Undertray / belly pan build - VW Golf MK3 1.9 Turbo Diesel
 

Today I set about starting my undertray build, I wanted to build and strengthen the tray before mounting it to the car for ease of removal.

I bought 4 large sheets of 2mm correx board (see pics below) with the intention of using a couple of layers to get the correct strength. Unfortunately the rigidity was awful so I had to strengthen the tray with some pieces of roofing plastic. You can see this in the picture as the ligher plastic.

The car has a front tray fitted from the factory (which I did a little correx repair work on this morning) so today I made the mid and rear sections. Because a MK3 Golf has a full width rear axle I had to build a separate rear section. You can see in the pictures how the tray is notched to allow movement of the suspension. You will also see that I have split the mid tray in half to allow the hot exhaust pipework to run down the centre un-covered.

The rear tray is a little longer than it needs to be as I will trim it once I have the correct angle for the "diffuser" part at the back. I think this will have to be spaced 20mm - 40mm below the bumper for optimum effect. It will also cover the exhaust back box as it will be cooler at the rear of the system.

I hope to have it fitted soon and put up some more pics, just a matter of fabricating some brackets to attach it to some convenient places under the car.

How did you cut your material? How did you lay out the cuts?

Hi there,

Because the material is so thin I used a stanley knife. The sheets are almost exactly the width of the car so, I dragged a sheet under there and used a jack to hold it against the floor of the car. Then I just cut around the various suspension components and exhaust.

With the rear I did the same, I just had to keep in mind where I'd be attaching it to when I build some brackets. I also left it a few extra cm longer than I needed so I can choose a length at a later date.

Hi Sallen, Looks like a good start. I need to do this to my Golf. My Mk IV has a bunch of 15mm plastic plugs in the underbody. I keep staring up at these on my back trying to come up with a way to attach my belly pan without drilling into the sheet metal. Please post more photos as you progress and let us know if testing shows any improvement in mpg.

Will do, I have the same plugs in the bottom of mine, have resisted the urge to tap screws into them tho...

I don't know about the MKIV but the MK3 has 5 holes down each side in the "lip" under the sill (the bit the jack locates into). I'm going to use these as mounting points for the outside edge of the tray via some tiny 90 degree brackets.

The front will be connected using brackets going to existing bolts for the wheel arch liner, and screwed to the trailing edge of the factory tray.

I think the inside edge nearest the exhaust will be bolted to existing bolts around the exhaust tunnel. Once I have brackets made I'l post pics of the setup. I'm aiming for the tray to be easily removable leaving minimal brackets in place under the car.

It's sweet that you have the TDI Golf though, it's easily 10% better FE than my IDI engine, and has loads more power and torque. That new bluemotion Golf is amazing though, 110hp and 75mpg!

Great work on the undertray so far, look forward to installation. Can I ask where you purchased the correx? I have yet to found any local stockists, and delivery costs are a factor for these big sheets.

Hope to undertake a similar project on my Polo TDI soon. Will the undertray cover the jacking points, or will these be left exposed?

Regards, Grant

Hi there, I think the jacking points will be exposed enough to use the jack.

I got my correx style sheet from Travis Perkins builder's merchant. It is far thinner than sign writing correx, so it's better to make a frame from something and "skin" the tray with the correx.

Excellent, there is a branch local to me

One plan was to incorporate wheel "strakes" into the undertray, but they are ideally placed at exactly the same places as the jacking points :confused: Perhaps tyre spats and side skirt channelling will suffice; small sections hinged at the jacking points to allow the jack to slide under

Thanks, Grant

I have no connection with these guys here but it might work:Blind rivet nuts - ACUMENT-GLOBAL-TECHNOLOGIES - Nut
I found a few at work the other day and thought i could get them to work in the existing holes in the chassis.

ollie

Right... Finally got the bugger on!

I built some brackets and bolted them to the tray, then bolted all three panels under the car. I mostly used existing bolts for the exhaust etc to attach the brackets to.

At the rear I spaced the rear edge of the tray down from the bumper using long bolts and bits of copper pipe.

Because the tray covers the rear silencer (with a hole cut out for the exhaust pipe tip) I built a heat shield with a hammer, out of some scrap aluminum I had lying about. It separates the plasic from the silencer.

I've tested it to motorway speeds and nothing has fallen off yet...

Looking forward to results.

please delete this double post. I do apologize for my negligence.

