Tam's Geometry Experiment - Hack Bike Derby 2016

Feb 15, 2016
by BTR Fabrications  
The 2016 Hack Bike Derby is an invitational race weekend for frame builders to let off steam and race shonky bikes which would otherwise have no place in their workshop. Let me get a couple of things straight before we get into this:

1. I (Tam) never weld a production BTR
2. This bike build represents some of the ideas I have, not the standard to which I feel it is acceptable to build a bike.
3. I don't think the BTR workshop has ever been dirtier than it was during this build.
4. No animals (except for me!) were harmed in the making of this bike.

Organised by Andrew Denham of The Bicycle Academy, the first ever Hack Bike Derby kicked off on the 6 - 7th of February 2016. The event consisted of a dig day on Saturday, followed by a (badly) floodlit night dual slalom race that evening. On Sunday, there was a DH race, followed by a Le-Mans-style mass start fire-road race. All of which was to be raced on the one (generally) unsuitable bike of your own manufacture. The event winner was the rider with the best overall finishes in all three events.

So you think you ve seen extreme geometry
So you think your bike has extreme geometry?...think again.

The bikes themselves had relatively few regulations; they were to be raced as klunkers, so that was a vague guideline which most followed, their bikes sporting extra top tubes, extra fork blades, crazy seat stays and wild handlebars - I think 900mm wide was the norm! The maximum build budget was recommended to be less than £300 all in, which seemed to be widely adhered to, though I did spot a set of brand new Shimano XT M8000 cranks! There were to be no gears, and brakes had to be rim (preferably not vee), drum, or coaster. I was let off with my disc brakes since I made every part of them except bolts, discs and cables. Bontrager stepped up to supply tyres for the event; all XR4 Team Issue, 26". I was pretty pleased about that; they're way above klunker spec! Oh, and you were obliged to allow any competitor to race on your bike if they requested. We really should have made better use of that rule!

Anyhow, with an almost completely blank canvas I couldn't help but let my mind run riot on the design - particularly the geometry; there was an idea that I'd been wanting to try out for ages, and it promised to result in a truly ridiculous bike. It was on! Two head tubes and linkage steering, coming right up! The next thing playing on my mind was brakes; I had every other component somewhere, but I had no rim brakes and all of my 26" wheels are disc only...I needed a plan. My initial thought was to make a vee-belt pulley that would mount to the disc flange of my wheels, and run the brake as a tensioner which pulled a fixed belt into the pulley...it sounded shady, and I wasn't convinced it would provide significant stopping power. I eventually settled on making my own cable operated disc brakes, and had a pair of used car handbrake cables to connect them up with. Bound to work!

A brake caliper body nearing completion
First brake caliper body nearing completion
And the disc fits
...and the disc fits! Bonus.

2 finished calipers
Two completed brake caliper bodies
Frame tubing scrap ERW a bit of T45 mild steel box section...
Scraps, offcuts, ERW, and a very little T45 are the main components of Tam's frame

Now, under event rules I had to make the whole frame myself; dropouts and all. This presents a problem, since my welding is not up to scratch for working with any sort of decent frame tubing - I definitely can't TIG weld to save myself. Dirty old MIG welding it is then, and thick-as-you-like mild steel ERW tubing to save me from blowing holes everywhere (and to save the pocket a bit!). A scrabble through The Bicycle Academy's tubing rack and scrap bin yielded the necessary tube and box section...

Most of the frame tubing cut to length
Tam's Hack Bike Derby frame tubing looking a bit more rational than when it started
The main frame taking shape in the Academy Tools jig
Tam's hack bike frame starting to come together

Main mid traingle tubing notched and cleaned
Tam's hack bike middle (?) triangle tubes

Main triangle welded up
Mid/front triangle done!
Multipass MIG weld
Multipass MIG weld?

Man alive I need to get some welding practice in!

Stay tubes complete...gnarly
Tam's hack bike stays!
Chain stay notch...massive!

Behold a frame So heavy so gnarly
Behold! A frame of epic mass!

How gnarly is that!?!? I have to admit though, I'm rather proud of it; it's the first frame I've ever welded entirely myself! At this point it felt like I was getting through it pretty quickly - it all seemed to be coming together. Onwards with the forks...

