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Acceleration MPH

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João Araujo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote João Araujo Quote  Post ReplyReply Direct Link To This Post Posted: Feb/09/2011 at 5:21pm
In the first seconds, accelerration is limited by the available traction, then it's limited by power.

a Baja Car that weights that can reach up to 62 km/h in 300 feet, they got close, they have a good transmission...

Fei isn't invincible, in fact, the finished 3rd on the regional competition, below Poli-USP and EESC-USP

The acceleration record is from UFRN from 2004 i believe, they reached 3.6 in 100 feet.. but their car was not very fast in top speed.



Edited by João Araujo - Feb/09/2011 at 5:22pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Feb/09/2011 at 5:33pm
Not really a big deal...

Our old car has increased from 60km/h during the endurace. he did not get to it in 300ft, but with a straight track long enough it arrives. 
and it doesn't have a refined design as the FEI's.

In these jumps(at 1:40 and 2:24) it goes as up to 60 (Our dashboard mark the peaks)


Maybe becouse brazillian gas has 25% ethanol


Edited by Pedro UFPBaja - Feb/09/2011 at 5:39pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mod Squad Quote  Post ReplyReply Direct Link To This Post Posted: Feb/09/2011 at 7:34pm
This thread was poised for a nice discussion of governor effect on top speed calcs. However, it quickly derailed...

Merging with the "Acceleration MPH" thread.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote João Araujo Quote  Post ReplyReply Direct Link To This Post Posted: Feb/12/2011 at 8:18pm
Before 2010 FEI wasn't one of the fastest cars, actually, in 2008 they reached 48kph/30 mph

And won the midwest competition, same for 2007.

now they are getting faster.
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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: Feb/14/2011 at 3:37am
Originally posted by João Araujo João Araujo wrote:

Before 2010 FEI wasn't one of the fastest cars, actually, in 2008 they reached 48kph/30 mph

And won the midwest competition, same for 2007.

now they are getting faster.
 
I know I should leave this alone.  What do you think is the absolute top speed of a Baja car?  If a jump from 48kph to 58.5kph doesn't raise any concerns, what would?  I'm not talking about down hill with the wind at your back.  What is the absolute top theoretical speed on a flat smooth surface?  A speed which no un-modified Baja car should exceed.

 

CVT efficiency 88%

1 gear/chain reduction efficiency 98%

300 lb car with a 100 lb driver

Let’s just say the engine makes 10 hp. (I've never dyno'ed one) (anyone feel like sharing?)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote CLReedy21 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2011 at 4:40am
I think the generally accepted number is in the high 30's, maybe even 40, which seems consistent with the results the Brazilian teams have referenced.  I don't have any personal experience with the subject since we've always valued an abundance of torque and geared for the low to mid 30's.

It's hard to nail down a theoretical number with all outside factors considered like tire rolling resistance, frontal area, drag coefficients, real CVT/gear/cv/wheel bearing efficiency.  I ran some rudimentary aero calcs that came back with something like 40 or 41 but that didn't factor tires and cv's and such.

For what it's worth my experience with the Briggs Model 20 has shown they make between about 8.7 and 9.5 horsepower in like new condition in the real world.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2011 at 4:44am
Lets say they modified the engine ...
The engine has the rpm measured on the first day of competition and immediately after it is tied with wire and receives a seal, so can not be replaced before the evet dynamic.
After the Endurace the engines have measured the rotation again.
At the end of the competition cars with better performance have completely disassembled the engines and checked by representatives from briggs.

Namely, this modification would want done only in the head and carburetor or very discrete for not be detectable by briggs, or reversible without anyone noticing and without the use of tools during the queue for the measurement of rotation at the end of endurance.

Unlikely?


Edited by Pedro UFPBaja - Feb/14/2011 at 4:56am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2011 at 4:51am
well, our new car came to 39 mph during testing (about 1/2mile track).
with a engine that came out of the box straight to the car.



