Modified Trailing Arms

[J P Stein]

2100 lb minimum with a 6 up to 2.8L.as of 2002.

[clubracr]

After a closer read I see that this must be FP SCCA Solo. I was thinking FP Club Racing. Glad I was wrong. We don't need another weight increase.

[Jim Smolka]

Carolina Speed Werks in NC has mfr a custom trailing arm setup for a 914. Requires relocation of the stock mounting points and a cage. This setup is used on Andy's car.

Jim; how could I get more info on what Carolina Speed Werks is doing?

[Jim Smolka]

Drew,

The best way would to be to call them, 336.475.6570 in Thomasville NC.

Andy's 914 is amazing. Very fast turbo GT 914. Back when the car was a PCA GT3 car, it was passing most of the other 'faster' Porsche in the higher GT classes. Now that it is a Turbo car, WOW.

[M. Hendrix]

If unsprung weight is so important, why is there not a chassis mounted spring system to counter it?

I know that's more weight.. but you could compensate all over the place..

-throws hands up-


M

[Jim Smolka]

The additional spring would add another 'forward transfer' function to complicate the whole controls scheme of things. Mechanical and Electrical Engineers are taught a bunch of silly math to model controls (springs, electrical stuff, etc). Then a feed back loop is added to 'control' the fwd transfer function.

Anyway, the more stuff added, the more complicated the model becomes.

[J P Stein]

Compensate for inertia?...."Scotty, is that inertia compensator ready yet?"....

Unsprung weight must be important ......otherwise *they* wouldn't have invented a term for it.....I figure.

[914GT]

I'd think most of the 'unsprung' weight is the wheels and tires, which nobody has mentioned yet. Is two pounds on a trailing arm all that significant compared to the total mass of the arm, hub, stub axle, wheel, and tire? Obviously bigger wheels and tires improve traction but at expense of added weight. I guess I'm just trying to understand whether or not a couple pounds here or there makes any difference when considering the whole system. And even if you had almost zero unsprung weight - wouldn't you have to compensate for that with stiffer shocks/springs? In other words, the mass of these parts provides for some of the desired dampening effect, at least up to a certain point. If you could choose where to shave off a few pounds, would it be best at the wheels/tires since this is rotating mass? Or doesn't it make any difference - mass is mass whether the engine is trying to turn it or push it forward?

[M. Hendrix]

I just imagine a shock that lays down on the same axis as the car, just fore of the middle of the arms, attaching to the body, and the control arms. -shrug-

They do that on rock climbers.. n' mud dragsters.. kinda..

M

[J P Stein]

I did wheels & tires last year:

Went from 225 X 50 Kumho V racers on BBS 7 X 15 .....43 lbs ea
to 225 X 45 Hoosiers on 7 X 15 Fuch.......33 lbs.....Izat gud?
I gotta admit that 2 lbs ain't much, but it is over 10% of the weight of the naked arm. If one could do that with the whole car, 220 lbs, eh?

Just wait till you see my "new" doors.
I'm workin' at getting down to miminum weight for my class....2100 lbs. Since I added a 10 point cage...80-90 lbs it becomes a bit of a challenge.

There is no advantage to unsprung weight.....zip, nada.

The only "inertia compensators" I'm aware of were the active suspensions of F1 cars in the mid/late 80s (IIRC). It was done with hydraulics & computers. It worked so well .....when it worked....that they banned it. From what I've read, it got reeeal ugly when it went berserk.

[Dave at Pelican Parts]

Originally posted by 914GT
I'd think most of the 'unsprung' weight is the wheels and tires, which nobody has mentioned yet. Is two pounds on a trailing arm all that significant compared to the total mass of the arm, hub, stub axle, wheel, and tire?

Yes, as a matter of fact it is. Unsprung rotating mass in particular is extra bad stuff. That's why we have alloy wheels instead of bullet-proof strong steel wheels. That, and they look Super Kewl... And one of several reasons that Hooser autocross tires kick ass--they're significantly lighter than any of the other tires out there.

The thing is, going light weight on wheels and tires and such involves compromises. A lot of people aren't willing to live with the consequences of such compromises. (Try running over a beer bottle with a Hoosier autocross tire some day. You won't like the result...)

--DD

[clubracr]

I've heard, and read, that removing one pound of unsprung weight is equal to TEN pounds of sprung weight when it comes to handling. So if you can get 2 pounds safely off of your arms it seems worth it to me.

[J P Stein]

I'm afraid I suffer from DWD......"while I was there" I figured it couldn't hurt. I just got done painting the other side, now I gotta start putting it back together. Clean stuff is more fun than dirty stuff......another rerun pic. ........



Those bearings were the real focus of this exercise.

[M. Hendrix]

Man, that is ugly!



M

[sammyg2]

JP, what kind of wax you gonna use on that thing?
(trying to resist the whine and cheese influence).