Front Torsion Bar Repair

356 Porsche-related discussions and questions.
Message
Author
JohnPierce
356 Fan
Posts: 166
Joined: Sun Sep 29, 2013 6:48 pm
Location: New Hampshire

Front Torsion Bar Repair

#1 Post by JohnPierce » Wed Mar 06, 2019 4:58 pm

I've finished the metal work on my 1963 S90 SR coupe after 5.5 years and have begun work on the front suspension. One of the first items is to clean out the old grease from the torsion tubes and from the torsion bars. Both front torsion bars have surface rust at the ends. One bar has a leaf that has come undone at the area of arc welding. The other bar has a leaf that is cracked several centimeters from the end. 

I could just leave the one that is undone at the end as the trailing arm will hold it in place, but what are people's recommendations for the other cracked leaf? Weld it together? My worry is that with the surface rust if one leaf is cracked the others may also be at risk of failure. Should I try to find a replacement set? New vs OEM and where?

IMG_8753 copy.jpeg
IMG_8753 copy.jpeg (199.89 KiB) Viewed 487 times
IMG_8752 copy.jpeg
IMG_8752 copy.jpeg (203.22 KiB) Viewed 487 times
IMG_8751 copy.jpeg
IMG_8751 copy.jpeg (207.86 KiB) Viewed 487 times
1963 B T6 Project

User avatar
Phil Planck
356 Fan
Posts: 1462
Joined: Wed Mar 26, 2008 7:21 pm
Location: NE Michigan, lower penn.

Re: Front Torsion Bar Repair

#2 Post by Phil Planck » Wed Mar 06, 2019 5:19 pm

John
I replaced mine with nos that came with my 55 coupe purchase years ago. Will see if I saved old ones. If so you can have them. Nothing cracked or broken that I recall, but ends may need rewelding.
Phil Planck

User avatar
David Jones
356 Fan
Posts: 3415
Joined: Tue Mar 25, 2008 6:32 pm
Tag: I wish I knew as much as I think I know.
Location: Harrodsburg, Kentucky

Re: Front Torsion Bar Repair

#3 Post by David Jones » Thu Mar 07, 2019 10:45 am

If all else fails you can find a set from a VW beetle in a junkyard. Must be millions of them out there.
If I had known I would live this long I would have pushed the envelope a little harder.
Cymru am byth
David Jones #9715

JohnPierce
356 Fan
Posts: 166
Joined: Sun Sep 29, 2013 6:48 pm
Location: New Hampshire

Re: Front Torsion Bar Repair

#4 Post by JohnPierce » Thu Mar 07, 2019 11:20 am

Thanks guys. Very generous, Phil!! I did a Google search but couldn't find anyone in the USA selling them. Does anyone know a source for new ones?
1963 B T6 Project

Brad Ripley
356 Fan
Posts: 3378
Joined: Tue Mar 25, 2008 9:28 pm

Re: Front Torsion Bar Repair

#5 Post by Brad Ripley » Thu Mar 07, 2019 12:48 pm


JohnPierce
356 Fan
Posts: 166
Joined: Sun Sep 29, 2013 6:48 pm
Location: New Hampshire

Re: Front Torsion Bar Repair

#6 Post by JohnPierce » Thu Mar 07, 2019 1:03 pm

Thanks Brad. That's some expensive metal. $460 for one. I think I'll try welding mine first.
1963 B T6 Project

User avatar
David Jones
356 Fan
Posts: 3415
Joined: Tue Mar 25, 2008 6:32 pm
Tag: I wish I knew as much as I think I know.
Location: Harrodsburg, Kentucky

Re: Front Torsion Bar Repair

#7 Post by David Jones » Thu Mar 07, 2019 1:58 pm

If I had known I would live this long I would have pushed the envelope a little harder.
Cymru am byth
David Jones #9715

Martin Benade
356 Fan
Posts: 3588
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: Front Torsion Bar Repair

#8 Post by Martin Benade » Thu Mar 07, 2019 7:50 pm

Is the stiffness a function of the cross section, set by the trailing arm hole size? In other words, are all front torsion bars that fit going to perform the same?

User avatar
Ron LaDow
356 Fan
Posts: 6206
Joined: Wed Mar 26, 2008 11:45 am
Location: San Francisco

Re: Front Torsion Bar Repair

#9 Post by Ron LaDow » Thu Mar 07, 2019 9:07 pm

Martin Benade wrote:
Thu Mar 07, 2019 7:50 pm
Is the stiffness a function of the cross section, set by the trailing arm hole size? In other words, are all front torsion bars that fit going to perform the same?
Yep.
Steel alloys suitable for that use are very consistent in their elastic values.
I have no idea how to calc the torsional rate of a laminated T-bar, but any laminated T-bar of that size will have the same rate.
Ron LaDow
www.precisionmatters.biz

User avatar
Don Gale
356 Fan
Posts: 35
Joined: Wed Jul 26, 2017 6:27 pm
Location: Albuquerque
Contact:

Re: Front Torsion Bar Repair

#10 Post by Don Gale » Thu Mar 07, 2019 9:12 pm

The stiffness is a function of the sum of the cross-section of the individual "slats/strips". Stiffness = GxJ/L where G = the torsional modulus, which will be the same for most grades of steel. J is the polar moment of inertia of the cross-section, which for a rectangular shape with a 6:1 aspect ratio, will be approx. .299 x t * w, where t = thickness and w = width of the individual slat, L = length center-to-end minus a token amount to account for the effective location of the grip area in the trailing arm. Judging by the picture and the break, it looks like it is made from a high strength cast steel. One could substitute cold rolled high carbon steel such as 1090. A generous radius on the outside edges of the slats will greatly increase longevity. The break was likely caused by an abrupt hit as in a wreck, pothole, or hitting a curb, or fatigue from high mileage.

