1001 replies to this topic

[Tom Hampton]

"3.8 HP seconds".  I like that unit. 

[Todd Green]

Incidentally, shaving your tires accomplishes something similar to shaving flywheels in the grand scheme of things... except it normally happens at lower rotational speeds.

 

It also marginally improves your gearing.  (My super-fast paper napkin math says its like going from a 4.3 to a 4.4x diff.)

[Steve Scheifler]

Posted in wrong topic...

[Tom Hampton]

True about tires, and driveshafts etc. But, anything after the trans is different in that it spins up much more gradually in direct proportion to car velocity so the time factor is amplified compared to the engine which changes speed relatively quickly and then repeats the toughest part for each gear. Not to say lighter wheels and tires don't count, they do. The diameter is large, which is a big factor, and of course there are four of them, but you can't directly compare them pound for pound with the flywheel. The other popular part to swap is the driveshaft. When you consider the diameter, weight, and being after the trans, it's hard to see why anyone would risk getting caught with a slightly lighter one.

 

Ok, I took the trouble to get a sense of scale on the tire/wheel combo.  The tires require ~4% of the total energy to accelerate the car (non-conservative forces still being ignored).  I had to make an assumption on the mass in the sidewall and tread.  I estimated 50% of the tire mass is in the tread.  The inertial moment of the tire doesn't change much as the distribution is varied, basically +/- 0.5% from 0% to 100% mass in the tread.  

 

So, everything scales by ~95% from above.  As Ralph points out, drag dominates the equation above 65 mph and will scale things down by 25% to 50% as speeds go up. 

 

All that is to say, that you would be lucky to see an improvement of 1 ft-lb of effective torque in second gear, and you can essentially divide that by 3 for every gear after that to account for gearing and drag factors. 

[Steve Scheifler]

Tom, excellent, but the latest numbers are per pound from the tread? (shouldn't matter much whether from one tire or a little off each, so per pound combined weight shaved from tread).

[Tom Hampton]

Steve-

 

I wasn't trying to compute anything related to tire-shaving.  I was simply calculating the moment of inertia for the wheel/tire combo and the kinetic energy stored in the assembly.  I'm modeling the wheel/tire assembly as three pieces, a single disk for the wheel, a separate disk for the sidewall, and a hoop for the tread.  In reality there is also a hoop for the wheel rim (versus the spokes), but I have no data to estimate the distribution of mass between the rim and the spokes, so I left that as a simple 13.1 lb disk.  I didn't want to do that for the tire, however, because the tread is at the outermost diameter, and likely had the largest inertial contribution. 

 

I initially calculated the moment using a 50/50 mass-split between the sidewall and the tread.  Then I recalculated at 0/100 and 100/0 splits to assess the sensitivity of the moment to variations in sidewall/tread mass split.  The Moment varies between 0.66 Kg*m2, and 0.88 kg*m2.  But, since the Kr equation is dominated by w2, the moment doesn't play that much of a role in the amount of energy stored in the wheel. 

 

So, the Kr stored varies between 3.2% and 4.3% of the total Vehicle kinetic energy gain in each gear depending on the exact weight distribution between sidewall and tread. 

[Brandon]

 

[Johnny D]

https://www.youtube....h?v=-FIMvSp01C8
J~

[Johnny D]

J~

[LarryKing]

So... will any of this have an influence on the SMAC's recommendations to the CRB. Two or the four "improvements" seem to be a whole lot of nothin (and the other two ain't much).

[Johnny D]

Well wasn't it a couple pages back someone said the SMAC ran the fly wheel idea past Mazda and they shot it down ?

I wasn't sure if you guys were running numbers for fun or for a aftermarket wheel or something.

 

Is the header a plug and play to the SD exhaust or a mod ?

Then does SD have development on their exhaust header combo or they good to go ?

J~

[Tom Hampton]

Well wasn't it a couple pages back someone said the SMAC ran the fly wheel idea past Mazda and they shot it down ?

