MK4 Drop Trunk Mod

[BobsMK4]

I’m interested in modifying my trunk with the “drop trunk mod”. I’ve seen the finished drop trunk mod on several threads. Does anyone have more pics of the actual construction of this mod? Also, measurements would really help. My car has been finished for a couple of years now, so I will be attempting this mod on a finished trunk with carpeting. Any help would be appreciated.

[initiator]

I think most people buy kits from Russ Thompson or Darkwater Customs. Here's a recent post with some pictures that might help understand the construction. This was a Russ Thompson kit.

https://thefactoryfiveforum.com/showthread.php?27099-Fabb-s-MK4-Coyote-Build-Thread&p=325561&viewfull=1#post325561

[phileas_fogg]

I did both the lower trunk mod and the upper trunk mod. I fabricated the pieces for both myself, using aluminum from online metals (http://www.onlinemetals.com/merchant.cfm?pid=1240&step=4&showunits=inches&id=76&top_cat=60). The lower box is 17 1/2” x 26” x 5”, and the upper box is 9” x 18 “ x 5 1/2”.

For the lower trunk mod, I began by deciding I didn’t want to modify the frame. I’m not a mechanical engineer and have no automotive experience. So despite looking at dozens of frame modifications, I just didn’t have the knowledge to get comfortable with anything I saw.

With that decision made, I cut off the upper wall from the lower trunk panel.

https://farm5.staticflickr.com/4301/36248926256_cd5cf1c343_k.jpg (https://flic.kr/p/XecdTb)IMG_1759 (https://flic.kr/p/XecdTb) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

Then I cut the opening for the box.

https://farm5.staticflickr.com/4298/36248925896_8a42942443_k.jpg (https://flic.kr/p/XecdLY)IMG_1761 (https://flic.kr/p/XecdLY) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

I then fabricated the various panels for the 17 1/2” x 26” x 5” lower trunk floor. I chose to fabricate the lower trunk box rather than just buy the Russ Thompson box for a couple of reasons. First, I wanted to practice on something a little more complex than the upper trunk box before tackling the passenger foot box expansion. Second, if you’re going to leave the rear frame members in place, you have to clearance the RT panels yourself. If I’m going to do that, I may as well make the panels. Third, I wanted every available inch of trunk space. That meant custom panels.

https://farm5.staticflickr.com/4401/36185663232_1a224f856f_k.jpg (https://flic.kr/p/X8AYZo)IMG_1846 (https://flic.kr/p/X8AYZo) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

https://farm5.staticflickr.com/4293/36185665302_b9063e9032_k.jpg (https://flic.kr/p/X8AZB5)IMG_1843 (https://flic.kr/p/X8AZB5) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

https://farm5.staticflickr.com/4413/36185666642_d5bafecf93_k.jpg (https://flic.kr/p/X8B11b)IMG_1842 (https://flic.kr/p/X8B11b) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

https://farm5.staticflickr.com/4374/36185669052_8e2c8fe82b_k.jpg (https://flic.kr/p/X8B1HJ)IMG_1838 (https://flic.kr/p/X8B1HJ) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

https://farm5.staticflickr.com/4318/35518749534_ab5174caa8_k.jpg (https://flic.kr/p/W7ESMm)IMG_1834 (https://flic.kr/p/W7ESMm) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

[phileas_fogg]

https://farm5.staticflickr.com/4330/36217569561_09ec38ffc7_k.jpg (https://flic.kr/p/XbqvCv)IMG_1833 (https://flic.kr/p/XbqvCv) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

https://farm5.staticflickr.com/4385/36217570531_421253bfb5_k.jpg (https://flic.kr/p/XbqvVe)IMG_1828 (https://flic.kr/p/XbqvVe) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

https://farm5.staticflickr.com/4316/36217571431_d775d63837_k.jpg (https://flic.kr/p/XbqwbK)IMG_1825 (https://flic.kr/p/XbqwbK) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

For those of you inclined to do this mod yourself, here are some lessons I learned:

1. The trunk framing is NOT square. For me, the left side of the 3/4” tube running between the rear roll bar mounts was a full 1/4” higher than the right side. I know this because when I fabricated the lower trunk wall panels they didn’t join. At first I thought I’d made a mistake, but subsequent measuring and inspecting and assembly off the car showed I had in fact made a square, non-joining-up box. I ended up remaking the lower left wall and sloping it so that I got a non-square box. If you don’t tell I won’t.

