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Absolute Top Speed of Competition Ready Car

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Akron 1998 to 2004 View Drop Down
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    Posted: Jul/02/2012 at 4:31pm

Vehicle weight + Driver 450 lbs (force) 2001.699 N
Rolling resistance coeifficent 0.08
Aerodynamic coefficent 1
Crossectional area 13 ft^2 1.207739 m^2
RHO "air" 1.22 Kg/m^3
Power Engine 9.775 hp 7289.336 W
Drivetrain eficency % 0.83 %
Power at wheels 8.11325 hp 6050.149 W
Drivetrain loss 1239.187 W
Rolling drag 2662.813 W
Aero drag 3387.336 W
Max vehicle speed 37.20 mph 16.63 m/s



Edited by Akron 1998 to 2004 - Jul/06/2012 at 11:53am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/02/2012 at 5:06pm

Vehicle weight + Driver 550 lbs (force) 2446.521 N
Rolling resistance coeifficent 0.08
Aerodynamic coefficent 1
Crossectional area 13 ft^2 1.207739 m^2
RHO "air" 1.22 Kg/m^3
Power Engine 9.775 hp 7289.336 W
Drivetrain eficency % 0.83 %
Power at wheels 8.11325 hp 6050.149 W
Drivetrain loss 1239.187 W
Rolling drag 3105.946 W
Aero drag 2944.203 W
Max vehicle speed 35.50 mph 15.87 m/s



Edited by Akron 1998 to 2004 - Jul/06/2012 at 11:55am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sandres913 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/02/2012 at 5:15pm
I want to start off saying that I do agree with your main point,  A baja car can only go so fast.

Our team always jokes about the "parachute" of a firewall being the largest limiting factor, so I completely agree with what you're proving. I'd say a baja car is for the most part, less aerodynamic than the Caterham 7 (from the other thread, I looked! Smile

Area I'll give you, more or less, but I'll agree with it. Drivetrain losses of up to 20%, fine there. Rolling resistance coefficient, okay, Rolling Drag, not so much, looks a liiiiiiittle high.

The technology guys on the team always give me a hard time when I start talking "theory" and aerodynamics. Without a firewall and body panels, we had a car that is in line with your max speed here (soooooo many more losses than I'd like to account for), but it was quick. But our car with everything installed was 683 lbs (yes, almost 700 lbs) WITHOUT a driver, estimated at 750 without body panels/firewall/hood and with my scrawny ass in the driver's seat. Now substitute our tank frame for a lightweight car with no body panels/firewall/hood and with my scrawny ass in that driver's seat and I would expect it to do 40, no problem. I understand that, as laid out here, it mathematically is impossible, but it's possible. Laval's acceleration run at Wisconsin was unbelievable, I think there's a youtube video of it, check it out. By that math, they must have been hitting close to 35-38mph at the end of the run. 150 ft in about 5 seconds, give or take due to the starting method.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/02/2012 at 5:48pm
Originally posted by sandres913 sandres913 wrote:

Laval's acceleration run at Wisconsin was unbelievable, I think there's a youtube video of it, check it out. By that math, they must have been hitting close to 35-38mph at the end of the run. 150 ft in about 5 seconds, give or take due to the starting method.
 
Laval did 150 ft in 5.588 seconds.  As you can see below my car did that like 10 years ago w/ the engine at only 3,600 RPM and I wasn't going 38 MPH to do it either (about 27 MPH is required).  Are you guys even trying to figure any of this stuff out on your own?
 
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Vehicle weight + Driver 500 lbs (force) 2224.
 N
Rolling resistance coeifficent 0.135


Aerodynamic coefficent 0.63


Cross-sectional area 10 ft^2 1 m^2
RHO "air" 1.22 Kg/m^3

Power Engine 9.5 hp 7084.265  W
Drivetrain efficiency % 0.891 %

Power at wheels
hp 6311.84  W





Drivetrain loss

772.
 W
Rolling drag

4773.  W
Aero drag

1544.
 W





Max vehicle speed 35.5
 mph 15.9
 m/s


Could you show your certainty for your coefficients?  Granted I stacked them up all 10% in a less conservative direction, but can you show data or references (for baja cars) that prove that this is impossible or unlikely?  For that matter what about 20% off of your coefficients?  20% would give you around 40.5 mph


Edited by Waffles - Jul/02/2012 at 6:57pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sandres913 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/02/2012 at 7:37pm
Akron, I am full-heartedly agreeing with you that there is a terminal speed for a baja car, but I don't believe some of the coefficients were right, though I acknowledge they will vary car to car.