You could screw into them, OR remove them and put in an appropriately sized expanding rubber freeze plug and put the belly pan material between the big top washer and the nut. You might want another washer in there.... Or, I'd probably use the freeze plugs to mount some simple aluminum cross-pieces and then attach the belly pan to the aluminum.

Thanks Bill, Since I posted that response I have done the coroplast belly pan, screwing into the plugs with screws for metal roofing. I have been a slug about starting a new thread but I do have photos uploaded. If you click on COcyclist above my avatar, it will take you to my profile and then open the album "belly pan". Of all the aero mods I have done so far, this seems to make the most difference.

Oh, cool. I like it very much. The wooden blocks on the jack stands in picture 5 are a little scary... But the belly pan is very nice. Did you do anything to protect it from the heat of the exhaust?

The coroplast is in direct contact with the exhaust pipe in a couple of spots and it hasn't been a problem. The catalytic converter is open to the air but diesel exhaust is much cooler than gas cars.

Nice work

i wonder why did you space the tray from the bumper?
Is this for cooling or venting or to get a certain angle? i have a similar rear tray, but the spare wheel tub extends below the rear buper that's fairly high, and so the tray curves up to meet the bumper

I didn't know that about diesel exhaust. Interesting. My Honday Element's exhaust will melt coroplast allllll the way out to the tip of the tailpipe...

I agree with CoCyclist, my exhaust is not really hot enough to melt it...

If you have a turbo car with an intercooler, which I don't (yet, watch this space for the addition of a Mini Cooper S intercooler) then the exhaust gas temp is drastically decreased.

I like your full width tray CoCyclist. I may extend mine in a similar fashion (leaving the cat open).

Just a 500 mile drive on motorways (nice and hot for engine + exhaust) to test it out, and I'll probably have some MPG results.

@lunarhighway, the spacing is only to allow me to change the angle, my fuel tank sits low and without spacers, the tray goes straight to the bumper at quite a steep angle. The rear portion of the tray is pretty much sealed at the front edge, so not much venting will happen.

The reason for the current gap is simply that was the length of bolt I had lying about. I probably need longer.

Thanks, the 4' x8' salvaged sign went from the back bumper to the cat and almost from seam to seam on the rocker panels. I got lucky.

There is a thread on here about diffusers and some charts showing that drag is lowest at about a 4 degree rise to the back edge of the diffuser (belly pan).

Nice work! Looks like a politicians race car now underneath. My question is, how much of a difference did it make? Could a mod like this cut down on road noise too?

... and the result is what?

I like your mod too btw.

It is not scientific since I can't really do back to back testing of the belly pan. But there is a trip I regularly take where my previous best mpg was about 55 with careful driving and I got into the low 60s after the belly pan, on the Scanguage. I didn't really notice any difference in noise though.

It'd be great if you have a template/trace out of your undertray :-)

Where you get your plastic plugs? Those things are expensive in the auto stores

Also, be careful about how you fasten your copper tubes. Galvanic corrosion will rust your car.

If that question is directed to me, I just drove the roofing screws into existing plugs in the VW underbody, didn't even pre-drill. Make sure the leading edge is fastened securely and no problems with the rest.

how you mount these panels? did you have to drill holes in the body?

http://ecomodder.com/forum/member-co...mg-0134sm.html

If you look carefully at this "before" photo you can see a round plastic plug just to the left of the catalytic converter and a couple more on the frame type reinforcements (look where the fuel line snakes around).

This photo shows what I used to attach it. NO new holes were drilled into the sheetmetal.http://ecomodder.com/forum/member-co...-img-0142.html

Undertray Results
 

Hi guys,

Have finally made my way through 87.5 litres of various flavours of diesel (83% Bio 17% Dino) with my undertray on, and I'm showing a 4mpg (uk) increase from 58 to 62mpg. That brings me into the 50's in US mpg.

Initial observations:

• Noise levels seem higher (tho this could be my hard tyres);
  
• I can hear more 'wind noise' around the A pillars (perhaps the road noise has dropped so I can now hear this);
  
• This is only the first full tank with the tray on, and it is pretty roughly built;
  
• I have been driving VERY VERY slowly tho I was doing that last tank too;
  
• Due to the price of fuel in the UK (about $2.28 per litre), the motorway network has slowed down, with many lorries now limited to 50 by their owners; and
  
• I need to tune the exit angle, it is way too steep as it is.

I have also just scored a scrap garage door, so will remake my undertray from Sheet Metal to make it smoother.