Unsprung Mass

[Thiago Gabardo]

Hello Guys,

So this week I got myself in the middle of a dilema about unsprung mass.

-The more mass attached directly to the wheel more continuous load at the tires, therefore more grip.

-But reducing the unsprung mass the pitching moment reduces as well, improving cornering.

Obs: The idea I presented to my professor is 4 in-wheel eletric motor.

My conclusion was as I have the ability to control torque at wich wheel, the gain in pitching moment could be
neglected.

What do you guys think?

Best Regards,
Thiago Gabardo

Puc-Pr
Brazil

[onemaniac]

Originally posted by Thiago Gabardo:
Hello Guys,

So this week I got myself in the middle of a dilema about unsprung mass.

-The more mass attached directly to the wheel more continuous load at the tires, therefore more grip.

-But reducing the unsprung mass the pitching moment reduces as well, improving cornering.

Obs: The idea I presented to my professor is 4 in-wheel eletric motor.

My conclusion was as I have the ability to control torque at wich wheel, the gain in pitching moment could be
neglected.

What do you guys think?

Best Regards,
Thiago Gabardo

Puc-Pr
Brazil

I don't think heavy unspring mass increases the grip. Although it can help improve the ride quality on a rough surface (but that's different than grip)

[Jay Lawrence]

Why would unsprung mass affect your pitch? The only way unsprung mass can affect your pitch is because you often need to run different damping to control your unsprung mass, which can in turn affect your pitch acceleration.

You need to have a look at some tire data to sort out your misconceptions about load, too.

[MCoach]

You will have a higher force in the contact patch of your tire due to having a higher Fz load on them, yes.

But the difference between sprung and unsprung doesn't really change that, except dynamically when your corner weights are adjusted from load transfer.

The idea of higher grip however, is false. You will have lower grip.



Think of it in terms of efficiency between what you are able to put into the tire and what you get back out.

If you put in 100lbf Fz and get out 100lbf in whatever direction. That's one value.
If you put in 500lbf FZ and get out 400lbf in whatever direction. Here's the second value.

As you can see, the higher input has a higher output, which is more load, but less grip (lower efficiency of load).
Going through tire data you will also see this relationship.


It's always better to reduce the unsprung mass if possible.



*SIGH* need to leave a disclaimer here... Unless you run into stiffness problems of the unsprung assembly. Then you will start to lose out due to not being able to point the tire where and how you'd like to.

[Max Trenkle]

Well guys, it's not quite that complicated... http://fsae.com/groupee_common/emoticons/icon_wink.gif

I mean sure tire data tells you a lot of things, but you don't really need to know that tire efficiency decreases with load in order to figure out what unsprung mass does.

Thiago, you just gotta ask yourself the right questions!

What happens when a tire goes over a bump?
How is suspension related to keeping the tire on the road?
What does more or less unsprung mass do to the suspension?
or, How does suspension react to more or less unsprung mass?
How does this affect the tire's contact with the road, and the forces involved?
*Is the track I'm going to compete on even have significant bumps?*

If you can find the answers to just these questions, you will see the light!

[Thiago Gabardo]

Thank for the answers!!!

http://ars.els-cdn.com/content/image/1-s2.0-S096706610800141X-gr2.jpg

this graph kind explain what I was trying to say when a said grip. (Bad word choice).

Dynamically speaking having more mass attached to the wheel will improve the cornering caoabillity. Is that right?

One more thing at this configuration it's right to say that the wheel have a faster velocity in Bound than in Rebound? (I don't if the term is correct whatI want to say is: Faster when the wheel go down and slower when the wheel goes up)

Best Regards,
Thiago Gabardo

Puc-Pr
Brasil

[onemaniac]

Originally posted by Thiago Gabardo:
Thank for the answers!!!

http://ars.els-cdn.com/content/image/1-s2.0-S096706610800141X-gr2.jpg

this graph kind explain what I was trying to say when a said grip. (Bad word choice).

Dynamically speaking having more mass attached to the wheel will improve the cornering caoabillity. Is that right?

One more thing at this configuration it's right to say that the wheel have a faster velocity in Bound than in Rebound? (I don't if the term is correct whatI want to say is: Faster when the wheel go down and slower when the wheel goes up)

Best Regards,
Thiago Gabardo

Puc-Pr
Brasil

Well, it's probably not wrong to interpret lateral force as 'grip' in the literal sense.
However your graph shows exactly what MCoach is trying to say.
As your Fz increases, see how much smaller the Fy increment becomes? So how effect is it?
Mu decreases as load increases.

