Independent Rear Suspension

Why?

In an FWD car, the rear is pretty much just along for the ride. FWD cars are designed with very little weight at the rear also.

In a rwd car, more weight is back there and keeping both wheels on the ground is critical because they are delivering power. IRS works there with better suspension geometry and reduced unsprung weight compared to a solid axle which is much much heavier than a twist axle that doesn't have to transmit drive torque to the wheels.

The geometry issue is moot when forces are purely lateral, the total grip is determined by the coefficient of friction (sticktion actually i think) and the normal force which are essentially both fixed. The amount of rubber on the ground has almost no effect and is actually determined by normal force divided by tire air pressure.

When it comes to unsprung weight, the IRS might have an edge while three wheeling because only 1/2 the unsprung weight is actually moving at that point but it loses because the complexity equals more weight almost every time.

Lightening the twist axle, adjusting the torsional resistance or adding camber will all have an effect but changing to a custom IRS would be very expensive with questionable benefit.

No one will do it except AWD conversions like Olsbergs.

I thought I covered this:

It will be expensive.

It will be heavier than a twist beam guaranteed.

The only two things you can do with IRS that a twist beam can't is give it a toe curve for "rear steer" in corners and give it a camber curve which is probably no better than static camber when the rear is lightly loaded and body roll isn't huge.

Everything else is worse except unsprung weight might not be as bad as the extra weight suggests.

It does almost nothing beside add cost, complexity and weight and there is a cottage industry of suspension kits designed to eliminate "rear steer" because toe changes can feel bad in a corner.

As for high end performance cars, how many are front wheel drive? I'm going to put that number at zero.

As Fun as a Fiesta ST is, its a fwd hot hatch. Its not in the same segment but Ford figured out how to make it handle like it is and a light weight twist axle is part of their successful plan.

IRS on a Fiesta is change for the sake of change unless you are adding AWD and your drive shaft wants to go straight through that twist axle.

I can agree with all that you have said except.

wash said:

If you take the twist beam out of a twist axle rear suspension the wheels will flop around wherever they want.

Click to expand...

That is just wrong because you could/would not build a rear arm on a single mount point.
It would have to be built so that it could not flop around.
For example the classic Mini, it uses a longer bolt/axel to prevent the arm from twisting and has no anti-sway bar.
So assuming the arms did NOT flop around, the geometry would also be unchanged if the beam was removed.
Then of course you would have independent arms.

In conclusion the Fiesta's rear suspension is two independent arms tied together with a beam to eliminate additional mounting hardware.
In addition the interconnecting beam acts as an anti-sway bar.
This of course makes it an Interdependent Rear Suspension.

Dave

Ataru said:

Since the Fiesta and the FiST have a twist beam rear suspension:

Click to expand...

Take out the twist beam from ^^^ that ^^^ and then explain to me how toe and camber are controlled.

If you say "its the bushing" you get an F in remedial suspension geometry class.

In traditional suspension analysis, the arms are broken down in to links, so when you hear about a "five link rear suspension", each side has five links but you can also count an A-arm as two links making two A-arms and a toe link a "five link rear suspension".

In a trailing arm independent rear suspension the trailing arm is always a two link arm. That constrains the motion in to an arc contained in a set plane.

A single link only constrains motion to a spherical surface in a 3D space rather than a plane. In layman's terms, it flops around wherever it wants.

When a twist beam axle still has its twist beam, you can ignore one side and call the twist beam an A-arm with motion constrained to an arc in a plane perpendicular to the line drawn between the two bushings. That's the typical very simplified analysis of a twist beam axle, you ignore half and leave out torsion entirely. If you cut out the connection to the other bushing, now you only have one link, no A-arm and it flops around wherever it wants.

So what does that tell you?

Don't cut out the twist beam is a good start.

Again professor wash, you have proved my statement and the ONLY thing, I have said.

The Fiesta's rear suspension is two arms tied together with a beam and one cannot work without the other.
This of course defines Interdependency.

I truly have enjoyed your lectures.
I am very glad that you have posted them so I did not need to take notes.
However you really do read too much into people's statements and then overstate the obvious.
(more sarcasm)

Thanks again, but I will have to leave you now.
I have a life to get back to.

BTW, getting back to the whole point of this topic.
No, I don't think an IRS will be built as an aftermarket part for this car, due to cost and very limited potential sales.

Dave

I have to laugh a bit when I'm reading about how a IRS would be heavy and costly. I used to have a 1st gen Dodge Neon and it did have a independent rear suspension. It was cheap, simple and worked well. Here is a pic. I also believe that the Honda Civic has one too, another inexpensive car. Now adding a IRS to a car that didn't come with one from the factory, that's another story all together if that is what you guys are talking about.