FWD vs RWD: Adders, Subtractors, and Weight, Oh my...!

[Greg Amy]

I am sure that I am missing something but don't the numbers above show that the HP level DOESN'T matter THAT much?

Just to ensure you understand, the chart on the left is hp, chart of the right is weight. At 150hp (120 stock) the lap time difference (same weight) is 8/10 second per lap; at 200hp (160 stock), 1.2 seconds; at 250 hp (200), 1.6s.

Given equal 200 hp (160 stock) a 2500 pound FWD car would need a near 200# weight advantage to make equal lap times. And, this assumes a 60/40 balance; I'd *kill* to get my FWD car near that value (most are 80/20).

You don't consider that significant? I most certainly do.

But why not use weight? Keep in mind that since we set weights based on horsepower, we can only choose one variable from which to derive subtractors. It's my position that the base assumption from which everything else is derived, horsepower, should be the factor used.

That's some big numbers, Andy, and supports exactly what I've been trying to convince you of for a couple years...[/quote]

 

[Greg Amy]

Sorry for my ignorance but what is LapSim and what is tGA?

LapSim is a program written by Bosch some years ago, to simulate conditions and how car changes would affect lap times. It's a pretty g-d impressive program, but it's limited by, well, being a program. But I've heard that it's good enough to where some high-level teams have re-coded it with little changes for helping to setup cars such as BTCC and such. I've personally never had the time to deal with it (though the downloads have been sitting on my computer for years....)

"tGA" is an inside joke. Years ago I was sitting on the grid at HyperFest and someone came up to me and introduced themselves, asking if I was "THE Greg Amy?" Much to my misfortune, Joe DiMinno was standing there at the time, and I've never lived it down...the Sandbox got word of it and created the "tGA" moniker. - GA

 

[Knestis]

How does Lapsim work? What are the inputs? Does it have a built in modifier/handicap for FWD? Why do we consider this statistical significant when the lap time differences even in the high hp range are around a second?

And why are we considering using a simulator when we are already so distrustful of dyno sheets?

I suspect we could use that same simulator, with the appropriate human inputs, to show similar detriments due to poor aero, or brakes, etc. Do we want to go there?

I trust sim software because it just does math: It has no agenda and it doesn't let perceptions skew what it tells us. I think this has potential.

And again, I'm NOT advocating - nor will I seriously consider, knowing what I currently know - using more factors than we already have on the board. NOR am I suggesting that we run every car through the simulation app...

I'm simply saying that we can hone our current model, using exactly the factors we use now, to (1) take opportunities for subjectivity out of play, and (2) get a better degree of granularity for the factors that we agree matter.

K

 

[Knestis]

... My guess? FWD no longer becomes a factor around 100-ish hp, as long as the classified weight is also small; i.e., ITC territory. And, it really starts to make a difference around 150/160-ish stock hp. ...

That's about what my early morning napkin math told me, too...

Edit - and I agree that we should start with HP, since we derive weight from the same data. I completely accept that there are some inherent issues with starting with manufacturers' quoted power figures but I'm willing to trade them for consistency - except in the very special case of the rotaries.

KK

 

[JeffYoung]

I understand, but sim software is written by....people. Who have perceptions and agendas.

Do we know if this program already skews results against FWD cars? Meaning, it is set up to produce the results we "expected" to see?

I trust sim software because it just does math: It has no agenda and it doesn't let perceptions skew what it tells us. I think this has potential.

And again, I'm NOT advocating - nor will I seriously consider, knowing what I currently know - using more factors than we already have on the board. NOR am I suggesting that we run every car through the simulation app...

I'm simply saying that we can hone our current model, using exactly the factors we use now, to (1) take opportunities for subjectivity out of play, and (2) get a better degree of granularity for the factors that we agree matter.

K

 

[Greg Amy]

I understand, but sim software is written by....people. Who have perceptions and agendas. Do we know if this program already skews results against FWD cars?

But, who do you trust more: us, or Bosch engineers? Who is more motivated to produce consistent repeatable, accurate, objective results?

Hey, don't let the fact that it agrees with my position cloud your judgment...

 

[Ron Earp]

Thanks for the explanations Greg.

