boxed trailing arms?

[captjm]

I am wondering if it is necessary and at what Horse Power a 914 stock trailing arm would be expected to twist or fail? Does anybody have a picture of a failed trailing arm? Thanks-

[rfuerst911sc]

I am new to the 914 arena so my opinion probably has little meaning but I'll give my 2 cents. I am planning on a 911 6 cylinder conversion on my 75 chassis. I'm not sure what size engine but I think 2.7,3.0 or 3.2 so a fair amount of torque and horsepower. I initially thought I wanted to box the trailing arms and I still might but after lots of reading online many folks feel that the chassis and trailing arm mounts can benefit from aftermarket stiffening kits but the trailing arms should be left alone. The general consensus seems to be let the trailing arm "sacrifice" itself by bending in a hit or severe torque twist vs. ripping out the mounts or worse. I am definately going to stiffen the chassis and the trailing arm mounts but have not made up my mind on the trailing arms. Some guys think just drilling holes in the arms and welding in metal tubes stiffens the arms enough. I'm curious what others have to say.

[Brother]

I thought this was supposed to be worthless.

[Woobn8r]

Y'know, I'm for stiffening.....anything you can!

If the argument is to let the suspension fail at the weakest point when you hit a curb or other immovable object....I say
1) don't hit the curb.
2) why sacrifice strength you WILL USE, for an event that may never happen.
3) if the stiffer arm prevents failure in a moderate event and the force is dissipated in the suspension....you just avoided a failure.
4) the added weight in negligible...eat less twinkies if you're that concerned.

Just my $0.02 worth.

Sean

[Dave at Pelican Parts]

Racer Chris from Tangerine Racing has had some pretty good success with strengthening the trailing arm internally--with a bulkhead across (or partway across?) the interior of the arm. I also know people who gusset the main part of the arm to the pivot tube, which would seem to me to be one of the places that would flex the most.

I don't think the reinforcements are worthless, but I think the above would be preferable to the conventional "boxing" of the arms.

And on a street car, I don't think it's needed until you've gotten to a pretty wild car. I've done the curb thing before, and I definitely prefer to live with a little more flex in the trailing arms than to run the risk of killing the tub.

--DD

[Brant]

Chris' measurements (actual measurements) showed that the old style stiffening kit is significantly helpful.

in fact nearly as helpful as his new style kit.

also, that a stock unstiffened arm does in fact bend and twist significantly.

I know my favorite 914 race shop in the country won't build a 914 race car without the kit!

brant

[Eric_Shea]

:agree:

You're talking 4lbs for 40% more stiffness.

I've experimented with welding in two 1" steel tubes and gussetting the pivot tube. This is a zero weight gain measure.

Regardless of what you do you'll probably want new race bushing to take advantage of the newfound stiffness.

[captjm]

So it seems that nobody has had a trailing arm fail? Correct? How about an axel that failed? Has anbody had an axel fail? and at what horse power? Thanks guys for all the imput.

[Joe Sharp]

I have had in my hand a bent trailing arm. You could see the waves in the sheet metal. I have heard that the C.V.'s have a point that they fail do to too much H.P. kinda like the 901 tranny.

[captjm]

Thank you Joe, I have sent you a PM. I would like to see where the arm bent. What horse power bent the arm or was it due to a collision. - I have also read that the 901 transmission can go up to 300 HP if one is careful.

[Dave at Pelican Parts]

I bent a trailing arm on a curb. Spinning out and over the curb, actually. The trailing arm was twisted about 30 degrees (lots of positive camber, wooot!). The wheel was wasted, the brake caliper and rotor were broken, the CV (or both?) on that side were broken, and the mounting ears on the back of the transmission broke off.

The tub was fine. A whole bunch of new (or new-used) parts later, and the car was driving again.

