Rear Lower Control Arm Design (LCAs): H-arm VS. A-arm

[260DET]

What did you do to remove anti squat? I realize you are talking about a ZX model but the discussion probably is still worth it.

 

Spacers under inner control arm pivots?

 

Mine is a real HybridZ it has S14 suspension all round. To get rid of the rear anti squat the rear cradle/subframe was tilted forward ie the front of it was lowered.

 

No rear ARB either, helical LSD, lays the power down great and does not understeer at sensible speeds.

[heavy85]

Lateral load transfer is based solely on CG height and track width - the RC location doesn't have anything to do with it.

 

Maybe I'm missing something but thinking about free body diagram here. Input force is at CG times distance from CG to RC = roll moment. This then has to be resisted by tires at the track width. So if the CG and RC were on top of each other there basically would be no roll to resist so equal loading on both tires. This is obviously oversimplified but is my understanding.

 

Cameron

[tube80z]

Lateral load transfer is based solely on CG height and track width - the RC location doesn't have anything to do with it. If you are having issues with putting power down then you need to know what the dynamic alignment changes look like under bump. Does the outside rear toe out or in under bump? How much? Whats the anti-squat look like on these cars (I don't know but I have a feeling its a rather large percentage - but I could be wrong - its happened before )

 

Not to pick on you but the RC position does matter in these cars. I'm sure it's related to other factors but this was an area of experimentation last last year with a friends LS-2 powered car running FA rear tires setup for autox/hillclimbs. If we raised the RC to get the car to turn in better it lost bit coming out of corners and was much quicker to overheat the rear tires and cause the handling balance to shift towards oversteer. If we lowered the rear RC the car put down power much better and didn't overheat the rear tires but wouldn't turn in that well. We played with spring changes and moving the front RC to try and solve this but there appeared to be a sweet spot for this car with the rear RC. We ran out of time before trying to add a rear ARB to see if that would help to get the car to turn better. Unfortunately we didn't have a complete model of this car to understand what else we were changing.

 

Cary

[JMortensen]

Maybe I'm missing something but thinking about free body diagram here. Input force is at CG times distance from CG to RC = roll moment. This then has to be resisted by tires at the track width. So if the CG and RC were on top of each other there basically would be no roll to resist so equal loading on both tires. This is obviously oversimplified but is my understanding.

You're missing weight transfer. You can have weight transfer with no roll, and it will still unload the inside tires.

[heavy85]

You're missing weight transfer. You can have weight transfer with no roll, and it will still unload the inside tires.

 

Obviously there are fundamental physics here where unless the CG is on the ground there's a overturning moment trying to put your car on it's lid regardless of suspension goemetry, but in the context of minimizing transfer through moving around CG is what I'm pondering. Cary brings interesting testing that seems to say the opposite of what I was thinking.

 

Cameron

[TheNick]

Not to pick on you but the RC position does matter in these cars. I'm sure it's related to other factors but this was an area of experimentation last last year with a friends LS-2 powered car running FA rear tires setup for autox/hillclimbs. If we raised the RC to get the car to turn in better it lost bit coming out of corners and was much quicker to overheat the rear tires and cause the handling balance to shift towards oversteer. If we lowered the rear RC the car put down power much better and didn't overheat the rear tires but wouldn't turn in that well. We played with spring changes and moving the front RC to try and solve this but there appeared to be a sweet spot for this car with the rear RC. We ran out of time before trying to add a rear ARB to see if that would help to get the car to turn better. Unfortunately we didn't have a complete model of this car to understand what else we were changing.

 

Cary

 

 

 

You aren't picking on me We're having a discussion And I didn't say that RC's don't matter - I just said their vertical position doesn't effect load transfer.

 

That is actually incorrect - I went back and looked at my reference material and double checked the equations

[Lazeum]

Lazeum, you're braking scenario is not complete, you also need an X-direction force applied to the spindle.

 

Also, the bending reference was not talking about the toe-link, it was referring to the entire H-arm. If the strut to control arm pivot axis is not parallel to the control arm to body pivot axis, then there is going to be a moment acting on H-arm trying to twist it, and the only things reacting that force will be the torsional stiffness of the arm, and the strut itself, which will cause more friction in the strut. The A-arm design eliminates that problem.

 

I agree with you regarding braking case as well as the fact regarding A-arm

 

(Regarding Y-load on cross member) Not so sure about this. Regardless of which arm type you're looking at, the front bushing handles most of the braking load and the rear handles most of the lateral loading, as in either scenario the front pivot is stouter and the rear is weaker. I guess I'm not seeing how the rear pivot is more stressed in the A arm. I think it is more isolated with monoballs or rod ends in use vs bushings and gets more directional force vectors from the lateral loading, but I don't think it has more loading in the Y axis in either design.

