Lets Talk Lower Control Arms

[Bowie]

Particularly in a FWD application where there is drive shafts and suspension in the road. He. Road.

Just trying to understand the load these humble triangles end up with.

Apart from locking the wheel in location, and with the aid of a ball joint allowing it (the wheel) to rotate left and right, whilst the inside edge of the control arm mounted to the chassis, provides for rotation up and down, I quickly see the lower control arm is really a clever little sod that does a great deal.

Under acceleration, would it be fair to say weight is transferred to the front chassis locating point of the control arm as the drive shaft tries to twist everything forwards, and under deceleration the rear as the twisting force is stopped? I confess I'm struggling to see how the lower ball joint could transfer much weight either direction but images of really beeffy box section style lower control arms, would seem to indicate to me there is significant load bearing these parts are required to handle?

 

[seasink]

Simplistic version: Think of the suspension as an inclined stick, shape unimportant, supported at the top bearing, stayed by the arm attachment near the bottom, and standing on the road. The arm attachment cannot support a vertical load (it is a hinge). We are interested only in loads external to this system.

The vertical load from the body is applied at the top of the strut, and resisted by an upward force applied by the road. If the stick were vertical the arm would be decoration. It is actually a little inclined, so the road pushes sideways, resisted by the arm.

If we accelerate, the road pushes us forward and the arm resists horizontally. In braking the road pushes backward with greater force. In turning, the road pushes or pulls sideways. More heavy horizontal loading on the arm attachment.

All these external forces may happen at once, and are resisted by couples formed by the arm attachment and the strut bearing.

We can look at the lower arm separately the same way, with various horizontal loads applied to the end of a cantilever fixed at the chassis.

 

[PeterT]

If you slam the brakes on, the wheel is slowing down but the rest of the car has momentum and tries to keep going forward. Thus the rear mount is in compression, the front mount is in tension?

Depending on the angle of the arm at any given moment, will determine magnitude and direction of the force from the spring/suspension in the arm. Thus the total force on a locating point could be an addition of tensile forces, an addition of compressive forces or the sum of compression and tensile forces.

 

[schlitzaugen]

Particularly in a FWD application where there is drive shafts and suspension in the road. He. Road.

Just trying to understand the load these humble triangles end up with.

Apart from locking the wheel in location, and with the aid of a ball joint allowing it (the wheel) to rotate left and right, whilst the inside edge of the control arm mounted to the chassis, provides for rotation up and down, I quickly see the lower control arm is really a clever little sod that does a great deal.

Under acceleration, would it be fair to say weight is transferred to the front chassis locating point of the control arm as the drive shaft tries to twist everything forwards, and under deceleration the rear as the twisting force is stopped? I confess I'm struggling to see how the lower ball joint could transfer much weight either direction but images of really beeffy box section style lower control arms, would seem to indicate to me there is significant load bearing these parts are required to handle?

I am not sure I understand what you're asking, but here's two pictures of a simple suspension setup, off a BMW 2002.


http://frogtowngarage.com/album_2002/BMW2002_FrontSuspension/images/img_1743.jpg

http://lh6.ggpht.com/_3zEDxv9mp2U/SadNe8BLB6I/AAAAAAAAEuY/xd2uwUgOi4I/s640/IMG_1382.JPG


Top is the front lower suspension arm, bottom is the entire front lower setup (only the struts are missing - the setup is of course a McPherson strut arrangement). Front of the car away form you.

Now then. The lower suspension arm is very flimsy as you can see. I had one of these cars, and I can assure you, if you stand on top of the arm, you can bend it with your weight.

So how does it cope with the strain of a car much heavier than your R12 (which has seriously beefy wishbones - a much stronger setup to start with)?

Well, the forces are never up-down on it. The arm only makes sure the wheel is x cm away from the chassis to the side. The radius control rod (see pic2 - you can see it under the sway bar) is what locates the arm fore-aft. That is where the strain is when you accelerate or brake. See in pic 2 the control rod? That is cast steel, solid rod about 2cm diameter.

Okay, I hear you, but the balljoint has to take the same stress, because that is where the force applied by the ground meets the force applied by the chassis (mentioned by ss above) and fight each other. Yes. True. But you can see in the pictures above, the ball joint is a separate item, it is made from special materials, and is captured in a cast steel fixture, and the ball itself is some strong alloy (I think at least on par with OL60 (D3?) - something like that judging by how it rects when you take a file to it - it is harder than your file). That means it can take a lot of shear stress, which is what it has to do. The bolts securing it to the arm are grade 12 anyway, so they take some shear stress with no problems.