I don't know anything about LapSim but I'll accept that the program is good and folks in the industry use it to setup cars.

There are engine dyno programs that are also accepted in the engine building trade and used extensively to simulate and optimize engine builds. We can purchase them and they are not very expensive. Are we willing to use these programs to predict IT horsepower gains and use that information to set weights? The program will produce data that is in line with what we expect.

I imagine we are not ready for this step.

So why are we willing to accept the output of a car simulation program to modify our IT rules set? Because it produces what we expect or want to see? What if we find another simulation program that produced results that are not inline with what we expect?

Justing playing the bastard this morning. As mentioned, I know nothing of Lapsim.

 

[JeffYoung]

Bastard No. 2. here.

I just sent an e-mail to Bosch on this, as after reading the LapSim manual (which is extensive, and it seems like an amazing program), I see nothing that accounts for FWD v. RWD other than corner weights.

I am, admittedly, perplexed and amazed that we have gone from [stock hp X 1.25 X class multiplier] to doing what we are doing in this thread and in the specific output thread. I find so dangerous that I am astounded we are even discussing it.

The goal it seeks to obtain - perfect repeatability and accuracy and ultimately fairness - in IT car classing is not only unrealistic, it's impossible. and once we use Lapsim to set the FWD modifier, the next guy is going to use that program to argue why his wheel base, or his brakes, or his camshaft, or his aero, or any of the other inputs it allows justifies a modifier to his car.

THIS is the slippery slope. It sure as hell ain't washer bottles.

 

[Greg Amy]

Are we willing to use these programs to predict IT horsepower gains and use that information to set weights?

I'm not suggesting that you use a program to set the FWD subtractor (though, what is an Excel formula, other than a "program"?)

And...

I see nothing that accounts for FWD v. RWD other than corner weights.

Meaning, of course, that the handicap when one does consider the drive layout will be even worse...

...I find so dangerous that I am astounded we are even discussing it....THIS is the slippery slope. It sure as hell ain't washer bottles.

Jeff (et al), remember my original contention: that the FWD subtractors being used in IT are insufficient and do not accurately reflect the handicap that FWD carries versus RWD. All of the above discussion is nothing more than supporting opinion and evidence.

Again, as I noted to Ron, I'm not asking you to take a software program's output as an article of faith, I'm simply pointing out that I'm not standing here all alone, trying to convince you of my position solely on my own experience (as vast and all-conquering it may be ). I'm asking for consideration to rethink how you're doing this "FWD versus RWD thing" and open your mind to the possibility that it's misguided.

You asked for formula examples, you got 'em. You asked for experienced opinion(s), you got 'em. You asked for supporting evidence, you got 'em. If you truly are interested in reasonable parity between these dissimilar platforms, and you have not descended into an easily-inferred mindset of "we can never attain it thus we shan't try", then we can move forward on the concepts. Otherwise, if all I'm going to get is gruff and heel-digging every time I adequately answer your queries, well, then we're wasting our time here... - GA

 

[JeffYoung]

Greg, I'm listening and I appreciate your efforts.

I think my structural issue with this is being lost in the focus on FWD.

ALL IT cars have advantages and disadvantages. While I could not do it as well as you, I could give you numbers to support a performance disadvantage to a particular brake setup, and then I could use LapSim to support it.

I could then do that for wheelbase.

I cold then do that for aero.

Is that where we want to go? I FULLY ACCEPT that FWD is a disadvantage. But why are we choosing to address that one versus others?

I am slowly moving back to just using stock hp and power multipliers and dump ALL of the other subjective factors because there is no rational means of sorting through which ones we should account for and which ones we shouldn't.

 

[Greg Amy]

Is that where we want to go? I FULLY ACCEPT that FWD is a disadvantage. But why are we choosing to address that one versus others?

Simple: because front-wheel-drive is far more significant (versus rear-wheel drive) in terms of performance than any other factor you note. In fact, I'd suggest other than horsepower and weight, nothing else really matters if the drive layout isn't "right"... - GA

 

[dj10]

"I FULLY ACCEPT that FWD is a disadvantage. "

Not necessarily in the shorter races we run.
Why are the ST FWD dominating in Grand AM?