 

To summarize:

Fore/Aft loading ==> Front inner pivot

Lateral loading ==> Rear inner pivot

 

Regarding the rear pivot stress, I think this was true until I've understood Rear pivot is not reacting according to Fore/Aft direction.

 

For loading condition, bushing rate (finite for rubber bushing, kind of infinite for monoballs) have very little influence on loading. There is less damping in metal rod end than rubber ones, thus dynamic/very suddent loads might be more important but not that much I would think. this would be especially true in case of pothole for instance.

 

All in all, is H-arm better than A-arm...? I don't know anymore

I still believe H-arm would be less stressed than A-arm except if twisting, as Flexicoker, you were explaining, is severe.

 

Anyhow, this discussion makes me think a lot about the suspension. I've learned quite a lot with this thread. time to change stock H-arms???????

 

 

Discussion about Roll center and jacking physics have been good recalls since I've forgotten them

From this point of view, we should not choose lowering springs, wheels diameter, sway bar the same anymore neither.

[260DET]

Yeh, these discussions are invaluable, always something to learn or refresh the memory on

 

On roll centers, some info from the late great Carrol Smith, who incidentially has said that on most road courses a rear ARB is a bad thing.

 

Rear roll center too low/ front RC too high relatively.

Roll axis too far out of parallel with mass centroid axis, leading to non linear generation of lateral load transfer and chassis roll, as well as the generation of excessive front jacking force. Tendancy will be towards understeer.

 

Rear roll center too high/ front RC too low relatively.

Opposite of above, tending towards excessive jacking at the rear and oversteer.

 

I assume the 'mass centroid axis' he mentions is the mass center line in the horizontal, if it may be put it that way.

 

 

 

.

[JMortensen]

Yeh, these discussions are invaluable, always something to learn or refresh the memory on

 

On roll centers, some info from the late great Carrol Smith, who incidentially has said that on most road courses a rear ARB is a bad thing.

 

Rear roll center too low/ front RC too high relatively.

Roll axis too far out of parallel with mass centroid axis, leading to non linear generation of lateral load transfer and chassis roll, as well as the generation of excessive front jacking force. Tendancy will be towards understeer.

 

Rear roll center too high/ front RC too low relatively.

Opposite of above, tending towards excessive jacking at the rear and oversteer.

 

I assume the 'mass centroid axis' he mentions is the mass center line in the horizontal, if it may be put it that way.

I like Carroll Smith's books, but people get too hung up on some of his stuff. He said detroit lockers aren't good for road racing, yet they have a LONG history in road racing. He said in Tune to Win that Ackerman is bad, and anti-Ackerman is good. I think he reversed himself on that one later on. And he said that a rear anti-roll bar is bad. I think that is a general statement, not an absolute one, and there are plenty of occasions where a rear anti roll bar is a useful thing. Plenty of formula cars (even modern day F1 cars) still use rear anti-roll bars despite Smith's advice.

 

By the way mass centroid axis is essentially the weight of lateral sections of the car. If you can imagine the car sliced like a loaf of bread laterally, and the center of gravity plotted in each one, the graph you would have plotting those centers of gravity would be the mass centroid axis. I guess it turns from the mass centroid plot to the mass centroid axis when it is simplified and made a straight line from front to back. He shows a diagram on p30 that suggests the RAI should be roughly in line with the mass centroid axis. That model doesn't work so well when translated to a front engine sedan. With his formula car the cg in the front is very low and moves more or less upwards as you travel back until you get to the engine, then goes down towards the back of the car. This is diagrammed on p29 if you're looking at the book.

 

In a Z for example, the CG is higher in the front where the engine is than in the back where the differential is for the simple reason that the engine is a lot taller than the diff. If you were to follow Smith's guidelines and try to match the RAI with the MCA, the front roll center would be a lot higher than the rear. We know that this doesn't work in practice. I'm sure if you could dig him up and ask, he'd probably say that the issue with the Z is that the engine is in the wrong end of the car, and maybe he's right. But assuming you want to work on making a tin top handle, it's probably best not to take everything Smith says literally.

[TheNick]

http://zzyzxmotorsports.com/library/roll-center-myths-and-reality.pdf

 

 

Found an excerpt from the Bill Mitchell paper I was talking about

 

 

Also some other good links if you are in the mood to pay for SAE papers

 

Asymmetric Roll Centers

http://www.sae.org/technical/papers/983085

 

Force Based Roll Centers

http://www.sae.org/technical/papers/2006-01-3617

 

Good PDF File on FRC's and their application

https://aerade.cranfield.ac.uk/bitstream/1826/2097/1/Allibert-2007.pdf