BFG All terrains and fuel economy

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Ladies and Gents
Last January I had to replace two tyres and since Dunlop no longer make
all terrain "Grand Treks" I swapped to BFG all terrains on the rear
axle. They have a much chunkier treat pattern than the Dunlops, and this
was the week before the heavy snow where they definitely helped, so no
complaints there.
However I have noticed that the truck's fuel economy has dropped by
about 1 or 2 mpg (ie perhaps 5%, it's hard to be more precise) over the
last few months, and from what I've read this is not wholly unexpected
when fitting tyres with a chunkier tread. I have noticed that there is
a slight warm rubber smell from the BFGs after longer journeys, which
sort of confirms this.
As far as I can tell it's going OK, the engine sounds fine, oil
consumption is negligible and it only smokes a bit under full throttle
acceleration when on full boost @ 2000rpm or higher (as it always has
done). It has done 153k miles and is on the original and unserviced
fuel pump & injectors.
Would the consensus of the forum expect this sort of mpg loss from
chunkier tyres, or should I look harder for other problems?
Also should I get excited about the fact that the BFGs on the rear
(nearly new) are about 1cm greater diameter than the Dunlops on the
front (25k+ miles)? I know one is supposed to rotate tyres regularly
to keep wear even, but I can't help feeling that the centre diff and
viscous coupling should handle that sort of minor difference without
trouble. Or could the difference in rolling radii, causing front/rear
fighting in the transmission, perhaps contribute to the loss of fuel
economy?
Christopher Bell
Devon, UK
1996 1HD-FT
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I have fitted BFG AT onto our 100 series and also lost 3 MPG.
The old girl only does 15 now......
I think their a fantastic tyre but obviously at a price....
Cheers
John
-----Original Message-----
From: [Email address removed] [mailto:[Email address removed]] On
Behalf Of Christopher Bell
Sent: 06 May 2009 11:49
To: [Email address removed]
Subject: [ELCO] BFG All terrains and fuel economy
Ladies and Gents
Last January I had to replace two tyres and since Dunlop no longer make
all terrain "Grand Treks" I swapped to BFG all terrains on the rear
axle. They have a much chunkier treat pattern than the Dunlops, and this
was the week before the heavy snow where they definitely helped, so no
complaints there.
However I have noticed that the truck's fuel economy has dropped by
about 1 or 2 mpg (ie perhaps 5%, it's hard to be more precise) over the
last few months, and from what I've read this is not wholly unexpected
when fitting tyres with a chunkier tread. I have noticed that there is
a slight warm rubber smell from the BFGs after longer journeys, which
sort of confirms this.
As far as I can tell it's going OK, the engine sounds fine, oil
consumption is negligible and it only smokes a bit under full throttle
acceleration when on full boost @ 2000rpm or higher (as it always has
done). It has done 153k miles and is on the original and unserviced
fuel pump & injectors.
Would the consensus of the forum expect this sort of mpg loss from
chunkier tyres, or should I look harder for other problems?
Also should I get excited about the fact that the BFGs on the rear
(nearly new) are about 1cm greater diameter than the Dunlops on the
front (25k+ miles)? I know one is supposed to rotate tyres regularly
to keep wear even, but I can't help feeling that the centre diff and
viscous coupling should handle that sort of minor difference without
trouble. Or could the difference in rolling radii, causing front/rear
fighting in the transmission, perhaps contribute to the loss of fuel
economy?
Christopher Bell
Devon, UK
1996 1HD-FT
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John
Thanks, I'm glad to hear that. I can afford a couple of MPG but not an
injector pump rebuild!
CB

| I have fitted BFG AT onto our 100 series and also lost 3 MPG.
| The old girl only does 15 now......
|
| I think their a fantastic tyre but obviously at a price....
|
| Cheers
|
| John
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The way I understand this and I stand ready to be corrected.
If the rolling circumference of the tyres is greater, the speedo
will read and trip will read slightly wrong.
I guess you could calculate the difference between front and
rear tyres and divide by the square route of a dead cockroach
or whatever it is, but all this boils down to is maybe your mpg
is still the same, just your trip reading slightly out.
Regards
Les
--------------------------------------------------
From: "Christopher Bell" <[Email address removed]>
Sent: Wednesday, May 06, 2009 12:16 PM
To: <[Email address removed]>
Subject: RE: [ELCO] BFG All terrains and fuel economy
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Les
I thought of that, but rejected it as an explanation because:
The diameter of the tyres is about 80cm (by eye), and the difference in
diameter about 1cm. Circumference is Pi.D, so the difference is
proportional to difference in diameter, ie about 1/80th.
Since the front tyres are unchanged and the distance covered is - I
assume - the average of front and back that would be 1/160th (your dead
cockroach factor), or less than 1%. Even if the backs were doing all
the work it would still only be a smidgen over 1%.
