Track choice (FSAE)

[delacf]

Hi, folks. I don't belong to any team but I'm designing a suspension for a formula SAE. I'm a mechanical engineer but I'm self taught in automotive engineering.

I'm on the choice of track:

I decided to take, for the rear track, the minimum track width that passes the roll-over test. I've done my calculations and a track of 47 inches passes the test. The front track must be higher. But, how much? My intention is to take the front track maximum possible but I don't increase the frontal area for aerodynamic reasons. Is it a good reason? The front area will be minimal. I speak of this:


Regards

[Greg Locock]

Unless you have some really weird requirement I'd maximise the track at bothe ends given the opportunity. (what is your counter argument?) There may be a point of diminishing returns, but off hand I can't see one. I suppose in the slalom type events there may be an advantage in a slim car.

[delacf]

Yes, Greg. You usually want a track as big as possible but the vehicle must be tested for slalom and autocross in which the turns are tight and the road is narrow.

[Jersey Tom]

Pick an average track width. Front to rear split in TW is just baking in a FLLTD bias into your design. How much you want is arbitrary, up to you. Could make it marginally larger in front and get the added understeer of an otherwise huge front track width by other means.

[Greg Locock]

So, you can either build some sort of simulation of a typical slalom turn or event and see whether the tighter course your narrow track width allows compensates for the increased weight transfer, or you can guess, or you can copy what everybody else is doing, or you could run a real life experiment. Speaking as a one time FSAE_A judge, I'd be variously impressed by those approaches. And you'll gain more points for an intelligent approach that leads to the wrong answer than you will get for the right answer selected by the wrong method.

[delacf]

I hope I understand you right, Tom. I’m not sure. My English is not good enough.

The width of my tires let me use a 48 inches front track while the rear track is 47 inches. From 48 inches, my track front is lost 12.5% power (drag) per inch increase with regard to more aerodynamically efficient configuration in which the front area of both tires is minimal.

What I have understood you is that a difference an inch can be enough because there are other ways to achieve more desirable behavior. For example by the sway bar. ¿?

Regards

[delacf]

So, you can either build some sort of simulation of a typical slalom turn or event and see whether the tighter course your narrow track width allows compensates for the increased weight transfer, or you can guess, or you can copy what everybody else is doing, or you could run a real life experiment. Speaking as a one time FSAE_A judge, I'd be variously impressed by those approaches. And you'll gain more points for an intelligent approach that leads to the wrong answer than you will get for the right answer selected by the wrong method.

Ok, Greg. Thank you for your answers. I will continue thinking about this. Regards

[bill shoe]

Go with the narrowest track width that reliably passes rollover.

FSAE autocross courses are extemely tight and narrow. The difference between a 55 vs 45 inch track width is huge from the driver perspective. It makes the difference between passively threading the car between cones vs actively driving the car. This dominates the influence of lateral weight transfer. It's a driving issue, not an engineering issue.

Another example of FSAE sim vs reality-- Sims predict similar laptimes for discreet gear ratios vs CVT's. Then the students run actual cars and the CVT is much better. The sim can't model the engine hesitation from step-rpm changes. It can't model the awkward fore-aft weight transfer that comes with right foot braking. It can't model the subtle hesitations of a driver who isn't certain of getting the next downshift right. It can't model the fact that a driver with only a few hundred laps of experience had to spend too much of that time driving through awkward gear changes instead of learning car control.

[Greg Locock]

excellent points. So, which brave soul has ballasted their car to get the cg lower so they can run a smaller track?

[Lycoming]

It's not necessary to run track THAT short. most cars are around 45-50 inches track, which seems to be adequate for an FSAE style track. Its certainly not necessary to ballast to pass tilt with this sort of track width. I don't think there's much to be gained in terms of negotiating cones by dropping track by much more than that.

[Jersey Tom]

I hope I understand you right, Tom. I’m not sure. My English is not good enough.

The width of my tires let me use a 48 inches front track while the rear track is 47 inches. From 48 inches, my track front is lost 12.5% power (drag) per inch increase with regard to more aerodynamically efficient configuration in which the front area of both tires is minimal.

What I have understood you is that a difference an inch can be enough because there are other ways to achieve more desirable behavior. For example by the sway bar. ¿?

Regards

Aerodynamic considerations - particularly drag - on a FSAE car would be very low on my list of initial design priorities. Time at wide open throttle is probably quite small these days. In any event, I'd put mechanical balance higher on the priorities list.

If you make a very wide front track width relative to the rear, you are going to be forcing a very high front load transfer distribution and would require lots of rear spring or bar to balance it out. More sensible to me to have the front only a little wider than the rear so your mechanical balance isn't as extreme.

[delacf]

Ok, perfect, Tom.

I suspected that in this case aerodynamics considerations were very poor but I intend to justify my decisions. You have given me some very interesting supports. I will focus on the transfer front load distribution.

Cheers

[Greg Locock]

Lycoming - in my opinion you have missed the entire point of FSAE which is to make people think. I'd far rather the OP came up with some weirdo justification for say a single front wheel and a fat rear axle, and then demonstrated the correlation between his predictions and the actual performance, than that he just fit the same old track width as the previous winner.

[Lycoming]

the point of stating what is the norm for the series is not to say to simply copy that. If that was my point I would give a number, not a range. Even within that range, there is a difference between 50 and 45 inches of track. And what about front vs rear? Do you run them the same, and balance load transfer with the ARB? do you make the front wider or narrower? If you want to make it wider, by how much? There's still plenty to think about.

Besides, the reason I pulled those numbers is to show that you do not need to have a track width so small that you require ballast to pass tilt.

[mep]

What do you think about having a short wheelbase?