All past achievements or experiences aside, could we really discuss this? I love to discuss the equations and the theory for automotive applications, really I do. My team is mostly freshmen (now going-to-be sophomores) so there's a lot more instruction and guidance (Correct And Reinforce Daily LOL) than discussion and debate.

My best guess, at this point, with no calculation, back-of-the-napkin or spreadsheet, would be right around in the range of 40mph, including firewall, body panels, driver, everything. Tech ready/approved condition, in other words. Haven't seen anything much higher than that. I'll probably get to a calculation tomorrow afternoon on my lunch break.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/02/2012 at 7:46pm

Drive train efficiency can definitely be improved 10%.  I'm running a CVT with double chain reduction.  Gear reduction "could" easily be more efficient. 

 

Rolling resistance could go up or down a bit depending on ATV tires.  It was estimated based on radar data for one set of low profile race ATV tires @ recommended inflation. 

 

Aerodynamic coefficient is a lot more likely to go up, based on published data for open wheel race cars. 

 

You can SWAG some for your car but don’t finagle the numbers to get the speed you are hoping for.  Your car doesn't actually get faster just by moving all the variables 20% in one direction.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Waffles Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 10:06am
Originally posted by Akron 1998 to 2004 Akron 1998 to 2004 wrote:

Drive train efficiency can definitely be improved 10%.  I'm running a CVT with double chain reduction.  Gear reduction "could" easily be more efficient. 

 

Rolling resistance could go up or down a bit depending on ATV tires.  It was estimated based on radar data for one set of low profile race ATV tires @ recommended inflation. 

 

Aerodynamic coefficient is a lot more likely to go up, based on published data for open wheel race cars. 

 

You can SWAG some for your car but don’t finagle the numbers to get the speed you are hoping for.  Your car doesn't actually get faster just by moving all the variables 20% in one direction.



And you can't prove that your coefficients are correct without proof.  Part of being an engineer is using data, not the seat of your pants.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 11:32am
Originally posted by Waffles Waffles wrote:


And you can't prove that your coefficients are correct without proof.  Part of being an engineer is using data, not the seat of your pants.
 
Your impotent response proves nothing.  Ciao! (the most irritating form of goodbye)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 11:54am
What is your Point Akron?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sandres913 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 12:07pm
Originally posted by Akron 1998 to 2004 Akron 1998 to 2004 wrote:

Originally posted by Soccerdan7 Soccerdan7 wrote:


Again with the I. You do realize this is a team competition, don't you? 
At one point in time I questioned if it was fair for me to go against a team of 30, then I realized they probably didn't have room for more.

Probably something along the same lines as this was
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 12:10pm
Originally posted by ErikHardy ErikHardy wrote:

What is your Point Akron?
 
Answering the question many people are asking, "Absolute Top Speed of Competition Ready Car"!!!
No B.S. about driving next to some other vehicle to gage top speed.  No B.S. about some outlandish scenario like bald tires, no firewall, and a midget behind the wheel.
What is your point?  You got something better than I've presented?


Edited by Akron 1998 to 2004 - Jul/03/2012 at 12:11pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 12:26pm
Originally posted by Akron 1998 to 2004 Akron 1998 to 2004 wrote:

Originally posted by ErikHardy ErikHardy wrote:

What is your Point Akron?
 
Answering the question many people are asking, "Absolute Top Speed of Competition Ready Car"!!!
No B.S. about driving next to some other vehicle to gage top speed.  No B.S. about some outlandish scenario like bald tires, no firewall, and a midget behind the wheel.
What is your point?  You got something better than I've presented?

