TT Mk3

It has been ten years since I designed and built the TT Mk2; since the pandemic I have mostly retired from racing, just doing a few local events and riding time trials on my road bike. But I do sometimes miss the pure speed of a TT bike! When Enve asked me if I might be interested in building a bike to showcase some new aero products they had, I took the bait.

My design goals with this bike were to take what I learnt from the Mk2, add disc brakes and accommodate a modern aero position. I also wanted to make everything as narrow as possible, whilst retaining compatibility with standard wheel spacings.

The starting point was the crank – after measuring up the Vision Metron aero crankset, I determined that I could make it narrower which would both reduce the q-factor and improve the chainline for a single-ring drivetrain. This proved a little challenging – the axle is permanently fixed to the driveside crank, so it took some careful work to split the axle, remove the amount required but leaving a toothed interface for pressing the axle back together (with an internal reinforcement sleeve) so the cranks would be perfectly aligned. The 16mm removed meant that the crank now fits into a 68mm BB shell with internal bearings.

The narrow crank led to the next problem – shaping the chainstays to fit in the small space between the chainring and the rear wheel, and also avoiding the heel of the shoe. A combination of ovalising and bending found a solution. For the mainframe, I had some 4130 cromoly tubing custom drawn to a NACA aerodynamic profile which is just 24mm wide. I used this tubing for the downtube, headtube and seattube. At the top of the integrated seatmast is an eccentric for the saddle mount that allows for 15mm of adjustment for fine tuning the seat height.

Then the front end….. For forks, aero theory seems to suggest to either go very narrow or very wide. With the idea that going wide may also help the airflow transition onto the rider’s legs, I opted to try that this time. So the fork blades remain at the 100mm width from the front hub right up to the basebar. There is no conventional headset, rather two angular contact bearings in the fork crown and stem, which bolt to the headtube (with the upper one also incorporating an internal steering limiter).

The strength and stiffness of steel means that narrow sections can be used – the tubing used for the fork and handlebar is only 13mm wide. But also means the internal space is small – running the hydraulic brake hoses through the fork/bar was a little tricky! At least I only need to do that once.

To complete the frame structure I needed seatstays – I ovalised some small diameter tubes and then bent them to mirror the shape of the chainstays. When looking at how best to join them to the seattube whilst keeping tire clearance, I ended up using a small section of the fork tubing to create a little aero bridge.

Thankfully wireless shifting means no other wires or cables to deal with! The prototype Enve aerobars accept the SRAM blips directly. The bars are adjustable for length and angle so I can experiment with the position once I get this on the road.

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