Mid v Front Engined Cars

Dale,

Until I have time to post later.

Go to: http://ferrarichat.com/forum/showthread.php?t=39217 and scroll half way down the first page where we discuss the "spin" characteristics of mid and rear engined cars.

Regards,

Jon P. Kofod
www.flatoutracing.net

 

I have no racing experience to contribute here but would like to say that the traditional handling differences between mid-engined, rear engined and front engined cars have been somewhat mitigated with clever suspension tuning.

E.g. The Ford Focus RS steers neutrally and has a hint of oversteer even though it is FWD. The 996 has been designed with a dash of initial understeer to warn unsuspecting drivers.

 

Ok, I’ll give you the basics.

For a race car these are the characteristics you want to have.

1) Understeer for high speed turns. High speed we’ll define as anything above 60-70mph per the Prost and Senna driving books. Why? Because it’s much easier to control a car that has a little push at those speeds than one that has a back end coming around on you. Driving fast isn’t as much about having the fastest set up as having a set up you can drive fast with! This is where wings and other aerodynamic devices come into play. Set up your car for mechanical (springs, shocks, swaybars, ect) slow speed oversteer and as the speed increases the wing downforce increases. An over steering car at low speeds with a rear wing adding downforce to the rear of the car becomes one that has extra rear grip and becomes an under steering car at the higher speeds.
2) Oversteer for low speed turns. Low speed we’ll define as anything below 60-70mph per the Prost and Senna driving books. Why? Because for these slower turns having the rear of the car rotate in a controllable fashion can help you carry more speed through the turn and onto the following straight.
3) Neutral. This is the nirvana of car handling. When both your front and rear wheels drift/push out at the same rate. With this set up you can steer by both steering wheel and throttle with ease. The only road car I’ve ever driven that had this characteristic was the Datsun 240Z which came out of the factory with a perfect 50/50 weight distribution. Trim your wings (if you’ve got them) however you like. You don’t have to worry about cranking more rear wing to get extra downforce to help in high speed turns. You can back off on that wing and use your reduced drag to increase your gear ratios to increase your top speed and your overtaking chances. Or crank it on and use the extra downforce to get through the turns faster (this of coarse applies to track cars).

A mid engine car usually has a weight distribution in the neighborhood of 40/60 to 45/55. Having that extra weight in the back makes it much easier to rotate the back end. Approach a slow turn, come in with a bit of extra speed during the turn in, balance the throttle to shift the weight to the back and then squeeze it on and accelerate out. If there is still some rotation just apply some opposite lock to the steering wheel. Very easy to do at slow speeds. Once you start getting above that 60-70mph mark doing that kind of maneuver gets a little bit tricky. This is why most race cars are mid engined. It’s easier for a race engineer and driver to get that rear weight to work for you. Ideally you’d want perfect neutral handling. That usually requires quite a bit of set up work to achieve on most cars.

On to front vs rear engine cars.

If you’re doing a lateral G-Force skid test in the two different types of cars here is what’s going to happen when you’re diving around that big circle and lift off the throttle.

Both of the cars back ends are going to drift out on you. But…

The front engined car’s rear will be easier to control. Why? Because the majority of the weight is at the front. This places the center of gravity closer to the rear wheels than the rear. It’s easier to control the rear weight, i.e. bring it back into line, that if more of the weight were toward the back. The rear tires don’t have to achieve the same level of grip to control the weight.

Which is the case of the rear engined car. That extra weight on the rear wheels is going to be harder to control. Now how fast that back end comes around on you and how much opposite lock you apply to control it depends on the way the suspension is designed. I own a Toyota MR2 Turbo. Performance is on par with a Ferrari 348 for a frame of reference. I bring this up because the early 91-92 (MR2s) cars were known for “snap-oversteer”. The rear end would hold and then suddenly lose grip and come around on you. This was such a problem that Toyota released the 93 models with revised rear suspension. The trailing arms were a bit longer and the main arm was shorter. It also didn’t help that they released the 91 car with 14” wheels and 195/50 front and 205/50 rear tires. That was also changed and the 93 car had larger wheels and a more staggered tire diameters front to rear 15” 205/45 front 225/45 rear. They also lowered the car’s ride height by a ½ inch. This made the car much more progressive in its feel and handling characteristics. But, most guys racing the cars, autocross usually, preferred the 91 model. The car would actually hold a higher lateral G-load than the 93 model. Change to polyurethane bushings, sway bars, springs, wheel and tires and the 91 car could out corner the 93s. The changes also got rid of the “snap-oversteer.” It’s a really great car but like other mid engined cars there is a learning curve that goes into having confidence with the car. Which is the key to driving anything quickly.