My first fillet braze Fork crown to steerer tube
Fork crown/steerer assembly - my first fillet braze!

Billet machined 20mm front dropouts
Billet machined 20mm front dropouts

Machined 20mm front axle
Billet machined hollow 20mm front axle

Front 20mm dropouts TIG welded into the fork blades
20mm thru front dropouts TIG welded onto the fork 'blades'

Don't be fooled by that - there's no filler rod there at all; they're just fused on. Still plenty strong though, and looks half decent I reckon!

Linkage steering
Steering linkage...on a bicycle

Looks almost finished right
Looks almost functional and almost finished, but there was another solid day of work to get 'er done!

That was about 4am on Saturday. My newly revised plan was to finish it up in time for dual slalom racing that evening at 7...

...after three hours sleep, I crawled back to the workshop to try and get the brakes together. That's really all there was left! It seemed like a couple of hours of work, so I planned on taking four hours. Tiredness had taken its toll though, so progress was glacial and motivation was dwindling fast! I couldn't even face taking many photos now, I just wanted it finished...

Integrated rear brake caliper...fancy
Integrated brake caliper anyone??
Tapping the brake caliper arms
Brake caliper lever arms taking shape - big thanks to Matt Stitt for hacking these out for me!

Still so much to do even now - brake pads (just little discs of aluminium), handbrake cable stops, complete brake levers, all sorts of brake adjusters, remount my shifter to a truly inconvenient place (shh, don't tell the commissaire!), etc, etc... Motivation was at zero so I just sat down...

Finished at last
Finished at last...so epic, so heavy!

That was 8 pm on Saturday. I'd missed the start of racing, and there was no way I was going anywhere but my bed now!

The seat works too
Testing out the seat...seems to work - Roll on Sunday, LET'S GO RACING!! (on a horrendously heavy bike with bad brakes and the worst handlebars ever...)

Turns out that aluminium pads are actually pretty handy; the aluminium builds up on the disc, and generates a usable amount of friction! I could even do a skid**, which is much more important than it sounds!

I somehow managed to plough my way to 3rd place out of 18 in the DH race, I think mainly because I was scared that I would die if I crashed! In the mass start, I totally failed to utilise my selectable gears, and came stone last out of everyone who didn't stack. What a plonker!

Burf wrapped up the overall, having taken 1st in dual slalom, 4th in DH and 2nd (?) in the mass start. Tidy!

**using three fingers on the lever

Hack Bike Derby 2016
Hack Bike Derby 2016 'podium'

Now the important stuff; how does a bike with a 1225mm wheelbase and 75deg head angle ride? Honestly, it's hard to say yet. It's so heavy, the bars are so nasty, the brakes are horribly unergonomic and quite ineffective, and the trails were slop-fest or fire road. I'll figure out the necessary bits though and put in some time on it. It's a concept I'm keen to explore more, but don't worry; it's not scheduled for production any time soon!

Massive thanks to Andrew for putting the event together, and the team of people who made it all happen. Also to all the other builders, competitors, camera-folk and spectators. I'm also hugely grateful to everyone who encouraged me when I was totally ready to give up hope and go home before I'd finished my bike on Saturday afternoon! Finally, much respect to Break Fluid for awesome coffee, tea, and superlative granola...

A 2CV in the woods Must be Break Fluid
A 2CV in the woods?? Big thanks to Break Fluid for keeping us all fueled for the weekend!

Healing vibes go out to those unlucky enough to break themselves, and the ground wherever Ted James hit it. Keep your eyes peeled for total media coverage premiering at Bespoked UKHBS in April - I can hardly wait!

Same time next year?? Go on then...


MENTIONS: @BTRfabrications


  • 241 8
 Still looks better than that Pinarello at the London Bike Show.
  • 85 9
 still better than an Ellsworth
  • 24 60
flag Zimmer68 (Feb 15, 2016 at 11:21) (Below Threshold)
 Still better than a session
  • 41 4
 haha neg props, love em. P.S. The session is an awesome bike, lighten up.
  • 9 1
 The only bad press is no press...
  • 39 0
 This is actually Mondraker's new test rig for their new "Forwarder Geometry".
  • 3 0
 That's the future.
  • 3 0
 Gotta love the Brits.
  • 1 0
 No kidding
  • 77 0
 I think he forgot only one thing, the place to fit a bottle cage.
  • 10 0
 it s not like there's not the space ^^
  • 18 0
 or a SWAT "burrito" door...
  • 5 2
 or a weed hidden cage...
  • 48 0
 Well its Monday... everything is possible.
  • 13 3
 Looks like someone's got a case of the mondays!

