Modified Trailing Arms
 

Hi all,

Looking at ways of improving my tarmac rally car, it occurs to me that the 914 trailing-arm setup is less than ideal. They are damned heavy, especially if they are boxed (unsprung weight!) and the forward pivot points are quite close together.

(From behind protective asbestos shield ;) ) IMHO the 911 banana-arm approach is superior. Does anyone have any experience with modifications in this area e.g. light-weight/modified arms, adapted 911 bits etc etc?

Just a thought!

TIA

Mark

914/6

Anything is possible ..... given sufficient resources (mostly $$$$)

The 9eleben trailing arm is a totaly different set-up using a torsion bar. Personally, with the troubles involved in adapting that set-up, I would go with double Arms on a subframe. There is simply no room for torsion bars....they would be deep into clys 3 & 6. Frankly, trailing arms suck as do Macphearson struts....but that's what we have.

I am doing some grinding on the casting in which the rear wheel bearings mount. I was able to get 2lbs of weight off the unsprung end. The one naked arm now weighs 13 lbs.

Something else to consider. Most sanctioning bodies won't allow changes to the suspension pickup points.....or into some modified class you go.

JP, yes regulations will certainly limit what can be done. Actually, I'm not really unhappy with the handling as it is - the thought process is more related to trying to shed weight.

My car weighs 980kg (2156lb) and I'm trying to get it down to ~930kg. Since you need Arnie's muscles to lift them into place, it seems to be an area of potentially significant weight saving.

Since it's such on obvious area, I was thinking that someone out there in 'teener land might have come up with a lighter solution. Getting rid of 2lb off each arm is certainly worth the effort - but how do you do it without weakening them?

Cheers,

Mark

Grind areas that aren't stressed as much, around things.. grind flash off.... square looking things can be rounded.

I have seen a guy racing on arms I wouldn't drive to the store on... I am sure his cornering loads are at least 2-3 times what mine are, it's rare that I slide all four wheels, although there was this time last week...

M

I'll be interested to see if JP's arms hold up at AX next season. I nearly ripped my right side arm in half. I replaced them with boxed arms. Some people say by boxing the arms you're just transferring the stress to other areas and they are right. Since boxing the arms I've had to reinforce my console ear and add the chassis stiffening kit, all after seeing stress cracking show up in different placed at different times. I thoroughly inspect the rear suspension after every event now. I'd still rather be pushing more weight than have my suspension rip apart on the track.
As far as mods go, there was a post last year by ajserrano showing pics of his trailing arm mods. He did spherical bearings, heim joints and other radical changes. Maybe he will post here and tell us how they worked out.

Just clean up the arms like JP says. Don't box them as it transfers the stress to somewhere else and it rips or cracks it. The arms are strong but do bend somewhat which is good.
What rear springs are you using??

To lighten the car, get some fiberglass front and rear hoods and bumpers as that saves a ton of weight. Strip out all the soundproofing including that crap on the floor pans.


Geoff

I have seen a number of tubular steel substitues for our semi-trailing arms. Same mounting points and all, but steel tubing instead of mostly-square-section stuff. Supposedly saved weight that way. One was, I think, made from aluminum. Definitely saved weight. It also had some way of moving thewheel bearing carrier so as to change camber and caster right at the end of the arm, not by moving the whole arm the way we have to do it now.

I don't know if they sold more than one set of those...

One person around here somewhere has posted pics of a 911 rear suspension in his 914-6. Or rather, a lot of the suspension; I'm pretty sure he used coil springs instead of torsion bars. He had to build a subframe to mount the parts.

Not worth it IMHO. Just lighten it up as JP is doing, and follow Geoff's suggestions.

--DD

I didn't know how much weight I could get off them without compromising strength, but it was worth a shot, I figured.

Here's a rerun pic....http://forums.pelicanparts.com/uploa...1071171866.jpg

Not much contrast, I'm afraid. There are several spots that I went after, some of which are brackets that I no longer use.....ebrake,
dust shield. Then there are 2 casting risers that were gas axed off during manufacture. I also reduced the 4 bosses that are used for the bearing retainer bolts......and for strength, me thnks.
Maybe 1/8 inch off the radius.
All the weld spatter had to go...prolly a half pound right there:D
The flange around the CV hole sticks out bout a half inch...not any more, but I left all the weld fillet. Basicly ground, sanded the whole casting smooth and put good radii everywhere.....sharp edges are not gud.

On the suspension mounts: Long ago I welded (& re welded all the lousy beads) all the spots the factory missed. Then seal welded everything else....paid particular attention to these ears around the outside edges. http://forums.pelicanparts.com/uploa...1071172841.jpg

I inspect this area after every AX.