Why all that? Well, because you are right even if you can't believe it, it is the balljoints that take the stress (yes, plus some on the inner hinge pin of the lower arm, but mainly the balljoint). The lower triangle (in your case the lower wishbone) is very rigid parallel to the ground, but very soft in a vertical direction (get yourself one of those older R12 suspension arms without the boxed section and see how easy it is to bend it - I would hazard a guess they are about 37daN/cm sq steel (as opposed to 60daN/sq cm for the bolts and balljoints). Again, you can bend them vertically by standing on them. (okay maybe I exaggerate a bit, but it's not far fetched, you get the point). But in the direction of strain (imagine how they attach to your chassis) they are very tough, because of all those shapes they have bent into them, the vertical and horizontal profiles and most importantly the roundness of the transition from horizontal bend to vertical (The BMW arm is bent at straight angles, not ideal). The Renault arm steel gauge is at least double that of the BMW too. But the BMW arm compensates because the lower suspension triangle is much larger, at least twice the size of the Renault, so it can put up with higher strain because of the longer leverage of each triangle apex in respect tot he opposite side (apexes). The BMW also converts the twisting moment around the hinge side (attached to the chassis) to an extension force on the radius control arm (the steel rod). So basically the lower control arm under strain would try to elongate the control rod (car braking) or compress it (car accelerating), which is what steel is best at coping with and it's not going to budge.

Yes, all these forces are relayed to the suspension via the wheel bearings and balljoints, but I don't think you need me to tell you how tough the bearing steel is (I think you have some recent experience). Well, the balljoint steel is about as tough as the bearing balls' steel. It can take immense loads no problem. In fact most of the time you will find balljoints wear by "erosion" - the inner surface of the socket and the outer surface of the ball sort of grind each other as grit and shit gets in and grease gets out and the ball starts to wobble around in the socket. But you'll never see a broken ball (like a broken egg in half). Or a sheared ball (okay, you might see a stud sheared off the balljoint - that is a weak point and it will give eventually, but it will take some trying to get that to happen). I think I have seen more suspension arms fasteners sheared clean off the chassis with balljoint still intact and correctly attached at the end of the arm than balljoints pulled out of the socket or sheared off.

Still here?

You need a life.

 

[seasink]

If you want to roughly calculate the forces on the joint, ignore the internal structure of the suspension and look at the external forces acting on it. The largest usually are the reactions from the road during braking and fast turning. The magnitude depends on things like vehicle mass, velocity and wheel layout. It's the road reaction, friction, that pushes you forward, otherwise the wheels just spin.

Edit The basic equations derive from Newton, as taught in school.

 

[jo proffi]

I confess I'm struggling to see how the lower ball joint could transfer much weight either direction but images of really beeffy box section style lower control arms, would seem to indicate to me there is significant load bearing these parts are required to handle?

Look at the little pissy bit of pseudo chassis the mount is attached to, and then the pissy bit of steel that holds the mounts to this.
The loads are clearly not that great.

Slide your car sideways into a gutter and you'll see the loads increase dramatically and the box section collapse.

Jo

 

[Bowie]

That was amazing.

So, you can modulate design with different materials. Ie softer material if the triangle is large(r). But by nature it is (as a triangle) going to be most rigid on the horizontal plane, and relatively soft on the vertical.

Great! so as it really is just a triangle that is been pushed over, with a bit of measuring, with someone to weld, we could all go make something like this!

I'e. round threaded steel tubing, with screw in eyelets with appropriate bush sizes at the chassis mounting point, and we would have a control arm that should be extended or retracted.

That seems fairly straight forward.

 

[robmac]

with screw in eyelets

A lot more than that, called rod ends (or by the old codgers) heim joints.

And used extensively in aircraft motion control.

 

[Bowie]

Ah yes. Yes that's better language.

 

[PeterT]

That arm looks a bit rough and isn't adjustable on the car. This is how I make them. The rod end is LH thread and the hex bush it screws into is RH thread, thus camber and castor can be adjusted without taking the arm on and off. Material is cold drawn seamless.

 

[Bowie]

Ah! That's a clever application. Yes perfect.

 

[schlitzaugen]

That was amazing.

So, you can modulate design with different materials. Ie softer material if the triangle is large(r). But by nature it is (as a triangle) going to be most rigid on the horizontal plane, and relatively soft on the vertical.

Great! so as it really is just a triangle that is been pushed over, with a bit of measuring, with someone to weld, we could all go make something like this!

I'e. round threaded steel tubing, with screw in eyelets with appropriate bush sizes at the chassis mounting point, and we would have a control arm that should be extended or retracted.

That seems fairly straight forward.

Modulate as in get away with using softer cheaper steel. Yes. But racers in California box up those BMW arms with a top and a bottom plate and weld them all the way along (the original arms have a few spot welds at the ends and one in the middle).

