Rear upright design question

[greg V]

Is there a desired location for the lower control arm pickup point on the upright. I drew a sketch to help get my question across. I had placed the pickup point towards the front of the centerline. But now I may have to move my pushrod strut to the back of the axel and thought it would be better to then move my control arm pickup towards the back also. Does it matter?

[greg V]

Hmmm, .. lots of views but no responses.
Not sure if my drawing is confusing or if it's a dumb question.

[David Ferguson]

I think you really need to look at this in a suspension geometry program (like Mitchell's WinGeo) to understand what effect it may have on the dynamic suspension geometry. And it may be dependent on the strength/design of your upright. I believe the moment changes due to the relocation will result in different forces through your A-arms. Are they over-designed? If so, then you're probably OK.

So, I suspect no responses because we don't know the answer. And I'm only providing a bit of a clue as to how to find the answer (because I don't know the answer either).

[racerdad2]

Like Dave said... changing pick up points changes many other things... you may make things worse or may make things better ? You need a suspension guru to help you out... and that ain't me... this may require $$$ for a consult from a pro... someone on Apex will undoubtedly help you. It may take awhile... they're probably busy on their own suspensions. Good luck & be safe !

[kea]

The problem which I see might present itself, has more to do with a final alignment setup.
What if (big letters), to get the rear bump steer correct, the front lower edge of your pictured upright, needs to drop down (from the horizontal you drew), rotating around the axle center line.
Would the lower trailing arm now run into it before full extension is reached?
With the lower pick-up in the front, the only limiting facter would be the angle of misalignment the bearing allows.
Hope this is understood as I wrote it.

[Marty Nygard]

Harsh as this might sound, I'm guessing that if you can't do the force vector sums, you probably shouldn't be designing uprights or other suspension items. There are plenty of people that sell these things. Walk up to the winning end of the grid in whatever class you are interested in, take pictures and blatantly copy their design.
What are you interested in building anyway?
Marty

[greg V]

Thanks for the responses all. It's not really rocket science I don't think. I understand suspension pretty well. I've studied a lot of upright designs and there is no conclusive evidence on where the pickup points should be as a standard way of thinking. I just wanted to hear if anyone could add to this.
I know maybe it was too general of a question maybe.

I am building a late model mid engine Mini Cooper. Here is a thread about it if you car to see. There are some pics of the rear suspension mock up, so it will give you more of a clue on what the heck I am doing.
http://forums.bimmerforums.com/forum...i-Cooper-Build

[racerdad2]

Very cool project :thumbup: It may not be rocket science, but it is science

[stephen wilson]

It may be too late to change your design, but I think this is a better solution, with the Toe link on the LCA:
http://dsrforum.yuku.com/topic/9827/...7#.UgZlaBPD-mQ

[kerlampert]

Very cool project :thumbup: It may not be rocket science, but it is science

You may have the up-right uoside down.The camber and toe adjustment works better when they are on the bottom

[J Leonard]

Mount your lower control arm on the center line and use for camber adjustment. Use your upper camber arm for toe.

[greg V]

Thanks for the input guys.
The arms are set up the way you see them as there will be diffuser tunnels in between the chassis and the tires, with a height just below the driveshaft. Usually when doing this you try and limit the number of tubes in that tunnel. This is the same configuration that you would find on most of your cars here I might guess.

[greg V]

Mount your lower control arm on the center line and use for camber adjustment. Use your upper camber arm for toe.

Yes, I miss labeled my "Toe" arm as a 'camber' arm. It is in fact the toe arm and the camber is adjusted by the upper control arm.

[Modo]

would think bottom point would have to be on a line drawn thru the mid axle/bearing and a midpoint between upper a arm point and toe link point ..... if lower wishbone point is forward seems it would want to wander around the toe link point and rearward would want to toe in or wander around the axle/bearing center and upper front a arm point ...... just guessing

Just looked at my Tatuus rear upright and it does that, splits the distance between toe and wishbone pivots although toe is on the bottom and is built into lower wishbone splitting axle/bearing center ..... the top wishbone accomodates camber shims and is in line with center of those points and axle/bearing center for the most part ...... 3 points make a plane and u may want to load points kindof equally with respect to the wheel/tire loading the bearing as opposed to putting torque up on upper wishbone while toe stays still, vice versa, u can see it gets complicated as David said

[R. Pare]

Sorry to have to add this in, but your "square to the chassis centerline" inner rod ends is NOT the way to go - they are now in bending under ALL types of loading. To minimise their stresses, they should be directly in line with the arm tube.

