Roll resistance from spring, rather than bar

[exFSAE]

When tuning a racecar, which do you prefer.. moving roll resistance around with a lot of spring, or ARB? To be honest, the more I think of it I don't see an awful lot of advantage to using bar over spring.

Your single wheel bump rate and warp stiffness are going to go up in either event. Not sure if they'd go up by the same amount. Anyone run the numbers on that? And in a "blip" load on a corner, that roll bar is going to pretty instantly transfer load to the opposite end of the axle.

I can't think of many instances where you would have 2-wheel bump on an axle, where the rollbar wouldn't activate.

Only advantages I see to the bar are (a) being able to change it while driving as fuel burns off or tires give up [not a FSAE issue] (b) being able to run real soft pitch stiffness if you want to change your diffuser or wing AoA substantially on the brakes [not really a FSAE issue] and (c) being able to tune springs for chassis ride (the whole having a higher rear freq than front), though I'm not convinced this isn't more of a passenger car tuning item than anything.

But if you can get the car right with just spring, that's at least 2 less parts and less weight...

Thoughts?

When we tuned the 06 car we wound up using springs that were more than double the original design.. which dominated the car's balance and the set of ARB's we had wound up doing nothing.

[exFSAE]

When tuning a racecar, which do you prefer.. moving roll resistance around with a lot of spring, or ARB? To be honest, the more I think of it I don't see an awful lot of advantage to using bar over spring.

Your single wheel bump rate and warp stiffness are going to go up in either event. Not sure if they'd go up by the same amount. Anyone run the numbers on that? And in a "blip" load on a corner, that roll bar is going to pretty instantly transfer load to the opposite end of the axle.

I can't think of many instances where you would have 2-wheel bump on an axle, where the rollbar wouldn't activate.

Only advantages I see to the bar are (a) being able to change it while driving as fuel burns off or tires give up [not a FSAE issue] (b) being able to run real soft pitch stiffness if you want to change your diffuser or wing AoA substantially on the brakes [not really a FSAE issue] and (c) being able to tune springs for chassis ride (the whole having a higher rear freq than front), though I'm not convinced this isn't more of a passenger car tuning item than anything.

But if you can get the car right with just spring, that's at least 2 less parts and less weight...

Thoughts?

When we tuned the 06 car we wound up using springs that were more than double the original design.. which dominated the car's balance and the set of ARB's we had wound up doing nothing.

[HenningO]

I'd have to say I prefer changing it with an ARB.

It can be changed a lot quicker (depending on design) and it can be changed in much smaller increments. You also don't risk messing up your corner weight (of course you can measure all your ride springs and create appropriate shims for preloading them).

I've heard people take pride in not having to use ARBs, claiming their suspension is so well-designed/setup that they don't need it. I rather take pride in being able to quickly adapt to changes in track/tire/driver conditions.

Finally I believe running without ARBs in FSAE is ridiculous, reason, roll damping. But that is a completely different matter...

[VFR750R]

i almost agreed with you exfsae, but then i thought about bump handling in a straight line. Softer springs will be more compliant and follow the road better. You'll get less 4 legged table syndrome under braking and accelerating. Those things happen on corner entry and exit while you're still turning, but not at the limit. The only time i think you could stiffen the springs to the point of no arb is steady state cornering.
Doing one or the other may also be more susceptible to picking up an inside tire on entry or exit, which may or may not be desirable.

And like henningO said, dampers have a much broader use then just damping the spring you use, especailly in transitional zones. there's probably an optimum spring rate to go with that damping rather then the other way around.

[flavorPacket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by HenningO:
Finally I believe running without ARBs in FSAE is ridiculous, reason, roll damping. But that is a completely different matter... </div></BLOCKQUOTE>

What?

What exactly does the presence of a roll spring have to do with the presence (or lack thereof) of a damping element? Roll damping can be provided by 1) ride dampers 2) tires 3) a specific roll damper. Yes, there's also material damping from lots of places, but that's small in this context.

[HenningO]

flavorPacket,

Most FSAE cars use their ride dampers in roll. When running these ride dampers to maximize mechanical grip, in most cases you will end up with an over-damped roll mode.

