measuring torsional stiffness

[Michael]

Folks,

 

We've covered a lot of ground on the issues of roll cages, subframe connectors, floor pans, torque tubes, etc. But, I probably speak for many people when I say that the actual effect of these modifications is still shrouded in mystery. in other words, do we really know what increase in, say, torsional rigidity such modifications produce?

 

So, here's my question: has anyone attempted a "test" of their chassis's torsional stiffness, e.g. by shoving a long beam in the vicinity of the steering crossmember, clamping the rear fixed, loading the far end of the beam, and measuring deflection? Alternatively, does anyone have ideas on quicker and simpler methods?

 

The point is not to question people's structural designs, but to ascertain what's the maximum benefit for the minumum cost and effort.

[pparaska]

I had my car on a rotisserie once the subframe connectors were in and considered torsional testing it, but just didn't for some reason I can't remember. I think I was in a hurry to get it to the paint shop.

 

Yeah, I'd love to know whatthe effects of the subframe connectors are. Great point.

 

 

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Pete Paraska - 73 540Z - Marathon Z Project - pparaska@tidalwave.net">pparaska@tidalwave.net -

[Guest Anonymous]

When I get my spaceframe completed I will be doing the test you specified. this is definately a concern of mine but I realized that I don't have a baseline measure to compare my results to (i.e. my car was too far gone in the front end to do this test before ripping it apart). If anybody has done this already or is planning to do it soon please send me your findings so I can discover that all this work has probably gotten me nowhere!!

 

SpencZ

MonsterZ Cont. http://members.xoom.com/SpencZ/index.htm

[Mikelly]

Well a poor mans way of measuring stiffness could be this: Any Mods to the chassis could be measured by doing the mod, placing a jack under one corner of the vehicle and measure all other corners in 6inch incriments. One thing I noticed was when I put subframe sin the car, I could jack two wheels up from one corner of the car... When I put the cage in, I could jack up three wheels from one corner of the car... This is a MOST cruid way of check for stiffness, but it atleast shows you just what each mod does, to some extent... This is really backwoods engineering ....

 

Mike

 

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"I will not be a spectator in the sport of life!"

mjk

[Guest Anonymous]

Mike...that is even too backwoods for me...and I live in the backwoods!!! It definately would be effective in giving you a general idea of what the mods are doing...but with no numbers associated with it the usefullness of the procedure is questionable. This does bring into question how to keep the chassis tied down if the torsion bar is used to twist off the front end...has anybody seen jigs for this procedure?

 

SpencZ

MonsterZ Cont. http://members.xoom.com/SpencZ/index.htm

[Mikelly]

Usefull? Ah hel! SpencZ, now you tell me.... I was just using it as a way to justify the expense and headache!

 

Seriously, You are absolutley right, there is now way of measuring it. We had mine in a craddle before the floorpans and subframes were put in and we wondered them same thing!

 

Mike

 

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"I will not be a spectator in the sport of life!"

mjk

[Drax240z]

Well, ferarri and porsche have been doing something similar for years... One way they show off their rigidity is to jack up one corner of the car to the point that 3 wheels are off the ground, then open and close the doors.

 

Doesn't help you measure it, just thought someone might be interested.

 

Drax240z

[Al Keller]

SpenZ,

There was an article in the Jan 1989 Circle Track called "Charting Your Chassis" that showed one way of fastening the chassis down to obtain stiffness numbers. Basically, they chained the chassis to the jig in the rear-end area and used a long beam to torque the front end. The measurements were then converted from inches to lb-ft. I just got a scanner and could TRY to send or I can copy and mail or fax the info if you're interested.

 

Al Keller

[DennisK]

I know I read SOMEWHERE that the Germans have a test for ridgity in chassis. As with all tests, a benchmark needs to be established for comparison. I don't know where I read this(old age), but the conclusion of the article was similar to the other responses. They put 4 inch blocks under opposing corners of the car and open/close the doors. If they don't bind, go racing!

[pparaska]

For the last 5 or ten years, the big 3 US car makers (that includes the old Chrysler) and many of the other OEs have been touting how the new model of car X now has a YY% increase in torsional rigidity. I don't know if they do that by analysis or test.

 

The way I see it, for handling, the biggest deal would be to know if the car were stiff enough (don't know how stiff that would be) at the 4 places that the struts mount to the unibody in the towers. If cornering loads or driveline reactions could twist the car so that the line connecting the rear two strut towers and the line connecting the front two towers changed from the ideal parallel to some skewed angle between them (envisioned along the longitudinal axis of the car), then that would be what mattered.

 

Of course, you'd need to baseline a Z and apply a certain amount of twisting torque on the body, and measure the angle change of these lines. Then do you chassis mods and measure again.

 

Even if the car was complete, you could use the "jack up one corner" method to assess any modification's stiffening effect. I might do that once the car is started and I have all the body work back one it. It has no cage now, and I'm thinking I want to do a bolt in cage some day.