Process Driveline Loss Assumptions

[tnord]

How do we know we're right on this?

I have recently learned that when the ITAC uses "what they know" to change the multiplier to a number other than 25%, they use the assumption of 15% driveline loss in a FWD car and 18% in a RWD car. Where did these numbers come from? How have they been verified accurate?

It makes a big difference in the minimum weight of a specific vehicle, and sure fits with my belief that FWD cars are favored in ITA and down. I think the arguement is that there are more moving parts in the drivetrain of a RWD car and thus greater loss since the LSD and FD are all housed inside the transmission casing of a FWD car. I could argue that switching to better lubricants in the RWD car minimizes the loss moreso than switching in a FWD model.

if we take a hypothetical vehicle....

140hp * 1.25 * .85 = 148.75whp
140hp * 1.25 * .82 = 143.5whp
140hp * 1.3 * .82 = 149.24whp

140 * 1.25 * 14.5 = 2537.5
140 * 1.3 * 14.5 = 2639

you can see here that the ITACs assumption of driveline loss effectively adds 100lbs to all RWD vehicles if they examine a car based on "what they know." does that just not sit well with anyone else? maybe it's right, but i'd have to see some data to be convinced of such.

 

[Greg Amy]

The only significant different in driveline rotating mass - and thus the only difference in moving parts - in FWD vs RWD is the longitudinal driveshaft. Everything else is pretty much the same. Especially given that this additional rotating mass is minimized and is most certainly made as small-a-diameter as possible to minimize rotational inertia (i.e., as one would do by lightening a flywheel) I suggest the 3% difference is more than adequate.

However, in the large-scale scheme of things (i.e., 30,000-foot view) having that (additional) mass centrally-located in the chassis (i.e., less frontward-biased) plus being able to split the driveline mass between front and rear (taken to extreme = Porsche 924/944 with the transaxle in back) is a pretty significant chassis performance advantage...

It's a wash.

 

[Greg Amy]

140 * 1.25 * 14.5 = 2537.5
140 * 1.3 * 14.5 = 2639

you can see here that the ITACs assumption of driveline loss effectively adds 100lbs to all RWD vehicles.

...and yer mixin' yer metaphors. If you want to compare FWD v RWD in a vacuum, you need to compare 85% versus 82% at one adder or the other, not by mixing both together:

FWD: 140hp * 1.25 * .85 x 14.5 = 2157
RWD: 140hp * 1.25 * .82 x 14.5 = 2081

Ergo, the RWD car actually gets a 76-pound weight ADVANTAGE, despite same stock horsepower and better chassis performance on so, so, SO many levels.

Still wanna go here...? - GA

 

[MMiskoe]

Not that I disagree w/ tGA but I am curious to see what people have done for actual confirmation of the percentages. Its a question that I have often asked, never heard a response.

My next question asks if it is in fact a linear progression as the motor makes more HP? Seems that the smaller motors are going to lose a bigger percentage than a more powerful one.

Take a 100hp motor losing 15% = 85whp. Now do all the tricks and the same motor is now making 200 hp. Is it now going to use up 30hp in the same driveline? Me thinks not. Does the 5000 top fuel dragster actually use 750hp to spin up the tires? If so, where is all that energy going?

 

[Ron Earp]

I wasn't aware that rear wheel or front wheel hp even figured into the process. As far as I knew it was based on a IT gain and target hp/weight for the class. I've never even heard wheel results come into it. Just basing this on my involvement with the ITR discussions.

Ron

 

[JeffYoung]

Driveline loss numbers are only required if we are trying to figure out the percentage gain in IT trim from a particular motor based on taking whp dyno sheets and comparing to stock crank hp figures.

 

[Greg Amy]

I am curious to see what people have done for actual confirmation of the percentages.

Likely not much. Those numbers have been around so long God-only-Knows where they came from or if they're accurate. Back in the Good Old Days you used an engine dyno as there weren't any chassis dynos; Today we use chassis dynos 'cause there ain't many engine dynos. It's a rare shop that has - and knows how to use - both.

That's unfortunate, because I - like Travis and Matt - would really like to know how they truly relate today. Absent that, we use the Old Wive's Tale "rule of thumb".

My next question asks if it is in fact a linear progression as the motor makes more HP? Seems that the smaller motors are going to lose a bigger percentage than a more powerful one.

