Intro: Bouncy Chair With MTB Suspension
Ever wish your garden chair was more, uh, bouncy? Me too. So I set about making myself one.
Step 1: Stuff You Need
About 9 metres of 25mm square tube.
About 2 metres of 20 x 5mm or similar for the slats to rest on
Wooden Slats (I used left over shiplap from my shed build)
2 x M8 bolts at least 60mm long. I used ones that were only threaded at the ends, but you could use fully threaded.
MTB spring suspension. Amazingly, these are only about £6 on eBay. The springs are a bit too powerful on the cheap one that I bought, but if you want more bounce you could spend more and get less powerful springs. Or maybe get one for a child's bike. You can replace the springs easily.
Pieces of wood as temporary props
Welder (if you don't have access to one you could just bolt it together)
Angle grinder with cut discs and sanding flap disc
Step 2: Plan
Before you can get on to the fun parts of cutting and welding you have to come up with a plan.
I used sketchup, the free Google thing. This was my first go and it took me ages because of my impatience and insistence on trying to figure everything out for myself. Anyway...
I knew roughly what I wanted and used the rough measurements from a comfy chair I have. While it was annoyingly painfully slow to use sketchup, at least once it was done I could just read the measurements off to make a cut list.
The red bit is where the spring will go.
Step 3: Cut
Here's the cuts I used. In retrospect the base could be much shorter, but here's what I did anyway:
25mm sq tube
Ideally you should cut these at 45 degree angle for welding.
Base (800 x 500)
2 x 800
2 x 500
Crosspiece beneath the spring 1 x 450
Uprights 2 x 450, 1 x 400
Seat (450 deep x 500 wide)
2 x 450
2 x 500
Back (600 high x 500 wide)
1 x 500
2 x 600
Attachments for suspension 2 x 140 approx
10 x 5 steel
2 x 550
2 x 320
Step 4: It Is a Base Thing..
Weld (or bolt) your base.
As you can see from my welding, I am new to welding. That is why the base was welded with square butt joints, rather then the 45 degree angles I should have used. Oops...
At least I made sure everything was clamped properly and the angles were good.
Don't do the crosspiece yet for 2 very good reasons:
1) We are going to use the inside of this base as an excellent jig for the uprights.
2) As the spring supports need to be pretty straight we are going to bolt it all together to eye the correct placing of the crosspiece.
Step 5: Uprights
Curse my impatience! I got carried away and forgot to photograph this stage. Grrr! Hopefully you can see the uprights in this photo and make sense of my crap drawing.
Put a slight curve on the top of the 2 uprights. I did this with an angle grinder and then a bench sander. If you don't have these you could use a hacksaw and a file.
Drill an 8mm hole near the top of the uprights on the apex side of the curves (see photo). This is for bolting to the seat, so both need to be level with each other.
Using the inside of the base as a jig as in my crap drawing, clamp the 2 uprights at the side of the base and weld the crosspiece with enough clearance for the bolt holes.
Weld the uprights in an, uh, upright position at the front of the base. You can see this in the photo. It is important that they are both properly upright and equal as they will be carrying weight and a bit of bounce. I achieved this with welding magnets, clamps, combination squares, timber props, swearing, shouting, praying, cigarettes and patience. It is obviously important to weld tacks first and keep checking. You also need to make sure these welds are strong.
Step 6: Seat
Weld the base together first, 450 deep, 500 wide.
Then weld the 3 back pieces ensuring they are properly square (bit obvious that one, sorry). I'm sure there are lots of clever welding ways to achieve this. I don't know them. I used clamps and a piece of 450mm timber clamped in the empty end to keep it square (see next step to see the timber in place).
Step 7: Even Seatier...
Weld the back to the seat.
Once again, I'm sure there are lots of excellent techniques for achieving this. Unfortunately once I get cracking I'm too impatient to look them up and just figure out a clumsy way of getting the job done.
You need to figure out the angle you want the back to be in relation to the seat. I went for a straightforward 90 degrees as it was simpler and my seat is going to be leaning quite far back.
Having done the build, 90 is not enough for a relaxing chair. I would suggest 110 degrees. I'm going to cut the back off and re-weld it at 110.
As you can see from the picture, I held the base in a vice then clamped the back to it using steel props to keep the back correctly aligned and the timber prop (mentioned in the previous step) to keep the back uprights correctly spaced.
Step 8: Angle Schmangle
30 years ago I did advanced mathematics. Quadratic equations, calculus, the whole thing. That was a very long time ago. The only thing I remember was the teacher's haircut. So, instead of using maths to figure out the required angles we are going to use a piece of wood...
Bolt the chair to the uprights, position the back of the chair into a pleasing angle then mark a piece of wood, cut it to size and use it to prop the back up.
This gives you the correct spot to put the crosspiece on the base, mark the sides of the base.
Get another piece of wood and hold it against the side of the chair, parallel with the back. Using the seat as a guide draw a line across the wood, giving you the angle you need. Cut this angle and try propping the chair with this piece of wood. You may need to try a few times until you get the angle perfect so that the prop meets the bottom of the chair perfectly and is perfectly plumb. This piece of wood now has the measurements for the spring supports and the angle for the top of the top spring support to meet the chair.
Step 9: Springtime in London
Use the wood to mark the correct angle on a piece of square tube and cut it. Check it fits nicely.
Use the length of the wood to give you the length of the spring supports (remembering to allow for the 25mm of the base crosspiece, and to measure the distance between the bolt holes on the spring unit and the supports, NOT the total length).
Cut both supports to size and drill holes suitable for the suspension bolts.
Weld the square support to the base cross piece and the angled support to the seat (making sure it's centred and you angle it the right way round!).
Weld the crosspiece to the base using the marks you made earlier.
Grind/sand/cut the spring unit slightly so that it just fits inside the supports.
Use the bolts that came with the suspension to bolt it to the supports. I had to use a couple of washers to get the correct thickness.
Step 10: Strutting Your Stuff
Now we need to weld some struts to support the slats.
Mark the seat and back at the thickness of the wooden slats. This is so that the slats sit flush to the steel.
Cut the steel to size, allowing a bit of a gap next to the uprights so that it can still move.
Mark the struts at suitable places for screw holes for the struts. I allowed 1 hole per slat for the seat, 2 for the back.
Drill the screw holes.
Clamp tightly and weld in place.
Step 11: Finishing
I like a nice shiny finish, so I sanded it with an angle grinder flap wheel, then sprayed it with clear coat.
Step 12: Slat'll Be the Day.
Cut your slats to size, screw on.
For now I used some shiplap left over from my shed build. I will probably replace it with something that looks a bit better eventually, but for now I wanted a quick finish so used what I had.