Didn't Cornell already post data that shows they exceeded your "Absolute Top Speed of Competition Ready Car" during an endurance race?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 12:40pm
There will never be an absolute top speed of a baja car!! Do you really think the cars 10 years ago will be the same as the ones today? Has everyone just been sitting on their thumbs? Will todays cars be faster than the ones 10 years from now? Not a chance, unless if there is a significant rule change. Why are you getting so butt hurt that the cars you and your team were associated with aren't the pinnacle of baja. Its natural progression dude. Things are suppose to get better over time...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 12:51pm
Originally posted by dillon_b12 dillon_b12 wrote:


Didn't Cornell already post data that shows they exceeded your "Absolute Top Speed of Competition Ready Car" during an endurance race?
Cornell's data said 34 MPH.  My calculation at the top says 34 MPH. 
Are you paying attention or just arguing with me because you don't comprehend there are people much better than you at doing most everything.  My above calculation is a pure gift of the highest order and you doubt its validity simple because you wish it to be wrong.  I don't believe it gets much simpler.
If you've got wind tunnell data, either post it or go jerk it in San Francisco!
Vehicle weight + Driver 450 lbs (force) 2001.699 N
Rolling resistance coeifficent 0.15
Aerodynamic coefficent 0.7
Crossectional area 10 ft^2 1 m^2
RHO "air" 1.22 Kg/m^3
Power Engine 10 hp 7457.122 W
Drivetrain eficency % 0.81 %
Power at wheels 8.1 hp 6040.268 W
Drivetrain loss 1416.853 W
Rolling drag 4552.202 W
Aero drag 1488.066 W
Max vehicle speed 33.91 mph 15.16 m/s
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 12:56pm
[QUOTE=ErikHardy]There will never be an absolute top speed of a baja car!! QUOTE]
 
Of course there is an absolute top speed, bloody sperm bubbles for brains. 


Edited by Akron 1998 to 2004 - Jul/03/2012 at 12:57pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 1:10pm
Everyone needs to bring it down a notch and calm down...  This is a good engineering topic, lets have a good engineering discussion with data and rationale - if people want to post opinions then take it to the lounge... 

To EVERYONE, lets keep this forum professional and polite.  This forum is a reflection of you, your team, and SAE, and whether the poster is foreign or alunmi or noob, they need to be treated with respect.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 1:13pm
Really? Is everyone mandated to have the same tires, bodywork shape, firewall area, drivetrain efficiency, driver weight, etc... Why is cornells car faster than yours? I get it, your car is the holy grail of baja and can never be exceeded because everyone is below you. You remind me of that VX1 guy. And on that note, Beer timeBeer
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 1:18pm
Originally posted by Akron 1998 to 2004 Akron 1998 to 2004 wrote:

Originally posted by dillon_b12 dillon_b12 wrote:


Didn't Cornell already post data that shows they exceeded your "Absolute Top Speed of Competition Ready Car" during an endurance race?
Cornell's data said 34 MPH.  My calculation at the top says 34 MPH. 
Are you paying attention or just arguing with me because you don't comprehend there are people much better than you at doing most everything.  My above calculation is a pure gift of the highest order and you doubt its validity simple because you wish it to be wrong.  I don't believe it gets much simpler.
If you've got wind tunnell data, either post it or go jerk it in San Francisco!
Vehicle weight + Driver 450 lbs (force) 2001.699 N
Rolling resistance coeifficent 0.15
Aerodynamic coefficent 0.7
Crossectional area 10 ft^2 1 m^2
RHO "air" 1.22 Kg/m^3
Power Engine 10 hp 7457.122 W
Drivetrain eficency % 0.81 %
Power at wheels 8.1 hp 6040.268 W
Drivetrain loss 1416.853 W
Rolling drag 4552.202 W
Aero drag 1488.066 W
Max vehicle speed 33.91 mph 15.16 m/s

I'm not sure I've ever claimed to be the best at anything, on this site or at any other time.  I don't even have an engineering degree, which is why I usually abstain from discussions of this nature. Math is not a strong suit of mine.

In your second post in this thread, you basically confirm your numbers with the real world by saying that the results in your car matched the calculations.  So, the rolling resistance coefficient you used was for asphalt correct?  I'm assuming this since this is where you confirmed the numbers in your car.

