Lower Roll Centre at Front Than Rear

[VT-R]

Hi guys...

Sorry if this has been asked/answered before, but after a good hunt around with the search function I still couldn't turn up an answer.

I keep reading (mainly on the internet) that the front roll centre should be lower than the rear roll centre seemingly regardless of the CoG position, however I've not yet seen a convincing explaination of why this is.

My understanding would be that the position of the mean roll centre would be more important than the relative heights of the front/rear roll centres.

Cheers

Ed

[MilanT]

With equal front and back roll stiffnesses and track widths, a lower roll center height on one axle will equate to less lateral load transfer on that axle. So having a lower roll center in the front would mean that less of the total lateral load transfer is happening on that axle, which is a way to combat understeer.

[Goran Malmberg]

Originally Posted by VT-R

Hi guys...
I keep reading (mainly on the internet) that the front roll centre should be lower than the rear roll centre seemingly regardless of the CoG position, however I've not yet seen a convincing explaination of why this is.

My understanding would be that the position of the mean roll centre would be more important than the relative heights of the front/rear roll centres.
Cheers
Ed

Straight question, and I understand your confusion. Rc has been discussed in a way that would confuse anybody. The term Rc imply that it is a centre about which the car roll. The old way of determing the Rc is the intersection of the force lines, which it is not. That said, we could look at the forcelines.
The angle of the forcelines times the load of the wheel in question during cornering, determines the ammount of geometric load transfer. The lower the forceline angle the less the geometric load transfer, and thereby the Rc, as they are related. This means that the axle with lower forceline angle-Rc will transfer more load mecanically over the suspension system. The geometric loading will hit the tire grip hard like a bar was directley connected to the Cg, another name is antiroll. This has a lot of bad sides, jacking or lift of the entire chassis is one and harch contact patch loading is another. However, the outside wheels will create + jacking while the inner wheel makes - jacking. This means that we may have zero chassis lift while still having rock hard geometrig weight transfer present.

The lower front Rc is a very old idea, and I am not a fan of it with todays advanced A-arm and shock absorber tecnology. I set the Rc close to the ground, both front and rear, in order to have all load transfer taken care of by the suspension sytem and thereby be able to tune.

Rising the Rc is a harsh way of tuning the car, but possible to use. But we must have a very good reason for not using som more sofisticated way of balancing the car. Or, be very clear over that the chocking contact load is what we are looking for. Chocking load may give an initial planting of the tires, as needed in a dragracing start.

Regards
Goran Malmberg

[Goran Malmberg]

Originally Posted by Goran Malmberg

The angle of the forcelines times the load of the wheel in question during cornering, determines the ammount of geometric load transfer. The lower the forceline angle the less the geometric load transfer, and thereby the Rc, as they are related. This means that the axle with lower forceline angle-Rc will transfer more load mecanically over the suspension system.
Goran Malmberg

I see that I turned the words backways here. "mecanically" was not the word to use here.
Goran

[VT-R]

Cheers for that Goran, very informative!

[ubrben]

For maintaining contact patch load variation to a minimum low roll centres are a good thing. Having one higher at the rear than the front has an important function though in altering the rate of load transfer at the rear as opposed to the front without needing to alter the springs or bars.

As I understand it the relative rate of front and rear load transfer has a big impact of the driver's perception of US/OS in transients.

Goran: I think dogmatically having front and rear RCs on the ground is a bit limiting. Most good cars I've come across have a very low front RC and the rear at around 50mm above ground.

Ben

[Goran Malmberg]

Originally Posted by ubrben

Goran: I think dogmatically having front and rear RCs on the ground is a bit limiting. Most good cars I've come across have a very low front RC and the rear at around 50mm above ground.
Ben

Ben.
Please let me explain my theory.

By raising the Rc we will have a corresponding part of the load transfer to become geometric. With zero front and + rear Rc we will have part of the load transfer to without delay to hit the tire contact patch, while the rest of the Wt will pass thought the springs and shock system as normal.

As we turn the front wheels the loads will at first hit the front wheel, and by having part of the rear Wt to become instant by passing over the geometry, there will be some feel of a load transfer equalizing effect.

