# Thread: Are these poorly designed A-arms

1. Saw this indy pic on facebook and thought...

parallel equal a-arms!?

I'm a first year rookie suspension guy but I was under the impression unequal length non-parallel (SLA) were superior.

with SLA you get a lower camber/roll coefficient and lower roll center.

Am I wrong?

2. Saw this indy pic on facebook and thought...

parallel equal a-arms!?

I'm a first year rookie suspension guy but I was under the impression unequal length non-parallel (SLA) were superior.

with SLA you get a lower camber/roll coefficient and lower roll center.

Am I wrong?

3. <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by OspreysGoSWOOP:
with SLA you get a lower camber/roll coefficient and lower roll center.
</div></BLOCKQUOTE>

So then the design judge might ask you:
How much do you think this car rolls while cornering?
Where is the roll center for parallel equal length a-arms?

4. <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Crispy:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by OspreysGoSWOOP:
with SLA you get a lower camber/roll coefficient and lower roll center.
</div></BLOCKQUOTE>

So then the design judge might ask you:
How much do you think this car rolls while cornering?
Where is the roll center for parallel equal length a-arms?

For parallel suspensions links, the roll center is at infinity. This always puts the roll center at ground level.

The ground level RCH will introduce a higher roll couple than say a RCH 1" off the ground, but this might be offset by their wide track width which would reduce lateral load transfer and decrease the camber gain caused by body roll.

5. The first year rookie design judge might ask...

Are those a-arms equal length?
If they were, what would be the compromises made on the steering geometry?

Hint: I don't think they are equal length. Look at where the lower pushrod is pointing. The lower balljoint looks to be an inch or two outboard of the upper balljoint.

6. <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by OspreysGoSWOOP:
For parallel suspensions links, the roll center is at infinity. This always puts the roll center at ground level.

The ground level RCH will introduce a higher roll couple than say a RCH 1" off the ground, but this might be offset by their wide track width which would reduce lateral load transfer and decrease the camber gain caused by body roll.
</div></BLOCKQUOTE>

That does not seem unreasonable, but I was thinking a slightly different direction.

I might guess that they are so stiff in roll the suspension does not move much (to keep the aero flat). If the suspension does not roll, camber gain wont do much of anything. Even if they are not equal length, as Nick suggests, camber gain is still nearly zero (assuming close to parallel) until the suspension travels significantly. Will it travel enough to make a difference? My guess is no.

As far as their choice of roll center locations, your guess is as good as mine. Might have very little to do with "optimal" suspension, it could be for aero, or it could be for packaging, or stiffness, or safety, or looks... It might not even matter that much.

It could be that making money was more important than vehicle dynamics. The Dallara provided IndyCar chassis are spec, they don't have to compete with anyone so there is not much incentive for them to over engineer things. Instead of designing a car to go as fast as possible, they design it to the spec desired by the governing body, which mostly means it needs to be safe, it needs to look/sound like a race car, and be as simple as possible to build and maintain.

Overall, I would say that they are not poorly designed, but it is likely they have different targets than we have in FSAE.

7. We were discussing this a few weeks ago in our garage. Definitely appears that the lower control arms are mounted outboard of the uppers. Our suspension guy was speculating that the asymmetry of the tires likely makes it possible for them to do this. No one could really figure out WHY they would do it, but they have. I'm guessing there's likely some packaging reasons.

8. Why a parallel arms layout should be considered a bad design? I see a lot of good things in it.

Of course if the car is really stiff in roll then camber gain is not anymore an issue, but also if the car is not so stiff you could anyway achieve similar camber control in different ways. Having at the same time other possible advantages that a non parallel (or nearly parallel) layout could not give.

9. When was the last time you checked on F1 suspension? There are things that matter more than suspension design on some forms of motorsport...

10. Absolutely nothing inherently wrong with it, just like there is nothing inherently "superior" about a unequal length non-parallel arrangement.

Nothing wrong with having high roll centers (or really, high jacking coefficients).

Nothing wrong with having 50% weight distribution, 55%, or 45%.

Nothing wrong with bump steer, in fact I'd say using bump steer can be quite advantageous if you know what you're doing.

It's all a choice of making the car what you want it to do. You specify some performance metric of your car (or any other item) and whatever is necessary to achieve that falls into place. No sense in debating whether or not individual piece parts contain more or less magic than another set of individual parts.