Does a dirt or mud track have a higher rolling resistance than asphalt?  Cornell posted their ~34MPH speed on an endurance track NOT asphalt. If you move that same 34MPH dirt track car onto a lower rolling resistance coefficient track i.e. asphalt, would it not pick up some speed?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 1:56pm
Originally posted by dillon_b12 dillon_b12 wrote:

 Cornell posted their ~34MPH speed on an endurance track NOT asphalt. If you move that same 34MPH dirt track car onto a lower rolling resistance coefficient track i.e. asphalt, would it not pick up some speed?
 
AMEN. There was not a smooth enough, long enough straight to get up to a true top speed and the rolling resistance is obviously very different. I think it is perfectly reasonable to use this to justify about 37 mph or so on a long flat paved straightaway (and we have done it). Leave the firewall testing out and our competition ready car is still easily doing 37.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 2:37pm
Originally posted by dillon_b12 dillon_b12 wrote:

Does a dirt or mud track have a higher rolling resistance than asphalt?  Cornell posted their ~34MPH speed on an endurance track NOT asphalt. If you move that same 34MPH dirt track car onto a lower rolling resistance coefficient track i.e. asphalt, would it not pick up some speed?
 

A dry solid dirt/clay track vs. asphalt would be a marginal difference for ATV tires, based on experience.  Mud & sand would be on the order of 0.3 for rolling coefficient, based on wiki.

I would implore someone to post a measured valve if they disagree with my coefficients.  Otherwise you are throwing stones without offering much in return.



Edited by Akron 1998 to 2004 - Jul/03/2012 at 3:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JeremyB Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 2:41pm

Anybody have test numbers on the Rolling Resistance of a baja car? All it takes is a fishing scale to eyeball the force required to move the car at a slow, constant speed. Yes, it varies wildly depending on surface, tire, and tire pressure, but isn't hard to get data for.

Anyways, made a spreadsheet to visualize things. Top_Speed
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 2:55pm
Originally posted by Akron 1998 to 2004 Akron 1998 to 2004 wrote:

[

A dry solid dirt/clay track vs. asphalt would be a marginal difference for ATV tires, based on experience.  Mud & sand would be on the order of 3.0 for rolling coefficient, based on wiki.

 
The Alabama track was not hard packed dirt and clay. A lot of it was fairly soft and loose dirt and grass. The start / finish area was mostly clay since that's where the old pond was but the long straight away to the tabletop was not ideal for a top speed run. I still think the overall bumpiness and inconsistancy of the surface was more of a top speed hindrance than the actual rolling resistance, but I do not have any proof.
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http://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html


3.0 for rolling resistance coefficient? These aren't values measured by me personally, of course.

In the beginning, I wasn't saying what you had was totally wrong, I was asking where the values came from, and you pulled the Laval thing out and compared it to your past Baja car, and totally ignored my comment on the value of rolling resistance. I'll admit, yes, I think it's impressive to have a car doing 150 ft in around 5 seconds, so congratulations (no sarcasm, I promise! Big smile).

In either case, we have two real-life examples of cars doing 150 ft in around 5 seconds. One was confirmed to not be over 30mph, given the nice plot of distance and speed, so it might stand that the other car wasn't over 30mph either. Bad speculation on my part. That now is irrelevant to the determination of theoretical top speed of a competition ready car. I'm the one that brought it into the picture, and I'm saying it's irrelevant. I'm not trying to put anyone down by saying that.

Now, can we discuss values without egos? If the numbers are right, so be it. But there are cars going faster than 33mph. Again, I'm not saying what you have is totally horribly wrong, I'm just asking about one value in particular, the rolling resistance.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 3:45pm
in this video the car reaches 41 mph on the endurace.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 3:56pm
Originally posted by Pedro UFPBaja Pedro UFPBaja wrote:

in this video the car reaches 41 mph on the endurace.
 
The ethanol content of Brazil gas has already been noted as making more power than it otherwise should.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 4:01pm
Brazillian gas has 25% of ethanol, but it does not help without a higher compression ratio.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 4:11pm
Originally posted by Akron 1998 to 2004 Akron 1998 to 2004 wrote:

Originally posted by Pedro UFPBaja Pedro UFPBaja wrote:

in this video the car reaches 41 mph on the endurace.
 
