Merits of chassis strengthening & boxed swing arm kits ? Options

[amallagh]

I'm doing a 914-6 GT conversion project and just trying to decide if it is worthwhile fitting one or both of these kits ?
Do they both make a real difference to the handling or does one of them make 90% of the improvement ?
(Money being no limit, then I would probably just fit both, but if only life was that simple ! - If both deliver a notieable improvement then I'll fit both)

Any advice welcome from all you racers out there !

[jdogg]

The best bang for your buck is the Engman chassis stiffening kit offered in the resources section. The GT kit would make a nice complement.

There was a long, detailed engineering debate on boxing the trailing arms last year. Racerchris was expirimenting with several different methods. Do a search for the thread, I don't recall the conclusion (or if there even was one), but it was shown the GT boxing did little more than add extra unsprung weight. The discussion also centered around the need to stiffen the trailing arm mounting to take advantage of the stiffer trailing arms.

[So.Cal.914]

I think it was Eric Shea that said something like this, What would you rather have

bent, your trailing arm or your tub/attachment points. A trailing are is alot

cheaper to replace and more available.

[J P Stein]

I seam welded everything from the fire wall back.
I have a cage that mounts in 10 places but is not(yet) tied into the suspension towers.
I have no stiffening kits and no breakage after 6 years of autocrossing on *very* rough venues with big sticky buns in each corner & stout springs. You may well ask why I would want 40ishlbs of road hugging weight. (IMG:style_emoticons/default/biggrin.gif)

[drew365]

I think it was Eric Shea that said something like this, What would you rather have

bent, your trailing arm or your tub/attachment points. A trailing are is alot

cheaper to replace and more available.

If I had my druthers I 'd pick neither one of them bending. I boxed my trailing arms because one of them nearly split in half. I didn't like that. I admit that since I boxed the trailing arms I've had multiple chassis flex/crack issues. I keep reinforcing from the roll cage to the shock towers with triangulation, circles and arrows and three part harmony, and I feel I've now got this tub stiff and safe. I really don't think Porsche engineers designed these cars for heavy racing on sticky Hoosiers. So, if it's not a track car I wouldn't box the arms. If it is a track car you better plan on fully inspecting it between each event because eventually somethings going to give.

[McMark]

The conclusion of that thread was that boxed trailing arms had one and ONLY one effect.... they added weight. The FEA analysis proved that boxing the trailing arms did not prevent arm twisting under load. Chris Foley can do more to strengthen a trailing arm with 8 oz of metal than the pounds added by the boxing kit.

[Krieger]

I checked the factory welds on the trailing arms for my six project. They were not anywhere near as good as the welds on my 75. So I spent some time (IMG:style_emoticons/default/welder.gif)

[Joe Bob]

Boxing the arms is a waste of time....

[Brando]

Spend more time and capital re-inforcing the tub than the trailing arms.

They're already pretty damn tough.

[effutuo101]

I put a kit in my last car. my next car will get one as well. I noticed a huge loss of flex. I thought the car handled better afterwards.

[Bleyseng]

Engman kit is the one to install. The other don't really do much at all. Do a search for detailed threads about "why".

[amallagh]

Engman kit is the one to install. The other don't really do much at all. Do a search for detailed threads about "why".

When you say the 'Engman kit' do you mean the chassis stiffening kit mentioned on some posts above, and therefore not bothering with the boxed swing arm kit ?

[brant]

The best bang for your buck is the Engman chassis stiffening kit offered in the resources section. The GT kit would make a nice complement.

There was a long, detailed engineering debate on boxing the trailing arms last year. Racerchris was expirimenting with several different methods. Do a search for the thread, I don't recall the conclusion (or if there even was one), but it was shown the GT boxing did little more than add extra unsprung weight. The discussion also centered around the need to stiffen the trailing arm mounting to take advantage of the stiffer trailing arms.

Jason, Mark, Zois...

I disagree with your interpretation of the Racer Chris testing.
the Thread proved that the boxed trailing arms were significanly better than nothing.. If my memory serves they were about 80% as good as what chris came up with.

