Renault bike suspension to provide easier ride

When Pascal Tribottet, a Formula I engine designer at Renault Sport, took up mountain biking, he was appalled at the hammering his body received over rocky terrain.

For years, mountain bike designers have tried to develop suspension systems that absorb the shocks, but professional riders, and many amateurs, have spurned suspension bikes because all suffered from a loss of power to the rear wheel.

By taking a revolutionary approach, Mr Tribottet designed what is claimed to be the first bike suspension that reduces excessive bouncing and hence suffers no loss of power. Renault was so impressed with the result that it has teamed up with Giant, the Taiwanese bike maker, to develop a range of cycles to be sold through the company's network of car dealers - first in France, then worldwide.

The technological breakthrough that Renault believes will help transform the comfort of off-road bikes arose from Mr Tribottet's expertise in engine technology.

Traditional suspension bikes have a tendency to develop excessive bouncing both on the flat and on rough terrain. The movement, Mr Tribottet calculated, leads to a loss of power between the pedals and the back wheel of 35 per cent. That amounts to some 20 watts - enough to cause a topclass rider to lose a race.

Bike designers had tried to solve the problem by balancing the forces. But whenever the rider pushed down on the pedals the frame would move in response. Mr Tribottet, however, happened to be an expert in the resonance problems of racing car engines. With a few simple calculations, he concluded that resonance was the problem.

A fit cyclist turns the pedals at a cadence of 45 to 90 revolutions a minute - or, in engineering terminology, 1.5 hertz to 3 hertz. He discovered the vibration frequency of an effective bicycle suspension system is between 1.5 hertz and 3 hertz.

When rider and suspension system were generating vibrations at the same frequency, the vibrations were amplified, leading to excessive bouncing, and power loss. "My answer," he says, "is that what is important is not to have a balanced system, but to have no resonance."

His challenge was to design a system that would block resonance between the "motor" and the suspension. Renault's suspension system looks much like any other. The layout of bars and pivots is much the same. But the lower bar that links to the rear wheel is a little more horizontal, and the uplink at a steeper angle.

"What is really different," says Mr Tribottet, "is the path of the wheel." When it hits an obstruction, and the suspension responds, the wheel describes a more forward arc, giving a different resonance. That tiny tweak has produced a striking change in bike behaviour. On the road, a bike with Renault's patented No Resonance System (NRS) suspension feels stiff, like an unsuspended bike, and the power loss is less than 1 watt. Off the road, the suspension soaks up bumps, without loss of power.

Cross-country tests with professional riders showed that a bike with NRS suspension was faster than both rival suspension bikes and those without suspension. In tests against a conventional bike without suspension on a cross-country circuit and using the same rider, says Mr Tribottet, the NRS bike was on average 2 per cent faster, despite weighing 10.8kg, compared with 9.8kg for the unsuspended bike. But the proof of the pudding will come when riders in the Giant mountain bike team uses NRS equipped bikes in races this year.

NRS suspension will be the technological cornerstone for Renault's drive to carve out a significant position in a European market estimated at 15 million bicycles a year. Its top-of-the-range mountain bike, with NRS, is likely to cost about FFr10,000 (€1,524). But this will be the flagship for a 21-model range that includes on-road models and a battery-powered, electric motor assisted bike.

For it is not only hardened cross-country cycle enthusiasts who can benefit from better bike technology. Mr Tribottet says his 18-month-old son now has a much more comfortable ride when he takes him out on a crossbar mounted seat.