Fred W B
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| posted on 18/2/05 at 06:12 PM |
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Thanks guys
I have been following your sites, but it was usefull to be reminded to go over them again, as a lot of things are much more relevant to me now I am
actually building something.
kb58 - that is one sexy jack!
Cheers
Fred WB
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kb58
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| posted on 18/2/05 at 07:00 PM |
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The jack is the cheap aluminum one selling here for about US$89. It's not regarded well by some people as I hear it doesn't work well in
the cold, and if the load isn't centered, the jacking point can break right off. But, at 35lbs, it's okay by me, for now.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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kb58
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| posted on 18/2/05 at 07:02 PM |
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Just remembered another thing. I do something that I hardly ever see, mounting the bottom of the rear shocks directly to the upright. Most people
put it on the lower A-arm which puts a lot of stress on it. It remains to be seen how smart my choice was.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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Ferg
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| posted on 19/2/05 at 07:15 PM |
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quote:
Originally posted by Fred W B
Where does a decent length rear upper wishbone go?
Perhaps we could hang the wishbones off the rear of the car in a sort of trailing manner??? 
 
Libra Rear Suspension
[Edited on 19/2/05 by Ferg]
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cymtriks
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| posted on 19/2/05 at 09:29 PM |
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quote:
Originally posted by Fred W B
where does a decent length rear upper wishbone go?
Either swept backwards to fit behind the engine or use a very wide wishbone angle so that the front pivot is in front of the engine and the rear pivot
is behind the engine.
Another way is to just make the car wider to fit everything in.
The transverse engined middy is simple cheap and compact with lots of potential donors. This is mainly why every mass manufactured or smaller/cheaper
middy has this layout. The MGF, MR2 and Elise were largely designed with using a mass manufactured drivetrain in mind to keep the cost down. That
forced a transverse engine on them.
As soon as the budget grows, or the design leans towards competition, car makers tend to switch to longitudinal to get better weight distribution.
For Locosters I reckon that for a smaller engined car a front engine layout would be faster mainly due to the weight distribution and partly due to
the higher centre of gravity of a transverse engine package in a middy. Mount the mid engine longitudinally and the middy wins. Make the engine
bigger, thus making it harder to preserve agility and keep the polar moment down in a front engined car and the middy wins, though the middy
competition for the bigger front engined car would nearly all be longitudinal engined. All this assumes equal engineering and design, not
budget.
Sooner or later we'll be left with only BMW or Mercedes donors for a traditional seven. The closer that time gets the more you save, and can
therefore spend on other things like tyres, dampers, brakes, etc, by going transverse middy. I reckon we're getting very close to being able to
get an equal car for an equal budget by using a transverse middy layout and that will be the clincher for us.
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kb58
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| posted on 19/2/05 at 10:04 PM |
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Good points, but help me understand this:
"...a front engine layout would be faster mainly due to the weight distribution and partly due to the higher centre of gravity of a transverse
engine package in a middy."
If the front-engine design achieves 50/50 weight distribution, maybe, though I think having 55-60% over the rear axle is better for traction and
braking. Having larger rear tires makes a difference too regardless where the engine is.
About the center of gravity, what makes a mid-engine have a higher CG? The engines are basically the same. The transmission? The vertical placement
of everything in the tranny is the same front or mid-engine. The seating position will be the same too as well as the gas tank. Givent that these
four components pretty much consistute the mass of the entire vehicle, I don't see any diffference.
Help me understand.
[Edited on 2/20/05 by kb58]
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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TheGecko
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| posted on 20/2/05 at 12:38 AM |
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quote:
About the center of gravity, what makes a mid-engine have a higher CG?
I'd also like to see some justification for this common
claim. At installed height, my 20-valve 4AGE will sit with the sump between 125-150mm off the ground and the top of the cam cover at about 750mm -
not hugely different from where it would in a font-engined Locost. Plus, not having an engine the front lets me put the battery at floor level in the
front bay and my fuel tank is at floor level between the engine and the passenger bay.
In the words of an (infamous) Australian politician: "Please explain?"
Dominic
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cymtriks
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| posted on 20/2/05 at 10:37 AM |
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With a mass produced transverse engine the gearbox has a higher engine input shaft height than output axle height. This puts the engine well above
ground level in a typical hatchback.
Move the engine into a mid engined position and typically you could lower the engine by a couple of inches, i.e. from about 6.5 to 4.5 ground
clearance simply because sports cars tend to run lower and don't need to cope with being piled high with four people and their luggage. If you
lower the engine you get odd drive shaft angles, tilt the engine to lower it and you make the engine bay longer and possibly mess up the sump oil
level etc. So you can lower it a little bit but that's it.
