Lawrence Link Suspension

[Foote]

In this month's issue of race car engineering, they talk about a lawrence link suspension design. The pictures aren't very good, but it just looks like a trailing link setup to me. Does anybody know anything about this design or its advantages?
just curious

[SnailRacer]

Yeah that perked my interest too. It claims absolutely stationary roll centers and 'perfect' camber compensation in roll; but I find this hard to believe. I can't exactly make out the geometry from the pics either, if anyone could enlighten us some more I'd appreciate it too.

Thanks
jsn

[mtg]

I looked at it also. The kinematics look really good, the drawback is in the mounting. The loadpaths are very inefficient. You'd have to run a big beam horizontally out from the chassis to support the highly loaded control arms.

[Z]

I have also seen the article. Briefly - nothing special there.

Lawrence's car originally had VW Beetle suspension. The front tyres were losing grip because of adverse camber during cornering (wheels leaning sideways at same angle as car body). Keith Duckworth suggested to Lawrence to use "camber recovery" so he inclined the axes of the trailing arms so that they intersected close to the centreline of the car. Hence, "fixed roll centre" and "perfect camber compensation".

The thing is, this can also be done with normal wishbones - check your kinematics books!

More importantly, it can also be done MUCH MORE EASILY, and with BETTER STRUCTURAL EFFICIENCY, and with MORE SIMPLICITY, by using a good old-fashioned PURE SWING-ARM.

This is effectively a beam-axle that is cut in half, with each half mounted to the chassis with a longitudinal "hinge" that is close to the centreline of the car (this is how these swing-arms first evolved - eg. Tatra 1920's). Or continue your normal wishbones towards the centreline of the car, then join the upper and lower inboard joints together. The hinges should be low on the car, say at floorpan height, to minimise jacking (eg. quite successful rear suspension of many Mercedes', including GP racecars, of the 1950-60s).

The main disadvantage of pure swing-arms (ie. lateral arm with longitudinal axis), or the Lawrence link, is significant camber change with body heave and pitch, and with single wheel bounce. So bad wheel camber if there is body pitch under accelerating/braking, and horrible gyroscopic forces on bumpy roads. This is the penalty you pay for having 100% camber compensation. On smooth tracks (eg. FSAE) the heave/pitch camber change can be minimised by using stiff "axle-bounce-mode" springs, eg. stiff Z-bar, or stiff lateral leafspring, connecting left and right wheels. This, in effect, returns the system to a slightly bendy beam-axle pivoted at its centre to the chassis.

Low-pivot, pure swing-arms would be a simple, cheap, easy suspension for FSAE cars, that would have more than adequate performance characteristics for FSAE conditions.

Of course, you only need 100% camber compensation if you are expecting a lot of body roll, perhaps because of soft springs. Can you have soft suspension and still have no body roll, and consequently no need for camber compensation? Sure, but that's another story...


Z

[DJHache]

As you pointed out yourself Z, there are horrible gyroscopic forces on bumpy roads. Given the track conditions at the Silverdome and in most parking lots where these cars drive, the disadvantages are significant. The bumpiness of these tracks also prevents us from going with an really stiff suspension.

And bending is not really structurally efficient.

[Z]

DJ,

(I've just added some to my previous post.)

Re: "gyroscopic forces due to camber change". If you do the maths (which is quite easy) you will find that the gyro forces on a FSAE track are really very small. On a desert racer, with big heavy wheels, big, BIG bumps, and 200mph speeds, then you don't want any camber change over bumps, though some desert racers still put up with it. FSAE tracks, even the Silverdome, are really quite smooth in comparison.

I do agree that stiff suspension is bad on FSAE cars, but that's another matter.

Re: "bendy beam-axle with central chassis pivot". My comment there was meant to explain by way of an analogy what a stiff "axle-bounce-mode" would look like. I wouldn't do it that way in practice (a rigid beam-axle, softly sprung, would be better all round).


Z

[Ché]

Justin, it seems all things eventually end up back at a Tatra!