--DD

[Brant]

I haven't seen Joe's picture...
but the purpose of stiffening isn't to prevent full failure.
the purpose is to eliminate flex (that creates inconsistent suspension and handeling)

they put roll cages in race car for the same reason... not because the chassis' fails. because the chassis' flex

really only necessary on a track oriented car.

regarding the 901 transmission.
the actual cost/benefit ratio for failure occurs at 250hp when on a track

with track use the 901 holds up for an adequate service life at 250hp

but the 915 becomes cost effective above 250 due to the reduced service life of the 901 at that output.

[Woobn8r]

Well I agree and disagree....

The purpose of a roll cage is SAFETY and the bi-product is a reduction in chassis flex.
YUP, boxed arms flex less....making suspension set up and handling more predictable...the bi-product is strength to withshand increased (or unforseen) force(s)....

However to finish first, you must first finish so preventing failure is even more important especially if a weakness has been identified and is easily corrected....l

On the street this same identified weakness(s) can be corrected and is more of a "no brainer" as you are less concerned with rules or weight or class structure, but are probably more concerned with reliability....unless your tow vehicle and trailer follow you everywhere.

I don't understand why anyone would not consider modifying their trailing arms, unless their class rules prohibited it or it was for concours purposes. It is simply a better arm.

[Brant]

Woobn8r,

good point
a roll cage is for safety.
but yes flex also.

anyways, the arms don't fail
they only flex
again, I have not seen Joe's example but the other failed ones I have seen were a result of accidents/curbs and such.

the Chassis do fail at the swing arm pick up points.
that happens even without an accident
that happens as a result of the chassis not being able to withstand the side loads of racing tires.

it also does not happen on street cars
it is not really dependent upon horse power
but it does happen on race cars with race rubber.
it results in total failure of the suspension point.
it is not addressed by swing arm strengthening.
it is an entirely 2nd subject and 2nd thread
it is addressable
and again it is not necessary on street cars.

brant

[captjm]

I asked on a new thread about what can be done about swing arm support stregthening. I was looking for some pictures of any failed arms or suspension conection points that can be analyzed.

[Woobn8r]

anyways, the arms don't fail they only flex -agree. Is flexing good? Would a vehicle handle better if this flexing were reduced?

failed ones were a result of accidents/curbs and such. -Stuff happens on track and off...

the Chassis do fail at the swing arm pick up points. -Agree

it also does not happen on street cars - Are you sure?
it is not really dependent upon horse power -Agree
it is not addressed by swing arm strengthening. -Agree

I was strictly adding my $0.02 to the swing arm debate and not that of chassis design/shortcomings.

If a part can be improved (at a reasonable cost/reward) do it.

Non flexing arms are better for suspension set up and handling...
strengthened arms will survive more punishment (curbs, potholes etc).
Failure by contact or overloading is still failure.

Some street cars are used at DE/lapping days and are not pure racers....I say swing arm modification is a definite.

I'll save my Chassis opinions for the other thread.

[J P Stein]

Somehow my car has survived without boxed arms....for 5-6 years or AX on a very rough venue. 4 lbs each of unsprung weight goes against my grain....unless I can see a need.
I took 2-3 lbs out of my swingarms, it fact

I have well above average chassis stiffness, tire, power, springs, with pratically no rubber in the suspension....rear shock tower rubber only. No " chassis stiffening kits", tho.

The reason I bring up chassis stiffness is because it is intertwined with the suspension. The more chassis stiffness, the more load is absrobed by the suspension bits.....the same with the lack of rubber in the suspension. The suspension is supposed to do the work, not the chassis.
The metal bits in the suspension hold the alignment.
This bit of info comes from a guy out flogging his car, not just talking about theory.

I've done a bit more chassis work this winter to improve it's
stiffness. The front & rear shock towers are tied into the cage.

[Brother]

anyone have a picture of a boxed trailing arm?

[84toy]

[ahblain]

There's got to be a reason why it was part of the 914-6-GT kit in 1970. Secondly, I'm curious what thy did to the trailing arms when Porsche stuffed the 908 engine in two 914s. As for the CV Joint, they growl before going. I blew one out once - it leaves you really stranded.