roll center location vs camber gain's effect on haNdliNg

[Tmoose]

Someone on a brand specific mailing list referred to one Of Niki Lauda's books as a having a chapter or two with technical content, but could not remember which book that was. Possibly " art of gran prix driving " or " turbo age."
1- Does anyone know which book he might have been remembering?

He also offered the opinion that "handling is affected more by roll centers than camber gain."
2 - I don't know how "correct" that opinion is, but would be interested in hearing comments by folks here.

FWIW the suspension being discussed is 2nd generation Corvair FRONT suspension which uses upper and lower control arms and was developed long before the TransAm days so is probably biased toward minimizing tire scrub etc.

Also FWIW I suspect handling for most folks is how a car feels rather than best lap times or skid pad numbers. But I have not probed that question with the above mentioned someone. And possibly never will.

thanks

Dan T

 

[JackAction]

Any suspension with the roll center above the ground will experience a jacking force; That's the whole point of resisting the rolling motion.

But if the roll center is above the CG, the jacking force becomes larger than the weight transfer and thus the body actually lifts off. That is when it's an issue as you mentioned.

Swing axles are particularly bad at this because the problem is not the roll center height at rest but their short lever arm. As the wheel moves down, the roll center goes up rapidly and excessively. It's not good on any vehicle that experiences the slightest lateral acceleration, race car or not.

 

[Tmoose]

I think maybe that Spitfire's drive is off the gas too. That is when I got into trouble.
As do early Porsche owners, and even BMW owners with semi trailing arm suspensions.

The story in Denis Jenkinson's Race driver book has a section on Sir Stirling Moss driving various Mercedes swing axle or maybe low pivot swing axle race cars. He was able to make them initially with tons of steering lock, but later applied lots of throttle when the tail started to come out. Supposedly some German engineer said, yah we make it that way on purpose, but only very knowledgeable drivers know how to use it. Stirling was apparently 1 of 2 factory drivers that figured it out.

 

[BrianPetersen]

"Hark, the Herald axles swing!"

All independent suspensions with instant centers above ground level will experience some jacking effect. Trouble arises not only if the instant center is above the CG, but also if the spring rates are too soft. As the body lifts up, the jacking effect goes up, that increases the "jacking" effect, and if the "jacking" effect is increasing faster than the spring forces are falling away, up and over she goes!

Some swing axle suspensions were fitted with something called a "camber compensator" which was an extra spring tied between both sides with almost the opposite of an antiroll bar. The idea was that if both sides of the suspension were in droop, it added some spring force to resist further extension into droop.

Swing axle suspensions of the normal design have the roll-center above axle centerline; the M-B low-pivot system was a bit different. That is a very high roll center by modern standards.

Beam axle suspensions can get away with having a high roll center because the force that would otherwise lead to jacking is (correction: should be, if the suspension is properly designed) contained within the axle, it (mostly) doesn't pass through the bodyshell and thus affect the ride height.

"Side scrubbing" is another side effect of a high roll center, and that doesn't matter if it's beam axle or IRS. Go over a one-wheel bump, and the tire contact patch kicks sideways. You feel it in the vehicle as a sideways kick. People used to being in modern cars will feel odd ride motions if they are in a 4x4 vehicle with beam axles front and rear, and this is part of it.

 

[NormPeterson]

Good discussion. See if the attached makes sense.

Bob

The sketch does make sense, but it needs to be shown with a more realistic amount of roll. Drawn closer to 4.5° than 45° would provide a better sense of the magnitude of actual CG migration being relatively small.


Norm

 

[BUGGAR]

Norm,
You've never followed an overloaded mini-pickup in TJ?

Actually I was trying to figure this out late at night and used the principle of exaggeration to make it clear.

Bob