2015 FSAE Rules

[MCoach]

The restrictor functions on a difference in pressure differential. Usually it is referenced to atmospheric pressure, which then defines your maximum flow rate compared to the vacuum drawn by your engine. If you move that downstream of the turbo, you then end up in a situation where the restrictor no longer is limited by the vacuum, but by how pressurized you can get the air to force it through that tiny restriction. Not regarding efficiency and eventually melting turbos, power becomes unlimited. Essentially, your mass flow rate is no longer limited, which makes it an easy decision to run a turbo in that configuration over NA.

Turbo-609.9cc here I come.

To circle back to the placement of the throttle, a leaky throttle in an naturally aspirated application allows more air to flow by circumventing the throttle while in a boosted application it would leak down presssure, decreasing mass flow rate, decreasing power. It actually seems pretty practical.


PS...
Please, don't go and prove what the downstream restrictor would practically restrict the engines to. I've had enough lower member swinging contests this week. Also, couldn't pass up a Powerthirst reference.

[Swiftus]

Oh Jay, think about it :-)
You want 300 horsepower turbo FSAE cars?

On second thoughts, sounds good to me :-) :-)

Pat

Could you imagine the top speeds in the straights? They would be hilarious! 5G braking into hairpins, all sorts of oversteer in slaloms. I would bet you could more than double the attendance at a Formula Student event if the cars all had 1hp per pound.



MCoach - My point about the throttle body placement was more to show that it can be trivial to prove a 'leaky' throttle in either case. Differential pressure sure is exciting, but I don't think the sarcasm came across in my post.

Start here for large differential pressures through restrictors:

http://en.wikipedia.org/wiki/Rocket_engine_nozzle


P.S. Please keep your member to yourself.

[MCoach]

Well, now. If I understood there was sarcasm in that post my response would have looked something like this:

downstream, eh? You mean we get to make formula cars with this much power:
http://www.youtube.com/watch?v=qRuNxHqwazs

1hp/pound would be very exciting.

[Z]

JurrienK,

"... both the electrical cars in the top 5 had problems getting a good time in endurance."

Ok, the results pages didn't mention that. I thought the E-cars might have been conserving their batteries, hence both having very similar (but slowish) times.
~~~~~o0o~~~~~

Mumpitz,

Yep. More below...
~~~~~o0o~~~~~

Troy,

"May I suggest spending more time around the drag strip...
... the quarter mile record is 4.58 seconds in a wheel-drive car as seen here"

Yes indeed, I should get out more often. You are right, and I WAS WRONG!!! (And you have just brought eternal joy to legions of FSAEers! )

Last time I attended a dragstrip the racing was ONLY 1/4 mile, and they only stickied-up the first 60' or so. I do recall the fastest times in the low 5s and headed for the high 4s, with sustained ~4-ish Gs.

I found a 1/4 mile (402 m) "record" of 4.42 sec which gives average 4.2 G, and your 1,000' (305 m) of 3.73 sec giving ~4.5 G. Given that the cars are travelling at half the speed of sound at the end of the runs, their Gs should be dropping off there, so must be higher earlier in the runs.

[Nevertheless, note to STUPID self: Z, do NOT trust Google! Must try harder... ]
~~~~~o0o~~~~~

Dylan,

"I would like to see support from your statement that the combustion cars are NOT doing accel "properly". On top of that, support that the electric cars ARE doing accel "properly"."

Right and wrong ways:

C-cars.
======
1. The fact that times have been static for ~30 years strongly suggests that something is very wrong. In open competitions of almost any type, the times always steadily drop.

2. Watching FSAE Acceleration first in 2002, and again last year, ALL the cars smoke their rear tyres at the start-line, keep them smoking for the next 5 to 10 metres, but NEVER make a great deal of forward progress. Having seen lots of other cars accelerating hard, it is blindingly obvious that the FSAE "wrongness" is a lack of rear grip, due to lack of rear weight.

When I question a team about this the response is usually something like:
"Well, this year we turboed the engine, so of course the tyres are going to be spinning more than last year..."

Then another team member adds,
"Yeah, but next year we're going to add Traction Control, so that should fix it..."

To spell out the multi-level-wrongness here.
1. Start with nowhere near enough grip from the driving wheels, for a MEDIUM powered FSAE engine.
2. $pend re$ources to add even more power...
3. $pend even more re$ources to SWITCH OFF the power you added in step 2!

E-Cars.
======
1. They must be doing something right because they are a half-second, and half of the 75 points, faster than the C-cars.

2. On the other hand, this thrashing of the opposition might be quite "wrong". The general rule in motorsport is that as soon as you start winning too easily, all the other teams start whingeing and moaning to the Organisers, who then change the Rules to ban whatever advantage you have. Yes, FSAE is not supposed to be "motorsport", but this nobbling of the E-cars is (part of) the topic of this thread.

I would much rather see both C- and E-cars competing on their merits. Restrict both to the current 85-90 kW for safety reasons. Let the E-cars have their perceived advantage of 4WD, because IMO they have a possible (?) disadvantage of heavy batteries. Certainly, if the Enduro is made much longer, then the E-car battery mass will become a problem.

The two areas in the Rules where the E-cars really do seem to have an "unfair advantage" is Cost, which doesn't seem to reflect their expensive batteries (? corrections welcome), and Fuel, which reads like a fairytale!

Given that the E-car energy is likely coming from a coal-fired steam-engine, with umpteen transmission/conversion losses between there and the car, I honestly can not see how they can be rated as using less than half the CO2 of the C-cars. You can grow ethanol (E85) in the back paddock. Fully sustainable, closed-loop carbon-cycle, etc. But perhaps best to leave that argument for another time.
~~~~~o0o~~~~~

