Learn me: rear engined cars and handling

I'm finally seeing the light at the end of project frankenfiat's tunnel, and now I'm getting a little worried.

I'm mixing together a 67 fiat 850 spider, Porsche 915 transaxle and subi twin turbo ej20.

Problem is I've never driven anything fast that was rear engined. My dads 84 vw vanagon wasn't exactly a track day star.

A little more about the car. The front transverse spring is gone and replaced with an abarth a arm and Coil over setup. The previous owner had raced this in scca solo for years. The car is tiny, wheelbase is 79.8 inches and track width is 47 finished weight should be less than 1400 lbs. Miatas look obese next to this thing. It will be riding on 13" Hoosiers.

I have no idea what this thing will handle like, any insights, tips, or hints?

I think my first trip to the track will be scary

Have you ever thrown a dart backwards?

icaneat50eggs wrote: I think my first trip to the track will be scary

It will be. Start slow, once you get going fast with dimensions like that, things happen REALLY quick. Also, turn the boost down/off at first...that adds another level of trouble while working out the kinks. I still haven't made time to start improving the suspension on my N600, but I know there's a hell of a lot of room for improvement to make it anywhere close to modern expectations for handling/safety. If you figure it out, let me know.

Where can I see this project...are you posting pictures or a build thread anywhere?

Bryce

Add a huge front sway bar to bias handling towards understeer as much as possible.

Remember the GRM Berkeley? (The car, not the explitive). The width was simply not maneageable in terms of getting the power to the road. Although you have the weight on the preferable end of the car, you'll experience similar issues. If it were me, I'd widen the track abunch and reconcile yourself to big-ass flares. Otherwise it'll be tip-time.

Also, I'd run bigger rubber in the rear than front.

Very cool project though!

Don't lift.

Flares/ bigger wheels and tires are the first on the list of mods once I get it up and running, 13 inch tires are hard to come by so I'm going to try to move up to 15

What problems or where can find the berkely project? it scares the Berkeley out of me to be compared to something that is now used as an expletive

I'll put together some pics and a crude build thread my other sites have been wanting the same thing

I'll stay tuned in here for sure. (I have an affinity for AC VWs and have long considered building a VW-powered Locost 7).

Good luck and hang on.

icaneat50eggs wrote: [It] scares the Berkeley out of me to be compared to something that is now used as an expletive

That is beautiful. I think it belongs in "Say What?" in a very special way...

Look here:

http://grassrootsmotorsports.com/project-cars/berzerkeley/

I don't want to say it couldn't be awesome since Abarth did bolt a 2 liter into the back of a few 850 chassis cars, but by installing a monster of an engine back there you have taken an excellent handling car and probably ruined it. The stock engine weighed ~125lbs. What is back there now? 300?

My advice is to flare the rear enough to fit some 245's, go with something closer to 195's in the front and don't even think about lifting off the throttle in turns. Move any weight you can to the front. Gas tank, radiator, expansion tank and perhaps even add ballast. That will be an awfully high polar moment of inertia though.

This is what Abarth did with the tire technology avaliable in the late 60's

They also did a double coilover "pendolare" rear suspension

The cars had a lot of development and never went into production so no one really knows what they were like to drive.

Dude. You gotta post pics of this!

Regarding your original question, be quick on your hands to manage power-on oversteer. Expect to spin a couple of time. Should hopefully be fun.

Treat it like a 930 - very slow into the corner, but use the additional grip you get out of the corner.

Whatever you do, don't lift mid corner unless you like to get the panoramic view of who's behind just before you make a hole into the tire wall with the rear of the car.

BoxheadTim wrote: Treat it like a 930 - very slow into the corner, but use the additional grip you get out of the corner. Whatever you do, don't lift mid corner unless you like to get the panoramic view of who's behind just before you make a hole into the tire wall with the rear of the car.

A former co-worker and PCA racer/instructor always said most 930 crashes saw the driver "backing into things"...

Doesn't surprise me - even in the "normal" 911 of that vintage you are very aware of the weight in the back, add to that the notmuch-notmuch-underwear change power delivery of the 930 and you're in for an interesting time.

The problem is that you can feel the weight in the back in corners on these and at least I have to actively fight the urge to backing off.

Bah.

I need another 911. Badly.

It took Porsche many, many decades to tame it so don't expect to get it right the first time out. Do what you can and then go test.

If it's still in one piece make some changes and test again. Hopefully you won't test to destruction.

The major issue with 911 SC and 930 handling problems is terrible rear suspensino geometry. They have dynamic toe out so when you lift, the car tries really hard to put rear wheels ahead of the front wheels. What's your suspension geometry? What's the weight balance front to rear? What's the suspension frequency front and rear?

Start out with what you have. See what it does and that will give you an idea as to what it needs. It will be a learning process .

I think GRM had a story on how to handle a Porsche 911 around a racetrack. I don't remember which issue, though. Might be worth a look.

it's going to have a steep learning curve.. but as long as you do not damage the car, should be fun. Just be prepared for a damaged ego

I am watching this thread with much interest.

Much interest indeed...

Boxhead Tim is right, treat it like a 930 your first time out. I swapped ends viciously in a 1969 911 Targa because I tried fast in, correct speed and position, fast out, let up on the throttle at what seemed the right time and all KINDS of stupid stuff happened real quick.

From a mechanical standpoint, if you can get access to scales see what the corner weights are. Then maybe you can move stuff around (battery, ECU, etc; it adds up). If you move the fuel cell, remember that fuel weighs ~8 lbs/ gallon and as the cell empties your weight bias can change big time! Try to keep the weight between the axle centerlines.

Porsche put lead weights inside early 912 bumpers to help with weight distribution, so don't be afraid to add if needed. It may very well be that the weight bias correction done by adding 100 lbs in the front of the car (again keeping it inside the axle centerlines) far outweighs any loss of acceleration due to that additional weight.

Go play with karts, even if it's at your local Malibu GP or something. Seriously. They won't prepare you for the power, but they will prepare you for the balance.

Hey guys, I started a build thread

frankenfiat build thread

thanks for the interest, I'm going to pull some comments off this thread and address them in the build thread. I know I'm going to need lots of help getting this thing to perform!