Yes and no. These "losses" we try to accommodate are primarily due to friction, loss through gearing and/or direction changes (e.g., changing from longitudinal to transverse rotation), and the acceleration of the mass itself (which *should* be accommodated properly on a controlled dyno such as the Dynapack). As such, the drivetrain will absorb XX horsepower, thus using a higher-horsepower engine on the same drivetrain will absorb less ponies as a percentage.

However, it's on odd situation where engines of significantly-different torque will use the same drivetrain. Lower-torque engines will be mated to lighter-weight (less robust) components and will thus absorb lesser energy.

In the end we're back to that old "rule of thumb" thing.

GA

P.S. Ron, it was news to me to read this today as well, that different percentages are used; I'm assuming Travis has info this from a horse's mouth...

 

[tnord]

The only significant different in driveline rotating mass - and thus the only difference in moving parts - in FWD vs RWD is the longitudinal driveshaft. Everything else is pretty much the same. Especially given that this additional rotating mass is minimized and is most certainly made as small-a-diameter as possible to minimize rotational inertia (i.e., as one would do by lightening a flywheel) I suggest the 3% difference is more than adequate.

so what you're saying is that in the instance of this hypothetical vehicle, the presumed 5hp disadvantage from driveline loss is greater than the actual disadvantage in practice?

However, in the large-scale scheme of things (i.e., 30,000-foot view) having that (additional) mass centrally-located in the chassis (i.e., less frontward-biased) plus being able to split the driveline mass between front and rear (taken to extreme = Porsche 924/944 with the transaxle in back) is a pretty significant chassis performance advantage...

that is already accounted for in the 100lb weight break the FWD guys get, it has nothing to do with power numbers.

 

[Knestis]

As Jeff mentions, this isn't an aspect of the typical classification/specification process.

It *might* be part of a conversation if evidence for considering nonstandard power multipliers is in play, and in that instance would be put into the mix re: how confident each member is in the entirety of the evidence examined. There's no way that a few percentage points one way or the other in this math will bear DIRECTLY on a weight specification.

But recognize that these cases represent only a very small portion of our work.

K

 

[Greg Amy]

so what you're saying is that in the instance of this hypothetical vehicle, the presumed 5hp disadvantage from driveline loss is greater than the actual disadvantage in practice?

That is my assumption. IMO, a "theoretical" -5hp - especially when multiplied across our per-class factors to something around 75 pounds in ITA - is an advantage.

...that is already accounted for in the 100lb weight break the FWD guys get, it has nothing to do with power numbers.

I agree: it has nothing to do with the power numbers, and is a non-sequitor to your original question.

However, one cannot look upon this in a vacuum (thus my "30,000 foot" comment), especially when trying to equalize performance with these numbers. It's not just a (presumed) 3% greater loss in applied torque/hp; with that RWD "disadvantage" comes a buttload of advantages that FWD doesn't have (e.g., better chassis balance in corners, better ability to apply power under acceleration, better balance for braking, etc). I'd say it's a more-than-fair trade-off...

But, to your original question: I don't think anyone can say with much confidence that these multipliers are wholly accurate; they're just rules of thumb, presumably based on SOMEONE'S prior experience (certainly not mine; I haven't used an engine dyno in 25 years, and we didn't have access to a chassis dyno at the time...shit, I don't even know if they even EXISTED then... )

Side note: I tend to agree with you that FWD *can* be an advantage in the lowest-hp cars; thus our long diatribe(s) of a few months ago. I'm not convinced about ITA, however; I think we've reached whp values in that class (certainly at the pointy end) to where FWD is at a goodly DISadvantage.

GA

 

[tnord]

...and yer mixin' yer metaphors. If you want to compare FWD v RWD in a vacuum, you need to compare 85% versus 82% at one adder or the other, not by mixing both together:

FWD: 140hp * 1.25 * .85 x 14.5 = 2157
RWD: 140hp * 1.25 * .82 x 14.5 = 2081

Ergo, the RWD car actually gets a 76-pound weight ADVANTAGE, despite same stock horsepower and better chassis performance on so, so, SO many levels.