I'll give my Acceleration numbers tomorrow, promise...

Z

[Z]

Meanwhile, here are some pics to go with Mumpitz's.

Typical F-5000 car (late 1960s to early 1980s). This was a low-cost, entry level Formula series, using inexpensive stock-block, but ~500hp, 5 litre V8s. Highish power + heavyish engine = more rear weight, => so also needs bigger rear tyres => so accelerated faster off the start-line and out of slow corners than contemporary F1 cars.

~o0o~

Typical F1 Turbo era car. Renault started it, but this first 1977(?) car wasn't too successful (took them a few more years...), and had nowhere near the 1,000++ hp of the later turbo cars.

~o0o~

Not really relevant, but interesting. F1 Tyrell P-34 "six-wheeler" (1976/7). Design was mainly to reduce aero-drag by tucking front wheels into bodywork. Only moderately successful (1 win, several other podiums...) because Goodyear (?) didn't do enough development of the front tyres. Avon now make tyres for these 10" front-wheels which has led to these cars winning many "Old-F1" races. I think (?) Avon also make these ~40 cm diameter tyres in the sticky hillclimb compound, possibly very suitable for FSAE?


Williams and March also made 4R(tandem):2F-wheeled experimental cars, and Ferrari experimentally fitted 2 x tyres on each rear corner, similar to the rear-engined (~600 hp) Auto Unions of the 1930s.
~o0o~

IMPORTANTLY, all the above cars were designed when the Rules were much more open than now, and the goal was to build cars that go as fast as possible in a straight line, AND AROUND CORNERS. Most modern Formulae have the tyre-sizes, and hence also the weight-distribution, tightly specified by the Rules. Modern racecars do NOT look like they do because "it works better".

Z

[Dylan Edmiston]

Dylan,

"I would like to see support from your statement that the combustion cars are NOT doing accel "properly". On top of that, support that the electric cars ARE doing accel "properly"."

Right and wrong ways:

C-cars.
======
1. The fact that times have been static for ~30 years strongly suggests that something is very wrong. In open competitions of almost any type, the times always steadily drop.

2. Watching FSAE Acceleration first in 2002, and again last year, ALL the cars smoke their rear tyres at the start-line, keep them smoking for the next 5 to 10 metres, but NEVER make a great deal of forward progress. Having seen lots of other cars accelerating hard, it is blindingly obvious that the FSAE "wrongness" is a lack of rear grip, due to lack of rear weight.

When I question a team about this the response is usually something like:
"Well, this year we turboed the engine, so of course the tyres are going to be spinning more than last year..."

Then another team member adds,
"Yeah, but next year we're going to add Traction Control, so that should fix it..."

To spell out the multi-level-wrongness here.
1. Start with nowhere near enough grip from the driving wheels, for a MEDIUM powered FSAE engine.
2. $pend re$ources to add even more power...
3. $pend even more re$ources to SWITCH OFF the power you added in step 2!

E-Cars.
======
1. They must be doing something right because they are a half-second, and half of the 75 points, faster than the C-cars.

2. On the other hand, this thrashing of the opposition might be quite "wrong". The general rule in motorsport is that as soon as you start winning too easily, all the other teams start whingeing and moaning to the Organisers, who then change the Rules to ban whatever advantage you have. Yes, FSAE is not supposed to be "motorsport", but this nobbling of the E-cars is (part of) the topic of this thread.

