How does weight affect economy?

TDI Steve

Veteran Member
Joined
Oct 18, 1999
Location
LI, NY
TDI
2000 Golf
Just wondering what benefits can be had from reducing the weight of the car.

Actually... i'm researching and considering dropping a tdi into a 1200 pound car... about 1800 pounds lighter than my car, if I am correct.
 

think diesel

Veteran Member
Joined
Jul 20, 2000
Location
Northern Virginia
Interesting question, Steve. I think it would be dependent on how much advantage you TAKE of that reduced weight--- for example, if you altered the gear ratios and/or final drive ratio in the transmission to run at a lower RPM @ a given speed. In other word a lighter car needs less power @ a given speed so it can rev lower--- that would afford some signifigant fuel mileage savings. I think another factor to consider is where the car would be driven and how. I'd imagine weight differential would be less a factor in a relatively flat area when you accelarate slowly as opposed to pushing it up a mountianous areas. There are lots of variables there. Maybe one of the more scientific minds here can give you some solid data or a formula or something.

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1999 New Jetta GL TDI
Atlantic Blue/Gray studio

Mods: VW rubber floor mats, Thule roof rack, Nørsk Pine scented air freshener.
 

Powder Hound

Top Post Dawg
Joined
Oct 25, 1999
Location
Under a Bridge, Crestview, FL, USA
TDI
'00 Golf 4dr White 5sp, '02 Jettachero 5sp, Wife's '03 NB Platinum Gray auto(!)
You are exactly correct, think diesel, which is why the highway mileage, where you don't bleed off energy from the brakes and then have to replace it with the engine to get going again, is better than around town stop and go driving.

TDI Steve, let us know if this project actually gets started. You'll learn a lot since you'll have a lot of work adapting the new engine to the old chassis. It won't be quite an 1800 # weight loss (you are adding the new drivetrain), but there will be a significant weight loss.

Do you have a source for the wiring and ECU? What is the target chassis? An older Golf or Rabbit? Just curious...

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"I love the smell of [burning diesel] in the morning. It smells like ... victory!"
 

Dieselgeek

TDIClub Enthusiast, Vendor , w/Business number
Joined
Oct 3, 2000
Location
Golden, CO
TDI
2016 Golf TDI
Steve:

I have a stripped A1 Rabbit that weighs 1830 with a G60 motor (weighs roughly the same as TDI). The A2 will weigh no less than 2000 stripped (no interior) with euro bumpers. I can get you a VNT-15 powertrain if you so desire. But unless you plan on removing sheetmetal, your weight won't go much less than 2000. Weight reduction is everything though.

jim

I am going to be using an a2 golf as the donor car (easy to find)... I don't have a source for the wiring and ecu (not so easy to find).

I am planning on selling my car to pay for this... but then again maybe i should just turn my car into a parts bin... what do you think? I should contact Jason Crabtree (I think that's his name) and see what advice he can give me. When I'm ready to get started I'll make a post asking for advice and documenting my progress.[/B][/QUOTE]
 

TDI Steve

Veteran Member
Joined
Oct 18, 1999
Location
LI, NY
TDI
2000 Golf
Yes, i am going to be learning alot..

I don't even know where to start...

I am going to be using an a2 golf as the donor car (easy to find)... I don't have a source for the wiring and ecu (not so easy to find).

I am planning on selling my car to pay for this... but then again maybe i should just turn my car into a parts bin... what do you think? I should contact Jason Crabtree (I think that's his name) and see what advice he can give me. When I'm ready to get started I'll make a post asking for advice and documenting my progress.
 

Dieselgeek

TDIClub Enthusiast, Vendor , w/Business number
Joined
Oct 3, 2000
Location
Golden, CO
TDI
2016 Golf TDI
Turbo Steve:

I'd use an early Scirocco '75-'81 since it has a lower frontal area than the Rabbit or Golf. It weighs in between a Rabbit and an A2 Golf but is aerodynamically better than both and it has a little more room for an intercooler in front of the radiator than a Rabbit. That dude that has a '74 Scirocco TDI in Norway would probably let you copy his intercooler setup. Sadly, Euro bumpers are expensive for that chassis but weigh much less than the U.S.-spec bumpers.

jim

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
I am going to be using an a2 golf as the donor car (easy to find)... I don't have a source for the wiring and ecu (not so easy to find).[/B]<HR></BLOCKQUOTE>
 

jorpet

Veteran Member
Joined
Oct 31, 2000
Location
West Seattle, WA
TDI
2001 Jetta - 2015 Golf SW
I have a formula for weight to rolling resistance, it is
Rolling resistance (lbs) = vehicle weight (lbs) x 0.012 to 0.015

The constant is based on tire type, tire pressure, wheel bearings...

So, if you drop the weight of a car from 2000 lbs to 1000 lbs your rolling resistance (everything else being equal and using a constant of .013) would drop from, say, 26 lbs to 13 pounds. Compare that to a 2001 Jetta going at 60 mph and find that air resistance is 57 lbs and at 80 mph it goes up to 101 lbs.

The reduction in weight is small potatoes in comparison to air resistance with speed changes. Getting a "slippery" car is much more important since the air resistance is a function of frontal area and coefficient of drag. For the new models these numbers are:

Style``````````Cd````````````fA
Jetta``````````.3````````````20.6
NB`````````````.38```````````21.4
Golf```````````.31```````````20.7

The Cd numbers are coming straight from VW, but the fA numbers I am guessing at, if anyone knows the fA numbers I would be interested.

To get a fairly accurate number for the rolling resitance, you can use a bathroom scale. Place your car on a flat smooth surface in neutral, no parking brake (duh!). Then push on the bathroom scale on the bumper of the car. You should get an intitial high number that is the force needed to get the car moving and then a constant number that is the force of the rolling resistance...

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2001 Jetta GL TDI, 5 spd,
Black/Black,

Shift early, shift often!

[This message has been edited by jorpet (edited November 03, 2000).]
 

ort5

Well-known member
Joined
Nov 2, 2000
Location
Jaffrey, NH
TDI
Golf GL, 2000, Red
Perhaps the most significant gains from an decreased mass of the vehicle will be seen in the acceleration and handling numbers. Assuming you have a constant amount of force (from your motor) the acceleration of the vehicle would vary inversely with the mass
acceleration = force/mass
This would help with economy around town I suspect because of all of the changes in velocity (ie. acceleration). Highway mileage would be better, but not as much as pointed out by the people above (drag).
Fun stuff though.

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VR6 driver with TDI envy
 

shirish_bh

Active member
Joined
Nov 9, 2000
Location
Auburn Hills , Michigan
Well...just to take this point further....

Force = mass * acceleration.

For the same acceleration, a heavier car would need higher force, hence a higher engine torque (everything else remaining the same). This means more fuel consumption.

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God did not make spark plugs!
 
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