiments like a scientist.

There's nothing there to indicate that he can't or won't or won't remember to compensate for the larger tires in his odometer readings.
DarthSurplus is offline   Reply With Quote
Old July 6th, 2005, 13:46   #37
vwxyzero
Veteran Member
 
Join Date: Jul 2004
Location: Los Angeles, CA
Default Re: Low Rolling Resistance Tires...Which is Best?

Quote:
Quote:
And over-sizing your tires does what to your odometer reading?
A false idea of your actual MPG achieved?
Come on. You aren't paying attention.

Ernie posts all of these ideas, calculations and data, the samples of which, that I've observed, do nothing to detract from his claim of being a physicist who experiments with fuel economy. He writes like a nerdy engineer and he plans experiments like a scientist.

There's nothing there to indicate that he can't or won't or won't remember to compensate for the larger tires in his odometer readings.
And your the one that made the comment about being carefull about sliding off the road with tires that when over inflated to the limit essintially become motorcycle tires?
S----------S
-L--------L-
--I------I--
---C---C---
----K-K----


Come on what

My point was that the effect it has on your odometer reading is not being addressed at all
vwxyzero is offline   Reply With Quote
Old July 6th, 2005, 14:39   #38
DarthSurplus
Member
 
Join Date: Mar 2004
Location: Indiana
Fuel Economy: low?
Default Re: Low Rolling Resistance Tires...Which is Best?

I'm sure he as taken the odometer issue into account as a matter of course, and not found it worth mentioning.
DarthSurplus is offline   Reply With Quote
Old July 7th, 2005, 00:50   #39
Ernie Rogers
Veteran Member
 
Join Date: Oct 2001
Location: Pleasant Grove, Utah
Fuel Economy: 49/ 45/ 40
Default Re: Low Rolling Resistance Tires...Which is Best?

Okay, Okay, I'll mention it, but VWXY0 will be sorry!

When I made up the examples, I was thinking about actual speeds and MPGs, not what your gages read. And, I only took into account the effect of the tires directly, no adjustment for changes in engine efficiency when changing tire size. (Bigger tires get lower RPM and higher torque, so better engine efficiency, actually very important.)

So, according to the request, here's what happens if you read the gages and don't think about the consequences. (Fixed engine efficiency.) Assume that the mileage for case A is what the gages give you, but the speedometer was reading 5% high, and the odometer was right on, no error--

Case A:
Travel 47.5 miles at "60" (actually 57.1) and use 1.0 gal.
You think you got 47.5 mpg, and you did.

Case B: Bigger Tires
Travel "47.5" (actually 49.5) mi. at "60" (actually 59.5). The actual car speed increased by 2.4 mpg, causing increased aero drag. At the same time, the rolling friction dropped by a factor of 24/25 = 0.96 for the tires. The resulting fuel use is 1.09 gal. Dividing, 47.5 /1.09 = "43.5 mpg" about. That's what you think you got. The actual mileage was 49.5 /1.09 = 45.4 mpg. At any rate, the car seems much worse with the new tires. The careless gage reading sunk us.

In reality, if the two tests had been done at the same true speed, the Case B mileage would have been about 0.6 mpg better with the bigger tires simply because they have less rolling resistance.

Sorry, it's probably about as clear as mud.

Ernie Rogers

Quote:
I'm sure he as taken the odometer issue into account as a matter of course, and not found it worth mentioning.
Ernie Rogers is offline   Reply With Quote
Old July 7th, 2005, 05:05   #40
vwxyzero
Veteran Member
 
Join Date: Jul 2004
Location: Los Angeles, CA
Default Re: Low Rolling Resistance Tires...Which is Best?

Thank you Ernie!

Sorry I asked, not at all.
Muddy, not even.

I did some calculations of my own based on the circumference of the tires mentioned: Case A and Case B and came up with similar numbers, but from a different angle.
24” tires: 840,338 revolutions per tire for every 1000 miles driven.
25” tires: 806,745 revolutions per tire for every 1000 miles driven.

Where I was off and concerned was in not knowing the discrepancies in the speedometer/odometer equation.
Thanks for clearing that up, and making it an aspect of this topic.