PS
All Brazillian cars increased speed after they changed the engine 205432 (sold in Brazil until 2009) by the model 205332.


Edited by Pedro UFPBaja - Feb/14/2011 at 5:20am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote CLReedy21 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2011 at 5:13am
Pedro I'd be willing to bet that you car falls well within the 95th percentile of the "small/light/efficient" category so I'd believe 39 under ideal circumstances.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2011 at 5:37am
we have our tricks.
Small tires, lightweight wheels, use of plastic, aluminum and composite materials in innovative ways, etc. ..





Edited by Pedro UFPBaja - Feb/14/2011 at 7:57am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2011 at 10:19am
Originally posted by Mod Squad Mod Squad wrote:

This thread was poised for a nice discussion of governor effect on top speed calcs. However, it quickly derailed...

Merging with the "Acceleration MPH" thread.
 
I'm a fan of Akron's thread hijack champion statusThumbs Up...
 
and... back on topic
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2011 at 10:40am
Originally posted by CLReedy21 CLReedy21 wrote:

I think the generally accepted number is in the high 30's, maybe even 40, which seems consistent with the results the Brazilian teams have referenced.  I don't have any personal experience with the subject since we've always valued an abundance of torque and geared for the low to mid 30's.

It's hard to nail down a theoretical number with all outside factors considered like tire rolling resistance, frontal area, drag coefficients, real CVT/gear/cv/wheel bearing efficiency.  I ran some rudimentary aero calcs that came back with something like 40 or 41 but that didn't factor tires and cv's and such.

I think 40 is definitely possible, but, like you mention, at what cost to acceleration and torque?  I don't want to be stuck on the track somewhere thinking, "If only I wasn't stuck...I'd be soooo fast!".

Tire rolling resistance, IMO, makes a HUGE difference.  We have a set of street tires in the same size as our dirt tires, and the car is a rocket ship with those on.  Again, it's a trade-off between traction and rolling resistance.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adrive7 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2011 at 2:34pm
Alumni always claimed the 2005 car with it's 5 speed gearbox out of an ATV could hit 45 eventually in the giant off-campus parking lots. That's eventually in a gear far too tall for it's own good.

I only ever drove that car in third, so I can't comment on fifth gear performance.
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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: Dec/21/2011 at 6:31pm
mod note: Merged from Show Your Junk: 2012 Edition

Originally posted by schooter schooter wrote:

yeah, bends like that usually take a few tries or someone with a bit of experience. Unless you have bending software and a bender that's calibrated.

I like the idea of using bends like you got, its something you can actually justify in design judging. Your reducing the frontal area and increasing top speed. I estimated last year that we were loosing close to 3 hp with a speed in the low 30's. Every little bit of that firewall you can get rid of helps.
 
All the aerodynamic drag of a Baja car @ 30 mph might add up to 0.3 hp.  I'd give you style points for the bent tubing but not worth the effort from an aerodynamic point of view.


Edited by Mod Squad - Dec/22/2011 at 3:25pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote schooter Quote  Post ReplyReply Direct Link To This Post Posted: Dec/21/2011 at 9:29pm
Let's not start a debate\argument. Calculate it for yourself with some typical numbers and see what you get. I'm not the first to acknowledge significant aerodynamic losses. http://forums.bajasae.net/forum/acceleration-mph_topic161.html

Besides, if it wasn't for aerodynamic affects these baja cars would be going a lot faster than 30 or so mph. Most of these cars have terminal speeds in the low to mid 30's (I actually confirmed it with some back road testing).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote collinskl1 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/22/2011 at 7:05am

The mid-engine cars without the typical firewall on the RRH don't go much faster (if any)... not to say that there isn't an aero loss, I don't think it's really that parasitic.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Dec/22/2011 at 10:16am

I would put my money on rolling resistanceSmile

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Post Options Post Options   Thanks (0) Thanks(0)   Quote frinesi2 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/22/2011 at 10:17am


FEI had some interesting colorful fancy diagrams showing the reduction in drag just from running half of a firewall. It was an appreciable reduction, but I don't recall the numbers.

edit: found it!


