Stressed vs. Non-stressed Engine/Gearbox

Hey, you know how some supercars have the engine bolted directly to the chassis? What are the advantages of doing that? Also how come some cars like the F50 have it while others like the McLaren F1 don't?

In the case of the F50 it was a good marketing trick.
The Mclaren was a little more converntional, it also used an engine from another manufactor, that possibly wasn't able to surport the weight of a road cars ass.


By useing the engine and gearbox as part of the chassis, you do away with those parts of the chassis, and so save weight.

the engine/tranny/drive train is a rather large solid chunk of metal that is very rigid. If you incorperate it into the frame design, it will make the frame more more solid without adding any material to the frame.

Where as a rubber mounted drive train is the opposite. The frame has to be boxed in and reinforced to deal with all that weight and torque movement acting apon it, so the frame will be much more substantial. The advantage to rubber mounting is less vibration transmitted to the frame, which will not only make the car more creature friendly, it will also reduce metal fatigue.

Motorcycles, for example, use the engine assembly not only for a large portion of the frame, but in some examples will also make it the pivot for the rear swingarm and base for the front steering assembly. One of the most extreme built in this way was a race bike made by John Briton. It literally had no frame, everything was built off the engine/tranny assembly.

o, so thats why the F1's chassis is so long at the back, while the F50's just stops infront of the engine. I was looking at the F50's engine and the R390's engine...the F50 has two rods connecting the chassis to its rear subframe, while the R390 has to rods cennecing the chassis directly to the engine. Is there any reason why one's bolted to a subframe while the other's bolted onto the engine?
http://www.splitsec.com/spike/Ferrari_F-50/F50-Engine-Detail.jpg
http://www.usm.maine.edu/~salhany/R390-eng.jpg

Also how come car's like the F50 have only two connecting rods from the chassis to the subframe while others like the Koenigsegg have like 6? and just one more thing, can you use a stressed engine layout with a tubular chassis?

yea so why do some manufacturers bolt the chassis on to the engine, while others do it to the subframe?

packaging and space involved.
it's common sense.

oo, I thought it was for performance reasons. Thanks for that

in a sense.... but in a round about way.

the less metal there is, the less weight there is and hence the faster you can go.
however, in some race formats, some things are regulated such as air intake sizes, power figures and whatnots. all of this means that there is an optimum size for things that you can use which in turn means that there is a minimum weight you can aim for
i.e if you don't need a bigger heavier turbo, then use the smaller lighter one.

in the case of road cars, there are no such regulations so the packaging can be different and there is not the need for extra lightness.

looking at the R390 model, i can see that the engine of the 390 actually sits on/in the bottom half of the semi-monocoque. as far as i can remember, the engine on the F50 is bolted onto the back of the F50s.
this difference goes to explain why both cars use members at different locations; to prevent twist in certain directions.

oh true. Doesn't the R390's engine sit on the underbody? Because with the Sauber C9, the Engine sat on the underbody, and the area which it sat on was covered in aluminum.

underbody=the bottom half of the semi-monocoque

look for details of the ferrari 365 GT boxer or miura and see how that was made.
essentially, it is a shell type structure that is also rigid and acts as both body and chassis.

oh, so once the underbody is bolted to the chassis, they both act as one piece?

not quite but yes the underbody IS part of the chassis.
it's basically a shell that is structurally rigid.

technically, a monocoque is a strutural shell (simplified description); think egg where there are no other structural elements apart from the shell itself.
a semi-monocoque is a shell that has secondary structural elements such as ribs along the inside or a strut across the top.
(so yes, it is the underbody attached to the frame - but really, they are both designed as one so it's not just an arbitary attaching of shell to a frame)

true. But even though the underbody/semi-monoque and chassis/monoque are designed as one piece, they come as two peices which have to be put together to function as one piece right? Also can you use a stressed powertrain layout with a tubular chassis?

most race cars are tube frames that the drive train is hard mounted.

not quite.
the monocoque will start life as more than one piece, yes
but it isn't like a structural frame attached to an underbody.
the secondary elements are designed to make the part-shell rigid.
without the shell there, these elements might not actually do anything.

like i said, it is a single thing that is designed at once.
it isn't simply putting a shell/underbody onto a tubular frame, although the end results usually looks like it is.