I would not have posted in this forum if I wasn’t interested in the topic. For the record I run Biodiesel and am interested in economy issues/topics too.

Also for the record, I think overly inflated tires are a hazard to the driver as well as others on the road if the tires in question don’t hold the road.

And, as a final note, I am very curious how oversized a tire anyone can run before it puts to much strain on the drive train, clutch, etc. etc.

Thanks again for your clarification, and your efforts, vwxyzero
vwxyzero is offline   Reply With Quote
Old July 7th, 2005, 07:44   #41
Chris Bell
Veteran Member
 
Join Date: Feb 2000
Location: Santa Barbara, CA
Default Re: Low Rolling Resistance Tires...Which is Best?

There are other consequences of fitting larger OD tires besides obvious clearance issues. To name just two: assuming the same wheel offset and alignment specs., a larger tire OD will increase negative scrub radius (distance from SAI road intersection point to the tire's contact patch center, when viewed from the front or rear) and increase caster trail distance (from SAI road intersection point to tire contact patch's center, when viewed from the side). Increased negative scrub radius is desireable from a safety perspective, less desireable from a performance perspective. Increased trail distance is generally desireable from both.

As to higher tire pressures, another consideration besides a narrowed contact patch is that some tire tread construction will not tolerate significantly higher pressures in terms of even treadwear. OEM TDI tires generally perform well in this respect, but not all tires do. I had a dramatic example of this; I had set of Firestone Firehawks in a correct +1 size for one of my cars. Running 40 PSI (rated @ 44 max) for 5000 miles wore the center of the tread down to the wear bars while the outer edges were not even down a 1/32" from new on all 4 tires (car had been correctly aligned right before tire installation). These tires were quite highly rated for real world treadwear according to Tirerack's owner survey (and UTOG was 500). I think this case may be unusual, but I've read reports of less dramatic uneven treadwear with other tires.
Chris Bell is offline   Reply With Quote
Old July 7th, 2005, 08:37   #42
DarthSurplus
Member
 
Join Date: Mar 2004
Location: Indiana
Fuel Economy: low?
Default Re: Low Rolling Resistance Tires...Which is Best?

Chris,

Good point about the uneven tread wear. Changing tire size from OEM also can affect handling, in the form of tendency to drift (the road camber induces the wheels to turn) and in the sensitivity of the steering wheel.


Ernie,

Quote:
Assume that the mileage for case A is what the gages give you, but the speedometer was reading 5% high, and the odometer was right on, no error
This confuses me. I have no knowledge of Volkswagens. Is this possible on them? Is it possible to alter a computer parameter to adjust the odometer for a different tire diameter, but the speedo to still be off? This implies a separate variable parameter for the speedometer, or a fixed constant. How are VW's computers set up in this regard? I simply don't know. Ford and Chevy correct both the odometer and the speedometer with one parameter. I'm aware of another vehicle, exactly what escapes me right now, that has a correctable speedometer, but the odometer can't be adjusted.
DarthSurplus is offline   Reply With Quote
Old July 7th, 2005, 09:08   #43
Chris Bell
Veteran Member
 
Join Date: Feb 2000
Location: Santa Barbara, CA
Default Re: Low Rolling Resistance Tires...Which is Best?

Not to speak for Ernie, but I assume he was making a hypothetical case for the purpose of illustrating his point.

You're right...the VSS signal is the only driver for both speedo and odo, and their response to the signal cannot be split electroncially without re-engineering the cluster, but it can be split mechanically for a certain type of error. I have corrected the typical VW 3-8% speedo optimism by reindexing the speedo needle on its spindle. As it happens I have also experimented with larger OD tires and installed a pulse frequency adaptor inline with the VSS signal to correct the odo readings as well as my (previously corrected) speedo readings.
Chris Bell is offline   Reply With Quote
Old July 7th, 2005, 10:25   #44
Ernie Rogers
Veteran Member
 
Join Date: Oct 2001
Location: Pleasant Grove, Utah
Fuel Economy: 49/ 45/ 40
Default Re: Low Rolling Resistance Tires...Which is Best?

