For maximum confusion, I recommend the username 'Jersey Tim'Originally Posted by Xfsae
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For maximum confusion, I recommend the username 'Jersey Tim'
This indeed does look like some good fun! I've tinkered with this idea myself but more-so in an effort to make a car that fits me! (I'm about 77" tall and 240lbs).
Personally, If I had to design a "weekend fun" "extreme FSAE" type car....
-dual wishbone: This is mostly because I think it would be easier to design and there's more information out there "readily available" on the design and setup. Plus there's more parts out there that are readily available as well. I think a solid axle simple car design like Z pushes would be a fun project but my big concern there is the manufacture of the rear axle. something like a quad would probably work o.k. and would be a viable option....but what about compliance? I find that design to be a bit more difficult to keep the compliance down on (could be completely wrong there)
-obvious space frame
-whatever motor you know the most about/can get your hands on easiest: After spending hours on hours on hours with our R6 and a dyno I'd probably choose an r6. Might not be the best "design" option for FSAE but I know every little trick about that motor. When its happy, when its not. Just the sheer knowledge I have about the internals of that motor would make it a better choice for ME. Plus finding a wrecked sport bike typically isn't that hard. Someone's constantly rear-ending/low side/high siding one somewhere. They're relatively cheap and are reliable if you know what you're doing.
Also I'd probably increase the FSAE restrictor imposed. Scotty's diff is rated at something like 200ftlbf IIIRC and as long as you've got the torque in mind designing around it shouldn't be too bad. The reason for the increase is just some more drivability. None of these motors are optimized to run through that small of an orifice and with the power limitations we typically will gear them deeper than the bikes to get more acceleration. Get that choke flow outside of the powerband and I'll be happy.
-Big aero: Simply for the reason that wings don't work.Also running a bit stouter motor should allow you to completely neglect the drag from it.
All of these thoughts are not the most "correct" design process but in reality if you're looking for something fun to autocross its going to give you the best options. There's a lot there to optimize out but why worry about that when you can be having fun!
South Dakota State University Alum
Electrical/Daq/Engine/Drivetrain/Tire guy '09-'14
Go big, Go blue, Go JACKS!
@ Zac C .Ok mate next thread will be Jersey Tim build active suspension fsae car and a cartoon blowing in a pitot tube and the fsae car moving up and down like mad
@ exFSAE all you said is appreciated. i do have a day job as engineer but lets keep the extreme seriousness on things and knit picking out of this thread please. Lets be positive, have some engineering fun , and try /learn stuff that otherwise cant be done in FSAE competitions nowadays
I want to see this Cornell car so badIf this is so immature and overly enthusiastic so be it
@ Harry . Thanks for the welcome mate
I have sourced some typical stuff like brake master cylinders. callipers,rotors, sprockets ,some used moto engines(among them some small ones maybe to be used as auxiliary fan units ) , wheels 13in , steering wheel & seats and some meters of frame tubing in 3 dimensions so i am not in step zero (that doesnt mean i will rush anything - just means that i have some components that maybe be useful to work with ) .Also have my small backyard shop with a tabletop milling machines, small lathes, welders , cnc router and a manual pantograph
One thought is to build it as an 1/5 model first or similar to test the final concept but i am not in a hurry i want to enjoy the design and analysis process with you guys and gals
Who knows, maybe through personal or department contacts and associated thesis projects we can get some help for the full scale model. who knows , if i see people really interested ,helping and involved i might donate the creation to a department afterwards ( i said might
Last edited by Xfsae; 11-02-2013 at 03:24 PM.
exFSAE (the old one
That really is expert advice!
(Err..., from "ex" = "a has-been", and "spurt" = "a drip under pressure"... (Z thinks - Oh, no, no, no... you're just digging yourself a deeper hole! Quick, move on...
~~~~~~~~~~o0o~~~~~~~~~~
Xfsae (the mysterious, unknown one),
Taking the above very good advice, here are some thoughts.
GENERAL, BIG-PICTURE STUFF - As indicated on the "Sucker Car" thread (link back on page 1) and covered in more detail below, an FSAE sized active-aero (fan) car should be able to get AT LEAST a ton of downforce, at any speed from zero to Vmax. In fact, I think 3 tons would be quite easy, and 10 tons would be feasible!
The important point here is that with these numbers the big-picture changes radically. Things like tuning "under/oversteer handling balance at the limit" become irrelevant, because YOU WILL NEVER REACH THE LIMIT.
Let's consider roughly where "the limit" is. Take the middle figure above of 3 tons downforce (technically ~30 kN downforce). Assume that because of the extra strength that you have to add to the car (more below), the car+driver weighs 1/2 ton (technically ~500 kg mass). And assume racing slicks good for about Mu = ~1.5. The tyres thus have the capability to exert a ~45 kN force horizontally, which will accelerate your car+driver mass at 45,000/500 = 90 m/s.s, or ~9 G!. A lower mass car, with greater downforce, and maybe stickier tyres, are all possible, so even higher Gs possible...