*Immediately punched in the face*
  • 15 0
 What do you think comes after Sunday? Sunday Jr.?
  • 7 1
 I'd rather have that than bright orange shoes!
  • 6 1
 Frankenbike. "Please.... kill..... me....."
  • 4 0
 @bigtim "Every moment I live... Is Agony...."
  • 29 2
 Awesome content Pinkbike, keep it up!
  • 9 0
 yeah, love this stuff. Let's see pics of all the other bikes in this "race"!
  • 23 1
 Totally want to see more on this (like how the brakes work) and the other bikes that were built for the Hack Bike Derby!
  • 14 0
 This looks like a bike for Burning Man.
  • 12 0
 Mad respect to anyone who builds their own ride. From a pile of tubing to the race track is what its all about.
  • 9 0
 Pretty cool! The linkage steering is pretty neat. It allows you to make wheel base and reach independent of head angle. So you could make a bike with a 90 degree head angle (and negative fork offset), that has no wheel flop and much less fork crossloading, but still a long wheelbase. I've been wondering for a while how such a bike would ride.
  • 5 0
 Nail-head. Spot on! Smile
  • 2 1
 would understeer like a bitch above 5km/h I'd imagine.
  • 11 0
 Messing around with bikes. That's what it's all about
  • 9 2
 LOL I'm currently mig welding a steel MTB frame. I'm using thin walled tube though and a technique I call farmer tig. Just a series of short but HOT tacks real close together. You get better heat control vs running a solid bead and the finished product sorta resembles a tig weld or a nice braise.
  • 7 0
 AKA ... Muffler welding
  • 4 0
 Or stitch welding
  • 1 1
 i didn't know that chromoly steel could be joined with MIG? or did you use mild steel DOM?
  • 2 0
 @reallybigmantis chromoly steel is still steel and just like mild steel DOM tubing any steel filler will join them safely (er70s-6, ER70S-3, stainless 208, 309, etc. etc.). Also MIG welding is just another way to weld, vs TIG, ARC, etc. you can switch out the wire, gas, polarity (AC to DC with the proper machine) and weld pretty much everything with a MIG you wanted (granted somethings like aluminum you may need a spool gun for). TIG, ARC, and oxy-acetylene all have their advantages but all machines you can get it done if you are crafty/skilled enough.
  • 1 0
 @reallybigmantis What @only1mikey said really... Also this frame is mainly mild steel ERW (welded/seamed) tube. Smile
  • 11 0
 Agricycle! Looks like a prop from Mad Max
  • 8 0
 oh man wish I had any engineering skills..let alone these skills
  • 8 0
 That was fantastic. Good work and sense of humor.
  • 8 0
 haha yes Tam 51lbs of man bike
  • 4 0
 ...is about what all our Freeride/DH bikes weighed 10-15yrs ago..I don't miss that!
  • 7 0
 awesome !!
  • 5 0
 Love the 2cV. I drove one all through high school. Get a lot of crazy looks in rural Ontario in the 80's.
  • 5 0
 Pretty badass! Could use a cattle-guard though.
  • 2 0
 more detailed article please. Would love to be a bike builder for events like these! I assume it will rust into oblivion, sadly
  • 2 1
 I'm curious what most riders consider "extreme" geometry besides the GeoMetron. My size large bike has a 1218 wheelbase and 475 reach.
  • 2 0
 I think for most riders a trail bike with a wheelbase over 1200mm and a head angle slacker than 66º is extreme. For me extreme geometrey aka awesome geometry would be GeoMetron and Pole bike geometry eg head angle 62-63.5º for a trail bike with 1300mm or more wheelbase. Reach of at least 480mm.
  • 2 0
 For me 'extreme' would be pretty much what @SintraFreeride has said. But what I think a trail bike should actually look like is 64-65° HA, ~1200 WB, 440mm chainstays, 450mm reach - on a medium that is. Oh and the all important seat angle about 76°. Still extreme in comparison to most of what's out there but we're getting there slowly.
  • 1 0
 My Rallón R4 has a 1204 WB, 467 reach, 420 mm CS and a 65.5° HA on Size L. The seat angle is a bit forged to my taste, since it feels slacker than the advertised 74°. Anyway it's a killer bike on the downhills and a (fat) mountain goat on the ascends.
  • 1 0
 I think the Rallons have very slightly offset seat tubes? Could be why it feels slacker than advertised if so..? Always thought they looked awesome but haven't had chance to ride one yet.
  • 2 0
 What do you mean by "slightly offset"? The seat tube bends a bit to clear the rear wheel at bottom out indeed, but it's also laterally asymmetrical near the BB. If I understood you correctly, I think you feel the same I do, and the kink they put there to clear the rear wheel slacks out the ST single.
  • 2 0
 Yeah that's exactly what I meant dude :-) it's quite common these days - most seat tubes have a kink or some even connect to the down tube rather than the BB (unfortunately the manufacturers showing this in their geo is less common). So there's a difference between the actual angle that your seat post will be and the 'seat angle' from the BB to the centre of the seat collar. The kink in the Rallon doesn't look huge but it could be why it feels slacker than the stated angle from Orbea. I don't think these offset seat tubes are necessarily a bad thing, they often have benefits on the geometry and kinematics on the whole but I wish there was more clarity. Lots of us rely solely on geometry chats and reviews so accuracy and clarity is key.
  • 2 0
 I agree. Plus the "theoretical" or "factory measured" ST angle would imply a certain saddle height,while that height would be different for everyone with the bike in the wild.

