Can FWD "feel" Like RWD

[johnc]

An interesting question from Mark Ortiz's latest newsletter:

 

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OPTIMIZING ENGINE-OVER-DRIVE-WHEELS LAYOUTS

 

My question concerns taking a front-wheel-drive car and giving it the response of a rear-wheel-drive car.

 

It seems that for large sedans the trend in the market today is to build rear-wheel-drive platforms. The ideal seems to be to emulate the handling characteristics of the BMW and Mercedes RWD sedans. This is becoming a big deal as first Chrysler, and now Hyundai and the Chinese (new Cherry V8 of 4.8 liters), are heading down the RWD path.

 

Recently GM put the Pontiac GXP into production with a 327 cid V8 driving the front wheels. It can cut a 13.8-second ¼ mile and it’s an automatic. It’s faster than a standard Mustang! I discovered that the GXP was ready to produce ten years ago but GM couldn’t afford to tool special parts for it then! Nor could they justify retooling the platform of the time in order to move the suspension hard points to more favorable locations. The recent new model introduction allowed the V8 option as they completely revised the platform for all model variants (Chev, Pontiac, Buick) and all engine options. It’s an excellent car by all accounts (available only as a wrong-side-of-the-road model, so they can’t export it – tsk tsk). [Questioner is in Australia.]

 

GM makes the point that front wheel drive is better in winter when traction is poor, for example in snow. Large parts of America (and many other countries) have difficult winters with snow and ice, so that’s a valid argument.

 

This got me thinking about chassis response. I have a question about handling. Would it be possible to make a front-wheel-drive car behave like a RWD car (or feel like it had similar response) by applying some active rear wheel steer? I don’t mean like Honda’s mechanical system (where rear steer was a fixed function of front wheel steer regardless of speed, weight transfer, yaw rate, available traction, or chassis balance), but active. It wouldn’t have to be much, perhaps just a degree or maybe even two at appropriate moments. What do you think?

 

This question implicitly raises a larger question: how can we get the most from an engine-over-drive-wheels two-wheel-drive car? To answer this, we need to look at the advantages and disadvantages of the whole engine-over-drive-wheels idea.

 

But first, to answer the question, yes it certainly is possible to use active rear wheel steering to point the rear wheels out of the turn in response to inputs from a steering position sensor, a throttle position sensor, a yaw acceleration sensor, wheel speed sensors, and any other inputs we can get a control computer to read and process. This would provide a yaw increase, or car rotation, in response to throttle application, as when throttle steering a rear-drive car. Getting exact reproduction of rear-drive behavior would probably be impossible, but getting the car to point the tail out when the driver applies power would be possible.

 

One reason that it would not be possible to duplicate rear-drive behavior is that we would have different behavior at the front wheels. At best, we’d be simulating the behavior of a throttle-steerable all-wheel-drive car, rather than a rear-wheel-drive car.

 

Another problem is that steering the rear wheels is not the same as powering them. Powering the rear wheels does not simply produce a slip angle increase; the effects are somewhat more complex. Light application of power actually plants the rear end, with front drive or rear drive. With rear drive, as throttle application increases, sooner or later we reach a crossover point, at which the use of the rear tires’ friction envelope for propulsion starts to erode their lateral force capability faster than the normal force increase increases the lateral force capability. Then the tail starts to point out. With modest static rear percentage, this crossover point occurs relatively early. In tail-heavy rear-drive cars, there is a substantial range of throttle application in which power actually makes the tail stick rather than slide.

 

So the question arises: what sort of rear-drive response would we be trying to mimick? Rear-drive cars aren’t all alike in their response to power application.

 

Regarding the Pontiac referred to: actually, the current Grand Prix GXP is the second V8, FWD car to bear the GXP designation. The 2004-2005 Pontiac Bonneville GXP has an engine closely related to the Cadillac Northstar V8, which has been used in FWD layouts branded as Cadillacs for many years now. This is a 4,565cc (279 cubic inch) engine with 4 cams and 4 valves per cylinder.

 

The current platform, using the 327 cid LS4 engine, is shared with the Chevrolet Impala and Monte Carlo. It is the first use of a pushrod “small block” family engine in a transverse front-drive layout.

