SM Build: Attempt at a Front Running Car for Under $15k all-in

Decisions, decisions.  Should I work on the cage or the engine tonight?  I'm thinking cage is a better choice.

Cage when your full of pi$$ and vinegar and engine when otherwise. On the other hand you have all the stuff required to do/finish the cage as in no lead time required, with the engine I'm sure lead time required to have machining completed.  

All, can we please be done with any posts related to 'dollars spent' for a while please?  As much fun as it was to rile everybody up, I think it's derailing this thread and taking it somewhere I don't want it to go. 

 

Much appreciated!

 

 

Decisions, decisions.  Should I work on the cage or the engine tonight?  I'm thinking cage is a better choice.

Cage, it sounds warmer.

Cage when your full of pi$$ and vinegar and engine when otherwise. On the other hand you have all the stuff required to do/finish the cage as in no lead time required, with the engine I'm sure lead time required to have machining completed.

 

 

Cage, it sounds warmer.

 

And I'll get a free workout pulling the handle on the tubing bender    Roll cage it is then.

Let's take a look at how wheel mass affects acceleration.  In SM, we are limited to a minimum of 13lb wheels.  How much would it slow us down if we used 16lb wheels?  Let's find out.

 

• The power being absorbed by a rotating and accelerating object is equal to its moment of inertia multiplied by its rotational speed multiplied by its rotational acceleration.  Power=Inertia*alpha*omega
• This quantity of absorbed power is power that doesn't make it to the ground, thus reducing the vehicle's forward acceleration ability
• How much power is actually absorbed?
• How much does that lost power affect acceleration?

 

-I don't have moment of inertia data for an actual wheel, but we can get close enough by calculating it based on the geometry of the wheel and it's mass.  We can assume the wheel to be a combination of a disk and a hoop, with 65% of the mass in the hoop. 

-For the 13 and 16 pound cases, vehicle ballast is adjusted to the car's total mass stays the same, just the wheels are different.  Just like how it would be in real life.

 

 

A copy of the code I wrote to calculate the inertial power being absorbed by a wheel.  It uses a simple model to predict acceleration of a miata for 25 seconds starting from 45mph. 

 

 

 

 

 

Here are the results!  From the plot below, you can see that the absorbed power reduces as speed increases.  This is because even though rotational speed is increasing, the acceleration is decreasing more quickly.   Magnitude wise, we are talking about HP of between 0.2 and 0.7 for the 13lb wheel setup (these numbers are for all four wheels combined).  So, when we swap over to the 16lb wheel setup, we see a slight increase of absorbed power, in the 0.1 hp range.  In fact, this ratio of the power for the two cases is equal to ratio of the wheel moments of inertia.

 

 

Ok, so there is a bit of difference in hp that makes it to the ground for the two cases.  How much does this affect the car's acceleration?  For this case, the two cars both started at 45mph and accelerated for 25 seconds.  Final speed was 119mph for both of them.  The 16lb wheel car traveled a distance of 3462.6 feet.  The 13lb wheel car traveled 3463.8 feet.  So, the 13lb wheel setup has a 1.2 foot advantage.  Pretty negligible!  For reference, adding 5hp to the car for the same test resulted in an extra 52 feet of distance traveled, or about 4 car lengths.

How are these numbers affected with going up a hill?

How are these numbers affected with going up a hill?

 

 

Slower change of velocity means less power lost to inertia.  So going up a hill brings the curves closer.

Let's take a look at how wheel mass affects acceleration.  In SM, we are limited to a minimum of 13lb wheels.  How much would it slow us down if we used 16lb wheels?  Let's find out.

• The power being absorbed by a rotating and accelerating object is equal to its moment of inertia multiplied by its rotational speed multiplied by its rotational acceleration.  Power=Inertia*alpha*omega
• This quantity of absorbed power is power that doesn't make it to the ground, thus reducing the vehicle's forward acceleration ability
• How much power is actually absorbed?
• How much does that lost power affect acceleration?

-I don't have moment of inertia data for an actual wheel, but we can get close enough by calculating it based on the geometry of the wheel and it's mass.  We can assume the wheel to be a combination of a disk and a hoop, with 65% of the mass in the hoop. 
-For the 13 and 16 pound cases, vehicle ballast is adjusted to the car's total mass stays the same, just the wheels are different.  Just like how it would be in real life.
 
