Torque Steer and the Nissan GTR LMP1 Nismo

"Torquelife" (torquelife) 03/14/2015 at 16:45 • Filed to: None

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So, yeah. I love the idea of this car, in so many ways.

As said in the blurb above, this is the first front engine, front wheel drive (commonly referred to as FF) car at Le Mans. One of the primary benefits of running an FF chassis layout is better traction,but generally FF vehicles suffer from what is known as torque steer. Torque steer is basically based on the different shaft lengths. More specifically, it is based on the different rotational inertias of the two drive shafts. This is because, as the engine suddenly increases its torque levels, the differential has to try and keep up.

A differential works by allowing one shaft to have less torque delivered through a series of gears or viscous clutches in order to adjust the power being delivered to each wheel. This was created in the early 20th century in order to help vehicles go about a turn without the wheels losing traction. However, due to the inertia of the two drive shafts being different in most FF vehicles, the torque will affect the two wheels in separate ways under extreme changes in torque. This effect causes one of the wheels to have the power delivered more rapidly, forcing the vehicle to move in the opposite direction.

This is why torque steer is generally felt when accelerating heavily from a dead stop, as the change in torque on the drive shafts is the highest at this point. This can, however, be felt at any speed so long as the change in torque is high enough.

Some trucks have offset differentials, though, so why do drivers of those vehicles not feel the effects? Well, they do, but the mass of the vehicle combined with the connected rear drive shaft cause the effects to be muted - generally to unnoticeable levels.

But why do the tires need different levels of torque under normal circumstances? As you go around a turn, turning with the front wheels, the arc-length from the center of the turn to each tire is changed. This is done in a rather odd ratio summed up very well in the SAE handbook on automotive engineering.

As the difference in the arc length changes, the effort on the wheels on the car changes, thus changing the torque that the differential needs to supply to each wheel. This is exacerbated in the front due to the fact that the those wheels are pivoting, unlike the rear wheels which are fixed. However, this is the reason that a differential was created - to supply different levels of torque to different wheels connected on an axle.

The differential will balance the smooth transitions of torque required to each of the wheels by allowing limited quantities of slip to the inside wheels. This is done through magic, by which I mean that complex gearing or viscous clutch set. Either way, the complex issue known as torque steer (which I personally love to death) is caused by high levels of flux in the torque between the two wheels, which is generally attributed in most FF vehicles by the difference in the drive shafts, as the shafts have the same density, but different lengths, giving them different rotational inertias.

Nissan GTR LMP1 Nismo engine assembly. Image: Top Gear/Greg Pajo ( !!!error: Indecipherable SUB-paragraph formatting!!! )

The Nissan GTR Le Mans vehicle, however, does not suffer from different shaft lengths. This vehicle has a centrally mounted differential, with even drive shafts driving each of the front wheels, as seen above.

That being said, this vehicle should have as much torque steer as any other LMP1 vehicle on the field.

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Cover image: Nissan promotional image introducing the GTR LMP1 Nismo. Image: Nissan ( !!!error: Indecipherable SUB-paragraph formatting!!! )

Santiago of Escuderia Boricua > Torquelife 03/14/2015 at 20:33

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It seems like all us engineers like this car.

I didn't know that torque steer happened because of the shaft lengths. Why not just add a bit of mass on the shorter one to even them out?

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Santiago of Escuderia Boricua 03/14/2015 at 21:07

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My Alfa Romeo 164-S did exactly this in combination with a fixed non-jointed intermediate shaft from the offset differential location to the other side of the engine... But even with an attempt to even the mass left to right, there are still differences in rotational flex because one shaft is longer than the other and even a system like this does not 100% eliminate torque steer. The only way to truly eliminate it is to have a centered differential (or axles that take into account not only mass but flex in their balancing design, which is VERY expensive...)