Long or Short A-Arms???

[tnttim]

So, I was wondering what others opinions were of Long or Short A-Arms for the rear independant suspension.

 

Since longer A-Arms would be more leverage for accelleration and twist for braking, would it be a real advantage or should a shorter arm be more advised?

 

Tim

 

 

[Alaskossie]

Certainly, there has to be some more sophisticated comparative analysis of IRS designs than this???

[slngsht]

tnttim, by "long", do you mean fore and aft? in other words, a symmetrical A shaped arm, vs one where the front leg of the A is much longer and attaches to the chassis WAY forward of the rear wheel?

[slngsht]

tnttim, by "long", do you mean fore and aft? in other words, a symmetrical A shaped arm, vs one where the front leg of the A is much longer and attaches to the chassis WAY forward of the rear wheel?

[Ian7]

The really long forward links which I believe slngsht is describing is often called a trailing arm design, very common on 60's vintage formula cars.

Given an A-Arm designed and manufactured with appropriate strength and more importantly stiffness, there's no geometry advantage I know of to a trailing arm design.

Of course, other compromises come into play, like packaging the whole works in a confined space...

[tnttim]

tnttim, by "long", do you mean fore and aft? in other words, a symmetrical A shaped arm, vs one where the front leg of the A is much longer and attaches to the chassis WAY forward of the rear wheel?

 

 

Well, I mean the length of the A Arms themselves. Inner and outer. Such as the distance from the Outer ball joint to the inner pivot point near the Diff. Based on the bottom of the legs (inner Bushings) being the same distance apart.

 

Of course with 'proper' geometery for roll center and caster and all the other junk. I am specifically asking about the actuall length of the arms.

 

 

Given the outer ball joint and upright does not move. would I rather have the inner ball joints (bushings) closer to the Diff. or closer to the Side of the car?

 

Shorter would be less likely to flex and twist. But is limited on motion. Techinally the angle of the arms moves more the shorter they get. Given a fixed vertical motion of the upright.

 

Have I confused everyone yet???

 

tim

 

 

I'm not refering to any trailing link.

 

[Arya Ebrahimi]

I'm with you. Don't know the answer, but at least I understand the question

[bsimon]

There are a number of things to consider here, one of which is the transfer of rotational torque to the chassis. (flex and twist of the wishbones)

 

Since the diff is mounted to the chassis, the wishbones are not transferring any acceleration torque, however if the brakes are mounted to the uprights you still have to transfer braking torque. If the calipers are mounted to the diff the wishbones transfer no rotational torque. Inboard brakes will allow a substantially flimsier system to be used.

[slngsht]

 

 

Shorter would be less likely to flex and twist. But is limited on motion. Techinally the angle of the arms moves more the shorter they get. Given a fixed vertical motion of the upright.

 

Have I confused everyone yet???

 

 

 

Make the links the size they need to be for optimal geometry.

 

Any flex in the link will be negligible - if your link flexes any appriciable amount, it will fail from fatigue anyway. Remember your links are primarily in tention / compression (with exception of spring forces.

[Arya Ebrahimi]

There are a number of things to consider here, one of which is the transfer of rotational torque to the chassis. (flex and twist of the wishbones)

 

Since the diff is mounted to the chassis, the wishbones are not transferring any acceleration torque, however if the brakes are mounted to the uprights you still have to transfer braking torque. If the calipers are mounted to the diff the wishbones transfer no rotational torque. Inboard brakes will allow a substantially flimsier system to be used.

 

Anybody here using inboard brakes? Can you comment on the slop in the axle shafts? Do you feel it when you go from accelerating to braking?

[tnttim]

 

Anybody here using inboard brakes? Can you comment on the slop in the axle shafts? Do you feel it when you go from accelerating to braking?

 

Actually I work on another Members car that has the inboard brakes and that is what got me thinking about this.

 

The other reason is I am using a turbo powered engine which when done will have about 350 Hp in it, and I would rather not rip off the rear end of the car.

