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

[Greg Amy]

Over the last few months, there have been thread hijacks in regards to the adders/subtractors for FWD vs RWD car. I wanted to create this topic to bring the idea to the fore on its own.

To summarize, there are subjective class-specific "subtractors" applied to all FWD cars. Would someone in the ITAC please post exactly what those are? As I recall the value for ITS and ITR cars is 100#; is it 50# for the rest? The outstanding question is, are these numbers reasonable, and are they sound?

So, why a "subtractor" for front-wheel-drive (FWD)? In racing, FWD carries with it several disadvantages. First, because of mechanical design (the drivetrain is almost exclusively transversely-mounted, ahead of the front wheels) there is a significant frontward weight bias. In most cases, as much as 80% or more of the weight of the car is on the front wheels. On the other hand, with longitudinal mounting and the axle assembly in the back (and in some limited cases, even the transaxle in the back and possibly the engine mounted front-midships) a rear-wheel-drive (RWD) car has a much better weight distribution. As a result, you can easily imagine that the two front tires in a FWD car will be called upon to do most of the sideload (g-forces) work around a corner, whereas in the RWD the loads are more evenly distributed.

Second, in both a FWD and RWD car, the front wheels steer the car via higher slip angle (i.e., the front wheels turn, the rear don't). Therefore, in a FWD car not only do the front tires have to deal with a lot of the side loads around the corner, they also have higher slip angles to steer the car.

Third, obviously a FWD car has to apply forward acceleration force through the front tires to accelerate the car, whereas the RWD car applies forward acceleration force through the rear tires. So to make matters worse, not only does the FWD car have higher side loads on the front tires, and has to steer the car with a higher slip angle, it also need to apply forward propulsion!

Fourth, weight transfer. As a RWD'er is applying that acceleration force, g-forces are transferring weight onto the driven wheels just as they need that traction, and off of the steering wheels just as they are no logner needed. At the same time, the FWD'er is applying acceleration forces and transferring weight off of the only tires that are doing anything!

That's a LOT to ask for two small patches of rubber. And through all this, the rear tires are wondering "uh, what am I supposed to be doing?" It's been said - I think Kal Showket was the first to quip this - that there's only two purposes for the rear tires on a FWD car: to make it look good on a used car lot, and to keep the fuel tank from sparking on the ground while it's driving...

If you can imagine all this, then you can imagine why racing a FWD car seems an exercise in futility; it's almost like racing with only two tires instead of four (and in pretty much all cases of FWD prep we intentionally destroy the rear tire grip in order to offer some semblance of handling balance). There's only so much that a pair of tires can do. Yet we do race FWD regardless. Why? Because normally FWD cars are given some "breaks" in attempt to even the performance disparity. Those "breaks" are almost always weight reductions; in some cases of pro racing they are power advantages. But it's generally accepted that similar-weight, similar-power FWD cars simply cannot compete in parity with RWD cars.

Keep in mind the three things that FWD tires do: side loads during cornering, slip angles during turning, and acceleration forces under power. The only one of the three where RWD and FWD are in parity is the steering; under side load the FWD carries a greater percentage of the total load, and under acceleration the tires have to share acceleration loads with the side loads. Remember the friction circle? Very simply put, there's only "so many g's" available from those front tires; if the fronts have to resist more mass they can't put out as many g's, and if they also need to accelerate they have to give up even more of that g-loading to share with the engine.

Try to do both too much and they give up and the car understeers. To compensate a FWD'er will, as noted, hose the rear grip of the car. I don't think there's really one person that thinks this is a performance advantage, but it's better than getting into the "waiting game" of an understeering car, waiting for it to stop sliding before you can then accelerate/turn. An experience FWD'er will "toss" the car into the turn, starting a large yaw of the chassis in advance. This large slip angle from the rear tires will allow less slip angle from the tires to turn the car, thus leaving more g-force for side loads and/or acceleration. Every successful FWD car drives this way to some extent, known or not, from nearly imperceptible all the way to full-up "oh my garsh". The only other option is to drive slower through the turn such that the maximum available g-force of the tire is not exceeded.