Now I mentioned the 240Z earlier. There are other cars out there that have that 50/50 characteristic. I’ve noticed the Z06 mentioned in many threads. That car I believe (someone correct me if I’m wrong) has a 50/50 distribution. I’ve hated Vettes but when GM released the C5 they were truly on the right path. The engine is behind the front wheel centerline and the transmission is in the rear of the car just in front of the rear wheel centerline. I also believe the Ferrari 550 is the same. This is a set up Porsche had used in the 928 series and 944 (951) series to good effect. If you’ve never driven any of these cars you should. If the 951 (944 Turbo) didn’t have so many issues I might argue it could be one of the best GTs ever made. Blindingly fast that perfect 50/50 weight distribution and a big rear hatch to put a lot of luggage. The same I suppose could be said for the 928s4. I’m sure the 550 fits right in here and could possibly be the best. Only time will tell.

I hope this little primer helps.

 

Dale,

The first thing you have to think about is that cars don't "snap and spin" by themselves. It's all about the driver's inputs.

In steady-state, at-the-limit cornering, most, if not all, road cars understeer. Call that the lawyer factor. Front wheel drive cars ALWAYS understeer and I've never encountered an AWD road car that oversteers--forget what automotive "journalists" say. Very rarely does a rear wheel drive road car exhibit oversteer in steady state cornering. Race cars are a different topic altogether.

Now back to the driver. Oversteer typically occurs when a driver lifts out of the throttle and transfers weight to the front of the car. This reduces the size of the tire contact patches at the rear and increases the front. And with most drivers--voila!--instant oversteer.

On the track, I'd rather drive a mid-engined car than a front-engined one. Generally versus a front-engined car, a mid-engined car may be more prone to extreme drop-throttle oversteer behaviour, but it is also more sensitive to steering and throttle application, making it more of a "driver's" car.

Ultimately, there should be no surprises at the limit. A driver should know his car's behaviour, plus, when driving at the limit, you had better be attentive enough to correct oversteer.

FWD and AWD cars are completely different. Generally, all a driver needs to do is apply MORE throttle to correct for oversteer. Think about that one for a minute.

Hope this helps!

 

HAHA!

Charles, you're story reads quite well until the part about the Datto 240Z!! Priceless.......

Chassis balance is totally subjective and there is not one correct answer for any specific question.

The biggest factor is springs/shocks and aero. NOT weight distribution.

Bottom line is "most" roads cars understeer through safety factors put into it at the set up stage, and "most" race cars go a little better if they oversteer.

Reason...... understeer requires a lift of the throttle to correct, whereas over steer can be steered away.

My most recent set up cars are 360C and N-GT. These can be turned from O/S to U/S in a pit stop if you know where to go. Most open wheelers react the same, many road cars can be trouble...

 

I don't know as much about cars as I'd like to, but my educated guess would be that because the center of gravity is closer to the rear in a mid or rear engined car than a front engined car (obviousely), as soon as you loose rear wheel traction, momentum causes the car to want to swap ends. Think of throwing a dart backwards, it's going to instantly swap ends because the front (the rear if you're throwing it backwards) is heavier. Of course it's not that extreme in a car, but maybe allong those lines?

I'm not sure if any of you know anything about aviation, but you'll see that this is the same reason a taildragger will "ground loop," and is much harder to control on the ground than an airplane with a nosewheel.

 

Let me throw something else in to muddy things up. I used to have a Fiero (waaaay back when.... Fun and roooooomy car - I'm 6'4" and I could sit in it with legs straight out!) that I did some upgrades to in suspension, tires and brakes. The car exhibited the "lawyers understeer" and was pretty tame and predictable - if not boring. I read an article about some diferent alignment geometry for better handling performance. It also came with a disclaimer about being VERY VERY carefull and to learn the limits before doing something stupid on a tree-lined road. Well, long story short - the new alignment made me wish for 5 pt. restraints and a racing seat! HOWEVER, when it let loose there was no turning in/back/or otherwise! You just let go of the wheel and wait for the dirt to stop flying - it was wicked fast coming around. Funny thing too is that it turned a pretty good snow car into a useless road marker. There is a lot more to the good Dr.'s question than would appear!
Jim

 

First, note that a 50/50 car is only 50/50 when sitting still.
Under heavy g-forces, the weight transfers to one or two tires.
Two 50/50 cars when sitting may be very different braking into a corner...
one may transfer far more weight than another.

