Modified Trailing Arms

[bumble]

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

[J P Stein]

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.

[bumble]

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

[M. Hendrix]

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.

[Bleyseng]

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

[Dave at Pelican Parts]

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

[J P Stein]

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....

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
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.

I inspect this area after every AX.

[bumble]

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 .

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

[JayDarden]

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. . .

[Zeke]

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.

[Dave at Pelican Parts]

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

[M. Hendrix]

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

Roof skin


M

[bumble]

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

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 . Gotta go - men in white coats have just arrived.... Agggghhhh


Mark

914/6 - heavy...

[Eric Fry]

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?

[Bleyseng]

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.

[J P Stein]

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.

[clubracr]

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

[Brant]

club racer....

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

thanks
brant