Roll axis.

[hemipanter]

After reading the "Mass Centroid Axis Revisited" the following come up in my mind, but I guess it is better suited in a new tread.

In his last newsletter Mark Ortiz answered a questioner about a roll axis phenomenon. Question was, if the roll axle was tilted so that the roll arms at each axle change its length in relation to the new ”jacking arm” length, will the roll stiffness be affected then?

The answer was, under the circumstances given, only the geometric load transfer should be altered.

Now, I recently read a tread where it was stated that some team use the Rch as a tuning tool,
in this case moving the upper A-arm attachment to the chassis up or down. It is easy believed that altering the Rch in this manner would not affect elastic load transfer then.

To me this is not necessarily so.
Depending on suspension design altering instant centre arm will more or less influence the wheel rate. I don’t see that altering Rch in a way that keeps elastic rate the same during Rch change is that easy, other than using very short scrub radius. The whole method seem a little tricky to get track of to me.

Any ideas here?

Also, what exactley is the advantage of adjusting balance using geometric load transfer?

Goran Malmberg

 

[GregLocock]

So far as I know both V8 Supercars, in Oz, and NASCAR, adjust the vehicle's balance by raising an lowering the rear axle's RCH by moving the pivot point of the lateral linkage, during a race. Now, either they are messing about with things they don't understand, or at least two racing series take small changes in RCH moderately seriously.

However, in both those cases they will not affect the elastic roll rate very much.

With an SLA it is very easy to believe that a significant change in RCH, brought about by moving the UCA inboard pivots vertically, will affect the linkage ratios, and so will affect the elastic wheel rates. Obviously if your spring and roll bar are mounted well outboard this is less of a concern. Equally as you say, if the scrub radius desn't move much then that will help desensitize this.

On the other hand, are small shifts in the elastic roll rate very important?

I'll have a play around today and see if it makes any sense.







Cheers

Greg Locock

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

 

[hemipanter]

My way of looking at it...
The roll axis model on my site shows that if we raise the Rc at one end of the car the chassis roll will no longer be parallel to ground. This means that the roll arm is 90 degree to the roll axis. So, if rising the Rc at one end it has to be lower at the other end (how much depend on weight distribution) in order to keep elastic roll the same.

Using geometric weight transfer in place of elastic makes for a more direct response, if this is the reason for use in these racing classes. Or maybe rules.

A small amount of elastic weight transfer is not very influential in most cases.
Regards
Goran Malmberg

 

[Knap]

The equations we use in the WTW (since 1999) are the same as used by Claude Rouelle.

Geometric Weight Transfer - due to the component of lateral force, applied directly at the Roll Centre (RC). Geometric WT is reacted directly through the suspension linkages, and does not induce body roll.

It is clear that low roll centre give little geometric wt and most of the weight transfer goes through the springs (elastic wt), and is therefore delayed by the time it takes for the vehicle to take a set. Conversely, with high roll centre most of the weight transfer precedes the body roll, leaving a smaller amount of weight transfer to go through the springs.

The location of roll centre heights and the affect on geometric weight transfer vs elastic weight transfer is of importance in the set up of the car. Geometric weight transfer is a major influencer for cars of high front weight percentage and/or for FWD. Also for RWD with live rear axle. Also for current open wheelers with high downforce and little suspension movement.

In current open wheeler racing, geometric w.t. can be used because of the reduction in jacking affect: small suspension travel, wide track, long suspension arms to stop the RC height moving around so much relative to the chassis ie you don't get "progressive" jacking as the car rolls more. In fact, you need the geometric w.t. to help reduce the roll angle and suspension travel, while using less rear anti-roll bar, sometimes none at all.
From

http://www.racing-car-technology.com.au/wgttransex.htm

 

[hemipanter]

That is interesting. "Moving around Rc" not is desirable as it creates jacking. I have a theory here that in fact uses a moving Rc to neutralized jacking. If the Rc is kept fixed in the middle of the car, positive and negative jacking forces will not balance the inside-outside wheel load during weight transfer thuse creating jacking.

A geometry that moves the force line interception to the inside wheel to the same degree as load is transferred to the outside wheel will have zero jacking movement in that the less loaded inside wheel get a greater force line angle and the outside wheel higher load and less force line angle. Provided the Rc height is kept the same.

I should read the article carefully during the day.
Thanks
Goran Malmberg

 

[GregLocock]

I'd just add that jacking is useful in production cars.

Cheers

Greg Locock

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

 

[gt6racer2]

Greg,
Being involved in rollover studies, I was surprised by your comment, since of course this raises the cg. What are the positive attributes ?

 

[GregLocock]

Steering

Cheers

Greg Locock

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