Previous Project Hellraiser Update

You know you are in trouble when your wife casually asks how hard it would be to add one more seat to the buggy. Actually, we have talked about having another child on and off for a while.  My biological urges were satisfied when we had our son but the female biological urge is apparently much stronger.  Anyway, where does that leave the buggy? Well, adding enough room to seat 4 has some challenges the biggest being packaging the rear seat and providing enough leg room for people to sit back their in relative comfort. I started looking into the issue with the following goals in mind:

1.  I would like to maintain or improve on the buggies performance in the rocks (overcoming the effects of the added weight and wheelbase will be tough).

2. Make the rear seating comfortable enough for a small adult to ride out back to and from the trail. Realistically speaking need enough room to seat a 3-5 year old and a child's car seat but the option for mom to ride out back could come in handy as both of us could keep an eye on one child.

3. Utilize the existing chassis and drive train (although an axle swap is an option if needed).

After some tinkering with my Project BMP model I determined I should be able to add rear seating and get 12" or so of leg room with a 5-6" chassis stretch.  A custom rear bench seat (with the seat area shortened by a few inches) looks to be a better choice over dual pre-teen sized seats as it eliminates the wasted space between the seats and would allow an adult to sit sideways if they needed a lift to or from the trail. 2 more full sized adult seats would require at least 12" of chassis stretch which is something I am not willing to do and I don't think I need.

The front end of the buggy will remain unchanged from the back of the driver seats forward. Everything behind the drivers seat will be removed starting with the upper, middle and lower main hoops that make up the rear end. These will be replaced with wider hoops that splice into the existing cage.  The new hoops will go straight back for 6 or so inches before gently tapering back to the rear. The existing shock mounting bar needs to move back at least 6" and is the biggest driving force behind how much leg room the rear seat will have. I plan on changing the 16" long coilovers out for a shorter 14" travel coilovers to allow for the shock to be mounted below the rear "deck" line.  This should simplify the upper tubing since I won't need a shock mount and associated bracing sticking up behind the rear seat. 

Clearance between the rear tire, rear seat and shock will be critical.  I won't know till I re-position everything whether or not I have a clearance issue but I can provide more clearance using either wheel spacers, a wheelbase stretch and by adjusting the coilover mounting position. I would like to go no more than 110" on the wheelbase if possible (currently at 108"). I considered adding a rear steer axle at this point as it would help compensate for the need to stretch the wheelbase out but getting a steering axle out back would be tough as you need enough room to turn the tires while the are stuffed at maximum compression. So for now I'll re-use my existing rear axle and try the longer wheelbase.

Obviously the fuel cell is a problem and I plan on having a custom cell made to optimize the space behind the rear seat and sit as low as possible in the chassis.  Whatever space is left above the fuel cell will be the designated ice chest zone.

Another problem is the swaybar. In my model things look too tight to fit one so I will have to play that by ear.  Since I will be adding some more weight up high I think body roll will get worse so I plan on playing around with the upper links in an effort to get a higher roll axis. I am able to change the upper link mounting positions at the frame end by changing the exhaust so it exists via a hole in the passenger boat side just below the nerf bar. The first picture above shows my present link geometry and the second picture shows the proposed geometry.  You can see that by adding a bit more triangulation to the upper links, then moving them upward I was able to move the theoretical roll axis up about 3 inches.

I had considered going with a  3 link & panhard bar which would have allowed me to easily raise the roll axis 6-9 inches and possible eliminate the need for a swaybar but After some consultation with friends I came to the conclusion that the 4 link was going to be a more robust option.  The biggest downfall of the 3 link is the fact that there is only one link on the axle controlling axle wrap  which puts a lot of stress in that links mounting brackets.  Super light moon buggies can get away with it but a 4000 lb trail rig is another story.

   Back to the tubing; once the new rear hoops are in place I I'll cut out the cross bracing behind my seats and replace them with two diagonals that run up from the corner of the chassis to a point on a new cross bar that is roughly centered above the front seats.  What this does is opens up the rear legroom while still bracing the cage from side hits. i also plan for a diagonal cross brace behind the rear seat.

That about covers the plan.  As far as timetables, since our club is pretty much done wheeling for the season I plan on getting started right away.  This schedule seems aggressive until you consider that I only have 12 weekends to work on the buggy before the fall wheeling season starts and my wife is working 6 of those weekends.  Let the fun begin!