WOW! I'm really impressed by the contributors on this topic. The case I chose was simple, but close to my perception of reality. Checking notes, my car with factory tires was reading 1.1% low on the odometer and 3.4% high on the speedometer. After changing tires to the larger size, the speedometer appears to be right on and the odometer is reading 4.6% low.

So far, my Michelin Energy MXV4 S8 tires appear to be wearing evenly, more variation from tire to tire. I'm depending on the service guy for this info. My tire pressure is about 40 psi cold.

We didn't talk much about the effect of tire size on engine efficiency. According to my engine program, based on the engine map and polynomial fit posted here somewhere, here is the efficiency effect, assume about 60 mph:

24 in. tires, assume 2100 RPM: Eff = 0.332 at 3.4 kW /cyl
25 in. tires, now at 2016 RPM: Eff = 0.339 at 3.4 kW /cyl

That translates to about +1 mpg for me.

By the way, the power per cylinder translates to 18 hp from the engine, a pretty close guess to power required at 60 miles per hour (true speed-- man, this gets complicated).

Ernie Rogers
Ernie Rogers is offline   Reply With Quote
Old July 7th, 2005, 11:09   #45
SUNRG
Veteran Member
 
SUNRG's Avatar
 
Join Date: Oct 2003
Location: Roanoke, VA
Fuel Economy: 28mpg@74mph: 1995 Volvo 850 Turbo Wagon
Default Re: Low Rolling Resistance Tires...Which is Best?

to return briefly to the thread topic ...

I received this response from Nokian regarding the Nokian WR:

my email: <ul type="square"> On the www.nokian.com website it states:

Nokian WR is the world’s fastest friction tyre…

What does this statement mean? Low rolling resistance?[/list]their response: <ul type="square"> Dear Rob

That statement was made when we introduced W-rated (max.speed 270 km/h) Nokian WR and that time it was the fastest friction tyre in the world. I really recommend to test Nokian WR cause it`s really is enviromental tyre ; made without any harmful high aromatic oils ( first in the world ), have very low rolling resistance and wet grip ( because of silica tread compound).

Best Regards :

Matti Morri
Manager
Technical Customer Service
Nokian Tyres plc
matti.morri@nokiantyres.com
phone +358 3 3407621
mob. +358 40 517 4444[/list]<ul type="square">

Nokian WR – the world’s fastest friction tyre

Nokian WR is the world’s fastest friction tyre that has been designed especially for the changing winter conditions of Central Europe.

Safety in all weather conditions

Nokian WR has good grip in the rain and also when you get surprised by snow. Nonetheless, its driving comfort is excellent on a dry road as well. The tyre’s groove angle has been tailor-made for each size, providing more accurate driving and effectively preventing aquaplaning.

A tyre for heavy use

Nokian WR’s new tread can withstand a lot of wear. With the advanced HAKA sipes, the tyre’s properties remain excellent throughout its service life.

Quiet driving pleasure

If you believe a friction tyre to be quiet, Nokian WR will gladly confirm your belief. The tyre’s drive-by and cabin noise are quiet and pleasant.[/list]<font color="red">the tread depth is 12.5/32 compared to 9/32 on new Michelin Energies - that's 38% more tread!</font>

the cost is identical to Michelin Hydroedge ($100 ea + free ship at www.discounttiredirect.com) Nokian WR ($99.99 + free ship at www.tirefactory.com) - and i think i'm leaning toward these, since i absolutely love our Nokian winter tires, these seem like they'll roll well, provide great traction / safety, last long, and they're environmentally friendly.

the Nokian NRHi seem good too, but they're not as long wearing - and at 35-40k/year - we <u>need</u> long wearing.
SUNRG is offline   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -7. The time now is 12:22.


Powered by vBulletin® Version 3.8.5
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Copyright - TDIClub Online LTD - 2017
Contact Us | Privacy Statement | Forum Rules | Disclaimer
TDIClub Online Ltd (TDIClub.com) is not affiliated with the VWoA or VWAG and is supported by contributions from viewers like you.
© 1996 - 2017, All Rights Reserved
Page generated in 0.18068 seconds with 10 queries
[Output: 131.54 Kb. compressed to 111.67 Kb. by saving 19.87 Kb. (15.10%)]