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:
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...)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.
