Due to a career as a mechanical engineer, working with a factory V8 Supercar team designing composite, fabricated and machined components, Mark Hester is very much a numbers man. He possesses an intrinsic knowledge of how stress and structures relate to each other.
Working under the banner of Prova Cycles from his home in the Australian capital of Canberra, he’s been applying those engineering skills to making exemplary vehicles of the two-wheeled variety. His latest creation is a 29er adventure road bike that he just rode from Canberra to Melbourne.
Mark explains the theorem behind his new frame: “The design brief was a bike capable of performing both the duty of an endurance road bike while also being capable of tackling adventures on typical Australian rural dirt roads.
“I resisted the temptation to design for 650bx2.1 clearance due to the intended use being bias towards on-road capability and not pure off-road — that bike is next. The result was a form uncompromised by excessive clearance when running closer to 700x30mm and bang on with 35mm tyres.
“To start the design process I use a method learnt from my time working on the design of race cars and after identifying what the bike needs to be capable of, spend hours playing with numbers in a spreadsheet that estimates characteristics that are influenced by the behaviour of the vehicle, like mass distribution for both seated and standing positions at different inclines.
“For this bike, with longer chainstays for tyre clearance, this allowed me to tune the design to get the front-to-rear distribution I wanted and, in doing so, match the riding behaviour of bikes I have benchmarked.
“Benchmarking both your own bikes and the best out in the field is crucial and a practice undertaken by all good vehicle manufacturers. Ideally I would use a single software for this and the geo/rider fit but currently, it means BikeCAD + spreadsheets.
“The numbers are;
Head tube angle: 72deg
Fork offset: 47mm
Seat tube angle: 73deg
BB drop: 72.5mm
Head tube length: 147mm
Seat tube length: 550mm
“The bike is designed to cover long distances in a speedy manner so the position is closer to that of a conventional road bike. The axle-to-crown of the ENVE gravel road fork is great, as it allows for a good-lengthed head tube when using a lowish BB and fatter tyres. Using a slightly sloping top tube allows for balanced comfort from more exposed seat post compliance and allows sufficient space for a frame bag.
“The frame is fillet brazed using Columbus Spirit HSS tubing with a Reynolds 853 oval top tube. The use of the large section down tube (GX shape) and oval top tube gives a torsionally stiff frame that feels responsive to the rider when cranking hard on the pedals and precise when cornering at high speed, while the steel ride quality is maintained by the use of a traditional 28.6 seat tube with 17mm seat stays.
“The seat clamp is an integrated stainless steel design with replaceable barrel nut, made by Bentley Components in Yorkshire. I have used this design a few times with both carbon posts and dropper posts with no slippage. The bottle bosses are all stainless and have a large diameter face to help distribute the forces from the bottle into the very thin wall HSS down tube.
“I chose the Syntace x12 thru-axle format in the rear with Paragon dropouts. On the rear of a steel frame, it is debatable that this will assist greatly with stiffness. I plan to perform some FEA (Finite Element Analysis) on this as it only reduces it to localised stress — a small amount can greatly change the fatigue life of a steel structure. For me, it also results in a more user-friendly bike and you know the disc will be aligned bang-on every time you replace the wheel.
“Using the new T47 bottom bracket format has allowed for internal routing through the downtube and into the chain stays with a 30mm spindle. The internal routing uses full-length stainless tubing and the main tubes are reinforced at each point it enters to frame tubes to reduce the risk of fatigue failure from the increased stress caused by the hole.
“As the internal tubing is stainless, this meant using a silver rod. Due to the higher amount of stress experienced here, I used Fillet Pro from Cycle Design due to its excellent mechanical properties.
“In addition to accommodating a 1-1/4 tapered steerer — with an external 44mm bottom cup for additional front end stiffness and strength — the tapered 34/44 head tube provides, to my eye, the correct proportions. It’s a slight nod to aesthetics but a 44mm HT on a road bike appears a little large for many people.
“It uses a full SRAM Force 1 groupset with a 42t chainring and 12-36 cassette, which results in a completely usable speed range of 8-50kph out on the road… more than sufficient for a regular single speeder!
“The wheelset is an ENVE 3.4 Disc with Chris King hubs, converted to tubeless using the new Silva tape and valves. Tyres are Clement X’PLOR MSO 36mm for gravel and dirt and 35mm Compass Bon Jon Pass for speedy road use.
“I had a custom half frame bag made by Stealth in NZ. I also used their front dry bag harness. I used a Wildcat seat pack which, with its quickly-removable tapered dry bag, means you can chuck it in the tent.
“There’s a pic here from the ride I did a couple of days after finishing the bike. Went on a solo mission to Melbourne, taking in some parts of the wilderness I hadn’t been to before. Did a bit over 800km in 5 days with almost 200km of dirt and 13km of climbing.”
That’s a helluva story, but it demonstrates the level of consideration Mark has for his designs. Keep an eye out for him at the October 2016 Single Speed World Champs, to be held in Woodend, where he’ll be racing his Prova Cycles 29er MTB.
Steve at Sun Graphics in Melbourne applied that sublime paint, with topographic details that refernce the mountains at the end of Mark’s street.
Special thanks to Lightbulb Studio in Canberra for the photos.