Help with Mid Engine Design

[veloce]

Just need to say thanks again for all the info and if you find anything interesting please let me know.

It has been awhile from this post....just wondering if you have been able to start on your dream project?

[SIMPLETON]

If your not dead set on using a V8 then you could use a subaru flat 4 or flat 6 with its tranny and block out the rear drive. These engines are really short front to back but might be a little wide depending on your design constraints.

[cmcpokey]

It has been awhile from this post....just wondering if you have been able to start on your dream project?

Last Activity: 06-13-2004 02:15 PM

hasnt been on for a little bit. dont think he's checking this anymore

[Mr-Supercar]

im thinking the same as you. im designing my car now its a cross between a pagani zonda, noble m15, ferrari 360, and a super touring car it touchs the zonda and super touring car the most i was thinking of using the 572 chevy big block but it would have had to much power so im dropping it to 600hp im just having trouble finding the engine. im no mechanic but im looking into it suspensions etc you know the drill. if anyone could help me in explaining the parts needed i have the money to dip in but im just not sure how this works btw 800 thats gonna be a beast. good luck who knows we might end up producing our cars in the future

[Rockefella]

im thinking the same as you. im designing my car now its a cross between a pagani zonda, noble m15, ferrari 360, and a super touring car it touchs the zonda and super touring car the most i was thinking of using the 572 chevy big block but it would have had to much power so im dropping it to 600hp im just having trouble finding the engine. im no mechanic but im looking into it suspensions etc you know the drill. if anyone could help me in explaining the parts needed i have the money to dip in but im just not sure how this works btw 800 thats gonna be a beast. good luck who knows we might end up producing our cars in the future

A 572 probably isn't the best choice for something in the realm of a 'Pagani Zonda'. You'd be better off going with an LS7 if you have money to dip into.

[Mr-Supercar]

yea but i was looking in the region on 600+ hp naturally aspirated engine but maybe the LS7 isn't so bad do you know the price range ?

[Simon_Tuman]

Lets bump this thread back up, but lets try to bring it back down to earth. I'm in the process of building a car Ive dreamt about for years. Its an A1 chassis Volkswagen. It also has a big hole cut in the rear. Ive always wanted to build a mid engine/ rear wheel drive Jetta Coupe. I saved this car from someone else who gave up on the project. He got as far as cutting the hole in the back. The jetta coupes are fairly rare, but they are basically a rabbit (or golf) with a trunk...

Ive done a bunch of research so Im not asking how to do everything, but more about how I can achieve that elusive 50/50 balance.

Heres the basic plan. Take the entire motor and drivetrain and move it into the back. simple right?

Some rough numbers to get us started. Factory curb weight is right around 2000lbs and roughly a 60/40 fr/rr bias. so lets say thats 1200lbs in front and 800 in the back. Im estimating the motor/ trans combo to be around 400 lbs so by moving that to the back we can flip the 1200fr/800rr to 800fr/1200rr and shifting the bias to 40fr/60rr. obviously this isnt a real world number, but it should give us a place to start... so now we need to move 200lbs back over the front wheels to get it to 50/50. i know it will be difficult getting it to 50/50, but thats my target, the short wheelbase will already make it tail happy. i really want this car to handle and it will probably be a track only car because it might be hard to make a car like this street legal.

ive already decided on a few things. radiator will be mounted up front in the stock location.the fuel tank will be right in between the strut towers in front and the battery will also be in the front. i dont think thats enough weight in front though...

motor and trans will be mounted to an A2 subframe and I will be fabricating frame rails in the back so the whole subframe can be dropped effectively getting the entire motor and transmission out without too much difficulty.

i can go on, but ill end this first post now and post some pics of the project car

[Simon_Tuman]

heres where it has been for the last 5+ years



the hole


another view from when we pulled the planned motor out


thats a VR6, but Im planning on starting with a 2 liter 8valve motor with a 16G turbo from the mitsu EVO8. i already have a ton of parts ive been gathering.

as far as suspension goes, in the front i will raise the strut towers and just run a stock A1 setup up front and I will be fabricating new struts towers in the back using essentially an A2 front suspension in the rear. koni adjustables and ground controls, front and rear anti-sway bars. full cage, gutted interior, with a dash and one seat... i think that covers the idea behind the car

ok so ill stop here and wait for some responses, but here are the overall goals of the car. as close as possible to 2000lbs wet, 50/50 power should be around 300bhp, though I think I can get to 300whp without getting too wild on this first setup.

[culver]

Why do you want 50:50?

[Simon_Tuman]

why not? the few people who have converted FWD VWs into RWD VWs all complain about how tail happy they are so Im trying my best to build it properly. most are hack jobs or have everything in the back and nothing in the front. do it right or do it twice right?

[culver]

Again, why 50:50? What makes you think that is the number to shoot for? What makes you think the problem with the other cars was a rearward weight bias vs poor suspension?

[Simon_Tuman]

Culver, thanks for challenging my thinking. I decided I better do more research and found a great archive

http://www.ngnl.net/hall.pdf

Though I didnt understand everything stated 100%, I will be reading it over and over until I do. I also have some books I have been reading about suspension design and race car dynamics. The article above dispelled my determination for 50:50 bias. Thinking about it more, I am willing to bet most of the people who gave up on the rwd VWs werent used to having a cars ass swing around nor were they designing a car to be a competitive track car, but more for the purpose of rear wheel burnouts lol. ... i must admit years ago that was most of my motivation too, but as the years have progressed and Ive gotten older (im turning 30 in a few weeks) I really want to build a car that satisfies my love for racing and my love of vws and having a dedicated FWD race car just doesnt seem to fit the bill, so now this is the car that will.

