How can I join this? - Bike Coursework/project - Woodworking Talk - Woodworkers Forum
 
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post #1 of 4 Old 02-13-2018, 01:00 AM Thread Starter
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Question How can I join this? - Bike Coursework/project

Hello everyone,
I am currently making a children's balance/walk bike for my final project. It is made from wood and steel. The front frame is made from five 12mm CNC routed plywood parts which have been glued together and sanded to shape. It is then connected to two solid wood cylinders at the bottom at top which holds the seat post. Connected to the seat post cylinders is a steel frame.

My problem is how I can create a strong enough joint for the top cylinder shown in the images. The bottom cylinder connects with glue and screws which should be strong enough to take the weight and pressure from a young child. The top doesn't have any obvious points for screws and glue its self is not enough. What can I do?

I've thought about a knock-down fitting but because of the curved edges, it would be very difficult to do cleanly.

Please help?

Note:
- The joint is wood to wood (thick plywood).
- The seat post cylinder is 90mm in diameter and roughly 50mm in height.
- The front frame is 40mm in height and 60mm in length (five 12mm plywood pieces glued together to create a solid piece).

Any feedback would be very helpful, I am avoiding changing much of the design as I have already started building and have deadlines. Please reply if you have any ideas as soon as possible.

Thanks a lot,

Alex
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post #2 of 4 Old 02-13-2018, 05:13 AM
where's my table saw?
 
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Did you study physics in school?

You should have made a mockup in scale to determine the types of forces acting on the joints so you could better prepare for what you are asking us. I can tell by looking at it, the bottom wood laminate wants to pull away from the cylinder section, so steel straps or "U" bots would work better than screws which will pull out over time. Any joints in "tension" should have steel reinforcements. Compression joints should be fine with screws. Joint that are subject to rotation or twisting forces need reinforcement also.

That design should be subjected to a free body diagram of the forces before you start your design and construction phase. Bicycles have many triangular sub sections because they are the strongest shapes, but the joints must also be secure which is why a bicycle frame has brazed socket type joints OR it's a monocoque type construction, no separate pieces. Like this:


Many more ideas here:
https://images.search.yahoo.com/sear...g&action=close

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 02-13-2018 at 07:03 AM.
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post #3 of 4 Old 02-13-2018, 06:29 AM Thread Starter
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Thank you for your feedback.
The design is slightly different from the CAD drawings shown. So what are you suggesting for the joints? Using steel straps or U bolts? How would they fit?

The bicycle is made for a young user, so it doesn't have to take too much weight. Testing has been done on a model but i'm now looking for the most effective way to strengthen the weakest points.

--
Alex
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post #4 of 4 Old 02-13-2018, 12:12 PM
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modify the clamp on the seat tube so you can attach the wood.
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