Also when you compare two designs with respect to one specific variable you should keep everything else as the same (or controlled).
Otherwise how can you quantify your reasoning?

Consider two cars with equal 'total weights'.
Car A has heavier unsprung and lighter sprung
Car B has lighter unsprung and heavier sprung

At equilibrium, do you think Car A should have more Fz on the contact patches of the tires?
How about under acceleration (lateral and longitudinal)? Maybe less load transfer, but how effective is it to help the overall vehicle performance?

[Francis Gagné]

As load on the tire (FZ) Increase you have more lateral force (Fy). True. But, load is not mass. And mass, whether is sprung or not, will affect the load on the tires.

In all cases, increasing the mass of the car (Sprung or not) will create bigger load (FZ) and in return bigger lateral force(Fy). But the cornering capability (which can be described as the lateral acceleration) is ay = (sum of Fy of the 4 tires)/(Sprung mass + Unsprung mass).

Therefore increasing Fy augments cornering capability, but increasing mass lowers cornering capability. It is conflicting and could or could not change ay. What you need to understand is how Fz influences the coefficient of friction of the tire (MUy), and then only can you look at how your ratio of sprung mass to unsprung mass affects Fz (and therefore MUy).

You should look into the Tire Test Consortium (TTC) if you're not in it. This has to be one of the best FSAE investement to make for a team http://www.millikenresearch.com/fsaettc.html

[Thiago Gabardo]

So what i realised is that lighter unsprung mass more rapidly the tire will overcome the bump and the vibration will be damped faster.

Thank you guys I think i got all the info that i needed.

What was trying to conclude is: the addition of 4-5kg at wich wheel will mess with the handling of the car. But as I mentioned before the 4 in-wheel motors will give us the possibility of developing a ESC.

As the TTC this investiment will be the first next year. We need the green light from your university.

Thanks again guys.

Best Regards,
Thiago Gabardo

Puc-Pr
Brasil

[SteveHarasym]

It's also worth noting that doing as you suggest, increasing unsprung mass by moving items (motors) to the wheel assembly will increase yaw inertia. Not so good for cornering.

[Owen Thomas]

But Steve, does your yaw inertia matter as much when you have active 4WD? I'm guessing an in-hub 4 wheel electric motor setup would turn on a dime regardless. Just a thought.

Also, while it is generally frowned upon to increase unsprung mass at the cost of sprung, this may not be a bad option. You will reduce overall mass by removing the need for drive axles. If you can manage to have a very clever control system for your active 4WD, the car won't feel so sluggish with the heavy wheels either.

[SteveHarasym]

Originally posted by Owen Thomas:
But Steve, does your yaw inertia matter as much when you have active 4WD? I'm guessing an in-hub 4 wheel electric motor setup would turn on a dime regardless. Just a thought.


I would still say it matters. Even if they managed a super amazing control system so that the perfect amount of power was always applied to all the correct wheels, the car still needs to rotate and accelerate through that rotation about the z-axis. More yaw inertia = lower angular acceleration.

[Thiago Gabardo]

But reducing wheel rpm of the wheels in the inside of the corner would give a really good cornering capabillity. Just like a tank maneuvering.

Best Regards,
Thiago Gabardo

Puc-Pr
Brasil

[Owen Thomas]

Of course, inertia always matters. T=I*alpha 4eva, but just saying it wouldn't matter as much since you can introduce a larger couple from the wheels. You would have to do the math to validate it still, since this is just speculation.

Yes Thiago, like a tank, or a riding mower. But don't mention that to the judges http://fsae.com/groupee_common/emoticons/icon_razz.gif.

[MCoach]

If you think that the in-hub 4WD motors won't be able to be as much of a gain as lowering unsprung weight, I would suggest that you check out TU Delft.

http://www.youtube.com/watch?v=u9e6I5D7Z3c

http://www.youtube.com/watch?v...u0dw&feature=related (http://www.youtube.com/watch?v=wwz_7zcu0dw&feature=related)

[SteveHarasym]

Originally posted by Owen Thomas:
Of course, inertia always matters. T=I*alpha 4eva, but just saying it wouldn't matter as much since you can introduce a larger couple from the wheels. You would have to do the math to validate it still, since this is just speculation.