A "well set-up" FWD car will also have minimal rear grip mid-corner to corner exit, such that as the pilot starts to feed in the throttle there is sufficient g-force left to accelerate out of the corner. This is unusual in most cases except in extreme car setups; in most cases a FWD'er will have to "wait" for the mid-corner side loads to begin diminishing, before he can begin to feed in the throttle, all in frustration as his RWD counterparts are already using their rear tires to full advantage, accelerating out of the corner.

So, to compete on a reasonable level with a RWD car, a FWD will car need at least one of three things: a power advantage, a weight advantage, or a grip advantage. A power advantage will allow the car to make up ground lost in the corner and on corner exit; a weight advantage will reduce the loads on the tires through the corner thus allowing the driver to corner faster and begin accelerating sooner on par with the RWD'er; a grip advantage (e.g., bigger tires) will give the FWD'er a larger friction circle to work with.

"But wait," you might ask. "I've gotten my butt kicked by FWD cars in low-grip conditions such as in the rain or ice racing; how can you make such a blanket statement?" This is a good point, but is not germane to the discussion of the 95th-percentile track condition. I won't go into the specific details of why a FWD is an advantage in low-grip conditions because we classify and weight our cars based on dry conditions, which is what most of us encounter most of the time. But, generally speaking , in condition of reduced coefficient of friction (Cf) this vertical load over the wheels becomes an advantage. Think pounds-per-square-inch; anyone that has driven a pickup truck in the snow knows that a few large bags of feed in the bed is a big advantage.

So how do we apply these concepts to Improved Touring? How can we determine reasonable and sound numbers? Right now the numbers are arbitrary, almost "throw a dart and see where it lands", with one (known to me) adjustment for higher horsepower classes (ITS and ITR). This tells me two things: one, it is truly subjective; and two, we at least recognized once that they weren't "right". Personally, I still think they're not quite there.

In another topic, One poster wrote, "We can debate whether FWD may in fact be an advantage at lower hp levels..." And interesting concept, given the above discussion; FWD as an actual advantage? I responded with:

There are two primary factors that make this so [in lower-horsepower clases]: increased weight of the RWD system vs. FWD as a percentage of the car's total weight, and the ability to put power down to the ground. In ITC, the marginal "cost" of a heavier RWD system far exceeds its benefits, and because of the lower power and torque [of ITC cars] there's less likelihood of breaking traction under power. [P]lus, [with the low torque] there just not a whole helluva lot of weight transfer (which is one of RWD's primary advantages). I'm sure this would float like a lead balloon, but the FWD "subtractor" in ITC could probably be eliminated.

In ITS and ITR, however, the marginal "cost" of a RWD system is far, far less a factor as a percentage of total weight, and [in FWD cars] the power available can easily exceed the traction capability of the front wheels...IMO, the "subtractor" for FWD on those two classes should be increased.

(Cont'd next message due to space restrictions...)

[Greg Amy]

Let's discuss that idea a bit further. Let's take, for example, a pair of 50 HP ITC cars that weigh the same by "the process", one FWD one RWD. Keeping in mind my original concepts, the tires of the FWD car have to carry more of the side loads, but it's a lightweight car so not nearly as bad. Both cars use the front wheels to steer, though the FWD has a tad bit more slip angle due to the increased side load. But, with only 50 ft-lbs of torque to apply, geared, to the front wheels, The FWD'er doesn't really have to slow down nearly as much in the corner to apply full power coming out, so it's not too much of a disadvantage. Give him a few pounds' weight reduction for increased front tire loads and increased slip angle, but this one's easy.

Now, let's go to the other end of the spectrum, and compare an ITS Integra GS-R versus the Mazda RX-7 (I used these two cars not because it's what I drive, but because I don't know a RWD ITR car comparable in power to the Type R. If you do, please put it up as an example). These cars are both significantly heavier than the ITC example - but only have 1" more wheel width - and both put out significantly more power. Both are comparable in weight as classified. Both have much higher weight-to-available-tire grip levels than the ITC cars. As I believe you can easily imagine, the ITS FWD'er simply does not have the flexibility of available relative grip that the ITC'er does, and in most cases will have lower cornering speeds and will have to wait much longer before applying power on corner exit. As his RWD counterpart is rolling through the corners and rolling on the power to exit the corners, the FWD is slower through and out of the corners, managing the front tires.