 
A copy of the code I wrote to calculate the inertial power being absorbed by a wheel.  It uses a simple model to predict acceleration of a miata for 25 seconds starting from 45mph. 

 
 
 
 
 
Here are the results!  From the plot below, you can see that the absorbed power reduces as speed increases.  This is because even though rotational speed is increasing, the acceleration is decreasing more quickly.   Magnitude wise, we are talking about HP of between 0.2 and 0.7 for the 13lb wheel setup (these numbers are for all four wheels combined).  So, when we swap over to the 16lb wheel setup, we see a slight increase of absorbed power, in the 0.1 hp range.  In fact, this ratio of the power for the two cases is equal to ratio of the wheel moments of inertia.
 
 
Ok, so there is a bit of difference in hp that makes it to the ground for the two cases.  How much does this affect the car's acceleration?  For this case, the two cars both started at 45mph and accelerated for 25 seconds.  Final speed was 119mph for both of them.  The 16lb wheel car traveled a distance of 3462.6 feet.  The 13lb wheel car traveled 3463.8 feet.  So, the 13lb wheel setup has a 1.2 foot advantage.  Pretty negligible!  For reference, adding 5hp to the car for the same test resulted in an extra 52 feet of distance traveled, or about 4 car lengths.

Thanks
That is less than I thought.. when we put the rains on, they seem way slower on the straights than dries.. Now I am going to have to go weight them

Thanks
That is less than I thought.. when we put the rains on, they seem way slower on the straights than dries.. Now I am going to have to go weight them

 

Yup.  The rain tires also weigh a decent bit more, and that weight is located at a much larger diameter (tire OD) which raises the moment of inertia of the wheel/tire combination a significant bit.  Maybe tomorrow I'll run the difference between dry and rain tires.  Thanks for the idea 

 

Edit, the rains are also 0.7" bigger diameter according to Hoosier.  While that doesn't affect acceleration a whole lot, it does make the car feel slower as it reduces RPM for a given speed and pushes shift points farther down the track, etc. 

And I'll get a free workout pulling the handle on the tubing bender    Roll cage it is then.

 

That sounds medieval...

 

 

With 1.5 x .095  hand pull is not that bad.  When you start looking at 2 x .120 4130 it's a bit more of a heft.  The pic shows 1 x .065, you could bend that with bare hands and pliers.    On my first cage I hand pulled the main hoop then got the ram and pump.  Swag Off Road has a neat little conversion kit (he didn't have it when I got mine 5-6 years ago) that uses an HF ram. http://www.swagoffro...Mount_p_53.html   If  your buddy is looking to upgrade his rig the Swag with the HF ram is a couple hundred.  I got the SPX pump used on ebay  and the ram from another guy new and a new hose for under $500.  These days new it's about $950.

 

What are you going to use to notch it?

And cage progress is going forward at what rate, stagnant because of extracurricular activity, still in acceleration or reached peak and leveled off because you've over stressed your self?

 

EDIT:

Dave, Jayson is a young buck, don't need no stinking power other than Armstrong.

 

 

Dave, Jayson is a young buck, don't need no stinking power other than Armstrong.

 

I'd like to see the numbers, maybe he has a graph...

The power absorption calculation for the wheels, does that  .1HP solve for one wheel or all 4 with regards to the difference in acceleration of the car?

Your estimate of 65% in the hoop sounds high for a 7x15, but still your calculations come up with very low values, even lower than I might have guessed off the cuff. You probably came in too late for the latest flywheel weight debate, but suffice to say that I was on the side doing the calcs to prove that the impact was FAR below common expectations, and this is further support of that. The difference from a reduction of 12lbs at the wheels is lost in the noise. Of course heavier wheels also sightly Impact braking, and the unsprung weight impacts handling, but in terms of forward acceleration the difference is near zero.

I'd like to see the numbers, maybe he has a graph...

Something tells me he might

The power absorption calculation for the wheels, does that  .1HP solve for one wheel or all 4 with regards to the difference in acceleration of the car?

That's for all four wheels combined.

 

 

 

Your estimate of 65% in the hoop sounds high for a 7x15, but still your calculations come up with very low values, even lower than I might have guessed off the cuff. You probably came in too late for the latest flywheel weight debate, but suffice to say that I was on the side doing the calcs to prove that the impact was FAR below common expectations, and this is further support of that. The difference from a reduction of 12lbs at the wheels is lost in the noise. Of course heavier wheels also sightly Impact braking, and the unsprung weight impacts handling, but in terms of forward acceleration the difference is near zero.

