[Randy Chase]

This was in response to someone that PMed me asking about the difference in safety for a midengined car:


If you throw an object with the weight in the front forward, the object will tend to want to follow the weight. In the case of a midengined car, a lot of the weight is behind the driver. Like throwing a carpenter's hammer with the head in the back. It will rotate.

In a front engined car, when you go around a corner, the front of the car will want to stay in front. In fact, it will resist turning, so that means the car tends to "understeer" which means it turns less than you want. If you go into a corner too hot, you increase the understeer. That is not good, because you can run off the road with the front of the car. But it is somewhat controllable and you can slow the car down in the turn if you are too fast.

The physics of cars is that if you go into a corner too hot in a front engined car and hit the brakes, you slow down and actually can transfer grip to the front tires, since the car experiences more weight shift. You unweight the rear tires, so you can increase oversteer, which may reduce the understeer. In any case, if you wreck, you will probably do so nose first. But you can handle things by slowing the car down with brakes.

This is why most production cars are understeer biased. It's safer for the general public. Front engined cars are probably safer also for people who are not skilled also.

Now here is the difference in a midengined car. The weight is behind you. And it always wants to be in front. There are some good things in physics about the weight being behind you in racing. The ability of the car to rotate. Weight over the driving wheels. Launching. Weight transfer under braking. All better.

The first thing to understand is what we mean when we consider the ability of a tire to maintain traction with the road surface. To keep this under 1000+ words, I will just say that it's very important, and when you exceed the ability of the tires, you lose control. Many things come into play that determine what the ability of your tires are. These things may be:

Speed or g-forces. Sideways forces take their toll.
Weight pressing down on the tires. If you unweight the tire, it will lose grip.
Road and stickiness. Ice of course is bad, so is a patch of wet or some gravel, or even a small bump that upsets the weight balance.

The main trick with some of those things is to not get too close to your limits of adhesion, because none of us are superhuman and can forsee everything possible ahead of us on the road. You can play with the limits where it is safe to do so. But a public road is not the place.

However, what typically happens is this. Driver goes into turn near the limits of the tires ability. We call that driving near 10/10ths. Most non-skilled drivers can't feel the difference between 8/10ths and 11/10ths.

Something happens. That patch of wet. Something in the road. Evasive maneuver. A squirrel. Something. Or maybe just realizing you are going into the turn too hot. Probably the most common reason.

You can do certain things BEFORE the turn. Remember, the mass behind you wants to go straight in front of you. If you are in a straight line, that is not a bad thing. If you are in a straight line, hit the brakes hard. But in the turn....now you have an issue.

The normal driver reaction is to panic. And panic is not good. If you abruptly lift on the throttle, jerk the steering wheel, or worse, slam on your brakes (which most people do).... you cause some things to happen. You transfer weight to the front of the car so now the front increases it's grip, but the rear loses grip. And the rear wants to lead. The rear end will start a very bad oversteer where the car starts to spin. Even if you can catch it, often the correction will induce a huge spin in the opposite direction.

And if you are spinning, you have *almost* no control. You will have no idea where you will end up. You can spin into things or opposing traffic.

The solution for this is to understand and learn car control. One of the best (safe) places to learn car control and what 10/10ths is, is a local autocross. Here you can spin out over and over. And many people do. Even on a safe racing track, it's not always safe to spin out and go off the track.

Famous racing quote: "Understeer is when the front of the car hits the wall. Oversteer is when you hit with the back."

Hope that helps!

Randy

 

[MattG]

Good writeup. The part about panicing in a corner is one that really needs to be emphasized. It's one that I've had a hard time with learning, even after owning my MR2 for over eight years, as it's counterintuitive.

Basically, once you're in a corner, you're committed. There's absolutely no way that braking or using the engine to slow you down will save you *if* you're cornering at the limit. If you're *not* already cornering at the limit, then there's no point in attempting a braking action as the car can handle what you're giving it.

Of course, if you're faced with a decreasing radius corner, this becomes more complicated, and it may, in that case, be best to attempt a braking action before the radius decreases (the same physics apply, but chances are the car is not as close to its limits as the driver thinks it is). There could also be random obstacles you encounter, like rocks or bicycles in the road, that require a braking move mid-corner. For this reason, it's always best to enter a corner well below your car's limits and accelerate out of it once you can view the exit.

