Front Torsion Bar Repair

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JohnPierce
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Front Torsion Bar Repair

#1 Post by JohnPierce »

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?

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Phil Planck
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Re: Front Torsion Bar Repair

#2 Post by Phil Planck »

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.
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David Jones
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Re: Front Torsion Bar Repair

#3 Post by David Jones »

If all else fails you can find a set from a VW beetle in a junkyard. Must be millions of them out there.
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JohnPierce
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Re: Front Torsion Bar Repair

#4 Post by JohnPierce »

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?
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Brad Ripley
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Re: Front Torsion Bar Repair

#5 Post by Brad Ripley »


JohnPierce
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Re: Front Torsion Bar Repair

#6 Post by JohnPierce »

Thanks Brad. That's some expensive metal. $460 for one. I think I'll try welding mine first.
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David Jones
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Re: Front Torsion Bar Repair

#7 Post by David Jones »

If I had known I would live this long I would have pushed the envelope a little harder.
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Martin Benade
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Re: Front Torsion Bar Repair

#8 Post by Martin Benade »

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?
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Ron LaDow
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Re: Front Torsion Bar Repair

#9 Post by Ron LaDow »

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.
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Don Gale
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Re: Front Torsion Bar Repair

#10 Post by Don Gale »

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.
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Martin Benade
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Re: Front Torsion Bar Repair

#11 Post by Martin Benade »

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?
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Re: Front Torsion Bar Repair

#12 Post by Don Gale »

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.
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David Jones
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Re: Front Torsion Bar Repair

#13 Post by David Jones »

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.
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Mike DeJonge
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Re: Front Torsion Bar Repair

#14 Post by Mike DeJonge »

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.
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Martin Benade
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Re: Front Torsion Bar Repair

#15 Post by Martin Benade »

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.
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