Discussion on adjustable rose-jointed trailing arms

[LeeVuong]

Despite the many ways people have tackled adustability on Helm-jointed trailing arms, and after having spent hundreds of hours working on solutions, things still don't add up. I am not talking about deciding if camber adjustability shall be done on the inner or outer pick-up point (I think that camber shall be done on the inner point and toe on the outer point), but the fact that when threading in or out a rod-end in order to gain/reduce (say) caster, the movement is linear on the rod-end, while the trailing arm moves in a very slight rotation. Therefore, if tolerances are perfectly tight, there's no room for movement and it requires some compliance somewhere or a stress is created on the rod-end or trailing arm.

Here are some threads on the subject of adjustable trailing arms

http://www.s14.net/forums/showthread...m-Thread/page7
http://forums.bimmerforums.com/forum....php?t=1344968
http://www.s14.net/forums/forumdispl...and-Track-Talk

[Gregg]

Lee,

I'm not 100% sure that the suspension has to bind (if that's what you're implying by the stress creation comment at the end).

One method for evaluating suspension geometry that I've used is to CAD model the various parts (trailing arm and subframe pickups, shock, half shaft) with appropriate constraints (degrees of freedom; i.e. rotation, translation). Then, in an assembly of the parts, you move them through their range of motion.

In college, as part of the SAE Baja competition, we did a five link rear instead of the usual swigaxle design with all joints using sphericals. Modelled in Solidworks, we were able to check suspension kinematics (and control arm lengths for a rough alignment, which actually came pretty damn close to what we were shooting for). As it was an off-road vehicle we were also concerned with axle plunge (or, er, unplunge...) thru the arc that the upright travelled in bump/rebound.

Anyway, this method should give a quick way to see the 'problem areas'. With rubber bushings (and their compliance) that's pretty hard to model, but with sphericals, there is an assumed ideal location of all nodes, and the CAD model excels at this analysis.

Hope it helps,

Gregg

[JrIIIRacing]

Lee, are you referring to the DTM style trailing arm setup with the toe/camber adjustment done on the outboard leg and the inboard leg just a floating pivot? If the car has been converted to a true coil-over setup with mono balls on the top and bottom of the shock, then there should be no binding in the trailer arm, correct? If care is taken during assembly to ensure that the mono balls are loose and free in the vertical direction and that the trailing arms and subframe are sufficiently stiff then binding should be at a minimum. The trailing arms, subframe, upper shock mount and sway bar mounts should all be reinforced to try to eliminate flex then the mono balls should never get bound up to the point where they snap. This might be a simplistic view because it might not be possible to get the factory parts sufficiently stiff enough to ensure limited compliance but in principle it should work. Also, if you use spring mounts that help to eliminate lateral loading of the spring, that will greatly help to eliminate bending moments on the lower shock mount and thus stress through the whole trailing arm.

I know you have shared pictures of the late '92 rear subframe and tube frame Trailing arms, do you think those generate stress through the monballs or do you think that setup works? I believe that the inner leg on that set up allows it free movement laterally, is that correct? If so shouldnt most of the misalignment in the system be taken up in that sliding joint and not snap the outer leg heim joint?

I am thinking of converting my trailing arms over to the DTM style setup with toe/camber on the outboard leg and with separate roll center adjustment as well. Mine will be with a true coil-over setup with reinforced trailing arms and subframe tied into the cage with hydraulic Hypercoil spring perches. This should eliminate a lot of compilence in the rear end and allow the roll bar and damper/spring to control the tire, not the flex in the trailing arm.

Is that along the lines of what you are thinking?

[sprytel]

a quick drawing will reveal that with a heim jointed trailing arm, the distance between the inner and outer pivots will increase and decrease as the toe is adjusted. You are essentially creating circles with different radii. if this isn't accounted for, there will be a point at which the trailing arm will no longer fit in the fixed location of the pivot points.
I compensated for this by allowing some room on the inner pickup point. The trailing arm is located and fixed in place by the outer rod end, which pivots on a vertical axis, while the inner pick up point rotates around a horizontal axis, which allows for side to side movement during toe adjustments.
sorry if my reply confuses the issue. its difficult to put into words what is easy to diagram in a drawing.

[LeeVuong]

You have properly expressed the problematic. ;-)

[e21jason]

Intresting ever looked at the ford sierra set up, the outboard pivot could be adjusted for camber and roll center, the inboard for camber and toe.



And on the e21 i just went with adjustemnt on one end

[TBest90M3]

Sprytel, are you saying that your inner rod end / pick-up point is utilized for toe adjustment? That's how my arms are.... Made them a long time ago and will be the first to say that fabrication came prior to fore-sight. My assumption was that once I set the trailing arms to the ideal position I would leave them there and check periodically... Or perhaps adjust down the road when setup or conditions required... Pleased to see this board has a number of technically-savvy enthusiasts..

-Terry

[LeeVuong]

Interestingly. When the inner mounting point is used for toe, it basically moves the wheels in and out because the trailing arm will rotate around the outer pivot, reducing or enlarging the track. If the toe is corrected on the outer pivot, then the wheel moves front and aft. Changing the wheelbase at the same time. Basically both methods can work and will require compromises. Pick your poison.

