Wednesday, 26 November 2008

Reinventing the wheel

Profile Racing disc compatible bmx hub in all it's glory. Mdf wheel jig, bolts adjusted to position rim on centre line of hub.
Black tape on spoke identifies the 'key spoke'
Lacing up as per the instructions
All spokes laced up
Spokes fully tensioned
The finished wheel.. just needs a frame
As shown in the diagrams in the last post one of the main features of the frame design is that it uses a bmx rear hub with its 110mm dropout spacing to reduce the width of the parallel swing arm design, allowing it fit within the crank arms without fouling them while pedaling. When built the rear wheel will be a bit of an oddball.. 26in mtb rim laced to an 110mm bmx hub.

It took a while finding the right hub as I wanted a bmx hub that was disc compatible and they’re bit of a rarity. Finally I found that Chris King and Profile Racing each make one. The CK one is bloody expensive where as the Profile one was just expensive. I went for the Profile hub with a 14mm bolted axel to provide a really solid connection that should stop the sliding dropouts slipping and help stiffen the swing arm laterally. Luckily when I ordered the hub the exchange rate was pretty favorable.. still got done on the import duty though.. damn!

A mate donated a rim to the cause.. cheers Tony. It was used but still true. The local bike shop worked out the spoke length required for the 26in rim / 110mm hub combo.

If you’re thinking on building your own wheels and haven't done so before check out Sheldon Browns guide: , its really informative and takes you through not just how the wheel is built, but the why too.

As I didn't have a wheel building jig and I couldn't use the frame to check alignment I knocked one up out of flat sheet of mdf and some bolts. The bolts allow the rim to be lifted to the centre line of the hub and by flipping the wheel over you can double check alignment.

It took a little longer than the instructions suggested but by following them I was able to build my first wheel with out too many troubles. Its just a case of take your time, don't rush ahead and start tensioning spokes too quickly like I did, and if things start to go wrong go take a timeout.
As far as I can tell the wheel has come out pretty well. I'll probably get one of the local bike shops to check it over before it gets used for real, but I think it should be OK.. in fact it’s in a hell of a lot better condition than all my other wheels!
All I need to do now is build the frame otherwise all I've got is an expensive wheel that doesn't fit anything.. useful!

Monday, 3 November 2008

The frame design: reasons for the way it is...

Above are some diagrams outlining the design aims and the design feaures of the frame. Also an exploded diagram of the frame shows how all the bits will hopefully fit together.

Monday, 20 October 2008

Workshop tour

Now i'd love to proudly show you around an extensive workshop.. you know the kind, lots of useful tools hanging off the walls, a sturdy workbench, with a lathe or mill in one corner and maybe a knackered old arm chair next to the beer fridge in the other but unfortunately that's not going to happen (anyway i wouldn't know what to do with all the fancy equipment). Instead take a look at the sharp end of the WMW setup:

Inventory so far:
Collapsible workbench - two workmates and a reinfor
ced sheet of mdf
Tool box - collection of tools slowly growing :-)
Pillar drill - bought while trying to drill holes in some ba***rd tough steel sprockets
Two gas torches (more on them later)

Workspace - backyard with tarpaulin stretched across it

I've only recently rigged up the foldaway awning after getting the idea from the Instructable website. Previously the seemingly default wet weather in Wales had been making working out in the backyard at weekends a bit unpredictable. Also the addition of an inspection lamp has given a bit more flexibility as the nights draw in. But it wont be long before i'll be wanting a more robust setu
p.. maybe with a solid roof and walls. The only reason i haven't invested so far in some kind of shed it because i'm currently in rented accommodation and don't really want to put money in to something i can't easily take with me.

Lastly the welding kit.. Ideally i'd like to have got myself an oxy-acetylene welding kit because that is what i have learnt with at college and i know i can achieve some decent fillets. But living in a residential area i didn't really want the responsibility that comes with the acetylene gas. Instead i have bought two other types of gas torch which still require respect in terms of safe use and storage but they aren't as scary as a bottle of acetylene.