Come to think of it, I had a pair of R12 lower arms (the old model, single skin) in my hands and one of them was twisted so bad, I binned it. Not sure what I did with the other. You couldn't see the twist but if you put the arm next to a good one, it was obvious. That twist was from braking forces, no doubt. Mad men racing R12 in countries where that's all they have, attach a radius control arm to the lower wishbone to locate it more rigidly. The rod points back and it is attached at the other end to the chassis rail, right about where the back gearbox mounts are. On the arm, the rod is bolted on the lower balljoint studs/bolts.

The vertical loads (applied as per design) don't even strain the suspension arms because they just move up and down wherever the suspension movement takes them. If you exceed the limits of the design, of course they'll bend (quite easily as Jo points out).

One exception is the double wishbone setup of the R12 (and similar) where the top arm is subject to spring load (check out yours). That is where the weight of the car rests and you can see the arm is quite thick gauge metal and reinforced around the bushings both at the bottom of the shock absorber and the end where it connects to the chassis. It also has a complicated profile so the metal is not just a flat sheet. It has all sorts of curves to make the strain disperse along the thickest cross section achievable with the steel gauge they used.

That arm in your picture must be properly designed to take strain in the horizontal plane, and it doesn't really look like it's up for it. You also have to realise that extending and shortening those balljoints (rose joints?) at the chassis end will change more than just the arm length. They'll also change camber and caster (and by way of consequence toe) and probably have a big impact on your car's handling. The angle between the two looks a bit wrong as well. Depending on how much you want to extend the arm, you might run out of room in the brackets, because to balljoints move out further away form each other as you unscrew them.

Find Scotty's posts (old now) and see his suspension setup. He went to a McPherson setup on his R12 but can't remember what he did for the lower arm.

 

[Bowie]

Ah you have stumbled upon my intention! Yes a 12 with camber and caster adjustment please.

I have those 17 lower control arms out of the car, now would be a great time to measure. I think I have a network available to me of skilled dudes whom like dumb sh1t. I reckon if I plan it, provide the materials, I could talk them into assisting me.

#Edit, I tried to look for Scotts ol' posts but kept coming up nought. I wonder if that was part of the great server purge.

 

[schlitzaugen]

To adjust the camber, R12 drivers used to slot the bottom balljoint bolt holes out a bit. Make up a template to measure how much it changes, but don't go overboard. About 2-3 degrees of camber should do it. If you can measure the distance balljoint to balljoint (centre to centre) on the front hub carrier that will tell you how much to move for the desired outcome. Also keep in mind that ride height (i.e. suspension compression) is going to change the numbers (you will gain camber as the suspension compresses) so perhaps you should first work out where you want the car to ride and then decide how much you need to slot the holes. It's a simple year nine geometry problem. The real problem is to measure accurately the distance between balljoints. In fact, you should measure the length to hinge points on the arms and the distance hinge pin to hinge pin and then you can draw a model to scale to calculate accurately the changes in suspension geometry. Faint memory tells me you could find all these data somewhere in one of the manuals (most likely a french factory one).
Don't know about the rear end, it's too complicamacated.

 

[jo proffi]

My fuego received lots of mods, and to be honest most of them were a step backwards.
I suppose you want to make these mods yourself but don't expect every one of them to be a step forward.

Sure increasing (-) camber theoretically makes you able to corner harder, but it also makes you braking and acceleration worse and shreds the inside of your tyres.

On that note, the best mod you can make is shoding the car in the best tyres you can get……not exactly something you want to shred from too much neg camber!!

jo

 

[Bowie]

Yes fair enough. No illusion to what the 40yr old POS is, and I suppose much you all you before me, enjoying understanding, learning, and eventually doing.

Something adjustable seems sensible just to correct the mess I make when I start messing with springs and shocks.

 

[Bowie]

but since we are speaking of 12 lower control arms and ball joints. Folks have just drilled these out when replacing?




And Schlitzaugen, people used to enlarge the openings and simply bolt the lower ball joints further out in the arm?

 

[PeterT]

It's more important to increase caster than camber. You then get camber when you turn the wheel. Like an old Merc.

 

[schlitzaugen]

but since we are speaking of 12 lower control arms and ball joints. Folks have just drilled these out when replacing?


And Schlitzaugen, people used to enlarge the openings and simply bolt the lower ball joints further out in the arm?

Yeap. Correct and correct.

The geometry of the 12 is a little old school. By that I mean that the suspension goes to positive camber as the wheels droop. And the car has a lot of droop. I never actually checked to see what the camber gain was when the suspension compresses (the opposite of droop, right) but I suspect the camber would go negative or stay zero (from pictures of lowered cars I would say zero most likely).

That needs to be checked however. Because you want to know how much the camber will change with the stock setup if you just drop the car say a couple of cm or so.

I would leave caster alone for a start especially if you keep driving the car on the road. Can make the steering very twitchy.

 

[jo proffi]

It's more important to increase caster than camber. You then get camber when you turn the wheel. Like an old Merc.

And as a bonus, texting ay high speed becomes safer as the car gains more directional stability!!!