We have implicitly agreed with these concepts by having lower subtractors for lower-hp classes. However, I don't think this "process" addresses the situation well at the extremes. Thought it's been suggested as above, I think you'll find a few (but not many) people that think the subtractor in ITC is excessive. But, 50# is a fair number, given that FWD'ers should get SOMETHING. Further, most will agree that ITA and ITB seem to be well-classified; the subtractor works out well for both ITB and ITA simply because of the wheel/tire rules: the subtractor is good at ITB horsepower levels with 6" wheels, and the increased wheel size in ITA accommodates the increased horsepower.

But, however, I think you'll find a lot more people think the subtractor is insufficient in ITS and possibly in ITR (though I recognize there's a wheel size increase; I don't know that it's been shown that FWD cars have the flexibility of taking taken advantage of the increased wheel sizes). And, I believe, that breakdown comes somewhere around a value of 150/160 (stock) horsepower.

For example, Kirk's ITB Golf puts out - what? - 115/120 hp? Once you've got that car turned into the corner you can pretty much flat-foot the throttle from the early apex out. My ITA Nissan was putting 155 ponies to the ground; through most corners I was just barely feathering the throttle from mid-corner out, and once I started opening up the wheel I could put the throttle down on most corners (except uphill exits like T3 at NHMS). The ITS Integra, on the other hand, is a strong case of "weight/wait management" (another Kal Showket term): I have to manage the forward-to-rear weight transfer significantly, and I have to wait-wait-wait before I can put the throttle down. All the while, competition is already on its way...

So, I hope that I've illustrated how the "problem" of FWD is one that is directly related to vehicle weight, available grip (tire/wheel size), and available power. Well, what to do about it?

I haven't begun to seriously think about the mathematics, but I believe the answer is not stepped and subjective subtractors, but numbers based on the physical characteristics of the car's classified weight and horsepower (not necessarily horsepower-to-weight). The more horsepower a car has, the higher its subtractor (and, inferentially via "the process", its classified weight). I suggest given our wheel size changes from ITB to ITA that a logarithmic scale may work, keeping ITC/B/A similar to what it is now (maybe giving the rear-wheel-drive cars in ITA a slight relative disadvantage) but increasing significantly as stock power outputs exceed 155/160 or so, giving ITS a better advantage, with ITR even more so.

"Wait," you may say. "Aren't we doing the same thing with the stepped subtractors?" Yes, we are. However there seems to be a whole lotta subjectiveness going on here. A reasoned, mathematical formula would ease a lot of minds on both sides, and melt resistance to the obvious-to-me need for increased subtractors in ITS and ITR. If you want to do it with a subjective stepped number, fine; but I think you'll end up with a lot of people arguing about it. In the end it was an objective mathematical formula that eroded opposition to the "Great Realignment"; there's just something about math and objectivity that people really like...

FWD in higher-horsepower cars is a significant disadvantage; go find a pro racing series with evenly-powered and -weighted FWD and RWD cars competing in parity. You won't. in all cases where FWD and RWD compete head-to-head successfully, there is some significant advantage being handed to the FWD'ers. And I can tell you from experience, it's not just 90 pounds...

GA

[Andy Bettencourt]

I would suggest using this letter as a basis for your math excersize, and submitting it for consideration.

[JeffYoung]

Ditto, and I'd like to see the math.

[Gary L]

FWD in higher-horsepower cars is a significant disadvantage; go find a pro racing series with evenly-powered and -weighted FWD and RWD cars competing in parity. You won't. in all cases where FWD and RWD compete head-to-head successfully, there is some significant advantage being handed to the FWD'ers. And I can tell you from experience, it's not just 90 pounds...