[culver]

I'm glad you caught on to what I was saying. Many people read the marketing stuff and assume 50:50 is ideal. At one point I believe most of the marketing stuff as well (Hp/L, 50:50, etc). Once you start asking WHY you find that often it isn't what we think. I think my favorite is the Corvette guy who replaces the leaf spring with coil overs because he knows he want's "independent" suspension. Then he asks the forum if he should remove the roll bar while he's at it!

Here's a post I did a while back.

I put this post together to look into the whole matter of 50:50 weight distribution. I’ve seen this come up many times and I suspect that many people don’t know why it is good or when it may not be good. I have a friend with a BMW who swore the car handled well because of the 50:50 distribution. He seemed to have overlooked so many other reasons and stuck with the one delivered by BMW marketing. Of course please keep in mind that I am going into general terms and the handling devil is often in the details.

As for 50:50, it’s not really the best but there are reasons would want it. For straight line pure performance cars you would want a rearward weight bias. It helps in both acceleration and braking. It’s in cornering that we have to look at the details.

Start with changing direction. When changing direction the front tires act like a lever turning the car about the center of gravity. When dealing with a lever a longer lever always makes it easier to move something. So how do we get a longer lever, we either make the wheelbase longer or we move the CG back. A rearward weight distribution will move the CG back and effectively act as that longer lever. This is why mid/rear engine cars typically have very quick steering response.

Now that we have started to turn we need the tires to hold that turn. The next thing we need to know is that tires grip more with more weight (go figure) BUT the increase in grip is proportionally less than the increase in weigh. So if increase the load on a tire from 500lb to 1000lb you will not get twice as much grip. Also, grip is proportional to area. This is a surprise to no one. We all know wirer tires grip more than skinny ones.

So you want to pull some Gs while turning. Well if you are going into a turn without accelerating or decelerating the lateral load on the tires will be split between the front and rear wheels based roughly on weight distribution. So if you have 50:50 weight distribution half the cornering load will be on the front axle. That works out nicely for most cars as they normally have equal sized tires front and rear. However, if we have more weight in front we would want larger tires in front to balance out the grip (GM does this on the V8 Grand Prix). If we have more weight in back we want larger tires in the back. The larger tires, in combination with the heavier load effectively make it so that the front and rear tires have the same load/ square inch of contact pact. That will make the front and rear tires slide at the same time (neutral handling) when cornering. It may seam odd, but GM put larger tires in the front of the Grand Prix because that makes the car more neutral rather than understeering.

So what about braking while turning or accelerating while turning. Each of those will transfer weight fore or aft while cornering. Shifts in tire size or weight distribution can make the car under or oversteer more or less based on the same principle I mentioned above. Increasing the front tires in size or adding load relative to the rear will make the car more likely to over steer at the limit (not talking about initial understeer). Going the other way makes the car more likely to understeer.

So do we still want 50:50 weight distribution? Well that depends. If the car is FWD, 50:50 doesn’t help acceleration. That rearward weight shift moves too much weight off the front wheels and we get wheel spin. For that reason I might want my FWD car to be nose heavy. Of course I can deal with some of that nose heavy understeer with some larger sized tires in the front and suspension tuning. Then again, most people would prefer equal sized tires just for convenience. For the other reasons (braking, cornering) we would want a rearward weight bias even in a FWD car.

In a RWD car we don’t need weight over the front axles for acceleration so we want something like 50:50 or even rearward biased. If we are going to have equal sized front and rear tires we want around 50:50. That will help keep the car approximately neutral in most handling situations.

If we aren’t stuck with equal tires front and rear then a rearward weight balance can work quite nicely.

While many car companies love to brag about the 50:50 weight distribution, it’s really only good under certain circumstances. Depending on other design decisions it may be undesirable (FWD, RWD with a rearward weight bias and wider tires in back).

PS: The above is certainly not the end all be all on this subject. There are lots of detailed reasons why designers may want to do something different than what I have listed above. All that typing is largely meant to illustrate why something that many people think is a pillar of handling isn’t a pillar in all conditions.

[Matra et Alpine]

Another very important point when looking at cornering in particular is the "polar moment" which is sinmply where the weight is in relation to the centre of the vehicle.

So if in trygin to get 50:50 with a rear wngine you then try to put lots of stuff as far forward as possible - because that will get 50:50 then you ahve something VERY bad.
As much as possible you want all weigh in the centre of the car. So if the car is 1m behind the centre in "theory" you want to put the same weight 1m ahead. WHcih is half the weigth, 2m ahead. Not do that and that weight 2m out acts differently during cornering and is bad. So often you DONT go 50:50 so that you can keep the polar moment of the mass closer to centre !

But as this all depends on steering and suspension and at the end of the day are you wanting a car that "drives" or one that can win autosolos ? Because getting the weight and handling is the bigger challenge than just gettin gthe engine/drivetrain in which is going to be a hard enough task

Best of luck with it and look forward to reading further musings !!!

[pimento]

I often see 47:53 being referred to as perfect, I think the idea being that you have more weight over the rear for acceleration then when you brake the shift forwards tends to even things out. I think that was in regards to FR cars though.