 

This is little remembered now, but GM’s first FWD cars were big V8-engined designs. The first was the Olds Toronado, in 1966. That was a really big, heavy car. As I recall, it weighed close to 5,000 pounds. A Cadillac variant followed, which had engines as large as 500 cubic inches. These cars had longitudinal or “north-south” engine mounting.

 

So putting large amounts of power through the front wheels can definitely be done. Power steering becomes a necessity, but power steering has become so commonplace, even on light cars with rear wheel drive, that we are generally not accepting an increase in cost or complexity if we include it.

 

With a transverse V8, the engine compartment packaging really gets tight. This makes it hard to find room for controls on either side of the car, to provide both left and right-hand-drive versions. It also becomes hard to find room for large-section structural rails beside the engine. The rails have to be narrowed to make room for the wheels to steer, and then the only way to make them adequately stiff is to make the walls thick, which adds weight. GM is using strut suspension, which helps some as the loads from the top of the struts can be fed into the cowl area. However, with struts and wide tires, the steering geometry doesn’t work out very favorably. We have to accept either a large steering axis inclination, or a large scrub radius, or both.

 

Looking at the mechanics of putting ample power to the pavement, a FWD car doesn’t look that much worse on dry pavement and street tires than a RWD, front-engined car. In fact, if both cars are relatively nose-heavy, the front-drive car may be better. For example, suppose both cars have a center of gravity 18 inches above the ground, and a wheelbase of 108 inches. At .50g forward acceleration, 1/12 of the car’s weight, or about 8%, will transfer from the front to the rear.

 

If the front-drive car has 60% static front weight, it ends up with about 52% on the drive wheels. If the rear-drive car has 56% static front weight, it also ends up with about 52% on the drive wheels.

 

If the front-drive car has 62% static front weight and the rear-drive car has 58%, the front-drive car has 54% on the drive wheels at .50g, and the rear-drive one has 50%. That means the front-drive car will actually put power down better, assuming the coefficient of friction is such

that the .50g figure is realistic. With 50% of the weight on the drive wheels dynamically, that would be a coefficient of friction around 1.00.

 

If we put slicks on the cars, things change. Assuming wheelspin still sets the limit, rear drive starts to look better, because more weight transfers rearward as the forward acceleration increases.

 

Conversely, if the surface is so slippery that it’s a challenge to get the car moving at all, the front-drive car is clearly better. If the road slopes uphill, the front-drive car loses some of its advantage, but will generally still have an edge.

 

Note that we are comparing a fairly typical front-drive car to a decidedly nose-heavy rear-drive car. The more static rear percentage we have for the rear-drive car, the better rear drive looks.

 

 

Readers who like Porsches and Corvairs will be quick to point out that there is no law of nature that says an engine-over-drive-wheels car has to be front-engined. Assuming we are ruling out all-wheel-drive for reasons of cost, complexity, and weight, we really should be considering three layouts: front engine/front drive; front engine/rear drive; and rear engine/rear drive.

 

From the standpoint of propulsion, the rear-engine configuration has clear advantages over the other two layouts. The drive wheels can have 60% or more of the static weight, and this percentage increases when accelerating forward or climbing hills. Under hard braking, the front and rear brakes share the workload. In straight-line limit braking, the front wheels will do around 60% of the work, as opposed to 70% for a front-engined car with 50% or a bit less static rear weight, or 75-80% for a typical front-drive car.

 

The rear-engine layout can also be throttle-steered without any added contrivances. It can often do without power steering if desired. There is lots of room for the controls, making left and right-hand-drive versions relatively simple to accommodate.

 

The biggest problem with the two engine-over-drive-wheels layouts is the difficulty of achieving balanced cornering with four equal sized tires. This was more of a problem when tires were less reliable than they are now. Until recently, it was fairly important to have a single spare tire that would fit any corner of the car. Now, most cars have compact spares that are only suitable for limp-home use and don’t match any of the regular tires anyway.

 

With four equal-size tires, we can build a FWD car that understeers, or a front-engine RWD car with balanced handling, or a rear-engined car that oversteers. The first two options are clearly preferable from a safety standpoint.