The ethanol content of Brazil gas has already been noted as making more power than it otherwise should.


"The energy content of ethanol is about 33% less than "pure" gasoline, although this varies depending on the amount of denaturant that is added to the ethanol.  Thus, vehicle mileage may decrease by up to 3.3% when using E10."

How could adding something with less energy content make an engine make more power without other changes?  It's always been my understanding that one of the positives of ethanol was that it allowed you to run higher compression ratios before experiencing detonation.  I can't recall ever reading anything that said it made more power than gasoline in an engine originally designed for gasoline.


Edited by dillon_b12 - Jul/03/2012 at 4:15pm
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becouse you can compress more before the ingnition with ethanol.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 4:19pm
Originally posted by Pedro UFPBaja Pedro UFPBaja wrote:

becouse you can compress more before the ingnition with ethanol.

You just barely beat my edit. Smile I should have been more clear.  I'm aware of this scenario, but we are talking about an engine designed specifically for gasoline right?

Akron made it sound like Baja teams could make more power just by running higher ethanol content gas.


Edited by dillon_b12 - Jul/03/2012 at 4:20pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 4:22pm
Originally posted by Pedro UFPBaja Pedro UFPBaja wrote:

becouse you can compress more before the ingnition with ethanol.
 
But the engine is un-modified, thus brazil competitions should be slower yet they are faster.  It boggles the mind!!!  We had a whole thread devoted to this.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote collinskl1 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 5:14pm
Everyone knows gravity is stronger closer to the equator, and that makes everything faster...
 
Conundrum solved.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2012 at 5:37pm

Interesting solution to the “Brazilian Paradox”, sounds like the title to a XXX movie.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/06/2012 at 11:58am

Calculations at top of page have been updated.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote CLReedy21 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/06/2012 at 10:21pm
I hate to pull a trick out of my bag just to win an argument on the interweb, but I'm not above it.  So...

Per testing performed by the U.S. Army Corps of Engineers on a Suzuki King Quad ATV of similar weight and dimensions to a Baja SAE vehicle the rolling resistance value for an ATV on a hard surface is .0482.  (Wesson et al. January 2007, ERDC/CRREL:TR-07-1)

See other values in chart taken from the paper (second from right cited):


Basically this turns your whole argument on it's ear unless you want to pull a flip flop and change all your other parameters on me.  I've highlighted my changes in red from your edited first post.  Numbers calculated using your numbers in Jeremy's excel sheet (with decimals run out to 3 places).  Baja cars CAN theoretically do 40 (or perhaps more).

Vehicle weight + Driver450lbs (force)2001.699N
Rolling resistance coeifficent 0.048
Aerodynamic coefficent1
Crossectional area13ft^21.207739m^2
RHO "air"1.22Kg/m^3
Power Engine9.775hp7289.336W
Drivetrain eficency %0.83%
Power at wheels8.11325hp6050.149W
Drivetrain loss1239.187W
Rolling drag1732.380
W
Aero drag4318.291W
Max vehicle speed40.40mph18.03m/s

Now I'll do the same calculation using weight and frontal area from TTU's 2011 vehicle (in accel trim, aka girl 125lb girl driver).

Vehicle weight + Driver500lbs (force)2224.111
N
Rolling resistance coeifficent0.048
Aerodynamic coefficent1
Crossectional area12.5ft^21.1612880m^2
RHO "air"1.22Kg/m^3
Power Engine9.775hp7289.336W
Drivetrain eficency %0.83%
Power at wheels8.11325hp6050.149W
Drivetrain loss1239.187W
Rolling drag1921.103W
Aero drag4127.897W
Max vehicle speed40.33mph17.996 m/s

So, in conclusion:




Edited by CLReedy21 - Jul/06/2012 at 10:41pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote otto Quote  Post ReplyReply Direct Link To This Post Posted: Jul/07/2012 at 3:54pm
Originally posted by Pedro UFPBaja Pedro UFPBaja wrote:

in this video the car reaches 41 mph on the endurace.


lets not forget the differing firewall rules for brazil vs. north america - might account for some difference, too lazy to make up a cross sectional area number (if the rules are actually different? I'm only speaking from having seen a few cars from brazil with a smaller firewall)


Edited by otto - Jul/07/2012 at 4:02pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Akron 1998 to 2004 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/07/2012 at 8:45pm
Originally posted by CLReedy21 CLReedy21 wrote:

I hate to pull a trick out of my bag just to win an argument on the interweb, but I'm not above it.  
 
