Archive for October, 2008

Scissor lift prototyping

October 17th, 2008

Ok, with the cap out of the picture, it’s back to popup designs.   I’ve played with a number of schemes for the popup to happen.   Perhaps easiest is to do something similar to the “hinged” popup of Vanagons and Eurovans. They aren’t really hinged, but have a compound scissor lift which lifts the rear portion a bit (about 1 foot on Eurovans)

This works pretty well, and has the advantage of being relatively stable laterally because the low end is only raised a bit and the canvas acts as a shear preventer.

However, the angled top limits usable interior floorspace.   So, to do a full popup, I need 3 (or maybe 4) scissor lifts.   Using 1″ sailboat T track and track slides, I can create a simple scissor lift.   It’s made out of 6061 aluminum so it’s relatively light.  Here’s the prototype collapsed.

and extended

It’s quite smooth and is also fairly stable to torsional forces.    Side to side, not so much, so that’s why I need a 3rd scissor on the end (and possibly on both ends).   I will most likely add gas springs to make lifting easier.  Maybe even some compressed air driven gas pistions (as I’ll have onboard compressed air).

Now I have to figure out the mounting of this, as well as the placement of the canvas to keep everything flashed properly so it’s weathertight both when deployed and when driving down the highway.


It’s a drag…

October 16th, 2008

Sorry to give you a heart attack, Willem and hdegroot.

I’m concerned about the extra drag that the high cap will introduce.   Trog is already underpowered, with the B30A engine only producing 125HP or so.   So, I did some modeling of the drag calculations.

Google spreadsheet here.

I had to make a number of assumptions, some of which I think are a bit suspicious.   However, I believe the general shape of the curves are accurate.    See the spreadsheet for values and some notes on uncertainties.

Speed vs HP

I added 500lbs to the high-cap version to account for the new walls and windows.    At 60mph, the practical top speed of Trog due to engine RPMs, there’s an additional 16HP required to push that cap thru the air.   When laden with an upper limit of 1500 other camper conversion junk, it’s pushing 100HP.   I’m not comfortable running Trog that hard.

On a 6pct grade, Trog already bogged pretty severely, with top speed being around 30mph.   With the extra weight it’s much worse.   Since speed is reduced, the extra windage caused by the cap doesn’t matter as much.

I encourage readers to check my spreadsheet and look for anything that seems out of whack or missing.   (the 30% mechanical losses seem way high to me).

So, it’s back to popup designs.


Topless Trog

October 15th, 2008

I’m finally back from six weeks and 6000 miles driving all over the American Southwest.    It’s time to start tackling the popup.

Removing the top was surprisingly easy.   30 minutes to remove about 30 bolts and 12 screws.   The mastic seals parted with a deadlift.     The top is made of aluminum so it’s quite light; two people can easily lift it.

I’m thinking of permanently raising the roof 3 feet, to give me standing room inside.   It also makes interior layout much easier.   Here’s what it’d look like.

I think it looks pretty good, keeping the boxy military lines of the original vehicle.   The new walls would be framed in aluminum, possibly with Alucobond composite panels.   Alucobond panels are 0.020 aluminum skinned foam.   It’s a bit lighter than the 0.100 or so solid aluminum skin I would use.   Windows are Motion Windows 1600 series.