I don't think I can calculate MPG that accurately, and it is quite a lot
less than the difference I'm seeing.
CB
| The way I understand this and I stand ready to be corrected.
| If the rolling circumference of the tyres is greater, the speedo
| will read and trip will read slightly wrong.
| I guess you could calculate the difference between front and
| rear tyres and divide by the square route of a dead cockroach
| or whatever it is, but all this boils down to is maybe your mpg
| is still the same, just your trip reading slightly out.
|
| Regards
|
| Les
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80 * 3.14 = 251.20
79 * 3.14 = 248.06
That's 1.25% as the difference is times the short crust apple pie.
Speedo connection use to be on the rear of the transfer box output - no idea
where it is on fulltime 4x4.
If you changed the front as well you might be down more than 5%, wow that's
not good, don't forget you are comparing a worn old set vrs a new set with
full tread depth, like for like might be less of a difference and over time
you might not have noticed the mpg change.
As a side note, my wife's Crossfire with 18" back wheels and 17" front
wheels looks at all four wheels for the abs, esp and traction control. The
computer uses some clever logic, accelerometers and steering input to work
out if you spinning the wheels, skidding or spinning, applies the brakes to
straighten you out if required and from all that computes a speed for you to
view, if your looking at the speedo while the car straightens up and cracks
your head on the side window :)
Malcolm
Stafford (UK)
FJ45 '75 & FJ45 '76
-----Original Message-----
From: [Email address removed] [mailto:[Email address removed]] On
Behalf Of Christopher Bell
Sent: 06 May 2009 15:02
To: [Email address removed]
Subject: RE: [ELCO] BFG All terrains and fuel economy
Les
I thought of that, but rejected it as an explanation because:
The diameter of the tyres is about 80cm (by eye), and the difference in
diameter about 1cm. Circumference is Pi.D, so the difference is
proportional to difference in diameter, ie about 1/80th.
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I don't want to sound alarmist, but you may not be doing the viscous
coupling any good. Any difference in rotational speeds between the
propshafts will cause the fluid in the VC to shear, thus generating
heat, an excess of which appears to cause these units to fail.
Whether or not a constant 1.25% difference would be enough to cause
permanent damage I don't know, but the VC was seized on mine when I
bought it, and one of the tyres was much newer and of a different brand
to the other three, although nominally the same size. Coincidence,
perhaps, but then again there's no smoke without fire, and all that.
You could just remove the VC until such time as you have matching tyres,
then stick it back in. It's not a particularly big job, and at least
you wouldn't have to worry about it breaking. It would be interesting
to see how it affected the fuel consumption, if at all.
Cheers
Paul
1996 FZJ80
----- Original message -----
From: "Malcolm Bagley" <[Email address removed]>
To: [Email address removed]
Date: Wed, 6 May 2009 17:04:03 +0100
Subject: RE: [ELCO] BFG All terrains and fuel economy
80 * 3.14 = 251.20
79 * 3.14 = 248.06
That's 1.25% as the difference is times the short crust apple pie.
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When I had a Freelander I read about people removing the VC and getting
significant reduction in fuel consumption.
The VC did cross my mind but 10mm difference in height isn't much more than
new tires vrs old of the same make or does the manual mention anything about
matching the wear?
I was told the Freelander used slightly different diff ratios to get an
unequal split of torque, more to front, in normal driving. Don't know if
that is true but don't know how else the split could be unequal.
Malcolm
Stafford (UK)
FJ45 '75 & FJ45 '76
-----Original Message-----
From: [Email address removed] [mailto:[Email address removed]] On
Behalf Of Paul Harper
Sent: 06 May 2009 19:17
To: [Email address removed]
Subject: RE: [ELCO] BFG All terrains and fuel economy
I don't want to sound alarmist, but you may not be doing the viscous
coupling any good. Any difference in rotational speeds between the
propshafts will cause the fluid in the VC to shear, thus generating
heat, an excess of which appears to cause these units to fail.
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Malcolm
| The VC did cross my mind but 10mm difference in height isn't much more
| than
| new tires vrs old of the same make or does the manual mention anything
| about
| matching the wear?
Yes, it gives a recommended rotation pattern to give even wear. I
accept that any shear in the VC will warm it up and - by definition -
waste fuel, so I can see how removing it might improve fuel economy.
I did worry about the different rolling radii overheating the VC, but as
far as I can tell it doesn't heat up significantly - and nothing like
the heat that is generated in the transfer case when hauling heavy loads
at speed.
Ho hum, it will be interesting to see what happens when I replace the
fronts: will economy worsen again because of the extra tread, or improve
because of less VC shear?
