Stupid light design, only because entire drivetrain is stress mounted. I want to qualify my design. Frame and complete drivetrain is aluminum, so very
little flex permitted, (cyclic fatigue). A conventional T'-bucket frame has lots of flex, but is steel. Anyone with torsional #'s to fit this design ?
Thanks, Art.
Torsional Test for Roadster Frame
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Dan Timberlake
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Re: Torsional Test for Roadster Frame
Some seriously qualified folks over on https://www.eng-tips.com/ .
My first guess is the chassis 10X stiffer than the suspension.
I'm thinking the potential for stress concentration (that would bring on fatigue real quick) in the fabrication and geometry details would not be detectable with a basic torsional analysis.
Fatigue of welded joints sometimes is so detail driven that the mechanical properties of the weld filler and base material don't even come into play.
The "toes" of fillet welds often mess up the show, so fatigue resistance improving details like TIG re-melting the toes or grinding a carefully blended radius profile on the toes/edges can make BIG imrprovements.
https://www.comsol.com/blogs/how-to-pre ... -of-welds/
https://processbarron.com/wp-content/up ... es-403.pdf
My first guess is the chassis 10X stiffer than the suspension.
I'm thinking the potential for stress concentration (that would bring on fatigue real quick) in the fabrication and geometry details would not be detectable with a basic torsional analysis.
Fatigue of welded joints sometimes is so detail driven that the mechanical properties of the weld filler and base material don't even come into play.
The "toes" of fillet welds often mess up the show, so fatigue resistance improving details like TIG re-melting the toes or grinding a carefully blended radius profile on the toes/edges can make BIG imrprovements.
https://www.comsol.com/blogs/how-to-pre ... -of-welds/
https://processbarron.com/wp-content/up ... es-403.pdf
Re: Torsional Test for Roadster Frame
What type of vehicle is this? What is it's primary mission, i.e. asphalt, off road, road course?
What alloy aluminum is the frame made from?
Can you post any pictures, especially showing front suspension mounting points/welds?
It's been properly TIG welded, yes?
All welds are fundamentally T0 now so you have some decisions to make:
As for stiffness targets, that takes heuristics. You can read up in Milliken and Milliken's Race Car Vehicle Dynamics for some ballpark torsional stiffnesses. I believe there might be a number or two in Fundamentals of Vehicle Dynamics by Thomas Gillespie.
You will never achieve steel chassis torsional stiffness so it's all going to be relative for your application.
Frame locations at bushing interfaces, engine mounts and suspension, should ideally be 10 times stiffer in every direction than the rubber bit. You can get by with 5X in my experience if you don't mind some vibration feedback.
For suspension locations, bottoming is much more damaging than repetitive flexing due to the spring/damper forces. Figure out your unsprung mass and sprung mass on each corner and assume the spring/damper have slowed to 10mm/sec before bottoming and determine the input and reaction forces. Stresses over 50% of yield stress should be a hard limit. Of course, it all depends on how many times the suspension bottoms. This is where engineered jounce bumpers are really handy, whether internal or external to the damper. The focus of the design should be to reduce the tensile stresses at the welds. This is done with proper large radius gussets and tapered/layered doublers. Sticking a tab onto a tube usually does not lend itself to good durability when using aluminum.
I've driven some pretty flexible chassis over the years and there are advantages to flexing under some cornering situations. Even competition go kart chassis have allowable flex that is driver and inertial induced to raise the inside rear tire to minimize push and really rail corners.
What alloy aluminum is the frame made from?
Can you post any pictures, especially showing front suspension mounting points/welds?
It's been properly TIG welded, yes?
All welds are fundamentally T0 now so you have some decisions to make:
- Run it as is and hope it works.
- See if a local heat treat facility has room in their oven for your frame and solution heat treat it back to its original temper, i.e. T4, T6, T651, etc. Then bend it back into formation.
- Leave it sit for a year or three and let it naturally age.
- Remake the frame out of 7005 and it should age properly inside of a year.
- Add a pressure fitting or two and monitor pressure after every session, ala Porsche 917. Reweld as needed.
As for stiffness targets, that takes heuristics. You can read up in Milliken and Milliken's Race Car Vehicle Dynamics for some ballpark torsional stiffnesses. I believe there might be a number or two in Fundamentals of Vehicle Dynamics by Thomas Gillespie.
You will never achieve steel chassis torsional stiffness so it's all going to be relative for your application.
Frame locations at bushing interfaces, engine mounts and suspension, should ideally be 10 times stiffer in every direction than the rubber bit. You can get by with 5X in my experience if you don't mind some vibration feedback.
For suspension locations, bottoming is much more damaging than repetitive flexing due to the spring/damper forces. Figure out your unsprung mass and sprung mass on each corner and assume the spring/damper have slowed to 10mm/sec before bottoming and determine the input and reaction forces. Stresses over 50% of yield stress should be a hard limit. Of course, it all depends on how many times the suspension bottoms. This is where engineered jounce bumpers are really handy, whether internal or external to the damper. The focus of the design should be to reduce the tensile stresses at the welds. This is done with proper large radius gussets and tapered/layered doublers. Sticking a tab onto a tube usually does not lend itself to good durability when using aluminum.
I've driven some pretty flexible chassis over the years and there are advantages to flexing under some cornering situations. Even competition go kart chassis have allowable flex that is driver and inertial induced to raise the inside rear tire to minimize push and really rail corners.
Re: Torsional Test for Roadster Frame
Asphalt only street roadster. Rover V16, ( common 7075 T6 main girdle, deep Y blocks). Rear pivot of front composite mono, 1/4 elliptics load on front of girdle., with 1/8" inside corner r. 1/8" wall 6061 T6, 2"x4", mainframe. Backfill with argon using 5356 rod. No post treatment. Yes to pressure gage/ inter-connected joints. Girdle spreads compressive/flexure loads on top of frame with underside of girdle to encompass frame radii, both sides of rail. Girdle laterals act as load speadders upon top rails. Engines, trans and IRS diff are acting as united stiffening members. Target wait is 1,600 lb.-
Thanks, Art.
Thanks, Art.
Re: Torsional Test for Roadster Frame
The 1/4 elliptics are going to be mounted to the crankcase girdle or to the side of the frame tube?
If mounted to the frame rail, doublers with extended ends should be employed to prevent corners where welds will break. (Think diamond shaped with elongated ends) Look at the illustrations in US10507870B2 patent to see how there is no "straight" doubler ends where it is welded to the frame tube. In Figure 6, you can see where a tailed off weld extension is used to eliminate a pinpoint stress riser due to the vertical bending and frame twist that occurs in that area. I worked with the inventor a bunch to create nice stiffness gradients where stiffness discontinuities of traditional construction were formerly employed.
Solid front axle or unique IFS using the 1/4 elliptics?
If mounted to the frame rail, doublers with extended ends should be employed to prevent corners where welds will break. (Think diamond shaped with elongated ends) Look at the illustrations in US10507870B2 patent to see how there is no "straight" doubler ends where it is welded to the frame tube. In Figure 6, you can see where a tailed off weld extension is used to eliminate a pinpoint stress riser due to the vertical bending and frame twist that occurs in that area. I worked with the inventor a bunch to create nice stiffness gradients where stiffness discontinuities of traditional construction were formerly employed.
Solid front axle or unique IFS using the 1/4 elliptics?
Re: Torsional Test for Roadster Frame
Solid axle, spring mounts on top of girdle "wing". Internal support tubes, thru bolts, inside 2x4 main rail, (load top and bottom of tube).
Thanks, Art,
Thanks, Art,