 

However, if we are willing to entertain the use of bigger tires at the heavy end of the car, some interesting possibilities open up. If we are careful with the body shape so that we maximize aerodynamic stability, and if we use a longish wheelbase, we can have a rear-engined sedan that will out-handle any front-engined design, and outperform it in the snow. This approach would be well suited to a transverse V8 powertrain. The problem of finding room for structural rails would be greatly eased since the wheels wouldn’t have to steer. The car would be, just barely, mid-engined.

 

There would be some drawbacks. There could be two trunks, as in some mid-engined sports cars, but fold-down rear seats with a pass-through from the rear trunk would not be an option. The rear seat passengers would be subjected to more engine noise. Building a station wagon version would be problematic. Still, such a car could probably find an enthusiastic following among buyers who need to carry multiple passengers, yet give priority to performance.

 

Alternatively, we can also have a FWD car with little understeer. To make the most of this approach, we would want to have the engine well forward for at least 65% static front weight, bigger tires in front than in back, and the rear wheels way at the back of the car, as in a Citroen DS21. This requires the designer and the buyer to cast aside accustomed notions of how a car should look, but it offers the promise of better handling and traction than existing two-wheel-drive cars, with a large, quiet, unobstructed passenger and luggage space in the rear.

[Guest Z Draci]

RWD and FWD cars handle the same as long as you stay within the traction limits of the tyres. In fact, FWD cars understeer less than RWD cars in tight hairpins. In city driving, FWD might be more surefooted due to more weight being on the driving wheels.

 

FWD cars in the US market are PIGS. Europe has hot hatches like the new Golf GTI, 206 GTI, Astra VXR, etc. Many of these European FWD cars handle very well and are capable of out running RWD rivals. If properly engineered, FWD cars are better than improperly engineered RWD cars.

 

The ONLY problem with FWD cars is that there is a theoretical limit on how much power can effectively used by the front tyres. I've heard figures ranging from 200-250hp. If FWD cars didn't have this limit, I'm sure there would already be 1000hp LMP cars with a FWD layout.

 

I personally dislike the driving style used to make FWD cars useful on the race track.

[Guest grimlynsan]

The best ahndling car i've ever driven was a Proton Satria GTI with the factory lowering kit and Lotus suspension. Don;t know if you get them in the USA though...

[rudypoochris]

I think most european cars already have their suspensions tuned a bit more then American cars for sporty driving, so comparing a Euro FWD to an American RWD is apples and oranges, imho. Also, I think FWD cars can't outrun RWD cars unless like you said the RWD is poorly engineered. In a straightline though my money is on the RWD for a normal 3000 lb car with 200+ horse.

[Guest grimlynsan]

apples and oranges aside, how can you compare the fun to be had with a RWD (esp in the wet) compared to a FWD. Yes i have driven both extensively.

The Satria GTI is Malaysian BTW not European.

[zguitar71]

Back a few years ago the Acura integra was the hot car on the race track in what type of racing series I forget. They used the stock body and engine block and head. The cars were very modified other wise (suspension ect) though. There main compitition was the BMW 3 series. The Acura won a lot of races with its FWD v.s. proven RWD technology from BMW.

 

 

THe new MINI is amazing to watch in the hands of a skilled driver. At a NORPAC event this summer there were quite a few racing in H stock. They showed the ability and potential of FWD cars in stock form. I wish American car makers would put a little research into the handeling of there FWD cars. Non of the other american FWD cars could keep up with the Mini's, even ones in calsses that should be faster (some of that could be the drivers too).

[rudypoochris]

Bout the MINI's, I think it is primarily their size that makes them handle so well. IMHO they would be even better with RWD or AWD as far as handleing, but that is just me. Also, American manufactures should make something like the MINI, but it is really small for the American market, so it would be hard for them to sell it in large numbers. I would think any way.

[2126]

John's question....Can FWD feel like RWD? Of course, just drive it in reverse!!!

[JMortensen]

Back a few years ago the Acura integra was the hot car on the race track in what type of racing series I forget. They used the stock body and engine block and head. The cars were very modified other wise (suspension ect) though. There main compitition was the BMW 3 series. The Acura won a lot of races with its FWD v.s. proven RWD technology from BMW.

Racing is kind of deceptive in that way.

