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Comparing 2 cars quantitatively with some known variables...

Posted by ReluctantGolf 
ReluctantGolf
Kevin Andrade
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Comparing 2 cars quantitatively with some known variables...
February 12, 2016 12:41PM
Good afternoon folks!

My question is how to go about comparing the speeds at certain RPM of 2 vehicles if I happen to know the wheel size, RPM range (redline levels), HP and TQ figures and weight. I am hoping that there is an engineer or some other knowledgable member out there who knows of a spreadsheet or graphing tool which can show some visualtization of the following.

Knowns:

1. Max HP and TQ at a certain RPM in each
2. Gear ratios for each tranny
3. Wheel size for each car (in this case both are the same)
4. Weight of each car (2500lbs vs. 2900lbs)

I am essentially trying to gauge the speed/acceleration differences between one car that is slightly lower powered (as described in HP and Tq figures) but lighter then the other (by 400lbs).

How should I go about framing this analysis??

Thanks and happy to field some clarifying questions on the topic.
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Robert Culbertson
Robert Culbertson
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Re: Comparing 2 cars quantitatively with some known variables...
February 12, 2016 01:43PM
I'm going to make some huge generalizations on this.

F=ma

The Force is measured at the tire contact patch, and calculated through torque and gearing.
The vehicle Mass is known. You are solving for the Acceleration, which will be linear unless you want to get handy with a spreadsheet.

You can also use google and do energy calculations. Google is a crazy good calculator.

http://www.physicsclassroom.com/class/1DKin/Lesson-6/Kinematic-Equations
https://answers.yahoo.com/question/index?qid=20100130060655AAt1ZCe



Edited 1 time(s). Last edit at 02/12/2016 01:46PM by Robert Culbertson.
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john vanlandingham
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Re: Comparing 2 cars quantitatively with some known variables...
February 12, 2016 04:40PM
Quote
ReluctantGolf
Good afternoon folks!

My question is how to go about comparing the speeds at certain RPM of 2 vehicles if I happen to know the wheel size, RPM range (redline levels), HP and TQ figures and weight. I am hoping that there is an engineer or some other knowledgable member out there who knows of a spreadsheet or graphing tool which can show some visualtization of the following.

Knowns:

1. Max HP and TQ at a certain RPM in each
2. Gear ratios for each tranny
3. Wheel size for each car (in this case both are the same)
4. Weight of each car (2500lbs vs. 2900lbs)

I am essentially trying to gauge the speed/acceleration differences between one car that is slightly lower powered (as described in HP and Tq figures) but lighter then the other (by 400lbs).

How should I go about framing this analysis??

Thanks and happy to field some clarifying questions on the topic.

You can fuck around with "calculating" a bunch of stuff if that's what yanks your chain...
However every single time I see guys geek out and start "calculating" there is ALWAYS inevitably a number of complete guesses in fundamental places...then they go fuck with their Anal-ist tools and then eventually arrive at soem OFTEN totally fucked up "answer"..

I see it with springs constantly..answers 2 or 3 times what is reasonable
Injector sizing--and the geek-oids ignoring the little fact that some systems fire the injector every 360 degrees, others every 720 degrees, and they come up with totally fucked up answers..
Just for fun post up the data. the weights and the gearing I-5, final drive, tire diameters...
Ft/lbs of torque @ whatever the whole curve looks like.. and real HP curve for both..

If you have real, not assumed, values presented in 2 columns side by side. a quick glance ought to be enough to make a half assed guess which car might be whatever vs the other theoretically..



John Vanlandingham
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ReluctantGolf
Kevin Andrade
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Re: Comparing 2 cars quantitatively with some known variables...
February 12, 2016 09:05PM
Quote
john vanlandingham

Just for fun post up the data. the weights and the gearing I-5, final drive, tire diameters...
Ft/lbs of torque @ whatever the whole curve looks like.. and real HP curve for both..

If you have real, not assumed, values presented in 2 columns side by side. a quick glance ought to be enough to make a half assed guess which car might be whatever vs the other theoretically..