In answer to your question, yes, stiffness will be approx. the same for bar packets that fit within the same size of the trailing arm hole, if the bundle is closely packed without gaps or spaces.

I remember as a young dumb high school kid, I removed the torsion bars from my '65 VW beetle and chiseled the bundle apart, and removed 4 of the innermost slats and substituted short spacers, in a crude attempt to lower the front end. It worked well and lowered the front end about 2", at the expense of a significantly reduced spring rate and sacrificed much strength, which I didn't care about at the time. I put the slats back in when I sold the car.

When I built my Speedster replica, I cut the center of the cross tubes either side of the center bolt and rotated them x degrees and welded the tubes back together, to set the ride height the same as my coupe. I lowered the rear by clocking the inner & outer splines of the rear torsion bars. No clue if a '67 VW matched legit Speedster spring rates, but it was a blast to drive. Yet another car I never should have sold.
1958 1600 Super Sunroof Coupe
former 1966 Euro 912 Sunroof
former 1978 Intermeccanica Speedster w/'68 912

Martin Benade
356 Fan
Posts: 3588
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: Front Torsion Bar Repair

#11 Post by Martin Benade » Thu Mar 07, 2019 9:47 pm

When I was a dumb high school kid I cut just one of the center anchors out of my 55 VW and rotated it X degrees. It lowered it nicely, what did that do to the combined spring rate of the two bars?

User avatar
Don Gale
356 Fan
Posts: 35
Joined: Wed Jul 26, 2017 6:27 pm
Location: Albuquerque
Contact:

Re: Front Torsion Bar Repair

#12 Post by Don Gale » Thu Mar 07, 2019 10:57 pm

Martin Benade wrote:
Thu Mar 07, 2019 9:47 pm
When I was a dumb high school kid I cut just one of the center anchors out of my 55 VW and rotated it X degrees. It lowered it nicely, what did that do to the combined spring rate of the two bars?
The combined spring rate of both bars remains the same. The torsion bar with the uncut center anchor sees more torque and carries more weight of the front end. The trailing arms on the torsion bar in the cut tube pick up a smaller portion of the weight. in the end, the spring rate for each tube and the combined rate remains unchanged. The exception to this would be if the cut and rotated anchor is rotated so much such that the uncut torsion bar carries the entire front end weigh. In that case, at least for static ride height, it would reduce the combined spring rate to approx. 1/2 the original.
1958 1600 Super Sunroof Coupe
former 1966 Euro 912 Sunroof
former 1978 Intermeccanica Speedster w/'68 912

User avatar
David Jones
356 Fan
Posts: 3415
Joined: Tue Mar 25, 2008 6:32 pm
Tag: I wish I knew as much as I think I know.
Location: Harrodsburg, Kentucky

Re: Front Torsion Bar Repair

#13 Post by David Jones » Thu Mar 07, 2019 11:46 pm

On the Vees I would remove one full bar and replace with 2 half bars and also prebend the bars to get the right height. This we had to do until the SCCA allowed the use of adjustable torsion bars as the 356 has. If one used a complete set of bars the ride height was way too high and had too much travel like a desert racer. For me the right height was with the lever arms parallel to the ground with the car at racing weight.
If I had known I would live this long I would have pushed the envelope a little harder.
Cymru am byth
David Jones #9715

User avatar
Mike DeJonge
356 Fan
Posts: 676
Joined: Thu Dec 25, 2008 7:58 pm
Location: southern Ontario, Canada
Contact:

Re: Front Torsion Bar Repair

#14 Post by Mike DeJonge » Mon Mar 18, 2019 4:11 pm

On the VW leafs inside the torsion tube, the ride height is adjustable from the front, of the torsion tube, which is opposite of the 356 cars. You can weld the dimple drilled into the leafs, which are for the adjustment grub screw. Welding the dimple may affect the steel which is spring steel. Also the leafs are about 1/4 short on each side, this may effect the grub screw placement, althought the dimples on the VW match the location of the dimples on original leafs from a 356..
Or you can just take one of the leafs from a VW and weld it onto the leaf set of the 356. we will be making the leafs for the front torsion tubes of the 356 later some time this year then we will have a complete torsion tube assembly.
Mike dejonge
Restoration Design Inc.
52 Pre A Body Bumper X2
53 Pre A coupe
54 Pre A speedster
56 Speedster
60 D'letern Roadster
67 911
05 997

Martin Benade
356 Fan
Posts: 3588
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: Front Torsion Bar Repair

#15 Post by Martin Benade » Mon Mar 18, 2019 8:23 pm

Mike, what VW adustment are you referring to? The only adjustment I am aware of is if you weld in a new center section, which then has an adjustment on the front of the tube. The factory VW center grub screw is just to clamp the bar tight in the square hole, and handle cornering forces.
PS keep those body parts coming!

Post Reply