 

Here's another way to look at it. The BMW 3 series competes in ITS, Speed World Challenge Touring car, Speed World Challenge GT, ALMS GT series, and I would venture a guess that a 318 could probably fit into an ITC or similar class. Just because a car is getting beaten in a class doesn't mean the car that beat it was faster. It means the car that beat it was faster within the boundaries of that rule set. Of course the opposite argument is that you could make anything extremely fast if you put enough money in it, so it's a matter of where you draw the line. The Speed Touring car series is the series I think you were talking about. I think the 325's were pretty restricted power wise in that series IIRC.

 

I'm not calling the Real Time Acuras slow either. I'm just saying that those cars were built to a particular formula, and the RWD platform is capable of quite a bit more.

[auxilary]

you guys forget that europe and japan have better roads than we do. Our roads SUCK.

 

My mazda3 has pretty stiff suspension, and handles very well stock. It's the same chassis/suspension as the current Euro Focus, and the next generation ford focus will be.

 

However, every day on my way to work I get pissed at the ride quality because the freeway is so shoddy and I can feel every bump. It almost feels like riding my motorcycle

[johnc]

I think the 325's were pretty restricted power wise in that series IIRC.

 

SCCA tries to restrict the Touring category cars to a standard power to weight ratio. From some of the numbers I know its "around" 12 pounds to each horsepower. That actually helps our discussion of FWD vs. RWD by taking horsepower out of either side's arguement. It also helps that the cars run a spec tire (225 width Toyo Proxes RA1).

 

Since tires and horsepower are basically equalized it really comes down to vehicle preparation, driver skill, vehicle handling, and luck. Drive configuration really isn't a factor in the Touring class, except in how the vehicle is driven. At the end of each race, the BMW's rear tires and the Acura/Mazda front tires are greasy. The winner is generally the driver that conserved his tires the best.

[johnc]

Just because a car is getting beaten in a class doesn't mean the car that beat it was faster.

 

Well... maybe the winning car was driven better, maybe it was better preapred, maybe luck played a factor, but the winner of any race is ALWAYS the car that got aound the race track the fastest druing the race.

[JMortensen]

SCCA tries to restrict the Touring category cars to a standard power to weight ratio. From some of the numbers I know its "around" 12 pounds to each horsepower. That actually helps our discussion of FWD vs. RWD by taking horsepower out of either side's arguement. It also helps that the cars run a spec tire (225 width Toyo Proxes RA1).

 

Since tires and horsepower are basically equalized it really comes down to vehicle preparation' date=' driver skill, vehicle handling, and luck. Drive configuration really isn't a factor in the Touring class, except in how the vehicle is driven. At the end of each race, the BMW's rear tires and the Acura/Mazda front tires are greasy. The winner is generally the driver that conserved his tires the best.[/quote']

This is true and they do this in the interest of making the cars competitive with each other, which makes the racing more fun to watch. Racing isn't as exciting when its about who can throw the most money at their car. Watching Schumacher win over and over and over and over wasn't as exciting as watching Alonso win last year (for me anyway).

 

But hp is the argument of FWD vs RWD to a large extent. Up to a certain hp level FWD can be made to perform very well. It also does very well at autox, because autox isn't very hp dependent. When you get beyond that power threshold you don't see anymore FWD cars. I can't think of a single FWD car in Speed GT or any of the ALMS classes. Still see AWD, but no FWD. There is a reason why. Traction circles. It's tough to ask 2 tires to put down 300 hp and do all the turning and most of the braking.

[tube80z]

Racing is kind of deceptive in that way.

 

Here's another way to look at it. The BMW 3 series competes in ITS' date=' Speed World Challenge Touring car, Speed World Challenge GT, ALMS GT series, and I would venture a guess that a 318 could probably fit into an ITC or similar class. Just because a car is getting beaten in a class doesn't mean the car that beat it was faster. It means the car that beat it was faster within the boundaries of that rule set. Of course the opposite argument is that you could make anything extremely fast if you put enough money in it, so it's a matter of where you draw the line. The Speed Touring car series is the series I think you were talking about. I think the 325's were pretty restricted power wise in that series IIRC.[/quote']

 

Just as another data point here. In Speed World Challenge SCCA changed the weight balance of the BMW as well as changing the cars race weight. So as Jon points out this isn't necessarily indicated of the base performance of the car at question.