Still wanna go here...? - GA

yup.

i'm not very eloquent, but the maybe i can illustrate my point with a better example.

the driveline loss number is not used when calculating the initial spec line of a vehicle for classification. it is as you all thought, 140 * 1.25 * 14.5 = 2537.5lbs

it's when the car is "reviewed" for various power multipliers this issue shows up. to determine "what we know" multiple dyno sheets should be considered, which as you point out is 95% of the time on a chassis dyno.

the process tells us that this hypothetical car should make 175hp at the crank, but all that's being considered for data is wheel hp. so the standard number that is used to get back to that crank hp is the assumed driveline loss; 18% for RWD and 15% for FWD.

in this case, we should find that the 175hp would make 143.5hp at the wheels if the assumed 18% is correct. if the ITAC has some sheet in their hands that says the car makes 149hp at the wheels instead, guess what happens? it'll get run through the process at 30% rather than 25% because 140 * 1.3 * .82 = 149whp. but coincidentally, 175 * .85 = 149whp also.

so which is right? does a rwd car REALLY lose 18% in IT trim, or is it more like 15% and pretty dang close to what a FWD car loses?

 

[tnord]

However, one cannot look upon this in a vacuum (thus my "30,000 foot" comment), especially when trying to equalize performance with these numbers. It's not just a (presumed) 3% greater loss in applied torque/hp; with that RWD "disadvantage" comes a buttload of advantages that FWD doesn't have (e.g., better chassis balance in corners, better ability to apply power under acceleration, better balance for braking, etc). I'd say it's a more-than-fair trade-off...

sounds like this is your way of classifying cars on aspects other than what is explicitly included in the process, IN ADDITION to the 100lb gift you're already getting (and factored into this lapsim data you're pushing so hard to use for an increase in that weight break). all of those reasons you say RWD is better should be considered for the arbitrary weight break and that factor only. it really needs to be left out of this conversation.

But, to your original question: I don't think anyone can say with much confidence that these multipliers are wholly accurate; they're just rules of thumb, presumably based on SOMEONE'S prior experience (certainly not mine; I haven't used an engine dyno in 25 years, and we didn't have access to a chassis dyno at the time...shit, I don't even know if they even EXISTED then... )

i agree, but from where i sit it looks like a major factor in applying "what we know."

 

[tnord]

It *might* be part of a conversation if evidence for considering nonstandard power multipliers is in play, and in that instance would be put into the mix re: how confident each member is in the entirety of the evidence examined. There's no way that a few percentage points one way or the other in this math will bear DIRECTLY on a weight specification.

this seems contradictory to me. on one hand -

evidence for considering nonstandard power multipliers

but on the other.....

There's no way that a few percentage points one way or the other in this math will bear DIRECTLY on a weight specification.

i beg to differ. i'm pretty sure i just showed how a few percentage points change the weight of a car by ~100lbs. 100lbs of error is an amount the membership has already shown there is no tolerance for by the reaction to the +/- threshhold.

 

[Ron Earp]

IN ADDITION to the 100lb gift you're already getting (and factored into this lapsim data you're pushing so hard to use for an increase in that weight break).

Good lord, I hope we're not trying to set weight breaks in IT with a simulator. The top built FWD cars I raced against in ITS at VIR didn't need a weight break. Wait, those are on track results, we can't use those. But can we use results from someone's simulator? Really?

I digress. Back to your regularly scheduled program.

 

[tnord]

Good lord, I hope we're not trying to set weight breaks in IT with a simulator. The top built FWD cars I raced against in ITS at VIR didn't need a weight break. Wait, those are on track results, we can't use those. But can we use results from someone's simulator? Really?

I digress. Back to your regularly scheduled program.

oh don't worry, it's getting serious consideration.

 

[Knestis]

I guess what I'm not explaining is the thinking behind my putting DIRECTLY in caps.

Travis describes a process where we would do concrete math based on those percentage assumptions, translating those figures "directly" into a nonstandard multiplier: We don't. It's just one part of the entirety of the "evidence" considered, the result of which is a final decision on a power factor. Even plus or minus 5% on that single factor gets pretty much lost in the conversation.

I feel the need to reiterate that we're talking about what is just about the ONLY point now, where subjectivity can be brought to bear in "the process." I am on record as not being in love with that whole concept but I have grown WAY more comfortable with it as we've installed checks and balances, and documentation into our practices. I'm (truly) afraid that we DO have to use our judgment sometimes.

(Geez - Did I actually say that?)