Autocrosses are great, but track days can be very rewarding too. I've noticed my speeds increasing a lot after a couple 20 minute sessions on a track.

One last note about spinning - Randy's right about having no idea where the car is going to go. However, one technique I've heard of, but thankfully have never tried, is to step on the brakes as hard as you can to lock up all four wheels. At that point, the car will more or less continue in a straight line until it stops.

 

[Randy Chase]

A couple more points.

You can control a spin with some experience. Having spun out cars more than 400 times now I find I can catch spins that I would never have had the chance to before. More importantly, I can recognize when they are going to happen before they do.

When instructing and sitting in the passenger seat, I often see the spin before the driver does.

But yes, with some experience you can control the spin to some extent. You know the old rule, both feet in hard.

Clutch and brake. And close your mouth if you don't wear a full face helmet. Because you are probably going to be eating dust. Putting in the clutch saves your engine. Might as well be hard on the brakes too.

In a decreasing radius turn, you can carefully apply brakes, specially if you are adept at left foot braking and can transition the car between throttle and braking. Depends. You can also scrub off some speed by your line. But mostly, you are screwed.

 

[LarryB]

In my old age, I’ve become a lot more conservative on public roads. Now tend to do the bulk of the braking before going into the turn. Then push it through the turn, something mid-engines do well. It might be a good approach if your not use to them. Certainly agree with you Randy after a while (more so if you spun a lot) you’ll get a good feel for the limits & being able to catch it. When it was stock use to love kicking the tail out a bit just to put a exclamation point on it.

 

[JonOrangeElise]

Randy and Matt G,

OK, I've done two car control clinics (with skid pads) and three driving schools (all in my E36 M3, all through the BMW CCA). I fully understand the basic concepts of understeer/oversteer on an intellectual level, and I'd say I've honed my skills enough *not* to panic and do something stupid (a la lifting while going in hot). I am also developing a feel for weight transfer, and loading/unloading different parts of the car.

But I'm certainly not good enough to consistently catch and correct the car once I'm in a 10/10ths oversteer situation (I've only been at 10/10 on the skidpad -- where spinning is part of the learning situation)

So here is my question: My front engine M3 is spec'd for 50/50 weight distribution. It is also said to have neutral handling erring toward understeer at the limit. The Elise, meanwhile, is reported to have a natural propensity for understeer as well (at least recording to the reports I've read/seen). Also, when oversteer does creep in, it is supposed to be markedly controllable (at least according to the Top Gear video).

So: Given that understeer would appear to be dialed in by the Elise's chassis and stock wheel set up, wouldn't this negate the otherwise traditional oversteer concerns associated with mid-engine cars? I concede that *any* car can be made to spin; we can't shut down the laws of physics. But wouldn't the factory-dialed understeer negate the mid-engine oversteer characteristics?

ITo wit: I've driven a Porche 996, and it plowed like crazy -- and this is a *rear* engine car!

So what nuance am I just not getting here? Educate me!

Thanks!

 

[Randy Chase]

Good question and well stated.

Any car can be made (as you note) to understeer or oversteer in a normal cornering state. The issue is not that the Elise, or any midengined car, is set up to oversteer out of the box.

If it was, I would actually think in a way it would be less surprising to people, because they knew the back end wanted to come around.

I think the issue is that Elise is not 50/50 weight distribution. With the engine towards the back, you have that hammer being thrown effect.


Let's exaggerate the analogy a bit. The front engined car is a large hammer with the hammer head representing the engine end. You throw the hammer and the head end wants to go to the front. Normal. And predictable. You throw the hammer, with the head first, it stays first.

Now flip the hammer around. Toss is. What happens? The hammer rotates in the air. The head wants to go to the front, but there is now a force causing it to tumble and rotate as it moves.

You can set up cars to understeer by various methods. Larger rear tires vs smaller front tires. DIfferent alignments. The use of suspension bits like swaybars. Etc. But that does not change the physics of the hammer.

The main difference is, as long as you are operating within the "budget" of the tire's contact patch, those suspension bits and alignments are doing their job to keep the car neutral. It's when you exceed them that those things become less part of the equation and the hammer analogy takes over instead.

