Track width of FSAE cars

[MCoach]

It struck my curiosity of what drives teams reasoning for their selection of track width front and rear. Think of this as more of a poll of what you think is important. They vary around quite a bit from ETS with a very narrow track width to teams such as Uni Stuttgart that has a relatively large track width. After scanning through some old data and programs I've also noticed the trend of cars getting much smaller in overall dimensions even with the more strict implementations of the template rules.

Typically, what drives the selection on our car is dependent on the virtual swing arm for camber control, roll over considerations (no lift), roll stiffness targets, load distribution, and the little details that play into it, such as manufacturing tolerances for picking easy numbers to hit.

I'd just like to throw out the fact that in 2001 our car had a 57" front trackwidth.

What do you think is important in FSAE?

EDIT: brainfart, changed 2001 "wheelbase" to "trackwidth" :P

[mdavis]

At Cincinnati, we have a pretty hard maximum track width constraint given where our shop is. The track can be no wider than a standard double door. Our 2006 car pushes this dimension to where the car actually has to go out rear end first, then be rotated to where the wheel almost hits the wall on the opposite side of the hallway, crank the steering wheel to full lock and drag the car through the door.

As for the decision process this year, we tried to minimize car width while still passing tilt. There were some other factors that determine front to rear split and that kind of thing, but those are team secrets.

To me, minimizing track widths provides (inexperienced) drivers with more choices for driving line through tight courses, which is a plus. Limiting an inexperienced driver to a single line in testing/driver training may be a plus, but when it comes to the competition, I feel like they should have their choice of line (although where they are in the run order may dictate line choice more than they would like, but that is the teams fault for not picking the right time to run) as much as possible.

In terms of the other 2 dimensions, lowering height lowers CG, minimizing weight transfer and allowing for smaller track widths to still meet your lateral G targets. Minimizing vehicle length reduced PMOI which helps with getting the car to respond faster through the transition heavy courses that we've seen at Michigan. It will be interesting to see what type of course we see this year at MIS. 2011 was fairly slow, 2012 was fast as all get out. Only time will tell...

[BillCobb]

I'm guessing that most replies will state dimensional considerations: Tierod length, driveshaft length, slalom fit, swing arm length, driver comfort, etc. I'd add the additional burden of mass and inertial factors (radius of gyration and c.g. location). Actually, there can be 'optimal' value parameters based on these factors for a fixed mass that can make for high value dissertations. A surface plot showing the optimal potential well of max lateral capability vs. track and c.g. location is an excellant project with high value. A simulation to produce this needs to be able to perform adjustments of a number of other parameters which float with the the main ones, but you get the idea.

At least you don't have to make it thru a car wash or fit on shipping truck tracks like the very first competition. The GM PG car wash tore up quite a few cars in the WayBack era because of odd track(s) values. (That may be before some of you were born [or hatched]).

Whatever...

[Buckingham]

A=V^2/R (lat g, velocity, radius). The goal of racing isnt to maximize A, its to maximize V. Yes you need to look at how track width affects A, but it is EQUALLY important too look at how track width affects R. FSAE uses a constrained course, so how does overall vehicle width change R in a slalom, or in a 180 hairpin, or in a single lane change?

[Francis Gagné]

Originally posted by Buckingham:
A=V^2/R (lat g, velocity, radius). The goal of racing isnt to maximize A, its to maximize V. Yes you need to look at how track width affects A, but it is EQUALLY important too look at how track width affects R. FSAE uses a constrained course, so how does overall vehicle width change R in a slalom, or in a 180 hairpin, or in a single lane change?

In fact, in this case it's to minimze time t (=V/Length), since the length of the slalom depends on it's radius. We do something similar, we have a "lapsim" that account weight transfer, tire load sensitivity and adjust the radius of turns. Results varies with the track type, but it seems, for our tire, to indicate a narrow track. More sensitive tires may end up being "more optimal" with a wider track.

On the soft side, it is probably easier for the drivers too.