The short rear centre could certainly contribute to the slacker feel of the bike.

Anyway, if you have the chance to test a Rallón, DO IT. It's an amazing bike, and it really shines on the downhills.
  • 1 0
 Sort of a giant reign
  • 4 1
 looks like you had a good time Wink
  • 2 0
 You should sell it to the film makers of the next Mad Max film for a set piece, it would fit right in.
  • 2 3
 those welds are so bad, need to stick to the tack weld setting only!!! Mild steel would be so so heavy, flexy and rust like crazy, unless you keep it coated in WD40 at all times... why not use a patch of leather for the brake pad at least!? This thing is appaling and needs to be melted down, crushed and shredded
  • 2 0
 where do I get me some of that break fluid coffee!?
  • 3 0
 Cheers! coffee, mug and a press! love it
  • 2 0
 Great read! How about Ti @BTRfabrications... #BIGshiteatingrin
  • 1 0
 I'll leave Ti up to Burf; he's the man with the welding skills! Even then, it's so expensive and not very stiff...it would be great to be able to do away with paint though!
  • 2 0
 Looks just like a session.
  • 1 0
 Change the fork, brakes and handlebars for more "normal" ones and give us some feedback!
Very cool experiment by the way!
  • 2 0
 That's the next step! Well, I won't be changing the forks on this one, but that will happen eventually Smile
  • 2 0
 Sunday jr. Would be awesome... like a 1/2 day on Monday?
  • 1 0
 When I saw the numbers in the race I was like whaaatt??? But then I saw the oter bikes competing and it turned to aaahhh ok
  • 1 0
 The front part of the frame, can trick you to believe it is a Hossack-style suspension chassis...
  • 2 0
 Looks fun to barspin!
  • 1 0
 But.... Most of those are better than my bike..!
  • 4 3
 i just hope they didn't let it lay its eggs....
  • 2 0
 It will evolve, don't worry Wink
  • 1 0
 but how much more joy building it?
  • 2 0
 Weight weenie...
  • 2 1
 What the hell did I just read?
  • 1 0
 whats the wheel size ?
I can't take the suspense
  • 2 0
 Real wheels! 26" Wink
  • 1 0
 That's some serious brake hose there!
  • 1 2
 I just looked at the pics... So it is a bunch of neo bike builders, in the uk, trying to be all Portland/California like?
  • 6 0
 .... or people having fun? Wink Smile
  • 1 0
 what the hell is btr!
  • 2 1
 You need to do some research dude
  • 2 3
 WoW too long too slack;

by the way awesome idea
  • 9 0
 75º head angle is too slack for you? Are you riding a unicycle?
  • 1 3
 looks like a session... XD
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