2. Use a piece of scrap aluminum to get 90* bends on the longer panels with your 30” Harbor Freight metal brake. First, make your bend as usual. You’ll find that the only the outermost 2-3” are at 90* and that the part you just bent up is bowed such that the middle is only at ~75*. No big - just insert that scrap aluminum between the brake and the work piece:

https://farm5.staticflickr.com/4438/36185660502_66ca45737b_k.jpg (https://flic.kr/p/X8AYbj)IMG_1856 (https://flic.kr/p/X8AYbj) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

and bend a second time. Repeat everywhere you see the need.

3. There is no elegant way to notch the aluminum panels so that they clear the frame members. I used a jig saw and a lot of trial fitting. I probably spent an hour on each piece and they still look like crap. Good thing they got covered up!

4. I used a bunch of patch panels to cover gaps between the major panels and to provide strength in the corners. I numbered the patch and whatever panel they went on. Note the notch in patch #4 below; that’s a result of Lesson Learned #3 immediately above.

https://farm5.staticflickr.com/4316/36217571431_d775d63837_k.jpg (https://flic.kr/p/XbqwbK)IMG_1825 (https://flic.kr/p/XbqwbK) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

5. Round off the corners of your pieces with a file. If you don’t you’ll bleed.

6. Don’t get too concerned about putting the box on the car after go-kart. I tried to come up with an assembly method that did not require dropping the gas tank, but I ended up having to do that anyway. Just leave yourself enough feed/return hose between the tank and filter/hard line such that you can get the tank out of the way.

I can’t call this a lesson learned because I haven’t actually done it, but it MAY be better to fabricate a shallow tub for the bottom rather than use a flat piece of aluminum like I did. That would let you make the walls of the lower trunk box L-shaped instead of Z-shaped. The tradeoff is that with the tub method you’re guaranteed a row of rivets around the side walls of your box.

The upper trunk mod is much more straightforward (I started with that one to get some practice). Note that this mod IS NOT POSSIBLE with the 2015 IRS; the framing is such that a box like I built won't fit. The upper trunk mod reclaims some space between its floor and the differential. The box shown below is 9” x 18 “ x 6 1/2”, and it just barely fits. I later found out that a 6 1/2” box interfered with the routing for my parking brake cables. The depth had to be reduced to 5 1/2”. So if your parking brake cables run over your differential at all, go with the 5 1/2" depth.

https://farm5.staticflickr.com/4432/36255666571_97d4707dce_b.jpg (https://flic.kr/p/XeMLxv)IMG_1502 (https://flic.kr/p/XeMLxv) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

When finished, my trunk aluminum looked like this:

https://farm5.staticflickr.com/4414/36217573721_668ade6f98_k.jpg (https://flic.kr/p/XbqwSe)IMG_1815 (https://flic.kr/p/XbqwSe) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

Feel free to contact me if you have questions.


John

[Hacksaw84]

I am partly though the install of the Russ Thompson one. It is not listed on his website but if you email him, he has them.

Shoot me an email and I can provide some pictures.

michael.a.craven@gmail.com

[David Hodgkins]

Nice work John!

I went the easy way - Russ Thompson Drop Trunk Box Mod on both my builds:

https://thefactoryfiveforum.com/attachment.php?attachmentid=34951&d=1413997233

https://thefactoryfiveforum.com/attachment.php?attachmentid=49175&d=1451252628

I added a lid to ol' 5369:

https://thefactoryfiveforum.com/attachment.php?attachmentid=57742&d=1471886252

https://thefactoryfiveforum.com/attachment.php?attachmentid=57741&d=1471886251

:)

[Derald Rice]

I went the easy way - Russ Thompson Drop Trunk Box Mod on both my builds:




I am partly though the install of the Russ Thompson one.

