Why Yoshi doesn’t like solid rear axles

To start, any suspension engineer will prefer IRS (Independent Rear Suspension) to Solid Rear Axles and you should too. The key word is “prefer”. In some cases, other preferences or constraints will require a solid rear axle as a compromise. There are only a few up sides to a solid rear axle. This article will cover the general bases of the pros and many cons of the SRA (Solid Rear Axle).

The good sides of a SRA
There is a reason you see a solid rear axle in many vehicles. The predominant reasoning is that they are simple and cheap. There are (usually) fewer parts to maintain and the cost to manufacture a SRA is much lower than an IRS setup.

Due to the type of construction, SRAs are also stronger. They will stand up to more abuse; especially lateral hits (that sometimes happen in drifting). Although, once the rear is broken, the entire rear will have to be replaced. At least it’s cheap.

SRAs have 0 camber gain (camber added during suspension travel). This means that on a flat surface, if the suspension functions properly, the vehicle will have 100% of the contact patch on the ground throughout total suspension travel. This is only good under these ideal circumstances. Most track surfaces are smooth and hopefully the suspension is setup well. This is the largest pro of the SRA and why it’s commonly used in drag racing.

The bad sides of SRA
I’ll start with the biggest deal killer of the SRA – The weight. Not just that the axle itself is heavy but that it’s unsprung weight. As a result, the rear end tends to receive stiffer springs/shocks. But this can cause wheel hop issues. As a result of the weight, factory chassis tent to allow for as much as 7 inches of suspension travel on the up or down stroke. While this has nothing to do with drifting, this added space takes away from trunk/seat space in the rear of a solid rear vehicle.

Another fairly large down side is that the rear alignment isn’t truly adjustable. I’ve seen people get something like -0.1 degrees of negative camber but that doesn’t really help much. Toe is similarly not adjustable.

Most SRAs I’ve seen are panhard style. There’s also Watt’s link but I haven’t seen a drift car with one. Panhard bars will make a vehicle move differently left or right since the bar is asymmetrically mounted to the chassis from the rear axle. The Watt’s link is a bit more symmetrical but also adds more unsprung weight.

Coil spring SRAs can have axle locating issues from the factory. Stiff bushings and sometimes additional links are required to keep movement to a minimum. Similarly, leaf springs will bend into an “S” shape under load which can cause traction issues. There also are lateral force issues with leaf springs when drifting. Again, there are some extra provisions that can mitigate these undesired effects.

In the end, the solid rear axle comes down to excessive weight and lack of geometry. Geometry is more how the solid rear is located in the vehicle through suspension movement than camber/toe adjustment. This is mainly an issue since the weight of the rear unit lends itself to large amounts of travel. All of this can be mitigated but not eliminated. It will work, but it isn’t preferential.

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