Of course, kb58, you could lower it by 2 inches by using 13 inch wheels!
With a front engined car you can run both the axles and the drive shaft to the rear diff at a slight angle and still end up with a lower engine and
sensible shaft angles. Some front engined sevens do run with very low engines and usually all you need to make this work is a chopped sump.
So unless you make some drastic change somewhere the front engined car has a lower CoG than a transverse engined middy just because of the mass
produced parts that we have to work around.
Regarding weight distribution there is some debate about what "perfect" is. Most sources state either 50/50 or 55/45 (small rear bias).
With a front engined car like a typical seven you can easily get into this range with a small engine and with the weight of a driver included. With a
transverse engined middy you're probably going to end up with something closer to 60/40 by the time you've sat in it.
So for a small engined light weight car you usually get a lower CoG and a better weight distribution with a front engine.
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MikeR
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| posted on 20/2/05 at 11:10 AM |
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For road cars i've heard a very convincing argument that you don't want 50/50 balance. It makes the car too twitchy. If you have 60/40 in
either direction you will know what the car is going to do in a given situation and be prepared for it.
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kb58
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| posted on 20/2/05 at 06:25 PM |
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I understand now.
In our applications here though I don't think it's an issue. In my case for example, using 13" wheels and tires, axles at their
proper angle, there is 4.5" under the pan. This is about as low a ground clearance as it should be for a road-going car. If I install racing
slicks, there is now 3.75" under the pan. How low do you want to go?
OTOH, in the case of a true race car, yes, you are correct; we are not free to set the ground clearance to, say, 1". A FWD mid-engine cannot
work. I was assuming the case of a self-built road-going car, and in that case, the CG can be just as low as any front-engine setup.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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kb58
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| posted on 20/2/05 at 06:45 PM |
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I agree that 50/50 seems logically perfect; when cornering on a skid-pad, the fastest car will be the one with equally loaded tires.
Having said that, the first strike against it is that many sports cars have larger rear tires. Now the tires aren't equally loaded the same
front/back because there is a lower force/area on the larger rear tires. All things being equal, I would expect this 50/50 car to exhibit understeer
on the skid pad because the fronts do not have as much traction as the rear.
If the car has 50/50 weight distribution, AND equal size tires, I'd expect oversteer because the back tires are being asked to do more then the
fronts, cornering AND propulsive force, so they will break free first.
The second strick against the theory is, race tracks do not have many constant speed, constant radius turns. The car is always speeding up or slowing
down; rare is the case in a turn where everything is static.
Given this, when the car is accelerating, weight transfer shifts to the rear, a good thing (for rear-drive cars.) With a greater static rear weight
distribution there is even more weight on the back end, holding the rear wheels down and aiding acceleration. I will admit though that this condition
can cause understeer if taken too far.
When the car is decelerating, weight shifts to the front. In a front engine car, the rear tires are practically useless now because there is no
weight on them. With a mid-engine layout, the weight of the engine holds the rear tires down on the road, allowing them to add enormously to
braking.
At least, that's my personal theory... for now.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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kreb
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| posted on 21/2/05 at 01:06 AM |
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kb58 - I think you nailed it. A 50/50 weight distribution is a theoretically perfect condition which only has much of a bearing on a skidpad.
I'd also take issue with the idea that you need 250 + HP before the middie is ideal. The dynamic factors that you list are weight neutral -
weight transfer happens regardless of vehicle GW.
https://www.supercars.net/blog/wp-content/uploads/2016/04/1966_FiatAbarth_1000SP1.jpg
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sgraber
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| posted on 21/2/05 at 03:09 AM |
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Hear Hear KB - The Lotus Elise would be a prime example of a Sub 200Hp/Sub 1000Kg transverse mid-engine car that out-handles almost every other
production car known to man regardless of cost.
Steve
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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kb58
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| posted on 22/2/05 at 03:59 PM |
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I was thinking a bit more about this. When a mid-engine car has wider rear tires, it's an even further improvement. Dynamically the car now
approaches the advantage of 50/50 weight distribution because traction can be identical front to back. AND it still retains the mid-engine advantages
of better traction under acceleration, and better braking due to the rear weight bias.
What's we need are two Locosts using the same engine, one front engine and one mid, then compare performance under all conditions. Easy to
say...
[Edited on 2/22/05 by kb58]
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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cymtriks
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| posted on 22/2/05 at 10:02 PM |
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I've never heard of a 250bhp limit at which a mid engine becomes better than a front engine but it's probably a result of the issues I
mentioned above.
