Geometric and Elastic WT

[The Stigg]

I know that elements of this topic have been discussed before. But there are several questions resulting from theory that I can' explain by my self.

As suspension software with included force based RC modeling are far away of beeing cheap I worked out a simple kinematic tool in MS Excel.

I started to analyse the beaviour of elastic and geometric weight transfer regarding different RCH. In addition I looked a little bit further in situations with RC fixed to Chassis height and situtations with RC fixed to Ground.

It seems to me that if you want to control elastic WT you need to have a RC fixed to the Chassis. But you will also get a decresing geometric WT with bump because of the reduction in RCH relative to ground. If you have placed your static RC lower than the amount of bump the geometric WT changes its sign from positive to negative as the RC passes the ground (could this be??)

So if you place your static RC on ground level fixed to the chassis there will always be a negative geometric WT for roll situations increasing with roll angle.
(could this be??)

The theory doesn't make sense to me anymore.
I have to think about this a little more

Regards
Andy

[The Stigg]

I know that elements of this topic have been discussed before. But there are several questions resulting from theory that I can' explain by my self.

As suspension software with included force based RC modeling are far away of beeing cheap I worked out a simple kinematic tool in MS Excel.

I started to analyse the beaviour of elastic and geometric weight transfer regarding different RCH. In addition I looked a little bit further in situations with RC fixed to Chassis height and situtations with RC fixed to Ground.

It seems to me that if you want to control elastic WT you need to have a RC fixed to the Chassis. But you will also get a decresing geometric WT with bump because of the reduction in RCH relative to ground. If you have placed your static RC lower than the amount of bump the geometric WT changes its sign from positive to negative as the RC passes the ground (could this be??)

So if you place your static RC on ground level fixed to the chassis there will always be a negative geometric WT for roll situations increasing with roll angle.
(could this be??)

The theory doesn't make sense to me anymore.
I have to think about this a little more

Regards
Andy

[Trevor]

Here's my thinking on the subject.

Steady state cornering
Simplified one axle
For a RC fixed to the chassis:
If you have two wheel bump (or dive/squat) during cornering, yes, you would get less geometric WT due to the lower GC height. You would have the same elastic weight transfer because the sprung mass CG moment arm around the RC hasn't changed.
This makes sense, because we all know that a lower CG gives less load transfer, all other things being equal (track, weight, lateral acceleration).

If a bump puts your RC underground, you have reversed geometric WT. Total WT will be lower due to the lower CG.

But, it seems to me that in pure roll, the car will roll about the RC, meaning the RC height with respect to the ground shouldn't change, thus the same WT. When you add bump, the RC height would go farther underground and thus increase negative geometric WT. So more bump (for both wheels) would yield more negative geometric WT and lower overall WT. But in steady state, more roll should not mean decreased geometric WT.
One wheel bump seems like roll (unless you're not cornering, and then why are we talking about this?)

Right?

It seems like this might change if you're thinking about two axles. Then you have to consider if both are going into bump, or if you're pitching, what happens to the CG and the CGs at each axle (pitch center effects).

I'm still learning this stuff myself, so anyone more experienced should feel free to chime in.

[Steve O]

Well to answer about the positive/negative swap of weight transfer, that does not happen when you cross the ground, that happens when you cross your CG. If your roll centers are above your CG your car and actually roll into the corner... strange concept but true and your driver will probably freak out. Also this concept of fixed roll centers you are referring to is only in static fyi. Your RC will change as your suspension geometry changes. There is no particular reason to have it at ground or at chassis in the static position. Before I go into this too deep, what exactly do you know about roll centers and roll coupling relative to the CG? It will help me explain what I need to. For instance there is more than one type of RC you have your FBRC, your KRC (the one that can be found geometrically) and not really an RC but you have your force application point which is commonly what people treating their RC as which is not the case.

In response to Trevor one wheel bump is not the same as roll. There is a lot more going one which geometry changes in all 4 wheels in roll than there is with a one wheel bump not affecting anything else in the system.

One more note, before you try and use software try doing some geometric calculations, then hand calculations and use the software to help you from there. Look into Susprog3d if you are on a budget, 300 bucks, I think, and it will get the job done. The developer Robert Small is also very quick to respond to any questions.

Steve

[cmeissen]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Steve O:
Well to answer about the positive/negative swap of weight transfer, that does not happen when you cross the ground, that happens when you cross your CG. If your roll centers are above your CG your car and actually roll into the corner... strange concept but true and your driver will probably freak out.
Steve </div></BLOCKQUOTE>

Your geometric weight transfer(Acts directly through the RC) will reverse when the RC goes below ground. The elastic weight transfer(Caused by moment about RC) reverses when the RC is above the CG.

[The Stigg]

Hi Steve and all others,

thank you for your comments!
Fist I have to say that I'am investigating this topic for a long time now. I forgot to mention that my previous post refers to a special geometry. It is possible to design geometries that show no noticable differences in RC location for the Force Based and the Kinematic Method. (For our small movements)

When you analyse such a geometry regarding the two situations mentioned in my previous post you will see a different behaviour in the spring movement. This lead me to the suspicion that there must be a reverse in the geometric WT.
It acts like a anti-jacking effect !?

Regards Andy

[murpia]

For equal tyre forces, a geometric 'roll centre' below ground will have an 'anti-jacking' effect.

In the real world, with unequal tyre forces, dampers and component inertias all at factors, the result could be wildly different...

Regards, Ian

[exFSAE]

I'd be more interested in seeing how your weight transfer changes when the car pitched forward or rear (trailbrake corner entry, or power-on exit).

Was thinking about this the other day. Let's say for simplicity's sake that you have static RC's at ground level and your CG height doesn't change appreciably with pitch or roll.

When the car pitches forward into the turn, front RC goes underground, rear RC comes off the ground. Roll axis height at the CG remains the same. Net geometric WT stays at ~0, but there will be opposite non-zero WT's front and rear... magnitude of which being proportional to how much the chassis is pitching. I'd think that would effectively throw a little wedge into your car. (At least this is my theory. I could easily be completely wrong).

Depending on how load sensitive the tires are that could be a good or bad thing.