The first torch is an oxy-propane set (see pic below) that can apparently achieving a flame temp of 3050 deg Celsius in favorable conditions. This is a surprisingly small kit, and i've heard it will rip through the oxygen rather quickly, so i plan to use this only for laying the fillets and use a second propane torch for pre-heating the joints.

The second torch is a propane torch (see below) that is meant to be suitable for brazing (with idea conditions.. probably not a windy backyard then!), so hopefully between the two they will be man-enough to fillet braze the frame.

My plan is to prepare all the pieces of the frame and build the jigs in readiness for welding. Once this is done i'll then have a play around on any left over scrap metal as i need to practice a bit. Should it turn out to be a no go i'll go to plan B and see if i can weld it up at the local college using their equipment but with my filler/flux.

Wednesday, 15 October 2008

And so it starts...

Hello, and once again welcome to the Welwyn Machine Works blog! I'm going to be using this blog to document my first bike frame build, partly as a record for myself and hopefully as a resource for other novice builders (if only so they can avoid mistakes i'm bound to make along the way).

I've been toying with the idea of building a mountain bike frame for quite a few years now and only recently has it started to all come together. Back in 2003 i came across an article in the Sept/Oct issue of DIRT on Brooklyn Machine Works' FQ Mini link, and it just caught my imagination:

The expense of the BMW frames combined with a lack of riding prowess meant i could never really justify getting such a high end frame. However i enjoy designing and building stuff so maybe i could build my own version. Initially i was aiming to copy the main features of the BMW's race link - high pivot point, jack drive (two chain system) to isolate peddeling from suspension action and low centre of gravity - (see photo below.. you'll also probably have noticed where i got the idea for the name of the bike/blog :-)).

To cut a long story short between starting this project five years ago and where it is today there has been quite a bit of design progression - many ideas had and designs penned (i'll try to collect together info on the more intersting stuff at some point). Generally the designs were getting more complicated, which brings with it potential fabrication and maintanance issues.

The final round of last years Dragon Downhill series at Pen-yr-heol farm near Caerphilly , South Wales, really helped inform the design of the frame i am trying to build. The downhill race was held in mid October on top of a rain-swept hill. The course was freshly cut and it pissed it down all weekend, the track got broken in and then some. Basically it turned in to a mud-fest.. check it out: It was tricky riding, but damn good fun too. The mud was getting everwhere, clogging up wheels and destroying gears and grinding pivots. Massive mud clearance and drivetrain simplicity and minimal pivots would have been a real bonus here.

Basically i came away from that race thinking i should really try to simplify the design and construction as much as possible. Make it as easy as possible to fabricate with my limited resources and experience. I also took the decision to build this frame as a singlespeed - this was based on a few observations: i don't tend to shift through gears much on a downhill run (when i need to get back to the top i'll just push), and some of much cleanest runs i had during last years Dragon Downhill series were during the chainless seeding runs where the idea was to conserve and if possible increase momentum by pumping the track and laying off the brakes rather than relying on peddeling. This seemed to work for me as it forced be to think about whether i really needed to brake. Though occasionally it would have been useful to have had the ability to put a couple of pedal strokes in to get back up to speed if you slow right down e.g. exiting from a tight corner.

Therefore the revised design aims for this downhill bike were as follows:

  • 170-180mm travel
  • rearward wheel travel
  • single pivot simplicity (ignoring the fancy shock linkages)
  • maximum mud clearance
  • singlespeed - again for simplicity and quietness
  • constructional simplicity (nearly all the expierienced framebuilder advise on building a hardtail as first frame.. but wheres the fun in that!)

Finally.. below is an image (could almost be called an 'artists impression') showing roughly what the finished bike will hopefully look like. In a future post i may run through a few of the frame details and thinking behind them, until then this'll give you an idea of what i'm gunning for:

Sorry for the long introductory first post, hopefully from now on they'll be a bit more lightweight.. more pictures less text.