Jacking Forces

[blister]

Hi,

What is often written in literature is, that the higher the roll centre, the bigger are the jacking forces.

First: shouldn`t we use the angle between the instant center and a line between the instant center and the contact patch to determine the amount of jacking force acting at the contact patch?

Second: If this is true, is it possible to have a high roll centre and low jacking force? I could imagine a suspension with both A-arms pointing steeply up seen from the chassis. (Low IC, High RC, small swing arm length, much lateral RC movement)

[blister]

Hi,

What is often written in literature is, that the higher the roll centre, the bigger are the jacking forces.

First: shouldn`t we use the angle between the instant center and a line between the instant center and the contact patch to determine the amount of jacking force acting at the contact patch?

Second: If this is true, is it possible to have a high roll centre and low jacking force? I could imagine a suspension with both A-arms pointing steeply up seen from the chassis. (Low IC, High RC, small swing arm length, much lateral RC movement)

[exFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by blister:
Hi,

What is often written in literature is, that the higher the roll centre, the bigger are the jacking forces.

First: shouldn`t we use the angle between the instant center and a line between the instant center and the contact patch to determine the amount of jacking force acting at the contact patch?

Second: If this is true, is it possible to have a high roll centre and low jacking force? I could imagine a suspension with both A-arms pointing steeply up seen from the chassis. (Low IC, High RC, small swing arm length, much lateral RC movement) </div></BLOCKQUOTE>

Angle between a line from the contact patch to the IC, and the ground. This can be determined from the RC location (if you're defining RC as the intersection of those two force vectors from left and right sides)

Bear in mind. Jacking or anti-jacking isn't inherently bad.

[nowhere fast]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by blister:
Second: If this is true, is it possible to have a high roll centre and low jacking force? I could imagine a suspension with both A-arms pointing steeply up seen from the chassis. (Low IC, High RC, small swing arm length, much lateral RC movement) </div></BLOCKQUOTE>

The amount of jacking will be determined by the angle that the virtual swingarm (line from contact patch to IC) makes with the ground and not by the height of the IC.
So the suspension you describe would have a high amount of jacking since the virtual swing arm would be angled up steeply.

[blister]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> So the suspension you describe would have a high amount of jacking since the virtual swing arm would be angled up steeply. </div></BLOCKQUOTE>

I meant angled up steeply from the chassis to the upright.

I think with that setup it is possible to have small jacking forces (Small IC to ground angle) and high kinematic RC.

[exFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by blister:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> So the suspension you describe would have a high amount of jacking since the virtual swing arm would be angled up steeply. </div></BLOCKQUOTE>

I meant angled up steeply from the chassis to the upright.

I think with that setup it is possible to have small jacking forces (Small IC to ground angle) and high kinematic RC. </div></BLOCKQUOTE>

Not seein how that's possible. The "RC" defines the slopes of the left- and right-side force vectors.

High intersection ("RC") = high angle = high jacking.

Low intersection ("RC") = low angle = low jacking.

At least for the symmetric case.

The two are not independent.

[Rotary Sprocket]

Here's the link to an article about the relationship between roll centers and jacking forces. It's a good article, the only thing is that you have to buy it. Worth it though if you really want to understand what is going on.

http://www.sae.org/technical/papers/1999-01-0046