CorteX Racing partner and founder Filip Trojanek went deep for us, debunking the idea that independent rear suspension is always superior to a solid-axle setup. His response dropped so much knowledge on us we decided to share it here:
We often see solid-axle cars keeping up with or even outrunning their IRS-equipped competitors. Why? The reason is simple: An independent rear suspension doesn’t necessarily perform better than a well-designed solid-axle setup. This is especially true for vehicles with a lot of front weight bias, like a Mustang.
It all comes down to how a car shifts its weight backward when it accelerates. That weight needs to transfer to the rear tires so they can grip the road during power-down, but it takes time to travel through the springs and shocks–especially on a front-heavy vehicle. On a high-horsepower car, that time delay shows up as lost traction.
(IRS does work well on mid- and rear-engine supercars, since their back tires don’t need as much additional weight to put down the power. Even a Corvette gets pretty squirrelly when it has a 52- to 58-percent rear bias.)
The way to speed up the process is to increase the anti-squat geometry. This allows the weight to transfer directly, without waiting for the springs to load up.
However, an IRS with significant anti-squat generally has bad rollsteer characteristics, making this an impractical solution.
The same is not necessarily true for a solid axle. A torque arm design is good for both power-down and cornering because it allows for a relatively high anti-squat value without compromising other geometry. It’s also very tolerant of rear ride-height changes (IRS is extremely sensitive in comparison) due to the stable fore/aft instant center location.
If you drive on bumpy surfaces, it’s usually better to reduce anti-squat since it will let the suspension move more, allowing for more tuning and engagement of the shocks and springs. This can be accomplished by changing the angle of the rear lower control arms.
Everyone fixates on track handling, but on the street, what the car has makes an enormous difference in ride comfort due to the sprung-to-unsprung weight ratios. The lighter the car, the more pronounced it becomes and on Lotus Seven type cars (or even Miatas), it's honestly the difference between a comfortable car and one you don't want to drive on an even slightly bumpy road.
Well, i see i am not needed here 
I do think it's kind of funny that most of what people do to their cars to make them "handle better" is basically just turning their independent suspensions into more and more like a solid axle. For instance installing super stiff springs or sway bars to try to keep camber from changing through roll.
I have plans for building an fsae car for SCCA autocross in the future and I'm definitely going to be using solid axles (with probably a de Dion in the rear) for sheer simplicity. There are actually tons of benefits to it compared to independent stuff. For instance, it takes far fewer suspension pickup points on the chassis. You also get consistent predictable camber behavior. It also makes a great platform to mount sprung arrow elements to, which is totally legal in that class.
kb58 said:Everyone fixates on track handling, but on the street, what the car has makes an enormous in difference in ride comfort due to the difference in sprung-to-unsprung weight ratios. The lighter the car, the more pronounced it becomes and on Lotus Seven type cars (or even Miatas), it's honestly the difference between a comfortable car and one you don't want to drive on an even slightly bumpy road.
I've put a solid axle in the rear of my V8 MG, which has Miata front suspension and very similar weight and balance to the Miata. I'm having a hard time getting the ride quality in the rear where I want it, and I think it's in large part due to that massive axle bouncing around.
In reply to Keith Tanner :
Based on the fun I had loading the Explorer axle for the Ranchero out of the van, I'm pretty sure it's having a substantial impact on the sprung:unsprung ratio... It's not apples to apples comparing it to the A1 VW knuckle on the shelf, but I can't help noticing when I pass them both...
A bit off topic, but loading into the van with the help of an exuberant but cavalier assistant reminded me how much I prefer doing that sort of thing myself. Even if the total time it takes is longer, the number of digits retained per unit time is better.
Solid axles can definitely be a problem in the unsprung weight department, but other than that, they're generally simpler to get setup in a way that works well. Even a solid front axle isn't a bad thing if it's done well.
I prefer IRS for the street. On a smooth track, I am not sure it really matters too much. On the street the suspension has way too much to deal with, potholes, camber changes, surface changes, dirt, ice, snow, rain, and that is before you try to turn
I’ll tell you this, my ford 8.8 lifts the inside tire on off camber corners on the street. It sucks in the rain or snow, since you can’t whomp on the gas to pull out in traffic.
freetors said:I do think it's kind of funny that most of what people do to their cars to make them "handle better" is basically just turning their independent suspensions into more and more like a solid axle. For instance installing super stiff springs or sway bars to try to keep camber from changing through roll.
In a way, that is actually making them the opposite of a solid axle, because bump on one end will result in droop in the other and vice versa, since the springs are by necessity not located over the contact patches. It is sort of like having a built in Z bar.
If it is not apparent, i consider this to be a virtue and not a vice. Especially when judicious amounts of anti squat are present and the goal is to accelerate as hard as practical over rough terrain.
In reply to Trackmouse :
Isn't that more of a tuning problem than a stick axle problem?
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