Yeah I erred on high side for hoop weight, that way the numbers for sure aren't underestimated.

 

Flywheels.  So the flywheel actually can have significant power absorption.  P=I*a*w.  Depending on which trans gear you're in, the flywheel spins significantly higher rpm than wheel rpm.  5.7 times faster in 3rd gear for example.  Likewise the fylwheel acceleration rate is also 5.7 times higher.  Therefore the flywheel absorbed power (for the a given inertia) matters more by a factor of about 32 for 3rd gear.  It really adds up fast.  Guys that do autocross or standing starts need to pay attention even more so, in first gear the overall drive ratio is 13.5, so the factor is 13.5^2, or 182. Plus the acceleration rate is even higher yet, because in first geat, the vehicle acceleration rate is much higher than it is at greater vehicle speeds.

 

But the power that the flywheel is absorbing isn't going away, per say.  It's being stored as kinetic energy in the flywheel.  So the question is, when you make a very fast shift and drop the clutch out, how much of that stored inertial energy is dumped through the clutch back into the drivetrain providing motive force?  Probably a decent bit.  So, for the flywheel case, maybe or maybe it doesn't matter as much???

 

 

 

 

Something tells me he might

Haha, I like graphs.  I could make a graph for just about anything 

You have recapped the basics of the FW debate, the key being that unlike drag racing and auto-x we spend very little time in the lower 2 gears and accelerate very slowly above that. Bottom line being that the cheap/easy option of removing a few pounds was so little benefit that we didn't want it mistaken for being the solution to "the torque problem".

Jim, checked the rain tires.  Didn't have a weight for them, so I made up that they are 4 lbs heavier.  They are also 23.5" in diameter instead of 22.8".  Given weight assumption, the rain tires absorb a full 1hp more inertial power than dry tires at slow speeds like 45mph, and that drops to a half hp by 115mph.  So I could see them feeling slower, with the ~0.75hp loss combined with the shift points being delayed by the larger tire diameter.  At 100mph, solely from the tire diameter effect, you shift 1.3 seconds and nearly 200 feet later!!

Jim, checked the rain tires.  Didn't have a weight for them, so I made up that they are 4 lbs heavier.  They are also 23.5" in diameter instead of 22.8".  Given weight assumption, the rain tires absorb a full 1hp more inertial power than dry tires at slow speeds like 45mph, and that drops to a half hp by 115mph.  So I could see them feeling slower, with the ~3/4hp loss combined with the shift points being delayed by the larger tire diameter.  At 100mph, solely from the tire diameter effect, you shift 1.3 seconds and nearly 200 feet later!!

They are considerably softer and have a much higher rolling resistance, if you add a static friction delta in the equation you should see even more of a difference.  

That's for all four wheels combined.

 

 

 

Yeah I erred on high side for hoop weight, that way the numbers for sure aren't underestimated.

 

Flywheels.  So the flywheel actually can have significant power absorption.  P=I*a*w.  Depending on which trans gear you're in, the flywheel spins significantly higher rpm than wheel rpm.  5.7 times faster in 3rd gear for example.  Likewise the fylwheel acceleration rate is also 5.7 times higher.  Therefore the flywheel absorbed power (for the a given inertia) matters more by a factor of about 32 for 3rd gear.  It really adds up fast.  Guys that do autocross or standing starts need to pay attention even more so, in first gear the overall drive ratio is 13.5, so the factor is 13.5^2, or 182. Plus the acceleration rate is even higher yet, because in first geat, the vehicle acceleration rate is much higher than it is at greater vehicle speeds.

 

But the power that the flywheel is absorbing isn't going away, per say.  It's being stored as kinetic energy in the flywheel.  So the question is, when you make a very fast shift and drop the clutch out, how much of that stored inertial energy is dumped through the clutch back into the drivetrain providing motive force?  Probably a decent bit.  So, for the flywheel case, maybe or maybe it doesn't matter as much???

 

 

 

 

Haha, I like graphs.  I could make a graph for just about anything 

 

This forum needs more engineers.

This forum needs more engineers.

We became engineers because we couldn't make a living racing.

Except MPR of course who is a real estate development guy who has been learning engineering words to use on this forum

CNJ