In your opinions, can this be done on a completed car ?

[Dave Howard]

I installed the kit from Russ after the fact also. PITA but the additional space was worth the effort.

[initiator]

Wow, John - that's some impressive work!

[Hacksaw84]

In your opinions, can this be done on a completed car ?


I don't see why not, the gas tank is in the way and needs to be lowered or removed. I think his instructions talk about new vs retrofit. The bottom part that he provides in the kit needs to come from the bottom up. So it has to go in above the tank then attach to the floor.

Its obviously going to be WAY easier earlier in the build. The part I guess that would be the hardest would be cutting your existing trunk floor aluminum.

[Avalanche325]

I did mine after I built my car. It is not hard at all to retrofit. My friend came over and we made two of each part so he had a kit to take home and do his. Also a retrofit.

I did mine with five pieces like the picture of Russ'. A Harbor Freight metal brake is all you need. to make the parts.

[BobsMK4]

https://farm5.staticflickr.com/4330/36217569561_09ec38ffc7_k.jpg (https://flic.kr/p/XbqvCv)IMG_1833 (https://flic.kr/p/XbqvCv) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

https://farm5.staticflickr.com/4385/36217570531_421253bfb5_k.jpg (https://flic.kr/p/XbqvVe)IMG_1828 (https://flic.kr/p/XbqvVe) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

https://farm5.staticflickr.com/4316/36217571431_d775d63837_k.jpg (https://flic.kr/p/XbqwbK)IMG_1825 (https://flic.kr/p/XbqwbK) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

For those of you inclined to do this mod yourself, here are some lessons I learned:

1. The trunk framing is NOT square. For me, the left side of the 3/4” tube running between the rear roll bar mounts was a full 1/4” higher than the right side. I know this because when I fabricated the lower trunk wall panels they didn’t join. At first I thought I’d made a mistake, but subsequent measuring and inspecting and assembly off the car showed I had in fact made a square, non-joining-up box. I ended up remaking the lower left wall and sloping it so that I got a non-square box. If you don’t tell I won’t.

2. Use a piece of scrap aluminum to get 90* bends on the longer panels with your 30” Harbor Freight metal brake. First, make your bend as usual. You’ll find that the only the outermost 2-3” are at 90* and that the part you just bent up is bowed such that the middle is only at ~75*. No big - just insert that scrap aluminum between the brake and the work piece:

https://farm5.staticflickr.com/4438/36185660502_66ca45737b_k.jpg (https://flic.kr/p/X8AYbj)IMG_1856 (https://flic.kr/p/X8AYbj) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

and bend a second time. Repeat everywhere you see the need.

3. There is no elegant way to notch the aluminum panels so that they clear the frame members. I used a jig saw and a lot of trial fitting. I probably spent an hour on each piece and they still look like crap. Good thing they got covered up!

4. I used a bunch of patch panels to cover gaps between the major panels and to provide strength in the corners. I numbered the patch and whatever panel they went on. Note the notch in patch #4 below; that’s a result of Lesson Learned #3 immediately above.

https://farm5.staticflickr.com/4316/36217571431_d775d63837_k.jpg (https://flic.kr/p/XbqwbK)IMG_1825 (https://flic.kr/p/XbqwbK) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

5. Round off the corners of your pieces with a file. If you don’t you’ll bleed.

6. Don’t get too concerned about putting the box on the car after go-kart. I tried to come up with an assembly method that did not require dropping the gas tank, but I ended up having to do that anyway. Just leave yourself enough feed/return hose between the tank and filter/hard line such that you can get the tank out of the way.