To understand that, you must move beyond simply "where is the center of gravity". You must look at "where's the polar moment of inertia". Or, how far from the center of gravity is most of that weight?

Imagine holding over your head a 6-foot long bar that weighs 10 pounds has 20-pound weights at each end. How hard is it to start rotating it? Once you have it going in a circle, how hard is it to stop it? Now move those two 20-pound weights to the center of the bar. How much quicker can you start and stop its rotation now? The weight is unchanged. The center of gravity is unchanged. The polar moment of inertia is what changed.

Similarly, the beauty of a mid-engine car is that the majority of the weight is actually in the center of the car... very near the center of gravity. Thus, under heavy cornering or braking, the weight shift is much less than in front- or rear-engine cars... meaning the car stays more balanced... meaning you have higher limits.

HOWEVER, when your tires do break loose, the car can start rotating very quick... its easy to rotate the car... the snap you mention. FORTUNATELY, it is also easy to stop the rotation of the car... its both easier to start and easier to stop the rotation... soooo, although the car will snap (begin rotating quickly), you can more easily catch it. But it will all be happening quickly, so programming those reflexes is important.

Hope that clears things up for you, Dale.

 

I must admit my experiences have been coloured by the cars I have owned.

I had an early 80s BMW 320i which was the car most likely to snap and spin that I've had.

I've also had 928s which were much more controllable, and also now my 208 which is also more controllable than my 320i ever was.

Having more weight at the back has 2 advantages:
1. Better Traction
2. When the rear end breaks away it generally happens slower, though can be harder to get back.

I found with the 320, that you would catch the first slide, but then it would regain grip suddenly and likely as not pitch you into a spin in the opposite direction than the first slide! I found the solution was to let the wheel slide through my hands while steering on the throttle, then grasp it when straight ahead was achieved.

So I guess that while engine position and weight distribution has an effect, this can be more than overcome by suspension settings...and presumably aerodynamics at higher speeds.

 

The best of both worlds....front/mid-engined with a transaxle.

Like the 612 Scaglietti, SLR MacLaren and forthcomming 550/575M Maranello replacement.

These cars have the engines set low and behind the front axle line and a gearbox mounted in a rear-transaxle.

Gives slighty rear biased weight balance and great handling. I feel this is the next wave in exotic road cars. It has all the benefits of a mid-engined car without its disadvantages. Plus it retains that classic, aggressive long hood/short trunk lid look we all love. Everyone who has driven a SLR or 612 gives them excellent reviews on the track AND road.

 

Also describes the C5/C6.

 

What are you talking about? The Datsun's the best part!

I'll second that.

I'm going to have to disagree with you there. The basic platform/equation that you're working with is how the car naturally distributes its weight. Springs/shocks and aero and such is a function of that basic platform/equation.

I'm in agreement with you again.

I'm getting the itch to turn some wrenches again. My point is that it's easier to move from one to the other (O/S to U/S) if you've got a car that inherently has a 50/50 weight distribution. There is less weight to jack around to get from one to the other.

Also, I forgot to mention this, one of the wonderful things about that Datsun was the driving position. Due to that long nose housing that long straight 6 (sounds wonderful with 3 Webber 40mm side drafts by the way) you were just about sitting on the rear wheels. Because you're so close to them you can feel what the back end is doing much better than a mid engined car where you're sitting farther forward. I know that comment will bring even more responses on what really makes a car give a driver good input to what it's doing.

Just my .02 cents... which to everyone outside the US is worth a little less with the falling dollar.

 

At the limit, mid-engine cars (due to their smaller moment of inertia) shift less weight, and thus maintain their balance better... giving them that better grip you mention. But when they do finally lose grip, they rotate much quicker/faster (due to their smaller moment of inertia). However, if you react properly to catch the spin, you can more easily catch/stop the spin in the mid-engine car (due to the smaller moment of inertia).