Well played! 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Jul/08/2012 at 2:01pm
That video is from 2010, despite the FEI have made a car with half  firewall  that year, the rule did not allow it yet, the rule of the firewall was modified in 2011.
With open firewall,  2012 car of my team reached 42 MPH on a flat 3/4 mile asphalt road (not at the competition but complete as runs at).
2012 car




the 2010 car was shaped to reduce the drag force





Don't forget that North American cars with side engine has no firewall




Edited by Pedro UFPBaja - Jul/08/2012 at 4:52pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote otto Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2012 at 1:10pm
Drag coefficient may also be slightly higher than 1, Cd for a flat plate is 1.28.  the front of the car may help out a little over the flat plate, but I'd still say Cd will be larger than 1 for most of the cars out there.  Has anyone done testing to get their drag coefficient?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote collinskl1 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2012 at 1:23pm

Have any of y'all seen the 18 wheelers that have the fold up fairings behind the 53' box?  I know I've never seen a baja car with bodywork behind the firewall... I wonder if having some panels on the rear bracing of the frame wouldn't help out with some aero.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote sandres913 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2012 at 2:00pm
Originally posted by otto otto wrote:

Drag coefficient may also be slightly higher than 1, Cd for a flat plate is 1.28.  the front of the car may help out a little over the flat plate, but I'd still say Cd will be larger than 1 for most of the cars out there.  Has anyone done testing to get their drag coefficient?

We haven't done testing on ours, but I was thinking about exactly that. The thing is though that the body work will affect that (i.e. our firewall was at 5-10 degrees, and our stance made the firewall at a 90 with the ground, if that makes sense) So say you have your firewall installed at 20 degrees backwards, you'd do sin(90+20)x1.28=1.20. Granted, this is a smaller change than going from 1 to 1.28, but it's something to keep in mind. It can be the difference of about 3mph, which to the top teams is a lot. But a lot of the lower teams (I'm including my own team in that, given the last couple years) they're not going to worry about that as much, most of the time they're more worried about the tires staying on, or the transmission not exploding because a bearing or 2 were accidentally seized >_<

I'm going to dabble some software-based analysis. Autodesk came out with Simulation/CFD tools, and, well, I kinda have a *thing* for Autodesk Heart Thumbs Up I'll probably do that tomorrow night or Thursday night, if I find anything useful I'll post it here
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2012 at 3:41pm

fairings at the car back do helps reduce the aero drag,




The vacuum created behind the firewall causes more drag that the front area.

POLI USP uses fairings even on the engine area to reduce the vacuum



That's why the shuttle used to fly with this coverage back when it was being transported




Edited by Pedro UFPBaja - Jul/09/2012 at 3:45pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote otto Quote  Post ReplyReply Direct Link To This Post Posted: Jul/10/2012 at 3:20pm
Originally posted by Pedro UFPBaja Pedro UFPBaja wrote:


fairings at the car back do helps reduce the aero drag,

The vacuum created behind the firewall causes more drag that the front area.

POLI USP uses fairings even on the engine area to reduce the vacuum




Curious to know the difference this makes, you have any more details pedro on their reduction in pressure differential/drag or a streamline pic? 

edit: looks like we're becoming more of a prism as per this article/photo so somewhere in the neighborhood of 5% should be attainable?



Edited by otto - Jul/10/2012 at 3:23pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JeremyB Quote  Post ReplyReply Direct Link To This Post Posted: Jul/10/2012 at 5:12pm
Originally posted by otto otto wrote:

Drag coefficient may also be slightly higher than 1, Cd for a flat plate is 1.28.  the front of the car may help out a little over the flat plate, but I'd still say Cd will be larger than 1 for most of the cars out there.  Has anyone done testing to get their drag coefficient?
Anybody?!

