Chassis mod list for autocross car

[JohnJL]

somehow Ive managed to get all my other projects running as envisioned, so im turning my attention to the last one, a 1978 sc rolling body that will become a lightweight canyoncarver and basic autocross car. i aim to get the weight under 900kgs and already have cf hood, fenders and bumpers.

the question i have here is to cast an eye over the fabrication mods i plan to complete before paint. i will do the work myself, the car will be on a hanging rotisserie for ease of access.

im checking tho local pca autocross rules as well as my roads authority too, but would appreciate some checking of my list....

shave the driprails - ok this is aesthetic really
patch the sunroof
weld all the seams
install raised and adjustable rear suspension pickups
fabricate front camberboxes
reinforce shocktowers
cut front trunk wall and weld in ducting for front oil cooler
gut the doors
cut and reengineer pedalbox area to accomodate dual master brake pedal
cut off misc unused tabs and brackets
cut transmission and engine front access door and weld in dzus tabs for closing fasteners

anything i missed?

thanks!

[JohnJL]

Forgot

Reinforced sway bar mounts

[Flieger]

If you are going to have a roll cage you will need the fiberglass or carbon doors and rear quarter panels too, along with Lexan windows if you want to get under 900 kilos. My car is 2040 pounds with half a tank of fuel (without me in the seat) and it has fiberglass everything, gutted interior, Lexan, and roll cage. Magnesium engine and transmission but steel trailing arms and front crossmember. It is a 1971 911S non-sunroof.

[rothaus]

Hello John,

Looks like you would run in modified or at least improved class with pcs. Or FP by scca rules, like I do.

What engine you gona put in Gretchen? And what size wheels will you use. I wish I had 17" 3 piece wheels. I ended up with 7.5 and 9 / 16 and 245/45 275/45 19 Hoosier A6.

Keep us posted.

Cheers
Engelbert

[JohnJL]

Originally Posted by Flieger

If you are going to have a roll cage you will need the fiberglass or carbon doors and rear quarter panels too, along with Lexan windows if you want to get under 900 kilos. My car is 2040 pounds with half a tank of fuel (without me in the seat) and it has fiberglass everything, gutted interior, Lexan, and roll cage. Magnesium engine and transmission but steel trailing arms and front crossmember. It is a 1971 911S non-sunroof.

Hi,
i am going to skip the rollcage for now. I agree it makes my weight target very difficult to achieve, i dont envision high speeds with this car (the track car is for that) and its been a challenge to get road cert in Australia with a cage and we may end up back there again soon. im hoping the seam welding and reinforcements along with the light weight mitigates the stiffness missed out from a cage.

i am also replacing all but the windsheild with makrolon.

Originally Posted by rothaus

Hello John,

Looks like you would run in modified or at least improved class with pcs. Or FP by scca rules, like I do.

What engine you gona put in Gretchen? And what size wheels will you use. I wish I had 17" 3 piece wheels. I ended up with 7.5 and 9 / 16 and 245/45 275/45 19 Hoosier A6.

Keep us posted.

Cheers
Engelbert

hi Englebert,
im waiting on my pca autocross lead to send the rules. they arent posted on the ucr site and ive asked him twice now with no joy...id obviously like to max out the mods without bumping myself into a class id be uncompetitive.

i have a spare 3.0 i mothballed from the race car, it got 240hp at the wheels on webers. im going to go itb this time.

i have spare sets of 16" fuchs in 7", 8", and 9". in the theme of low unsprung weights i was going to try the 7s in front and 8s in back and see how that did.

i havent got the gearbox yet, it will be a 915 of some sort.

[JohnJL]

This weekend I built a dual master with balance bar pedal cluster. I like a firm pedal and the firewall on these cars flexes so I welded in some reinforcements. The plates are 12 gauge steel and are intended to spread the load. Don't laugh at the welds, I am getting better and I do have good penetration even if they aren't pretty.

If using a stock pedal assembly you need to use an early clutch pedal or gusset and cut yours which is what I did. Not pretty but effective.

[911pcars]

A good friend has a purpose-built early TA Camaro that he autocrosses. An engineer, he's singularly into only what it takes to win w/o falling prey to the latest go-fast gizmos and technology many of his fellow competitors have embraced. He describes himself as "old school".



Since most auto-X courses are fairly short (~1 min or so) with only add'l laps for warm up and cool down (total - 3), he feels no need for weighty, superfluous items like oil coolers, dry sump lube systems, excessive ground effects, roll cage, etc. He runs stock front brakes, rear al. drums, and for a time, a rear child seat; and except for a modern small block (al. 427), sticky R tires, thorough chassis setup (w/leaf springs) and good driving, is surprisingly vintage, fairly light (w/o plastic panels - ~3200 lbs.), balanced compared to contemporary Detroit iron and fast. He manages to do quite well, and with his "fast, but good enough" attitude, has generated enough print in big block, non-drag racing magazines to pose a minute threat to Magnus.

However, he'll admit - if on a full track for more than 3 quick laps and it'll overheat (engine and brakes) - that's how purpose-built (stubborn) he/it is. On the road at legal speeds, it's fine. YMMV.

You might find the following thread interesting as it discusses chassis reinforcement w/o typical roll cage assistance:
Structural Reinforcement

Sherwood

[JohnJL]

Thanks Sherwood, yes that thread is interesting.

I buy the lightweight/built for purpose argument and am tryingto follow that. I am skipping the roll cage, but making some chassis mods to beef up stiffness like seam welding and gusseting some corners. Ditching the sunroof, etc. I dont want the $ or weight of big brakes and with such a light target, I think the stockers with some mods will do well.

The theme for this car is an emphasis on feel for autocross and backroad blasts. Hence my decision to have a small front oil cooler (engine puts out 240 hp and needs it). I am fussy with my brake feel so I thought it worthwhile to ensure there is no flex in the pedal/mount, allow some adjustability front/rear and ease of swapping out different masters if needed.

Also picked up a hydraulic e-brake. I know it wont pass street safety inspection alone, so I will leave the factory e-brake in place until inspected. After that I'd like to see how much unsprung weight can be removed by gutting the ebrakes.

[kjchristopher]

Originally Posted by 911pcars

A good friend has a purpose-built early TA Camaro that he autocrosses. An engineer, he's singularly into only what it takes to win w/o falling prey to the latest go-fast gizmos and technology many of his fellow competitors have embraced. He describes himself as "old school".

Looks like Gerald's car. Haven't seen him out in a while.