cornering force and jacking force

[the weldmeister]

ok, i've read a lot of conflicting posts on this topic, so i was hoping to clear it out.

is the cornering force (and thus the jacking force) reacted through the IC or the roll centre?

according to carrol smith, it is through the RC.

if reacted through the IC, jacking force would a) increase roll if the IC is above ground.
b) reduce (or oppose) roll if it is below ground

if reacted through the RC, jacking force would have no effect on roll.

am i correct?

[the weldmeister]

ok, i've read a lot of conflicting posts on this topic, so i was hoping to clear it out.

is the cornering force (and thus the jacking force) reacted through the IC or the roll centre?

according to carrol smith, it is through the RC.

if reacted through the IC, jacking force would a) increase roll if the IC is above ground.
b) reduce (or oppose) roll if it is below ground

if reacted through the RC, jacking force would have no effect on roll.

am i correct?

[J. Vinella]

In the terms you are talking about you are correct it is about the IC. But because your IC defines your RC you will often find jacking force calculations in terms of RC.

Roll angle change due to jacking = (2*JackingForce*lateral_location_ofRC)/(track^2*wheelrate)

If you want that in terms of IC then...add body roll moment reaction...somewhere http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

I agree it gets confusing. Carrol Smith says RC, The Millikens say IC when looking at a single side of the suspension.

Best of luck,

EDIT: I'll check that equation in the morning, however I feel it is right. If only Z was still posting to check my "maths".

[ben]

The kinematic roll centre is a fictitious point. Each side of the car is a separate mechanism and if you want to do it correctly you need to react the lateral force on each side at the respective IC.

If you assume equal lateral force on left and right wheels the jacking forces cancel anyway.

Ben

[the weldmeister]

but the lateral forces are never the same are they? the outer wheel provides most of the cornering force..

the point is, if the jacking force reacts through the RC, there's only a net upward (or downward) force on the sprung mass.

on the other hand, if it reacts through the IC, it should also contribute to (or detract from) roll.

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by the weldmeister:
but the lateral forces are never the same are they? the outer wheel provides most of the cornering force. </div></BLOCKQUOTE>
Precisely, see Ben's post. We might bust the myth of the 'roll centre' one day...

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by the weldmeister:
the point is, if the jacking force reacts through the RC, there's only a net upward (or downward) force on the sprung mass. </div></BLOCKQUOTE>
That would assume your 'roll centre' is a) on the centreline and b) is actually what your car rolls around, neither of which is likely to be true...

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by the weldmeister:
on the other hand, if it reacts through the IC, it should also contribute to (or detract from) roll. </div></BLOCKQUOTE>
Not if the ICs are both on the centreline...

If you haven't already done so, take a look here (http://fsae.com/eve/forums/a/tpc/f/125607348/m/47110403711).

Regards, Ian

[mtg]

Wait a minute, we have a motorcycle engineer talking about roll centers? http://fsae.com/groupee_common/emoticons/icon_smile.gif

I've said this before, I don't think kinematic roll centers are worth putting much thought into. As soon as you change your springs or ARB stiffness, the real roll center moves. Kinematics does not account for this.

I would venture to say most FSAE cars, mid-corner, have a roll centers that are on or close to the outside tires' contact patches.

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by mtg:
Wait a minute, we have a motorcycle engineer talking about roll centers? http://fsae.com/groupee_common/emoticons/icon_smile.gif

I've said this before, I don't think kinematic roll centers are worth putting much thought into. As soon as you change your springs or ARB stiffness, the real roll center moves. Kinematics does not account for this.

I would venture to say most FSAE cars, mid-corner, have a roll centers that are on or close to the outside tires' contact patches. </div></BLOCKQUOTE>

I need to change my sig - I'm back on cars in January http://fsae.com/groupee_common/emoticons/icon_razz.gif

Ben