[MCoach]

Maximizing acceleration (A) just get's the car darting around the track. Maximizing velocity(V), minimizes time(t).To maximize V several factors play into that including tire forces (and moments), load sensitivity, and balance of the car. (How's that Izz looking?) But focusing on minimizing t through several events such as steady state cornering (skid pad), low speed transience (slalom) and acceleration gives a good idea of how the car will perform overall. If the car has enough grip to accelerate well, then it should brake well. Assume using max mu *0.9 (no ones perfect) during braking events and we're well on our way there...

Back to the focus...

Passing tilt is definitely a bottom line for track width. If you aren't going to pass tilt, then there is barely a chance to be competitive and some design choices should be re-evaluated.
Tilt test only simulates 1.7Gs and these cars have proved to be capable of more than that.

On polar moment of inertia, there is something to be said about chassis and tire set ups. I don't need to open my big mouth too much, so I'll just leave this here...
Most teams build near the bottom line of wheelbase (60 in; 1524mm) so something could be said for making a more "balanced" car, but it's just so enticing to keep it a bit short.

A low CG is great for keeping load transfer under control however, the current common list of tires seems to not care whether you have a CG of 13" (330mm) or 9" (229mm)...but your chassis and your suspension does. Which can lead to lower track widths...or wider, depending if you end up shiny side down or not. Considering a slalom, track width affects R by the amount the car needs to input as a step steer to navigate the event. A lower track width value gives a lower R value.

I'm curious just how far the limits on these things can be pushed. Tires and cars are nowhere near as finicky as planes, if something fails they come to a screeching (or sound or grinding carbon) halt. They don't fall out of the sky. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Will C]

To throw some numbers out there, we have been running a track width of 1m (average) and a wheelbase a fraction over 1524mm since 2009. I’d be really interested to see who else is around this number.

It was found that having a narrower car could notably reduce lap times on the tight Australian tracks, by giving the driver a “straighter” line around the track.

Just throwing it out there, we’ve always had more trouble passing noise than tilt. All you need is some clever packaging http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Z]

Originally posted by Will C:
... we have been running a track width of 1m (average)
... All you need is some clever packaging http://fsae.com/groupee_common/emoticons/icon_wink.gif
Will,

That would be at the narrowest end of the FSAE range.

Without giving any secrets away regarding your "clever packaging", could you tell us roughly how high your CG is?

Z

[Will C]

Being an engine guy I honestly wouldn’t be able to tell you where our CG is at.

What I can tell you, is that our car was designed to be able to handle a 2G corner and that during our 3 months of testing last year I never saw the car on two wheels (when it had a proper set up).

[MCoach]

Doing some quick math on based on track width and your roll value, that gives me a good guess at an upper limit.

The maximum it could be is about 9.75in (248mm)but my guess is that it is somewhere from 9.0 - 9.5in (228 - 241mm)

The Auckland car is the narrowest that I am aware of.

[ThreeColours]

Firstly You must worry the car pass the tilt table test (Roll-over test). That depends on CG height, RC height, Roll rate, expected acceleration and track width. You have to do estimates, with more or less accurate of CG height, RC height, Roll rate and expected acceleration. You have to consider that It's possible... No... It's sure that your estimates could have deviations. I would choose the minimum track that allow you to pass the tilt table test. Regards

[rjwoods77]

To add to mdavis statement...does anyone set/check the track and wheelbase so it can fit in the back of a variety of pickup trucks and use that as part of your marketing and design strategy. Seeing as the car is designed for the average American autocross attendee, it would be wise to do so that the cost of a trailer/registration/insurance isn't part of the operating budget. A big issue for powersports is always the transportation and storage of it.

[Claude Rouelle]

Just calculate the part of the yaw inertia that the front and rear non suspended masses have compared in the whole car yaw inertia.....

[Sormaz]

Originally posted by MCoach:
Doing some quick math on based on track width and your roll value, that gives me a good guess at an upper limit.

