bcal
|
| posted on 5/6/04 at 01:23 AM |
|
|
Handling virtues/vises
I have a question regarding the handling virtues and or vices of mid engined cars.
I’m building my own design from scratch and recently I was talking to an engineer who warned me not to build a mid engined car unless I really knew
what I was doing.
He reasoned that in his experience conventional layouts were much more forgiving for the novice than middies and that handling vices in them were far
more easily rectified.
I know one of the assets of concentrating the weight towards the middle of a car are that it gives you low moment of polar inertia but does this make
the car far less forgiving if you don’t get it right?
I’ll be sitting well back from the front axle and I have concerns about a possible tendency for the front to lift at high speed. One way I thort of
countering this was by locating the fuel tank, battery, and radiator in the front so as to distribute the weight more evenly. The concern I have is
that with the radiator up front the only place air can exit is under the front half of the car. Do you think this may cause the nose to lift?
I am quite concerned about this because I don’t want to put a lot of time and money into something which may end up being difficult or even dangerous
to drive.
|
|
|
|
|
kb58
|
| posted on 5/6/04 at 02:36 AM |
|
|
There are no black-and-white answers, but I'll try. What he says can be true, but it's all by degree. If you don't know how
to drive, no matter what you get in, it'll be hard. If you're an F1 driver with the initials MS, you won't have a problem. My
point is, it very much depends on the person, in addition to the car. A mid-engine car can be harder to control if designed poorly, as can
any car. You see "conventional layout" cars on racetracks losing control all the time, so what does that mean? Hard to say.
Would you drive a 4WD Jeep on a race track? That's a "conventional layout" but it does lousy at that because it's not set up
for the track, so regardless of where the engine is, it's no guarantee of success or failure.
About moving the weight to the center, the best illustration is an example that you can perform in the grocery store. Put all your groceries toward
the front of the car and see how the cart "handles". Seriously, it really works and you don't have to go fast. (Six-packs of beer
work well, but I digress.) Now move the food to the back and repeat. See how different it is? With it at the front (front engine) it's harder
to spin, but once it starts it's harder to stop. With the food at the back (mid-engine) it's easy to turn (or spin) but also easier to
stop the turn, or spin.
About the front lifting, that's an aero issue, one solved by keeping air out from under the car, which should be done in any case. Putting a
heavy engine at the front to hold the car down isn't a good reason to put it there. Exit the radiator air up out the top of the hood or out, or
behind, the front wheel wells.
Again, it all depends, on a lot of things. How fast will you be driving? How big are the tires? Tire alignment, aero loading, driver ability...
what do you want out of this thing?
It's kind of like saying blondes are better wives then brunettes.... no.. just different. And what "better" means is different for
each person... just like cars. Anyway...
I just get spun up a bit when someone says someone shouldn't do something. Is it he who wouldn't want to do it, or is it that he knows
you so well, and knows your technical abilities, that he really feels he's helping.
Having said all this, yes, building a mid-engine, independant suspension car is more difficult to build then a straight axle front
engine car, but I truly don't think it's more dangerous or unstable.
[Edited on 5/6/04 by kb58]
|
|
|
bcal
|
| posted on 5/6/04 at 04:09 AM |
|
|
Thankyou for your reply KB58.
I know what I’m asking is probably a very subjective question and the answer depends mostly on the tastes, needs and wants of the individual. After
all I guess that’s why we build our own cars.
If you go to my www you’ll see the car that I drive now has a fairly low moment of polar inertia. Took me a while to get use to at first. Bit darty
like a gokart but now that I’m use to it she’s delight to drive in a winding road. Tho I do have to concentrate at speed with a crosswind.
I realise that with any steering responsive car that issues like bumpsteer and toe changes can make a huge difference.
With my current car I hadn’t allowed enough Ackerman in the geometry and it suffered badly from transient handling problems. As a quick fix I set it
for static toe out rather than toe in and the difference was amazing. Totally different car to drive.
So I guess that the moral of the story is what you can get away with on a large stable car you may not in a small responsive one.
I just thought though that there may be more traps for the initiated with a middie than a convential setup.
I’ll have to give some long hard consideration as to what to do about air getting under the nose of the car. Maybe there’s good reason why not many
people mount the radiator upfront.
The nose I plan to build will be a track style nose as found on many American circuit and salt cars of the 40s and 50s so it should be fairly
aerodynamic, well more so than the current brick that I’m trying to push thru the air 
|
|
|
Spyderman
|
| posted on 5/6/04 at 01:38 PM |
|
|
There is a fundamental difference in the way front and mid engined cars drive.
Do you like your car to be forgiving and tollerant of your inexperience or would you rather have it bite you when you overstep your capabilities?
Front engine cars are very forgiving and fundamentaly "safe". However it takes more to get them to perform competetively than a mid
design.
This is not meant as derisory to front engine cars.
A mid engined car should be capable of higher cornering speeds due to it's lower polar inertia. However when they reach their limit it is not
uncommen to break traction very rapidly and spin. Compared to the front engined understeer or powered oversteer.
I suppose it all depends on how you like your driving!
I like mid-engine cars and the way they bite. Reminds me what I'm doing and helps me to focus.
You may prefer something different and the Locost is as near optimum as possible.
I just had a look at your website, very cool!
I reckon you should go middy as you already have a front engine car for comparison.
You'll learn a host of new driving skills!
Mounting the rad up front should pose no real problems after all Fiat did it with the X1/9 and Toyota with the MR2. As KB said front lift is an
aerodynamic thing not related to the rad location or venting.
Even if all the cooling air that is ducted through the rad went under the car it should not cause lift.
Lift is caused by lower air pressure over the top rather than higher pressure underneath. As long as unwanted air is kept from going under there
should be no major issues.