[TBest90M3]

Thanks Lee, that's semi-comforting to read I say YOU pick the poison and let us know what you come up with...I'm sure by that time I'll be fed up of halfway removing my arms just to make a toe adjustment and will be ready to buy what you engineer/produce. I have thought about making tubular arms for a long time... Or over the top 7075 billet pieces (with neon lights attached for extra bling factor). But in all seriousness, I think there's an increasing need for such a piece for E30 M3s today. Even though a lot of people choose not to run in such a high class, the cars eventually get there (more and more 'D-mod' E30 M3s now then there are J-Stocks... 10-12 years ago the club racing fields here were filled with stock class M3s.). Anyway, I'd be curious what sort of weight savings there could be with a race-specific or custom hub assembly..

Lee, you build it and we will come

-Terry

[bre513]

Interestingly. When the inner mounting point is used for toe, it basically moves the wheels in and out because the trailing arm will rotate around the outer pivot, reducing or enlarging the track. If the toe is corrected on the outer pivot, then the wheel moves front and aft. Changing the wheelbase at the same time. Basically both methods can work and will require compromises. Pick your poison.

Actualy its not as simple since we are looking at two different arcs at different angles of the same triangle (namely inside pickup, outside pickup and attachment point at the wheel) depending on where the toe is adjusted. When it is adjusted on the outside pickup the arc is located at the inside pickup and the arm that moves is the long connection to the wheel which give more toe change per increment of change at outside pickup. The opposite happens when it is located on the inside pickup because now the arc is located on the shorter arm from the outside pickup to the wheel and there is less toe change per incremental change at the inside pickup. The end result then would be that there is a greater change in wheel base as opposed to track width per degree of toe change. But hopefully we are only looking at a minimal toe change so the resultant changes are minimal compared to the other benefit of adjusting the toe on the outside pickup.

However when the outside pickup is moved towards the front of the car to induce positive toe it also reduces the sweep angle of the trailing arm and results in lowering the rear roll center. This is very good. So it would seem that the toe adjuster should be outboard and the camber adjuster on the inside. Well it is not that simple.

When we place the camber adjuster inboard we effectively raise the attachment point of the trailing arm. When the trailing arm is tilted in this manner we effectively raise the rear roll center. Not a little, but a lot. The effect will be "jacking" or raising the car during lateral acceleration (aka turns). A high rear roll center also has the effect equal to a very stiff rear sway bar, lots of weight transfer in a turn and a loose rear or oversteering car. So it seems that the camber adjustment "must" be on the outboard pickup and the toe adjuster is either incorporated into the camber adjuster as in early DTM cars or placed inboard and the slight change in track tolerated or not even noticed by club racers.

There is an ultimate fix to all of this. The adjustment for toe and camber is placed on the outboard trailing arm, the outboard pickup is slightly massaged to a new location and effectively reducing the sweep angle AND the outboard pickup is actually made adjustable so it can also be raised. The net effect is a roll center that is lowered but still adjustable.

Next up would be what would happen if we placed the roll centers too low or below ground level. Save that for another day but here is a picture of a very very late DTM rear suspension :






References

1. 
   Tyres, Suspension and Handling, John C. Dixon, Cambridge University Press (1991)
2. 
   Vehicle Dynamics Terminology, SAE J670e, SAE, 1952, last revised 1976
3. 
   Racing by the Numbers, Wm. C. Mitchell Software, 125 E. Plaza Drive, Suite 117,
   Mooresville, NC 28115 USA

4.
Many long discussions with my neighbor and Ferrari F1 suspension guru Bill Milliken

[sprytel]

Sprytel, are you saying that your inner rod end / pick-up point is utilized for toe adjustment? That's how my arms are.... Made them a long time ago and will be the first to say that fabrication came prior to fore-sight. My assumption was that once I set the trailing arms to the ideal position I would leave them there and check periodically... Or perhaps adjust down the road when setup or conditions required... Pleased to see this board has a number of technically-savvy enthusiasts..

-Terry

Terry,
Actually, no, I don't use the inner rod end for toe adjustment although in theory I could, since they can be adjusted.
I make all adjustments to the outer rod end.
Jeff

[Jake]

Or... put adjustments into the trailing arm at the hub for minimal secondary effects. I believe this is Moufang's rally M3 but not sure. Original thread and pictures on S14Power.com.

















Jake

[LeeVuong]

Hi Jake

This is indeed what I have in mind when it come to correcting camber. I am working on rear trailing arm that is very like the late DTM trailing arms, but incorporating an adjustment for camber or toe) at the hub, using eccentrics. I haven't yet found a way to incorporate both camber AND toe adjustments though.

[ukrbmw]

This is a perfect thread to find. For the second weekend in the row I had to pack up early because my trailing arms keep getting loose. This of course renders the car practically un-drivable. Thankfully today I was able to nurse it home and still finish second.

From reading all this it seems most of the solutions are basically custom. Is that right? I'm not an engineer like most of you in this thread, so I'm curious if something better exists available for purchase. My car is a GTS4/CM car - not overly crazy on power, but enough to keep messing with my trailing arms. Somebody recommended using IE lockable kit, etc - but that seems like a patch, not a long term solution. I'd welcome any recommendations at this point. I've got a great car that I keep parking for the same issue over and over again.

[LeeVuong]

Rod-ends ans spherical joints. But you'll need yo go with rear coil-overs.