CHASSIS STIFFNESS

[golfpin]

GOOD DAY ALL,
MY QUESTION ... HOW REPRESENTATIVE IS A STIFFNESS TEST BASED ON OLD FASHIONED KNOWLEDGE EG CHASSIS BOLTED TO THE FLOOR SUPPORTED AT 3 POINTS THEN LOADED TO CREATE AND MEASURE DEFLECTION. THIS IN THE LIGHT OF NO COMPUTER PROGRAMS OR F.E.A. OR LIKE THAT, JUST OLD FASHIONED HANDS ON STUFF? HAVE A PROJECT ON HAND THAT I MUST SELL GETTING TOO OLD WOULD LIKE TO HAND IT OVER WITH A LITTLE MORE INFO. COMMENTS APPRECIATED.
GOLFPIN

 

[GregLocock]

Absolutely nothing wrong with a physical test like that. Chapman had a rule about the distribution of stiffness along the length of the car, I suspect to stop people throwing stiffness away through lazy design at bulkheads.

I'd admit I haven't seen a production car set up for that test in the last 20 years, but a modal test tells you that and far more. I have seen it used for a one-off, and we then used it to correlate and check the FEA model. Sadly that website got deleted.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[golfpin]

Many thanks for the reply Greg.
Golfpin

 

[GregLocock]

Incidentally, if it is for your own purposes it doesn't matter so much, but restraining the chassis correctly while under test has a great effect on the measured stiffness. I ended up modelling the rig as well to get good correlation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[dynatune]

The best way is to do an "NVH" test with a "hammer" and use sophisticated software to extrapolate from "dynamic" stiffness to "static" stiffness at O Hz. I have always seen a great influence of the test result by the way the car was constrained to the rig (already for instance a hydraulic clamp on the lower floor section would massively change results) . Since most people do not have the opportunity to calculate those effects, the testing becomes somewhat inaccurate (and unfortunately useless) because not knowing where the error comes from makes it all a juggling act. If you do not have other options, use the test as an indication. If you do have other options, like for instance running an FE simulation, use those as a well trusted reference (as also the FIA does) and forget all about testing.

Cheers,

dynatune,

http://www.dynatune-xl.com

 

[golfpin]

With thanks to Dynatune and Greg Locock
all the best to all for the Easter week.
Golfpin