Thanks guys! My car has had a lot of weight taken out already - just about anything that's superfluous or cosmetic has gone. Has fibreglass bumpers, lexan rear window, Boxter monoblock calipers (real light!) etc. etc. One of the main reasons I kept the 901 tranny is it's light weight!

If Santa leaves a digital camera, I'll post some pics ;)

I'm compiling a list of items that can save more weight such as:

- fibreglass hood/trunk

- lighter engine (currently uses early sandcast cases which are supposedly 10kg heaver than magnesium) and I'm going to build a (hopefully, killer) short-stroke 2.5L over the next year with mag cases...

- lexan side windows

- lighter top - does anyone know if there is such a thing available?

- I vaguely remember reading somewhere that from '73 the doors were lighter. Is that true, and if so by how much?

- Any other suggestions???

The trailing arms are already boxed but I've never seen an explanation of what problem this solves and where other problems are likely to appear. Any enlghtenment would be welcome :confused: .

Of course, the requirements for a tarmac rally car are somewhat different than those of an AX or track car. We don't sustain such high consistent g-forces but some of the roads are fairly 'ordinary' and the short-duration, high-amplitude forces must give these components hell.

Thanks all for the info and the pics....

Mark

914/6

Dave, I might be the one you are thinking of - I have 911 bits mounted up to coil overs and the subframe for my V8. I would love to drive it back to back with another conversion car. . .

Mark, it's the early doors that don't have reinforcing in them. And if you replace the 2 trunks with FG, you should peel off more than 50 lbs. Aluminum 911 cross member will help. Somebody has got to make a skin for the top. Mine is bolted on, so no brackets. With a skin, you could do the same. Headers are light too.

Up through early early 73, 914 doors did not have side impact beams. Those doors are significantly lighter (15 lbs each? More?) than the later ones.

Scrape the sound deadening out of the car--you should be able to get it lighter than that!!

--DD

http://www.rennspd.com/Products.asp?Task=STD&ModID=13&catID=11

Roof skin


M

Yeah - I agree dammit! it should be lighter. But - ahh... we might have got a wee bit carried away with the roll cage - sucker weighs 36kg (79lb) so we start out behind the eight ball :( . On the other hand the chassis is now real stiff :D

Pretty much anything that can be gone, is gone - including any sound deadening stuff. Doors are original 1970 items - pity, I was hoping there might be gains there - thanks for the info Dave...
Now the main gains will be had by adding lightweight body parts - thanks for the link to Rennsport. - and our $ is starting to look a bit less peso-like...

After that, we need more power... the more the better - right? Can't have too much power :p :p :p :p :p :D :p . Gotta go - men in white coats have just arrived.... Agggghhhh SmileWavy


Mark

914/6 - heavy...

I am curious about the weight of your guys cars. I have a 2.7-6 in a 76 body, early chrome bumpers, 8x15 fuchs, the easy out sound insolation (both sides of fire wall) but the floors have not been touched. Now with that said, in auto-x trim (empty w/light fuel load) mine weighs 2050.
Just lucky?

Eric, its the 79lbs of cage he's added. Go get the fiberglass hoods from Getty as they are as nice as the orginals made by Porsche for the GT's. You won't have to do 15 hours off repairs to make them fit like some of the other fiberglass pieces made.http://forums.pelicanparts.com/uploa...1071245665.jpg

A quick check of the PNWR rules says those will put you into the Mod class.....damn rules.....not to mention $1K to loose 30-40 lbs. They are nice, but not cheep.

BTW, Eric, you'd have to add 50lbs of ballast to be SCCA FP legal. I still haven't weighed mine.

You'll probably see about 100# variation between posted weights. Some of it because of the gas load. For comparison my car recently weighed 2090# with 1/2 tank of gas and no one in it. I have FG hood and trunk and full cage. I've stripped all insulation and back pad from the interior but added the chassis reinforcement kit and boxed trailing arms. That's with my 15" fake fuch's which weigh more than my 16" Fuch's I race with. I also have a Chalon body kit but I'm not sure that is saving me any weight. I think 2100# is pretty normal for a mildly prepped AX/track car until you start eliminating every item not needed for racing.

JP,

Have they changed the FP weights this year? I sat out for 2003 while building my garage. In 2002 my car had to weigh 2020 lbs at impound with driver. I already have to run triple digit ballast. Won't say how much though ;)

club racer....

how much metal do you have left at 2020 with ballast?
Is this a 4 or 6?
any pictures?

thanks
brant

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

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.

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?

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.

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

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.

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.;)

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?

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

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.

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

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.

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. ........

http://forums.pelicanparts.com/uploa...1071470139.jpg

Those bearings were the real focus of this exercise.

Man, that is ugly!

:)

M

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