As mentioned above, the other issue is the weight transfer in a midengined car. It magnifies the unweighting of the rear tires in a turn. A front engined car is designed to have the engine's weight over the front wheels. As such, the front tires are doing more of the work in a corner. When you brake in a turn, and unweight the rear tires or weight the front tires, you are only shifting incrementally more what the car normally does. In a midengined car, the rear tires are doing a lot more of the work. To brake in a turn forces the rear tires to unwieght and the fronts to take on the load instead. That adds to that hammer affect.

This is my grasp of it anyway.

The net is this. Any car can be made to oversteer and understeer. A driver can induce those conditions also. But in a midengined car, the result of sudden oversteer is more severe and can be catastrophic to the untrained or less conservative driver.

This means-

1. Use car pushing the car on turns where there may be traction changes. A wet spot. Dirt on the road. Ice/snow. Anything that can reduce the traction of the rear tires in a corner.

2. In slippery conditions, do almost all your braking/accelerating in straight lines. As much as possible.

3. Try to avoid sudden movements while turning. Specially anything abrupt that unweights the rear such as quickly lifting off the throttle or hitting the brakes.

4. You can possible do better in a turn with more judiciously applied throttle to maintain grip. Even a car with a lot of oversteer (think "drifting") can be controlled. A spinning car is much more difficult.

5. Practice with the car under safe conditions. I highly recommend parking lots and cones. There is normally an autocross somewhere near you. Experience sudden oversteer and see what it feels like. Over and over. Go into a corner near 10/10ths and tap the brakes. Jab the throttle. You can make the back end come out. Get a feel for what the car is doing.

Damn.. I wrote another novel. sorry! Hopefully someone else will add to this.

 

[MattG]

Jon, it sounds like you have more formal driving education than I, so I don't know how well I can answer your question! Hopefully Randy can pitch in with a more technical explanation.

I think it's entirely possible that the car could be designed to have enough grip at the rear to make a spin unlikely. The challenge would be to provide that level of safety while still giving the driver oversteer if desired. The Elise's low weight, short wheelbase, and midengined design would all help with that. If anyone could pull it off, it'd be Lotus!

 

[JonOrangeElise]

Gotcha, Randy. If I'm reading you correctly, the net statement you're making is: Once you've exceeded the limit, the weight distribution of a mid-engined car could make correcting/catching much more difficult than if you were in a front-engine car. Fair enough!

Speaking of driver's training:

- Does anyone know what our aftermarket roll protection will be for the Elise, and will it fit with both the canvas bonnet and optional hard top? (I would be hesitant to track the car without it; some schools demand it for open roof cars; and I wouldn't mind it for driving on public roads).

 

[Randy Chase]

Gotcha, Randy. If I'm reading you correctly, the net statement you're making is: Once you've exceeded the limit, the weight distribution of a mid-engined car could make correcting/catching much more difficult than if you were in a front-engine car. Fair enough!

Yeah, it just takes me a lot longer to say that.

I would go just a little farther though in that the midengined cars can have this pendulum affect. It's not the first oversteer that gets a lot of people, it is when it snaps back.

As I understand it, from what I have read, the built in rollbar in the Elise is very effective. I agree with you that it's an important issue for the track and a good idea for the street.

 

[LarryB]

Certainly agree with Randy. Get a good feeling for the limits in a safe setting. When it breaks it really is more of a sudden snap . Knowing the limits make it much easier to catch. The pendulum effect is a good way of thinking of at the limits... when I use to kick the tail out on a regular basis it would swing 3-4 times (lessening each time) before it would settle in. In a poor traction condition snap oversteer will happen at a very low threshold, you really need to drive like your on thin ice, this is where alot of people get into real trouble.

 

[Jay]

The Elise's weight distribution is more towards the back of the car, so a lot of the rules for driving a 911 will apply: straight line braking, don't lift, and learn how to throttle steer.

If at all possible, get your car out on a skid pad and learn how it behaves. Making a car turn using only a throttle is a breakthrough experience.

Midengine cars are a lot of fun (I've got a 914/4 2liter that is set up for autocross) and you can feel the car rotate around the weight of the engine. But, like any car, if you exceed that file line between braking and turning in the 'tire budget' you'll be punished.

