Why inline-four MotoGP bikes handle better than V4 MotoGP bikes

A V4’s crankshaft is shorter, more rigid and runs on fewer bearings, which helps in the search for more power. An inline-four’s crankshaft is longer, which makes the bike more user-friendly through the corners.

“That’s why it’s the engine that gives an inline-four its superior line through corners, not the chassis,” says a former Yamaha engineer who now works for a rival factory.

But why?

It’s largely down to something called moment of inertia, which can most easily be explained like this…

Imagine you are walking along the top of a garden wall. Do you walk with your hands by your sides or do you spread your arms out? This is the principle: spreading mass further from the pivot point increases the moment of inertia, which minimises your body’s ‘rotation’ around the top of the wall, thereby giving you more stability and better balance.

By the same token figure skaters pull in their arms to reduce their moment of inertia in order to spin faster.

Now apply this thinking to a crankshaft (which is your stretched-out arms as you walk along that wall). A V4’s shorter crankshaft rotates around its longitudinal axis more easily than an inline-four’s longer crankshaft. This is why V4 MotoGP bikes are twitchier than inline-fours and therefore more demanding to ride.

We see it all the time: when a V4 gets sideways on the power it’s more of a handful than an inline-four in the same situation. This is because the V4’s crankshaft has a lower moment of inertia, which allows the crank to turn around its longitudinal axis more easily, making the bike unstable.

On the other hand the higher moment of inertia of an inline-four’s longer crankshaft is slower to turn around its longitudinal axis, so the bike reacts less violently to any loss of grip.

A longer crankshaft doesn’t just make an inline-four more stable accelerating out of corners, it makes the bike easier to ride in pretty much every situation.

An inline-four rides through corners faster and turns better through the neutral off-brakes/off-throttle phase, thanks to the self-righting force of its longer crankshaft. This makes the bike want to continue riding its line, without wanting to drop further into the corner or stand up out of the corner. This effect is similar to the behaviour of a motorcycle with a lot of trail.

This same effect of self-righting and of damping down unwanted movements also improves grip and helps on bumps, because the bike is more difficult to disturb from its chosen path.

These are important considerations, especially over race distance, because they make the bike less demanding to ride, both physically and mentally, which allows the rider to spend his energy on going as fast as he can, not on controlling the bike.

Now you know why riders like Johann Zarco, Jorge Lorenzo and others struggle when they switch from inline-four MotoGP bikes to V4s.

The other big positive of an inline-four MotoGP engine is its dimensions. An inline-four is shorter and more compact than a V4. This gives chassis designers a freer hand when they design the motorcycle, so they can get the centre of mass exactly where they want it, fore and aft, up and down.

Of course, inline-four MotoGP bikes do have their negatives, because too much of any positive in one direction usually turns into a negative in another, so nothing in race-bike design is all good.

It should come as no surprise that a bike which is slower to lose stability will turn into corners slower and change direction slower. Engineers work to mitigate this negative by adjusting geometry and by tweaking crankshaft design. Although a longer crankshaft makes the bike friendly, too much moment of inertia makes it lazy. So inline-four engine designers concentrate much the crank’s weight at its centre.

Whatever the science and technology, nothing really matters in racing apart from what happens on Sunday afternoon. Then it’s not about who’s got the friendliest bike, it’s about who gets to stand on top of the podium.

The V4 is currently by far the most effective engine configuration in MotoGP, because Ducati and Honda V4s have won 44 of the last 50 MotoGP races. That may be due to multiple factors, including engine layout, chassis architecture, rider talent and tyre design.

All of these mechanical factors combine with the result that V4s and inline-fours make their lap times in their own ways. Simply put, the inline-four carves a U line through corners, while the V4 carves a V line.

The inline-four MotoGP bike takes a sweeping line to exploit its superior corner speed, which improves its corner-exit speed and compensates for the engine’s relative lack of grunt.

The V4 isn’t good mid-corner, so it uses a sharper, slower line, turning the bike more aggressively mid-corner to move into the corner-exit phase as soon as possible, so its superior horsepower can be unleashed. The V line also gives the V4 an advantage in braking, because the rider can brake right to the apex, while the inline-four needs to get off the brakes earlier, to get set up for its swooping arc through the corner.

There’s also the simple fact that it’s easier to overtake in a straight line than around a corner.

And an inline-four might be faster given a clear track, but MotoGP doesn’t often allow that.

These are the reasons that Suzuki’s and Yamaha’s inline-fours don’t have what it takes to consistently beat the V4s.