Paul Lew with the RZR 92 rear wheel
Beating its nearest competitor by around 20W at 50km/h (13.888m/s), the Reynolds RZR 92 wheelset is the fastest in the world. So says its designer, Paul Lew, who took time out during a recent whistlestop tour of Europe to chat over croissants and coffee with interested journos at London’s recently-opened Bespoke Cycling shop in Farringdon Road.
Paul Lew is something of a legend in the worlds of composites engineering, of bike wheels and, particularly, of bike wheel aerodynamics. His Black Hole front wheel concept – basically a rim and tyre with annular bearing carrying the rim and supported just inboard of it to obviate the need for a fork or hub – was so fast it was banned by the UCI in 1995, while more recently he designed for Shimano the Dura-Ace 7800 wheelset, which was ridden to victory by Oscar Friere in the 2004 world roads championships.
His views on wheel aerodynamics perfectly explain the results of various rough-and-ready tests on cycle aerodynamics I have done and, given that his day job – Lew is a consultant to Reynolds Cycling having sold his Lew Composites firm to them a while back – is designing unmanned aircraft for the US military, it may be assumed he knows a bit about airflow.
The RZR concept is, of course, about more than aerodynamics. First unveiled as the RZR 46, it comprises a deep-section carbon fibre rim attached to a carbon-fibre hub using carbon-fibre spokes, which sounds much like several other wheelsets out there.
Even the most cursory glance at an RZR rear wheel , however, says that something is different. Sitting on the hub centre line is a four-armed flange, each arm locating a spoke that runs at a tangent from the hub to the wheel while the remainder act radially. The central batch of spokes transmits drive torque to the rim while the others keep it axially and radially stable. The design, with no opposing spokes for those at a tangent, is only feasible with carbon fibre, which is stiff enough to allow spokes made with it to act either in tension or compression and with no preload.
Why bother with the central flange? Here Lew made a point that I’d never previously encountered; in any dished wheel, the spokes on the gear side act at a less favourable angle to the rim in terms of lateral pull so either they must be tighter or there must be more of them. Then there’s the current practice of putting radial spokes on one side of the rear wheel and tangent on the other.
In any case, the result is a rear wheel with the spokes on one side able to transmit torque more effectively than on the other. What then happens is that, as the wheel is driven, the less effective, usually radial, spokes tend to be wrapped around the hub, especially if they are on the drive side, pulling the rim to that side. Which is the Lew explanation for why the rear rim can rub on a brake block under hard pedalling. And applying the drive torque through the wheel centre line obviates the problem.
Meanwhile, RZR radial spokes aren’t under tension, acting instead in compression as the wheel rolls and otherwise being . This ensures that there is no force pulling inwards at all times on the rim, which can be made lighter – by around 200g according to Lew.
Interestingly, the deep sides of the rim are therefore thin and flexible enough to add a suspension effect of their own, which has allowed Reynolds test riders to run significantly higher tyre pressures – with accompanying reduction in energy loss to sidewall flex – without losing bump absorption.
Of the two RZR models, the 46 is obviously much lighter – 910g per pair compared to 1582g claimed for the 92 – but offers less favourable air penetration. This is where the 92 scores over even the best of the competition, according to both Reynolds’ own CFD (Computational Fluid Dynamics) analysis and to the outcome of validation tests carried out by Mooresville, North Carolina’s A2 Wind Tunnel.
While exhibiting the same amount of drag over a range of yaw angles – where a crosswind strikes the rider and cycle on the shoulder rather than from dead ahead – the RZR 92 is as good over a wider range of yaw. It therefore offers an advantage – some 20 seconds over 40km/25 miles – according to Reynolds in wind conditions where the wind hits the rider from more points of the compass.
And how does it do it? That’ll have to wait until we get a pair in to test.
Reynolds RZR 46 and 92 wheelsets £4,800 per pair, tubular only.