Child’s First Pedal Bike

Child’s First Pedal Bike

Having previously made a balance bike I now need to follow this up with a pedal bike. This has proved to be quite a challenge. Having come up with a design from the child’s existing measurements using BikeCad I needed 14 inch wheels. These are almost impossible to find as is a suitably small chain set. Eventually after much searching I decided to buy a donor bike for some of the parts. I wasn’t able to find a suitable one in a hurry on Ebay so headed to my local Argos (other retailers are available) and purchased  very heavy 14 inch bike. Like all similar bikes it has very fat tyres and weird caliper brakes that make contact on the undersurface of the rim. The chainset is basic press fit with cheap bearings and lightweight 1/8 inch chain. Surprisingly the bottom bracket was a standard adult width and the dimensions of the bottom bracket very close to a standard PF86, though not quite. Having started down this road it looks like I will have to bite the bullet and use the wheels, chainset and brakes and maybe the chainguard.
This has all lead to a series of first for me in terms of manufacturing custom parts and components. First off was the press fit bottom bracket which I had to make by boring out a solid mild steel bar. The press fit cups are from the donor bike.
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The next step was to make the forks. As I have fancied the idea of building a fork crown with two parallel plates for I while, I thought I would experiment with this. This involved using graph paper again to make a full size drawing, then using some flat mild steel bar which needed to be milled to flatness and bored at a slight angle (as per drawing) to take the fork legs, for which I used standard 25.4mm frame tubes. This was all done using a milling machine and was a LOT harder than I anticipated.
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I used a slitting saw in the lathe to cut the ends of the fork legs at a slight angle as this was the most accurate way I could think of. It was a relief that the tiny forks would fit in the fork jig for brazing.
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Probably over engineered!
The next problem I encountered was fitting the frame components in the Anvil Jig which is clearly not designed for small bikes. The bottom bracket fixture would not accommodate the diameter of the bottom bracket so I had to turn some spacers to fit.
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I could then proceed to mitre the tubes in position.
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The tubing was all Columbus Gara which is plain gauge so no worries about cutting off the butting. I brazed the seat tube to bottom bracket first in the jig, then tacked the down tube and brazed that in a stand before returning it to the jig to check straightness and tacking the top tube before brazing that in a stand. The jig required removal of some securing levers to reduce it’s size to accommodate the frame but it just about worked. I had also put in a brass tube in the top tube for internal brake cabling before brazing the top tube in.
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I had decided on 22.2mm tube for the chain stays, probably overkill but narrower tubes are harder to find and I thought 14mm a bit small, using this for the seat stays. Because of the fat wheels I had been forced to buy the chain stays would need bending so I would need a tube bender. Having purchased a “cheap” bender that would do up to 1 inch tubing, and paying heed to a video on YouTube I found the bending quite easy though the quality of the finished bends leaves something to be desired.
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I initially made a 90 degree bend in a section of tube which enables you to see the bend radius and the exact point the bend begins compared to the start point of your length of tube. Also by marking a series of lines along the tube say 1 cm apart, you can re-measure the same distance after bending and see how much the tube stretches to allow you to calculate how much length you will need, so this may save you wasting tubing. I also did a simple full size drawing of the rear wheel noting the desired straight line chain stay length and using the curved template tube was able to easily work out where to bend the tube, bearing in mind it would have to be bent twice. Bending it bit at a time and offering it up to the drawing until it looked right, then the second bend in a similar fashion. The second stay is a mirror image so the same bend degree can be used until it matches the shape of the first one. The problem with cheap tube benders is they do not have a degree scale so you need to memorise the position you got to with the first bend in order to repeat it.
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As the bend in the tube was near to the prospective mitre I was not able to clamp it adequately for a machine mitre in the lathe (which I had used for the main tubes) so it was back to paper templates. I brazed the stays one at a time because of difficulties clamping them adequately in the jig, but had another problem to overcome in as much as the rear axle spacing is 107mm! I then had to turn a new axle of 110mm to clamp the rear dropouts. I actually used the hooded style dropouts for the first time, usually used for Tig welding but easier to braze on this occasion because of the diameter of the chain stays.
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Next step is to check the spacing and adjust if necessary, also good idea to make sure the wheel actually fits, then move on to the seat stays. This also required a bend though not as drastic and again guided by a simple full sized drawing of the wheel.
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Having bent the seat stays I did the tricky mitres by hand then used the tracing paper method described elsewhere to transfer the mitre to the opposite stay to save a lot of time. Again to do this you need accurate centre lines on both stays to line up the transfer paper just like with any mitring operation. I have found the easiest and most accurate way to draw centrelines is to use a surface plate and height gauge. I can only accommodate a small surface plate in my space but find it very useful. You can scribe a line simply by lightly scraping along the top of the tube then running over it with ink, or measuring the thickness of the tube, moving the scriber down to half thickness and scribing along the tube lightly then again marking with ink if required. This last method isn’t in fact easy as the tube tends to roll but in the case where it has been bent it lies flat and doesn’t move.
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At this point I had thought the frame pretty much finished but on checking the spacing found the right chain stay had pulled outwards a bit. attempts to rectify this provoked a near disaster which is referred to in the Mea Culpa section of the site.
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Brake bridge added and yet again the fancy Anvil brake bridge holder was far to big to fit into the frame so back to the hard way.
After brazing that in and the seat collar, head tube rings and cable routing cover caps it is done. I forgot to take a picture of it cleaned up before I took it for painting.
I now plan to make a handlebar stem while it is away.

Aluminium Handlebar Stem
My first attempt at an aluminium stem for which I chose a ‘top loading’ BMX style design which I thought would be the easiest to machine.
Start with a drawing! (on paper in my case) then milling the aluminium block to absolute squareness.
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Followed by marking and drilling all the holes required.
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I then set up the handlebar and steerer holes on the lathe using the original drills to centre them. I used metric drill so the holes would be slightly undersize and to ensure they would finally be exact I finished them off with accurate reamers.
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I them smoothed the corners with a radius mill and set about cutting off the faceplate and the steerer slot with slitting saws on the mill. A bandsaw would be a lot easier but I don’t have one and although slitting saws do not give a perfect result they are a lot better than a hand saw.
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A little more shaping in the mill followed by sanding and a final polish. I found a tiny bit of light filing followed by a maximum of 240 grit emery strip to start was enough prior to final buffing of the aluminium.
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During assembly I ended up using the jig BB holder with the adapter I had made to press fit the oversize bottom bracket

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Final result

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In retrospect I would avoid a 14 inch wheel design and opt for 12,16 and eventually 20 inch which have a much better selection of wheels and parts.

2 Comments

Charlie Merivale

about 4 months ago

Hi Stephen, I hope all well. I had meant to drop you a line post Bespoked, but, as they say, life happened. I found this a really helpful article. I'm currently wanting to get a 650b in close, but at 42mm have clearance issues. Your article arrived just in time! I've also got lathe envy! Best wishes, Charlie.

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Stephen Hilton

about 4 months ago

Dear Charlie, I'll post my observations on the tube bender, may put you off! With 3 days at Bespoke you must have contracted the 650b virus. Stephen

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