So I'm glad Lotus went with the lightest possible engine that made sense. Otherwise it would be less of a mid-engined car.

 

[meat]

Throttle steering is the key to my Exige. I generally brake in a straight line and use a slight amount of trail braking (usually until the rear starts to step out!!), then I turn in and will usually just mash the gas and feather it all the way around the turn. The car responds very well to this. More throttle equals more track out and less throttle means that I'm not going to make. Lifting or even hitting the brakes could be bad. If you do get into a situation where you aren't going to make - just go with it and don't fight it. Try to go straight off the track and not go off sideways (don't want to flip).

 

[drrjv]

where can you find a skid pad? anyone have recommendations for driving schools or places to try stuff out on the east coast?

 

[brownsa5]

As stated before, try your local autocross. In the DC area, we frequently have skidpads as part of the course, and there have been a number of schools in the region this year too. An autocross is an excellent place to explore the limits of your car and learn how to control it in extreme conditions. Check your local SCCA region's calendar of events, or GRM. Also, many car clubs hold autocross events, open to anyone (try PCA or BMWCCA).

Steve Brown

 

[Randy Chase]

Yep. A skidpad has some value as a training tool, but not as much. It is a better car tuning tool. But be careful about oil on certain cars in sustained skidpad sideloading.

I second the recommendation to do some local autocrosses. Learn the limits by spinning out where it's safe to do so.

 

[Lotus Fury]

Lifting the Throttle during a turn can actually spin you the opposite way of the turn. If you're drifting (intentionally or not) through a turn and lift the throttle the back tires act like brakes and the car reacts like someone kicked the rear of the car in the opposite direction.

Example:

During a left hand turn I was massively oversteering the car. The rear of the car was spinning towards the right. As soon as I lifted the throttle the car immediately spun to the left. When I hit thewall I contacted with the front left bumper first then the car spun even harder to the left and hit the left rear corner. Very luckily no one got hurt and I managed to not even get a ticket for anything (despite me pretty much deserving one). That was a very expensive mistake.

 

[meat]

Lotus Fury

Did you have your front tires pointed where you wanted to go? If not what happened was that when you lifted the car slowed and transferred weight forward. If your tires were not pointed straight with the car's angle, the front tires regained traction and the snapped the rear around.

 

[Lotus Fury]

I was countersteering to the right and lifted... Front tires said "HEY! Thanks for the traction! I'll go that way!" I countersteered the left but by the time I did that I was already perpendicular to the roadway.

It was a case of "How bad can you drive?" I went in to the corner too fast, i hit the gas just before the apex & the tires spun & I was just a dumbass in general.

ugh... i need more track time...

 

[MOKOSpeeD]

Lifting the Throttle during a turn can actually spin you the opposite way of the turn. If you're drifting (intentionally or not) through a turn and lift the throttle the back tires act like brakes and the car reacts like someone kicked the rear of the car in the opposite direction.

Example:

During a left hand turn I was massively oversteering the car. The rear of the car was spinning towards the right. As soon as I lifted the throttle the car immediately spun to the left. When I hit thewall I contacted with the front left bumper first then the car spun even harder to the left and hit the left rear corner. Very luckily no one got hurt and I managed to not even get a ticket for anything (despite me pretty much deserving one). That was a very expensive mistake.

You know what? I've made the same mistake with a torquey and weighted car, Merc E430. However I lucked out and did not collide with anything except for the curb. No body damage at all! Just needed an alignment and new tired. One of my wheels got bent, but not enough to notice. I knew better than to go drifting on a public road. But school was tough that day, and so my head was not working.

 

[fastspider]

Light weight

The is one point that hasn't been made.

All things being equal, the front/rear tradeoffs are as described. However the big difference between an Elise and most other road cars is that it is much lighter. For me the effect of this is that the Elise is actually quite reluctant to spin, it steps out a bit then finds enough grip to keep going, since there is much less mass trying to make it spin around.

I ran an autocross in the rain soon after I got my car and learned a lot, but in the regular runs didn't even hit a cone. In some fun runs afterwards I pushed harder and harder but despite sliding all through the course, it really didn't slide far off line.

I've driven/spun a Ferrari 328 on a skid pan, and owned a Porsche 911 for a few years, and you can feel the extra weight trying to turn you around at a much higher level than the Elise.