On midengined cars and driving - Page 3 - LotusTalk - The Lotus Cars Community
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post #41 of 166 (permalink) Old 01-06-2004, 06:32 PM
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Jeff, great post! Thanks,

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post #42 of 166 (permalink) Old 01-06-2004, 06:57 PM
 
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No sweat. Not much to do but lurk and talk about such stuff until these cars hit our shores!
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post #43 of 166 (permalink) Old 01-07-2004, 01:43 PM
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I would be careful with generalizations about front engined cars though. My 7 definately does not understeer defacto, for autocross I set it up to slightly oversteer and do set it up to slightly understeer for track events. The weight distribution is 46% front 54% rear, this car is very, very easy to balance and set up to understeer or oversteer depending on the event. Remember also the Panoz LMP cars are considered mid-engine even though the engine is in the front. As someone mentioned earlier, it's not just weight distribution it is also polar moment of enertia and even radius of gyration. Don't forget the old college test question from dynamics, if you have two metal balls the same diameter, same weight, but one is solid and one is hollow and you roll them down and incline, which gets to the bottom faster? Or ask another way which one accelarates quicker? If you understand the answer you understand that while weight distribution is important, how that weight is distributed around the center of gravity is important also, polar moment of enertia.
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post #44 of 166 (permalink) Old 01-07-2004, 10:15 PM
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I would argue that a Seven is not front-engine as the entire engine is located between the axles. It is mid-engine but the driver is behind the engine!!! :-)
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post #45 of 166 (permalink) Old 02-25-2004, 08:12 AM
 
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A good representation of Trail Throttle Oversteer (abruptly lifting off gas) is that MR2 video. I'm sure some of you have seen it. I have a copy of it, if anyone wants to host. Good conversation piece
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post #46 of 166 (permalink) Old 06-12-2004, 01:09 AM
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I also track a se7en. The Front of the engine is 7 inches behind the centerline of the front suspension.
For Kart courses I drive the car as if it is a front drive car that is set up for trailing and lift throttle oversteer (toss and catch like a an older 911). The car is set up as neutral as possible then biased slightly to either under or oversteer with tire pressure.
On road courses the car is set up to understeer slightly, with the use of throttle modulation to destabilse the rear just after turn in if necessary.

Elise owners that track the car are going to alter the standard handling characteristics of the car in order to remove some understeer.

m.
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post #47 of 166 (permalink) Old 06-12-2004, 10:52 AM
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Quote:
Originally posted by Cary
A good representation of Trail Throttle Oversteer (abruptly lifting off gas) is that MR2 video. I'm sure some of you have seen it. I have a copy of it, if anyone wants to host. Good conversation piece
Cary,
Why don't you try PMing Randy to see if he can help you out? How large is the video?

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post #48 of 166 (permalink) Old 06-14-2004, 10:28 AM
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Re: On midengined cars and driving

Quote:
Originally posted by Randy Chase
The big difference is that which makes an arrow fly straight. The weight of the arrow is up front. If you throw an object with the weight in the front forward, the object will tend to want to follow the weight.
To throw a rock (okay, a hammer) at this analogy, it has only a little to do with the weight.

The physical properties at work in these cases is called aerodynamics. In both cases, the center of gravity (CG) is in front of the center of pressure(CP). The aerodynamic forces act on the CP. With the CG "in front" of the CG, the object will "fly" stable. If the CG is behind the CP, the object will flip around until the CG is in front of the CP. If the CP and CG are in the came location, the object will often tumble. That's why arrows have feathers in the rear - to move the CP rearward, otherwise, the CG and the CP would be in the same spot (the weight of the arrowhead doesn't move the CG very much at all). In the case of the hammer, the CG is most likely very close to (if not in) the metal head, but the CP is near the middle of the hammers handle - to be aerodynamically stable, the hammer will fly head first.

In the case of the discussion of front/mid/rear engined cars, the aerodynamics are not playing a significant part. The rest of the analogy is good, and the intended point is good - but in this part of the analogy, it's for the wrong reason...

The rest of the discussion is just fine and makes lots of good points - I'm just being picky...