My main concern was that there was something wrong with the engine, but
it seems that tyres can cause this sort of effect so I'm more cheerful
now - thanks everyone.
Christopher Bell
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Hi Guys
Have been watching this post and would just like to know what is the VC,
what does it do, and where is it located,
In the day when they (80 series) first appeared and all was new what was the
mileage expected from them back then.
cheers
john 92HDJ 80 !HDT
----- Original Message -----
From: "Christopher Bell" <[Email address removed]>
To: <[Email address removed]>
Sent: Thursday, May 07, 2009 9:24 AM
Subject: RE: [ELCO] BFG All terrains and fuel economy
Malcolm
| The VC did cross my mind but 10mm difference in height isn't much more
| than
| new tires vrs old of the same make or does the manual mention anything
| about
| matching the wear?
Yes, it gives a recommended rotation pattern to give even wear. I
accept that any shear in the VC will warm it up and - by definition -
waste fuel, so I can see how removing it might improve fuel economy.
I did worry about the different rolling radii overheating the VC, but as
far as I can tell it doesn't heat up significantly - and nothing like
the heat that is generated in the transfer case when hauling heavy loads
at speed.
Ho hum, it will be interesting to see what happens when I replace the
fronts: will economy worsen again because of the extra tread, or improve
because of less VC shear?
My main concern was that there was something wrong with the engine, but
it seems that tyres can cause this sort of effect so I'm more cheerful
now - thanks everyone.
Christopher Bell
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Hi
A VC, in this case, is a viscous coupling. It is used as a center diff
on the 80.
One thing I'm not entirely sure about is that a VC has only an input
shaft and an output, while a standard diff has one input, 2 output.
Out of curiosity is the VC mounted on one of the outputs of the
transfer case and if so which one? Or does the transfer case have a
standard diff in it with the viscous coupling complementing it to
create a LSD (limited slip diff)? Or am i just talking complete
rubbish here?
Rob
97 HDJ80
2009/5/7 john <[Email address removed]>:
the
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John
VC =3D Viscous Coupling.
It acts like a limited slip centre differential in that it sits in the
transmission between the front and rear axles and allows them to turn at
different speeds while still transmitting torque to both.
It acts by having plates attached to front and rear shafts that are
immersed in a viscous liquid like a very thick oil, and any difference
in speed causes shear (twist) in this liquid which transmits torque.
I would expect it to last the life of the vehicle unless it was
subjected to very heavy shear over a long period, which would cook the
fluid.
I've a feeling that it is only fitted to 80 series with ABS, and prior
to that just had a centre diff, but I'm not totally certain about that -
does anyone have a definitive answer?
Christopher Bell
| Hi Guys
| Have been watching this post and would just like to know what is the
VC,
| what does it do, and where is it located,
| In the day when they (80 series) first appeared and all was new what
was
| the
| mileage expected from them back then.
| cheers
| john 92HDJ 80 !HDT
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John,
The viscous coupling should last the life of the vehicle, except in the case
of the Land Rover Freelander where they don't seem to ;)
The viscous liquid is actually very clever, when the two ends of the shaft
are spinning at different speeds (like then the rear wheels are spinning)
the fluid warms up becomes thicker or I think in the case of thixotropic
silicone it actually becomes solid.
Malcolm
Stafford (UK)
FJ45 '75 & FJ45 '76
-----Original Message-----
From: [Email address removed] [mailto:[Email address removed]] On
Behalf Of Christopher Bell
Sent: 07 May 2009 11:10
To: [Email address removed]
Subject: RE: [ELCO] BFG All terrains and fuel economy
It acts by having plates attached to front and rear shafts that are
immersed in a viscous liquid like a very thick oil, and any difference
in speed causes shear (twist) in this liquid which transmits torque.
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Malcolm
I think thixotropic materials become *less* viscous under shear, like
non-drip paint or mud under Alaskan villages.
I don't know the word for the opposite effect, although there must be
such a term. [Later - Google does the trick, apparently it's called
"rheopexy".]
I remember reading somewhere that someone tried to make aviation fuel
behave like this (ie go solid under shear) so that it would solidify and
remain in the tanks in the event of a crash. Nice idea, but I don't
think it ever worked.
CB
| The viscous coupling should last the life of the vehicle, except in
the
| case
| of the Land Rover Freelander where they don't seem to ;)
|
| The viscous liquid is actually very clever, when the two ends of the
shaft
| are spinning at different speeds (like then the rear wheels are
spinning)
| the fluid warms up becomes thicker or I think in the case of
thixotropic
| silicone it actually becomes solid.
|
| Malcolm
| Stafford (UK)
| FJ45 '75 & FJ45 '76
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Toaddmy2cents-yes,almostright,Chris.Thefluidbehaviouryou
describeisdilatant.Thisisdifferentfromrheopecticinthata
dilatantsolutiongoesfrombeingviscoustoalmostsolidprettymuch
instantlywhensubjectedtoshearingforces.Arheopecticsolutionunder
shearincreasesitsviscosityovertime(i.e.itgraduallygetsmoreand
moreviscousasit'ssubjectedtomoreandmoreshear).Ineverthoughtmydegreeinchemicalengineeringwouldcomeinhandy!!