I can’t call this a lesson learned because I haven’t actually done it, but it MAY be better to fabricate a shallow tub for the bottom rather than use a flat piece of aluminum like I did. That would let you make the walls of the lower trunk box L-shaped instead of Z-shaped. The tradeoff is that with the tub method you’re guaranteed a row of rivets around the side walls of your box.

The upper trunk mod is much more straightforward (I started with that one to get some practice). Note that this mod IS NOT POSSIBLE with the 2015 IRS; the framing is such that a box like I built won't fit. The upper trunk mod reclaims some space between its floor and the differential. The box shown below is 9” x 18 “ x 6 1/2”, and it just barely fits. I later found out that a 6 1/2” box interfered with the routing for my parking brake cables. The depth had to be reduced to 5 1/2”. So if your parking brake cables run over your differential at all, go with the 5 1/2" depth.

https://farm5.staticflickr.com/4432/36255666571_97d4707dce_b.jpg (https://flic.kr/p/XeMLxv)IMG_1502 (https://flic.kr/p/XeMLxv) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

When finished, my trunk aluminum looked like this:

https://farm5.staticflickr.com/4414/36217573721_668ade6f98_k.jpg (https://flic.kr/p/XbqwSe)IMG_1815 (https://flic.kr/p/XbqwSe) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

Feel free to contact me if you have questions.


John
John, thanks for all the pics and explanation. Do you think it is possible to do the trunk mod on a finished car? I would like to do this without dropping the gas tank?

[BobsMK4]

I did mine after I built my car. It is not hard at all to retrofit. My friend came over and we made two of each part so he had a kit to take home and do his. Also a retrofit.

I did mine with five pieces like the picture of Russ'. A Harbor Freight metal brake is all you need. to make the parts.

Well I’m off to Harbor Freight to buy a metal brake tool. This mod will definitely be worth the extra trunk space. I thank everyone for your input and help

Bob

[CraigS]

I did mine on my mkII way after the fact. You make pieces and install them one at a time rather than building a box outside the car. It does take some planning as to which pieces go in what order. Also which way to overlap them so you have access for your rivet tool.

[phileas_fogg]

John, thanks for all the pics and explanation. Do you think it is possible to do the trunk mod on a finished car? I would like to do this without dropping the gas tank?

Bob: I hate to be the bearer of bad news, but you're going to have to drop the tank. The fabrication & installation will be sooooooo much easier you'll actually be happy you did.


Nice work John!


Wow, John - that's some impressive work!

Thanks guys! I just finished carpeting the trunk yesterday. The lower panels are only secured on one edge so that I can fold them over to access the fuel pump & sending unit.

https://farm1.staticflickr.com/878/40643547040_5e2e65e663_k.jpg (https://flic.kr/p/24VwNx3)IMG_4465 (https://flic.kr/p/24VwNx3) by jhsitton (https://www.flickr.com/photos/91016165@N07/), on Flickr

[Higgybulin]

Great job!!
Higgy

[brewha]

The drop trunk is a nice add on. Just be aware that getting to the body bolts becomes next to impossible after it’s in. So before you install the trunk make sure the body bolts are in or modify the set up so you never have worry about dropping the tank again. The combination nut solution some one else recently posted would fix the problem permently and easy to do while the tank and trunk floor are out.

[phileas_fogg]

The drop trunk is a nice add on. Just be aware that getting to the body bolts becomes next to impossible after it’s in. So before you install the trunk make sure the body bolts are in or modify the set up so you never have worry about dropping the tank again. The combination nut solution some one else recently posted would fix the problem permently and easy to do while the tank and trunk floor are out.

That is an excellent point. As you can see in the picture of my trunk carpet, I replaced the long upper quick jack bolts that came with the kit with shorter ones; these will serve as the foundation for the "Kleiner Mod". There is absolutely no way to remove even these shorter bolts (and it's going to take some effort to keep them from rotating when I tighten the couplers, 'cuz the wrench won't fit!). Also, getting the nut on the back of the #10-24 bolt that serves as the trunk latch pin is going to be a real bear.