Edited by frinesi2 - Dec/22/2011 at 10:35am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote CLReedy21 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/22/2011 at 11:01am
The car is so much faster without that firewall section that it's about to drive off the page!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/22/2011 at 11:10am
haha... marketing at its greatest
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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: Dec/22/2011 at 11:36am
Originally posted by schooter schooter wrote:

Let's not start a debate\argument. Calculate it for yourself with some typical numbers and see what you get. I'm not the first to acknowledge significant aerodynamic losses. http://forums.bajasae.net/forum/acceleration-mph_topic161.html
 
An industrial size fan has a 0.1 hp motor.  Does your car move more air than an industrial size fan?


Edited by Akron 1998 to 2004 - Dec/22/2011 at 11:38am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote J.Cremer Quote  Post ReplyReply Direct Link To This Post Posted: Dec/22/2011 at 3:08pm
What else is preventing further acceleration (beyond 35-40mph) besides air drag? 

The three forces that keep a vehicle from accelerating are rolling resistance, aerodynamic drag, and grade.  We are assuming here that the vehicle is on level ground.

Air drag is increasing at the rate of velocity ^2

Rolling resistance is pretty much not changing.
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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: Dec/22/2011 at 3:43pm
Originally posted by J.Cremer J.Cremer wrote:

What else is preventing further acceleration (beyond 35-40mph) besides air drag? 
 
You're fixated on things increasing as you go faster; you need to think about your decreasing torque at the rear tires as your gears ratio changes.
 
or
 
Realize rolling resistance force is constant but rolling resistance power is increasing.
 
You need to put everything in terms of force or in terms of power (NO MIXING)!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Dec/23/2011 at 2:23pm
Originally posted by Akron 1998 to 2004 Akron 1998 to 2004 wrote:

 
Realize rolling resistance force is constant but rolling resistance power is increasing.
 
Confused
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Post Options Post Options   Thanks (0) Thanks(0)   Quote varun Quote  Post ReplyReply Direct Link To This Post Posted: Dec/30/2011 at 12:18pm
hey guys my name is varun from India,
this will be be our first year participating in baja,
can someone please tell me what is the coefficient of friction between the tires and the ground
and whats the torque of the engine,
 
we have taken,
coefficient of friction=0.7
and torque as 18 Nm at 2000 rpm.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote varun Quote  Post ReplyReply Direct Link To This Post Posted: Dec/30/2011 at 12:19pm
this forum is very informative...
thanks in advance Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ErikHardy Quote  Post ReplyReply Direct Link To This Post Posted: Dec/30/2011 at 12:42pm
You're on the money with the torque. Friction coefficient is a crap shoot, don't worry about it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote p.lewis Quote  Post ReplyReply Direct Link To This Post Posted: Jan/01/2012 at 4:01am
Here's some kinetic coefficient of friction values from engineershandbook.com:
Rubber Asphalt (Dry)
0.5-0.8


Rubber Asphalt (Wet)
0.25-0.0.75


Rubber Concrete (Dry)
0.6-0.85


Rubber Concrete (Wet)
0.45-0.75

Since we're off-road, engineer for uncertain and extreme conditions. However, often the brake check is performed on asphalt where the friction is higher. So, make sure to design your brakes for enough power to lock the tires on dry asphalt.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graeter Quote  Post ReplyReply Direct Link To This Post Posted: Mar/12/2013 at 9:36am
Just as a note, we have gotten our car to 43 mph in a parking lot confirmed by data-logging GPS. We started from a roll around about 20 mph and had about 300 feet to get it to 43. Granted, we have a 6 speed manual which is beneficial at high end like this. I want to re-do this test with the local police and use their radar for confirmation.

With that in mind, we were in 6th for parts of Wisconsin (the back slight uphill section) but when we went to midnight mayhem, we could never get it going that high even on the oval.
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