The maximum it could be is about 9.75in (248mm)but my guess is that it is somewhere from 9.0 - 9.5in (228 - 241mm)

The Auckland car is the narrowest that I am aware of.
Don't forget about the width of the tire, assuming he is measuring track width from the center of the tire. for a 7" wide tire that adds 1.75" to the cg height limit(assuming 2g limit)


Originally posted by ThreeColours:
Firstly You must worry the car pass the tilt table test (Roll-over test). That depends on CG height, RC height, Roll rate, expected acceleration and track width. You have to do estimates, with more or less accurate of CG height, RC height, Roll rate and expected acceleration. You have to consider that It's possible... No... It's sure that your estimates could have deviations. I would choose the minimum track that allow you to pass the tilt table test. Regards

Maybe my brain is shut off again. all I need is a base (half track) and an angle (60deg) to draw a right triangle...who cares about RC height etc.

[MCoach]

Sormaz! I hadn't bothered to consider the extra width that the tire added.

Well, I think we've found one of the narrowest cars in the world, and as of many things in that section of the world, evolved to meet the demands that are placed on it from it's unique environment. The Aussie courses are typically made up of very tight slaloms and very little make up of the long sweeping sections that we see here at the US competitions. Therefore, the best way to get through those extensive slaloms is to build a car with a very narrow track width.


I agree with Sormaz that for a tilt table, RC heights won't matter. The suspension isn't moving, no jacking forces, therefore no extra loading. The way it's lifted is the ground plane moves rather than loading from one side.

[Pete G]

The Auckland car with driver had a CoGh of around 260mm last year, depending on what shape driver you installed.

It had a few issues with inside wheels lifting through slaloms, but that was mainly due to poor roll damping. By the time competition rolled round it was fine.

[Sormaz]

Originally posted by MCoach:
I agree with Sormaz that for a tilt table, RC heights won't matter. The suspension isn't moving, no jacking forces, therefore no extra loading. The way it's lifted is the ground plane moves rather than loading from one side.

Pushing forward with the pedantics, because I feel like this is something that is generally misunderstood (not implying by you, but by many)

'Jacking' forces have no effect on tire loading. Think of the car as a closed system, and these are internal reactions. The only way suspension travel could have an effect would be physical movement of the CG through travel (or a track width change).

Jacking forces will result in 'extra' loading of the spring, but this is being reacted by the a-arms, toe link....NOT the tire

[MCoach]

To clear a bit up there..
I was trying to say what you had managed to put in a much clearer way. :P
CG isn't moving (relative to tilt ground) and therefore the lateral loading condition is all the same.
Basically, no suspension movement means no CG movement. Nice catch though.

Any other input?

[Z]

Originally posted by Sormaz:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MCoach:
I agree with Sormaz that for a tilt table, RC heights won't matter. The suspension isn't moving, no jacking forces .....
'Jacking' forces have no effect on tire loading.... </div></BLOCKQUOTE>
Grooooaaaann.... http://fsae.com/groupee_common/emoticons/icon_mad.gif

No time for a full reply here, but...

IF the suspension is soft (especially in axle-bounce, or body-heave), then "RC height" and resultant "jacking" MOST CERTAINLY DO MATTER on the tilt-table!!!!!

And as ThreeColours suggested, if RC on ground (no jacking), then suspension roll stiffness makes a difference on the tilt-table.

Time for students to practice their FBDs...

Z

(PS. Just noticed that Crispy gets it.
If I was back in my footy coaching days, then I would demand that above players DROP AND GIVE ME 20 FBDs!!!! http://fsae.com/groupee_common/emoticons/icon_smile.gif)

[Crispy]

Originally posted by Sormaz:
Pushing forward with the pedantics...

'Jacking' forces have no effect on tire loading....

If we are being pedantic, wouldn't we say that jacking forces will effect tire loading, because of the resulting movement of the CG?


Originally posted by MCoach:
CG isn't moving (relative to tilt ground) and therefore the lateral loading condition is all the same.
Basically, no suspension movement means no CG movement. Nice catch though.

Any other input?

Again, being pedantic, why won't the CG move?

