Project chopper

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Dec 29, 2016
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Welded the fork together today and brought the bike out to see if it's strait.



Seems ok, but it's very flimsy, because most welds are only tacks.

 
Joined
Dec 29, 2016
Messages
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Location
Lanc's, England
Most of the welds got more tacks on today and a temporary seat and handle bars was added, so for the first time I put weight on the frame.
The frame seems rigid enough, but there is some flex in the fork.
When I sit on the seat the top of the fork drops about 10-20mm, it would give a comfortable bounce when you ride, but what would happen if you hit a pothole?
I knew that my material choice was on the weaker side, that was one reason I decided to design the under fork hold down from the head tube.



How much flex is allowed in the fork?
The remedy is to cut the fork in half and insert a 0.5m or so of 22mm tube and weld it all up again, but then you add more weight. Hmmm...
The bike as the pics but with temp. handle bar and seat weighs just under 20kg, so finished with battery, between 30-35kg? I'm pleased with that.
 
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Oct 19, 2012
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Are you certain it's the forks that flex or is it the main chassis or a bit of both? It's hard to see the lower fork mount but it looks fairly flimsy with scope to give.
 
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here is probably some flex in both, but mostly in the fork, if you bounce the front wheel you can notice a bend which springs back immediately.
The fork tubes are 25 x 1.6mm tubes which I thought from the beginning could be a problem.
I bent a sheet of 5mm steel into the lower fork holder and that is strong enough and also reaches lower down the forks than a normal brazing.



The flex in the fork starts under the lower support.
I have a 2m length of 22mm tube, I need about 1m for the handlebars and risers, so about 1 m is over.
One way to strengthen the fork is to cut the 22mm tube into about 15cm lengths and push them down the tube and leave 5cm or so between each piece, to keep the weight down and spread the support, the pieces will be welded from the outside through drill holes, just a thought.
I think I will continue with the build and face the problem later. But I do rather like the bounce.
 
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Given that info it does sound like you need some support down there. I'd expect a permanent curve to set in before long without it. It seems like a good place to make a composite. ie reinforce with ally or carbon tube rather than steel and glue it in. You'd get some of the give of the steel with some of the rigidity of ally or carbon, not unlike adding rebar to concrete to get a product superior to either on it's own.
 
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Because of the length you keep that flex. You can ad a second one next to it, that helps and you don't need to do it over the complete length.
Inside works also, but one next to it ad a small distance, works better.
 
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Great build , get it on the road ! you may be ok , mine looked very similar and rode fine for years :-



I liked the bounce brought a smile t my face watching it , It was so much that over time the weight of the cable nipple on the front brake cable fatigued and snapped the cable at the clamp however forks were ok.



Maybe it is the other end you need to worry about :whistle:

Paul
 
Joined
Dec 29, 2016
Messages
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Lanc's, England
Thanks popshot, emil and stormbird.
Went to bed with a brain storm and this morning my solution is to add a "downtube" from the seating area to the bottom bracket,
It will stabilise the frame and also simplify the fixing of the bottom brackets tube. Then to finish the build and if a problem with the bounce, replace the fork legs with new tube, with a 2 or 2.6mm wall.
 
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