Bottom line here is that YOUR PROBLEMS ARE MAINLY STRUCTURAL, both for the car and the driver!
So, toss those girly-boy 68xx thin-ring wheel bearings beloved of FSAE teams. You will need, at the least, wheel bearings off a largish production car. When you really start pushing the envelope you will need something off a light truck (hint - tapered-roller bearings).
Most definitely toss those Keizer wheel centres from the other thread currently running! Some stout, production-car, steel wheels might be a good start.
I am not sure what loads typical FSAE tyres will take, but you will certainly need higher than normal inflation pressures (eg. 2 bar, 30psi?). Le Mans style cars weigh ~1 ton, and develop 2 or 3 times that in aero downforce at high speeds, so those wheels and tyres should be strong enough (at your lower downforce numbers!).
Other chassis structure stuff should also be appropriately up-sized. Main point here is do NOT use typical FSAE-sized parts.
As for the driver, well cornering at 9 G would be like lying on your side while your favourite football team jumps on top of you! The sides of the cockpit should be very well padded. And lest the G forces rip the driver's head off his shoulders, I suggest a very effective "Head And Neck Restraint System" (preferably something much better than the conventional "HANS", which is just a few strings tied to the helmet).
"Centrepoint" steering would be good (ie. Offset/Trail/SAI/Castor = 0), but even then you will probably need power-steering. Think about how fast those slalom corners will be coming at you. Do NOT worry about "steering feel". Remember, you are unlikely to get close to the tyre's limits, and I doubt you would have time to notice if you were.
~~~o0o~~~
THE FAN - You have the choice of axial-flow fans (like an aeroplane propellor, or a domestic fan, or on the Chaparral 2J, or the (two-stage) Abrams-tank fan linked to by Rob on page 1) or radial-flow fans (like the compressor on a "turbo", or many industrial units). Either of these can be compounded (put in series) for greater pressure difference, and some fans are partly axial, partly radial-flow.
Briefly, I suggest a radial-flow fan for the following reasons. Generally, for pumping a given amount of air, axial-flow fans are smaller and more energy efficient (ie. less input power required). BUT (!!!), once the airflow is reduced past a certain limit (say, your "skirts" start to seal really well), the fan "stalls" and the pressure difference disappears! Also, the blades must be accurately "aerofoil" shaped, and the incoming airflow must be "straightened" for good operation (ie. to prevent stalling).
On the other hand, radial-flow fans are "STALL-PROOF". They operate by "centrifugal force", not by subtle "aerodynamic" means. As a result they can be made very simply, such as from fabricated sheet metal, like the majority of industrial "blowers". Even with very simple design (eg. just radial, sheet-steel vanes inside a sheet-steel, snail-shaped housing) they still have reasonable efficiency, and generally higher pressure difference capability than the axial-flow fans.
As confirmation of above, I have just conducted an extensive series of tests (err, while the jug boiled for my next cup of coffee...). My oldish domestic vacuum cleaner has a small radial flow fan in it (not sure which way the blades are curved, probably radial or backward). It has a hose nozzle of ~34 mm diameter, so about 9 sq.cm area. The suction from this vacuum-cleaner comfortably picked up a plastic drink bottle with ~1 litre water in it.
So the "stalled" suction is about 1 kg/9 sq.cm, or 0.11 bar, or 11 kpa, or 1+ ton per square meter. Keep in mind that a column of air 1 metre square at its base, and reaching from ground level up to the edge of space, weighs about 10 tons (or "atmospheric pressure" = 10 tons/sq.m). I put a few 1+mm thick toothpicks between the suction nozzle and the drink bottle, to simulate skirts that are "off-the-ground", and I could still comfortably pick up 0.5 litres of water. So still ~0.5 ton/sq.m lift (or downforce).
Google radial-flow fans and you should see that much higher pressure differences are possible with commercially available, off-the-shelf, relatively inexpensive fans. 3 tons per square metre (0.3 x atmospheric pressure) should be a feasible...
~~~o0o~~~
THE VACUUM-BOX - From the above, a "vacuum-box" of about 1 to 2 sq.m plan-area should suffice. This can have "skirts" that are either held just off the ground, or else allowed to lightly touch the ground and gradually wear away (say, made of something plasticky, like polycarbonate, UHMW-HDPE, or even plywood). For some comformability of the four wheels and the vac-box to uneven ground, and also for a smoother ride for the driver, I reckon it would be best to mount the vac-box "unsprung".
The easiest way I can think of to do this would be by using beam-axles front and rear. Longitudinal beams running down each side of the car (just inboard of the wheels) would hang off the beam-axles and form the side-skirts (possibly with flexible plastic extensions, like "side-splitters", for better sealing). Similar cross-beams at front and rear would seal the ends of the vac-box. The front might best be V-shaped (like a ship's bow) to help sweep away gravel, etc. In fact, "broom-like" skirts at the front might be a good idea.