I would much rather see both C- and E-cars competing on their merits. Restrict both to the current 85-90 kW for safety reasons. Let the E-cars have their perceived advantage of 4WD, because IMO they have a possible (?) disadvantage of heavy batteries. Certainly, if the Enduro is made much longer, then the E-car battery mass will become a problem.

The two areas in the Rules where the E-cars really do seem to have an "unfair advantage" is Cost, which doesn't seem to reflect their expensive batteries (? corrections welcome), and Fuel, which reads like a fairytale!

Given that the E-car energy is likely coming from a coal-fired steam-engine, with umpteen transmission/conversion losses between there and the car, I honestly can not see how they can be rated as using less than half the CO2 of the C-cars. You can grow ethanol (E85) in the back paddock. Fully sustainable, closed-loop carbon-cycle, etc. But perhaps best to leave that argument for another time.

If an open class will always have times steadily drop, you are implying that an FSAE car will eventually have a 0.000 second accel run. At some point it does begin to level off. There's a good chance we are already at that point (note, I am not saying we are, otherwise I would use data to back that up).

From my experince with accel tuning, it has proven to be advantageous to slip the rear wheels since the "hook up" RPM (for a 600cc motor) is ~mid way through the RPM range. Otherwise the car has a good chance of getting bogged down in a low RPM area that is strongly suffering from lack of power. Of course, this is operating on my team's car that does not have a huge rear static rear weight distribution like you think would be better (of course better for accel, but how will it affect the rest of the events?). For accel, my team does take measures to increase rear weight distribution and increase rear load transfer. Our poor results in accel this year were a cause of timing issues that caused us to lose a faster run (the rerun was then on cold tyres because we were notified after the driver got out) and hold us up in line which forced us to forefit our final two runs (with an improved LC tune). I do not have numbers on our accel setup (weight distribution and CG compared to our autocross setup), but it would be interesting to get those written down to discuss.

As far as Ecars being faster, what if the complaints are actually true? . And you are right, the general rule in motorsports is if one team is winning, they must have an unfair advantage so they should get some sort of restriction. The issue is, especially for student designed and built cars, what levels the playing field in regards to performance? When you look at some levels of motorsports, it is absolutely ridiculous. Being that some teams may be the only ones running a certain car, if they are doing poorly, regardless of the level of their engineers and drivers, they can convince the rules to decrease their weight restriction or power output to "level" them.

[Z]

FSAE ACCELERATION - INTO THE LOW 3s.
===================================

Below is a Velocity-Time graph that shows why the first few metres of the Acceleration event, the "Launch phase", is most important for fast times.

BTW, V-T diagrams are good for this sort of thing because the rate of Acceleration is given by the slope of the curve, and Distance travelled is area under the curve. The curve, of course, represents Velocity of the car at any given Time. So most of what you want to know (ie. A, V, D, and T) are all on the same diagram.

Some simplifying assumptions.
* "G" is conveniently rounded off to 10 m/s.s.
* The total mass of car and driver, including equivalent rotating inertias, is 200 kg. Why would anyone want more?
* There is an artificial Vmax "speed-limit" of 30 m/s (= 108 kph), perhaps set by the gearing of the car (or by the Rulemakers?).
* Maximum "drawbar" power of the car is 60 kW (~80 hp). This is the net power available to increase the Kinetic Energy of the car's mass, after driveline-friction (including tyre-slip) and aero-drag are overcome.
* Maximum distance required, from the FSAE Rules, is 75 m.

Four different "runs" are shown.

A - dashed line at left.
=================
This (unrealistic) run minimises the Time required for the 75 m, given the above constraint of Vmax. Perhaps the car got a "flying start", or else you parked it out on the main road, and a large truck travelling at 108.1 kph hit it from behind. Anyway, with the constraint of Vmax, the Time is minimised by squashing the given area of D = 75 m into a RECTANGLE, which gives an Elapsed Time of 75/30 = 2.5 seconds.

C - dot-dashed line at right.
====================
This run has a constant acceleration Ac = 1 G (= 10 m/s.s). Perhaps the car has tyres with Mu = 2 (ie. quite sticky), but because only HALF THE STATIC WEIGHT of the car is on the driven rear-wheels (so that, despite some rear-weight-transfer, the tyres are furiously spinning and losing that extra grip), the car can only accelerate at 1/2 x Mu = 1 G!

At time T = 1 second the car has travelled distance Dc = 5 m (... yaaawwwn ...), and requires power Pc = 20 kW to maintain this rate of acceleration (see graph for all these numbers). Much, much later ("... are we there yet ..."), at T = 3 seconds and Dc = 45 m, the car eventually becomes power-limited, with Pc = 60 kW. It has also just hit the Vmax speed-limiter, so then continues at 30 m/s to the finish line in an even ET = 4 seconds.