Foraneasyexampleofhowadilatantfluidworks,makeacornflour
solutionathome-addcornflourtoacupofwateruntilyouhavea
"runny"paste(slightlyrunnierthanyoghurt)bymixinginSLOWLY.Then
trystirringfastandseewhathappens;-)
Notsurethatacornflourmixturewouldmakeagoodsubstituteforthe
siliconeinyourV/Couplingthough-mightstarttosmellliketoast
afterawhile!
Cheers,
Andrew
'98LC80VX4.2TD
-----OriginalMessage-----
From:=[Email address removed][mailto:[Email address removed]]
OnBehalfOfChristopherBell
Sent:07May200915:42
To:=[Email address removed]
Subject:RE:[ELCO]BFGAllterrainsandfueleconomy
Malcolm
Ithinkthixotropicmaterialsbecome*less*viscousundershear,like
non-drippaintormudunderAlaskanvillages.
Idon'tknowthewordfortheoppositeeffect,althoughtheremustbe
suchaterm.[Later-Googledoesthetrick,apparentlyit'scalled
"rheopexy".]
Irememberreadingsomewherethatsomeonetriedtomakeaviationfuel
behavelikethis(iegosolidundershear)sothatitwouldsolidifyand
remaininthetanksintheeventofacrash.Niceidea,butIdon't
thinkiteverworked.
CB
|Theviscouscouplingshouldlastthelifeofthevehicle,exceptin
the
|case
|oftheLandRoverFreelanderwheretheydon'tseemto;)
|
|Theviscousliquidisactuallyveryclever,whenthetwoendsofthe
shaft
|arespinningatdifferentspeeds(likethentherearwheelsare
spinning)
|thefluidwarmsupbecomesthickerorIthinkinthecaseof
thixotropic
|siliconeitactuallybecomessolid.
|
|Malcolm
|Stafford(UK)
|FJ45'75&FJ45'76
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Andrew
| To add my 2 cents - yes, almost right, Chris. The fluid behaviour you
| describe is dilatant. This is different from rheopectic in that a
| dilatant solution goes from being viscous to almost solid pretty much
| instantly when subjected to shearing forces. A rheopectic solution
under
| shear increases its viscosity over time (i.e. it gradually gets more
and
| more viscous as it's subjected to more and more shear).
|
| I never thought my degree in chemical engineering would come in
handy!!
| For an easy example of how a dilatant fluid works, make a corn flour
| solution at home - add corn flour to a cup of water until you have a
| "runny" paste (slightly runnier than yoghurt) by mixing in SLOWLY.
Then
| try stirring fast and see what happens;-)
| Not sure that a corn flour mixture would make a good substitute for
the
| silicone in your V/Coupling though - might start to smell like toast
| after a while!
Well I learned about it (thixotropy that is) while reading Civil
Engineering, so it's a small world isn't it? I have a daughter reading
Chemistry at Bristol at the moment, but everything she says goes way
over my head.
I got cold feet and decided to re-measure the tyres, and have now
decided that the difference in diameter is more like 5mm - it's actually
quite hard to measure without a Tee square or something similar - and I
simply refuse to believe that a small difference like that will upset
anything. So I'm officially sticking my head in the sand on the subject
of cooking VCs!
The truck is due for an oil change this month, it tends to do better MPG
in the summer anyway (higher ambient temp? higher cetane rating in
summer diesel? Who knows!), and I think I'll put in a new air filter too
just in case. Hopefully that will help a bit.
CB
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You're probably right to make like an ostrich.
Still, at least my (misplaced?) alarmism has given us all an insight to
thixotropicity, rheopexy and dilatancy. Marvellous thing, shared
knowledge and t'Interweb.
Now, where's that corn flour...?
Paul
Christopher Bell wrote:
So I'm officially sticking my head in the sand on the subject
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Hi Guys
So the pre 92 80 series may not have the VC, if it does what does it look
like and where can it be found. Does anyone know what the original expected
mileage could have been.
I read a few weeks ago that Scania (I Think) have brought out a new artic
truck which will deliver much more than the cruiser when pulling 40ton, so
how can this be done when the cruiser can't achieve this.
cheers
john 92hdj80 1hdt
----- Original Message -----
From: "Christopher Bell" <[Email address removed]>
To: <[email protected]
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