The whole, roughly rectangular, vac-box should be allowed to twist to give more equal wheel loads on uneven ground (ie. a soft "twist-mode"). The several tons of down load on the roof of the box will help this, and should also be considered when building said roof! The rest of the car, namely chassis, engine, driver, etc., can now be softly sprung above the vac-box. So at least you get some vertical ride comfort, if not horizontal.
This whole arrangement is very similar to UWA's 2012 car, just with the active-aero added. Oh, and another couple of hundred kilos of structure, to keep it all together...
Enough for now...
Z
Last edited by Z; 10-29-2013 at 11:16 PM.
Z thank you for your post
just 2 quick questions the 1-3 ton of downforce you start your line of thought with is based on a fan(s) of what kind of diameter and what type? Also how much power you allocate to running the fan? unlimited ( 45 hp you mention somewhere else in the sucker car thread i think is not acceptable for an FSAE type car - it is like having a second engine equal to the main one for driving fans ,packaging & space available, weight and weight distribution - i think the vehicle gets heavier larger etc )
The Cornell car fans were driven by the engine with clutch like the BT46B?
Last edited by Xfsae; 10-30-2013 at 08:18 AM.
Consider the extra weight of everything. Consider packaging compromises. Consider that FSAE cars don't put out much mechanical power to begin with, and that current draw can be a challenge to manage when only using fans to help cooling.
To respond to an earlier post, I really don't think I'm nitpicking here, nor trying to be negative. I'm trying to be pragmatic and consider all the alternatives and consequences at an early stage here. That's IMO is good engineering. Now I will be blunt, and say that going into the design process with a singular vision of, "I need to have this gizmo on the car" before laying any of the high-level groundwork is.. poor engineering at best, and dangerous at worst. As I believe I mentioned earlier, that trap is not uncommon at the FSAE level and is a bit of a tough habit to break (I used to do it myself).
Back to a positive tone. If you want to make a "tech demo" car of sorts outside of any legal SCCA class and this concept has really grabbed your attention - then hey, go for it and CAD it up. But if you want to make a good racecar I think it really warrants taking a clean slate and more objective approach.
exFSAE - thanks again for your post. At this point the final product in my mind is a show/demo car that can be used in autocross events maybe in breaks between classes - car or other xtreme sports type events shows etc. But as i said this is not my primary concern now, i just want to enjoy the thinking , analysis and design process and learn something in the way now in company with the big community here
It is more about opening the door of exploring something that there is no point in researching normally because it is banned- that doesnt mean though that it is not interesting from an engineering point of view (please dont start me up with it is poor engineering etc...we all know it works in several type of vehicles - in the sucker car thread you see it worked in everything from karts to autocross monstrous prototypes - the physics are there - specifying the power you want to allocate to powering the suction system is key)
As i said also , for safety reasons i might do a model car first or even a kart next before arriving to the fsae type so that i can check things like failure of sealing etc even Deltawing or Lunar Rover followed that route-its good , exciting and good engineering practice in my opinion.Yes a lot of things can be simulated but it is only when you do a practical experiment like Z did with the vacuum cleaner and the bottle that the forces involved hit you
CAD work i do slowly because i have a normal work too. I post here because i believe FSAE community is a great think tank of talented and enthusiastic people (yes you need enthusiasm it is not bad) and more people can get involved and contribute in several levels (they might do their own version too - the sucker car club
Last edited by Xfsae; 10-30-2013 at 08:23 AM.
Well to be fair, the "Oh c'mon everybody knows 'X'..." is poor engineering. It's hand-waving. Alternative approach here is that if it's such a no-brainer huge performance gain which easily trumps all the challenges that come along with it, then it should be quite easy to demonstrate that objectively - no? The reason I make such a sticking point of this is because the hand-waving stuff is so prevalent in FSAE kids and young engineers, and it has no place in industry. Big pet peeve of mine, and a hard habit to break.
On a related note, enthusiasm can be great. Can be. I'd rather bring someone into a team that's over-enthused than under-enthused. But there is such a thing as too much enthusiasm. That drive has to be reeled in a bit by pragmatism. Again I find it prevalent among FSAE kids being so excited or interested in something that they just dive in before asking... (a) is this the most effective use of my time? (b) is this the best approach? (c) have I exhausted all alternatives? etc. When that comes into the professional workplace it can be "negative value added."
With all that said, this is your baby and if it's just your spare time and money - go for it in any manner you please. Probably worth thinking about making a little website or blog about it, some place to organize some thoughts and materials. I'm just taking the opportunity to point out that some of this drives ya nuts at the professional level, and I wish more students coming out of FSAE didn't have bad habits!
exFSAE you are just the fuel a grown man wants to spend time in the garage/shop after work...Keep it coming the you cant do it its a waste of time, grow up etc works for me well (i bet deep inside you is the 21 yo buried that wants to build the baddest FSAE car out there)
I keep the blog idea though but at the moment want i want is more interaction with others, brainstorming ideas ,examining concepts etc
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