longitudinal forces from braking vs. bumps

[formula94lt1]

There are a few suspension tricks I have seen utilized that control longitudinal compliance for the sake of ride quality without giving up very much, sometimes extremely little, in the way of overall wheel control. Some examples are the Acura NSX and its compliance pivot, the Lotus Elan M100 and its suspension raft, the McLaren F1 and its ground plane shear center subframes. I am curious though, in allowing longitudinal motion over bumps to decrease the shock of it that the passengers will feel, this obviously would allow for longitudinal motion in the wheel under braking. I would assume that braking force, especially at the front wheels, would be far greater than the force imposed by pavement junctions and lane dividers, even a pothole. Or is it?

 

[MikeHalloran]

We don't see many of those fancy cars here in N'awlins.

Which doesn't mean they're not here. There could be thousands of 'em in the potholes, and you wouldn't know.







Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Umm

Why does it matter if the bump steer is set correctly and suitable castor is retained.

Link pin VW type front ends were used extensively on off road racers. The were set to give a lot of longitudinal compliance and gave no problems.


Regards

eng-tips, by professional engineers for professional engineers
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[GregLocock]

Depends what you mean by a pothole. We have one called a square edge pothole (SEPH). The forces you get in that test are enough to break the suspension.

Typical longitudinal recession rates are 300-1000 N/mm.

You can certainly design around the softer end of that, principally by making sure your inner tie rod is in the right place.






Cheers

Greg Locock

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

 

[MikeHalloran]

One of the supermarkets here has square edge speed bumps. No kidding; ~2" high, ~2' wide, perfectly square edges, i.e. vertical faces. I thought the semicircular ones were bad enough..



Mike Halloran
Pembroke Pines, FL, USA

 

[formula94lt1]

Thank you very much Greg. Just to clarify, by pothole I mean like a 4-7" radius semicircular hole or there about in shape. I guess the impact depends on the tire dimensions and characteristics as well as whats being run into. Trying to gauge how different in peak magnitude, if any, braking and impact forces are in comparison to each other in the fore mentioned cars. On a bumpy roadway, ie. like a cobblestone road, would you suspect the longitudinal forces from the individual impacts would reach the forces that a maximum effort stop would on smooth concrete? Maybe greater in the rear but what about the front?

 

[GregLocock]

Sufficiently rough roads put much larger forces in than the brakes can, for non-downforce cars. The brakes are limited by mu at the contact patch.

A 2D model of a wheel in a pothole will not give a very accurate estimate of the forces, but if you put all the right parameters in you can get within a factor of two or so.

Full 3D vehicle models with non linear tire models will get much closer than that, see for instance

http://www.ftire.com,

or ABAQUS Explicit. I've never done that, it's on the list.





Cheers

Greg Locock

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

 

[Bourgest]

Hi there, newbie signing in.

To my knowledge the longitudinal compliance of the wheel centre is the important factor regarding impact harshness felt by the passenger. The braking forces act at the tyre-road patch. This could be designed stiffer longitudinally than the wheel centre point. Wasn't that the clever bit of the McLaren F1 front suspension?

Best regards
René le Grand

http://www.bourgest.com

 

[formula94lt1]

Yes it was, thanks for that comment actually!

 

[GregLocock]

I hope you can draw the free body diagram and so on for that!





Cheers

Greg Locock

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

 

[formula94lt1]

What do you mean Greg? (BTW I don't mean what is a free body diagram lol)

 

[GregLocock]

How can you arrange a set of springs and levers above ground level in side view so that a force applied to the contact patch produces a smaller motion at the wheel centre than a force applied to the wheel centre itself?

It's quite an interesting problem.





Cheers

Greg Locock

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[Bourgest]

The McLaren F1 has a "normal" front axle double wishbone suspension. The control arms where not directly attached to the body but where attached to a "subframe". Not your regular subframe but two positioned vertically and longitudinal, one at each corner. This subframe was now mounted to the body. This subframe had bushings arranged in such a matter that the whole corner module was stiff in translation (X,Z) but soft in rotation (Y). The rotational centre is placed at the contact patch. Thus a force in the contact patch results in a translation only, which is stiff but a force in the wheel centre higher up will result in a translation AND a rotation, the later being soft. Hope my explanation is clear It's another great Murray idea.

Best regards
René le Grand

http://www.bourgest.com

 

[formula94lt1]

I am a fan of his stuff as well!

 

[GregLocock]

Yes that's the handwavy explanation, although I'd usually say RY for rotation. Lotus proposed that 'raft' design in 1987 and used a version of it on the SID car. I did the bushes.


Cheers

Greg Locock

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[Bourgest]

Well excuse me , didn't know the level of expertise here so kept it simple. I'll be more precise next time. Do you mind me asking how big a difference was used between X,Z stiffness and Y stiffness?

Best regards
René le Grand

http://www.bourgest.com

 

[GregLocock]

Hi Rene

We are supposed to be professional engineers here, so there is no need to dumb anything techie down. I'm actually rather suspicious that the handwavy explanation is misleading in the extreme, suspension engineers often think that bushes in the real world behave as they would like them to behave, rather than as lumps of fluid that vaguely behave like a solid. I think I can guess what the handwavy setup would have been, and I bet no one ever worked out the ACTUAL recession stiffness at the CP and WC. Because they'd have had to admit that the claims are untrue. Big clue:compliances are additive.

No I can't remember every single bush stiffness in all 6 DOFs from the last 25 years of working with them.

SID was a bit odd as it had a spine chassis, containing the powertrain, with a raft (separate, vertical, subframe), at each end for the suspension, that also suspended the body. So by choosing the appropriate bushes you could go anywhere from isolated body on rigid skid unit, to rigidly mounting the subframe in the car and suspending the powertrain softly.

The raft was originally sketched out for the M300 supercar that I suppose morphed into the Bugatti EB110, but they used some fancy sliding linkages that would not have made it into production, or even onto a working prototype.


Cheers

Greg Locock

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[formula94lt1]

Maybe I didnt understand your post, but it seems like you are skeptical the McLaren F1's system of bushings canceling the braking longitudinal movement while allowing compliance in bump would really work? If that is the case it should be noted the bushings used are not typical rubber bushings. I have the book "Driving Ambition" where they briefly describe their construction using some sort of nylon windings. They are about 25 times as stiff radially as axially more or less allowing them to plunge. Being tangents to circles whos centers are the contact patch it would allow the subframe at each corner to rotate about the contact patch when a force above the contact patch exerts itself on the tire, yet holds it very true (they give 1g castor wind off readings of the McLaren vs. a few other cars in the book which I can post) under braking.

 

[GregLocock]

"Being tangents to circles whos centers are the contact patch" Yes that's the way I'd have expected them to do it. 25:1 is a good effort, off the top of my head 16:1 is the best we do with rubber.

The problem is, it might work very well in reducing the longitudinal compliance at the contact patch compared with the wheel centre, but it sacrifices the main function of a suspension - to go up and down. It is not a mechanism.

Secondly, what is so great about reducing the compliance at the contact patch? Is stability of the castor angle so important?



Cheers

Greg Locock

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[GregLocock]

Just thinking about it, that is not reallly like the raft system at all. The raft allowed longitudinal motion while preserving the suspension geometry, the mclaren setup prevents longitudinal motion of the cp in order to leave the suspension geometry unchanged.

Cheers

Greg Locock

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