So my interpretation of the testing was that chris' new and improved bracing was better than the boxed arms.. but that the boxed arms were much stronger than stock.

this doesn't answer the original threads question, but I think its worth noting that there may be different intrepretation of the results of actual testing.
My own personal opinion is based upon AJRS results that the boxed arms are very necessary and put onto every single car that he builds.

brant

[amallagh]

I put a kit in my last car. my next car will get one as well. I noticed a huge loss of flex. I thought the car handled better afterwards.

Your opinion seems to stand alone in a sea if derision regarding the benefits of fitting the boxed swing arm kit.
I'm intrigued - what made you think it gave you an improvement over and above just fitting a chassis stiffening kit ?
(Nothing like a bit of healthy debate !)

[Trekkor]

Engman kit thread

From the classics...

Great kit.

KT

[IronHillRestorations]

Ok, you can interpret Chris Foley's data however you choose.

Al Swanson, an aerospace engineer for Ratheon did computer modeling on the rear trailing arms and found that installing the rear trailing arm kit can actually weaken the rear trailing arms! Why? When you weld these on such a small part they aneal the steel, which weakens it.

This should not be confused with seam welding the trailing arms. There have been a couple instances that I know of where the bearing race (the part the bearing slides into) were not welded on the trailing arm adequately and came apart under stress.

[brant]

agreed...

everyone is going to have to form their own opinions.
did Al measure deflection or actual failure?

my learning curve has always been to see what the absolute fastest guys are doing and use their baseline as a starting point.
from that I've choosen to believe that the boxed stiffening kits are worthwhile.

I highly value the instruction and advice I get from AJRS. He absolutely insists upon them. I have ran a set with the kit for 14-16 years now and not had any failures or problems. I choose to trust his advice based upon results. For example I think AJ was running about 10 seconds per a lap quicker than Rich Bonatempi. at one of the PCA club races a couple of years back...

so everyone will have to form their own opinion.
but I'll try later to dig up Chris' testing thread.
I really remember that it showed an unboxed, or non-reinforced stock trailing arm had significant deflection (and less when boxed)

now interpreting how "significant" is really significant will be up to the individual user.

brant

[brant]

Ok... I found the link.


Chris test showed that his reinforcement was lighter (2lbs lighter than boxed)
and also superior/stronger (by about 20% than boxed)

however the boxed kit according to Chris is; "Yup. Actually more like 38%" stronger than a stock unmodified arm. (page 7 of the thread)

Chris' testing

[Eric_Shea]

The conclusion of that thread was that boxed trailing arms had one and ONLY one effect.... they added weight.

I'm with Brant and it wasn't I who said the "what would you rather have bending?" I think Chris' testing is the best we have to date. What I take from it is; for 6lbs. of weight you get a control arm that is 40% stiffer. I like that.

Your opinion seems to stand alone in a sea if derision regarding the benefits of fitting the boxed swing arm kit.

I also like:

* The factory chassis stiffening kit. All of it.
* Seam welding is a must.

You may well ask why I would want 40ishlbs of road hugging weight

I believe the chassis kit is something like 16-19lbs (JP is old and he keeps forgetting these things and he fudges numbers to support his delusion) (IMG:style_emoticons/default/w00t.gif)

Read in the link Brant posted above. Also find Jeroen's post about his semi-tube project... there's some good debate on the factory kit in there.

Old myths die hard... weed through and make your decisions. If I were in your position (which is what I'm doing now) I would (am) use the control arm kit along with the factory chassis kit (along with rear mounting ear braces and camber braces). I would also seam weld the entire lot. I spent a great deal of time snooping around the 914's at the HRS race at MM. All the top cars were seam welded. Ask Mike's buddy Jim Patrick what he thinks about seam welding. I don't believe there's a (race prepped) car that leaves his shop without it.

Flame away.

[Bleyseng]

Seam welding is a must as if you have ever cut apart a car you would see how many of the factory welds don't hold crap! The spot welds are hit and miss so the seam welding has to be done.

I thought Jeroen thread showed how the kit didn't address the shock tower/ long weakness which the kit doesn't address at all. Its a bandaid on the surface of the problem.
The Engman kit addresses the biggest weak spot on the car, the bulkhead to long connection.