[Modo]

For a "Looksee" ..... 98 Tatuus .... sorry bout cellphone pic, other camera ooo

[greg V]

Interesting comments all.
I was maybe looking for some definitive facts, or documentation somewhere about this. Yes, lots of thoughts and opinions, but few I would agree with. I have done a good amount of study and research on the topic, and have not found any such documentation, but have in fact seen a whole lot of examples on a lot of cars. I find it more on the rare side to find a control arm mathematically placed at the center point of the opposite control arm and toe arm. In fact I have seen it more times as I have done it originally. So I don't buy any theory I have heard here, at this point in time. Hence why I was asking. I'm here to learn, but to learn from facts.
R. Pare, I follow what you are saying and yes that was taken into consideration. But these hiems make the Hiems that most of you all use look like fishing tackle. They are 5/8" thread. Not the best way to have them, but I don't believe these will fail due to bending. I would though 100% agree with you if they were placed on the vertical rather then the horizontal position.

[provamo]

just following this...............

[JerryH]

If the lower rear ball joint is off center it will create a torque load on the upright which will be resisted by the toe link. If they are designed with that load in mind, then it will work ok. Same thing goes for the push rod load....if it is on the upright and not the a arm. put them both on the same plane one in front and one in rear of the center and the calculations are a bit more difficult, but the loads are usually less.

The above is for an upright with upper points as in your sketch and mockups. Camber/toe adjustments on top are common....and work fine.....but usually with shims for camber. That way toe is not effected with camber changes.

If you are going to leave the rod ends as pictured (not in line wth the tube as richard suggested), then connect the a arms together between the rod ends.....forming a triangle. That will take some of the bending load off the rod ends (not all tho). Rod ends fail in the threaded portion when subjected to repeated bending loads (as they will be in your design).

You have done a lot of work, I hope it does what you want it to.

Jerry

[greg V]

Thanks Jerry.

The main reason for the rod ends being like they are was to get them around the chassis so I could keep the control arms with some decent length. I understand the concerns for sure and will of course keep thought into this.

The main reason for the post was that I am now moving the suspension pushrod pickup point from an original design of it being a pull rod design. So I have to rethink the loads on the upright and control arms. The new pushrod pickup point will be as drawn below, on the back end side of the upright. So my thinking was to get that lower control arm pickup as close to it as I can, circled in green, as I now have it at the leading edge of the center line,
I believe that the design will handle loads fine, but am more wondering about geometry. I'm under the impression that geometry plays not that much of a roll in the rear, but for roll centers.
Again, thanks for the input here.

Oh, and yes, there will be an anti intrusion rod connecting both ends of the lower A arm near the inboard pickups, as I have done in the front

[greg V]

Here is a picture of a DP rear suspension. My chassis is designed from the DP chassis. As you can see the lower pickup is forward of the center line by a good amount as it is also where the pushrod pickup is.
Just sharing.

[provamo]

Here is a picture of a DP rear suspension. My chassis is designed from the DP chassis. As you can see the lower pickup is forward of the center line by a good amount as it is also where the pushrod pickup is.
Just sharing.

who is this manufacturer?

[greg V]

Fabcar. Porsche powered. Was the older chassis.

[CGDerian]

Greg,

I haven't driven an adjustable suspension on a big track in a while, but I have a little knowledge on the matter. Please take what I’m about to say with a grain of salt since I don't know your car’s details (hp, compliances, tires, etc.).

The short answer is: it probably won't affect your car much if you only have an inch or so to play with longitudinally, especially if you have a lower powered car.

Here’s the long answer: Caster in the rear is the same as caster in the front, it’s the side view analysis of the steer axis (a.k.a. king pin axis). And the higher the caster angle the more camber is gained when the tire steers; again the same in the rear as in the front. Though having the UBJ forward of the toe link creates negative caster which means you’ll lose camber as you steer. Chevy did this to the front suspension of the Vega in the mid 70’s, though only so they didn't need power steering.

Case 1: If your rear caster angle is near vertical and intersects the ground in front of your tire contact patch (Fig 1) you’ll gain less camber with rear steer (depending on your bumpsteer and compliance). This can help reduce tire wear and maintain aero platform control though it’s generally less stable because your driving force (tire CP) is behind the steer axis.

Case 2: Putting the LBJ in the middle is common (Fig 2)…it keeps the caster axis behind the tire CP and is a safe choice. The resulting caster angle will depend on your wheels size (plumb-bobs and protractors are invaluable here).

Case 3: Having the LBJ rear of the tire’s centerline (like you’re planning) further reduces (makes it more negative) your caster angle but brings the caster axis closer to the wheel center which can help reduce loads on the toe link (Fig 3)

Having your pushrod axis on the rear of the knuckle and not in-line with the steer axis will actuate your spring/damper when the rear tire steers, whether through its designed movement (i.e. bumpsteer) or its compliances.

As always, the best thing to do to learn about what your car is doing on track is to take some on-board video. Point it at the rear suspension when you’re done and see if it’s controlling the tires well.

Now let’s talk about the lateral location of your LBJ. Well…maybe next time.

Good luck. I’m jealous; I haven’t been able to build something in a while.

-Craig

[greg V]

Craig ,
Wonderful, .. thank you. I will read this over a many times and study the diagrams.

I was looking for zero bump steer in the rear, as I don't know enough to understand playing with that. So being you bring this up, I'd love to understand more about the benefits of having some bump steer in the rear, if in fact there are.