GA

So, what are the basic numbers in WC Touring that make the Acura and Mazda FWD cars run with (and sometimes, away from) the BMW? I realize we're talking more hp here than the typical ITS car, but it should provide a clue as to where the outer limits might be, relative to the faster IT classes.

[Greg Amy]

Ditto, and I'd like to see the math.

Well, ironically, the problem with the objective math in this application is...it's subjective. The end failure of such an exercise - and the root of my comment in another location stating it's likely pointless - is that it has to pass someone's subjective sniff test.

It's "easy" to use math to describe a natural phenomena: you measure many points of observation and use appropriate mathematics to predict it. Continue with enough points and eventually you're proven right or wrong. Math is nothing more a descriptive/predictive tool.

Using math in vehicle classifications is no different, except now you're asking me to predict in advance of observation, and at the same point of changing the observations from the existing norm. Said simply, for example, I'm telling you I want to make the subtractors higher for ITS, and possibly even higher than that for ITR, so I can assure you my mathematics will fit that premise. You can bet that any formula I provide to you will fir that base assumption. Problem is, I have no doubt my premise varies widely from yours, so you will, no doubt, reject the idea of formulation rather than rejecting the specific formula itself.

I presented the above treatise for two reasons: one, it's snowing outside and I'm bored; and two, I want the idea breached that the FWD subtractors in ITS and ITR need adjustment. "How much" is open for discussion, and once we have a few of those specific observations I'll be glad to provide to you a mathematical formula through which to remove subsequent subjective decisions.

For reference, provided as a subjective example, I think any FWD car with over 160 hp should have a minimum of a 150# weight advantage over a comparable RWD car if not more (and I'm talking in terms if dynamics only, given I cannot illustrate that premise with any reasonable and comparable on-track performance). Given no experience with ITR cars, I cannot speak to that; but I suggest given similar dynamics a formula can be derived using existing ITA/B/C examples, the above-suggested ITS example, and have that extended forward to ITR.

As I said, this could very easily go back to the whole ideal of stepped subtractors per category or horsepower bracket, especially given our tendency to "round down" to the nearest hundred (a different issue within itself). But the idea should be given consideration. - GA

[Greg Amy]

So, what are the basic numbers in WC Touring that make the Acura and Mazda FWD cars run with (and sometimes, away from) the BMW?

Gary, WC is no longer a good example (at least, until 2009). They long-ago abandoned a standardized ruleset with weight adjustments, instead going to individual make/model performance adjustments, and significant mods varying from our IT prep. As such, their 2008 model doesn't work for us.

However, in 2009 they are slowly beginning a return to standardization and "minimal" modifications, and in the long run may offer us reasonable examples.

But, totally from memory, I recall from the late-90's a weight difference along the order of 200-250 pounds between the E36 BMWs and the Real Time Acura Type Rs (with prep reasonably-comparable to our ITR rule set). But that memory is fuzzy and subject to being completely out-and-out wrong. If someone has access to those rulesets, something from the 96-99 eera would be useful... - GA

[lateapex911]

The biggest issue I see with drawing comparisons between WC Touring and IT is the cars use substantially different cages, and suspensions. The cages ties the structure together, so that incredible spring rates and dampers are feasable, and up to now, suspension points were far more open than IT.

[dickita15]

I think real men drive RWD race cars and weight breaks for FWD is affirmative action.

[ddewhurst]

Has anyone checked the FWD/RWD equator model used by the PAC/CRB for Production cars. Why not use the same model for the day when IT person wants to move to Production so that the move is transparent.

[Knestis]

Well, ironically, the problem with the objective math in this application is...it's subjective. The end failure of such an exercise - and the root of my comment in another location stating it's likely pointless - is that it has to pass someone's subjective sniff test. ...

But that's a policy issue, not a technical issue.

We HAVE to make some operative assumptions if we're going to have a repeatable process. We KNOW we won't be perfect. We've demonstrated that the membership we've heard from want us to be "more accurate" than we currently are. The policy question is, "How do we get appropriately close?"

