Effects of CofG & Roll Stiffness

[T.Donaldson]

Here's the difference between your two senarios (RWD only):

Acceleration: the forces in the tire patch are all parallel to the direction of travel, meaning that the additional force being implemented on the tires is solely from above, which helps the tires find more grip. The contact patch cannot change because the wheel is round in that direction.

Cornering: no static difference. Here you have a force in the forward movement and an additional force perpendicular to the tire (parallel to the axle). This perpendicular force causes the car to roll, which also moves the CoG outward. Now your tire's contact patches are subjected to forces in two directions. The perpendicular force also changes the geometry of the contact patches, causing the outer tires to carry more load, and moving them toward the friction limit.

http://www.rmcbmwcca.org/DrivingSchool/ ... nsmall.jpg

By increasing the height of your CoG you increase the leverage on the shoulders of the outside tires, until you reach the tipping point. If the friction force in the tire patches is not exceeded by the centrifical force on the car and the tipping point is not reached, there is no major difference in the higher or lower CoG, since the increased pressure on the outside tires is offset by the decrease on the inside tires (torsion bars help against this problem, different subject though). It doesn't feel very comfortable for the driver when the car rolls excessively however, and this roll can cause dynamic problems (i.e. reaching the tipping point earlier).

I hope I managed to be somewhat clear, otherwise I'll try to translate it again.

This was a very helpful post and seems more logical than anything else I've read, thanks!

[donskar]

T.Donaldson, take a look at Carroll Smith's books -- Tune to Win and Prepare to Win. I think they will help you a lot.

[Tim.Wright]

Here's the difference between your two senarios (RWD only):

Acceleration: the forces in the tire patch are all parallel to the direction of travel, meaning that the additional force being implemented on the tires is solely from above, which helps the tires find more grip. The contact patch cannot change because the wheel is round in that direction.

Cornering: no static difference. Here you have a force in the forward movement and an additional force perpendicular to the tire (parallel to the axle). This perpendicular force causes the car to roll, which also moves the CoG outward. Now your tire's contact patches are subjected to forces in two directions. The perpendicular force also changes the geometry of the contact patches, causing the outer tires to carry more load, and moving them toward the friction limit.

http://www.rmcbmwcca.org/DrivingSchool/ ... nsmall.jpg

By increasing the height of your CoG you increase the leverage on the shoulders of the outside tires, until you reach the tipping point. If the friction force in the tire patches is not exceeded by the centrifical force on the car and the tipping point is not reached, there is no major difference in the higher or lower CoG, since the increased pressure on the outside tires is offset by the decrease on the inside tires (torsion bars help against this problem, different subject though). It doesn't feel very comfortable for the driver when the car rolls excessively however, and this roll can cause dynamic problems (i.e. reaching the tipping point earlier).

I hope I managed to be somewhat clear, otherwise I'll try to translate it again.

This was a very helpful post and seems more logical than anything else I've read, thanks!

Except that it contains a few massive errors. Using that theory, changing the roll stiffness distribution will have no effect on US/OS which is of course not true. The key point is that the decrease in grip on the inner wheel is MORE than what you gain on the outer wheel so the result is a net LOSS of cornering grip. The higher the CG, the more grip you lose.

JT has explained the situation using reasonable numbers. If you don't want to do any research yourself, at least try understand the calcs he did.

[T.Donaldson]

Here's the difference between your two senarios (RWD only):

Acceleration: the forces in the tire patch are all parallel to the direction of travel, meaning that the additional force being implemented on the tires is solely from above, which helps the tires find more grip. The contact patch cannot change because the wheel is round in that direction.

Cornering: no static difference. Here you have a force in the forward movement and an additional force perpendicular to the tire (parallel to the axle). This perpendicular force causes the car to roll, which also moves the CoG outward. Now your tire's contact patches are subjected to forces in two directions. The perpendicular force also changes the geometry of the contact patches, causing the outer tires to carry more load, and moving them toward the friction limit.

http://www.rmcbmwcca.org/DrivingSchool/ ... nsmall.jpg

By increasing the height of your CoG you increase the leverage on the shoulders of the outside tires, until you reach the tipping point. If the friction force in the tire patches is not exceeded by the centrifical force on the car and the tipping point is not reached, there is no major difference in the higher or lower CoG, since the increased pressure on the outside tires is offset by the decrease on the inside tires (torsion bars help against this problem, different subject though). It doesn't feel very comfortable for the driver when the car rolls excessively however, and this roll can cause dynamic problems (i.e. reaching the tipping point earlier).

I hope I managed to be somewhat clear, otherwise I'll try to translate it again.

This was a very helpful post and seems more logical than anything else I've read, thanks!

Except that it contains a few massive errors. Using that theory, changing the roll stiffness distribution will have no effect on US/OS which is of course not true. The key point is that the decrease in grip on the inner wheel is MORE than what you gain on the outer wheel so the result is a net LOSS of cornering grip. The higher the CG, the more grip you lose.

JT has explained the situation using reasonable numbers. If you don't want to do any research yourself, at least try understand the calcs he did.

Apparently I quoted the wrong post! I did in fact mean Jersey Tom's post - but I appreciate input from anyone

I have done a TONNE of research on this topic.

[PhilS13]

Apparently I quoted the wrong post! I did in fact mean Jersey Tom's post - but I appreciate input from anyone

I have done a TONNE of research on this topic.

If you read a text that goes the opposite way of what Jersey Tom explained just remove it from your research sources. Pretty basic stuff very well explained above. Higher CG -> More lateral weight transfer -> Less total cornering grip available. I remember Carl Lopez's Going Faster! Mastering the art of race driving covers that in a very simple (almost non-technical) way.

[Tim.Wright]

Yea further to that, my advice is to make your own mind up about anything you read. There is a LOT of BS in the field of vehicle dynamics, some of it coming from well known people too. The only thing that will get you past that is a good education in physics/mechanics.

[CBeck113]

Thanks for correcting me, Tim - especially for T. Donaldson...I'll stop answering posts when I don't have time to read them thoroughly. Sorry!

[T.Donaldson]

Apparently I quoted the wrong post! I did in fact mean Jersey Tom's post - but I appreciate input from anyone

I have done a TONNE of research on this topic.

If you read a text that goes the opposite way of what Jersey Tom explained just remove it from your research sources. Pretty basic stuff very well explained above. Higher CG -> More lateral weight transfer -> Less total cornering grip available. I remember Carl Lopez's Going Faster! Mastering the art of race driving covers that in a very simple (almost non-technical) way.

Thanks, I'll have a look at that

Yea further to that, my advice is to make your own mind up about anything you read. There is a LOT of BS in the field of vehicle dynamics, some of it coming from well known people too. The only thing that will get you past that is a good education in physics/mechanics.

Yeah, I've quickly discovered that myself - thanks for the heads up!

[T.Donaldson]

Thanks for correcting me, Tim - especially for T. Donaldson...I'll stop answering posts when I don't have time to read them thoroughly. Sorry!

No worries