[ThreeColours]

Originally posted by MCoach:
I agree with Sormaz that for a tilt table, RC heights won't matter. The suspension isn't moving, no jacking forces, therefore no extra loading. The way it's lifted is the ground plane moves rather than loading from one side.
I don't agree. Force of gravity is not normal to the tilt table. In my opinion, if you want to ignore RC height you must have ---> RC height= CG height or Roll rate equal to zero, that is to say, you have infinitely stiff springs. Regards

[Tim.Wright]

Originally posted by Sormaz:
'Jacking' forces have no effect on tire loading. Think of the car as a closed system, and these are internal reactions. The only way suspension travel could have an effect would be physical movement of the CG through travel (or a track width change).

Jacking forces will result in 'extra' loading of the spring, but this is being reacted by the a-arms, toe link....NOT the tire

I don't agree there. Thats wrong even before the pedantic level. You have got the theory almost completely backwards. If you have lateral loads on the tyre, (and a non zero rch) you will have jacking forces. These act between the contact patch and the chassis and the bypass the spring.

Anyway, don't take my word for it. As Erik said, do an FBD in front view and you will see your self. Apply a lateral force at the CP, and you will see you need a vertical force to satisfy force equalibrium.

[Sormaz]

Originally posted by Crispy:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Sormaz:
Pushing forward with the pedantics...

'Jacking' forces have no effect on tire loading....

If we are being pedantic, wouldn't we say that jacking forces will effect tire loading, because of the resulting movement of the CG?
</div></BLOCKQUOTE>

Hence the 'dot dot dot' part:

"'Jacking' forces have no effect on tire loading. Think of the car as a closed system, and these are internal reactions. The only way suspension travel could have an effect would be physical movement of the CG through travel (or a track width change)."

And further,
I was mainly targeting the comment of "no jacking forces, therefore no extra loading" (implying 'extra' tire load)
I did not say no jacking forces on the tilt table. In fact, I did not mention the tilt table in that reply at all because that was not the point I was making.

Regardless, I agree with your comment and regret the invocation of that pesky word.

Z,
Two cars go into a corner with the same lateral g...do the tire reaction forces care whether load is internally reacted through spring or a-arm?
I agree that the tires may care how high the cg is, and the cg height may depend on certain geometry and resulting body roll angle....but that would just be being pedantic http://fsae.com/groupee_common/emoticons/icon_cool.gif


Tim,
Thanks you for clearing that up. I agree with you. Now please show me where my backwards idea of the 'theory' is in contradiction with yours?

[Tim.Wright]

Originally posted by Sormaz:Tim,
Thanks you for clearing that up. I agree with you. Now please show me where my backwards idea of the 'theory' is in contradiction with yours?

Well you said 3 things, 2 of which I thought were backwards:

Jacking forces will result in 'extra' loading of the spring... Don't agree with that. Jacking forces bypass the spring and act directly between the contact patch and the ground.


...this is being reacted by the a-arms, toe link Agree


....NOT the tire Disagree there too. Jacking forces are seen at the contact patch for sure. In fact "geometric load transfer" as Claude names it, is just the jacking forces which occur from the lateral forces at the contact patch.

[Goost]

Two cars go into a corner with the same lateral g...do the tire reaction forces care whether load is internally reacted through spring or a-arm?


Not sure about this Sormaz. Perhaps you would be right that they don't care in the mid-section of a long turn, but you "into the corner", so:
Even if they weigh the same and have the same CG height but different RC heights, they have very different contact patch loads during transient maneuvers: The car with the high RC immediately transfers weight (jacking) to the outside tire as soon as the car accelerates laterally. The car with the low RC only transfers a little instantly (jacking), but the rest transfers relatively softly as the car rolls into the turn.
For those few moments the low RC car is faster.
Those few moments happen nearly 40 times on an autocross track.

[Goost]

And there I've done nothing to clarify the fundamental disagreement here which is whether RC height affects weight transfer. A FBD would be the only help there. Just remember transient maneuver is the key: this is not a static problem it is a dynamic problem - it has two degrees of freedom and you have neglected one of them.

[Buckingham]

For those few moments the low RC car is faster.

In FSAE its possible there are more 'few moments' that the high RC car is faster.

