Thinking of rebuilding my Street Fox, looking for ideas.

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Sep 14, 2013
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eire
Hi all,
I built a Street Fox about 6 years ago, but never really got much use out of it because I found it just to heavy for the hilly conditions around here. I was planning on getting it bnack on the road again this year and low and behold but didn't a storm come along and collapse the shed it was in on top of it. This damaged the back end but most of the structure survived.
So my idea is to use the existing frame as the basis of a complete rebuild. My idea is to cut out the difficult to fabricate bits and use them as end bosses for a glued and riveted Aluminium stock rebuild. My framing is 40mm square so theoretically I could use 45mm square aluminium tube and mount it on the outside of the existing steel "bosses". Given a 20mm overlap and PU glue (the stuff used in car body paneling) and then rivets on all four faces. This should afford sufficient strength and act like a brazed boss. I would have to replace the back forks with an aluminium set of forks as this is where a lot of my weight problems come from as I used a real cheap Chinese donor bike.

At some point in the future I would like to add either a 250w electric hub motor or alternatively a nice 4 stroke petrol. The design would have to account for this extra speed potential and I was never happy with the utility of the front side pull brakes which would often allow the back end to lock up way before the front two engaged. So I would look to some sort of brake upgrade at the front - center pull at the least.

So just shooting ideas out there at the moment.

Shoog
 
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Oct 19, 2012
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I'd be wary of using aluminium in such a fashion. Aluminium diamond frame bikes have all the stresses sent down the tubing in either compression or tension where the material is strongest. The std diamond frame shape is an almost perfect one from an engineers perspective. All the welds have to do is keep the tubes together with little stresses being actually taken by them. A streetfox uses it's metal as beams where it's weakest and in such a design the welds themselves take an enormous amount of stress. Using aluminium as a beam is asking for failure and that failure will likely be catastrophic and without warning. When steel fails it often takes a long time to fail totally with cracks showing long before it fully lets go. Aluminium is much less forgiving going from fine to snapped very quickly. If you do go ahead with it I'd forgo the rivets. They'll add almost no structural integrity but will act as stress concentration points to initiate any failure. I'd expect a modern glue to do a good job without the rivets.

A quick calculation for deflection of beams shows you'd need to replace 16swg 40mm square steel with 45mm 8SWG aluminium with the aluminium sliding over the steel bosses to get the same amount of deflection or in other words the same static strength which appears to be what you are proposing. That would save a paultry 44g per foot allowing for one boss per foot or so on a Streetfox design with 20mm overlaps. Add in the glue and even that meagre saving is eaten into if not erased. On top of that, unfortunately it isn't static strength that's important but dynamic strength under loading and where steel bends a fair amount before deforming aluminium doesn't and is therefore a much worse performer.

A better bet would be to use TIG welding and make a thinner steel frame but I appreciate that TIG equipment is not cheap and there's also a learning curve to it. With care and practise you can MIG the thin stuff too though it's a harder job than with a TIG. Another way to loose a lot of weight is with a thin plywood frame with the ply made into a lot of complex box sections for strength.
 
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Jan 16, 2011
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323
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Vilvoorde / Flanders / Belgium
Speedholes are another way to save weight. But that requires or a fancy ass CAD program, or a lot of experience and gut feeling.

For brake upgrade. Sturmey Archer 90mm hub/drumbrakes ain't the end of the world in €€.

TIG doesn't have to be very expensive (but it never will be cheap). I'm doing most of my TIG welding with a €209 TIG
 
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You can save weight by changing a few things on the frame. Also the materials used, like for the seat and the rear.
For the rear, you can change it to an aluminum part from a bike.
I changed the steering design on my new trike and use an aluminum rear. It isn't a street fox, but I save a lot of weight. I also go for an other seat mot from wood any more.
The steering handle will be aluminum, instead of steel. Main part is still steel, but it is way less than I used on my other bike and because of the other design and also other wheels, I get o lot lighter frame.
 