However, most of the time a car is filled up with angles to get all kinds of effects, without being questioned. I am a spokes man for what I call a "zerocar", a car that has no angles like anti: s, sai or scrub, which is an abstract car to get once mind straight. Let say we like some "turn in" to the car. If the car is a zerocar, we must ADD something to accomplish this. If the car is a "normal" car we might be better off by taking something AWAY that has the opposite effect from turn in, like lowering the antiroll of the rear.
But most of the times the action will be to add something to the front suspension system.

Therefore I usually design a car to be as much "zero" as possible, and add adjustability for what I think will be needed. This will minimize the risk of making tuning adjustments to become a "compensation" for already inbuilt "not necessary" angles.

I like to design the A-arm geometry with controlled forceline intersection geometry. Raising the rear Rc interferes with this goal. But, I am leaving the inner pivot point adjustable in height, so the lower A-arm could be raised slightley if needed. Look at the A-arm to chassis joint.

Regards
Goran Malmberg

[retro_msport]

I can understand Goran's thinking about the 'zero' .. in simple terms you only have one way to adjust, ie halving the options which also haves the mistakes.

In my world though, we have front RC at or just above the ground and rear RC of anywhere between 11" to 13" from the floor. (McPherson front and Live axle rear) On my cars i use a low RC watts link to get the RC's closer to each other as i belive the old saying .....

"if you corner on 3 wheels there is less rolling resistance"

which used to accompany pictures of Mk1 and 2 Escorts going around corners with the front inside wheel in the air is wrong..

I like all 4 tyres to stay in contact.

Now Goran, i've just had a thought pop in my head .... what about an RC above the C of G .. i'm thinking a pendulum effect, as the car goes around the corner the CofG wants to slide out from under the RC to the outside so transfering load to the inside wheel (i'm sitting here making triangles with my fingers and thumb and rolling them ) ...or should i go and get some coffee?

[Goran Malmberg]

Originally Posted by retro_msport

INow Goran, i've just had a thought pop in my head .... what about an RC above the C of G .. i'm thinking a pendulum effect, as the car goes around the corner the CofG wants to slide out from under the RC to the outside so transfering load to the inside wheel (i'm sitting here making triangles with my fingers and thumb and rolling them ) ...or should i go and get some coffee?

As I am jus drinking coffee here, I better wait a momemt to make a comment...
Goran

[retro_msport]

sort of like that ..... and yes i'm just putting the kettle on

my low RC Watts link

[Chris Y]

"Made in Harlow from girders" - some nice fabrication there!

[Goran Malmberg]

Originally Posted by retro_msport

Now Goran, i've just had a thought pop in my head .... what about an RC above the C of G .. i'm thinking a pendulum effect, as the car goes around the corner the CofG wants to slide out from under the RC to the outside so transfering load to the inside wheel (i'm sitting here making triangles with my fingers and thumb and rolling them ) ...or should i go and get some coffee?

Long coffee brake here.
The Wt=(cgh*w*g)/Tw, no matter what the pattern of roll movement are, or the proportions of geometric vs sprung Wt. But of course, as the body roll, the cg will to some extent alter its location, but I usually dont take this in to account.
In fact I heard about a such theory way back in the 60, dont know what it turned out to.
Regards
Goran

[retro_msport]

So when we building a car then Goran

[retro_msport]

Originally Posted by Chris Y

"Made in Harlow from girders" - some nice fabrication there!

Girders .. cheeky bugger , that lots lighter than the normal Rally Type Gp4 watts that you see on Escorts .. and it has less unsprung weight too..

But thanks for the thumbs up on the fabbing.. perhaps you'll like the inside ..




But getting back on topic .. is the lower front RC rule of thumb a direct link to rear engined single seaters and the C of G's along the axles of these cars.. now i don't go near singles (5'17" tall ) but would i be right in saying that the rear has a higher CogG than the front... and the lower RC at the front is the same distance from the CofG as the rears RC is from its CofG ... hence the "common" wording of "lower front RC"

???