Was that any clearer?

 

Actually no it does not. The C5/C6 engine is NOT behind the front axle line.

 

Brian is right about describing polar momentum - it's the same reason that figure skaters can speed up in the middle of a spin by pulling their arms in. You have to think in very dynamic terms. The reason front-engined cars give you a second chance to correct for oversteer is because most of us mortals are not fast enough to catch the action of the mid-engined car when it starts to swap ends. With the bulk of the weight sitting in the middle of the car and virtually no weight over the front wheels, the mid-engined car is significantly easier to rotate.

The best comparison I can think of the miata vs the elise. Both are considered fun-to-drive and capable, but the elise is the fastest low-speed car on the planet because it holds all its weight next to the center of gravity like a twirling figure skater. The miata, on the other hand, is used as a entry-level club-racer. It is balanced like the elise, but it has an iron lump hanging out over the front wheels and a torque-tube connected to a big metal diff in the back. The miata is the figure skater letting her arms out to slow down the spin.

 

Sorry, thought I read something which said that.

 

Dok,

I have spun on racetrack many times. I drive a 348 regularly and a Z06. Both are about 58/42 weight but in opposite ways. Your perceptions are correct. The tail heavy car breaks faster than the nose heavy. Then just after the break all that mass in the rear is harder to control because it is heavier just like balancing a cantalope in your hand versus a watermelon. So when your tail is about 45 degress from the direction you are going you have reached the point of about terminal oversteer...just let it go and let go the wheel and the caster of the car will straighten you out and if you spin put both feet in (clutch and brake full on). Before the 45 degree point you can correct with steering. The problem is that with a tail heavy car all you got is steering since you have to keep your foot on the gas. So I disagree with the early poster because if you are driving 10/10ths you are using all the track. And if you use all the track then you run out of track of you have to correct with steering. But with a nose heavy car going 10/1oths and you make a mistake you can modulate the gas back just a bit correct with steering and scrub speed then hit the gas when you hook up. In fact the nose heavy mass is still moving in your desired direction and when you break the back end even if you do nothing the front mass will want to keep you going in the chosen direction. That is why nose heavy car spins feel slower on initiation and also because they are slower you have more of a chance to correct for it. In the real racing, leMans etc.. aren't they using the 575 as the platform to compete with the Corvette and the Viper? Even if you don't a gree with me...I think Prodrive's choice of the 575 as their race platform says it all.

 

Oh and by the way you guys really have to take a close look at the C5 Z06 or the new C6. Vettes use a torque tube so you have the weight going fore and aft like the 944/51/28 variants. They have a great design. I wish Ferrari could do what corvette does and I would pay twice the price. The corvette is the girl you marry. The Ferrari is the prom queen who sits at home and just eats bon bons.

 

AFAIK engine and transmission account only for approx. 20% of the weight of the car. I assume b/c of this the impact of engine position can´t be compared to your 10 lbs. 6-foot bar

I think speedy_sam is right that nowadays "the traditional handling differences between mid-engined, rear engined and front engined cars have been somewhat mitigated with clever suspension tuning." Why on earth would Ferrari have changed the engine position in the successor of the 512 M íf that meant sacrificing performance?

However, for top performance a mid-engine layout might still be the first choice.

 

Talking about weight distribution, I tried to pull out some data:

Verified front/rear/total:%/%
Ferrari 360 Modena 603/824/1427:42.3%/57.7%
Porsche 996 TT 606/956/1562:38.8%/61.2%
Source Sport-Auto, verified weight with all liquids full, no passenger(s).

A Maranello is said to be 50/50 for 1690kg.

However, weight distribution does not reflect moment of inertia, just tells you where the center of inertia is at constant speed, under no longitudinal acceleration.
Not sure these static figures tell much on dynamic behaviour
Useless post <sigh>

 

360C can be set up to be 45/55. 550 road car is nowhere near 50/50, but I haven't had one on the scales yet to see for certain. Ex Rafanelli 550 race car is about 42/58 with oil tank and alternator and electric steering pump in the rear.....

Balance at the limit is mostly influneced by suspension/tyres/aero. Beyond the limit the PMofI determines "catchability", but the faster car is always an oversteerer, despite being harder to catch.

If it keeps spinning out, tune it some more, and then if that doesn't work.....put a better driver in it!