Ok, here is the data that i have:


Car#1:

Wheels are 175/70/15

Gear ratios:
1st = 3.78
2nd = 2.118
3rd = 1.36
4th = 1.029
5th = .837
R&P = 4.235

HP = 115 @ 5,200RPM
TQ = 122 @ 2,600RPM

Weight = 2,900lbs

Car#2:

Gear Ratios:
1st = 3.846
2nd = 2.038
3rd = 1.28
4th = .95
5th = .756
R&P = 4.07

HP = 120 @ 6,350RPM
TQ = 112 @ 5,000RPM

Weight = 2,500lbs

So given the 2 above car parameters which one will get me down stage faster? or which one seems suited more to long gravel stage type of events in the RA and NASa series....Thanks Jon!
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Anders Green
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Re: Comparing 2 cars quantitatively with some known variables...
February 12, 2016 09:18PM
Here's the data I have:

they one you can build fastest and care about denting less will go significantly faster. winking smiley

Both of those cars have essentially zero power. *grin* So the lighter one will go faster. Build it and stop worrying. Worry about driving more.

Anders



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ElectroTech
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Re: Comparing 2 cars quantitatively with some known variables...
February 12, 2016 09:38PM
The one where the throttle is to the floor the most and the brakes are merely stabbed occasionally, covers a way larger horsepower and torque gap than what you posted, I'd worry more about where that weight is and it looks like Mk2 vs Mk3 VW stock 8v numbers so I'll say you would be faster in the Mk2, purely from a connected to the road surface feeling perspective. That 400lbs is civilization weight.



Power means nothing if you cannot control it!
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NoCoast
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Re: Comparing 2 cars quantitatively with some known variables...
February 13, 2016 11:51AM
Gear Ratios:
1st = 4.2
2nd = 2.49
3rd = 1.66
4th = 1.22
5th = 1.0
R&P = 4.1

HP = 190 @ 5300RPM
TQ = 210 @ 3950RPM

Weight around 2650.

Sounds more fun to me. :-)



Grant Hughes
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john vanlandingham
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Re: Comparing 2 cars quantitatively with some known variables...
February 14, 2016 12:26AM
Quote
Anders Green
Here's the data I have:

they one you can build fastest and care about denting less will go significantly faster. winking smiley

Both of those cars have essentially zero power. *grin* So the lighter one will go faster. Build it and stop worrying. Worry about driving more.

Anders

so solly lound eye. more torque times more gear times more final drive is much more powerz multiplied than the little 16% weight difference.

But my first impression was "neither car could be described as quick, so the question is moot. Were the question, which of these two cars would be less agonizingly glacial than the other" then maybe we could look at the datat..

I wonder what the whole point of this fap-tastic fap-tha-alon is?



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turoc
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Re: Comparing 2 cars quantitatively with some known variables...
February 14, 2016 09:14PM
Quote
NoCoast
Gear Ratios:
1st = 4.2
2nd = 2.49
3rd = 1.66
4th = 1.22
5th = 1.0
R&P = 4.1

HP = 190 @ 5300RPM
TQ = 210 @ 3950RPM

Weight around 2650.

Sounds more fun to me. :-)

Can you get em that low weight? That means mine can be a good 100-150lbs lighter smiling smiley



rally gods would turn in their graves if they ever knew Lada's were now part of EU rallying!!!
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Cosworth
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Re: Comparing 2 cars quantitatively with some known variables...
February 14, 2016 09:51PM
Unless you're choosing between rental cars at the airport, I would give preference to the vehicle that has the most aftermarket support and rally specific parts. Suspension, brakes, final drives, LSD diffs, engine, etc... And then the availability of spares.

But since its obvious car#2 is a 1.6L Fiesta, I would chose the Car#1.... because its obviously a 2L VW and it will make more sense than another $20k car in contention for last place.
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john vanlandingham
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Re: Comparing 2 cars quantitatively with some known variables...
February 14, 2016 10:23PM
Quote
turoc
Quote
NoCoast
Gear Ratios:
1st = 4.2
2nd = 2.49
3rd = 1.66
4th = 1.22
5th = 1.0
R&P = 4.1

HP = 190 @ 5300RPM
TQ = 210 @ 3950RPM

Weight around 2650.

