Which rear control arm does what? I got a pair of adj arms and need to verify which arm controls the camber, the upper or lower?

Maybe I should just go ahead and get another pair to replace them all?

 

Lower is camber

 

If you get all four, make sure you tell your alignment folks to adjust toe via the typical method - not with the new upper adjustable arm.

 

Right - adjusting toe via the subframe adjustments is much better.

 

That's as I expected. So replacing the upper one would mostly be for strength purposes then.

Thanks !

 

The upper and lower arms are essentially the same, with the lower being galvanized and the upper one being just painted since it's not subjected to the same amount of road spray. On '04 and later cars, an adjustment was incorporated in the trailing arm where the lower control arm mounts to allow for camber adjustment. Camber was fixed until that change. All MINIs have had toe adjustable via the forward trailing arm mount; loosening the bolts allows the mount to slide laterally, giving toe some good adjustability.

Replacing the control arms with adjustable ones allows camber adjustments in pre-'04 cars, and makes for a wider range of adjustability in later cars. Only one arm per side is needed to have camber adjustability, this can be either upper or lower, but the lower position benefits most by having the adjustability, and the lower arm bears most of the lateral loading in turns so an aftermarket high-strength arm does best here.

The upper and lower arms can both be replaced with adjustables, this can be used to adjust toe, but is nowhere near as effective as using the forward trailing link adjustment. A little known secret about using upper and lower adjustable arms is that the wheel/tire can be positioned laterally (changing rear track) by the arms and the toe adjuster to help prevent some minor rubbing when fitting big wheels and tires and not having misalignment.

 

Right you are, but adjusting the length of the control arms to increase or decrease track will also influence toe/camber curves.

I built a scale model of this set up just to see what was happening. If I remember, the camber curve becomes more aggressive as does the potential for rear steer affects if the arms are shortened. This may or may not be desirable depending upon where you drive. I don't like rear steer affects on a track, but this might be one of several helpful techniques for those who autoX.

Camber compensation as the car leans is typically a function of spring rate...and to a lesser extent, damping. You have to anaylize where the wheel is in steady state; too much compensation and the rear tires corners on the inside shoulder...not enough and it corners on the outside shoulder. There is no one correct correct setting for everyone, just one correct or best compromise for a given setup.

 

The long length of the control arms and the fact that they are equal length and tie together close together and to near the center of the subframe makes the whole system work much like a swing axle, with the resulting tire movement being pretty much in a simple arc. If these were unequal length or if the bars would have been more parallel, the camber and toe would have been a much more complex motion and more affected by arm length and offsetting the front trailing arm mount. Being that the arms form side to side are not parallel with each other, there is a slight change to be expected by moving the trailing arm laterally outwards, but with the arm angles versus the overall length, this should be pretty much negligible, especially since in practice the adjustment would only be able to be made a few millimeters inward or ourward at most without some modifications to the trailing arm mounts. Such slight movement of the wheel might be enough to reduce or eliminate a nagging rub though.

Some people see a swing axle design as limiting, though I see it as being more predictable and adjustable and able to cope more with things like lowering and spring rate changes. An unequal length control arm setup can move into strange motions at the extremes of movement, especially if the car is lowered. This can be a benefit, since on the outside wheel, the control arm angles work to increase the negative camber in severe compression, but in extreme rebound in the inside wheel the camber can move negative and have that tire put too much on it's inside edge limiting traction.

 

There are a couple of things going on here; first, the instantaneous center determines the swing arm length - though you are referring to an actual swing arm suspension design. If you draw two lines, beginning where the control arms attach to the hub (on center) and extend these until they intersect somewhere in space on the opposite side of the car's centerline, you will find the instantaneous center and the fulcrum from which these two arms move. It is curiously high... Since this point is quite far away, and, there are two arms, I would say these do not act like a swing axle; a swing axle's fulcrum is its point of attachment on the body and therefore create huge differences in camber with small changes in suspension stroke.