John

[boat737]

That is an excellent point. As you can see in the picture of my trunk carpet, I replaced the long upper quick jack bolts that came with the kit with shorter ones; these will serve as the foundation for the "Kleiner Mod". There is absolutely no way to remove even these shorter bolts (and it's going to take some effort to keep them from rotating when I tighten the couplers, 'cuz the wrench won't fit!). Also, getting the nut on the back of the #10-24 bolt that serves as the trunk latch pin is going to be a real bear.


John

I also did the Kleiner Mod, with a "Modified Kleiner Mod" for the trunk catch bolt. Same idea, just smaller scale. Hard to see in the pictures, but it's there.

BTW, my trunk mod is the Dark Water Customs.

[shark92651]

I'm still in the early stages of my build, but this is what I am doing in my trunk. I cut off both of the PS tank strap brackets and made a new one out of 1" x 1/8" bar stock and going to weld it at the same height as the DS tank strap bracket. Of course this will require modifying the PS tank strap as well. I will cover the hole in the trunk aluminum with a plate so it will look very clean. I also decided to cut the diagonals so that I will have full access to the drop trunk box. Here is how I plan to re-weld some supports into the trunk. I will also weld another support down below the box on the DS assuming there is enough room between the tank and the bottom of the box. I will mock that up in the next couple days. I bought the drop trunk kit from Russ Thompson.

https://farm2.staticflickr.com/1722/41665968794_478034ba55_c.jpg

https://farm2.staticflickr.com/1760/42389627721_081bd2c785_c.jpg

[Avalanche325]

Don't sweat dropping the tank. I saw threads about mods so you never have to and wondered what all the fuss was about, so I timed it when I needed to drop mine. Not counting jacking the car up.......12 minutes! And I have hard lines going to my tank, so probably a little harder than most. I didn't bother with access holes for the fuel level sending unit. I can spare 12 minutes if I need to change the fuel level sending unit. Which I did have to do.

I did do an access plate for the left side body bolts. But it's not like that is coming off any time soon.

[Gromit]

Maybe not for a retrofit but I wonder what happened to the "Factory" dropped trunk. If you look at the announcement video for the MK IV right around 4:12 the prototype had it built in. What's interesting is two things they dropped the back crossmember so it's more like a shelf and the frame shots leading up to it show no diagonal braces.

https://m.youtube.com/watch?v=6ckgfkPU8Ss

Now I get this was the prototype and maybe you don't want to drop the cross member but that trunk space looks so much bigger and convenient without the lip/wall created by the full height cross member. Maybe it's a good idea to leave the back cross member so stuff in the trunk doesn't shift back during hard acceleration?

Maybe someone with a few older builds can answer what happened to this original idea

Chris aka Gromit

[David Hodgkins]

Maybe not for a retrofit but I wonder what happened to the "Factory" dropped trunk. If you look at the announcement video for the MK IV right around 4:12 the prototype had it built in. What's interesting is two things they dropped the back crossmember so it's more like a shelf and the frame shots leading up to it show no diagonal braces.

https://m.youtube.com/watch?v=6ckgfkPU8Ss

Now I get this was the prototype and maybe you don't want to drop the cross member but that trunk space looks so much bigger and convenient without the lip/wall created by the full height cross member. Maybe it's a good idea to leave the back cross member so stuff in the trunk doesn't shift back during hard acceleration?

Maybe someone with a few older builds can answer what happened to this original idea

Chris aka Gromit

If I remember correctly they had an issue in welding related to that box where the chassis warped so that design was quickly dropped.

:)

[edwardb]

Maybe not for a retrofit but I wonder what happened to the "Factory" dropped trunk. If you look at the announcement video for the MK IV right around 4:12 the prototype had it built in. What's interesting is two things they dropped the back crossmember so it's more like a shelf and the frame shots leading up to it show no diagonal braces.

https://m.youtube.com/watch?v=6ckgfkPU8Ss

Now I get this was the prototype and maybe you don't want to drop the cross member but that trunk space looks so much bigger and convenient without the lip/wall created by the full height cross member. Maybe it's a good idea to leave the back cross member so stuff in the trunk doesn't shift back during hard acceleration?