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post #49 of 166 (permalink) Old 06-16-2004, 01:10 AM
 
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Again: ( also for Tim - Degree in Aerospace Engineering-)

Moment of inertia
Rotational equivalent of inertia:
The tendency of an object to resist rotational acceleration. or remain in ----

The car `s polar moment of inertia:
Its moment of inertia about a vertical, the yaw axis through the centre of rotation has considerable influence on cornering ability, steering, and overall handling response.
To minimise the polar moment of inertia all masses of the car have to be as close as possible to the centre of gravity of the whole car. Then the tires can more easily alter the course, as if the main weights = engine, gearboxes, are located closer to the ends.
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post #50 of 166 (permalink) Old 06-16-2004, 08:05 AM
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Not disagreeing with the information on the car dynamics, but for arrow and the hammer, it's all about aerodynamic stability...

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post #51 of 166 (permalink) Old 12-10-2004, 03:48 PM
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Car Control is definitely best practiced in a controlled in a controlled environment. Having said that, here are some bits of what I've learned:

Skip Barber has a pretty decent way of teaching how to handle oversteer. Bobby Ore has a MUCH better way!

Bobby teaches you to release the steering wheel as you initially hit oversteer. I know, it sounds scary, but it works. An unattended wheel will generally correct itself quicker and more accurately than with someone sawing away at it. As the wheel finds its "sweet spot" in the slide, gently grasp it with your fingertips to coax it to where you want.

It's really quite a religious experience and you should try his course at Camarillo Airport. Go to bobbyoresports.com. It's a fraction of Skip Barber or Bondurant, plus you learn how to slide a car sideways into a tiny parking space!!!

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post #52 of 166 (permalink) Old 12-10-2004, 04:46 PM
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YIKES!

The ONLY time I can think of where it's a good idea to let go of the steering wheel is just a moment before your car is about to hit a wall or something hard!

I once went to a school that said the same thing. All I can say is "It doesn't work for me".

When you release the steering wheel the steering will re-center due to the caster in the front suspension. The great the caster, the more quickly it will re-center. That means your wheels will be pointing straight ahead. Having them point straight ahead may or may NOT be the best position for the circumstances.

I'm guessing others will chime in... so I'll stop here.

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post #53 of 166 (permalink) Old 12-10-2004, 06:07 PM
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It looks a bit risky even getting into this discussion. Whole books have been written on automobile handling. One or more would probably be a good idea for most Elise dirvers.

TimMullin is absolutely correct about the hammer and the arrow. And aerodynamics does have a significant impact on automobile handling. It is not as significant as the tire forces, but it is significant. The tire forces get much more complicated. The lateral force a tire can generate depends on tire patch, slip angle, weight on the tire, camber, coeficient of friction, accelleration or braking force and probably a lot of things I haven't listed. And it is a transient force. It is continually changing.

Randy is right about the front weighted car generally being more stable in a straight line, but it is because the combination of aerodynamics and tire force moving the effective Center of Pressure to the rear of the Center of Gravity. It is possible to set up a rear weighted car so the combination of aeordynamics and traction are behind the Center of Gravity. It is just not as easy to keep it there!

Also, to khamai, I think the caster will try will try to line up the front wheels with the direction you are going rather than where the front of the car is pointed. I can't say that I've tried Bobby Ore's method, but there is some logic to it.
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post #54 of 166 (permalink) Old 12-10-2004, 06:58 PM
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I ran into this thread the night before I go to track days (possibly going to rain) so THANKS to all who contributed. This thread is exacly why EliseTalk really makes a difference. With all this info in my head, Im bound to take some time off my laps and have a more mental control over the cars balance that I might not have had.

A little off subject but the low temp tomorrow is going to be 35 degrees and the high 49 and cloudy (might rain). Running SSR's and Yok A048's, what tire pressure should I start off with? I'm thinking 22-24F and 25-27 rear.

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post #55 of 166 (permalink) Old 12-10-2004, 08:18 PM
 
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eliseowner2b, are you going to the Panoz school at Road Atlanta this weekend? I'll be there both days (driving on Sunday).

Jim
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post #56 of 166 (permalink) Old 12-10-2004, 08:52 PM
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Since I've dug myself a hole on this I'll might as well dig deeper...

