KEVIN'S INSIGHTS: Ducati’s MotoGP Turning Problem

More work is needed...

jorge lorenzo catalunya qualifying action
Jorge Lorenzo, Grand Prix of Catalunya Qualifying 2017Courtesy of Ducati Team

Any thought that the arrival of corner-speed stylist Jorge Lorenzo at Ducati would solve Ducati's turning problems has been eighty-sixed – more work is still needed. Andrea Dovizioso said this last week;

“Turning is one of the points for sure, and especially with the feedback and the arrival of Jorge this has become a little bit stronger but it’s not something new for me. I’ve had a lot of teammates, and like I said last year I’m very happy about Jorge’s arrival because he has won a lot, he is one of the best riders in the world. It showed to a lot of people some limits of the bike. Many people thought maybe some riders could perform better with this bike but I don’t think it’s possible at the moment. Jorge showed the reality, and I’m happy for this because it can give us the possibility to put more effort in a different place than where we have been working in the last few years to improve the bike.”

At Jerez, Octo Pramac Ducati rider Danilo Petrucci said, “…the bike has some issues to solve. I don't feel the front on long corners and I don't feel the confidence. Even in the tight corners like one and two I have to go in with a lot of brake, but when I release the brake, the bike still doesn't turn.”

Riders are saying that when the brake is released the bike doesn’t turn well. Why should that moment be special?

danilo petrucci catalunya qualifying paddock
Danilo Petrucci, Grand Prix of Catalunya Qualifying 2017Courtesy of Octo Pramac Racing

Note that there are two general styles of corner entry – the separate brake, then turn corner-speed style of riders like Lorenzo and Vinales, and the combined braking/turning style of most others.

In combined braking/turning the front wheel may initially carry 100% of machine & rider weight, with the amount decreasing as the rider brakes less and turns more and more during corner entry. But when braking ceases, the front end carries only its normal weight, so there is a change at this moment in the load the front wheel carries. Why should that make a difference in a bike’s turning ability?

What changes at brake release (which is gradual) is the vehicle’s unsprung weight ratio. This takes some explaining. A vehicle without spring suspension – such as the rigid rear end of most 1930s motorcycles - must rise as a whole over every bump. This becomes a problem in corners because bumps easily cause the wheel to lose grip over bumps and “step out”. If the step-out becomes too big, grip is not regained and the bike goes down. It’s not even necessary for the wheel to “get air” over a bump in a corner - just the lighter tire contact on the far side of a bump may not be enough to maintain corner grip.

With no suspension, there is only the downward acceleration of gravity – one ‘G’ – to pull the wheel down after a bump lifts it.

andrea dovizioso catalunya qualifying action
Andrea Dovizioso, Grand Prix of Catalunya Qualifying 2017Courtesy of Ducati Team

When you add spring suspension, things change. Instead of the whole vehicle being lifted by the bump, only a small fraction of the vehicle – the wheel, tire, brake and moving suspension parts – rises over the bump. How fast can it respond to a bump? Acceleration is driving force divided by moving mass. The driving force is the weight carried by the suspension spring and the moving mass is the wheel, tire, and associated parts. At the front of Dovizioso's Ducati the driving force is the load on the fork springs – roughly 270-lb at rest – while the wheel and associated parts weigh about 30-lb. What is the maximum acceleration this suspension can achieve in following road surface irregularities? It is (300-30)/30 = 9 G (I've subtracted the 30-lb weight of the front wheel from total front end weight because the wheel is on the ground). What this means is that with suspension, this bike's front end can follow irregularities nine times faster than can a rigid front end. This acceleration is also the "unsprung weight ratio" of that end of the bike –the ratio of the weights on opposite ends of the springs.

Now we see what changes when the rider releases the front brake during corner entry. When braking at maximum, the front tire carries all 560-lb of machine, fuel, and rider, so the unsprung weight ratio is (560 – 30)/30 = 17.6, a bigger number. In this condition, front suspension performance is improved by the fact that it's carrying extra weight, keeping the front tire in firm contact. The rider reduces braking force gradually as he leans the bike into the turn, keeping the sum of braking force plus turning force always near the tire's grip limit. But we can see that unsprung weight ratio and the front tire's ability to follow road bumps both decrease steadily during corner entry, as the rider reduces braking force…

jorge lorenzo catalunya qualifying action
Jorge Lorenzo, Grand Prix of Catalunya Qualifying 2017Courtesy of Ducati Team

The result of this might be that the front tire grips adequately during the hard braking phase, but as the unsprung weight ratio drops as the rider reduces braking effort, it may grip less and less well – possibly resulting in a fall as described earlier this year by Andrea Iannone.

All of the above relates to the machine's normal suspension. It works well when the motorcycle is upright, but as it leans over for a turn, it loses effect because it is being laid more and more on its side. No matter how wonderful that suspension is, it cannot absorb vertical pavement bumps when it is itself close to horizontal (the 60-degree angle-of-lean we see in MotoGP)

Now chassis lateral flexibility must take over from the normal suspension, because it is the only thing that can prevent the leaned-over vehicle from losing grip like a rigid-framed bike of the 1930s (or like an Assyrian ox-cart of 2000 BCE).

andrea dovizioso catalunya qualifying action
Andrea Dovizioso, Grand Prix of Catalunya Qualifying 2017Courtesy of Ducati Team

If the chassis is laterally very rigid, a bump tosses the whole bike upward just like the no-suspension case, and tire grip is lost because gravity's one 'G' is too weak to make the tire follow the pavement.

If the chassis is flexible, the front wheel hitting a bump flexes the chassis rather than tossing the whole front-end upward bodily, and it is the springiness of the chassis that quickly accelerates the tire back downward into contact with the pavement.

Is this what Dovi is talking about when he calls for “more effort in a different place”? A more laterally flexible chassis that behaves less like an ox-cart in mid-corner? At the moment, the Ducati seems to need to be ridden in the combined braking-and-turning style that improves unsprung weight ratio during corner entry, and Dovi’s new teammate Jorge Lorenzo is having to learn that style.

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