K

 

[Knestis]

Good lord, I hope we're not trying to set weight breaks in IT with a simulator. The top built FWD cars I raced against in ITS at VIR didn't need a weight break. Wait, those are on track results, we can't use those. But can we use results from someone's simulator? Really?

I digress. Back to your regularly scheduled program.

No, no, NO. Please don't spread panic.

We used information gleaned from some Lapsim runs to help finalize current FWD subtractors. Our other option was to POOMA them, and we collectively decided we way less confidence in that option.

K

 

[Greg Amy]

so which is right? does a rwd car REALLY lose 18% in IT trim, or is it more like 15% and pretty dang close to what a FWD car loses?

Dunno. IMO they're not going to be significantly different. In point of fact, I have done all of expectations, calculations, and requests based on an equalized 15% for all platforms.

HOWEVER, having RWD at 18% instead of 15% is actually an advantage when back-calculating for the purposes of the "what we know" adjusters. Example: assuming your 1.8L Miata puts out 130 whp on a Dynapack; if we were to assume 15% then we'd back-calc that to an assumed 152.9 crank hp (130 divided by 0.85). However, if we assumed 18% instead then we'd back-calc that 130 to 158.5 crank hp (130 divided by 0.82) and adjust your process weight accordingly higher.

So, regardless of the 15/18% thing is accurate or not, using 18% instead of 15% for your RWD is yet ANOTHER advantage for you...so, honestly, I'm not clear as to what your concern is (from a personal level).

If it were up to me, we'd use 15% for everything (which is what I thought we were doing...)

...this lapsim data you're pushing so hard to use for an increase in that weight break...

To be clear, go back and read that stuff; I wasn't The LapSim guy. I made some FWD/RWD comparison predictions based on my years of experience, and someone else came up with Lapsim data that happened to agree with me. Were it up to me, we'd use GregSim instead...

Good lord, I hope we're not trying to set weight breaks in IT with a simulator.

As I understand it (I'm not involved in the process), we're using sim to support contentions about RWD v FWD weight "adders/subtractors". Nothing more/less.

GA

 

[tnord]

I guess what I'm not explaining is the thinking behind my putting DIRECTLY in caps.

Travis describes a process where we would do concrete math based on those percentage assumptions, translating those figures "directly" into a nonstandard multiplier: We don't. It's just one part of the entirety of the "evidence" considered, the result of which is a final decision on a power factor. Even plus or minus 5% on that single factor gets pretty much lost in the conversation.

I feel the need to reiterate that we're talking about what is just about the ONLY point now, where subjectivity can be brought to bear in "the process." I am on record as not being in love with that whole concept but I have grown WAY more comfortable with it as we've installed checks and balances, and documentation into our practices. I'm (truly) afraid that we DO have to use our judgment sometimes.

(Geez - Did I actually say that?)

K

i guess i have a lot less confidence that the multiplier isn't directly calculted should it happen to fit the mold of what ITAC members, or the population as a whole wants to believe.

i agree that this boils down to a judgement issue, and i argue with myself all the time on if we're better off with or without it.

 

[Greg Amy]

HOWEVER, having RWD at 18% instead of 15% is actually an advantage when back-calculating for the purposes of the "what we know" adjusters.

Scratch that whole paragraph from above; I'm even confusing myself here...

18% is actually a DISADVANTAGE within the IT process for RWD. What you (Travis, and all other RWD'ers) want is to have everyone at 15%, that way when ACTUAL whp is back-calculated to chp, the numbers - and thus the percentage above Process - come out lower.

Example: 140 stock crank hp (chp) car. Standard process says the car "should" get 140*1.25 = 175 chp after IT mods. Car is dyno'd on a Dynapack; actual measured whp is 150.

Using 15%, calculated chp is 150/0.85= 176.5 = 100.8% of process = no "what we know" adder.

Using 18%, calculated chp is 150/0.82 = 183 = 104.5% of process = +5% "what we know" adder.

So, is there actually a difference in drivetrain losses between FWD and RWD? If so, as that percentage-number-used grows the disadvantage (in terms of The Process) for RWD'ers grows. On the other hand, if there truly is a larger disparity than what we're actually using (spnkzss mentions 15/20% for drag racing, another number I've heard tossed around) then the larger the actual disadvantage for FWD cars.

I'll toss in the simplicity bone and contend that there is no significant difference in drivetrain loss between FWD and RWD, and thus 15% should be used for all cars in IT.

Did I get it right this time...? - GA