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eire
Thanks for the input so far.
Ideas so far with cognizance of input:
-handbars changed to aluminium
-back frame swapped out for aluminium
-brake upgrade using better quality dual pivot calipers (any pointers here appreciated)
- chop the end of the front tube down to the adjusted crank mount and weld the crank in place with top plate alone.
-some sort of ultralight seat, possibly a plastic bucket with vent holes
- replace stearing coupling with aluminium square section

Still not given up on bossing idea but point taken about its potential minimal benefit. However if I went internal mount of new aluminium (2-3mm walls this would allow me to change out the seat support with aluminium stock which could bring savings. Will have to strip to the frame and get some accurate weights and calculate what savings are possible.

Keep those ideas coming.

Cheers.
 
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OK got a chance to take a good look at the Fox today. Considering that half a ton of roof pancaked on it its not in to bad a shape. Back wheel is buckled and I found that the steering cross members main weld is failing which may or may not be due to the impact. Its going to need a complete strip down and repaint to get rid of some rust areas.
However by my rough calculation the square section steel only accounts for about 6kg of the overall weight so there seems little point in trying to replace it.
However there are a few areas which can yield weight savings:
  • cut about 20cm off the front tube (I am six foot so its unlikely that it will ever need the front bit)
  • head rest support is made from two front forks welded together, so a 20mm square section would replace it for a neater job and a weight saving.
  • seat can be replaced with three smaller pads at bottom, small of back and back of head.
  • the steering assemblies have quite a bit of excess metal which can be trimmed back with no loss of function.
  • handle bars can be replaced with aluminium ones, allow for a better position.
  • back wheel frame could be replaced with aluminium if I can find one that fits
  • the crank peddles are cheap steel so can be upgraded to aluminium
  • grind back the thread on the steering rebar, this will allow for a neater look and for better paint - currently its bare and rusted
I think I will move the steering cross member back a few cm before welding it back on, the combination of its current position and the down-curve of the handlebars meant they were always a bit of a stretch.
I am wondering if its worth keeping the triple gear on the front crank as it never really worked that well and made the chain a pain to route.
An upgrade to the front brakes to a dual pivot, I have already extended the twin pull brake lever to give me more mechanical advantage.
Fit a 250w rear motor.
 
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You will never get it as light as the factory made trikes. But I noticed that thanks to the amount of gearing, I could get up bridges as fast as with my city bike. I know that race bikes will go up easier, but that is just the weak point of a trike.

As you want tork, than get a mid motor. They have more power to help you in the hills than a hub motor.
I have the tsdz2 250w motor, but for hills, is a befang probably better. It has more power.
 
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2.7kg in the crank assembly - plenty of weight saving to be had there.
Mostly stripped down and did a little trimming of the wheel assembles - 0.7kg a piece. There are a few more areas where weight savings can be made. Back frame not as heavy as I thought it would be so keeping that.
Front wheels running very notchy so a new set of barrings for those.
 
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Tyres can make a huge difference in weight but going light usually means narrow which means a harsher ride. Light often equals less puncture resistance too.
 
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Sorry I meant the actual wheel mount assembly itself was 700grams each - not the wheel and tyre.

Motors are expensive so I am discovering, and difficult to mount.
 
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I changed my front wheel mount assembly and save a lot of weight.


It is a lot lighter than I have now.

An e assist isn't that expensive anymore. For less than 300 euro you have a mid motor. Only need to ad a battery and you are ready.
Also is it very easy to mount. Befang is a bit more expensive but I am getting one from 330 euro.
Batteries are easy to build and cheap.
 
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A Bafang midmotor is easy to mount. A one on one replacement of the original crankshaft.
My kit took 2 months to arrive in 3 seperate boxes , but for €750 I have a 750W BBS02 , all the controls, 48V 17.5Ah "cheapy" battery and top end display.
 
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