Whatever you decide on if it turns out as good as your Rod it should be interesting.
Terry
Spyderman
|
|
|
sgraber
|
| posted on 5/6/04 at 02:51 PM |
|
|
quote:
Originally posted by Spyderman
I just had a look at your website, very cool! Whatever you decide on if it turns out as good as your Rod it should be interesting.
Terry
Very true! Nice Rod.
IMHO - One of the intriguing things about the middies is that you have to be more precise with them at the limit. But then again, how often will you
be at the limit of performance of a car like this? I suspect that on a common motorway - never. At least you 'shouldn't' be driving
like that when grandmas and teenagers occupy the same road. 
Graber
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
|
|
|
bcal
|
| posted on 6/6/04 at 07:12 AM |
|
|
Thanx for your input and kind words fellas.
Much appreciated
I rarely drive a car beyond my own limitations so I guess I should be safe in that regard. What I’m looking for is something that handles nicely on a
winding road and can keep up with my fellow rodders in a straight line.
I think a middie weighing around 800kg with a sooby EJ20 should do that with ease.
I have no plans to race the thing so an auto box is the most likely way I’ll go.
Thanx also for your advise re the radiator. Having would make life much easier because I need all the room I can get for cat converters and mufflers
etc to keep it under 94db. I’ll probably make up some sort of air duct that directs the warm air towards the rear of the car. May even scavenge a bit
for the cabin to keep me warm in winter.
|
|
|
Spyderman
|
| posted on 6/6/04 at 05:52 PM |
|
|
Wadya' mean keep you warm in winter?
Ya great wus!
Your winters are equivalent to our summers!
I heard that you sometimes have to put on a sweater in winter!
My point about handling was merely to point out that if and when you find yourself on the limit, which can happen with spirited driving, you have to
use a completely different approach to conventional layout driving.
You can't lift off if you find the corner tightening more than anticipated. You have to keep the power on and drift it through the bend.
Once you have done that you will either need to change your underwear or try it again!
Steve I'm suprised at you as you have some lovely back roads up north of Phoenix. Definately the sort for trying the handling out!
Terry
Spyderman
|
|
|
bcal
|
| posted on 7/6/04 at 07:07 AM |
|
|
quote:
Originally posted by Spyderman
Wadya' mean keep you warm in winter?
Ya great wus!
Your winters are equivalent to our summers!
I heard that you sometimes have to put on a sweater in winter!
This is the second week of our winter and I'm sitting here at 5 in the arvo wearing just shorts and a T shirt.
Yep we sure are spoiled down here
Sounds like I'm gunna have to build this thing just so I've got an excuse to change my underwear occasionally
|
|
|
Alez
|
| posted on 7/6/04 at 09:46 AM |
|
|
quote:
Originally posted by kb58
It's kind of like saying blondes are better wives then brunettes....
Blondes are better wives than brunettes.
|
|
|
JoelP
|
| posted on 7/6/04 at 09:48 AM |
|
|
quote:
Originally posted by Alez
Blondes are better wives than brunettes.
damn, i new i picked wrong...
|
|
|
Alez
|
| posted on 7/6/04 at 11:41 AM |
|
|
quote:
Originally posted by JoelP
damn, i new i picked wrong...
Time for DIY colouring, quick!!
|
|
|
sgraber
|
| posted on 7/6/04 at 11:52 PM |
|
|
quote:
Originally posted by Spyderman
Steve I'm suprised at you as you have some lovely back roads up north of Phoenix. Definately the sort for trying the handling out!
Terry
Ahem, a slight difference between 'shouldn't, couldn't and wouldn't'! Grandmas and teenagers watch out when I'm
drifting sideways through the intersection!  Or was that Alez drifting sideways? I can't remember.
And as Alan B. will confirm, the roads north of Phoenix are exactly what a sportscar is designed for. I can't WAIT!
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
|
|
|
Alez
|
| posted on 8/6/04 at 07:16 AM |
|
|
quote:
Originally posted by sgraber
Or was that Alez drifting sideways? I can't remember.
I was just trying to avoid that post that suddenly appeared in the middle of the road where that kerb was!!
|
|
|
sgraber
|
| posted on 8/6/04 at 02:45 PM |
|
|
quote:
Originally posted by Alez
quote:
Originally posted by sgraber
Or was that Alez drifting sideways? I can't remember.
I was just trying to avoid that post that suddenly appeared in the middle of the road where that kerb was!!
Ahh Yes... But had you been in a middy, you would have spun out way before reaching that signpost...
(You HAVE to have a sense of humour about these things! )
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
|
|
|
Alan B
|
| posted on 8/6/04 at 03:03 PM |
|
|
quote:
Originally posted by sgraber
.....And as Alan B. will confirm, the roads north of Phoenix are exactly what a sportscar is designed for. I can't WAIT!
Oh yeah.....VERY nice roads indeed....
|
|
|
cymtriks
|
| posted on 10/6/04 at 09:16 PM |
|
|
Just a thought or two...
Elise rear
roll centre is 75mm and camber gain is 0.45 dgrees per inch. This implies an effective swing axle length of 120 inches. Spring CPM is 98.
Elise front
roll centre is 30mm and camber change is 0.31 degrees per inch. This implies an effective swing axle length of about 170 inches. Spring CPM is 90.
A tip I heard recently is that a bit of toe in at the rear helps cure the middy handling vices. Toe out is normally reckoned to be bad news on middy
rears.
Staniforths books on car suspension explain things like CPM and swing axle length if this post isn't clear.
|
|
|
bcal
|
| posted on 11/6/04 at 09:53 AM |
|
|
Thanx cymtricks.
I'll have to pick a copy of them books.
|
|
|
