However, it is possible that inline-fours don’t only struggle to produce race-winning horsepower because of the length of their crankshafts but also because the engine’s greater width makes it difficult to use big-enough throttle bodies and induction trumpets.
Using larger throttle bodies and trumpets to get more power would most likely make the motorcycle too wide, which would be bad for aerodynamic performance. In theory a V4-powered MotoGP bike should be narrower than an inline-four, but this isn’t the case because radiator size is now the determining factor in overall width.
V4 engines are narrower but that doesn’t necessarily make V4 bikes narrower
The easier packaging and mass centralisation of an inline-four MotoGP bike can make Yamaha’s YZR-M1 and Suzuki’s GSX-RR faster when track layout allows them to use their superior corner speed, but this doesn’t always help in race situations, because it’s easier to overtake on a straight than around a corner.
V4s don’t enjoy a huge advantage on the straights, but enough to make a crucial difference during a race. At last year’s Qatar GP the fastest V4 was Marc Márquez’s RC213V at 218.7mph/352km/h, about 2mph/3.5km/h better than the fastest inline-four, Joan Mir’s Suzuki GSX-RR.
At Mugello, the fastest V4 was Andrea Dovizioso’s Desmosedici at 221.6mph/356.7km/h, about the same difference over the best inline-four, again Mir’s GSX-RR.
This advantage helps a rider catch a draft off the bike in front. It also allows him to jump out of the draft and set up an out-braking move. It also gives riders more options in their tactics, because they’re not playing catch-up all the time.
Gaining metres in a straight line also helps race-distance performance, because the rider doesn’t need to hammer the front tyre so much on the brakes, nor destroy the rear tyre by getting aggressive with the throttle. And, of course, he doesn’t have to risk crashing by entering corners trying to regain the metres lost on the straights.
V4s can also work well around the whole racetrack, not just on the straights. A V4’s narrower crankshaft reduces the gyroscopic effect of the fast-spinning crank, so the rider can tip the bike into corners more easily and pick up the bike out of corners more easily.
An inline-four, with its extra crankshaft gyro, is more difficult to divert from its trajectory when the rider wants to tip into a corner, but once the bike is in the corner that gyro helps the rider maintain his line. This is one reason inline-four MotoGP bikes are faster through corners.
But MotoGP’s current technical regulations don’t help inline-fours, because Michelin’s rear slick can overheat if riders overuse the edge of the tyre. This is another reason V4 engines are currently so successful, because the rider who can turn his motorcycle quickly into the corner and quickly out of the corner abuses the edge of the tyres less.
It wasn’t entirely coincidental that Ducati started winning races when the Michelins arrived in 2016, after going five seasons without a victory.
However, last year’s Michelin rear compounds were stronger than the 2018 rubber and the 2020 rear tyre is all new and stronger still; so will it help the inline-fours close the gap?