Sounds more fun to me. :-)


Historically most Golves came in around 2450 with no effort. Now a few more tubes of questionable need and usefulness add some pounds but still...

Can you get em that low weight? That means mine can be a good 100-150lbs lighter smiling smiley



John Vanlandingham
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turoc
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Re: Comparing 2 cars quantitatively with some known variables...
February 14, 2016 10:35PM
Building a beem-er e-dirty, John
Grant was pointing out the e36



rally gods would turn in their graves if they ever knew Lada's were now part of EU rallying!!!
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john vanlandingham
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Re: Comparing 2 cars quantitatively with some known variables...
February 14, 2016 10:57PM
Quote
turoc
Building a beem-er e-dirty, John
Grant was pointing out the e36

Oh sorry, don't read what Grant writes anymore, he's far too hip and his experice is so much vaster than mine we just can't reach the same plane...
Weight isn't the problem with the Bayerisch schrott, getting anythiong vaguely near useful gearing in them is..
You can HEAR that more and more of the Finns are getting back to what we expect in gearing, closer ratios and short final drive...

What rear stuff you going to use? Like Dave Kern's? Seems safest. You have a bunch of diff stub shafts? The output flage thing? Word is the kill the spines and twist off pretty routinely...
On guys that actually have competitive times in ya know rallies.



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NoCoast
Grant Hughes
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Re: Comparing 2 cars quantitatively with some known variables...
February 15, 2016 12:34PM
Let's take a look at the venerable highly spoken of T-5 ratios.
2.95 1.94 1.34 1.00 0.63
We can ignore 5th gear because it's nearly useless except for maybe putzing around on the highway on transits. Back in the 80s it might have been nice to have that kinda gear for getting up into the mountains to "practice" but I myself am in my 30s with two kids and businesses and houses and such and am not willing to risk that kinda liabilty not to mention the militarization of police that could easily led to a practicing rally driver getting shot.
So,
2.95 1.94 1.34 1.00
Since anyone using is probably driving a Mustang, Volvo, or XR4ti with a Supra diff, we can say that you'll probably have a 4.56 final drive.
So effective ratios of
13.452, 8.846, 6.11 and 4.56

Now let's look at the BMW ratios:
4.2 2.49 1.66 1.22 1.0
Much like 5th was pretty much useless, if you have a decent final drive, say the 4.1 that are pretty dang easy to find, I've got a few, never paid more than $65 for them, then first gear is a trailer loading gear.
10.209, 6.806, 5.02 and 4.10

Guess what aftermarket dogbox 1-4 ratio commonly used in these cars in Finland and sold by Sellholm is VERY close to what the 2-5 ratio? Drenth MPG.

The stub shafts are a service item. Replace them every few events, no biggie. All cars have routine maintenance if you are racing them competitively.

Everything you need to rally them is available off the shelf. We are working on domestic production of some of the things needed to avoid needing to order from overseas. The amount of them raced and racing is huge so there is massive aftermarket support for them, anything you might need you can find and get quickly typically from nice engine mounts to upgraded water pumps to lightweight steel flywheels.
They are a dickbag lawyer or day trader car instead of a hemp bag toting lesbian mobile though. At least you won't puke in your mouth a little bit every time you see the car like some 80s cars.



Grant Hughes
http://rally.build
Denver, CO
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john vanlandingham
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Re: Comparing 2 cars quantitatively with some known variables...
February 15, 2016 12:58PM
Quote
NoCoast
snip--ranting

too bad you make such wrong presumptions Grant, and are so obnoxious about it while you're at it..

Wrong first presumption..final drive.

And IF you had ever done more than 3 shitty little rallies you would have encountered transits up to 60 or more miles. Then the 5th gear is not as you snottily dismiss it "useless' , it is perfect for cruising along.

Turn down your snottiness, Grant..

You don't take the car out to play because you don't WANT TO drive, simple as that.

And you push the BMWs because you obviously want to be the King of BMWs...



John Vanlandingham
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