 

Cary

[johnc]

While I don't have the BMW VTS handy and I don't know what the current weight distribution is, I talked with Jim at Sunbelt a while ago (who builds a number of the front running BMW Speed Touring engines) and he said SCCA stepped in on the weight distribution issue when a couple of the BMWs were found to be running a pretty big rear weight bias.

 

SCCA did the same with the Cadillacs this year after they stomped the field at Sebring. Seems that no produciton CTS-V was ever delivered with a 43F/57R weight distribution...

[tube80z]

I'd have to look it up but I believe they are forced to run 52F/48R.

 

We had a couple of the Speed WC guys in the motec seminar this year. It sounds like a number of new cars are supposed to be coming. I was chatting with one of the caddy crew in Portland and they were moaning about the reduced revs and additional restrictor they got for that race. I wouldn't buy a caddy but I have to admit they are probably one of my favorite cars at the moment to watch.

 

Cary

[Drax240z]

Hey Cary, was that Claude Roulle's seminar? What did you think of it? I kept meaning to do it when I was in FSAE but couldn't come up with the coin. Still pondering doing it...

[johnc]

I found the VTSs:

 

Acura RSX: http://www.speedvisionwc.com/competitors/vts/05-vts-acura-integra.pdf

 

Mazda 6: http://www.speedvisionwc.com/competitors/vts/05-vts-mazda-6-rev4.pdf

 

BMW 325i: http://www.speedvisionwc.com/competitors/vts/05-vts-bmw-325-rev2.pdf

 

A quick summary...

 

Acura RSX - 2.0L, 2,400lbs w/driver (1,200 lbs per liter) 56F/42R

Mazda 6 - 2.4L, 2,700lbs w/driver (1,125 lbs per liter) 56F/42R

BMW 325i (E46) - 2.5L, 2,750lbs. w/driver (1,100 lbs per liter), 50F/50R

 

There is no weight distribution penalty for the E46 325i in Touring.

[tube80z]

Hey Cary, was that Claude Roulle's seminar? What did you think of it? I kept meaning to do it when I was in FSAE but couldn't come up with the coin. Still pondering doing it...

 

Yep, that's the one. Claude is really good at explaining complex subjects in a manner that is easy to understand. For instance, he gives an explanation of a tire as a series of springs. And when it coil binds in any direction it slides rather violently. And then tells you about how not to do this.

 

The only issue with the seminar for me is that he's at a pro level and I'm just a lowly autoxer. So what I can use and afford is a lot different from what he uses and can afford. Aim PISTA (8 channels) versus Motec ADL2 (45 channels).

 

Three really big things came from the class for me (among the hundreds of others).

 

1. Kinematic changes have a much larger influence than springs or shocks. Claude shared some stories about his days in F3 to back this up and we did some simulation that showed weight transfer etc. For me this means major changes to the front end of my tube car so this can be easily changed.

 

2. It is absolutely essential to have linear pots on your suspension if you want to be able to set the car up. Claude shared a number of nifty tricks that he'd used. My favorite was the use of pressure sensitive paper and trailer tie downs on one of the Aussie touring cars. They pulled the car down to match what the pots showed in a problem area. Camber and pressure were adjusted to maximize the contact patch and the car was instantly 1.2 seconds a lap faster and won the race.

 

3. Trying to measure tire temperature is a shot in the dark with a handheld pyrometer. Claude showed data from one of the flying lizard cars and asked us what to tune according to the data (looked like too much camber). On car tire temp measurement clearly showed it needed more. So that's 13 sensors at $250 a pop plus a system that can record all the data.

 

What it really showed is that racing these days, even at an amatuer level, you really need data acquisistion. He uses the example of going to school and needing a report card to see how you're doing. In this case the driver has a report card and so does the car. And while expensive you can learn more with one of these systems in a weekend than most people can testing all year without (assuming you can understand what the data means).

 

I figure this class put me about 3 to 5 years ahead of where I'd have been figuring this out on my own. I would add that for people operating at an amatuer level I learned more from the ICP seminar put on by Richard Pare and Steve Lathrop and it was a fair bit cheaper.

 

Hope that helps,

 

Cary

[zguitar71]

The Speed Touring car series is the series I think you were talking about.

 

Yes that is the racing I was talking about. Unfortunatly I have not had the Speed Channel or cable for the last 6 years. Well the only unfortunate part is the lack of Speed Vision the rest of cable I could care less about.