Smaller volume cars need to rely heavily on mass produced parts. The Elise, MGF, MR2 etc had their layouts determined by what was available in the
mass produced parts bin. That ment fwd drivetrains. To get rwd the drivetrains were moved to the back giving a transverse middy layout.
250bhp is very roughly at the point where mass produced engines give way to purpose designed ones. It also corresponds roughly with the point at which
the mass produced and small cars give way to more exotic, lower volume stuff. The truth behind this limit is probably that it represents the
limitations of a typical corporate parts bin to provide a ready made fwd drivetrain ready to be moved rearwards.
For our kind of cars, a light weight mass produced engine gives slightly worse CoG and arguably worse weight distribution if you choose a fwd package
as I pointed out above. With a traditional Seven layout you can lower the CoG and get better weight distribution due to not being stuck with the
relative heights of the axle and engine crankshaft of a typical fwd layout. Of course the Polar moment of inertia is worse with the front engine so it
isn't entirely clear which layout wins out overall.
As the engine gets bigger the effect on polar moment gets worse for a front engined layout and the benefits of shifting the engine to the middle
become more marked. Also the market for bigger engined cars will justify designing a special driveline. I would guess that once you move from the
smaller V6s to big V6s or V8s then middy is best just on the basis of polar moment. Thinking about it this point isn't far off that 250bhp
limit.
Just a final thought. The MGF and MR2 have slogged it out with the MX5/Miata for years and judging by the road test reports it's hard to find a
clear winner. Likewise has anyone put an Elise up againt a Caterham with a similar power to weight ratio? Was there a clear winner?
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kreb
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| posted on 1/3/05 at 07:23 PM |
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From what I've seen the Elise will kick most 7's tails with a comparable power-to-weight ratio. The more extreme 7s will win by virtue of
superior output.
On the Middie versus frontie debate- my experience is that it's easier for a mediocre driver (such as myself) to get the most from a neutrally
distributed car. A superior driver will be able to wring more from the middie despite it's more extreme tendencies towards overster, due to the
superior traction and braking.
https://www.supercars.net/blog/wp-content/uploads/2016/04/1966_FiatAbarth_1000SP1.jpg
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locost_bryan
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| posted on 3/3/05 at 12:50 AM |
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Depends how well sorted the middie is.
It took the Stig on Top Gear a whold morning to get a fast clean spinless run in the Porsche GT, but only a couple of attempts in the Merc McLaren
Slur 
Bryan Miller
Auckland NZ
Bruce McLaren - "Where's my F1 car?"
John Cooper - "In that rack of tubes, son"
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v8kid
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| posted on 3/3/05 at 02:33 PM |
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Got to disagree with you chaps - no suprise there then!! - 50/50 is a myth.
50/50 is a lousy combination for the average sporting driver and car and its just not far enough for racing.
Here's my tortured reasoning - Just observe what your car does when coasting round a corner - the blessed thing slows down! Yup the combination
of aero, rolling and scrub losses slows the dashed thing down significantly. To keep our constant speed (another myth but anyhow) we need to apply
throttle. Observe yourself at speed and it is suprising just how much throttle is required. Well I was amazed when I first twigged it anyhow.
Now with 50/50 weight distribution the cenrtifugal force is going to make both tyres break away at the same time- so where is our traction force
coming from? Yup it's stolen from the rear wheels (assuming RWD) reducing the rears ability to hold the road.
Hmm - terminal oversteer - dangerous can't have that so the designer has to incline the roll axis, or whatever, to make the front break away at
the same lower limit. i.e. add understeer
Answer is to put more weight on the front wheels so there is spare tractive capacity at the rear wheels to maintain constant cornering speed. Say
55/45.
All the above only applies if all the wheels are the same size which is the case for most road cars (apart from ferrarri et al).
Make the rear wheels bigger and its a whole new ball game - now we can load up the rear to get traction without loosing out on cornering.
Still I would never choose 50/50 though if you are prepared to do without a spare or an ineffective spare at best its a no brainer to go the whole
hog.
Cheers
David
PS Centrifugal force is another myth!!
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kreb
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| posted on 4/3/05 at 08:54 PM |
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Huh?
Well it's nice to know that nose-heavy pigs are great handler's after all. Well I guess you showed all those dummy engineers at Porsche,
Mazda and BMW. Good on you!
Now if you'll pardon me, I'm heading off to the garage to add some balast to my front end.
https://www.supercars.net/blog/wp-content/uploads/2016/04/1966_FiatAbarth_1000SP1.jpg
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