This, and drivetrain efficiency are two things I've yet to see data on. I'm sure data's been taken, but not necessarily shared.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/10/2012 at 5:29pm
We don't have the data you are looking for but we did some basic streamline testing with the 2007 and 2008 cars with little string tufts to show flow direction and turbulent areas. Having lower rear panels taper back with the frame behind the firewall didn't seem to help as the inflection point still created flow separation and our little strings went wild. We used that testing to justify reduced and eventually eliminated rear panels according to the logic that the air is going to be turbulent anyway and the slight increase in drag without panels will blow turbulent cool air across the entire drivetrain and help cool the Briggs. Upper rear panels are a different story, but with most rollhoops reclined, it is not really possible to smooth out the abrupt change over the top as the flow is going to separate anyway unless you have a USF-sized piece of aero out there.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sandres913 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/10/2012 at 9:29pm
Originally posted by JeremyB JeremyB wrote:

This, and drivetrain efficiency are two things I've yet to see data on. I'm sure data's been taken, but not necessarily shared.

Here ya go! Thumbs Up (for belt drives at least) Courtesy of Carlisle!


I think I read in Shigley's that v-belt drives had a power loss around 20%, which lines up with Carlisle's data, and a 2% power loss for steel gears (spur and bevel).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rolox Quote  Post ReplyReply Direct Link To This Post Posted: Jul/11/2012 at 12:43pm
This one gives the CVT efficiency on page 38.
http://poisson.me.dal.ca/~dp_06_6/IVT-Report2.pdf
If you look on Google Scholar you can find numerous papers with similar results for snowmobiles.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote otto Quote  Post ReplyReply Direct Link To This Post Posted: Jul/17/2012 at 5:50pm
Does everyone's cross sectional area include the tires? (roughly 8*22 x 2) as well as the contribution from shocks and suspension links being away from the body?  Just making sure these numbers are comparable.  Our 2012 firewall was 1305 sq. inches (9.0625 sq. feet), including all the tires and suspension brings it up between 1700 and 1800 sq. inches for frontal area of our 2012 car.

Also, 9.775 HP seems a little high.  I would put it closer to 9 HP to be on the conservative side.

All that being said, power limited top speed is very near 40 MPH unless you start to deviate heavily from the current cookie-cutter baja vehicle.

Looks like a mid engine with a high efficiency drivetrain (90-95%) could hit 45...
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Post Options Post Options   Thanks (1) Thanks(1)   Quote schooter Quote  Post ReplyReply Direct Link To This Post Posted: Jul/18/2012 at 1:08pm
I collected some gps data a couple years back on some old cars. One has a firewall larger than rules (don't ask why) and the other has a nearly rules spec firewall. They each obtained terminal speeds in the 31-34 mph range on hard pack gravel with the engine running steady at 3100-3300 rpm.

I agree that there's the possibility in terminal speeds near 40 mph with a firewall even over 40 mph w/o a firewall. However that takes a well built and setup car. Little things like drivetrain mis-alignments and vibrations, tire alignment, tire inflation, wheel run-out, wheel bearing drag, brake drag....etc. They all add up and have a significant impact on majority of cars. That and there's plenty of engines out there with dirty carbs that aren't letting the engine make max power.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote M.Henry Quote  Post ReplyReply Direct Link To This Post Posted: Jul/27/2018 at 12:13pm
You all discuss the aerodynamic drag created by the firewall having such a bad CD and surface area, but you also need to remember the aerodynamic effects of the tires. A rotating body up against a plane all in fluid flow will do weird things. You have to account for the high pressure regions created in front if the tire and the vertices being shed axially from the tires. This probably contributes a mph difference if you were to have the tires shrouded.

Ohtrying to putting simple rear panels on is not going to do a huge effect because your gonna get flow separations across any abrupt surface changes. Same thing with the front panels.

Now if one of the top teams were to do some proper work in aerodynamics in reducing flow separation on a proper front and rear fairings I wouldn't be surprised to see a 2 or 3 mph gain in top speed.
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