C' - dot-dashed extension to C at top-right.
==============================
Team Testosterone want to go faster, and they figure it is all about MORE POWER, ... and SPEEEEED! So they hyper-mega-turbo-boost their engine, and ... ahem, "fix" the speed-limiter. The grip stays the same (probably less, given tyres are now molten puddles), so with the same A = 1 G all the way they hit the finish-line at V = 139 kph, and ET = 3.87 seconds (= sqrt(15), from "distance = half-Aye-Tee-squared").

Despite needing almost 30% more peak-power, and having almost 30% more top-speed, the Elapsed Time drops by ONLY ~3%!



B - solid line around cross-hatched area.
==========================
Meanwhile, Team Toothless concentrate on the "launch" (see below for details). Doing so, they manage a very brief, but also very rewarding, initial acceleration of Ab = 2.5 G. This only lasts for T = 0.4 seconds, and covers a distance Db = 2 m (ie. less than the length of the car!), but it is where they win.

At the end of this 2 metre launch-phase the car is at V = 10 m/s (36 kph), and requires Pb = 50 kW to maintain this rate of acceleration. So the Gs drop off to only Ab = 1.5 G. At T = 1 seconds the car is at V = 20 m/s, has covered Db = 11 metres (I fudged the curve a bit here to keep the calcs simple), and has again hit peak power = 60 kW.

So the driver changes up a gear, and the reduced wheel torque means the car only accelerates at Ab = 1 G, requiring Pb = 40 kW (ie. engine is away from peak power). By T = 2 seconds the engine is again at its peak Pb = 60 kW, Db = 36 metres, and the car has hit the Vmax = 30 m/s speed-limiter. So the car coasts at constant speed to the Db = 75 metre finish-line with an ET = 3.3 seconds.

This time would be a World Record for C-cars and would win most comps. I believe that an E-car once did a T ~3.2 s (memory???). Continuing the above "B" curve above V = 30 m/s, but still keeping the power limit below 60 kW, would reduce the ET to below 3.2 seconds, for an outright WR. More engine power, and a transmission that keeps the engine near that peak power, should allow an ET in the high 2's.