Also, adjusting either arm to correct camber after lowering essentially make these unequal length arms. The camber curve will change. I prepared a few drawings and contructed a very basic and perhaps inaccurate model of the rear setup. If I adjust camber by making the upper arm longer ( to kick out the top of the wheel) it seems as though camber strength is better - meaning the camber curve appears not be eroded by much. however, if I make the lower arm shorter ( pulling the bottom in ) the camber curve appears to weaken...not really a good thing.

What I or someone needs to do is plot a toe and camber curve for a given wheel offset. This is straight forward but time consuming; construct a cardboard panel to fit within the front of the fender well, remove the spring/shock and plot a curve along the cardboard by moving the hub up and down via a jack. Key points; define ride height and wheel offset so the tire's actual center line can be used to construct the curve. The camber toe curve will tell us which link is really the best one to fiddle with. My intuition tells me that the bottom one is better even though we give up some camber strength because it preserves toe changes during the full suspension stroke. Said another way, if adjusting camber with the upper link maintains the camber curve, but cause more rear steer, then it is a bad choice...for a road course anyway.

The fifth control point - every suspension system has five - is the forward mounting point for the trailing arm. The trailing arm determines the amount of toe thru the suspension stroke - in the Mini's case. Altering the length of the arms affects dynamic toe. Though i imaging the flexibility in the bushing have something to do with toe changes, as well as its age. I think this is a more important relationship to keep track of since toe can have a stablizing or de-stabilizing affect on handling.

I feel like I'm being argumentative, but not really...my 99si has an SLA ( short and long arm or unequal length A-arms) set up. This car did not rquire big changes in static camber to achieve better cornering power. 1.25 -1.5 degrees neg up front is optimum and helps to eleiminate the tires from cornering on the inside edges on any corner. The rear was especially critical since this car posseses a very aggressive rear camber curve.

 

My take on all of this is a rather practical one.

Bump steer is bad. BAAAAAAAAAAAAD.

We can enjoy some discussions about the benefits of slight toe changes in the rear suspension when cornering, et cetera, but as a general rule, cars that do not bump steer benefit from simplified vehicle dynamics, making them both more understandable and more predictable.

Someday, I'm going to take the time to pull the coilovers off of my MINI and take some precision measurements for various points (obviously the front would still need struts). There are several programs out there which will then allow me to exercise the suspention virtually and generate camber and toe curves for different settings (Carsim Educational is one that I'm familiar with, although the same data could be generated by constructing a simple solid model in Solidworks).

I would always trade away some negative camber gain to achieve a healthy decrease in bump steer. The roll centers in the MINI feel about right, although the rear roll center is perhaps a bit high (certainly that is speculative).

 

...about 11" from my own crude measurments. That's the static position. Where it moves to while the car is in motion is another question. Given its realtively short distance from where the CofG feels like it should be (in vertical) I cannot imagine it exerts a lot of leverage on the CofG.


I cannot remember my measurments exactly, but I think the 11" above is correct for my lowered car, not a stock ride height.

My fanny dyno tells me the Mini has some pretty damn good bumpsteer control.

Thanks for replying txwerks.

 

Wow, 11"? Interesting. No wonder I keep taking rear bar out as I get the front to stick better. Wish I had the time to investigate further. Maybe this summer durring the summer track lull.

 

Keep in mind, this was a crude measurement...it may turn out to be an inch less. Wheel offset will change this too - the tire centerline moves farther away so don't forget to factor in where the tire's center line is.

Interesting too, that the roll center moves up as track width is increased. So, we have a wider track which resists roll, and, a roll center that is now closer to the CofG - a good thing.


Have you checked the new front A-arm setup on the 2007 Formula 1 cars? A big departure from traditional layout. Some speculate this was done to generate more tire heat faster, in addition to better aero.

 

I agree, F1 car design is so aero-driven that no feature on an F1 car can be taken as a rule of thumb. It has to be considered as part of a vehicle that is so far removed from passenger cars that the particular feature may be exactly the opposite of what would work for "normal" vehicles.

I just got my latest Racecar Engineering in the mail, and I haven't opened it up yet. Also I've had my head full of propulsion system valves for the last month, so I haven't kept up with the latest F1 tweaks. I'll get there, though.

 

Last time I read race car engineering they devoted 3 issues to pneumatic and mechanical trail...interesting stuff - mostly over my head.