Thanks a bunch.

[CGDerian]

Greg,

It's common to have rear bumpsteer toe-in under compression. Having the outside rear toe-in under compression prevents the car from oversteering during corner entry. It's easier to think about the alternative: if the outside rear were to toe out when your first turned into a corner, it would help the car rotate. This rotation is usually too much and delays when you can get back to the throttle, which is why it's advisable to have some toe-in with compression travel.

-Craig

[greg V]

Great, and quite simple Craig. Thanks once again.

[Modo]

beautiful day in the mid-atlantic, kind of a beer n' donut day (like Alpine myself, not mid-atlantic, yeaaa stuck, mums from Montana and I lived in Europe), ssoooooo ..... CG, correct my abstraction, in the case of a front upright, castor, is easy to vision, the tilt of two points and a lever to rotate the gismo around those two points, ......... (Breeeathe) the rear seems different because of a relatively fixed side of the upright--hubcarrier--wheel thing-a-majig, ....... that is, in Greg's example of the Porsche powered FabCar, the toelink is fixed to the upper wishbone, and those two points (wishbone/out-bearing-toelink bearing points), to my thinking, now describe the upper castor point, namely a point in between those two points, and that point would be to a small amount complex with respect to those two points being spherical bearings which move, basically I'm seeing in between as the point of kingpin inclination or castor definer and not the wishbone spherical bearing which you show in the three diagrams, (diagrams neat I might add and nifty-sifty discussion) ...... anyway I'm missing on that and was wondering if you can point me in the right direction!! ......

[greg V]

As far as the Fabcar setup goes, I don't even know that bumpsteer can come into play. ?? Maybe the raising and lowering of the one or the other inboard control arm pickups.??

[Modo]

controlling the car with the front or steering, you may not want the car's rear to have a mind of it's own or swing/anti-swing, it would be designer's choice, we feeble powered formula car folks, I would think, aim for minimum scrub and straight follow from the rear (straight follow with minimum scrub that is, is that different per-track??, oui-veh(I borrowed, east coast, namely Brooklyn, first wife type of deal), over my head??????...... Steve L, "Hep me Steve, Hep me" ?????

[CGDerian]

Mike,

I'm not campaigning for more rear bumpsteer, just trying to describe what happens if you have it. Certainly if you have minimal bumpsteer, much of this discussion becomes moot.

Note my recommendation about taking video though, your rear suspension is not only controlled by the designed movements, but also by the component and resulting system stiffnesses. Maybe nothing is moving in unwanted motions, but maybe not. Taking video can help quickly diagnose issues, this is especially important in a 1-off creation.

-Craig

[Modo]

following CG on the 'Looksee thing' I'm buying this as I can't get my Tatuus to stick back 'der' http://www.amazon.com/ContourROAM-Wa...=action+camera


just keep a bag of peanuts in the cockpit and throw them at the 'start button' to record, call it 'remote control' (I know, you actually have help at the track to push the dang button, u don't need peanuts)


PS "Goup 7, car 09 to the tower", Chief Steward, " Why you throwing peanuts on my grid" "Welll, errr, it's a long story, u got ears"

[CGDerian]

Also, Mike, you raise a good point about scrub radius. I would suggest it's more important that rear caster.

-Craig

[GAC]

Another thing to keep in mind when the LCA point is not in the centerline is that a toe change or bump steer is going to change your track. Instead of pivoting around the center of the wheel, you have added a fore/aft pivot arm that will cause the wheel center to move out and in with toe. You will normally get a little bit of lateral wheel centerline movement even if the point is in the center due to king pin inclination and offset, but moving off center adds to it. Changing the track, known as scrub when it is dynamic, affects your motion ratio and load transfer as well as adding a little heat to the tire. So, it is usually common practice to put your arm on the non-toe link end in the center. If memory serves me correctly, the Reynard Indycars all had a center LCA mount.

[CGDerian]

Putting your LCA directly below the wheel center will not guarantee that your track width doesn't change. The steer (caster) axis has to go through the tire contact patch (CP, simplified as the center of the tire on the ground) for there to be no steer-scrub. The distance between the axis and the CP is shown in the figures above as the mechanical trail. Of the 3 figures above, the trail is certainly lowest when the LBJ is directly below the wheel center.

Again, we’re grasping at straws, the lateral wheel movement with suspension bump and rebound travel is much more dominant than with steer. And that scrub (due to bump/rebound travel) is a result of your instant center height and swing arm length.

-Craig

[Roux]

Richard Pare gave you some good advice and all is well for the first few years. but....

I was shocked on a 27 year old car to have a 1/2 inch thread in the gear shift linkage break off during a routine gearshift. The repeated forces from a human hand working an MK9 gearbox over and over again in an environment where there could have been some corrosion assistance still resulted in a sudden fracture of what appeared to be a truly oversized part. lf you have any reason to rework those A arms, please go for the robust design solution

steve