Maybe someone with a few older builds can answer what happened to this original idea

Chris aka Gromit

I don't recall hearing or reading an official reason for the change. But not too long after the Mk4 was released, Factory Five changed to the increased bracing in the trunk area now shipped as standard. In the process, made the trunk one level as opposed to the stepped and lowered version in this early Mk4 video. Makes sense that the additional steel is stronger and more rigid. Perhaps a manufacturing issue. Or perhaps their computer modeling showed the structure wasn't strong enough.

[chjort]

That is an excellent point. As you can see in the picture of my trunk carpet, I replaced the long upper quick jack bolts that came with the kit with shorter ones; these will serve as the foundation for the "Kleiner Mod". There is absolutely no way to remove even these shorter bolts (and it's going to take some effort to keep them from rotating when I tighten the couplers, 'cuz the wrench won't fit!). Also, getting the nut on the back of the #10-24 bolt that serves as the trunk latch pin is going to be a real bear.


John

Question about this. If you can't access the body bolts after the drop trunk is in place, can you use overriders (bumperettes, if you prefer) on the rear? the body bolts have to thread into the overriders, so I think you need access to the body bolts and I don't think you could use the combination nut solution. Has anyone dealt with this?

[Mark Eaton]

Here is what I did with mine

https://thefactoryfiveforum.com/attachment.php?attachmentid=78889&d=1515465848

https://thefactoryfiveforum.com/attachment.php?attachmentid=77111&d=1511674635

[phileas_fogg]

Question about this. If you can't access the body bolts after the drop trunk is in place, can you use overriders (bumperettes, if you prefer) on the rear? the body bolts have to thread into the overriders, so I think you need access to the body bolts and I don't think you could use the combination nut solution. Has anyone dealt with this?

Chjort: Take a look at Carl's build log (https://thefactoryfiveforum.com/showthread.php?9583-Carl-s-Mk-4-Roadster-Build&p=282079&highlight=rider#post282079), post #677. Basically, you make a plate that screws onto the overrider, and that has a hole in the center for the all-thread. The crux is getting the plate sized correctly so that the hardware is hidden.

Bob: I hope you don't mind how far we've strayed from your original post, and please follow up with pictures of how you installed your drop trunk on a completed car!


John

[BobsMK4]

Don't sweat dropping the tank. I saw threads about mods so you never have to and wondered what all the fuss was about, so I timed it when I needed to drop mine. Not counting jacking the car up.......12 minutes! And I have hard lines going to my tank, so probably a little harder than most. I didn't bother with access holes for the fuel level sending unit. I can spare 12 minutes if I need to change the fuel level sending unit. Which I did have to do.

I did do an access plate for the left side body bolts. But it's not like that is coming off any time soon.

I finally got around to doing the drop trunk mod. It took me a while, but I finally completed it. It would have been much easier to do this work with the body off, but I decided to this mod after my car was done. It was pretty straight forward once the trunk panel was cut using a couple of different tools. I bought the same aluminum that was supplied with the kit. I made the box 14” x 24”. I bought a metal bender from Northern Tool and it worked pretty good. I installed the side pieces first and then the bottom panel. I had to drop the gas tank in order to install the bottom panel. That was the hardest part of the job. Before recarpeting, I put down some thin sound insulation. I know this was probably overkill, but everything else is insulated. I also installed aluminum angle on the 3/4” square tubing that run over the top of the drop trunk area. It looks a little more finished off. After putting my tool box, jack, tire repair kit, mallet, rags, gloves, I have so much extra room for my car cover and other misc. stuff. I want thank all you guys for responding to my post with photos and advice on how to do this mod. If you’re going to do much traveling in your roadster, you need to do this mod.912299123091231

[phileas_fogg]

Nice touch with the aluminum; well done!


John