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Old 02-22-2010, 01:43 PM   #1 (permalink)
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Big bumper or small? (VW Golf Mk2)

I have a plan of taking a VW Golf Mk2 and converting it to a 1.9TDI, it will be as empty/light as possible, I might also do carbon fiber hood, hatch etc.

Anyway I have a slight dilemma. From my reading (for instance Top gear USA Rabbit TDI etc) it appears the later larger bumper would be to prefer with some smoothing. But it also weighs a lot more than the European small bumper (shown in picture below) especially an aftermarket one which usually use thinner/less/lighter metal.

Here are the two next to each other:


Picture from here (and also full size pic available):
Flickr Photo Download: AIMG_8878

For those not familiar with this car model/bumper here is a picture of the big bumper (here with fog light holes but no fogs in them, also available with those holes sealed)

More pics Flickr Photo Download: low right

If I use the small European bumper I would most likely go for this spoiler:


More pics Flickr Photo Download: Golf GTI 16V

So the question is which would be to prefer the BIG BUMPER (with more weight but most likely better aerodynamics, and potential for even better with some mods) or the SMALL BUMPER (with less weight but not so great aerodynamics?).

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Old 02-22-2010, 04:34 PM   #2 (permalink)
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Isn't the difference in the bumper cover which is made of plastic?
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The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
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Old 02-22-2010, 05:07 PM   #3 (permalink)
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Better aerodynamics trump weight (within reason - and your case is definitely within reason).

The fairings on this motorcycle...



... added 88 lbs to the stock motorcycle, which only weighed 231 lbs to begin with ...



The stock bike can get 114 mpg (US). The faired (and re-geared) one can get over 200 mpg (US), despite the 88 extra pounds.

This is an extreme example, but unless you're only operating at very low speeds in stop & go type driving, aero mods win.

214 mpg with DIY aerodynamic fairings on a Honda 125cc motorbike | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com
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Old 02-22-2010, 05:49 PM   #4 (permalink)
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I'd start with a light bumper, and make a light, full-aero front tray.
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Old 02-22-2010, 05:54 PM   #5 (permalink)
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Quote:
Originally Posted by COcyclist View Post
Isn't the difference in the bumper cover which is made of plastic?
Well technically yes the bar under it does show the very same partnumber but it's a pain usually to take off the skin off of them and especially getting them back on to stay on.

VW does show this to be the part under the skin regardless of big or small though yes.


Secondly none of the aftermarket makers of these bumpers sell them separately like that, also to this is the fact that I'd want matching rear setup of course. Are you suggesting having both to swap them? If I went with the large one I'd try to make some brakets to just hold the skin in place (for weight).
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Old 02-22-2010, 05:56 PM   #6 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Better aerodynamics trump weight (within reason - and your case is definitely within reason).
And I've read (I think on here) that weight trumps aerodynamics in an overall perspective, while yes aerodynamics does so on the highway at long higher speed constant stretches.
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Old 02-22-2010, 06:14 PM   #7 (permalink)
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Quote:
Originally Posted by Bicycle Bob View Post
I'd start with a light bumper, and make a light, full-aero front tray.
Yeah that's what I am leaning towards, well I'll confess too that within the VW community there are so many styles, and while my goal is to have as great MPG as possible I do want to do so in style. No I am not looking for wide tires or anything like that, I plan on using 14" alloy wheels that are 4kg (around 8.8lbs each). The reason for this question before even starting the project is there is a major style difference between the two bumpers and the styles that go with them.
Sorry if I bring up style on a forum that maybe doesn't focus that much on that.

There's essentially two different styles I have been contemplating:

A) Older "retro" look something like this:

Which can work very well with the relative simplistic and smaller wheel look.

B) Alternatively then something more "modern" yet simplistic like this:
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Old 02-23-2010, 02:36 PM   #8 (permalink)
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Quote:
Originally Posted by G2TDI View Post
weight trumps aerodynamics in an overall perspective, while yes aerodynamics does so on the highway at long higher speed constant stretches.
It's a common misconception that aerodynamics are only an issue at highway speeds.

Enter your vehicle's stats into this tool...

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

... and you'll see that air drag consumes about half of the power (fuel) needed to drive your car at speeds as low as around 35 mph.
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Old 02-23-2010, 04:42 PM   #9 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
It's a common misconception that aerodynamics are only an issue at highway speeds.

Enter your vehicle's stats into this tool...

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

... and you'll see that air drag consumes about half of the power (fuel) needed to drive your car at speeds as low as around 35 mph.
The link you show asks for a number of values I have no clue on, on the vehicle I would do this on (stock), not to mention would have a hard time figuring out the values after modified. Lighter, lowered etc etc.

Also that example itself speaks for itself, remove 200lbs for instance from the example and see how your 35mph values change. The % is not the whole picture. If you have a 50% sale, a $2 item ends up being $1 with $1 saving whereas a $200 would be a $100 saving. So % sometimes is misleading. Also the WEIGHT is not a directly listed number in that link, it is indirectly hidden in other value(s).

The 1800lbs is rather light to begin with (the range I'd want to try for actually) so the % you see there is off of a very low number (l/100km) so even though the % might be HIGH the end value isn't that high.

Also let's be realistic here, how much reduction can you realistically change on the aero? How much can you do to the weight? I believe I can remove a good chunk of the weight, whereas even with the most serious attempts won't affect the drag factor much. Also many of the aero mods actually add weight. No I am not wanting to debate this I want to see where my balance should be in MY case with THIS type of vehicle.

Last edited by G2TDI; 02-23-2010 at 04:57 PM..
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Old 02-23-2010, 04:45 PM   #10 (permalink)
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i think the main reason for the bifferent bumper is that the golf was a long running model that needed a new face... aerodinamically it might incorporate more of an airdam than the original "clasic" configuration, but with the adition of an airdam i think both setups will likely be comparable. the main thing indeed would be a front undertray.

also with a front undertray a very low airdam might not be needed. aerodynamic modern cars often have a raised airdam in the center, alowing a more smooth transition to the undertray, rather than be harch to the air. so if you go for modern aero, the old bumper might give you more freedom, while the "modern" bumper has the 'low tech' airdam function build in (vs more advanced undertray)... on the other hand, you can't be sure untill you test it that with a relative rough underside, a classic airdam isn't better than a partial undertray... or that perhaps the plastic airdam provides easier undertray attachment...

the advantages of the newer bumper might be better crash protection, to a small degree... and also cosmetic protection (the lower metal work might dent but the plastic might not on a minor impact, and if it's destroyed it can be replaced)... not fe related, but something to take into account perhaps.

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