What's the fundamental difference . . .

What's the fundamental difference between a front suspension and a rear IRS suspension?

When you look at so many cars the rear seems to be simpler than the front as if it has to do less.

Some might say it's that the wheels pivot, and while I might be wrong, I don't really think that's really it or maybe I should say totally it.

I am working on a suspension for a Locost middie type of vehicle and I have all the right books as well as a design the car was intended to use. The more I read and the more I work on it the less I know if I'm trying to improve the existing design or redesign a totally new suspension. At this stage a clean sheet design is fine with me if it nets me something that works well.

The way my brain works I have found that if I can understand basic concepts or differences then it's much easier for me to absorb and understand the details. It just seems I'm missing some basic point about the difference between the 2.

If there's not a difference then why do the front and rear differ so much in mounting points and geometry. The rear seems to have more room for packaging and yet the suspension seems smaller or simpler somehow.

For the purposes of this thread and my question, I'm not interested in measurements or looking at one car's design vs. another, I'm only trying to understand the basic concept of why the rear differs or else why it works differently. Once I have this in my head many things will fall into place.

some suspension design considerations which drive differences between front and rear:

• is it FWD or RWD
• what's the F/R weight distribution
• how much (if any) anti-dive and/or anti-lift is designed into the front suspension
• how much (if any) anti-lift or anti-squat is designed into the rear suspension
• how much roll-understeer is built in (either front or rear)
• what are the other packaging constraints of the original vehicle for which this suspension was designed

all of these things will drive the size, shape, and attachment of the suspension components.

AngryCorvair wrote: - how much (if any) anti-dive and/or anti-lift is designed into the front suspension - how much (if any) anti-lift or anti-squat is designed into the rear suspension

And one thing to remember on the above- for the most part, the front will lift and the rear will squat under acceleration. So the wheels need to react differently depending on which wheel is being driven.

BTW, the car will lift/dive/squat- whatever you want to call it- based on wheel torque- so make sure you factor that in- it will add or subract from movements due to mass under braking, acclerating, turning. It's interesting to see a car move around when it's on a chassis roll.

NO, what I'm looking for is the basic difference in concept between the front and the rear IRS suspension. Anti-dive and such are details. I'm not looking for details and the car is a middie.

One end steers and the other doesn't?

Apparently, the suspension on some formula cars is designed so they use the same upright carrier at all four corners. Racerparts Wholesale sell an example.

In almost every case the geometry is different front to rear and the rear is simpler. I'm trying to figure out why. The rear usually has more room for packaging as well.

The steering geometry will play a large role in the front suspension. Instead of three inputs (bump, roll and pitch) for the suspension, there are four (steering). Needing to consider that and a number of other details (weight distribution, anti-features, trackwidth etc.) will drive the differences between the front and rear suspension.

Edit: IMHO a front suspension is harder to design because you have to consider that fourth input.

carguy123 wrote: In almost every case the geometry is different front to rear and the rear is simpler. I'm trying to figure out why. The rear usually has more room for packaging as well.

One part has to do with freedoms of motion. In three dimensional space you start with 6 (movement in x, y, z plane and rotation about x, y, z, axis). In one side of an IRS, you're only wanting to allow 2 (discounting rotation of the wheel); vertical motion and "body roll". In one side of a front suspension, you have those two, plus steering. So, while it seems like a simple addition, you've gone from locking down 66% of the freedoms of motion and controlling movement in the other 33%, to only locking down 50%, and having to control the other 50%.

Now combine that with tire physics, and attempting to keep the contact patch ideal under braking, acceleration, and all different manner of cornering loads.

So maybe it is the fact that the front wheels pivot afterall. It just didn't seem like it added that much to the mix.

The roll center in the rear should be higher than the front.

Also in general the trackwidth should be larger in the front then rear.

93EXCivic wrote: Also in general the trackwidth should be larger in the front then rear.

WHAT? I don't think I've ever had a car that had the front track wider than the rear.

It was an article in Racecar Engineering. I will check the reasoning again.

Every car I've owned has had a wider track in front than the rear.

My E150 had a wider front track than the rear - it seems a lot of Ford vans did. I understand it was done to reduce the turning radius.

carguy123 wrote:

93EXCivic wrote: Also in general the trackwidth should be larger in the front then rear.

WHAT? I don't think I've ever had a car that had the front track wider than the rear.

Take a good look at a Volvo 240. It is different by a significant amount.

A wider front track is more stable in sharp turns too.

Think of slicks & skinnys but reversed.