 

[JeffYoung]

That'd you need to support with numbers, and it can't be just one lap -- the FWD "disadvantage" appears to me to manifest itself over time as the tires can't handle the extra heat generated by the extra work.

I don't buy that the "FWD" disadvantage is "worse" than a 240z or yes, my car, that basically gets 3-4 laps with brakes then work and then after that it is a nightmare. I've also seen 1 second or more differences per lap due to aero differences.

And on edit, I think we need to ditch all of the adders/subtractors except torque, which makes the process work for cars like mine since it would be far too light otherwise. We should avoid this discussion on suspension type, brakes, aero, chassis, etc.

 

[ekim952522000]

I think the process should be simplified as well, I think all you need is.

FWD subtractor
TQ adder

To me the whole Double wishbone adder/strut subtractor is silly because there are good and bad designs of both.

 

[Ron Earp]

I don't buy that the "FWD" disadvantage is "worse" than a 240z or yes, my car, that basically gets 3-4 laps with brakes then work and then after that it is a nightmare. I've also seen 1 second or more differences per lap due to aero differences.

The difference is that you, I, or the 240Z pilot will be told

1) You aren't doing it right, you must manage your brakes or set your car up better.

2) Your brake ducts and cooling apparatus is not good enough.

3) You should have picked a different car to race. There is no guarantee of competitiveness in IT.

Now why we discuss dropping weight instead of options 1-3 when we talk about FWD cars, well, I'm not sure. I suppose because this thread is all about dropping weight and how much.

 

[trhoppe]

Wait, while we're here and running LapSim, plug in some numbers from the classes right now and see where we end up in lap times

 

[quadzjr]

I know that we have kinda got off the subject a bit about Lap Sim, but what was mentioned before.. but the human interface while working with the program can easily change the outputs. It is the same reason why many people don't trust dyno sheets (including myself). Hell just adding or removing fans in front of the car will give noticable results on the dyno.. not to mention the "human interace" with the dyno program setting it up". Ambent air temp, density, and simple water temp calibrations can make huge differences.

This discussion seems to have been very biased. Ican understand that everybody is looking out for themselves atleast a little bit. Or if you don't currently campaign a car you know your best friend drives xxx or you plan on getting into yyy and you don't want to handicap them or you. Is it possilbe to find a third party that is completely un-biased?

 

[Greg Amy]

Now why we discuss dropping weight instead of options 1-3 when we talk about FWD cars, well, I'm not sure.

Taking that logic to its extreme, Ron, why should there should be ANY differential between the cars, even one based on horsepower? Set a minimum weight per class (or, hell, maybe not) and "run whatcha brung".

Hey, after all, "no guarantee of competitiveness", right...?

But we don't do that, do we? No, we don't, because we have at least a passing desire to create some semblance of parity, and we recognize that there are some macro factors, such as horsepower, that really do make a difference.

And that's my whole point: second only to ponies, the drive layout is the most important factor affecting performance in otherwise-similar cars. Yet we handle the layout as nothing more than a quickie afterthought, when in fact, it has nearly as much to do with results as does the engine...

Whatever, guys. I seem to be repeating myself, becoming redundant, over and over again, constantly saying the same things. And, maybe not so coincidentally, I seem to be doing it mostly to members of the ITAC. I've said my piece; the present members of the ITAC are either convinced or not that this is something that should be addressed. If that's "yes" then let me know and I'll be glad to work with you on it. If no, then "thanks for your input."

GA

 

[madrabbit15]

Reading through this, this is all interesting stuff.

I read through all of this rather quickly but I did not see where anyone mentioned the FWD advantage. FWD powertrain losses are generally much less than that of a RWD powertrain. Powertrain loss on a FWD car may be 10-12% vs. 18 to 20% on a rwd car. Obviously varying from make to make. I think some with dyno experience with mulitple cars can give you more accurate info, this is just what I have always heard. You are changing the direction of the power and it creates more a loss, not to mention more moving parts.

I think this needs to be included in any calculations where we are looking at stock hp vs. improved hp and weight. Your thoughts?



Derek

 

[jrvisual]

Bastard #3 here

tGA,
If the current subtractor is so far off how is the you currently hold at least 3 track records?