But I stress again that the initial ACCELERATION AT LAUNCH is most important for good times (ie. it squashes the D = 75 m area towards the left). And mega-power is NOT necessary for good points in the other FSAE events (ie. SP, AX, E), nor does it make a big difference here.
~~~~~o0o~~~~~

LAUNCH TECHNIQUE.
===================
So, how do you get those high Gs at the start-line?

Let's start by dismissing the idea of putting only half the car weight on the driving wheels, and then hoping to find some magical Mu = 5 tyres. That is just wishful thinking, so STUPID.

The obvious first step is to have the driving wheels carry as much of the gravitational downward Fz force of the car as possible. Assuming RWD, and depending on CG-height and tyre-Mu, this might ONLY require about 60% static-rear-weight. Any forward acceleration causes "weight" to be transferred to the rear, which gives the rear-wheels a greater capability for forward acceleration, so more rear-weight transfer, etc. Generally, the higher your CG, and/or the higher the tyre-road-Mu, then the less R% you need, and vice versa.

(Note that with a 50F:50R Electric-4WD car, with typical CG-height, wheelbase, and tyre-Mu, during acceleration the rear-wheels might carry about 75% of the Fz loads, so require about 75% of power sent to them. Conversely, during hard braking the front-wheels might have to do about 75% of the "regenerative-braking". So the front-motors are 75% for "regen", but only 25% for "acceleration".)

But the BIG STEP to better launches (as all the THs know), is to have the CG of the car LIFT as it comes off the start-line. The only way the CG can move upwards is by INCREASING the road-to-wheel Fz forces. This Fz+ increase, which is above the static gravitational "weight", gives the car a greater forwards tractive Fx force capability.

Here are some ways to increase rear-wheel Fz forces:

1. Pop a "wheelie". Obvious really! Here the rear-wheels necessarily carry ALL the gravitational Fz loads (because fronts in air), and the UPWARD acceleration of the CG adds even more Fz loads. So much greater Fx forces, and more forward Gs. To maintain control you can let the front-wheels droop down so they stay on the road, while the rest of the car lifts. But remember that this launch-phase "wheelie" only has to last about 2 metres. Also, differential choice is quite important (and NOT so obvious...).

2. Set-up the rear-suspension with >100% anti-squat (ie. longitudinal n-lines sloping steeply up-to-front). This forces the rear-wheels down as the CG LIFTS. More grip = more forward Gs.

3. Do both of above. Basically, the higher the CG LIFTS, the more forward G capability. 4WD cars could have lots of anti-squat at both F&R suspensions, but eventually they run out of suspension travel, or all the weight is transferred to the rear anyway. Easier just with RWD.

4. DO NOT have a car with very high Pitch MoI. Dragsters are very long to increase their Yaw MoI, which slows down any Yaw motions. But they have frames that are flexible in bending so that the front of the car does NOT prevent the heavy engine and centre section from LIFTING upward. Fortunately, FSAE cars come standard with lowish Pitch MoI, but try not to let your "wheelie" go the full 180 degrees...

5. On the aero front, have a large undertray that, together with the lifting car, generates UNSTEADY aero forces that suck the car down. Note that your magical "Crayons For Drawing" are unlikely to help you understand these unsteady flows (which, BTW, can generate BIG forces!). Instead, quickly lift a large sheet of plywood up off the floor, and note how it "sucks" down. Also note that by the end of launch (ie. 2 metres!) the car is going fast enough to generate significant aero-downforce.

And a bunch of other stuff...

But as a last suggestion, look at the sprint races at the current Commonwealth Games. The runners squat down low at the start-line, then push DOWN and back as they come up and out of the blocks. Similarly with startled horses or antelopes, who always lift their front legs off the ground when accelerating hard.

Enough for now...

Z

[stever95]

Z,

I look forward to going through your post in more detail.

That being said, there are a couple things I notice:

1. I think your total inertia is quite off: 200 kg means a sub 300 lb car. Including rotational inertia in that figure is even more false, especially considering that your examples require a 4 cyl from what I can tell (>80hp).
2. For a typical CG, wheelbase, and 2.5G of accel, I'm not finding that there's a whole lot of weight transfer to the rears, 5% at best. Maybe I'm doing something wrong here..
But that would mean you'd need a larger % of the weight on the rear wheels statically. Or to raise the CG under accel pretty dramatically. Or have super sticky tires.

So I agree with your physics, but I don't think the realities are there to make this possible. Unfortunately I can't comment on the aero or CG raising techniques, I just don't know much about plausibility there. From what I can see... there's still a good case for 4WD in a straight line.

[DMuusers]

So I've ran your numbers Z. From the simulation the 130kg(!) car with 70kg driver with a mu of 1.5 (which is according to our tests quite viable) and a 60% CoG at 300mm above the ground. Without any drag but also no aero forces and driving constantly on the tire limit (like only electric cars can, since shifting and power and torque curves and such). With load transfer. No tire load sensitivity. The 2WD car completes the 75 meters in 3.705 seconds (stuttgart did 4.04 at FSUK2014, the fastest combustion).
Now, when maximizing the load on the rear tires without the car flipping over, for the same CoG height, we need 70% static weight distribution. Then the 2WD car completes the 75 meters in 3.545 seconds.
For it to get near the 3 second mark, the mu should be at least 3. And in that case you'd have to shift the CoG down or forward to prevent the car flipping.

I'm sorry if I missed something. But I don't think it's physically possible. The lightest car ever built was (I think, correct me if I'm wrong) the DUT06 at 120kg with a single cylinder (45bhp). I'm not sure what the lightest 4 cylinder (which has more power) was, but I'm guessing it's not less than 175kg.

On topic of the proposed rule changes, I guess this monday they are releasing the draft version which I think is quite exciting. I wonder how they will implement the rules for the rear wheel steering aswell, besides all the aero, power, etc. changes.

[Kevin Hayward]

There are another couple of things to consider, some of which can be rectified by design, but which are not there today namely:

- It takes time to perform a gear change, during this time there is a period of no acceleration. This also occurs quite early in the run, which has a large effect.

- Secondly even though only modest power is required for the early acceleration it does mean that there needs to be an rpm differential between the rear wheels and the engine for the given gear.

The first and second can be solved by a CVT, the second also by using a clutch controlled launch phase rather than rpm based. I am assuming that most teams have a simple acceleration sim to run these sims.

I'm not sure I have been convinced that we have tyres that can produce a mu of 2.5-3 for a period of 0.4s at low speeds.

The line that is missing here is one for the electric AWD cars. In that case lets assume fairly normal tyres that can produce 1.5g accel throughout the 75m, although I have heard figures of constant 1.7g. The cars have sufficient power throughout the run, and have no need for gear changes this means 2.58s. The only problem with that is that it takes 90kW at the 30m/s barrier. That is not too much more than the mandated limit. So the car accelerates at 1.5g up to a little lower than 30m/s then continues with a lower acceleration. Not in a place to do the calcs, but it ends up with low 3s.

Kev