**chasing them typos**

ah I see, wait so can you use a tubular chassis with a stressed layout?

well... you can do what you like.
it also depends on some other things too.
dimensions of the engine, what ancillaries there are, what kind of body you are working with, construction method of body etc etc.

the old F1 cars of the 60s-70s were more or less an engine (that held the rear suspension) bolted onto a tubular framed main chassis that had body panels attached (onto the frame).

Drunken monkey's right. Monoque is like a boat's design. The ribs help the cored hull.
I would like to know why the McLaren F-1 doesn't use this type of construction. It uses a cad designed 4130 chromoly steel frame to support it's highly touted carbon fiber body. All the body does is transfer downforce to the chassis. But it weighs something like 4.000lbs. This is probably due to using a large v-12 motor. It's output is approximately 600hp, so, o.k., it's good at top speed.
If a carbon fiber unibody (monoque) chassis was used, along with a more efficient 4-banger like a modified VW direct injection turbo motor, 2000lbs with over 400hp. could be easily acheived. This, with similar downforce charachteristics incorporated into the design would yeild a MUCH better product.

s far as i know, the F1 does have a monocoque chassis at its heart and uses secondary structures (also made from carbon fibre) attached to the monocoque (mainly at the front) to serve as crumple areas.
it also weights 1140 kgs which is not 4000lbs.

you're thinking of the SLR.
but then that doesn't have a V12 so i now have no idea what you're talking about.

yea man, what car are you talking about? The F1 was made all in Carbon Fiber, no steel frames were used. About the stressed engine layout with a Tubluar chassis, wouldn't it just depend on the chassis being a bit stiffer than usual, and the engine/gearbox being beefed up to support the weight? Also back in '89 Mercedes used a stressed chassis layout with a Tubular frame in thier LeMans winning Sauber C9.

not sure what you mean.
the point is, the teams aim to have lowest posible weight but this is not the only determining factor. Weight, weight distribution, centre of mass, polar axis, aerodynamics (allowed and not allowed) location of large masses all play a part in how you make the car.

you don't need to beef up the engine/gearbox because it is more or less a solid bit of metal. It is rigid enough as it is.

as i said, there is no single best way.
you do what you need to do to acheive what you want.

Something nobody mentioned, that may have played a part in the design process is that some of these cars are designed to break appart into pieces when wrecked.

oh true. What I meant about stiffening up the chassis was that you would probably need to make a tubular chassis abit more stiffer and stronger in a stressed layout than in a non-stressed layout, so it can support the wieight of the entire powertrain of the car. And then I gave an example of a car with a tubular chassis that used a stressed engine/gearbox layout...the Sauber C9.

here's a pic of the chassis
http://truesdell.eng.uci.edu/scale_models/reference_photos/Sauber_C9/images/c9%20aeg%20monoque.jpg

oh, that pic is of something else entirely.
that's more of a reinforced tub with roll-over bars.
a tub chassis can be taken to be a more fancy tubular frame but the way it deals with shear forces is different enough to make a different type of chassis.

it isn't really the engine you aim to support;
you make sure that the connection between the engine and the main chassis is solid and that any forces that acts between the two are diverted safely.
look at the roll-ever structure and where it connects on the main tub and you'll see what i mean.

Up untill the late 60s all a cars chassis had to do was keep the engine off the ground, and make sure all 4 wheels stayed in the right place relative to each other.
Then someone decieded race drivers were valuable and a chassis had to be able to protect them in a crash, and things started to get complicated.
So most of the strength in a chassis now days is aimed at protecting the occupants during a crash, with only a small part aimed at the basic job of keeping the engine off the road, and the wheels on it.

Add in all the different class rules in differnt types of racing, and there is no longer one type of chassis design that is superior to any other.
Start talking about road cars and it gets even more complicated as you have to start thinking about things like pedestrian safety and noise and vibration.

Throw the incrediabl versatility of modern materials like carbon fibre and bonded aluminuim and you can build what ever structure you like designed to behave in what ever way you like.

oh sorry for the late reply, but yea good point, you can do alot with modern matierials.

and going slightly differently, the porsche carera Gt has a full carbon fibre monocoque passenger cell and carbon fibre frame on the front and rear; essentially a chassis made of different ways for different parts for different features.

Yea I heard about that. Isn't it like the 1st car ever to incorporate plastic into the chassis