Thursday, May 16, 2013

Part 2: Home-built Xtracycle/Big Dummy seat

Seatbelt magic resides in the holes.
We left off last time with a more-or-less finished deck seat assembly. I used the seat as it appeared in that last post for over four years without an integrated seatbelt system. Instead, I used a length of one-inch wide utility webbing and an adjustable buckle purchased from REI; something very similar to, but not quite exactly like this. I ran the webbing under both v-racks (over the rear fender), with each end of the webbing coming up behind each side panel of the seat deck assembly, then over her lap with the buckle in the middle of her lap. My eldest daughter was three years old when she started riding in the seat, and this configuration worked fine for her at that age. Yet, I always thought there was room for improvement.

A piqued interest of my youngest daughter in riding the Big Dummy recently hastened seatbelt improvements. She is now about 15 months old, and although she has enjoyed riding in our old Burley trailer, she wants to do what the big people are doing and has very recently become very energized about riding on the Dummy. She did a few rides with the old seatbelt system, but it seemed that a five-point harness was in order.

I didn't want for screw heads or other hardware protrusions to stick out of the areas where her body would be in contact, so I devised the following system using easy-to-source webbing and Fastex-type fasteners. There is perhaps some detail missing below, but this is a new development and still in the process of being honed. As I get it dialed in, I'll post more photos.

Here we go. I used a one-inch diameter drill bit to make holes through which I inserted 3/4-inch webbing. Note that I would have preferred to use one-inch webbing, but 3/4 was what I had on hand. depending on how things work out, I may reconfigure the webbing, which shouldn't be a big deal. In any case, the procedure follows.
Pilot holes first. For the shoulder straps, I drilled two holes six inches apart on center, centered across the backrest. Each hole is 2 1/2 inches down from the top of the backrest. 
After drilling the pilot holes, I drilled with the 1" bit part of the way through the back...
...and the rest of the way through the front so as not to rip up either the front or back face of the wood upon exit.
For the seat belt, I located the hole center about 2 1/2 inches above the deck surface, centered laterally on the side panel. Same drilling procedure as above. 
After the holes were drilled, I sanded and smoothed their edges to reduce the potential for wear on the straps. Next came installation of the straps. I used some 3/4-inch nylon webbing that conveniently already had buckles installed, with one side adjustable and the other side fixed. Three sets of webbing and buckles are needed, as are 2 Fastex-type three-bar sliders, 1 D-ring, and 1 snaphook. First the seatbelt.
Sorry for the photo that doesn't quite focus on the seatbelt, but hopefully the threading pattern is clear. The goal is a snug fit at the passenger's hips, just below the iliac crest; not at the waist. A webbing/buckle combo is shown in the foreground. 
This is just a reference photo, showing a three-bar slider. These are used to attach the webbing with the fixed side of the buckle to the shoulder strap holes. I made sure to configure any non-smooth or hardware side of the strapping system away from body contact with the passenger.
The anchor strap for connecting the shoulder harness to the deck passes between the legs of the passenger. It's necessary for it to be quite secure. I could have opted to use a hole-based system as with the other straps, but instead I used a bolt. Based on some measurements and test fittings, I drilled a hole 6 1/2 inches back from the nose of the deck. I fastened stainless steel hardware, in this order from the top down: bolt, flat washer, webbing (two ends of a simple loop doubled over with the d-ring at the apex of the loop, and with holes pre-drilled for the bolt), wooden deck, flat washer, lock washer, nut. This will make more sense with the following photos.
Drilling the anchor bolt hole. Anchor bolt is in the foreground.
Anchor strap installed. Note the position of the d-ring. It's easiest to drill the webbing when it is sandwiched between two scrap pieces of wood. Making the anchor strap adjustable might have been nice, but is not entirely necessary.
Underside of the anchor bolt.
I measured and cut a slit in the foam pad through which to work the anchor strap.
Everything in place, but yet to be adjusted. The Fastex snaphook clips to the d-ring, and has a length of webbing running through it. Each end of this piece of webbing has the adjustable side of each of the the two shoulder strap buckles. The two three-bar sliders are at the top of the backrest, holding the shoulder straps to the backrest. 
A view from the back. Note the back of the seatbelt runs behind the seat, and the shoulder straps loop through the backrest. The goal of no hardware protruding into the passenger space has been met.
The first ride with the new strap system. It apparently gets a thumbs up. Note that I left the straps long for adjustment. Once they're dialed, I'll trim them. The handlebar holds a bell and a bottle cage; both well appreciated by diminutive co-pilots.
Hopefully, this presentation has made some sense and is not too difficult to follow. I can affirm that there will be at least one more installment of this series, with some more detail and updates regarding the strap system, among other things.

If this all seems like a lot of effort, well, it is to a degree. However, I personally consider the ability to safely carry passengers the defining element of my Big Dummy experience. Carrying kids and cargo has radically reshaped my biking life and made for some of the best times I've had on two wheels.


  1. Nice work, Andy. Looks awesome! If I ever get my Big Dummy over to the States, I'll make an order!

    1. Thanks, Cass. I still have a few kinks to work out on this version, but it's a great tinkering project.