solid axle, MacPherson struts

[grakson]

I am learning about car's suspension, and there are many sites, none that I found has a good explanation, also names of the suspension are not the same from site to site.

But I really don't understand movment of coil spring solid axle, because from the picture I can see say that both wheels can move only the same way, because otherwise if one will move up and other down, the axle should move right or left, because mounting of the axle can't move vertical up and down but it swings in arc, so if the left wheel moves up it should also steers left, and right wheel will then move down and right. And another misunderstanding about solid axel is that if it swings up or down it shoul rotate where carrier is mounted to the axle.

Then aslo I don't understand how can you turn a front wheel if (according to the picture) shock absorber is mounted stiffly to the "wheel holder" (don't know the word)

 

[grakson]

pictures

 

[grakson]

pictures

 

[GregLocock]

Your picture of a coil sprung solid axle is missing key features.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianPetersen]

I don't understand most of the questions but I'll answer the one that I can understand, and I'll take a guess at an answer for another question you are trying to ask.

For the MacPherson front suspension, you don't understand how the spindle can steer the front wheel if it is securely attached to the bottom of the strut; this is the question that I understand. The answer is actually very simple. The strut is attached to the vehicle at the top using a thrust bearing that allows the entire strut to rotate as a whole as the wheel is steered. (This mount has some designed-in flex which also allows the angle of the strut to change while this happens and as the suspension moves up and down.)

For the axle type suspensions, I don't understand the questions you are asking. I think you are not understanding how the axle moves over a one-wheel bump. Most practical rear suspension linkages DO cause the axle to shift sideways slightly as this occurs. Rear suspension linkages that use a "panhard rod" DO swing in an arc, and DO cause the axle to shift sideways slightly even in a two-wheel bump. Correct layout of the panhard rod will minimize this effect in the expected normal range of suspension movement, so that it is not noticeable to the driver.

In addition to the sideways movement, practical rear suspension linkages DO "steer" the axle as it goes over bumps or when the body leans while cornering. This is not necessarily a bad thing. Good designs can use this to create a little bit of "roll understeer" that partially offsets the natural "slip angle" of the tires while cornering.

 

[MikeHalloran]

The part that grakson appears to have missed is that the center of the solid axle is _NOT_ attached to the car directly; not rigidly, and not by a pivot.

Grakson, the axle is prevented from leaving the car sideways by means of a "Panhard rod" or "track bar" (same thing) or (rarely) by a "Watts linkage" and much less rarely by the diagonal arms of a "four link" suspension, that I won't attempt to explain right now.

The track bar is typically nearly as long as the axle, and is hinged to the car at one end and to the axle at the other end... so as the axle moves up and down relative to the car, the axle end of the track bar describes an arc relative to the car... but the angle is not great, so the lateral displacement is also not great.

Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

The rea suspension ilistrated had no way to prevent the axle housing rotating on the lower control arms. There were no upper control arms to restrain this rotation.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[GregLocock]

As Pat implies, while somebody may sell such an axle, it is not the sort of thing much seen. Mitsubishi and LandRover have used a similar concept, without the U bolts, and almost every Hotchkiss uses the U bolts, but with leaf springs.

One way or another every car suspension I have seen has somehow arranged for roll motion.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[grakson]

Thanks for useful answers!
Now I understand the single wishbone or MacPherson design.

But about the rear solid axel, I made some CAD 2D scheme so I can easier describe what throubles me.

The thing is that if "carrier" (connects axel to the frame) is connected to the frame via a pivot (3D scheme), than it swings up and down in an arc, not verticaly. So that means that when left wheel goes up (hits a bump), left carrier swings up in an arc path, so this means that whole axel should steer a little to the right.
But than again its a dependant suspension, and because we have solid axel I might say that when you hit the bump axel moves as a unit. so when axel on one side goes up it goes down on the other side for exact same lenght, meaning that carrier makes exact same arc left and right, so axel doesn' t steer in only move litle front or rear.

So please explain my thinking...

 

[MikeHalloran]

Ah. Now that I've bothered to look at the illustrations.. the axle is pretty much just plain wrong, not least because "independent" is misspelled and is the wrong word anyway. Perhaps it was scaled from a plastic model.

First problem: u-bolts can't possibly carry normal braking torque or normal ring gear torque.
Second problem: the links fight each other when the car rolls. At least one of the trailing arms needs a hinge at the axle end.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

You are correct. So say the left wheel moves up it moves forward and so the axle steers to the right, which since we are talking about the rear axle, steers the car to the left. Twist beams suffer from the same sort of problem. You might work out the angles involved before spending too much time on this, it is not the biggest fault with that architecture.

Your various drawings do not allow the suspension to work in roll, but real suspensions do. I suggest you look at one and figure out how they do it.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[grakson]

so what about any schematics. It will really help here, as they said pictures sais more than a 1000 words.

if you have any links to the solid axel shematics, please make it visible!

 

[gt6racer2]

Grakson,
I think the mistake you are making with the illustration you show for the solid axle is that you are assuming the parts don't have compliance. Without compliance the illustration cannot roll as you say - but think of a door with loose hinges - then you can get the roll freedom. The compliance can be designed into the mounting bushings, the fore/aft arms, and even the axle tube. The compliances in the suspension ( compliance steer)can be used to balance the mechanically driven geometry changes (roll steer).

 

[BrianPetersen]

Am I the only one for which the illustration is a red "x"?

I suggest to the original poster, to go someplace where it's possible to look at real suspension designs. Sometimes, illustrations in books are useful. Hot-rod books and magazines and parts supplier catalogs often have suspension kits that hot-rod builders can use.

Regarding suspensions that theoretically don't work in roll, you folks should have a look at the rear suspensions used on trucks that use air-bags (these will generally be quite visible underneath the truck). It is pretty much just a trailing arm on each side with the axle bolted to the trailing arm via bushings, and the air bags mounted on each trailing arm behind that, and often on a frame that joins the trailing arms. At first glance, this design has infinite roll stiffness, because the entire axle and both trailing arms are all locked together and can only go up and down, pivoting around the trailing arm attachment point to the frame as a unit. In reality, the trailing arms are bolted to the axle through two rubber bushings and the frame behind the axle (on which the air bags are mounted) has some torsional flex designed into it, to allow some relative movement left-to-right. Still, this design does provide compliance on two-wheel bumps with extremely high roll stiffness in body roll or on one-wheel bumps ... necessary on an 18-wheeler with a center of gravity that is extremely high by automotive standards; you do not want that to start getting into compliant body roll. On a one-wheel bump, it almost forces the whole axle to go up and down, which is why they ride rough despite the air bags. You would not want to use a design like this on an automobile.

 

[patprimmer]

It's also similar to what Drag Racers call a ladder bar suspension. Ladder bars have little compliance ad are very good at transmitting drive to the chassis with minimal compliance and thus giving good straight line stability under savage acceleration. They are very poor at negotiating one wheel bumps or curves.

I am not a suspension guy, but my knowledge sees well above yours. Are you a student, possibly a CAD system student rather than an engineering student.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[GregLocock]

Well to be honest that is what LandRover do, and they get good axle articulation in roll. It all comes down to the bushes.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[LionelHutz]

You are finding out what others have known for a long time. That is a very poor suspension design.

For that suspension, try bringing the arms together so the bushings are together at the frame end. Also, make the arms longer at the same time. Then, install a panhard bar. Give the arms and bushings some compliance (flex) and it will work closer to what you want. Search for info on a "truck-arm" rear suspension for examples.

Here's a link showing some of the different popular suspension designs.

http://www.popularhotrodding.com/tech/0604_rear_suspension_guide/index.html