I'd WAY rather make ONE set of assumptions, then let the numbers fall out for the individual cases, than to keep making new assumptions specific to make/model cases as they come up.

K

[lateapex911]

Has anyone checked the FWD/RWD equator model used by the PAC/CRB for Production cars. Why not use the same model for the day when IT person wants to move to Production so that the move is transparent.

Because they are different cars, with different suspension allowances, different tires, different power, and different chassis (cages). I'd think that the variables are enough to skew the result. If that's the case, then the benefit (ease of transfer) isn't worth the cost (applying an inaccurate factor, and screwing the category), especially when the transfer rate isn't all that high.

However, I would be interested to learn what the PAC process is. Can you share?

[Andy Bettencourt]

This is a tough one to swallow. In order for their to be a change, we need to understand why 100lbs is 'wrong', and why your 150lbs is 'right'. Until we can make an educated decision on that, I don't know how anything can change.

I am all for getting things right but after all those words, I see no data to move forward with. I read everything you said and I agree with what you are saying in terms of subjectivity. The current 100lbs is subjective. Your 150lbs is subjective. The 50lbs it was prior to my proposal was subjective. Trying to suggest a new adders without reverse engineering your desire is going to be tough.

I will admit, 150lbs seems like a whole lot of weight given equal power and suspension design...but that is just my gut.

Even in Koni Challenge they monkey with too many factors to draw comparisions. The MX-5 we run is 2350lbs (without driver). The Mini Cooper S is 2200lbs. IIRC they both start right around 170hp. Problem is the Mazda is allowed open cams and an open ECU and the Mini is allowed a boost bump but an OEM ECU - and alternate rear control arm kit...

[Gary L]

I think real men drive RWD race cars and weight breaks for FWD is affirmative action.

Affirmative action my arse... it's communism!

[tom91ita]

after reading the differences and advantages between RWD and FWD, i feel like an idiot for racing fwd.

except that i love my honda...

isn't there any track simulation software that could help with this on the math level? or maybe even some DL-1 data that could be reviewed?

just thinking out loud. i think it is an interesting math problem. if we thought that the fwd vs. rwd was worth x% g-force, we could start estimating corner speeds, etc.

also, is there any perceived straight line braking advantage to either car? my rear brakes do very little but i can also stab the brakes relatively hard and not have to "wait" for weight transfer to take place to let the front tires "load" up...

[jlucas]

I'm not versed in it but Bosch LapSim could answer these questions. I've heard it can use DL1 data but I don't know the details.

It's free download but takes some time to figure out.

Bottom of the page:
http://www.bosch-motorsport.de/conte.../html/3589.htm

[tom91ita]

Jeremy,

i think this was the one i was thinking of but could not remember the who/when/where. thanks,

tom

[MMiskoe]

FWIW, I don't see that it is out of line to at least look at this in terms of base weight of the car.

There are some 2700# FWD ITA cars, why do they get something different than the ITS car w/ the same weight? Or more likely the question is why do they get the same 100# as all the ITC cars at 2000# or less?

Not that I have an answer, as it is subjective at best. But then again so are any of the adders. I will say that what ever anal grab bag these numbers came from, its at least in the right ball park. Otherwise you'd never have anything even close.


Here's your starting point Greg. Go get yourself some vehicle like the Mitzu Eclipse or 3000GT which was offered in pretty much same trim but w/ FWD or AWD. Build up the two of them to a full IT prep, then go do some testing. You won't have a direct comparison to RWD, but you'll be on the right track. Maybe Ben will help you, he'll need another car before this time next year. All snide comments aside, I can't come up w/ any better plan to provide quantifiable answers.

Matt

[JeffYoung]

That's true isn't it? Curb weight/race weight is going to have a significant impact as well, just like horsepower. A heavier FWD car 'suffers" more, right, regardless of power?

[Xian]

Assuming both the cars run the same size front tire. Personally, tire size options is the main reason I think an ITR RSX (can fit 245's or possibly larger up front) has a better chance than an ITR Type R (225's up front are pretty much the max).

Christian