If the high RC car transfers weight faster, it might achieve steady state sooner (has a lower time constant). If the course requires quick changes in direction (step inputs) the car with the lower time constant should be easier to control by the driver (faster).

I'd take the car with the high RC in the slalom (square wave) any day.

[Francis Gagné]

Originally posted by Buckingham:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">For those few moments the low RC car is faster.

In FSAE its possible there are more 'few moments' that the high RC car is faster.

If the high RC car transfers weight faster, it might achieve steady state sooner (has a lower time constant). If the course requires quick changes in direction (step inputs) the car with the lower time constant should be easier to control by the driver (faster).

I'd take the car with the high RC in the slalom (square wave) any day. </div></BLOCKQUOTE>

With a high RC, you have a very fast time constant, granted. But you have no control over it. A low RC car can be adjusted to have a fast or slow time constant. Plus, the low RC car with fast (stiff spring/bar) can be adjusted to have good roll damping. The high RC car will always have poor roll damping.

The point Goost is making is that steady-state cornering is slower, I agree that this should be the case considering tire load sensitivity (TLS). It might not be significant with some of the SAE tires though.

Buckingham, I get the impression you are saying a soft car will change direction slower than a stiffer car (in roll). Of my understanding, it is not the case, it is only the suspended mass weight transfer that is transiently slowed. The lateral force time constant is relatively unchanged. Considering TLS, the car during transition will make more lateral force than in steady state.

Also, damping in bound, rebound at front and rear can be adjusted so to slow the transition on the front and make it faster at the front, contributing to more yaw acceleration in turn-in. (and can be adjusted for more stability in turn-exit).

This is something you cannot do with a high RC. As for the time constant, our 50mm high RC had a 10hz natural frequency if I remember well. The car was fairly stiff in roll to prevent our huge sidepods from scraping all over the place... 10hz is much faster than any human can actively control.

[MCoach]

http://fsae.com/groupee_common/emoticons/icon_razz.gif

Back to basics...

Ha, I am wrong in my original answer, as ThreeColours mentioned, RC height must be at the CG, and/or have infinitely stiff springs! Of course springs are going deflect. Doh! As an FBD shows, the tilt table represents what cornering forces are on the track because the Z-axis force of gravity is now the lateral component force when tilted with respect to the vehicle. With the RC height at CG height, all roll forces will be jacking forces.

This is the exact reason "infinitely stiff" springs are supposed to be swapped in when checking the CG height of a vehicle. The CG WILL move unless these steps are taken. In the case of the tilt table, the roll rate will affect how much everything moves, as in when lifting the front of the car, the pitch rate will affect how much everything moves there.

Freudian slip of the tongue and mind.

Z, I hope one FBD, a palm to the forehead, and 10 push-ups helps calm you down. :P

However it does make me curious as to how much that CG actually moves on our chassis when sprung...need do some more testing. I'm thinking it isn't a spectacular distance.

However, the conclusion that I'm coming to as that, even with springs deflecting, assuming that CG height change is negligible (don't have data to say whether it is or isn't right now), is that tilt table is still based on just CG height and the component Y-axis force...

[Z]

Recipe for a fit and healthy body = 100 x squats, push-ups, pull-ups, and sit-ups, per day.

Recipe for a fit and healthy engineering mind = Much easier!!! I reckon 1 x FBD per day should do it. http://fsae.com/groupee_common/emoticons/icon_smile.gif

If someone could convince the local newspapers to put a new FBD into the puzzle pages each day (along with the xwords, sudoku, etc.), then I reckon we'd all be flying around in zero emission skycars in no time at all! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Z

[Buckingham]

. The high RC car will always have poor roll damping.

Let's say that the roll center is sufficiently high that no CoG-RC couple exists and no kinematic roll deflection occurs during cornering, regardless of spring rate. (We could also just use a cleverly designed suspension that allows independent tuning of each individual mode and 'lock out' the roll mode.)

What are we left with? In Roll we now have a go kart. Is a go kart inherently flawed because it has no roll damping? If you have no roll motion, why are you concerned about damping? Do the tires provide sufficient roll damping? How much roll damping do you need?