Thoughts about PHB relocation

[Grant 302]

I hope nobody is thinking I have anything against lowering the PHB and also reducing the amount of rear bar. I don't, not where you might have a clear reason for doing them together, anyway.

Offhand, the only reason that comes to mind where you would do those together is where you're specifically looking to improve acceleration traction on corner exit. Not for tuning understeer-oversteer handling balance unless you're also planning on running significantly more rear spring to make up for what you gave away in bar.


Norm

This is exactly why I’ve been interested in the topic. With specific interest in keeping the roll steer properties.

Having two cars that are setup on different ends of the spectrum and driving them back to back make me wish for tuning that allows for more of both roll steer and ability to put power down.

 

[Grant 302]

I'm not at all convinced that zero here should be a design goal.

It is not a goal. Just a constraint for relative ease of analysis and comparison when it is not zero.

 

[Grant 302]

I think what you're neglecting is the reduction in rear geometric load transfer.

I don’t think I am. The rear axle assembly free body includes it with the different RC and changes in pickup points.

 

[Norm Peterson]

This is exactly why I’ve been interested in the topic. With specific interest in keeping the roll steer properties.

Having two cars that are setup on different ends of the spectrum and driving them back to back make me wish for tuning that allows for more of both roll steer and ability to put power down.

Perhaps a little work with UCA inclination will give you more of the initial forward bite that you want without screwing up the level of roll steer that you like.


Norm

 

[Norm Peterson]

I don’t think I am. The rear axle assembly free body includes it with the different RC and changes in pickup points.

Can you make this sanity check . . . add up the total amounts of load added to the outboard tires and compare that to the total load transfer as [mass] x [CG height] x [lat-g] ÷ [mean track]. Do the same for the inboard tires where it'd be the load removed from them. They all should match.

On edit, mass would be total mass, sprung + unsprung, and CG height would be that for the total mass.


Norm

 

[Norm Peterson]

I don’t think I am. The rear axle assembly free body includes it with the different RC and changes in pickup points.

I may need to see a sketch of this. It sounds a little like what Bob Bolles talked about, though I can't pin it down to it being from him that I heard of a two-sprung-mass approach.


Norm

 

[Grant 302]

I may need to see a sketch of this. It sounds a little like what Bob Bolles talked about, though I can't pin it down to it being from him that I heard of a two-sprung-mass approach.

That doesn’t sound unlike what I’m doing. I have been separating the sprung body from the tire sprung axle assembly.

Can you make this sanity check . . .

Found my error. Swapped a transfer value for a spring perch value. Transfer at the tires remains the same with the bar kept horizontal.

Perhaps a little work with UCA inclination will give you more of the initial forward bite that you want without screwing up the level of roll steer that you like.

No, that’s not going to work for various off-topic reasons. Mainly, because I want to use more roll to achieve other geometric changes.

——-

I did find that there are unrealistic (for a Mustang at least) ways to ‘force’ less roll with the lower roll center. High spring rates and low tire spring rates. Like a dragster or funny car. Perhaps possible in other open wheel formats.

But that aside, and back to Mustangs -reducing the need for rear bar is difficult to prove so far without other changes. Like coupled with more rear spring or less front spring or front bar. Changes in themselves that would ‘need’ less rear bar. Or perhaps looking at the whole car and lowering the roll axis front and rear reduces the need for front and rear bars.

I think the fact that more elastic transfer happens per g is almost proof enough. Changing nothing else but a lower roll center, this does cause a greater damping reaction...a change in itself toward understeer. Maybe that was the simple answer all along.

 

[Norm Peterson]

Found my error. Swapped a transfer value for a spring perch value. Transfer at the tires remains the same with the bar kept horizontal.

Good to hear it was something simple.

No, that’s not going to work for various off-topic reasons. Mainly, because I want to use more roll to achieve other geometric changes.

Got it.

Some years ago I knew a guy (over on Corner-Carvers, FRRAX, and a couple other sites) with a 4th gen Camaro who also tinkered around with lowering the PHB on that car (and was into driving the big tracks). Haven't heard much from him in years, so I have no idea how any of it past an initial impression turned out. I do remember that he wasn't in favor of having much axle roll steer, though.

I think the fact that more elastic transfer happens per g is almost proof enough. Changing nothing else but a lower roll center, this does cause a greater damping reaction...a change in itself toward understeer. Maybe that was the simple answer all along.

Lowering the roll center(s) does make the car at least relatively more sensitive to tuning the handling balance via tweaking the elastic elements. A ground-level pair of roll centers makes all of the sprung mass LLT 'elastic LLT'. The unsprung component of LLT, well, not much you can do about that term except make it smaller by taking weight out mostly from the round things that rotate.


Sorry for my tardiness. Notices of reply aren't always showing up again, not even in my spam folder.


Norm

 

[Grant 302]

Lowering the roll center(s) does make the car at least relatively more sensitive to tuning the handling balance via tweaking the elastic elements.

I think that may be the more accurate way to describe what I’ve been thinking. And it seems to me, that lower roll centers tend to ‘favor’ more spring and less bar because of that ‘sensitivity’.

Sorry for my tardiness. Notices of reply aren't always showing up again, not even in my spam folder.

Please, no apologies. Email notification issues and lack thereof were high on the list of reasons to quit the moderator post.