Caster & self-centering - the more caster you have the "higher" the car lifts itself as you turn the wheel. It's gravity that pulls the front wheels to center since it is in that position that the car's ride height is lowest. A car with zero caster would have zero self-centering.

Side to side weight transfer, as in a corner or slide has no affect on self center since it's the weight of the car on the front wheels that is doing the work to self-center.

As examples, go-karts have a lot of caster, you can see the front end rise as you turn the steering and you feel it in your arms. Most of the open wheel cars I've own have had little caster, thus as a driver you have to steering the car straight on corner exit and can't rely on "self-centering" since there's little caster.

IMHO a driver should retain a hold of the steering even in a slide with reverse lock on the steering. This allows the driver to "know" which way the front wheels are pointed at all times.

My fear is that if you release the steering wheel then, how would you know when to grab it again and would you know where the wheels are pointed? In the right direction?

The other thing that can happen, especially in a race, is that you get tagged while trying to recover, which could set the car going in some unpredictable direction. If your hands are off the wheel you've got ZERO chance of recovery.

The only reason I can think of that it's suggested to release the steering wheel at the onset of oversteer is because an inexperienced driver is likely to over correct by inputing too much steering lock. Potentially that's worse, a car may come around in the opposite direction and then there's likely spin.

Okay... deep hole is dug...

Kiyoshi

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post #57 of 166 (permalink) Old 12-11-2004, 10:48 AM
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Yes, letting go of the wheel "sounds" wacko, but the next time you're on an autocross or skidpad, try it. I guarantee you'll just start giggling to yourself about how easy and effortless it really is.

This morning on Mullholland Drive I ran into a wet patch of pavement as I was tracking-out of a corner. I had plenty of warning (eyes are always out and up) so I decided to give it a little welly.

Just a quick stab on the loud pedal to break her loose and let go of the wheel for a fraction of a second...

It was beautiful...a graceful little slide with the revs climbing ever so slightly. Shuffle steer back to center as the traction returns...

I cannot say it any better than, "This car is like time-released orgasms."

If you find yourself alone, riding in green fields with the sun on your face, do not be troubled; for you are in Elysium, and you're already dead!
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post #58 of 166 (permalink) Old 12-11-2004, 10:58 AM
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Sorry about the last post...I got so enamoured with the slide that I forgot to answer my detractors.

When you release the wheel at the onset of oversteer, it does not try to point the front wheel forward, but rather to the point of least resistance. In this case, that point is towards the oversteer. It is very easy to over-correct into a slide (you'll hear the chattering of tires - affectionately known as car farts).

At this point you really have to use the throttle to either continue the slide or slowly peel out of it. DO NOT LIFT!!! If you lift at this point you're going around because you just took all of the weight off the back.

PLEASE do not try this on the open road. It must be practiced in a controlled environment with nothing to hit (namely, me!).

At the end of the slide you begin feathering the throttle back ever so slightly as you re-center the steering wheel - the key is smooth. The Elise is extremely twitchy in untrained hands, she'll bite you back if you're not gentle!

If you find yourself alone, riding in green fields with the sun on your face, do not be troubled; for you are in Elysium, and you're already dead!
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post #59 of 166 (permalink) Old 01-02-2005, 04:59 AM
 
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Interesting thread this is.

Some of the Hong Kong Members just came back from the circuit testing our Elises today with personal bests.

Not a single time do we find our best laps with sliding antics.

I am indoctrinated from formula racing to search for cornering speed at the limit of adhesion and it definitely doesn't not allow any sliding to be fastest around the track.

If you like power oversteer, I think it is very very easy to do one that is aesthetically pleasing in road tyres but with very little effect on reducing the laptime.

Such antics, we usually reserve for the last session of the day when we have finished our testing schedules and is looking for fun from left over tyres lying around as near carcasses or to take the lady out to impress her. I am not sure what is its relevance on public road or in a race for the Elise or any car, because, we are only scrubbing off excess entry speed by going sideways and maintaining a larger slip angle at the rear than the front while controlling the attitude of the car with throttle steer and steering lock input. Not that difficult really.
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post #60 of 166 (permalink) Old 01-02-2005, 05:01 AM
 
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Now, it would require a clear head and more talent than I have got to be able to feel the limit of adhesion from braking to cornering and exit consistently for best laptimes and to win races.
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