I'd argue that's a completely undesirable feature (considering the tracks we race on). To solve this you have two options,

1. Decrease overall damping which will hurt your TLV and thereby mechanical grip.

2. Increase roll stiffness, by adding an ARB

edit, 3rd option, lower ride spring rate and ride damping, more chassis movement, results: larger changes in dynamic wheel angles (could hurt tires' performance) and increased risk of chassis hitting the ground ie. need for increased ride height

I hope this answers your question! http://fsae.com/groupee_common/emoticons/icon_smile.gif

[flavorPacket]

Roll will only be overdamped (meaning more damping than optimal, not more than critical) if you are trying to get correct pitch damping. And honestly, what FSAE car needs good pitch damping?

Also, I'd be very interested to see what proof you have about your first point. Are you using a simple nonlinear 1/4 car model, or do you have frequency response, pressure lag, friction, etc? I'm struggling to see how your high-speed tuning (for grip) is interfering with sprung mass control.

[HenningO]

I'm basing all my opinions on simulation results, more specifically a 7 DOF virtual "shake rig" running in the time domain which allows for non-linear damper curves. Friction is not modeled, neither is the dynamic response of the dampers (which I guess you mean with "pressure lag"). The simulation model have to some extent been (successfully) validated against real life shake rig results.

Let's agree that at a certain relative damping / damping ratio you'll have a minimum of TLV. Let's use that as a measurement of mechanical grip. What I've seen is that at that level of damping your typical FSAE car will be overdamped (relative damping / damping ratio &gt; 1) in roll (due to a fairly small MMOI in roll with respect to to roll stiffness).

To avoid this you shift more of the roll resistance to the (undamped) ARBs and thereby avoid the over-damped behavior in roll.

Furthermore, I'd say mechanical grip is not solely dependent on the high-speed behavior of the dampers which you are implying and unless you are running a very very degressive damper curve you'll still be over-damped in roll.

That's what I found and I'll gladly take part of your point of views! http://fsae.com/groupee_common/emoticons/icon_smile.gif

[flavorPacket]

OK, that makes sense. But consider this option: why not run your ride springs and ride dampers for correct roll dynamics and let pitch and ride be damned?

We run on smooth tracks and we don't have aero. And with roll frequencies around 8-9 Hz, I have not had many problems getting close enough for student work anyways. By this I mean that when designing for roll dynamics, we have not had a big issue with ride or pitch problems. With this approach we were able to reduce the roll response and yaw response of the vehicle by orders of magnitude (whether that's good or bad is up to you).

Yes, we are underdamped under braking. Yes, we are throwing mechanical grip out the window. But let's be honest, our drivers aren't that great, and not messing around with all kinds of settings at the track allows them to get more experience.

I also agree that high speed damping is not a cure all for TLV tuning, but what else can you do? At this level, without hours upon hours on 7 posts, we don't really have the tools to do it right. We both know that the tire's load is still changing in low-speed events, but you need to have proper inertial control.

[exFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by VFR750R:
i almost agreed with you exfsae, but then i thought about bump handling in a straight line. Softer springs will be more compliant and follow the road better. You'll get less 4 legged table syndrome under braking and accelerating. Those things happen on corner entry and exit while you're still turning, but not at the limit. The only time i think you could stiffen the springs to the point of no arb is steady state cornering.
Doing one or the other may also be more susceptible to picking up an inside tire on entry or exit, which may or may not be desirable.

And like henningO said, dampers have a much broader use then just damping the spring you use, especailly in transitional zones. there's probably an optimum spring rate to go with that damping rather then the other way around. </div></BLOCKQUOTE>

With regard to straight line bump handling.. I'd agree if both wheels on an axle go into bump at the same time. Don't know how often that happens but you'd think a bumpy racetrack would only be hitting 1 wheel at a time. Though maybe I'm wrong, maybe pure axle ride is more common than I think.

You'd think a stiff bar would be worse than stiff spring in that case. More load transfer to load variation on both tires, rather than just one.

I don't always buy into C. Smith's MO, but its interesting he is all about rear spring and little to no bar, to keep the rear end a little more compliant. Figure hey why not try that with the front axle as well.

Or if anything, all spring in the rear, mostly spring in the front with cockpit-adjustable bar to tweak balance for different drivers, fuel loads, tires..