Yes Thiago, like a tank, or a riding mower. But don't mention that to the judges http://fsae.com/groupee_common/emoticons/icon_razz.gif.

Yeah I see what you're saying. But it's better stated that having independant wheel control can increase the yaw moment acting on the car, while added mass at the wheels will generally increase yaw interia (unless if it were to be placed inboard it would need to be farther from the CG for some reason).

So in this situation, like you said, there is a trade-off that must be considered, more moment, or less inertia. The decision in this regard becomes easier when adding the unsprung mass does not increase yaw moment, because it will assuredly decrease angular acceleration. This, going back to Thiago's initial question of if more unsprung mass is (always) good, would result in a "no". In this situation, as always seems to be the case, it's a compromise. *sigh*


Originally posted by MCoach:
If you think that the in-hub 4WD motors won't be able to be as much of a gain as lowering unsprung weight, I would suggest that you check out TU Delft.

http://www.youtube.com/watch?v=u9e6I5D7Z3c

http://www.youtube.com/watch?v...u0dw&feature=related (http://www.youtube.com/watch?v=wwz_7zcu0dw&feature=related)

There are certainly advantages to be had with 4WD. It's still important to remember the drawbacks too though. Also, they must hate those green cones or something http://fsae.com/groupee_common/emoticons/icon_razz.gif

[MCoach]

Of course there is not decision that that is without compromises.

The motors could be placed inboard, but in the case of FSAE, there is the template rule that may cause bigger hurdles putting the motors inboard than outboard. So, I'm guessing this was already evaluated by Delft and therefore placed them outboard.

As noted, this does create some problems with yaw control by increasing unsprung mass. This is (assumed to be) overcome by increasing yaw control of the motors.

Re-reading everything, I didn't quite a address the indirect subject of 4 wheel control. Being able to have independent wheel control can do AMAZING things, assuming that you have the control strategies take advantage of it. Passive systems are a large compromise in any scheme because they work at peak efficiency at one point where an active control system can extend that to a line or area.

On the other hand, they are immensely more of an investment to develop.

I wish you luck on your development.

[Thiago Gabardo]

The difficult that I'm facing right is how much and wich wheel the controler has to interfere and if the Algorithm will do the calculations fast enough, but that is problem for the Computer guys. HHAHAHAHAH

Best Regards,
Thiago Gabardo

Puc Pr
Brasil

[MCoach]

Remember,

This is a team competition. Everyone needs to be able to contribute to and bring the total concept to completion. If a core system doesn't work, the car doesn't run.


It's always easy to say it, but hard to accept, but don't get in over your head.

[murpia]

Originally posted by MCoach:
... I didn't quite a address the indirect subject of 4 wheel control. Being able to have independent wheel control can do AMAZING things, assuming that you have the control strategies take advantage of it...
OT, enjoy:

http://www.youtube.com/watch?v=fJtpPyVM_y4

Regards, Ian

[SteveHarasym]

Originally posted by murpia:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MCoach:
... I didn't quite a address the indirect subject of 4 wheel control. Being able to have independent wheel control can do AMAZING things, assuming that you have the control strategies take advantage of it...
OT, enjoy:

Yaw inertia doesn't matter when you don't need to rotate! Perfect! http://fsae.com/groupee_common/emoticons/icon_wink.gif
http://www.youtube.com/watch?v=fJtpPyVM_y4

Regards, Ian </div></BLOCKQUOTE>

Yaw inertia doesn't much matter when you don't need to rotate! Perfect!

[Max Trenkle]

So what i realised is that lighter unsprung mass more rapidly the tire will overcome the bump and the vibration will be damped faster.

YES. All you gotta do is ask the right questions and you'll figure it out eventually. If you ever find yourself stuck on a problem again, remember this: if you can't answer the question you're asking yourself, then you have to ask a simpler question, answer that one, and ask the next question. In this way you will turn giant problems into small ones!

Now Thiago, think about how many bumps are on FSAE tracks... Now that you are exploring how unsprung mass affects the dynamics of your car, it's time to think about if it even matters or not!

Hint: If you aren't going over bumps, then rebound is not your problem, but the moments on the car will still affect you.

[wil oberlies]

In terms of minimizing load variation at the contact patch, i think keeping unsprung mass and damper rebound forces low, for a given spring rate, would be beneficial....