I wasn't sure if you guys were running numbers for fun or for a aftermarket wheel or something.

 

Is the header a plug and play to the SD exhaust or a mod ?

Then does SD have development on their exhaust header combo or they good to go ?

J~

You're familiar with the back button and search function, right?

 

But, in short...that "someone" was Dave Wheeler, and he was referring to a time in the past when he was on the SMAC and, at that time, Mazda did say "no".  The current SMAC seems not have the same issue. 

 

However, they indicated that someone had "done the math" and that it wasn't "much".  But, they elected to refrain from publishing the "math", and decided to include it in the proposed package anyway. 

 

I wasn't going to bother until Steve aske me to, because others who I consider to be equally (or more) knowlegeble than I had already pointed out that the gain was negligible. 

[Johnny D]

The current SMAC seems not have the same issue.

Seems ??

Carry on.
J~

[Tom Hampton]

Seems ??

Carry on.
J~

You're familiar with the back button and search function, right?

[Johnny D]

You're familiar with the back button and search function, right?

Yes, what are you getting at ?

It's fine. I go from stupid posts to smart. (this may be a stupid one)
Free country to go from "The Moment varies between 0.66 Kg*m2, and 0.88 kg*m2" to "seems, hope, looks like SMAC goes along"
J~

[Steve Scheifler]

My understanding, as communicated to me by someone closer to it, is that Mazda folks decline to give a formal blessing to such things (you don't need to be a lawyer to figure out why) but that once you buy a part it is not their concern or liability if you modify it. That certainly makes sense, but I can also imagine them wording it a little stronger if such mod were to be "spec".

[RWP80000]

The attached pdf file is a just completed analysis relating to the affect of reducing the flywheel weight.  I have walked through the analysis in a top to bottom approach explaining  methodology, the assumption and reasons for my approach.I showed the engineering units involved as well so you can evaluate the mixing of apples and oranges.

 

This analysis should help those interested in understanding the application of the physics and engineering and the units are in lb-ft and horsepower.  I also incorporate some personal judgement and recommendations related to how this impacts an SM race car and I end up with a table that shows the variation in HP saved from 1 to 4 lbs with varying acceleration rates which represent what is happening using different gears although there are no specific transmission ratio related calculations.

 

In general, my conclusion is that a 2lb weight reduction is confirmed to be slightly helpful from a 1/6 to 1/3 Hp depending on acceleration rate but in reality I do not think anyone could sense the change.  If 4 lbs were capable it would double the benefit.  I also address the impact on the up-shifting condition and show how much different the HP numbers are but due to the time reference I would consider totally insignificant.

 

Rich Powers

 

 

https://www.dropbox....ations.pdf?dl=0

[38bfast]

Just for reference, Over shooting performance and then adding a plate generally will shift the entire curve down. Not sure how it will effect the 1.6 it has not been tested. adding weight to the car is almost equivalent to taking torque away until aero drag becomes the opposing factor. The opposite direction we are looking to fix. 

[Tom Hampton]

Yes, what are you getting at ?

It's fine. I go from stupid posts to smart. (this may be a stupid one)
Free country to go from "The Moment varies between 0.66 Kg*m2, and 0.88 kg*m2" to "seems, hope, looks like SMAC goes along"
J~

 

1.  Just giving you a hard time...as you are typically "the guy" who searches everything out when others ask questions, but couldn't be bothered for your own question.   

 

2.  I have no further information as to why the current SMAC doesn't think that the problem mentioned by Dave is no longer a concern.  They weren't precise, hence my usage of "seems".  Maybe they are sworn to secrecy or some such, idk.  At the end of the day, its not my problem how they got it past Mazda.  In fact, I don't even care if they did. 

[Johnny D]

Ok,

FYI, One more hurdle is, "is it good for the class" even after all the numbers work, most are happy, etc, etc.

Cost, Implementation, etc.

 

Clarity from Ralph, Todd, etc from time to time is always good.

 

I'm not trying to kill your ideas, just show it's difficult.

Carry on.

J~