 

Or I can just run non-sticky, small, skinny tires in the back. NO traction = No twisting :ack:

 

Tim

[horizenjob]

 

I wouldn't expect you to feel any slop in the driveshafts. Unless your using those old rubber donut things, and then it would indicate replacement time.

 

Generally you would want longer then shorter arms. Just to keep the geometry from rapid changes at the ends of the travel. But without specifics it's hard to say!

 

Inboard brakes make a big difference in unsprung weight. How does that work on the car you work on, are there diffs that provide the mounting for that or is that a custom attachment?

 

 

 

[JohnK]

I think that in ANY current suspension design, all the links carry load in compression and tension only - no bending, not twisting. The link lengths and their origins and insertions on the chassis and hub relative to one another are carefully designed (hopefully) to make the wheel at the corner in question behave in a very specific manner when there are changes in ride height, roll, and pitch. The goal of the suspension designer is, among many other things, to keep the wheel vertical at all times and ensure that the track does not change when the car is driven - this is, of course, impossible.

If one has even a hint of compulsiveness as a character trait along with a fondness for cars that are particularly fun to drive around corners, I would avoid buying a copy of Wm C. Mitchell's WinGeo3. You can measure your car's suspension points, plug in the values and watch what happens to the wheels, say camber change, when your suspension sees a few degrees of roll, and what would happen if you lengthened (or shortened) a suspension member. Try Alan Staniforth's "Competition Car Suspension" ISBN 1 85960 644X for a very readable overview

[JohnK]

Actually I work on another Members car that has the inboard brakes and that is what got me thinking about this.

The other reason is I am using a turbo powered engine which when done will have about 350 Hp in it, and I would rather not rip off the rear end of the car.

Or I can just run non-sticky, small, skinny tires in the back. NO traction = No twisting :ack:

Tim

 

Was listening to a parts guy at a local Subaru dealership describe how he's turned his VW Passat into a fire-breather and I asked "How are your driveshafts?" He said, "Yeah, I'm saving up for a a set that're up to the power I'm making." Turns out this is a growing industry with all the bolt-on goodies one can buy, and the stock car engineered for just-that-much-torque and not much more. He said there were three different grades he could buy for his VW depending on the amt of power (torque, really) he's anticipating. You might think of your driveshaft too, along with the half-shafts. I'll bet the local industrial driveline place would be of help here (don't wear a coat and tie for the first visit :-) .

[JohnK]

Anybody here using inboard brakes? Can you comment on the slop in the axle shafts? Do you feel it when you go from accelerating to braking?

 

I took my half-shafts to a local Subaru dealer and had a mechanic I felt knew his stuff check them out. I was certain something was amiss from the amount of free play I felt in them, being suspicious about how the flange for mounting the inboard disk had been welded to the inboard CV joint carrier (seals had been melted when I looked inside the diff where the output shaft fitted). But, he was thorough in his checking and said they were fine. Major items in the rear end assy (diff, hub, half-shafts) are from a Subaru WRX which has been around forever and, from what I know, is big in the rally and off-road world - so perhaps that's just the way that setup runs. I'll be interested when it gets on the road to see what it feels like. Can you say "drive-line snatch" ?

[tnttim]

 

Inboard brakes make a big difference in unsprung weight. How does that work on the car you work on, are there diffs that provide the mounting for that or is that a custom attachment?

 

 

 

 

Yes they are attached to the DIff and the rotors are attached to the inner CV joint.

 

Once you take the Brakes out of the question then the A-Arms are really there just to support the upright, and there is hardly any rotation there from things like the brakes applying torque.

 

More or less of what I was looking for is the advantages of a shorter A-Arm as opposed to longer ones.

 

With the HP I plan to run and sticky tires, when I accellerate there will be a compression load on the front joint and tension on the rear.

 

Now with a given force at the wheel, the longer the A-Arm length is the more the force will be exerted on the joints.

 

Think of it like a lever......

 

 

_____1ft_______1lb = 1 ft/lbs of torque

^

 

 

____________2ft________________1lb = 2 ft/lbs of torque

^

 

 

The weight never chaged but it added alot more force at the pivot point.

 

Tim

 

 

[pierats]

TNTIM,

 

What do the F1 cars use?