How to build your very own Ti Gear Box Bike by Alan Hawkes
O.K where to start, the big question is why? Well why not? I've made downhill frames before using t45 steel so I thought if I was going to build a gearbox DH bike then I may as well do something special. There is probably no advantage in having titanium over alloy but if you're only going to do it once then why not make it different?Racing every weekend you see some sights, Big Hits with Super Monster Tee's, zero throw stems, 85 spacers under someones stem and even Wulf Sport clothing that would make your toes curl.
Luckily for you lot I still keep my eyes peeled and my old mate Alan Hawkes wheeled this little beauty past me. A quick karate chop and he was stopped in his tracks. He promised to get her polished and some pics and words how this little beauty came about..The following words by Alan Hawkes
I bought a G-boxx over a year ago after enquiring via Karl Heinz at Nicolai, so that was the starting point, after I'd decided on titanium the next problem was where to get the material as it can be quite hard to get the sizes you need hence some compromises.
I work in motor sport engineering, so I was lucky enough to get a piece of 60mm dia. ti tube for the down tube, which I then ovalised. The head tube is made in three parts, two rings for the headset to sit in and the center part is made from rolling up a piece of 3mm sheet ti, this was then assembled, welded, then final machined. The mounting for the gearbox is again made from 3mm sheet, folded carefully on a fly press just using eye to fit the contour of the gearbox, not easy!!
The most time consuming part was the swing arm pivots, machined from billet titanium, probably a good days work there! No CNC machines for me unfortunately. Even the seat tube had to be machined from a solid bar, if you were to make a few you could justify getting parts machined or water jet cut but for a one off it's not really viable.
The two top tubes are 19mm dia. and I even had to make a tube bender so I could get the desired angles. The actual assembly and building was not too difficult but then is my trade. The rear dropouts are fabricated from 3mm sheet with sliding alloy parts for the axle to go through.
I drew up the suspension layout on a board and made various pieces of metal until I got the desired amount of rear wheel travel for the shock length and to get a rising rate, then once that was done I could machine these parts from alloy, these are not yet really finished but I wanted to make sure everything works OK before I spend hours machining something just to make it look prettier!
So once I had all the parts all I had to do was put it all together, simple really. I made an 'L' shape jig into which I clamped both wheels at the intended wheelbase, then fitted the forks and head tube, set these at the correct angle, fitted the gearbox onto a mounting plate on the jig, then basically measured, cut, ground and offered up the down tube until it fit perfectly! Then I did the same with the rear end, then made a separate jig so I could weld the swing arm without distortion.
Refitted the swing arm then attached all the suspension parts, then made another fixture to hold the seat in the correct place and cut and bent the tube to fit between the seat tube and head tube. Then just a case of welding it all up and off for test rides.
The first proper ride I had on the bike was at the first round of the Caersws Cup 2009 so a baptism of fire really! Only a couple of minor issues with the chain really but these were soon sorted.
So that is how you build a titanium gearbox bike, I'm never satisfied so I have already started building another bike, this one is going to be more conventional though, made from t45 again and seven inch travel with single crown forks, hoping to get a lightweight downhill/freeride bike, hopefully by July, so I have a lot to do.
Alan, many thanks for your detailed words and inspiration to all Meccano players out there.
Si Paton.Descent-Gear.com Troy Lee Sprint Shorts 59.99GBP Posted